Medical Tubing Extrusion Made Easy(ier) …Author: Mike Ferrandino

Publication credit: A greatly abridged version of this text was published in back-to-back issues of medical device technology magazine in oct/nov 2004. The second paragraph was updated but the rest of the text was only subjected to a grammatical review prior to this writing. Originally dated 8/3/2004, republished on this website 8/7/2016

It is difficult to envision a catheter that does not include at least one piece of tubing. Actually, if you take two minutes to do a web search on the definition of the word catheter (definition +catheter), you would be hard pressed to find a definition dated in the past five decades that does not use either the word tube, or the word tubing. Historically, catheters have provided body port access for infusion, suction, and drainage.

Over the past 40 years, catheter development challenges have progressed from due diligence in the design of a less traumatic device for bladder access, to creating a means of performing or delivering non-surgical, lifesaving and life extending treatments from the lower leg to the brain and everything in-between. Entire businesses are dedicated to reaching each organ in the body for diagnosis and treatment; these businesses can start with the design of a single piece of tubing.

Millions of procedures are performed via catheter each month and still the market continues to expand. To support these business ventures, tubing is needed…a whole lot of tubing. Custom tubing is required for every custom application. Often times, it will take two, three, or as many as six or more different pieces of tubing to manufacture one catheter. Each of these tubular components is created to support a device or process function. Each tubular component has been designed to meet a need.

Understanding the basic functional needs of the device are usually fairly straight-forward. Delivering to the need, more often than not, takes a bit more work. Although it is necessary to bore down into each of the following two statements to really get it done well, when you come right down to it, success comes down to two things: focus on the right things and apply discipline in continuous execution of the right thing.

Regardless of the size of the organization, success will require the application of focus and discipline in two separate areas. One of these areas is the process and the other is the business. There has to be a vision that helps both of these elements flow in concert. One can focus solely on what is right for the process and fail because what was right for the business wasn’t in the plan. Likewise, focusing on the P&L and ignoring what is right for the process will also, inevitably, result in failure; potentially a more colossal failure, but a failure non-the-less.

Taking care of business has to do with meeting the needs of the market and the stakeholders while you properly manage business inputs and outputs. Doing what is right for the process has to do with understanding the process and understanding the needs of the material in, and for, the process.

When working with the process, it must be understood that production of an extruded product is the result of a compound process. To create tubing, consistently, with consistent properties, it is necessary to understand and control each phase of the process.

Yes, without conscious separation of each element of the process, is entirely possible to: place tooling in and on the extruder, heat it up, put material into the hopper, start the screw, string the line and make tubing that is the desired size. What it isn’t so possible, is to figure out why the results of the run, even though meeting ID and OD requirements, do not meet other requirements. That, or it isn’t possible to truly understand why ‘when everything is done the same this time as it has always been,’ that the component properties aren’t the same from run to run. Why the tubing doesn’t feel the same, why it doesn’t act the same, why it won’t fit the way it did the last time, why it doesn’t look the same as it did the last time.

If things are done right in extrusion, everything falls into place; when they aren’t, things often fall to pieces. It isn’t that extrusion is such a tough thing to do right; it is just that it is so easy to cause it to go wrong. Any little piece out of place can wreak havoc on any, or every, level of the process output.

If one looks at the process in an overly simplified way, material is procured and prepared. The pellets are then introduced to the extruder. The extruder turns the pellets into a homogenous, continuous flow of melt, pushing it through a set of dies used to size and shape the melt. The melt is pulled (drawn) away from the die set and through a quench to ‘re-solidify’ the material. It is cut to length on line, or coiled or spooled to the customer’s requirement.

Though not quite that simple, the process can be broken down into three main functional elements. Creation of the melt. Creating a stable flow of minimally damaged, clean, homogenous melt of

the proper viscosity. Channeling of the melt to and through the die set. Turning the pre-sized, pre-shaped melt into the desired tubular form.

If the melt isn’t under control, nothing downstream of the melt will be. If the tooling does not accommodate the melt, or the melt is not suited for flow to or through the tooling, the mismatch will result in process failures.

The integrity of the melt must not be compromised as the material flows to the die set. The pre-sizing and pre-shaping of the melt must be done in such a way as to deliver the dimensional and structural attributes in a tube that will support the designed function of that part.

To make consistent components, tube to tube, batch to batch, run to run, month after month and year after year, it is necessary to ensure that the melt that comes out of the die set is as consistent as possible.

Know the material, understand the process, control the basics, and keep the goal…the product… in focus.

Creation of the melt

To deliver a stable flow of clean, minimally damaged, homogenous melt from the barrel of the extruder, it is necessary to control the process inputs. One of the significant inputs is the material being introduced to the process.

The more that is understood about the material, the material’s process sensitivities, and how the process interacts with the material, the easier it is to generate process controls and standard operating procedures that result in refining the science of extrusion.

Immediately following is a condensed outline of what is involved in forming a foundation of material understanding. It will be followed by sections on the barrel and screw, and filtration…the other elements in the first functional element of this compound process...creation of the melt.

Knowing the materials

Suitability for processing

Know whether the material is friendly or unfriendly. Is the material process friendly? Materials should not be selected for use simply by looking at the physical property data sheets of the materials.

If a material is not process-friendly, it will be very difficult to achieve a level of process control that will deliver a consistent output. If the extrusion process is not in control, every function downstream that relies on extruded component consistency will suffer. If a material is extrusion friendly, it will also be assembly friendly; as dimensional and physical attributes will be much more consistent. This leads to more robust downstream operations and therefore more consistent product.

Finding the best material option to meet all needs in design and assembly is necessary to deliver an optimum product output, but keep in mind...if extrusion can’t process the material in a controlled fashion,

whatever product is approved, may not be manufacturable in a cost-effective manner. A product is no good if you can’t make it profitably. In selection of material, consider all process steps to create a best-fit the application.

Exceptions abound in the medical tubing industry and the area of engineered resins is no exception. When working with engineered resins, there may be that there is only one manufacturer that makes only one grade of a material that is suited to deliver the desired mechanical properties in the component. If that is the case, there isn’t much that can be done except to work with the supplier to ensure process controls are in place, there, to ensure control of this critical process input. If there are a number of materials to choose from, take the time to select the one(s) that will be the easiest to handle and process. If the processes are in mind during the material selection process, a basic element of design for manufacture is in play.

Material sensitivities

Whether a material is extrusion friendly or not, to gain control of the process it is important to understand the material’s processing sensitivities. There are four prominent material sensitivities: heat, moisture, flow, and UV.

Heat sensitive material

Although virtually all thermoplastics are sensitive to heat, some are much more so than others. When working in temperature sensitive thermoplastic applications, it is necessary to be conscious of the amount of heat energy being applied to the material or generated in the process.

Heat sensitive materials are prone to degradation. Gels, fish-eyes, black specks, black streaks, lumps and bumps, discoloration, and material property compromise are each signals that the effective life expectancy of the run has expired.

Poor handling or poor treatment of materials in processing will negatively affect product quality and production capacity. Degradation will inevitably set in at some point during processing.

The following are a handful of basic accommodations that will aid in minimizing the negative affects of the process on the physical integrity of the material, while maximizing the life expectancy of runs with heat sensitive materials.

1) Keep residence times down

Maximizing output on an extruder can result in minimizing the residence time in the extruder, which will result in the material ‘seeing’ less energy, if the higher output doesn’t generate an excess of viscous heat energy in the process. Reducing residence time is a large factor, but like so many other things about the extrusion process, it is a balance

2) Minimize flow resistance

Increased resistance to flow leads to degradation for three reasons. One, it increases residence time; two, it generally increases the amount of viscous heat generation in the process; and three, the point at which the resistance is generated is usually a place where the material can hang up.

Flow restriction within the barrel should be only enough to generate a homogenous and clean melt.

Flow restrictors such as dispersive mixers should only be used if product quality depends on the use of such a mixer.

3) Use tooling designs that have streamlined material flow paths

When working with heat sensitive materials, it is important to ensure that the tooling design provides for smooth forward movement and transition to, and through the die. Tooling design must help preserve the integrity of the material. Dead spots must be avoided.

Basic design assembly review can help identify physical ‘perches’, ‘ledges’, and ‘flats’ that will stop the flow of material, leading to degradation. Flow analysis can provide analysis of tooling designs that incorporate intricate or delicate transitions and flow diversion.

4) Double-check extruder and tooling setup prior to introducing the material to the screw

Controls should be put in place to ensure the material is not errantly exposed to an overabundance of energy. The process will suffer, the run will age quickly, the material will degrade and the extruded components that are produced can be cancerous for downstream processes and product performance.

Ensure all thermocouples (TCs) are bottomed out (the tip of the TC is touching the bottom of the TC well). This is, by far, the largest single contributor to across-the-board failures in every extrusion process.

Ensure all TCs are in the proper locations. Transposed TCs will cause one zone to be hot while the other is cold. Both of these situations are negatives for the material and the process.

Ensure that all heaters are in full contact with the tooling. Loose heaters will poorly deliver or inconsistently deliver heat to a zone. Loose heaters will lead to a minimum of two failures. One being that hot and cold spots will be present, affecting material flow; another being premature heater failure. Heating elements that are not in contact with metal will burn out, which will, ultimately result in the loss of the run.

5) Keep the heat history of the material to a minimum.

Minimize the number of times, and the length of time, the material is exposed to heat energy.

Expose the material to just enough energy to perform the task at hand. Heat in the material drying process Thermal energy from the heaters Pressure in the barrel

Every time the material is exposed to energy, there is an opportunity to damage the material, or add to the damage already present in a material. Every time the material passes through an extruder, the process degrades the material.

6) Incorporate thermal stabilizers into pre-compounded materials.

Often times in the medical industry, manufacturers of tubing utilize materials that have been pre-compounded with additives and fillers. The compounding process is a process that blends these ingredients in an extruder to create the pellets that are subsequently used by these tubing manufacturers. Often times these processes utilize dispersive mixing elements that can erode the physical properties of the material (degradation).

Heat stabilizers can, and should be added to the blend during the compounding process to help reduce the affect of process heat on the physical properties of the material. However, heat stabilizers are normally only an antidote for normal abuse of material during processing. No amount of heat stabilizer will provide protection for the compound, if the compound is exposed to a poorly characterized extrusion process. Nor will it protect the material (tubing) further downstream if the material is bombarded by extraordinary processing energies in the catheter assembly processes.

Failure can happen any where down the line. Good process discipline and characterization must be exercised everywhere the material is processed and reprocessed. Full control of the product cannot happen without full control of the materials going into the process, from raw material production and pelletization, through compounding and repelletization, through tubing extrusion, and all along through packaging sterilization and storage.

Moisture Sensitive Material

The affects of the moisture on the process will vary, process to process, material to material, and application to application. Moisture acts as a plasticizer. If a material has an elevated moisture level when it is exposed to the process, it will take less energy to process the material, if the moisture content is lower, it will take more energy to process the same resin.

Moisture is typically a negative for the process for two reasons, first, it is difficult to control the moisture level in the material. As moisture plays a part in the process, changes in the level of moisture will wreak havoc on the process. Secondly, when moisture is present in excess as it is processed, physical damage to the material will occur.

To preserve the physical integrity of moisture sensitive materials as they are processed, it is necessary to remove excess moisture before processing. To assist in gaining control of the process, it is necessary to at least attempt to control the level of moisture present in the material. The greater degree of control, the greater the opportunity for full process control.

Though moisture levels in resin or plastic parts have little affect its properties in a static state, moisture in or on a material that is being melted, or re-melted, can significantly impact the tensile properties and physical nature of the material, post-processing. Basic properties such as flex modulus and elongation can be severely and permanently affected if a material is moisture sensitive and that sensitivity is not understood and accommodated during processing.

If there is moisture in or on a material, that moisture comes from the atmosphere the material is, or was in, regardless of whether the material is in the form of a pellet, or the form is a bumper or fender on a truck. Myth: Moisture is only an issue in extrusion. Fact: moisture has an affect on secondary processes as well as primary processes. Myth: once the material is dried, it stays dry, even after it is extruded. Fact: The material, in any form, will take on water just as easily as it lets go of it.

Moisture does not magically affect process and properties only during extrusion and molding. Moisture plays a part in bonding, welding, ’California’ style assembly processes and any other process that provides enough energy to melt the material. While processing the material, know that if there is enough energy being applied to melt it, there is enough energy to damage the material…if the moisture level is high enough.

Extrusion processing of moisture sensitive materials

Obviously, high moisture content in extrusion processing is a bad thing for the process and the components manufactured with the materials in the given process. However, it can be just as detrimental to process materials in a given process when the moisture level is exceeding low. As stated earlier, it takes more energy to process materials with lower levels of moisture, the material has to be physically pushed ‘harder’ in the process to become a melt. This additional, physical push can, and usually does, drive the melt temperature higher.

As the melt temperature goes up, the melt viscosity goes down; too, the opportunity for degradation increases. If the viscosity is lower when the material leaves the barrel, the melt will not act the same way as it travels to and through the die set. If the melt does not act the same way as it is pushed through the die, it won’t ‘draw’ the same way when it is pulled from the die at the prescribed rate for the process.

Though the tubings produced with high and low moisture content in a given process may be dimensionally equivalent, the tubing properties will not necessarily be the same.

To control the tubing extrusion process when working with a hygroscopic material, strive to control the moisture level in the material being processed. At the very least, ensure that the preparation for processing delivers a material to the process that has a moisture level below that which is recommended by the material manufacturer.

Typically, over the past few decades, it has been enough just to keep the moisture level in a material during processing below a level that openly results in material failure. In today’s world of tighter requirements and more demanding applications (were not just making IV and suction tubing any longer) it is important to focus on ‘controlling’ the moisture level of the material so that the process outputs are more constant.

Obvious, physical flaws result from surplus of moisture in a material during processing. If a material has a gross overabundance of moisture the extrudate leaving the die set will actually be foamy and have little to no melt strength*. If the melt leaving the die set has numerous voids in the wall mass of the tubing, it is open proof that the moisture content is too high for the process.

How much is too much? While characterizing a process with a hygroscopic material, understand that as process and melt temperatures are elevated, the tolerance to moisture in the material, during processing, is reduced. Example: no physical damage (obvious or hidden) to the material may result from processing a polyurethane at 100PPM [parts per million (moisture) by weight] while processing the material in a given process at a set linespeed, at 400ºF, but at 450ºF, there may be obvious degradation or property loss.

Typically, as moisture level increases, the barrel pressure will start to decrease and the output will increase. It is one of the only times during extrusion that a reduction in pressure will be paired up with an output in volume. The reason is, as the material viscosity decreases, there is less resistance to the flow of the material through the extruder and tooling. With less resistance, more material can exit the extruder with less pressure behind it.

If using heat in the drying process, understand that this heat energy can take a little ‘life’ away from the material being dried. Drying heat adds to the heat history of a material, especially if the heat is applied for extended periods of times and the material is exposed multiple times to the drying process.

The material drying facet of the process is becoming more critical to those who produce tubing to support medical device applications. Manufacturers of material drying equipment are becoming more in-tune with the process need, but it is the tubing manufacturer that needs to lean on the equipment suppliers to deliver equipment that can prepare material for processing with less energy and greater moisture level control.

Processing of moisture sensitive materials, post-extrusion As stated earlier, any process that re-melts the tubing after extrusion should understand the affect on these secondary processes. Secondary process characterization should take into account the affect of moisture on the processes.

Yes, the material was dried prior to extrusion but that doesn’t stop moisture from getting back into the material after it is extruded.

Right after extrusion, the tubing again has moisture in it…usually lot of moisture. Does it have too much moisture for proper processing post-extrusion? It quite possibly may. Although secondary processes are generally more forgiving in regard to the amount of moisture in a material that can be tolerated, these processes are not exempt from the affects of moisture.

What can be done to control moisture in the material post-extrusion? Materials can be held until they reach a steady-state (Equilibrium Moisture Content, or EMC**)

**Plastics Engineering, June 1993, John W. Doub. Jr; Novatec, Inc.Plastics Auxiliaries, July/August 1998; John W. Doub, Jr.; Novatec, Inc.

Equilibrium Moisture Content (ECM) is a term that refers to the state at which the material in a given environment has absorbed or released all of the moisture it can in that given environment. EMC applies to all hygroscopic plastics in a raw, semi-finished, or finished form.

The EMC of the material is dependent upon the material itself, its saturation point, its absorption rate, the temperature of the environment, air flow around the material, and the relative humidity in the environment in which the material (or tubing) is kept. At room temperature it will take a material 5-7 days to effectively reach its ECM.

Although the moisture content will never be perfectly stable, unless the environment is perfectly stable, it will reach a state of relative stability. When answering the question, “how long do we need to wait after extrusion before we can use the material?”, the true answer needs to come from an effective Design of Experiments (DOE).

Ultraviolet (UV) sensitive materials

Many materials are sensitive to exposure to ultraviolet light. UV light can cause degradation. Materials that are UV sensitive should not be stored in areas where they are exposed to UV light.

The length of time of exposure to UV light is a topic of debate, but most people the author polled agreed that a month of exposure was enough to degrade the material. As UV light sources are used throughout most manufacturing areas and in most cath lab storage areas, it is certainly a good idea to add UV stabilizers to a material recipe when having materials pre-compounded for use in medical tubing extrusion, molding applications, and medical device packaging materials.

Flow Sensitive Materials

When processing flow sensitive materials, once the material is introduced to the process and processing has begun, the flow of the material should not be interrupted. Interruption of the flow of the material will cause degradation to immediately begin to take place. Example: aromatic PU processing; if the flow stops, the material will start to degrade, the degradation exhibiting itself in the form of small opaque crystalline chunks, the chunks growing larger and more frequent as the degradation spreads.

Typically, degradation will start at one point somewhere in the process; most often somewhere along a metal-to-material contacting surface, where the material typically moves ‘slower’ through the melt stream.When the degradation begins, it might be one very small area, but it can quickly spread to cover a larger area, as the degraded material acts as an obstacle to flow, causing more of the melt to stop moving, exponentially spreading the degradation until the melt is so full of crystalline bits and pieces and chunks that even the most forgiving of inclusion specifications is easily exceeded.

When working with flow sensitive materials, operational basics and tooling design must support the processing of the material. Extruders should not be shut down with the material ‘loaded’ in the machine, if the intent is to string the line again for processing. Tooling changes should be kept to a minimum during the life of the run…as the life expectancy will be reduced with each shutdown to change tooling; even if the shutdown is only for a minute or two.

Tooling alignment is important, as any mismatch upon assembly can create a ‘ledge’ for the material to hang up, where it will break down and cause a chain reaction for degradation. Equipment must be fully up to temperature prior to beginning the run. If a cold zone exists, the material will be held up along the cold tooling surface, initiating degradation.

It can help, when working with flow sensitive materials (ones that aren’t necessarily heat sensitive too) to increase the tooling temperatures 10 or 15 degrees F until the material is introduced to it, returning the temperatures to operating temperature as soon as the material is flowing smoothly out of the extruder.

Barrel and Screw: the primary process elementsThe barrel and screw are the heart of the extrusion process. The screw works with the barrel to convey, melt, and meter the material that exits the extruder. It is critically important that appropriate effort go into the selection and or design of a screw for use in any given extrusion application. By understanding each factor’s role in the process and understanding how variations to these factors can affect the process or processing, it is hoped that the reader will be able to more confidently select or design a screw for a given application or to address a processing issue.

L/D - Length to Diameter Ratio

The length to diameter ratio is the length of the working part of the screw divided by the diameter of the screw. It is the portion of the screw with the flight wrapped around it. The most common L/D ratio for extruders used for tubing extrusion is 24:1. Though the L/D for the extruder is called out at 24:1, the working length of the screw may be 3 or 4 flights ‘longer’ than the barrel as most smaller extruders, today, have feed ports that are not part of the barrel itself, but are integrated into a separate section called the feedthroat, which is designed not only to provide a port to introduce material, it also provides cooling at the point of entry to help keep materials from bridging in the feedthroat, providing uninterrupted flow of material to the screw.

Compression Ratio

Compression ratio (CR) is a factor in how much ‘push’ or force is being applied to drive the material forward and also how much restriction to flow, off the screw, is present.

Higher compression ratios are often used to increase viscous heat generation and ‘working’ of the material during processing. If a limited number of transition flights are available on a screw and evidence shows that the material is not ‘worked’ hard enough (cold gels, cloudy inclusions, fish-eyes that are not discol-ored, poor color consistency, etc.) or there are an insufficient number of flights on the screw to effectively create the melt, increasing the compression ratio may help.

Lower compression ratio screws are often used in applications that have simpler conveying and melting needs.

Typically, the compression ratio should be as low as possible for an application as a process will typically suffer more in regard to stability from over-working the material than under-working it. Additionally, the material or component being manufactured will more likely suffer in regard to physical integrity as a result of the exposure to the high sheer energy in processing on higher compression screws than in processes utilizing lower compression screws.

Remember, during screw assignment or assessment, that the compression ratio can be varied by changing the feed or metering channel depth. Picture the needs of the material flow and material considerations when deciding which channel depth to alter when increasing or decreasing the compression ratio.

Do many or most runs end due to material melting on the root of the feed flights on the screw over the full range of process conditions applied? Is this condition accompanied by periodic if not cyclical positive and or negative pressure spikes, over time? If so, the material may be melting too soon in the barrel and once the screw root has some material melt on it there could be a cyclical build-up and release of ‘chunks’ of pellets in the channel which result in the pressure spikes and increased process drift.

There may be inadequate melting capacity on the screw, or it may be that there is too much restriction for the material melt to flow effectively off the screw; the metering channel depth may be too shallow. The opposite could also be true.

Always consider the nature of the material, the process conditions being applied in the process, the overall screw profile, and the data you get from the ‘process output gages’ when making modifications to the process or deciding which direction to take for screw profile on the next set-up.

One can adjust the push, melting, and flow of the material through the barrel in a variety of ways. Compression ratio is a key factor, but it is still only one factor.

Feed Flights and Channel Depth

In the first few flights of the feed end of the screw the raw material or pellets are introduced to the process. The feed section of the screw has to be deep enough to accept the pellets being fed to the extruder.

The feed flights of the screw channel are responsible for delivering the raw material to the compression flights. There need to be enough feed flights to ensure constant feed; it is critical, however, to ensure too that only enough flights are allocated to feed as are truly necessary. The greater the number of feed flights assigned to the feed end of the screw, the fewer flights are available for melting and metering.

The depth of the channel in the feed flights can have a significant affect on the forward ‘push’ that is gen-erated during extrusion. A change in ‘push’ will affect how the material melts and flows as the material moves through the screw channel. Deeper feed flights will provide more ‘push’ for the material.

Transition Flights (compression)

Through the compression zone, there is a gradual transition from the channel depth in the feed flights to the depth of the metering flights.

The melting of the material must take place in the channel during transport through the transition flights. If the material is not fully molten prior to reaching the first metering flight, solids in the melt will create a restriction to flow. This restriction to flow will affect output quality. The reasons are two-fold.

First, short term pressure output instability will result. The unmelted solids will impede the flow of melt from the screw. The chunk of unmelt may or may not be small enough to travel into the metering channel.

If the chunk is not small enough to pass into the metering channel, it will sit at the end of the transition section. A blockage will result and the blockage will create an output pressure drop. It may be minor, or it could be a significant spike. It the chunk is very large, it could create a solid plug, effectively stopping flow.

If the chunk of unmelt is small enough to fit into the metering channel, it will push forward until it gets stuck. If mixing elements or dispersive flights are used, the chunk could easily get lodged in the flow path. If it stops here, the result will be a pressure drop. Again, it may be minor at first, but could become a significant problem in a relatively short period of time.

If the chunk of unmelt is small enough to fit into the metering channel and travel the entire length of the screw, it will leave the screw only to stop against the screenpack in the breakerplate. There it will sit blocking flow for the life of the run, or until there is enough backpressure built up behind the restriction to push it through the screenpack. If the chunk does get pushed through the screenpack, the unmelt would be cut into many smaller bits and pieces.

All of the resulting smaller pieces are in the melt heading for the die. The unmelts become inclusions in the extrudate. These inclusions are often referred to as gels.

Secondly, there will be some type of longer term disturbance to the process (or a permeating aesthetic problem with inclusions.)

If the process yielded one portion of unmelt to the metering section it will probably produce many more.

If the process cannot recover from the first unmelted mass before the second one arrives, the problem will increase in magnitude. Whatever the problem (s) is (are), as the additional chunks of unmelted material collect where the earlier one(s) lodged (or elsewhere in the system) the problem will grow exponentially.

If the chunk of unmelt clears the die before the next chunk of unmelt reaches the end of the transition sec-tion (highly unlikely) the problem could be simply that the run will be infested with inclusions. If they are tolerable in size and texture, a marginal process exists. Some adjustment to the process or screw profile must be made if the problem is to be addressed fully.

If the chunk of unmelt does not clear the die before the next chunk of unmelt reaches the end of transition section, it is a safe bet that before long the chunks of unmelt will start to agglomerate somewhere in the system. It could be in the head in a designed area of flow restriction; it could be in the screenpack; it could be at a mixing element or in a mixing flight; it could be in the metering channel. It could be that it just meets up with the other ‘chunk’ at the end of the transition zone, too large in size to pass into the metering channel.

The restriction to flow will increase. If subtle in nature, the operator may be able to overcome the shortage of material by increasing the speed of the extruder or slowing down the line. If feedback control is used, this will take care of the short-term problem (if it is flow only) for a while. As the force behind the blockage is increased it may in fact actually dissolve the mass due to higher viscous heat generation in the process. But when that blockage finally releases, there could easily be a surge of material forward, causing the need for further reactive adjustment.

The result of the ‘back-up’ of material on the screw, over time, can easily lead to material sticking to the screw in the feed flights. It is at this time that operators often ‘give up’ on a process and decide to clean the extruder out. The operator may notice that material is stuck to the screw in a couple of the feed flights....problem solved...right Not necessarily; as just explained, the problem of material sticking to the

screw too early can actually be a result of the screw not having enough melting capacity. Sounds screwy, but it is true. Piece all the information together when trying to solve the problem.

If the right troubleshooting adjustments are made for the wrong reason, the problem will get worse. To remedy the situation just covered, the operator may decrease the feed zone temp and or the feedthroat temp to keep the material solid longer in the barrel (so that it doesn’t melt too soon). If the root of the problem was unmelt resulting in flow restriction at the metering channel entry, this adjustment will make the prob-lem worse.

This is a classic situation. Long-term (hopefully in a relatively short period of time) it is necessary to cre-ate an understanding of the real reason the adjustment required was exactly opposite of what was expected. An understanding of the true root cause is essential in process development and refinement efforts.

This is why it is necessary for those operating and supporting the processes to be intimately involved in process observance. A constant watch over the process output gages (“windows to the process”) is required to fully understand the process and resolve any issues that form. Know when the problem is born. Know where the problem is born. Understand how it happened and why.

How are these phenomena avoided? Simply insure that there is the right balance in regard to the number of transition flights and the amount of ‘push’ provided. A safety net of a couple of extra flights of transition will usually solve melting problems, but at what cost. Remember, there are only so many flights available on the screw to ‘execute’ the entire process. If additional transition flights are added, they must be taken away from either the feed or the metering sections of the screw.

The force of compressing the pellets as the screw channel depth changes results in the compression of the material; which results in melting through viscous heat generation(shear).

In micro-bore medical extrusion, screw speeds tend to be much lower, therefore there is generally less viscous heat generation. With some materials there is little to no viscous heat generation. With these process/material combinations, the heat for melting comes from the barrel.

In situations where the melt temperature (measured at or near the end of the screw) is at or lower than the process temperature settings, heat for melting of the material is most likely coming from the barrel wall rather than from the process.

At slower speeds (without viscous heat generation) it may take more flights to effectively deliver a material melt to the metering flights; a melt that is free of solids.

Regardless of what material is being processed, and at what speed it is being run, to produce a stable pro-cess and components free of unmelts, the material in the screw channel must be fully molten before it reaches the end of the transition section.

Metering Flights and Channel Depth

The metering flights are used to meter the material being extruded. The metering flights of the screw help deliver a continuous flow of consistent melt to the die.

The metering channel depth and the number of metering flights create a restriction to the flow of material from the screw. This restriction to flow is necessary to create a truly homogenous melt, homogeneous in viscosity and temperature

The channel depth in the metering flights has a significant affect on the process and the material melt properties; those properties being melt temperature and melt temperature homogeneity, melt viscosity and melt viscosity homogeneity; melt pressure and screw beat, as well as long-term pressure drift.

Knowing the compression ratio (CR) and the channel depth are key to understanding a process output. CR is not the most important factor in a screw design, but it is one of the most important process effectors. Metering channel depth and CR, along with other process settings and variables, determine what properties the melt will have when it leaves the screw.

As the material exits the last flight of the screw, the melt should be: of even consistency, even temperature, and even pressure.

Shallower channels generally promote greater mixing, but create greater resistance to flow. Resistance to flow keeps material held back on the screw longer. When this occurs, the region where melting begins is pushed back toward, and or into, the feed flights of the screw (during this time, often causing output instability).

A deeper metering channel depth allows the material to pass through the zone with less resistance.

An increased number of metering flights offers an increase in resistance as the length of the resistance is increased. This provides a greater opportunity for a reduction in screw beat. What is the cost of each additional flight of metering? As with each of the other two screw zones, when you add a flight to one, you must reduce another by a flight to compensate (unless the intent is to change the L/D by changing the length of the screw and the barrel) for the extruder during the process of screw design modification.

A decrease in the number of metering flights used offers a reduction in resistance for the flow of material from the screw. Material mixing and metering will both suffer fairly significantly with the loss of every metering flight. Reduce the length of the metering section of the screw only if the feed and melting requirements of the screw outweigh the metering needs. Feed and melting are basic screw needs, but maximization of the effect of the metering zone is essential in creating a melt output that has thermal and viscous homogeneity.

Mixing

Mixing needs can be as simple and basic as creating thermal and viscosity uniformity (homogeneity) of the melt, or demanding, such as distribution and dispersion of additives or gels and unmelts.

Basic mixing tasks can sometimes be met through adjustments to the process parameters and or basic modifications to a single-flighted single stage (SFSS) screw. The previous segment dealing with the func-tion of the metering flights of the screw, covers the basic mixing functions of the screw.

When mixing needs cannot be effectively met within the bounds of a single-flighted, single stage screw geometry, mixing elements (flights/sections) may need to be added to the screw design.

Mixing elements are incorporated into a screw design to accomplish specific tasks in processing. The two sections immediately following this one cover Distributive and Dispersive Mixing.

Addition of mixing flights and basic screw geometry changes that improve mixing all have a tendency to restrict the flow of the material off the screw. Whatever type of mixing is used, the material melt is sub-jected to longer exposure to the heat and shear elements of the process inside the barrel.

To repeat an earlier warning, in all plastic processing situations it is most beneficial to keep exposure to all types and forms of energy to a minimum. Overworking a material can be much more serious an offense to the product than under-working the material. Get the product to look good without damaging the material.

Distributive Mixing - Re-orientation of the melt

Distributive mixing involves reorientation of the melt. What this means, basically, is that the material melt is mixed in a relatively passive manner. The desired end result is a material melt that has a homogeneous viscosity, appearance, and thermal condition (temperature).

To achieve distributive mixing, a variety of methods can be utilized.

The basic premise of distributive mixing is to mix the melt up, much the same way as one would mix the ingredients in a recipe. Whether a fork, a wisp, spoon, or other kitchen tool is used to mix the ingredients depends on what the ingredients are and what the end product needs to be, but basically the tool is used to mix the ingredients until it has achieved a uniform color, and or texture, and or consistency.

Mixing sections provides frequent re-orientation of the melt. In simpler terms the mixing elements help more evenly distribute the properties of the melt; much in the same way you would mix a bowl of cake batter after adding a few drops of food coloring. This mixing will provide an even dispersion of the additive throughout the material.

As with any other change to the process, when incorporating any processing aid to deliver or create one benefit for the process or value in the extrusion being manufactured, assess how the change or aid will affect the entire process and the material being processed. Always consider the consequences; whether the change is a change to the screw speed of a few percent, a change to the tooling, a change to a process temperature, use of a different screw...whatever...everything in the process is linked.

Adding additional metering flights and reducing the screw channel depth (whether or not the compression ratio is increased as a result) can provide some elemental distributive mixing action for the material. A positive resulting side benefit occurs; that being, the screw beat may also be reduced.

If the material melt exiting the extruder exhibits symptoms relating to non-homogeneity and the con-sistency of the extrudate cannot be addressed through basic changes, screw modification is often required.

Use caution when incorporating mixing elements into a screw design for use in medical extrusion.

Though it is imperative that the melt be clean for use in medical extrusion applications, mixing elements impede the flow of material through the screw channel and typically stress the material’s physical properties.

Always weigh the situation carefully and strike a balance with material, process, equipment, and tooling to achieve the most effective end result for the process, the component, and ultimately the device.

If work with mixing elements improves the appearance, texture, or physical nature of the extruded output and the result is that there is an unstable process, development efforts cannot stop there. The work must continue with the setup and process until a balance has been achieved in regard to aesthetics, physical values in the extrusion, and consistency of output. Without control of each of the process outputs, it is impossible to be entirely successful in this business.

Dispersive Mixing - High-stress dispersion of the melt

Dispersive mixing involves exposing the material melt to high stresses. Dispersive mixing is used when it is necessary to break particles in the material melt into smaller bits and pieces so that they can be better dispersed in the melt. The particles could be agglomerates of smaller particles, large particle additives, or inclusions in a raw material that are a result of the resin manufacturing process.

Raw materials can often contain bits and pieces of ‘unmelt’ (gels,) agglomerated clumps of ingredients that were not evenly dispersed during material creation or compounding.

Though dispersive mixing can ‘erase’ some aesthetic and physical flaws, prior to applying dispersive mixing stress to the melt, exhaust all possible reasonable process modifications in regard to single-flighted single stage screw application. Do not impose additional stress to a polymer if it not entirely necessary.

Always investigate the use of simple controls and solutions for processing problems prior to exposing the material to more stress than it can handle. Remember, when working in the medical extrusion field, the materials manufactured are almost always subjected to additional energy or ‘working’ prior to final packaging. If a material is damaged in any process along the way, it can fail. The more stress and energy any material is exposed to, the closer that material is to failure.

The result of high dispersive mixing stress may not be evident right after extrusion. If the extruded material is the finished product, there is little concern. If the extruded product will be exposed to additional heat, energy, or other changing forces in the production of the finished devices, it could be a concern.

The point is, as material degradation sets in, long before the material begins to ‘look’ bad, its physical integrity has probably been compromised. Degradation is not only present when one can visually ‘see’ black specks, dark gels, or dark streaks in the product.

General Purpose Screw

A GP screw is generally what a customer will order from an extruder manufacturer if they do not know what they need for the application the extruder will be used in. It offers a nominal place to start for development efforts.

If it is known what material will be processed on the extruder, a screw is typically ordered with the machine that has a design that will best accommodate the processing of that material.

Screw Characterization:

During process characterization efforts it is necessary to identify and monitor pressure variation and trends. It is through the analysis of these trends that more ideal process conditions and or screw configurations for application in the production of specific materials in specific extrusion applications are developed.

When working to identify an ideal screw profile and process conditions to most effectively control the process output, it is very helpful to enlist the aid of a data acquisition system. It is possible for a very observant and focused individual to cover tremendous ground in development efforts without data acquisition and display, but automated data collation is much more effective. The same troubleshooting basics will be applied regardless of the means of data collection but charted data is more easily reviewed and interpreted by all involved in the effort. Variation and trends that are charted are much easier to interpret.

Though melt temperature, melt pressure in the barrel, motor load, power draw for each of the heaters, melt pressure in the adapter or die, and dimensional and tensile values of the extrusion are all integral pieces of the development and troubleshooting, melt temperature and melt pressure are the two most critical outputs to monitor.

All ‘standard’ extruders are set up with one major tool used to gage process development for a given application. This tool is the pressure transducer in the barrel. It can be used for temperature and pressure monitoring, as melt transducers are available with a melt temperature option.

Though barrel melt temperature measured with a dual-purpose transducer won’t be an actual melt temperature measurement (because it is measured at the surface of the barrel ID) it is an effective tool for reference, as it is helpful to gage relative viscous heat generation. Those operating the equipment with this sole means of melt temperature measurement need to understand that the melt temperatures toward the center of the melt pool in the channel are generally higher than what is read at the surface of the barrel wall.

Screw development is one of the most challenging areas of process development. This is probably so because it is the difficult to ‘see’ what affect the screw has in processing the material. Although output results charted may be in black and white, the actual change to the process must be interpreted in some way to fully understand the results, long term.

The screw used in the extruder must be balanced for use in the process. Understand that any change to the material or to the process will affect the present balance that exists with the screw to the process, and potentially material and or component value.

Screw Beat, Melt Pulse, and Process Drift

During the extrusion process, as the screw turns, material is delivered off the end of the screw. With each revolution of the screw there is a cyclical variation in pressure. The term used to describe the delta between the high and low barrel pressure readings (behind the breakerplate) during each screw rotation is Screw Beat.

Pressure behind the breakerplate is generally measured over one of the last screw flights or between the last flight and the breakerplate, depending on the design of the barrel.

If the screw is turning at 30rpm, the full effect of screw beat can we witnessed in two-second intervals. In each 2 second interval a high and a low pressure reading will be seen.

When the screw beat is minimal, the process output is stable enough to potentially save product. When it is low enough and something else is wrong with the downstream equipment setup or with the handling of the extrudate, product cannot be saved. When the screw beat is not low enough, product cannot be saved and it won’t matter if every aspect of the downstream set-up is perfect. Process stability and capability begin with the extruder. Establishing minimized variation in the output is a critical element for success.

While monitoring the pressure gauge, one will often notice that the highs and lows aren’t always the same over time. There may be some variation from cycle to cycle or every few cycles.

One cycle may have a beat of 100 the next 90 the next 110 and so on. Some of this can be attributed to gage resolution as well as gage error. We aren’t necessarily as concerned with this elemental change in beat as we are with the trend or trends in pressure variation. The drift over time of the overall pressure level (and too, the screw beat) is the pressure drift.

Charting of the trends will allow the screw beat and process drift to be ‘seen’ in reference to one another and in regard to the process.

Any time adjustments are made to the process temperatures that influence the processing of the material in the screw channel, there will be a direct influence on what the screw beat is (and very likely the drift as well). Close observation of what happens as a result of certain adjustments can be applied to increase the understanding of the process.

The operator needs to understand what it will take to get the material melt to flow consistently on a given screw in a given setup.

When working the temperature profile, always make careful adjustments. This means being present when the changes made start to have an effect on the overall process and process stability.

While making changes to the process, it is imperative, after the result is experienced, that the question is always asked, ‘was the adjustment made the right one?

It is often difficult to make the right decision when it comes to long-term drift, as it would take 40 minutes or more to get a fair review of what affect the change had on the entire process. When a more ideal screw beat is identified, work can begin on the correcting the process drift, gradually.

Note: when making adjustments, keep in mind that it is usually safer to go down in temperature on the feed end of the screw and up in temperature on the metering end of the screw. Due to the fact that significant problems with stable processing of the material are: that the material is either sticking to the screw too early or that there is too much resistance to flow into, or through the channel in metering section of the screw.

No matter what is in an extruder for a screw, there is usually a ‘most-ideal’ set of conditions to use when utilizing that specific screw. If more than one screw design is commonly used for a process, this can lead to difficulties because what is ‘learned’ to work for a profile for one screw may be exactly the wrong set of conditions to work with another screw, even if the screws ‘look’ the same. Confusing and conflicting

‘theories’ of success will become a bigger part of the troubling issue and not help in troubleshooting efforts. Those working with the process must be aware of exactly what is in the extruder for a screw.

An ideal screw beat would be zero, but that is unrealistic. A target should be as low as possible, and cer-tainly the target should be 1-2% of the backpressure reading, but always keep an open mind. It is some-times helpful to mentally back out of the process from time to time to more objectively assess the situation. Although screw beat is a major focus need, don’t over-focus on it. There are times that a slightly higher screw beat may be better in a given application because it may be accompanied by a tremendously lower drift in the process.

Watch for interactions in beat and drift. They are invaluable keys to resolving complex processing issues.

Pressure Drift

Pressure drift is the long-term variation that is experienced in the process. It is the pressure variation that is seen with the process outside of the screw beat.

Using the previous example of a screw beat of 100 (pressure readings of 900-1000.) We know that typi-cally the run won’t always be from 900 to 1000, sometimes the high will be above 1000 and the low under 900.

If, over time the pressure went as high as 1100 and as low as 800, what is being experienced in regard to total process variation?

If the screw beat has stayed the same as the pressure floated up and down over time, the process drift is 200psi. The total range is 300, but 100 of that is screw beat.

Minimizing Variation

The same way we work to ‘control’ the product OD within a control chart, we should strive to ‘control’ the process. Whether we work to better control the pressure variation or not, the pressure variation is having an effect on the output of the process, and therefore the product and production effectiveness.

Even if it is out of your control to change the conditions that deliver the variation, understand what it is, why it is there, and how it is affecting the products you are producing.

While working to minimize the beat and the drift, if it is noticed that as the pressure drifts toward the low end of the cycle that the screw beat tends to be lower, it may be an indicator that the material may ‘want’ to run at a lower pressure. The reverse may too be true. If it is noticed that as the pressure drifts toward the high end of the long-term cycle that the screw beat is reduced, it may be an indicator that the material may want to be run at higher pressures.

Process drift can be two-fold. Virtually all processes will have some long-term drift, but short-term drift also plays a part in the performance and needs to be considered.

If during observation or review of the charts, it is found that if the highs and lows in most consecutive cycles are the same, it is an indication that there is no short-term drift in the process. Short-term drift is often a problem when trying to produce products for use in this industry. Short term drift makes it very difficult to keep the product within the tight control limits that exist on virtually every component extruded for the medical industry. When long-term drift is experienced in a process, it may be possible for the operator to make the necessary adjustments, periodically, to keep the run in control. With long-term drift, an option may also exist for the application; pressure feedback.

One observation that most will make after extensive work in the area of maximizing pressure control is that a shorter cycle will generally result in less overall variation downstream of the breaker plate. This is one reason for products to have a tendency to run better on the high side of the output potential for the extruder. Higher screw speed generally results in a shorter cycle in regard to pressure variation, beat and drift.

Consider, while working to create control with a given process, beat and drift as a percentage of the pressure present in the process. Look not only at the raw pressure delta but the percentage of the total pressure that that delta represents. A screw beat or drift of 50 PSI is very little (2%) when the backpressure (pressure behind the breaker plate) is running from 2450-2500 PSI. That same screw beat of 50 PSI represents a much larger (10%) overall variation in a process where the backpressure is running from 450-500 PSI. When working a process, don’t be blinded by screw beat alone. If not kept in total perspective, maximum gain from the development efforts will not take place.

Melt Pulse

Melt pulse is a term that refers to the delta of the high reading and the low pressure readings over the same cycle as the screw beat, but in the adapter after the breakerplate.

The melt pulse generally follows the beat of the screw, trailing it slightly as the melt is pushed through the screenpack and breakerplate. The breakerplate dampens the screwbeat, providing a more consistent flow of material to the die. Making adjustments to the breakerplate and the screenpack do have an impact on the melt pulse, but the greatest reduction in melt pulse will come with a reduction in screw beat.

Breaker Plate

Breaker plates are a primary processing aid in most extrusion applications. The breaker plate is placed between the screw and the head or adapter to the head. Although a basic element of the process with minimal maintenance needs, the breaker place serves a number of major function in the extruder.

The basic functions of the breaker plate are:

Provide support for the screenpack

Dampen pressure fluctuation caused by screw beat

Stops the spiraling motion of the melt

Provide resistance to flow of the material from the barrel

Breaker plates come in many different styles; some have holes, others are slotted. Some are designed to hold screens, others are designed to sandwich the screens between the breaker plate and the breaker plate sealing surface of the barrel. Some are thick and some are thin. They are put into the extruder to support the screenpack.

The breaker plate also works as an effective restrictor of screw beat. Through flow restriction, the backpressure is increased resulting in a dampening of the screw beat. Backpressure is a term that refers to the pressure behind the breaker plate in the barrel.

The breaker plate effectively stops the spiraling motion of the material that is exiting the screw channel. Without the breaker plate, the spiraling motion of the material would be allowed to affect the flow of the material through the adapter and into the head.

The breaker plate provides resistance to the flow of the material from the barrel. It provides a means of creation of some static mixing. It holds the material back, providing the screw with more time to ‘work’ the material. The breakerplate (and screenpack) as a static mixer Incorporating a mixing element in the screw design is usually a better means of providing adequate mixing for the material.

Breaker plates are often simple plates with holes bore through them. although this is ‘common’ to see in a design, this design provides multiple surface ‘dead spots’ that lead to degradation. A simple design consideration such as countersinking of the bores on the breakerplate can effectively reduce material hang-up and the degradation that comes as a result of the hang-up.

As with any element of the process, when making changes to the breaker plate it is important to assess the impact the change has on the process...positive or negative.

The Screenpack

The screenpack is an assembly of screens, generally stainless steel, generally varied in mesh value, that are placed in or against the breaker plate (on the screw side of the breaker plate.)

Screens are used in the extruder to:

Trap contaminants

Capture gels

Create additional backpressure

Provide static mixing

Most of the screens used in smaller extruders are loose screens. The screens, or screenpack (as an assignment of screens is often referred to) are stacked behind the breaker plate. Typically, there is a coarse screen closest to the screw with an assortment of screens of increasing mesh value following followed by another coarse screen which is placed directly up against the breaker plate.

Having a coarse mesh screen against the breaker plate keeps the finer mesh screens from being pushed through the holes in the breaker plate. If screens are pushed through the breaker plate they can lodge in the tooling causing a disruption of the flow of the material through the tooling, they can cause backup of material in the tooling leading to degradation, and or they can damage the metal-to-material contacting surfaces of the tooling and or dies.

Trap Contaminants

In medical extrusion, filtration of contaminants generally involves the trapping of ‘un-melts’ or other inclusions that are inherent in many pelletized thermoplastics. It is very rare to find a material, or even a single batch of a material that is perfectly ‘clean’ no matter where it came from.

It is desired to keep even these minuscule inclusions from being incorporated into medical device components. Because the size of some of this particulate matter in the melt is close to, equal to, or even greater than the wall or layer thickness for the component, the physical structure of the component can be compromised.

If a ‘gel’ or ‘unmelt’ has a diameter of .0005”, this can represent 1/4, 1/2 or more of the total structure thickness. In a critical application, such as an angioplasty balloon a ‘contaminate’ allowed to be incorporated into the component could lead to a failure due to a weakening of the structure of the wall. In tubing that is used in a ‘reflow’ application, this same gel is usually not a problem at all because the entire structure of the material is again melted in the reflow process

Gel Capture

In many cases gels are viewed as aesthetic flaws. Where they can be avoided, or filtered out of the melt stream, they should be. When the function of the component is not at risk and inherent flaws are known to exist in a material, a determination as to what level of acceptability needs to be established.

In all applications it is a really good idea to establish some type of acceptance criteria for gels and inclusions. It is irresponsible to set a limit of ZERO gels in all applications just to be on the safe side. Truly evaluate the need and come to a decision based on sound engineering and business principle. This is one element of Design For Manufacture that must be tabled when incorporating extrusions into a device. It may seem like a trivial matter when compared to the specifications such as burst strength, load-to-break, or even color...but it is truly a major factor when establishing manufacturing requirements and managing costs.

Create Additional Backpressure

Increasing the pressure behind the breaker plate can offer some benefits; it can also cause just as many problems.

Through an increase in the number of screens and or the mesh rating of the screens used in the screenpack, additional restriction to flow can be created. This restriction to flow results in increased pressure behind the breaker plate.

Backing the material up in the barrel will affect how the material melt is created and flows.

Some potential benefits to increasing the backpressure:

reduce the material viscosity

reduce the screw beat, as a percentage of the overall pressure, in the short term

mildly decrease the mid-term and long-term pressure drift

drastically affect the mid-term and long-term pressure drift

decrease the number of gels that are ‘unmelts’

Some potential pitfalls associated with increasing the backpressure

increase the screw beat, longer term

mildly increase the mid-term and long-term pressure drift

drastically increase the mid-term and long-term pressure drift

increase the number of gels as a result of material being over-worked

In reviewing the potential benefits and pitfalls of this process modification, it should be clear that using changes to the screenpack to influence the process is a double-edged sword. When utilizing this process tool, the operator has to be on close watch; short-term benefits can be tremendously outweighed by long-term negative side effects.

If a process needs to be ‘fixed’ inside the barrel, it is best to create the fix by modifying the screw profile used in the process rather than through use of changes to the screenpack as the primary function of the screenpack is to filter impurities.

Static Mixing Aid

When making changes to the screenpack, it is extremely important to assess the impact on the total process, not just the element of intent for the change. example: a run is infested with gels that are just a little too big for acceptance for a given product spec. The technician decides to add an additional tighter screen to the screenpack to ‘break-up’ the gels. The technician may be successful in using a tighter screenpack to bust up the larger gels, but needs to be aware of the change to the process that will take place. The process modification will ripple all the way back into the process.

A more effective way to ‘bust up’ gels is to use a screw with dispersive mixing action...get the screw to do the work, it is what it is there for (but don’t forget the potential pitfalls associated with dispersive mixing!)

Clean Melt

The term clean melt, as it is used in reference to the material melt thru the barrel, describes a material melt that is free of unmelts or degraded (charred) bits and pieces of material that are created during processing within the barrel.

If a raw material has foreign material embedded into it, there is little that can be done through barrel/screw or process modification to get it out of the melt. Filtration can sometimes reduce the level in the finished product, but if it is in the material it is usually in there to stay.

If a material has inclusions embedded in it that are ‘unmelts’ it may be possible to work them out of the material or break them into smaller pieces that are ‘tolerable’ for the component being manufactured.

If a material is inherently infested with such inclusions, it is a good idea to subject the melt to some type of dispersive mixing during its journey through the barrel (provided the physical integrity of the material is

not compromised). Some ‘unmelts’, which are typically higher melt point particles, in the raw material, just needs a little more energy to help it become part of a homogenous melt. Other unmelts need more energy than can reasonably be added to a given process.

Some unmelts can be cleaned out of the melt flow by way of filtration. This can be effective as long as a proper discipline is developed for addressing the need to change the filtration media (generally screens) and, or performing machine cleanouts in a timely fashion.

It is a common practice to add finer screens to the screenpack to filter out smaller impurities. It isn’t as common a practice to address the fallout created as a result. As the screens become clogged with these bits and pieces of ‘stuff’ the flow of the material through the breaker plate is reduced and the pressure behind the screenpack increases.

To address this variation, it is common practice to increase the screw speed to compensate. As more of the screenpack is jammed up the speed is further increased. Sooner or later some aspect of the run will fail. It could be that the pressure increases to the point where all this ‘crap’ is pushed through the screenpack, creating holes in the screens for more ‘crap’ to easily travel through. It could be possible that the bits and pieces that are backed up in the screens get broken into smaller pieces as they are forced through the mesh (many more pieces, but smaller in size). It could be that the flow gets so backed up on the material is now melting a little sooner on the screw; this could offer a ‘reprieve’ in regard to pressure buildup, but could quickly turn into a material flow issue as the material starts to melt too soon on the screw, sticking to the screw a few flights too soon; leading to greater long-term pressure instability.

Timely screenchanges and timely machine clean-outs can ease the problem some, but will not get rid of it. This can lead to more effective up-time, but increased down-time.

Tooling Setup: The system is only as good as its weakest link

As with any system, an extrusion system is only as good as its weakest link.

Having the right tooling is just as important to successful extrusion as having a consistent output from the extruder. Every piece of tooling that has a material melt contacting surface must be designed and maintained to accommodate the basic process and the material processing needs.

Poor design will result in an inability to manufacture acceptable products. Poor maintenance will result in an inability to continually produce acceptable products.

Tooling is used to guide and shape the material into the extrudate that will become components for catheter manufacture.

It is imperative that the design of the tooling accommodate the nature of the material and provide the proper guiding and forming for the material melt as it is shaped into the desired pre-form.

Thermocouples

A thermocouple, or TC, is used in plastics processing to continuously measure temperature at a given point in the process, typically the temperature of a zone. One of the most important things about the TC is that it needs to be installed properly.

When installing thermocouples, it is very important to ensure that the tip of the thermocouple firmly touches the destination metal. If it does not, it will read the temperature of the air between the destination metal and the TC tip.

If the TC is measuring the air temperature between the destination metal and the tip of the TC, the destination metal will have to be hotter than the desired temp to make the air around it be the right temperature. The difference could be as little as a few degrees Fahrenheit or it could be as much as 100º F. In either case, the process and the material will be affected. In the case of the latter, the material properties could be severely degraded.

Any time an incorrect temperature measurement is being used to aid in the ‘control’ of the zone temperature there are significant side affects on the material and the process.

One obvious problem is that the material is exposed to higher heat, making it more susceptible to degradation. Undesirable side affects such as discolored gels, black specks, black flakes or discoloration in the product quickly infest runs with heat sensitive materials. In medical extrusion applications it is entirely possible that the function of the finished device may be at risk as well.

Many of the extruded components incorporated into the design of medical devices have a specific function. The material has been chosen because it has the properties required to perform the function. In all medical extrusion applications everything possible must be done to ensure that the component material properties are not compromised.

Think about it. If a thermocouple is not bottomed out in a well, what is happening and what is going to happen.

The zone is hotter than it is supposed to be, but at first nobody knows. If the rest of the setup process went well, the operator may be able to start to save product. During that time or shortly thereafter, the material viscosity starts to be driven down as a result of the temperature being higher than it is supposed to be.

An aware and astute operator may immediately pick up on the resulting material viscosity change or slight change in appearance or handling that may occur, but most operators will not notice a change until that viscosity change has enough of an impact on the sizing, shaping, or control of the extrudate to push the product out of specification.

Troubleshooting efforts could follow many different courses of action, but regardless of what is done in troubleshooting from this point forward, there is one thing that will not change; that the run is aging....and quickly.

Often times, exposure of the material to the higher temp, even for just a few minutes, will initiated an irreversible problem. This generally requires a full and immediate clean-out. If this is the case, and the remedy is delivered in a timely manner, the loss to business and process is mitigated.

Whenever encountering an issue with a setup failure, it is necessary to contemplate the full extent of the failure.

The operator may notice some difficulty in maintaining control of the size or shape of the extruded material. If tensile testing is being performed on the material, the operator may be seeing a gradual change taking place in the readings.....he or she may not. In some cases, material will continue to be saved until the result of the process is out of control.

If product is saved when the material is being exposed to much higher energy levels than is ‘normal’ for the material/component combination, downstream operations or product function may be ‘at risk’.

Generally, when the failure is that a thermocouple is not bottomed out, the resulting issues are negative. Some thermoplastics are not easily damaged by exposure to high heat, but most are...in a big way.

Dies

Dies are used to shape the flowing plastic melt as it leaves the extruder. If more than one piece of tooling is used to shape the melt, the tooling setup may be referred to as a die ‘set’. A die set may consist of a die to shape the external perimeter of the melt flow and a ‘tip’ to shape the inner structural element(s) of the extrusion. The term ‘tip’ is generally interchangeable with these terms as well: mandrel, male die, tube tip, and others.) Tips design and function information follows directly after this sub-section.

Factors taken into consideration for the design and specification of the die:

the extrusion discipline to be performed using the die

the draw required for the material being run in the extrusion discipline to be performed

accommodation of the flow characteristics of the material being extruded in the extrusion discipline to be performed

the need to accommodate potential abrasive or corrosive material melts the die will be exposed to

Preparation of the tooling for use must always include inspection of the perimeter of the material exit.

Burrs, nicks, scrapes, and excessive wear of the face of the die where the material exits will always lead to a less than desirable outcome.

The internal land needs to be inspected for the same type of damage/wear. The internal land of the die is responsible for forming the outer surface of the extruded form. If the land is damaged, the surface of the extrusion will not be smooth. Too, the inside surface of the die (material contacting area) must be smooth. If the land area is shortened due to wear, shaping will be adversely affected.

Cleaning needs to be performed in such a way as to preserve the material contacting surfaces and the definitive edges worked into the design (orifice edge, land transition edge(s), etc.)

The die is the most important tool in the system where the outside perimeter of the material is concerned.

Thermoplastics are all somewhat visco-elastic (some much more ‘elastic’ than others.) when the material exits the die it will have a tendency to swell. The degree of swell is greatly affected by a number of factors.

Among those are: the length of the internal land of the die, the transition angle(s) of the material as it is being guided into the die, the pressure in the die, the temperature of the die, the temperature of the material melt as it reaches the die, the surface characteristics of the material-metal contacting surfaces within the die and upstream of the die, the size of the cross-section in comparison to the cross-section of the material melt volume behind the land area.

So how do you find the right die for the job? Past experience with known materials, known material families, known process disciplines and the like will often guide you to select a point at which to start. Some basic rules of thumb apply in every extrusion arena, but basically you pick somewhere to start and evaluate the tooling and process needs after the actual work begins.

It is often easy to find somewhere to start. It is not as easy to identify an ideal die set for a given extrusion, for a lot of effort needs to go into the development of not just the tool, but the process in which the tool is used. As understanding and development progress, everyone associated with the process needs to be involved with an evolution to an ideal die set for a given extrusion requirement.

During process development and refinement, understand that a given tubing die set will produce one ideal tubing (as long as the material melt consistency from the extruder is ideal). If you can find out what that ideal is from the die set that in the extruder, it is possible to extrapolate a die set that will produce exactly what is required. This is not only true for tubing extrusion, but for any extrusion.

Any tooling set that is well designed and in good condition will be able to produce a cross-section that offers a given shape and produces a given geometric cross-section. The trick is to be able to have that given shape and cross-section be what is desired in the extrusion you wish to make.

Tips

The term tip may not be familiar to everyone working in the extrusion industry. Other terms commonly used throughout the industry are: mandrel, male die, and pin.

There are a variety of tip types used in medical extrusion. The type of extrusion being performed, the material being extruded, and the type of heat that is being used are all critical elements that help identify the proper type, style, and size tip required for the extrusion run.

Typically, the tip is used to form the inner diameter(s) of the extruded form, though they can be used (in the case of substrate extrusion) to provide an access for continuous feed of core, core wire, or substrate into the cross-section of the material being extruded through the die set.

Design considerations used for dies also apply to tip specification. If the tip is being used to form a lumen inside the extruded form the tip is usually the same shape as the desired shape of the lumen. In extrusions other than standard tubing (with round inside and outside diameters) it is often necessary to incorporate a tip that is machined to deliver the desired shaped lumen or interior structures of the cross-section.

When extruding very small tubing, the tooling needs to be very small. It is important that the design and construction of the tooling provide enough substance to maintain its position in the material flow. If the material flow can ‘bend’ or otherwise change the position of the tip, the position and or shape of the inner lumen(s) will vary, resulting in poor control over the cross-sectional values in the extruded form.

Care needs to be taken during cleaning and preparation to insure the integrity of the outer edge(s) and overall shape of the end of the tip. All material contacting surfaces must be smooth and mar free. If the surface(s) that the material comes into contact are flawed, the material flow will be impeded, this can lead to aesthetic flaws in or on the extruded part. It can also lead to, or accelerate, other run issues.

All metal-to-metal contacting surfaces must be free of burrs, nicks, obvious damage, and uneven wear; as all of these factors affect just how well the tooling mates with other head parts. Poor fitment can lead to infectious tooling damage as tooling is exchanged between heads/machines.

Melt Pumps

A melt pump is a device with a designed intent of delivering output consistency of >99% from an extruder. There are a number of situations that a melt pump can be used to increase the stability of the output from an extrusion process. However, melt pumps should not be used as a band-aid for a bad process.

Extruder output is one of the three main variables involved in control of any extruded product. This is one tool that can help, but a melt pump should not be placed into an extrusion system setup for an extruded component simply because control limits can’t presently be met with the current state of the process.

Yes, theoretically the melt pump can help smooth out some of the dimensional ‘bumps’ and increase capability...when properly applied and where one is required. Prior to making the decision to use a melt pump, the application needs to be reviewed.

Has a thorough review of the adequacy of the screw, tooling, and process conditions taken place? Is the downstream setup and the equipment in the downstream setup appropriate for the material and component being extruded? Are all basic material preparation and handling concerns for the material being extruded in place? Do the people working with the material know what these are?

There are a lot of things that can go wrong with a setup. There are a lot of things that can go wrong with a process. Keep it as basic as possible, keep it as simple as possible, focus on what needs to be done, and instill the discipline to make the basic needs happen, time in and time out.

When every facet of every basic has been covered in FULL detail and additional control over the extruder output is required, put the right melt pump in place along with the support for it and its use. Remember, adding the melt pump is adding one more thing to the process that needs to be controlled. Every aspect of its setup, use, and maintenance must be addressed if they are to be used.

Where can they be used? Where can’t they be used?

If the material is heat sensitive, it is not a good candidate for processing through a melt pump. Adding any additional metal to material contacting surface into any extrusion system when working with heat sensitive materials is not usually a good idea. This does not mean that a pump cannot be used, but if a pump is used with a heat sensitive material it needs to be realized that degradation more quickly becomes a concern. The life expectancy of the run will be greatly reduced with the addition of the extra zone or zones that are added to the system.

When a melt pump is used with a heat sensitive material it is impossible to achieve a high ratio of run to set-up time. When evidence of degradation starts to show up the run will need to be stopped for a full maintenance on the equipment. Efforts in regard to rapid tooling exchange help, but costs for cleaning, repair, and replacement can be staggering.

If a material is abrasive, normal wear can create a situation where the effectiveness of the pump is diminished. Again, continuous improvements effort is inevitable in the area of controls for cleaning, repair, replacement.

If a material is flow sensitive the addition of a melt pump adds a number of additional surfaces where the material movement can be restricted or blocked. If a material is prone to ‘cross-linking’ in the extruder, it is a flow sensitive material and every effort should be made to create a process without the help of a melt pump, for delivery to production. Don’t apply additions to a process to address a symptom of a larger processing problem.

Hold-downs and Guides

Not all would agree that hold-downs and guides fit into the tooling category, but anything that comes into contact with the material melt and extrudate should be considered tooling.

Hold-downs are ‘tools’ used to hold the extrudate under the surface of the water, or to hold the extrudate in position under the surface of the water long enough for it to cool sufficiently.

Guides are ‘tools’ used to guide the extruded material somewhere. Guides are used sometimes used at the front of the waterbath to keep the material from moving around erratically, or to keep it from coming into contact with the tank dam. Guides may be used to hold the extrudate in position within the waterbath so that the material is properly guided through an ultrasonic measurement system designed to measure the wall mass of the material. They may be used to guide the position the extrudate in the ‘window’ for a laser measurement system being used to measure the OD.

Wherever they are used, guides come into contact with the material. This presents an opportunity for drag to result. As long as there is a smooth extrudate passing by or through the guide(s) there is a constant resistance. Any single event can cause a small but rapid change in size or structure of the extrudate. Someone bumping the line, a wave running through the waterbath, a renegade fan blowing against the die face, a piece of die buildup coming off the die and sticking to the surface of the extrusion....anything can cause that first dimensional flaw.

It is entirely possible (and very often is the case) that this flaw reaches a hold-down and causes a very slight change in the drag of the extrudate under the hold-down or causes the manufacture of a very slight ripple in the motion of the extrudate as it enters the trough which results in another small dimensional variation. Now there are two flaws, they can continue to multiply until the run is out of control. This phenomenon can be referred to as Accumulated Dimensional Instability, or ADI.

With the extrusion of some materials and some structures this is not a major concern. With other materials it is a highly significant concern. Materials that have a slick surface where the extrusion has a fairly significant mass tend to have very few problems with ADI. Materials that are soft and tacky are prone to this infection.

It is most prevalent when running materials that have low melt strength. Typically, these materials are those with relatively high MFI (melt flow index). It is much more difficult to achieve a high degree of control, dimensionally, when running low viscosity melts. Because environmental factors and events have a greater influence in regard to sizing and control with low viscosity melts, there is a greater opportunity for extrusion runs of these materials to become afflicted with ADI.

Cutter Bushings and Cutter Blades

Many look at cutter bushings and blades as standard components for extrudate cutting. The cutter bushings and the blade(s) used to aid in the cutting the continuous length can have a significant affect on the process if they are not properly set up.

During the course of process development, some reasonable effort should go in to evaluating the cutter setup needs for the component. Size, overall design, and material for construction of the bushings should be properly established for the cutting of the component. Identification of a blade type and style can be critical to assist in achieving, maintaining, and or improving control in regard to dimensional assurance for every run.

Not taking the time to identify the best setup parameters for the cutting operation will leave the door for failure wide open. It is a relatively small task to take the time to identify specifics for the setup that will insure that problems with dimensional control are not created or exacerbated with a poor cutter setup.

Now…take the above, solid extrusion processing foundation…apply Six Sigma and Lean initiatives and watch both return on investment and market penetration and expansion fertilize one another.

Remember…the average operation doesn’t make it in this industry. The demands of this business are above average. The demands on the products are above average. The demand for control is extraordinary. Close rarely counts in this field…at least it shouldn’t.

True, margins are much larger than those typical for the general extrusion industry. True, it is possible to ‘get by’ with a good culling. True, a company that supplies tubing to the medical market might be able to make money even if the runs have to be repeated two or three times before making acceptable material. True, once a tubing vendor is qualified to produce a component for a medical device, the supplier is often ‘locked in’ for the first half life of the product.

Truer...anything short of a capable process is a waste of resources. Everything about the business demands above average; settle for no less.

Today there are a lot of companies making a lot of money supplying tubing to the catheter industry. My personal experience has shown me that some of these companies do not necessarily deserve to be making the money they are making. In time, the current tide of change and the quest for continuous improvement will result in the strongest of the vendors of today becoming qualified suppliers to tomorrow’s market. The vendors that do, will create lasting value in and for the market they serve.