basic injection molding

Web

www.wm8c.com

Hobbies, Awards, Inspirations, and much more...

Alternate Menu for incompatible browsers

Up

Ham Radio Motorcycling

Hunting Building Our Pond

Raised Bed Gardening WM8C's Link Directory

Christian Pages Inspirations

I Quit Smoking Plastic Injection Molding I

Photography Morel Mushrooms Metal Detectors

DIY Enclosed Trailer Article Submission Questions

Astrids Embroidery Tribute to America

Win An Award Awards I've Won Vote Exchange PSP Tutorials

Pond Set PSP Projects Web Tools Web Rings Insurance

A Basic Injection Molding Machine

Process - How To Mold a Plastic Part

The Basic Injection Molding Process The basic injection molding process, which is the process taking a polymer or generically a “plastic resin” from a solid state, heating it and changing it to a semi-liquid state, forming it into something, and cooling it back to a solid state. Injection molding is an extremely versatile and popular form of molding. Other processes include extrusion,

thermoforming, and blow molding. This process can be used to create untold types of products including toys, everyday items we use, medical items, and car parts. It’s uses are almost limitless and are ever expanding day by day, as new technologies for both injection molding machines and engineered resins are developed and implemented. But, that’s not why you are here visiting this page. We could spend all day discussing the different types or resins, their uses and applications, the various types of machines and their processes but we won’t at this time. I hope that you are here because you want to understand the basic injection molding process and how it works. Based on that, we are going to take a very simplistic approach to this process and keep everything very generic. You can get training through most of the injection molding machine manufacturers and it can often be done as onsite training vs. offsite, but if you choose to do onsite training, it is important that you can create an environment where the staff that is being trained, can do so uninterrupted by their normal daily tasks. I have seen many failed attempts at using onsite training because proper arrangements were not made to cover the personnel involved in the training, and interruptions rendered the training useless. If you are looking for a more advanced explanation of injection molding, you will need to take some classes, seminars, workshops, or other more advanced training in the area of injection molding. You would then learn about basic polymer chemistry all the way through advanced polymerization and processing conditions and much more. This is not intended to be an answer to all your problems, nor will it be 100% applicable to all equipment or processes, but it should give you something to start with. So with that in mind, off we go.

Translate this site - in place of DHTML menu, use text menus on the bottom or bottom left of

each page to stay in your chosen language

http://partners.ipower.com/z/57/CD1029/&dp=641

http://www.iclass-sites.com/

http://www.wm8c.com/custom_injection_molding.htm

http://www.wm8c.com/ham_radio.htm

http://www.wm8c.com/motorcycling.htm

http://www.wm8c.com/hunting.htm

http://www.wm8c.com/water_gardening.htm

http://www.wm8c.com/raised_bed_gardening.htm

http://www.wm8c.com/links/themeindex.html

http://www.wm8c.com/christian.htm

http://www.wm8c.com/inspirations.htm

http://www.wm8c.com/i_quit_smoking.htm

http://www.wm8c.com/plastic_injection_molding_i.htm

http://www.wm8c.com/photography.htm

http://www.wm8c.com/morel_mushrooms.htm

http://www.wm8c.com/metal_detectors.htm

http://www.wm8c.com/diy_enclosed_trailer.htm

http://www.wm8c.com/article_submission_questions.htm

http://www.wm8c.com/astrids_embroidery.htm

http://www.wm8c.com/tribute_to_america.htm

http://www.wm8c.com/website_award.htm

http://www.wm8c.com/myawards.htm

http://www.wm8c.com/vote_exchange.htm

http://www.wm8c.com/psp_tutorials.htm

http://www.wm8c.com/pond_set.htm

http://www.wm8c.com/psp_projects.htm

http://www.wm8c.com/web_tools.htm

http://www.wm8c.com/web_ring.htm

http://www.wm8c.com/insurance

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_de

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_nl

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_fr

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_ru

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_it

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_pt

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_es

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_el

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_ja

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_ko

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_zh

http://66.94.231.168/babelfish/translate_url_content?trurl=http://www.wm8c.com&lp=en_zt

http://stats.xaraonline.com/ID.553806/product.Webstyle4CD/redirectid.WebstyleHome/affiliate.html

Bird Feeding The basic injection molding machine is comprised of a

few unique major components. The major components of an injection molding machine are: the feed hopper or hopper, the screw and barrel, the barrel heaters, the nozzle and nozzle tip, the platens, the tie bars, the clamp, or clamping mechanism (usually toggle type or hydraulic), the mold, and a whole lot of electronics, hydraulics, and/or servo motors and controls, to make it all do what it is designed for, over and over again. You also will have a number of external auxiliary equipment units, such as material dryers, mold heater or thermolators units for temperature control, hot runner controllers, valve gate controllers, and other items required for your unique processes.

[ Back ] [ Up ] [ Next ]

Written by: WM8C, July 28th, 2006. Not for use without written permission

[ Up ] [ Choosing a Machine ] [ Quick Mold Change ] [ Basic Injection Molding ] [ Basic Injection Molding II ] [ Basic Injection Molding III ] [ Basic Injection Molding IV ] [ Basic Injection Molding V ] [ Basic Injection Molding VI ]

[ Basic Injection Molding VII ] [ Basic Injection Molding VIII ] [ Basic Injection Molding IX ] [ Basic Injection Molding X ] [ Basic Injection Molding XI ] [ Basic Injection Molding XII ]

This site is rated

Home Ham Radio Motorcycling Hunting Building Our Pond Raised Bed Gardening WM8C's Link Directory Christian Pages Inspirations I Quit Smoking Custom Injection Molding Plastic Injection Molding I Photography Morel Mushrooms Metal Detectors DIY Enclosed Trailer Article Submission Questions Astrids Embroidery Tribute to America Win An Award Awards I've Won Vote Exchange PSP Tutorials

Pond Set PSP Projects Web Tools Web Rings

WM8C's Ham Links & More Copyright 2002 - 2006

All rights reserved. this site created and maintained by WM8C

http://www.wm8c.com/birdfeeder

http://www.wm8c.com/quick_mold_change.htm



http://www.wm8c.com/choosing_a_machine.htm


http://www.text-link-ads.com/?ref=19942

http://www.familyfriendlysites.com/FamilyFriendly/default.asp?nID=23370

http://www.wm8c.com/



























mailto:[email protected]?subject=I saw your page...

Web

www.wm8c.com



Up











A Basic Injection Molding Machine Process

Material Drying and Material Hoppers Material DryingDepending on the resin you are using and whether or not it is a “hydroscopic” resin (absorbs moisture form the air), you may have to first dry the resin. This is a process of removing any moisture from the individual plastic pellets, so as to make them usable in the injection molding process. This is done with a material dryer, which is provided my many different manufactures, and it’s sole purpose is to remove moisture from plastic. This is accomplished by circulating heated “dry” air through the resin inside a drying hopper at a manufacturer specified temperature. This causes the plastic resin to release it’s moisture and most commonly this moist air is run through the material dryer through a bed of molecular sieve (desiccant) which retains the moisture inside the dryer bed and sends dry heated air back out to the hopper. This air needs to be maintained at a dew point level somewhere between -10 and -40 degrees F. Any higher than this and you run the risk of having wet material. The material can be checked for moisture using a couple of glass slides and a hot plate, looking for small pockets of gas in the pellet. This is done by heating the slides on the hot plate to a high enough temperature to soften or melt the plastic you are checking, laying a few pellets on one of the heated slides and sandwiching the pellets between the plates. You can then inspect the flattened pellets between the plates for small gas bubbles which would indicate moisture. The other method involves a lab device called a moisture analyzer, which is considerably more expensive but much more accurate. Once the plastic resin is dry, which usually is anywhere from 2 -4 hours on average, it is ready for processing and we are going to feed the resin into the machine hopper to begin the molding process. The Machine HopperThese can be very small (shot hoppers) to very large in size but they all serve the same purpose and that is to hold plastic pellets or resin, which will be used in the injection molding process. The smaller shot hoppers, are typically used when you are drying resin either in a mezzanine or machine side drying hopper. These small hoppers only bring a small amount of resin each cycle to the screw feed area, thus controlling the likely hood of the pellets regaining moisture while waiting to be fed to the screw as can be the case with small shot sizes and larger machine hoppers that are not being dried on very humid days. The other very common method, but will require some reinforcement of the hopper mount, is to mount a sufficiently sized drying hopper directly over the feed throat of the machine. This method is commonly used but presents it’s own challenges as they are much more difficult to move out of the way when you have to inspect the feed throat. Some machine makers have added an air assisted hopper slide option to aide in the process of moving the hopper out of the way as needed. All of these options, should be mounted on a magnetic drawer base. The purpose of this unit is to try and eliminate any metal objects that may have gotten into the material, virgin or regrind, and yes this does happen from













































Bird Feeding time to time. Sometimes it’s a malicious act but most of the time it’s carelessness or disrepair that causes this to occur. Metal and screws don’t mix and it can make a mess out of your molding process depending on where the metal ends up. It can get lodged in the screw, nozzle or nozzle tip, in your gates, or simply scratch your beautiful mold surfaces should it make it’s way through the entire system. Each case has it’s related costs to repair and these magnetic bases can prevent much of it from ever happening.





This site is rated








































Web

www.wm8c.com



Up












Process Barrel, Screw, & Barrel Heaters

The Screw and Barrel The reciprocating screw in injection molding or just “screw” resides inside the barrel. It is made up of two components which are the injection molding screw and screw tip assembly. It serves two purposes, the first being to convey material forward in the barrel to the front of the injection unit for each new shot, and two, inject the plastic under high pressure into the mold. It is very important that these components are inspected for wear at

least yearly as they are very high tolerance components and very critical to the injection molding process. One way you measure your screw and barrel performance is to record your recovery time and screw RPM’s, when the screw and barrel are both new, and also note which resin that measurement was performed with. Then at 6 month intervals, check your recovery times, and make sure that they remaining relatively the same. If you start noticing a longer recovery time (20% or more), there is a chance that you are starting to get some wear. The only way to know that for sure is to remove the screw from the barrel, inspect them both and measure them with micrometers and compare the size to the print. The manufacturer can give you the recommended maximum where for your particular screw and barrel but usually it’s around .006 - .010 of an inch. There are three main zones to most standard injection molding screws and these are the feed, transition, and metering zones. You can also get specialty injection molding screws with mixing sections or other sections built to your own specifications as needed. Mixing or barrier wave injection molding screws would be an example of a different flight options for a screw. The screw, during the rotation phase, can also provide heat to the melting process through friction, improving the quality of your melt. The injection pressure applied by the screw is the heart of your process. Barrel HeatersThe purpose of the barrel heaters is somewhat self-explanatory, and that is to “heat the barrel”. The reason we need to do this is provide an environment in which to begin the plastic processing portion of our cycle. It is imperative that your













































Bird Feeding barrel temps are up to your “set temperature” for the type of resin you are using, before turning the screw. You will also want to add some “soak time” to this to be sure that all of the solidified resin in the barrel and on the screw has fully softened or melted. The result of not doing this is usually a broken screw tip assembly or in some extreme cases, even a snapped of screw. You'll notice I don’t mention how much time your soak time will need to be, and that is because it will vary by the material you are using and the efficiency of your barrel heaters. Normally, at least a half hour of “soak time” is recommended for most plastics and this is commonly a standard feature on most machines made in the last 10 -15

years. If you have the ability on your machine to “bypass” this part of the process, my recommendation is don’t and that is from experience with seeing the results of people who have tried that and failed. At upwards of $1500 for a screw tip assembly and upwards of $15,000 or more depending on the size of your screw, you don’t want to have to explain to your manager how that happened to you.


Written by: WM8C, July 28th, 2006. Not for

use without written permission



This site is rated








































Web

www.wm8c.com



Up












Process

Barrel Temperature Control, Nozzles & Nozzle Tips

Barrel Temperature ControlEach zone of barrel heat is individually controlled and range from a couple of zones to 6 or more depending on machine and barrel size. They are controlled through the use of thermocouples which are attached in a central location of each heating zone and provide feed back to a controller which turns electricity on and off to the heater bands, maintaining a constant

stable temperature for your process. Thermocouples come in two types on most injection molding machines, types J & K. It is always important to make sure you have the correct type of thermocouple when replacing these or you can have process issues due to inaccurate temperature readings. Same thing goes for heater band replacement. If you replace a 400 watt band with one half the size or double the size, it can dramatically affect your processing. You also need to pay attention to the voltage of the heater bands you are replacing as they typically will be one of three voltages which are 120, 240, or 480 volt. These bands are not simply interchanged. I have seen 120v heater bands hooked up to 240 volt circuits, and turn cherry red before they failed, which can be very dangerous. The opposite can also be the case where a 220 volt band is used in place of a 120 volt band and though these will not turn cherry red, you might never get enough heat to move plastic through it. Nozzles and Nozzle TipsThe nozzle and nozzle tip are the exit point for the plastic resin from the screw and barrel on it’s way into the mold. Nozzles come in a multitude of lengths and designs and you will need to know which is right for you and your process. The most commonly used are the two piece nozzle which incorporates a separate body and tip. The reason for this is that you have more versatility for use between the most common sprue bushing radii, which are ½” and ¾”, and also different material types which may require different tip styles such as a continuous taper or reverse taper for nylons resins and other high crystalline materials. It also allows you to easily change the tip opening size which can vary from mold to













































Bird Feeding mold. You always want to use a tip orifice size that is equal to or slightly less than the sprue bushing orifice in the mold to aide in leak prevention between these two areas. If you use a nozzle tip that has a larger orifice size than your sprue bushing, or is damaged, be ready to replace some heater bands as at some point you will likely leak plastic between the tip seat and the sprue bushing sea. This will then leak back over and cover your nozzle and eventually the barrel if it’s not caught in time, which causes unnecessary down time and cost to replace destroyed heater bands and thermocouples.. It is very easy to change tips during production with this type of nozzle either due to a radius change or if the tip has been damaged in any way. Nozzles and nozzle tips come in a variety of flow through styles as well, such as general purpose, continuous taper, and a few others.

[ Back ] [ Up ] [ Next ] Written by: WM8C, July 28th, 2006. Not for

use without written permission

[ Up ] [ Choosing a Machine ] [ Quick Mold Change ] [ Basic Injection Molding ] [ Basic Injection Molding II ]

[ Basic Injection Molding III ] [ Basic Injection Molding IV ] [ Basic Injection Molding V ] [ Basic Injection Molding VI ] [ Basic Injection Molding VII ] [ Basic Injection Molding VIII ] [ Basic Injection Molding IX ] [ Basic Injection Molding X ]

[ Basic Injection Molding XI ] [ Basic Injection Molding XII ]

This site is rated








































Web

www.wm8c.com



Up












Process

Platens & Tie Bars PlatensOn most machine there are two platens, referred to as the movable and the stationary platens. Most commonly you will have two stationary platens, one front and one rear, and one moveable platen. The platens serve a couple of functions. The first thing that it does is provide a place to attach your mold. The platens will normally be drilled and tapped with threaded holes, which in turn are used

to attach clamps to the mold, holding it in place during the entire molding process. Some platens in less common cases, are built with slots in them instead of drilled and tapped holes, and this is actually a more versatile way to attach clamps to the mold. This was more commonly used with European machines then it is on Japanese or American machines. The placement of these holes are usually determined by an industry standard unless otherwise specified by you for a given platen and machine size. The sizes of the holes in the platens are typically determined by the platen and machine size as well. The larger the machine, the larger the bolt sizes required to clamp the molds into them. I’ve often been asked how many clamps are required for a given mold. While I’ve not really found a “hard rule” about this, I can tell you from experience that if you have one clamp for every 18 inches of mold length, you will not drop a mold on the floor as long as it is properly attached. So, on a mold of 36” in length, you would have a total of 12 clamps, 3 per side, and 6 each per mold half. Now this will vary some in different molding facilities and is meant only to be a guideline. You should follow whatever rules your company has set forth for this procedure in your injection molding facility. There are also some “through holes” in the platen in a number of pattern options for ejector rods. The use of these will be explained in a later section. These holes are also laid out as per a specific standardized location as with the threaded holes for the clamps. There are multiple patterns available, which will handle 99% of all standard molds. The second function of the platen is to provide the steel to develop a uniform force on the mold and move the mold halves or cover and ejector, either to the open or closed position. In the open position, parts can be ejected and removed from the mold by means of an operator or by some other robotic means. In the closed position, tonnage is applied to the mold













































Bird Feeding

halves via the platens and this is when the plastic under high pressure is injected into the mold. The term “platen deflection” is derived from the amount of movement that is exerted on and results from this injection pressure being applied. A hydraulic clamp machine usually will resist the platen deflection issue better than toggle clamp press because in a hydraulic clamp press, the tonnage is developed by a large hydraulic ram located in the very center of the movable platen. On a toggle press, the attachment location of the toggles to the platen are more towards the outside edges and deflection in the center of the platen is more common place. This can make a difference when you are close to the press tonnage threshold on a given mold, and puts the mold at risk for flash problems along the parting line edges. Tie BarsThe tie bars are the mechanism that actually develops the tonnage of the press. Tie bars are made of steel. The size will vary from a few inches in diameter on small machine up to well over 20” in diameter on the very large presses and the length is determined also by the type and size of the press. The rods will be attached most typically with a very large nut on the outside of both the front and rear stationary platens. There will be four tie bars in the case of most common injection molding machines. The only place this won’t really apply is with some of the “C” type clamp mechanisms, which aren’t very common in most standard injection molding machines. The way this tonnage is developed is by moving the movable platen forward until the two mold halves contact each other and then high pressures are applied causing the tie bars to “stretch”. It is this stretch and the force being applied that will determine the final tonnage capability of an injection

molding machine. Often, this stretch is measured with a strain gage that is located on the end of one or even all 4 tie bars, and this stretch or “strain” is measured to deliver an output back to injection molding machine controller in order to get a “tonnage reading” from the machine. This is tie in with safety mechanisms to avoid over stressing the bars which can literally cause them to crack or snap, as well as feed back for the process itself. This information can also be used by many newer machines that have the option for “auto tonnage setting” of the machine, which takes most of the setup or technician requirement out of the function as it is automatic.





This site is rated








Web

www.wm8c.com



Up












Process

Mold Clamping Systems Mold Clamping SystemsClamping your molds on the platens for the injection molding process is a fairly basis principle and the methods used to accomplish this task are not all that different from each other. The most common method is the use of mold clamps. These clamps are

usually made of forged or cast steel and look like the picture shown here. While this is a fairly common clamp style, you can find a number of variations of this type of clamp. It consists of the clamp, an adjustment bolt, an attachment bolt, and a spacer or washer for use on the attachment bolt. The alignment of these clamps is critical to their function and they must be used properly. The result of not using these clamps properly, could mean the difference of a mold remaining in your machine and lying on the ground beneath it. The safety hazard of this is self explanatory, not to mention the damage that could be done to the mold or machine. It is very important that the clamp adjustment bolt is set the correct height which is level with the clamping surface of the mold. You can measure the clamp plate thickness and if it’s 1 “, set you adjustment bolt for 1” height. This provides the most surface contact with the clamp plate and results in the best clamping forces being applied. A small amount of error can exist without serious issues, but level is still the desired and safest approach. Another important factor in proper attachment of a clamp is the depth of the attachment bolt into the platen hole. The best rule of thumb used to calculate this is “one and a half times the diameter of the bolt being used should be used as a depth into the platen hole”. In other words, if you are using a ¾” inch attachment bolt, you should use a bolt length that will get you at least one and one eighth of an inch into the platen. .75 x 1.5 = 1.125 or 1 1/8”. The formula applies to all bolt













































Bird Feeding

sizes being used. Make sure you also account for the height of the spacer or washer you are using to determine the bolt length needed. Let’s assume the spacer is ½” in thickness. We already know we need to get 1 1/8” depth into our platen. To determine the length of the bolt required you need to make some measurements. First measure the height of your clamp plate. Then you need to know the thickness of your clamp and add the spacer thickness to that. Lets assume our mold clamp plate thickness is 1”. Our clamp thickness is also 1”. Our spacer is ½”. We total these amounts and then add the thread depth needed to determine our bolt length or, 1” + 1” + ½“+ 1 1/8” = 3.625” or 3 5/8” bolt length required for our task. This is important for a couple of reasons. The first is that if we are using too short of bolts, we will likely strip the threads right out of the platen holes when the mold is mounted on the platens. As the mold is opening, closing, and ejector system running, you could literally strip the mold off the platens and the mold could fall out of the press. Does this sound like the voice of experience? That’s because it is and I’ve seen it happen. The other problem I’ve seen that ends with the same result is using bolts that are too long. Each platen hole has a finite depth, meaning that you can hit bottom if the bolt is too long. The result of this if you don’t notice is once again a loose clamp and a dangerous situation, so pay attention these details and you’ll never have a problem. Keep your platens holes in good condition too, as this makes everyone’s job simpler and safer. Other methods of clamping a mold to a press is using holes in the clamp plates and bolting directly through them You will need to apply the same calculation to determine bolt lengths for this method as well. This method is easier and safer than clamps, but requires precise hole placement in the molds to line up with your press platen holes. The last common method is hydraulic clamping, which is the fastest, safest, and most expensive method but well worth it if the investment is made as this is a great time saver, saves wear and tear on the platens, and cuts down the risk of mistakes. These are the types of financial decisions your company will have to make for itself. Another less common and even more expensive clamping method is magnetic platens or clamping plates that are added to your existing platens and use extremely powerful magnets to hold the molds into place. These are typically only used where time savings and quick mold change are a way of life as their costs are tough to justify in many situations.




[ Basic Injection Molding VII ] [ Basic Injection Molding VIII ] [ Basic Injection Molding IX ] [ Basic Injection Molding X ]

This site is rated








Web

www.wm8c.com



Up












Process

Ejection and Core Pull Systems Ejector SystemThe sole purpose of your ejection system is to remove the finished injection molded parts from the mold or die. The ejection system is comprised of a hydraulically driven ejector plate, sometimes referred to as the butterfly plate, and some type of ejector rod or bar, which is used to drive the mold ejector plate forward, releasing the part from the cavity or core of the mold, or so that it can be removed by

an operator or a robot. Depending entirely on your mold and it’s requirements, there could be anywhere from 1 to 4 or more rods required to perform this task. One critical thing to remember is that if you are using more than 1 rod, it is necessary that these rods are exactly the same length. The reason for this is so that the ejector plate is moved forward in a uniform manner, to avoid causing the ejector pins to become scored and bound up. If you are using one rod, it usually is placed in the very center of the mold. If two or more are ejector rods are used, it is important that they a installed symmetrically into the mold, such as opposite corners, directly above each other, all four corners, or similar. These rods can be installed as loose or “floating components” or tied-in. As the term indicates when tied-in, the rods are attached at both the mold and machine ends of the rods, usually with bolts or threaded rod stubs in the rod. Molds that have return springs can be used with loose or floating rods, as the springs are intended to provide the return stroke of the mold ejector plate when the machine ejector plate is retracted, allowing the rods to also be retracted out of the way. This is especially useful if you run smaller molds with multiple ejection strokes and little or no actions built into the mold. With more complex molds, the tie-in method is more appropriate and more often used. The difference in using this method is that the machine is controlling the entire stroke and it allows you to safely control the ejection of parts from molds that have actions built into them such as slides, suicide core pins, or both. This also helps prevent damage to the tool, should something go wrong with the injection molding process or machine. When coupled with a robot or operator removal, the rods can maintain the ejector plate in the forward position until the robot or operator has removed the finished injection molded part from the mold.













































Bird Feeding Core PullsCore pulls do exactly what they sound like they would do, and that is to pull cores out of parts which otherwise could not be molded any other way. For example, If you wanted to create a hole in the top flange of a part and the hole was not in “die draw”, you would be able to use a movable core to create this hole. Just to clarify, die draw is anything that lies in the direction of “mold opening”, with sufficient draft for the part to be removed. In the case of a rectangular box, by adding some amount of draft, you can mold this box without the aide of anything special besides the ejector system to remove it from the cavity. But, If you would like to put a hole in the side of the box, we need to use a core pull mechanism to do this. The process is simple in the case of creating a hole. Once the mold is closed, we will use the core pull system to drive a pin into the core wall, creating an area that plastic must flow around during the filling of the cavity. This now creates a hole in the side of the plastic part. Now if we were simply to open the mold up, we would rip or tear away all the plastic behind this core pin during ejection, leaving us with a useless part. But, if we pull that core back up out of the way before we open the mold, we can safely eject the part with no damage to our hole. This is just one of the myriad of uses of the core pull mechanism on the machine. It can if desired, be used in certain mold design configurations be used as a hydraulic ejector system addition as well.





This site is rated








































Web

www.wm8c.com



Up












Process

The Machine Controller Machine ControlsThis area or component of the injection molding process can become quite expansive. There have been so many iterations of machine controls over the last 25 years, that it’s difficult to know where to start. Due to the fact that there are so many machines still in use today from this same time span, I will only touch on the basics of the main changes in order to keep this flowing and on topic as much as I can. The

process of injection molding in it’s basic sense has remained relatively unchanged since the beginning. Molten plastic is injected under high pressure into a mold, creating a non-Newtonian flow condition, which is cooled, and then removed from the mold. There are 100’s of control mechanisms to create these conditions but they all end with the same purpose in mind and that is to produce a plastic injection molded part. In the beginning, there were relays! Actually there were other things before that too, but we will start there. Relays, limit switches, relays, and more limit switches, were the flavor 25 to 30 years ago and I’m not referring to the newer solid state types, but the very large and now impractical mechanical relays used in many molding machines for years. As with now, these machines were provided to you with a hydraulic and electrical schematic or “ladder diagram” showing line after line of relays, some open, some closed, that all together made a required action occur. In many ways, I felt troubleshooting these machines were much simpler in a lot ways than with today’s solid state control systems. If your clamp wouldn’t close, you just looked for that line in the ladder logic, started at the end of that circuit which usually was a solenoid on a valve, activated the clamp close button and checked for voltage at that point. If there was none present at the solenoid, you back tracked to the next point along the circuit path which might be a junction block. No voltage there and back you go again until you found the first point where voltage was present, but not getting through to the rest of the circuit. That point often would be a relay or relay contact that had failed. Replace or re-build the relay and off you went again. Now that is a simplistic approach to circuits of old, but they worked. There weren’t any touch screens, or LED read-outs,













































Bird Feeding or circuit boards. Everything today is now touch screens, electronic control modules, programmable controllers, and the like. Trouble shooting is now more often done to the module level and then replaced if it’s not functioning properly with another. I would say that today’s machine controllers are much more reliable than those of old, but they are also much more expensive to maintain. Instead of replacing or rebuilding a burnt relay that cost a few bucks, you will likely have to replace a $2000 module. This is one of the costs of progress. In a nutshell, the controller is the brain of the injection molding machine. It controls all of the functions of the injection molding machine and insures that the machine does what it is programmed to do. In the case of the newest machines, they are comprised of many sensors, such linear measurement devices, thermocouples, proximity sensors, speed sensors and many more. Think of these sensors as the nerves of your own body, or your ears, eyes, nose, and tongue. You reaction to what your senses tell, just as with the machine. The controller allows you to input certain set points into it, which tells the machine what to do during the injection molding process. The sensors provide feedback to the machine controller so that it knows what’s going on and when, and what to do next based on a program that tells the controller what to do with the information that it receives. These set points that you have control over will be discussed more in a later module on process setting and controls.





This site is rated








































Web

www.wm8c.com



Up












Process

The Mold The Injection Mold | It’s Main Components and Function | Injection Molds ExplainedThe function of the “mold” or “mould” depending on which spelling you subscribe to in the injection molding process, is simply to be the vessel within which a plastic part is born. Think of the mold as the reverse image of your finished part. What I mean by that is this…if I took a ball and I covered the entire surface of the ball

with plaster and let it harden, and I then cut it exactly down the middle into two halves and removed the ball, what I would be left with is the basis of an injection mold. It is a void or space of some shape that is split into two sections so that we can inject plastic into that space to create a plastic part when it is removed from this mold. We refer to the halves of the mold as the cavity and core, or cover and ejector. The cavity and core can be either the front or back half of a mold depending on the design. Most molds will have the cavity or cover of the mold mounted on the stationary platen and core or ejector of the mold on the movable platen. In the case of molds that are built with cavity or cover ejection this would be reversed, and then we often would use the core pull system to run the ejectors forward. We will stick with the more common configuration of the ejector half of the mold being located on the movable platen and using the standard injection molding machine ejector system. The “ejector” or “ejection” half of the mold is what will ultimately cause the finished plastic part, to be ejected from the mold. It is comprised of a number of components, some of which will be the ejector housing, ejector plate, ejector pins, and water (cooling) lines. As design requires it, it may also have slides, cores, lifters, and other moving components that make up the the ejection system. It also is usually the core half of the mold. Besides being a critical part of the molded part, it’s only other purpose in life is part removal or “ejection”. Plastic shrinks to varying degrees, and because of this we must build molds with sufficient draft to allow the part to easily slip off of the core of the mold without damage to the parts plastic surfaces. Draft needed for a given material or resin can vary greatly and you should consult your material vendor for the specifications for the material you are using for your specific application. Draft can be as little as a couple of













































Bird Feeding

degrees, to something much more substantial depending the part design and material requirements. When a part is molded and then cooled, the ejector plate of the mold is moved forward causing the ejector pins and/or lifters to push the part free of the mold, allow it to be removed by an operator or robot, or to simply fall onto a chute or conveyor to be moved to it’s next operation. The cover or core of the mold, except when the part is cover ejected, does not typically have any moving parts. It does however usually house the entry channel for the plastic into the mold. This is done through a number of different methods, such as a sprue bushing and sprue, or a hot runner manifold. The channels of plastic flow can be directly into the part itself or flow through a “runner” system, which is the most commonly used method. The runner system can be part of the hot runner manifold or a cold runner system that is cut into the surface of the mold itself. We need a separation point

between the part and the runner and this is called the “gate”. There are many “gate” configurations and again, this all depends on your parts design and requirements as how this will be performed. This half of the mold will also require sufficient draft, as the part must release from the cavity as the mold opens which separates the two halves prior to ejection, with the part remaining on the ejector half of the mold. This half of the mold the mold will also contain the horn pins which move any slides the mold might have into place as the mold closes. The guide pins, which will located in two up to all four corners of the mold, perform the task of keeping the two halves aligned as the mold closes. When the two halves are closed together, they create a void that will be filled with

plastic to create the finished molded part. [ Back ] [ Up ] [ Next ]




This site is rated








Web

www.wm8c.com



Up












Process The Controller | Basic Clamp & Ejectors Setting Descriptions and Function Clamp settings:The clamp settings control the distance the mold opens and closes to, where it slows down and speeds up, and other miscellaneous settings where you might perform other actions such as core pulling and ejection start points. Because of the large variety of machine types, I will not attempt to cover every possible setting but only

the main ones that are common to most machines. Depending on the age and abilities of your machine, these could be digital settings in the controller interface on newer machines or limit switches on older ones, or in some cases a combination of both.. Clamp open fast – where you want to start opening the mold quicklyClamp open slow – where you want to start slowing down the clamp before reaching full open.Clamp full open - where you want the machine to clamp to stop after each cycleEject start – where you want to start your ejection movement if your machine has “eject on the fly capability”, otherwise it will usually be at the mold full open point.Clamp close fast – where you want to close the clamp at high speedClamp close slow – where you want to slow down the clamp when closing (usually before engaging the guide or leader pins)Mold safe – just before the mold halves first touch, usually a few thousandths of an inch before zero. This is the safety feature to help protect your mold from damage if it closes on a runner or part. The clamp is put into a low pressure state at this set point to protect your mold. This setting is one of the most critical to be set correctly and precisely.Clamp fully closed – this is the point where the mold halves make contact and high pressure is applied to build full tonnage.













































Bird Feeding

Clamp speeds and pressures – most of the above functions also have a speed and pressure setting. These will be set according to you needs and will vary somewhat for each mold. Ejector settings:The ejector settings control the distance and speed of the ejector rods during the part removal process from the mold. The settings are crucial to your molding process as they help determine a number of things such as whether or not the part is going to remain in place for robot or operator removal, or be ejected completely into a bin or chute. They can in some instances also help in determining part quality with how fast a part is removed in some more sensitive parts and molds, or ensuring the cores have been removed prior to ejection in order to protect the mold from damage. As with the clamp settings, depending on the age and abilities of your machine, these could be digital settings in the controller interface on newer machines or limit switches on older ones, or in some cases a combination of both.Eject start position – where the ejectors start their forward stroke.Eject forward position – the distance of travel the ejector rods will make during each stroke of ejection. Number of strokes – the number of times the ejectors will make a an entire ejection stroke each cycle, usually limited to not more than 5 repeats.Ejector dwell time or eject forward hold time – the length of time the ejectors will be held forward after reaching it’s forward limit. Usually only used with a single stroke condition.Eject retract position – the position the ejectors must be in prior to closing the mold for the next cycle.Eject speeds and pressures – most of the above functions also have a speed and pressure setting. These will be set according to you needs and will vary somewhat for each mold.


Written by: WM8C, August 8th, 2006. Not for use without written permission

[ Up ] [ Choosing a Machine ] [ Quick Mold Change ] [ Basic Injection Molding ] [ Basic Injection Molding II ]

[ Basic Injection Molding III ] [ Basic Injection Molding IV ] [ Basic Injection Molding V ] [ Basic Injection Molding VI ] [ Basic Injection Molding VII ] [ Basic Injection Molding VIII ] [ Basic Injection Molding IX ] [ Basic Injection Molding X ]


This site is rated








Web

www.wm8c.com



Up












Process The Controller | Injection Settings, Speeds, and Pressures | Basic Injection Control settings

Injection Settings Injection Phase High pressure | 1st Stage | Boost - Depending on the area in which you live, you will hear a number of different terms used for this phase of the custom injection molding cycle. No matter what it is referred to as in your

plant, it remains the same thing and serves the same purpose, and that is to inject melted plastic into your injection mold. This in many ways is the most important part of the basic injection molding cycle, as it has a great bearing on the initial cosmetics and size of your finished plastic injection molded product. You can control the hydraulic pressure and speed with which you perform this phase, and on most newer machines the “fill” or “injection phase” can be broken down into 10 segments or more to aide in precise control of this part of your injection molding process. We will get deeper into the profile settings, what they are, and what you can do with them in the troubleshooting segment of this series. Under normal plastic processing conditions we want to fill the part as close to the 95 -99% full cavity range as we safely can, allowing for the natural resistance of the plastic to absorb the remaining inertia of the screw. When we reach the injection transfer or “cut-off” point setting, we are doing exactly what it sounds like and that is transferring from fill or 1st stage pressure, to pack/hold or 2nd stage pressure. The machine settings controlling these functions are:Fill Pressure – the pressure setting for fill or 1st stage. In most processes today, this would be set to the maximum of the machines ability which is usually between 2000 and 2500 psi. The reason for setting this to maximum, is so that you do not “pressure limit” this part of the process.Fill Speed – The speed control for the fill portion process. We want to control the fill speed of our process and not the fill pressure.Fill Speed Profile – these are the fill segments we spoke of earlier. These profiles can have as few as 2 steps or many as 15 or more. 5 to 10 steps are usually more than sufficient to handle the processing conditions for most processes.













































Bird Feeding

Transfer, transition, or cut-off position – The point of transfer from 1st to 2nd stage injection. This can be done on most newer machines by position (most common), hydraulic pressure, time (least accurate and repeatable), and cavity pressure. Pack Pressure PhasePack or second stage pressure is used just as it implies. When used properly it’s purpose is to pack out the injection molded part. Assuming you followed the 95 to 99% fill rule, you will use the pack pressure to fill out the remainder of your plastic part, until reaching the desired cavity pressure for you process. Typically, there are only one or two segments for this function but I’ve seen up to five, and it will have independent time and pressure settings for each. Pack pressure if used correctly, will finish filling and “pack your plastic into the cavity. Often the 1st step of this pressure will be “equal too or slightly greater than” your transfer pressure so as to prevent the screw from bouncing back at transfer. Again, if you follow the 95 to 99% fill rule on the 1st stage filling process, you can even use a lower pressure pack in many cases without creating a screw bounce situation. This bounce is caused by two things, the first is overfilling the cavity in the 1st stage of fill and second, packing with too low of a pressure when the first condition exists. If the inertia of the screw is fully expended before the cavity is fully filled as should be, the likelihood of this happening is low.

Normal pack setting options, if your machine is so equipped are:Pack time – Typically, you will have one or two segments of time available if your machine has the pack option installed. This time should coincide with the amount of time to finish filling and pack the cavity to it’s peak pressure for the desired dimensions.Pack Pressure – settings for the pressure of each segment. Pack Speed – this is not an option on all equipment but very useful if you have it, especially if you are using your pack to finish filling and packing the part. This will be a low speed option and on some machines it’s even limited to 50% or less.





This site is rated








Web

www.wm8c.com



Up












Process The Controller | Injection Settings, Speeds, and Pressures | Basic Injection Control settings

Injection Settings Hold Pressure PhaseThe hold pressure phase of the process is meant to maintain the injection mold process pressure inside of the cavity until the gate or gates freeze off which then will be released, as pressure no longer need be applied

after that point. Once gate freeze off has been attained, no further pressure can be added or removed from the plastic inside the injection mold cavity, as the gates basically act as a cork after that point of the process. Once gate freeze has occurred, and the hold pressure has been released from the cavity, the cooling portion of the cycle may begin. Hold pressure on most newer machines is also provided in segments with individual time and pressure settings for each. There is typically only one hold speed for the entire hold pressure phase. This again is usually a very low speed of less than 30%, as we are not trying so much to move plastic at this point of the injection molding process as we are maintaining that pressure in the mold cavity until the gates have frozen preventing any further pressure loss. If your particular machine is not equipped with a pack pressure option, you can sometimes use the first one or two segments of your hold pressure phase as “pack pressure”. This again will depend on the abilities of your individual machine and controller capability. Most machines built in the last 10 – 15 years have at least 5 and 10 segments of hold pressure. The machine settings controlling these functions are:Hold Speed – this is usually on one speed setting that covers all of the hold segments.Hold Pressure Profile – each segment of hold will have an independent pressure setting and can be ramped up or down for each segment change.Hold Time - each segment of hold will have an independent time setting and can be changed up or down for each













































Bird Feeding

segment change. Use as few of these segments as necessary to provide a good molded product. Many molders have a tendency to take the approach that if there are 10 segments injection hold pressure, that we should use 10 segments of injection hold pressure. In reality, most injection molding situations will require less than 2 pressures segments and rarely more than 5. Some exceptions to that might be when you are using one or two segments as pack. Hold pressure also will most commonly remain the same over the profile or reduce to lower pressures in each phase when using multiple segments. Screw Rotate and Back Pressure PhaseThe screw rotate and back pressure portion of the process occur during the cooling phase of the injection molding portion of the cycle. Back pressure is applied as a resistance pressure to the screw as it rotates, while developing the shot for the next machine cycle. The purpose of the back pressure is to aide in providing a consistent density of the shot size as the screw rotates and conveys plastic resin towards the front of the screw. This is very important for the injection molding process “shot to shot” consistency that is required by today’s high precision injection molding process requirements. On most machines, back pressure is usually limited somewhere around the 25% mark of total system pressure. Back pressure in most cases is a very important function of the molding process, but it also contributes the wear of the screw and barrel components, thus keeping this as low as possible and still be able to produce a consistent and homogenous melt stream is very desirable. Depending on the resin you are processing, this could be very low in the range of 50 PSI or upwards of 300 PSI or more. Back pressure also contributes “mechanical heat” to the melt stream and the higher the back pressure is set, the more mechanical heat that will be added to the injection molding process through it’s use. The machine settings controlling these functions are:Screw Speed – This setting controls the RPM’s that the screw turns at up to the machine rated speed. This can be a few hundred RPM’s on a smaller machine, but typically will be less than 150 RPM’s on larger machines without special options. This setting also contributes to the time that it take to get the screw from it’s forward position during the injection molding process and the full shot size position. The ideal condition is to get the screw back to its full shot size position just prior to the opening of the mold at the end of the cycle, to prevent excessive idle residence time for the plastic in the barrel.Back Pressure Profile– This function on most newer machines, is also segmented with individual pressure and position settings for each segment.

[ Back ] [ Up ]



[ Basic Injection Molding VII ] [ Basic Injection Molding VIII ] [ Basic Injection Molding IX ] [ Basic Injection Molding X ]

This site is rated








basic injection molding

Documents