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Design Tipsfor Rapid Injection MoldingVolume 2

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Design Tips for Rapid Injection Molding

©2007 Protomold. All rights reserved. Volume 2 n DESIgn mATRIx n 2

Design Tips categorized by topicPage material

selectionDesign

guidelinesQuality

assuranceUnderstand the process

3 Side actions ñ ñ4 Minimize material to maximize options ñ ñ ñ ñ5 Consistent wall thickness can be a core issue ñ ñ6 A-side, b-side, how do you d-cide? ñ ñ ñ7 The cost of color ñ ñ8 A new slant on draft ñ ñ9 Keeping files in line ñ

10 Cut costs with rotational symmetry ñ11 Rapid Injection Molding 101 ñ ñ ñ ñ15 TGIT ñ16 Revision control ñ17 Getting the words right ñ ñ

TaBlE Of cONTENTS

External link to more information


In case you haven’t heard, rapid injection molding can now support undercuts in your part design. The details of what we mean by an undercut can be found in an updated version of the Protomold Design Guide, but in summary, the undercuts we can do are:

1 On the outside of the part geometry — not the inside.

2 On the parting line.

3 Must be perpendicular to the primary (A-B sides) pull direction.

4 Fit within certain size constraints (figure 2). As you are probably aware, undercuts in your part design are implemented through the use of what are called “side actions” in the mold. Figure 1 illustrates how a side action can be used to implement a simple undercut, and Figure 2 describes the maximum dimensions of the side actions that the Protomold process can currently support.

Due to mold size constraints we can have up to four side actions per part. As always, your ProtoQuote® will identify an undercut and let you know whether or not a side action can be used to accommodate it.

However, we can handle some pretty interesting geometries within these constraints (see Figure 3), so feel free to leave in some of those undercuts in your next part design.

Visit the Protomold Design guide for other helpful Rapid Injection Molding design information.

Side actions

©2007 Protomold. All rights reserved. Volume 2 n SIDE ACTIonS n 3

Due to mold size constraints we can have up to four side actions per part.

Figure 1: A side action used to create a through-hole undercut feature

Figure 2: Size constraints for mold side actions

Width Height Pull

≤ 8.419 in ≤ 2.377 in ≤ 1.445 in

≤ 1.523 in ≤ 1.445 in

≤ 1.038 in ≤ 1.639 in

≤ 0.553 in ≤ 1.736 in

Figure 3: Example of double undercut part

(courtesy of Vascular Solutions, Inc.)

http://protomold.com/DesignGuidelines.aspx



The accuracy of the rapid injection molding process is limited by our standard machining tolerances of 0.003” plus an additional tolerance for the molding process based on the resin you’ve selected (see figure 1).

Because these combined dimensional tolerances are highly dependent on the part design and resin selected, Protomold cannot guarantee that a specific tolerance will be met. Conventional injection molders address this through the use of iterative “steel safe” mold-making techniques. But with the automated, fast-

turnaround rapid injection molding process, we use the geometry specified in your 3D CAD model, incorporate a published shrink factor for the resin you’ve chosen and output CNC toolpaths to make the mold components.

So if you have features on your part that require tighter tolerances than our standard process can accommodate, consider a planned mold modification aimed at improving the accuracy of those features?. essentially a “simulation” of the conventional “steel-safe” approach.

The way to do this is to design your plastic features in the “minimum material condition” — because it is much easier to remove aluminum than add it (removing aluminum on the mold adds plastic on the part). As an example, consider the situation where the diameter of a through-hole is critical to the functionality of a design. As illustrated in the figure, it is better to initially design the hole in the part (and therefore the aluminum core in the mold) too large and then adjust to fit after some sample parts can be checked.

Because this type of mold modification and a subsequent run of new sample parts can typically be done for under $1000, by planning an iteration you might have a very cost effective way to get the greatest possible accuracy out of your rapid injection molded parts. Please note, however, that the modifications are still subject to our ±.003” machining tolerance.


©2007 Protomold. All rights reserved. Volume 2 n mInImIzE mATERIAl To mAxImIzE oPTIonS n 4

Minimize material to maximize options

Figure 1: Example additional tolerances for some resins

Resin Additional Tolerance

Polycarbonate or ABS 0.002 in/in

Unfilled Nylon 66 0.003 in/in

Polypropylene 0.005 in/in

Figure 2

minimum material on hole (maximum material for core pin)

Diameter you want



We’ve previously recommended that you maintain a consistent wall thickness throughout your part. One of the more useful techniques to do this is known as “coring out,” which involves cutting away blocks of material that don’t contribute substantially to the strength or function of the part. Of course we always try to hit it the first

time, but if the second rev needs strengthening, the mold can be modified relatively easily by removing metal. In this way you can add plastic to the weak areas, leaving the rest of the part with consistent wall thickness.

Figure 1 is an example of a simple part that was originally turned out of bar stock. When it came time to manufacture the part using injection molding, the designer cored out all the thick sections to a consistent wall thickness.

Another example can be found on the Protomold sample part (figure 2). It contains an example

of the shrinkage problems that can occur with thick features and illustrates how coring out the feature can resolve the problem.

The table* shows wall thicknesses Protomold recommends according to resin. Please note that thin walls only work on small parts and thicker walls are required where the resin has a long way to flow (Protomold makes parts with dimensions of about 0.25” to 15” or more).


* Adapted from www.manufacturingcenter.com

©2007 Protomold. All rights reserved. Volume 2 n ConSISTEnT WAll THICknESS CAn bE A CoRE ISSUE n 5

Consistent wall thickness can be a core issue

Cored geometry

Original geometry

Figure 1 Figure 3

Recommended Resin Wall Thickness (inches)

ABS 0.045 - 0.140

Acetal 0.030 - 0.120

Acrylic 0.025 - 0.150

Liquid crystal polymer 0.030 - 0.120

Long-fiber reinforced plastics 0.075 - 1.000

Nylon 0.030 - 0.115

Polycarbonate 0.040 - 0.150

Polyester 0.025 - 0.125

Polyethylene 0.030 - 0.200

Polyphenylene sulfide 0.020 - 0.180

Polypropylene 0.025 - 0.150

Polystyrene 0.035 - 0.150

Figure 2



One of the goals of rapid injection molding is to produce parts quickly. Proper design helps ensure that good parts will be produced on the first run. Determining how the part will be placed in the mold is critical. The overriding consideration is that the part must stay in the mold half that contains the ejector system.

In a typical injection molding machine, one half of the mold (the A-side) is attached to the fixed side of the press, and the other half of the mold (the B-side) is attached to the moving clamp side of the press (figure 1). The clamp (or B) side contains the ejection actuator, which controls the ejector pins. The clamp forces the A and B-sides together, molten plastic is injected into the mold and allowed to cool, the clamp pulls the B-side of the mold away, the ejection pins are actuated, and the part releases from the mold.

Let’s use a mold for a plastic drinking glass as an example. To ensure that the part stays in the mold half with the ejector system, we would design the mold so that the outside of the glass

is formed in the cavity of the mold (A-side) and the inside would be formed by the core of the mold (B-Side). As the plastic cools, the part would shrink away from the A-side of the mold and shrink onto the core in the B-side. As the mold opens, the glass will release from the A-Side, and stay in the B-side, where it can be pushed off from the core by the ejector system.

If the mold design were reversed, the outside of the glass would shrink away from the cavity in the B-side and onto the core in the A-Side.

The glass would release from the B-side and stick to the A-side where there are no ejector pins. At this point, we have a serious problem.

At Protomold, our design staff uses software tools and extensive experience to make the correct A-side vs. B-side choice. On some parts, it is difficult to predict in advance which side of the mold the part will stick to. Well thought-out part design ensures that the part will naturally stick to the correct side of the mold.

Let’s consider a rectangular enclosure with 4 through holes. The outside of the enclosure will be a cavity in the A-side of the mold and the inside will be a core on the B-side. Design for the holes, however, could be handled in two different ways: They could be drafted toward the A-side, requiring cores in the A-side of the mold, but this might cause the part to stick to the A-side of the mold. A better approach would be to draft the cores to the B-side, ensuring that the part would stick to the B-side of the mold. Similarly, any tab or strip sticking from the part or spanning an internal hole should be drafted to the B-side to prevent sticking in the A-side and bending or tearing off when the mold opens. And, of course, the design should also avoid heavy texture on the outside of a part without adequate draft, as this could cause the part to stick in the A-Side.


©2007 Protomold. All rights reserved. Volume 2 n A-SIDE, b-SIDE, HoW Do yoU D-CIDE? n 6

A-side, b-side, how do you d-cide?

Figure 1: A simple mold: A-side (cavity) on right, b-side (core) on left. Ejectors are in the b-side.

Figure 2: A part with 4 through holes and a tab drafted to the b-side.



©2007 Protomold. All rights reserved. Volume 2 n THE CoST oF ColoR n 7

The cost of colorHenry Ford famously offered cars in “any color…so long as it’s black.” His focus was on efficient production and low cost, and custom paint had no place in his production line. At Protomold, we’re all for efficiency and low cost, but we’re more flexible than Ford was. Color, however, can still be an issue.

The base color of most resins is either black or an array of lighter shades ranging from clear to brown and generically referred to as “natural.” This is why so many molded plastic products appear in those colors. But if you want color, there are several options available.

• Perhaps the simplest is paint. Not all plastics can be painted, but for those that can (and that will not be handled in such a way as to damage the painted surface) there is a huge variety of colors available. Painting is a separate process from molding and is not a Protomold offering.

• The simplest and least expensive way of getting color into (as opposed to onto) the resin is stock colorant like OmniColor. Protomold stocks about 40 of the 100 available colors whose palatte can be viewed here. The rest can be had at modest cost and can take up

to four weeks for delivery. The final color can be skewed by the tint of the base resin, and because the color is blended in the molding machinery, the final product can show swirls and significant color variation.

• Standard pre-colored resins are available from resin manufacturers in a wide variety of colors. These are quite consistent in shade, but have lead times of four to eight weeks. They typically must be ordered in minimum batches of up to 4500 pounds and must be supplied to Protomold by the customer.

• Custom-colored resins can be precisely matched to a customer sample by vendors like RTP (http://www.rtpcompany.com) or GE ColorXpress (http://www.gecolorxpress.com/jsp/extranet/user/home.jsp). Protomold customers should deal directly with the supplier to obtain the necessary resin. Costs vary and lead times are typically two to four weeks.

Finally, a note on metallic colors. Protomold can work with customer-supplied metallic resins, but these can cause cosmetic problems when the suspended metal flakes orient themselves along flow lines in the molded

part. A better solution for achieving metallic colors may be applying metallic color paint.


At Protomold, we’re all for efficiency and low cost, but we’re more flexible than Ford was.


http://www.rtpcompany.com

http://www.gecolorxpress.com/jsp/extranet/user/home.jsp

http://www.gecolorxpress.com/jsp/extranet/user/home.jsp

http://www.clariant.masterbatches.com/mb/internet.nsf/vwWebFramesets/6A01F6EB5D2EDB5085256A73006D7784?OpenDocument




Imagine a piston in the cylinder of an engine. It seems like a tight fit, yet the parts slide smoothly with no damage. In fact, there are several reasons that a piston moves so smoothly: first, there is space between it and the cylinder; second, both have hard, polished surfaces; and third, the surfaces are liberally lubricated. Eliminate the space or the lubricant and engine damage is guaranteed.

The same thing can happen when you pull a straight-sided plastic part out of a straight-sided mold, because there is neither space nor lubricant, and plastic isn’t very hard. To prevent damage or drag marks on the part, surfaces that are parallel to the line of mold opening may have to be drafted — angled away from the line along which the part

will be ejected (see figure 1). This causes the part wall and mold wall to move apart during ejection.

Your CAD package will not tell you which surfaces should be drafted, but it may indicate which surfaces are drafted (or reverse-drafted). Many CAD packages have a draft analysis module, which you should use liberally if it’s available. When you submit your design for a quote, however, Protomold’s online ProtoQuote® software identifies, in colors, surfaces that need additional draft and/or thickness. Arrows show the direction in which the feature should be drafted (see figure 2).

Most features require draft to facilitate ejection, and many features need draft for Protomold to be

able to machine the mold. Also, textured surfaces have a particular tendency to stick to mold walls, so light (T1) texture requires 3 degrees of draft, and heavy texture (T2) requires at least five.

In figure 3, a protruding arm of the bottom mold half (the telescoping shutoff) that forms the face and hook of the clip must be drafted by 3 degrees both to protect the part and to prevent mold wear due to rubbing of the shutoff against the top mold half.

Finally, proper drafting allows the cutting of deeper mold geometries at lower cost. A good rule of thumb in parts design, according to Protomold’s Kevin Crystal is, “When in doubt, draft it.”

©2007 Protomold. All rights reserved. Volume 2 n A nEW SlAnT on DRAFT n 8

A new slant on draft

Figure 1

DraftedUndrafted

Figure 3

TOp VIEw TOp VIEw

NO DRAFT RESUlTS IN SlIDINg paRallEl mOlD SURfacES

DRAFTED SHUTOffS mINImIzE wEaR aND ExTEND mOlD lIfE

Figure 2


It wasn’t long ago that most injection molds were cut manually by machinists working directly from drawings. Then, CNC (computer numerical control) came along and automated milling, working from digital input. Today, Protomold has added another layer of automation to the process, but however you get from model to mold, the accuracy of the original design determines the quality of the finished product.

At Protomold, all parts begin as 3D CAD files, but we’ve learned from experience that some files work better than others (and some don’t work at all). Of course, we’re always willing to work with customers to adapt files for production, but following a few simple guidelines will speed up the process and help you get exactly what you want without unnecessary delays.

The file formats we can use include:SolIDWoRkS nATIVE (.SlDPRT)

PRo/EngInEER nATIVE (.PRT)

IgES (.IgS): INITIAL GRAPHIC EXCHANGE STANDARD

STEP (.STP): STANDARD FOR THE EXCHANGE OF PRODUCT MODEL DATA

ACIS (.SAT): ANDY, CHARLES, IAN’S SYSTEM (REALLY NO KIDDING)

PARASolID (.x_b oR .x_T)

One common file format that we cannot use is STL. This format is designed for stereolithography and, though many CAD packages offer it as an output option, it does not contain enough data for rapid injection molding. Because data lost in saving a file in STL format cannot be recovered, the problem of data loss cannot be solved by opening an STL file and resaving it in an acceptable format. So if you want to submit a design for rapid injection molding, don’t save it as STL. Similarly, 2D files, wireframe models or .dxf (Drawing Interchange Files) do not contain all the information needed for the rapid injection molding process.

We realize that a part design often requires rework, but if you must edit a design, it is better to undo whatever needs changing than to patch it. For example, if you create a hole that you later decide you don’t need, plugging the hole is not the same as deleting the feature and recreating it. Patching can create internal surfaces, which can confuse the automated milling software.

You can, however “join” separate parts to create a single part. If you design a single part by assembling separate pieces, you must join them within the software; otherwise, your

design will include internal faces and, when you submit it, you will receive a message reading:

The parT below appears To be modeled as more Than one componenT. In order To quoTe your parT we wIll need The componenTs combIned InTo a sIngle model.

Conversely, if you are designing two or more parts and submit an “assembly file,” that is, a file showing parts that are intended to be separate, you will receive a message reading:

The parT below appears To be modeled as an assembly. In order To quoTe, we wIll need a fIle for each IndIvIdual parT ouTpuT In one or more of The followIng fIle formaTs: solIdworks naTIve (.sldprT), parasolId (.x_T or .x_b), sTep (.sTp), Iges surface (.Igs), or acIs (.saT).

When you export your design, set export tolerances as high as possible. 1/1000th is good; 1/10,000th is even better. This ensures maximum accuracy in your final part. In closing, keep in mind that modeling software is very complex. For a variety of reasons, saved files may occasionally appear incomplete. This can often be resolved by resaving the file in a different format. If this appears to be a problem with a file you submit, you will receive a message reading:

The parT lIsTed below has mIssIng feaTures. please Try To exporT your parT In a dIfferenT formaT. (e.g. sTep, Iges surface.)

Some of our customers send their designs in two different formats just to be safe. But whatever system you use and whatever you send, we will do everything we can to get you the parts you need in a timely manner.

©2007 Protomold. All rights reserved. Volume 2 n kEEPIng FIlES In lInE n 9

Keeping files in line


This month’s tip is one you probably won’t get to use often, but if you do, it will save you money (and make you feel really clever). We’re all familiar with bilateral symmetry, in which the left and right halves of an object are mirror

images of one another. It’s easy to recognize, and we see it every day in living things and a variety of

everyday objects (See Figure 1). But while the two halves are “the same,”

they are definitely not interchangeable, as anyone who’s tried to put a shoe on the wrong foot can attest.

Rotational symmetry, on the other hand, may not be as obvious to the eye, but it involves halves (or thirds, quarters, etc., depending on the degree of symmetry) that are identical. The face cards in a poker deck are a familiar example. Rotate a king, queen, or jack 180° and the image is the same as the one you started with. Cut the card in two along any line passing through the center of the card and you will end up with two identical pieces.

What makes this significant for plastic molding is the fact that parts for a rotationally symmetrical assembly can be made from the same mold. If you are having parts made, that means less tooling

to create the molds and smaller parts inventory to store and administer. But before you can take advantage of rotational symmetry, you must first recognize parts with rotational symmetry potential and, second, maintain symmetry in your design.

Imagine a drinking glass. The glass has bilateral symmetry, which we don’t care about, but is also rotationally symmetrical. Cut the glass in half vertically, rotate one half 180° and you have two identical pieces. If, however, you add a handle to make a coffee mug, you’ve lost rotational symmetry and must make the two halves from different molds. But if you add a second handle, such as you might find on a sugar bowl, you have now resumed rotational symmetry and can again make both halves of your design from a single mold.

Maintaining rotational symmetry also requires attention. For example, in joining pieces, you lose rotational symmetry by equipping one piece of an assembly with pins that mate with holes in the other piece. However, if pins and holes are evenly divided between the pieces such that pins in one mate with holes in the other, symmetry can be maintained. The same is true of counterbores and bosses for screw-attachment, hooks, latches, and hinges.

The accompanying photograph shows one part of a two-piece box designed to mate with an identical copy of itself (See Figure 2). When the parts are assembled, the hook at the upper left corner will engage the pin shown at upper right to form a hinge. The vertical loop at the lower left edge will catch the tab shown at lower right to latch the halves together.

To design rotationally symmetrical mating parts, begin by creating a single part in your CAD system. Copy that part twice into an assembly, and then try to mate the two parts. If your system has interference checking, it will let you know whether your parts are mating correctly. Then, as your design progresses, keep checking the fit until you reach your final model.

©2007 Protomold. All rights reserved. Volume 2 n CUT CoSTS WITH RoTATIonAl SymmETRy n 10

Cut costs with rotational symmetry

Figure 2

Figure 1


ACIS — A standard computer file format for exchanging CAD data, typically from AutoCAD programs. ACIS is an acronym that originally stood for “Andy, Charles and Ian’s System”

A-Side — The mold half that mounts to the fixed side of the injection molding press, through which resin is injected into the part cavity via the sprue. Sometimes referred to as the cavity side of the mold, the A-side does not have ejector pins and for this reason it often produces the outside or cosmetic side of the part.

b-Side — The mold half that mounts to the moving side of the injection molding press. Sometimes referred to as the core side of the mold, the B-side has ejector pins to push the part out of the open mold. An analysis of the part geometry determines the optimal part orientation to ensure that it will remain on the B-side when the mold is opened.

boss — A cylindrical protrusion within a part, often designed to accept fasteners.

bead blasting — Using abrasives in a pressurized air blast to create a surface texture on the part.

bevel — See “chamfer”.

blush — Cosmetic blemish at the point of injection in the finished part.

bridge tool — A temporary or interim mold made for the purpose of making production parts until a high—volume production mold is ready.

barrel — The part of the molding press where resin is melted.

CAD — Short for “computer aided design”.

Cam — See “side action”.

Cavity — A concave feature on either side of the mold into which an opposing core enters when the mold is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the A-side of a mold is referred to as the cavity side, and in the case of a part like a drinking cup, the entire A-side will be a cavity.

Chamfer — Also known as a bevel, it is a flat truncated corner.

Clamp force — The force required to hold the mold shut so resin cannot escape during injection.

Contoured pins — Ejector pins with the ends shaped to match a sloping surface on the part.

Core — A convex feature on either side of the mold that will enter an opposing cavity when the mold is closed. The void between the cavity and core is where the resin solidifies and forms the part. Often the B-side of a mold is referred

©2007 Protomold. All rights reserved. Volume 2 n RAPID InjECTIon molDIng 101 n 11

Rapid Injection Molding 101This Design Tip consists of a collection of terms we use every day on the phone with customers, so we thought it might be useful to have them all in once place. Don’t worry, they won’t be on the Final Exam.

Rapid Injection molding glossary


to as the core side, and in the case of a part like a drinking cup, the entire B-side will be a core.

Core-cavity — The design of a mold where the A-side forms the outside of the part and the B-side forms the inside. The advantage to this approach is that the part will shrink onto the B-side so it can be ejected, and if the inside and outside are drafted with equal and opposite draft the wall thickness will be constant.

Cycle time — The time it takes to make one part including the closing of the mold, the injection of the resin, the solidification of the part, the opening of the mold and the ejection of the part.

Direction of pull — Refers to the motion of a part surface relative to a mold.

Draft — The taper of features in the direction of pull. It allows deeper features to be produced in three—axis milling machines and it also helps parts release from the mold during ejection.

Drying of plastics — Many plastics absorb water and must be dried prior to injection molding to ensure good cosmetics and material characteristics.

Durometer — A measure of the hardness of a resin. It is measured on a numeric scale with numbers ranging from lower (i.e. softer) to higher (i.e. harder).

Edge gate — An injection method that uses a gate on the parting line of the mold. It typically

leaves a vestige on the outside of the part and is sometimes referred to as a tab gate.

Ejection — The process of pushing a completed part out of a mold.

Ejector pins — Steel pins incorporated into the B-side of a mold that push out the plastic part.

End mill — A cutting tool that is used to machine a mold.

ESD — Stands for “electro static discharge”, an electrical effect that may necessitate shielding in some applications. Some special grades of plastic are electrically conductive or dissipative and help prevent ESD.

Family mold — A mold containing two or more different parts.

Flame retardant — A resin formulated to resist burning.

Flash — Excess plastic that flows into the parting line of the mold beyond the edges of a part and freezes to form a thin, sheet-like protrusion from the part.

Flow marks – Visible indications on the finished part that indicate the flow of plastic within the mold prior to solidification.

Food grade — Resins or mold release spray that are approved for use in the manufacture of parts that will contact food in their application.

gate — The location where the plastic enters the part. There is typically a visible vestige when the gate is removed.

gF — Stands for “glass filled”, it refers to a resin with glass fibers mixed into it. Glass filled resins are much stronger and more rigid than the corresponding unfilled resin, but also more brittle. Resins can also be filled with carbon fiber, stainless steel, etc. In general, filled resins can be very susceptible to warp.

gusset — A triangular rib that reinforces areas such as a wall to a floor or a boss to a floor.

Hot tip gate – An injection molding method that uses a heated gate on the A-side of the part to eliminate the creation of any runner or sprue. The gate vestige will be a small sharp bump that can be trimmed if necessary.

IgES – Stands for “Initial Graphics Exchange Specification”. It is a common file format for exchanging CAD data. Protomold can use IGES solid or surface files to create molded parts.

Injection — The process of forcing melted resin into a mold.

jetting — Flow marks caused by the resin entering a mold at high speed, typically occurring near a gate.

knit lines — Visible indications in a finished part, formed by the intersection of two hot plastic fronts. They are always formed downstream



of through — holes and between multiple gates. They are also known as weld lines.

living hinge — Very thin section of plastic used to connect two parts and keep them together while allowing them to open and close. They require careful design and gate placement. A typical application would be the top and bottom of a box.

medical grade — Resin that may be suitable for use in certain medical applications.

metal-safe — A change to the part design that requires only the removal of metal to produce the desired geometry. Typically most important when a part design is changed after the mold has been manufactured, because then the mold can be modified rather than entirely re-machined. It is also commonly called “steel safe”.

mold release spray — A liquid applied to the mold as a spray to facilitate the ejection of parts from the B-side. It is typically used when the parts are difficult to eject because they are sticking to the mold.

multi-cavity mold — A mold with multiple copies of the same part, typically used to reduce piece-part pricing for higher volume runs.

nozzle — The tapered fitting on the end of the barrel of the injection molding press where the resin enters the sprue.

Packing — The practice of using increased pressure when injecting a part to force more plastic into the mold. This is often used to combat sink or fill problems, but also increases the likelihood of flash and may cause the part to stick in to the mold.

Parasolid — A file format for exchanging CAD data.

Parting line — The location where the pieces of a mold come together. Typically a thin line is created on the part here.

Post gate — An injection method that injects plastic through an ejector pin hole. This gating technique leaves a gate vestige on the B-side of the part where it can often be less visible for cosmetic purposes. There is often gate blush opposite a post gate.

Press — The injection molding machine that makes the plastic parts. It holds the mold closed, melts the resin, injects it into the mold, opens the mold and ejects the part.

Process — The injection molding environment consisting of input variables such as temperature, pressure, injection rates and time that are controlled to fill the mold while optimizing the tradeoffs between cosmetics and dimensional accuracy.

Radiused — An edge or vertex that has been rounded. Typically this occurs

on part geometries as a natural result of the Protomold milling process.

Recess — An indentation in the plastic part caused by the impact of the ejector pins.

Reinforced resin — Refers to base resins with fillers added for strength. They are particularly susceptible to warp because the fiber orientation tends to follow flow lines, resulting in asymmetric stresses. Typically these resins are harder and stronger but also more brittle (i.e. less tough).

Resin — Synonymous with “plastic” as far as injection molding is concerned.

Rib — A reinforcing member of a molded part.

Runner — A channel machined into the mold that directs the resin from sprue to the gate.

Screw — The mechanical feature inside the barrel that forces the resin out the nozzle.

Shear — The force between layers of resin as they slide against each other or the surface of the mold. The resulting friction causes some heating of the resin.

Shrink — The change in size of the part during solidification, typically anticipated based on published material property data and built into the mold design prior to machining.

Shutoff — The surfaces where the A-side and B-side of the mold contact. The shutoff meets the part at the parting line.



Short shot — A part that wasn’t completely filled with resin, causing short or missing features.

Side action — A sliding cam arrangement within the mold that allows for the molding of parts with undercuts. The undercut-creating mold face is held in place during the injection process and then slides out of the way prior to ejection.

Sink — Undesired depressions in the surface of a part that are caused by the shrinking of resin as it solidifies. Sink is most common in thick sections of a part.

Splay — Discolored visible streaks in the part, typically caused by moisture in the resin.

Sprue — The route the resin takes from the point where it enters the mold until it reaches the runner(s). When solidified, it remains attached to the part via one or more runners and is typically removed in finishing.

Steel safe — See “metal safe”.

Sticking — A problem during the ejection phase of molding, where a part becomes lodged in one or the other half of the mold, making removal difficult. This is a common issue when the part is not designed with sufficient draft.

STEP — Stands for “Standard for the Exchange of Product Model Data”. It is a common format for exchanging CAD data.

STl — Originally stood for “STereoLithography”. It is a common format for transmitting CAD data to rapid prototyping machines and is not suitable for rapid injection molding.

Straight pull mold — A mold without side actions. It is less expensive than a comparable mold with side actions.

Tab gate — See “edge gate”.

Telescoping shutoff – An area within a mold where metal slides along metal, usually creating a hole in the part. A three degree draft angle is required on any related part surfaces.

Texture — A surface treatment applied to the mold to create texture on the parts. Protomold offers polished, sanded and bead-blasted textures.

Tunnel gate — An injection method that uses a small gate located off the parting line in one of the mold halves. It leaves a vestige a short distance from the parting line of the part.

Undercut — A portion of the part geometry that would prevent the part from being ejected from a straight-pull mold without a portion of the mold passing through (and destroying) the part. The simplest example of an undercut feature on a part would be a through-hole aligned perpendicular to the direction of part ejection.

Vents — A very small (e.g. 0.001” – 0.005”) opening in the mold cavity, typically at the shutoff surface or via an ejector pin tunnel, that is used to let air escape from a mold while the resin is injected.

Vestige — A visible mark created by the manual process of finishing parts when the gate is trimmed. Ejector pins also may leave a vestige where they impact the part (also see “recess”).

Warp — The curving or bending of parts that typically occurs after ejection as the part cools. Warp is often caused by glass filled resins.

Weld lines — See “knit lines”.

Wireframe — A type of CAD model consisting only of lines and curves, in 2D or 3D. Wirefame models are not suitable for rapid injection molding.



Timing, they say, is everything. From catching a bus to tuning an engine — from Comedy Central to Wall Street — you’ve got to have timing if

you want to get ahead. Of course, if your goal is real injection molded parts at a great price and blazing-fast, you always come out ahead at

Protomold, but here’s a bit of arcane knowledge that might help you get your parts even faster.

Like most everyone else, design engineers like to clear their desks before going off to relax for the weekend. For this reason as much as any other, we at Protomold often see a surge in RFQs and online orders on Fridays. Also like most of us, the service reps at Protomold — those hardworking folks who answer your questions, resolve any issues, and do final entry of your order — work

five days a week. So if you place an online order on a Friday you probably won’t hear back from us with an order confirmation and proposed gate and ejector pin layout until the following Monday. (We can’t start milling your mold until you’ve approved the gate and pin layout.)

So here’s the tip: If possible, try to place your online order by Thursday. That gives us a business day before the weekend to send you an order confirmation with a proposed gate and ejector pin layout to review. If it looks good, you can select your options and click on “Approve layout” before you head off to the lake. We will pass the job to our weekend crew and be two days ahead making your mold. Of course if you can find someone willing and qualified to stick around and approve

these things in your absence, you can get a jump on the weekend and head for the lake on Thursday. (Seriously, though, if you’re as busy as a lot of our customers, designating a backup approver when you place your order is a very good idea.) So place those orders by Thursday and spend your weekend with a clear conscience and a clear desk.

©2007 Protomold. All rights reserved. Volume 2 n TgIT n 15

TgIF T

Figure 1: Reviewing the gate/ejector pin placement from Protomold

Figure 2: Sample order confirmation screen from Protomold

Place those orders by Thursday and spend your weekend with a clear conscience and a clear desk.


This month, we’d like to suggest that you take a lesson from grilling machine mogul and former heavyweight champ George Foreman. Relax, we’re not suggesting that you step into the ring with Muhammad Ali or Evander Holyfield, but Mr. Foreman could teach us all a thing or two about version control. As you may know, The former champ has five sons, all named George. That’s not as confusing as it sounds, since the five “little” Georges are named Jr., III, IV, V, and VI, respectively. At Protomold, we’d like our customers to do something similar in naming revisions to their part designs.

The naming scheme can be any logical approach you chose, but if you use part names and numbers, please enter both in the “part number/name” field when you submit a design for a ProtoQuote®. That way, when your buyer orders part number 123-456, we’ll know that’s Widget, Plastic. Also, be sure that the revision designation is entered in the “rev (optional)” field and is part of your file name (see Figure. 1).

Naming and numbering conventions may be mandated by the organization or developed by the designer. Either way, it makes sense to give

a design a new name each time it is changed and saved. But even with such a system in place, changes can slip through the cracks. Suppose, for example, that a designer submits a saved 3D CAD model to Protomold for a quote. Later, the

designer reopens that file, makes some small change to the design — perhaps in response to a change suggested in the ProtoQuote® received from Protomold — and re-saves the file under its original name. The reuse of the name may not matter to the designer since there is still just one file for the design, and it contains the most current information. But if that design is resubmitted to Protomold, it could cause confusion. Here’s how.

In the interest of version control, if multiple versions of a file that a customer sends have the same name, we at Protomold add version designations to avoid confusion between them. Unfortunately, if the designer doesn’t accept our version designations, Protomold and the designer could end up calling the same file by different names. The simple answer is for the designer — you — to provide version designations, which we’ll be happy to accept and use. This ensures that when either of us

refers to a file, we are “all on the same page.” When you get your ProtoQuote® (or any other correspondence regarding a design), check to be sure that it references the correct name, number and revision. Take special care to do so when you receive an order confirmation.

In summary, give each revision its own unique designation. It avoids confusion and helps you get the right parts on time. That’s how George Foreman does it, and you wouldn’t want to be the one to tell the former world heavyweight champ that he’s doing it wrong. Would you?

©2007 Protomold. All rights reserved. Volume 2 n REVISIon ConTRol n 16

Revision control

Figure 1

Give each revision its own unique designation. It avoids confusion and helps you get the right parts on time.


A picture may be worth a thousand words, but sometimes a few words or lines of text are all it takes to get your point across. If those words are going to appear on your plastic part, there are some important points to keep in mind. First, there are two general considerations. One is that text is usually among the smallest details on a part, so all factors affecting small details apply. The second is that the mold is the inverse image of the part. Raised text on the part is recessed text in the mold, while recessed text in the part is raised text on the mold.

The lower limit on text size is determined by the diameter of the smallest size end mill that can be used. For a part with raised text on a flat easy to reach surface the smallest feature that can be milled is 0.020in. (see Figures 1 and 2). If the text is at the bottom

of a deep rib or next to a feature that requires the endmill to “reach” a significant distance, a larger diameter end mill may be required and consequently the text size may need to be larger.

In general, for ease of cutting and ease of reading, particularly at small sizes, bolded sans serif fonts are preferred. Also, whenever possible, add 2 to 5 degrees of draft.

Finally, because it is easier to polish around text that is recessed into the mold, for cosmetic reasons it is preferable if the text on the part is raised.

©2007 Protomold. All rights reserved. Volume 2 n gETTIng THE WoRDS RIgHT n 17

Getting the words right

Figure 1: In SolidWorks, Century gothic/bold/0.20” high will generally produce text we can mill.

For ease of cutting and ease of reading, particularily at small sizes, bolded san serif fonts are preferred.

Figure 2: This text is too small to mill.

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