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Design Tips

NOBODY’S FASTER IN THE SHORT RUN.®

for Rapid Injection MoldingVolume 4

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Design Tips for Rapid Injection Molding

�©�007 Protomold. All rights reserved. Volume 4 n DESign mATRix n

Design Tips categorized by topicPage material

selectionDesign

guidelinesQuality

assuranceUnderstand the process

3 Fun with cams ñ ñ5 Sizing: an in-depth examination ñ ñ6 Living in the material world ñ ñ ñ8 The orphan fillet ñ ñ9 What you don’t “C” can’t hurt you ñ ñ ñ ñ

11 When you really need to dodge the draft ñ ñ13 Night of the living hinge ñ ñ ñ14 Good vibrations — ultrasonic welds ñ ñ ñ15 Resist that sinking feeling ñ ñ ñ17 The inside scoop on outside threads ñ ñ ñ19 Sliding shutoffs (again) ñ ñ20 When things get rough ... ñ ñ ñ

TaBlE Of cONTENTS

External link to more information


3

In a previous design tip, we used the example of a house-shaped box with a “mousehole” doorway (See Figure 1). The outside of the house was formed by the A-side of a simple straight-pull mold; the inside was formed by the B-side of the mold. A shutoff, a raised pad on the surface of the B-side mold, formed the doorway. In this tip we’re going to complicate the process by turning the door shown in Figure 1 into a window (See Figure �). By adding material below the bottom of the feature, we’ve created an undercut feature that cannot be produced in a two-part mold.

Whereas the shutoff that created the doorway in Figure 1 can exit the doorway through the open bottom of the feature when the mold opens, a mold feature used to form the window in Figure � would be trapped when we try to open the mold. The solution is to create a third mold part that

moves perpendicular to the direction of mold opening (or parallel to the plane of the mold’s parting line). This “side-action cam” fills the space that will become the window. When a side action is used, mold opening drives the cam out sideways as the two primary halves of the mold open, after which the part is ejected. Sometimes Protomold will add other faces to the cam to eliminate parting lines on a critical face. We have done this with the whole front of the house to prevent parting lines below the door/window. You can discuss this with your Protomold customer service engineer.

While a wide variety of parts can be produced in straight pull molds, side actions literally open up whole new dimensions in part design. One of the most common applications is the production of through-holes, of which the window mentioned above is an example. Producing a through-hole in the process of molding saves the time and cost of a separate operation after the part has been

molded. In a straight pull mold, through-holes can be made in the direction of pull. They can also be made in other directions using sliding shutoffs, which work well for some applications, such as the dormer window in the house. See our tip at: Creating Through-Holes. When sliding shutoffs aren’t appropriate, side-action cams can create holes and other features other directions as long as the direction of cam travel is perpendicular to the direction of mold opening and the feature is on the outside of the part.

Figure 3 shows a part with several features that could only be made using side actions. The tan circular hole is similar to the house window in Figure �. The purple rectangular indentation can be thought of as a hole that doesn’t go all the way through the wall. But like a hole, it would be an unmoldable undercut in a straight-pull, two part mold. The side-action cam, however, is well out of the way before the part is ejected.

Fun with cams

©�007 Protomold. All rights reserved. Volume 4 n FUn WiTH CAmS n

Figure 2 Figure 3Figure 1

http://www.protomold.com/DesignGuidelines_SimpleStraightPullParts.aspx


4

In all of the previous examples, cams are used to create small features on a larger part, but this is not the only way they can be used. The part shown in Figure 4 uses cams to create the entire circumference of the part, while the indented top and bottom are formed by the A- and B-side primary mold halves. Alternatively the entire part could be rotated 90 degrees

making the sides in the diagram with the A and B mold halves and using side actions to create what are shown as the top and bottom.

In short, now that we’ve added side-action cams to our mold-making tool kit, Protomold is not just for simple parts anymore. Here are some guidelines:

We can build up to four separate side actions into a single mold.

While side actions must all be in planes parallel to the plane of the primary mold parting line, they need not all be in the same plane.

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Side actions can be used to produce features on the outside of a part but not (yet) on the inside.

Like primary mold sections, side actions may require drafting. This was discussed in the June �006 design tip.

If you have any questions regarding the application of side actions to your parts, feel free to contact us.

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

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©�007 Protomold. All rights reserved. Volume 4 n FUn WiTH CAmS n

Figure 4 Protomold is not just for simple parts anymore.

http://protomold.com/DesignGuidelines.aspx

http://www.protomold.com/ContactUs.aspx

http://www.protomold.com/Design_Tips/UnitedStates/2006/2006-06_DesignTips/default.htm


5

Figure 1

There’s a Wall Street saying often quoted to those who see no limits to a favorite stock’s prospects: “Trees don’t grow to the sky.” In other words, everything has its limits. And so it is with Protomold’s molding capabilities. We can deliver great parts incredibly fast and at amazing prices, but due to a number of factors related to our existing molding equipment we do have size limitations. We are, of course, always striving to expand our capabilities.

Until recently, our production was limited to parts cut no more than two inches deep into each mold half. In other words, the depth of a carefully designed part could be a full four inches, but only if the depth of the part were divided equally between the two mold halves. (See Figure 1)

With the addition of new technology, we can now produce parts with a total depth of six inches as long as neither mold half is cut more than three inches deep. (See Figure �)

Regardless of the depth of the part, its total volume cannot exceed ��.1 cubic inches. The reason is simple: that is the volume of

resin that our largest press can currently inject into a mold in a single “shot.”

The next issue is maximum part outline. Imagine that you sat your part on a flat surface running parallel to the part’s parting line. The shadow of the part projected downward onto the surface is the part outline or “projected area.” (Light shining through holes in your part doesn’t count toward the projected area.) For parts up to two inches in depth in each mold half, the part outline must fit within a rectangle measuring 7.5 x 14 inches. For parts up to three inches in depth in each mold half, the part outline must fit a rectangle measuring 6 x 8 inches. The reason for this limitation is the size of the raw mold stock we use for molds of different depths.

The final issue is total mold area. This is the actual area of the opening where the two mold halves meet, and it cannot exceed 75 square inches. This limitation is based on the maximum closing force our molding presses can exert. That force must exceed the injection pressure, typically measured in psi, of the resin multiplied

by the total mold area or the press will be unable to hold the mold closed during injection.

To summarize the data:

Finally, there is the issue of draft. A good rule of thumb is that parts should be drafted one degree for each inch of depth cut into the mold half. In other words, one inch of depth requires one degree of draft; two inches requires two degrees; three inches of depth gets three degrees. Parts of one half inch or less require a minimum of one half degree of draft.


©�007 Protomold. All rights reserved. Volume 4 n Sizing: An in-DEPTH ExAminATion n

Sizing: an in-depth examination

Figure 2

2” from parting line. 4” total Requires 2° draft

3” from parting line. 6” total Requires 3° draft

Maximum depth per mold half �” 3”

Maximum part outline 7.5” x 14” 6” x 8”

Maximum projected part area 75 in� 75 in�

Maximum part volume 15.75 in3 15.75 in3



6

We’ve all heard at one time or another—from a parent, a coach, or a teacher—a reference to “what you’re made of.” It probably referred to what you could do or withstand, but since we’re all made of pretty much the same stuff, the meaning of the phrase was more figurative than literal. When you’re an injection molded part, however, “what you’re made of” literally determines a great deal of what you can do or withstand.

At Protomold, we keep over 100 resins in stock and have access to hundreds more. But the three most often requested are ABS, polycarbonate, and 33 percent glass-filled Nylon. This is not to say that these are the three most widely used resins for injection molding, just that they are the most used by Protomold customers.

ABS is a good, inexpensive, general purpose resin. It is widely used for the cases of hand-held electronic devices, the housings of power tools and many other products we use every day. The material is tough enough to take a licking in everyday use, and while it may scuff from rough handling, it is less subject to breakage than a lot of other plastics.

Another plus for ABS is its excellent moldability characteristics. It is somewhat susceptible to sink and can be damaged by solvents, but if you design parts carefully, it is possible to produce well-formed parts without serious shrink, sink, or internal stress. It is important to maintain relatively even wall thickness in designing parts in ABS, though not quite as critical as with other, more shrink-prone materials. In general, ABS is opaque, although a clear version of the material is available.

Polycarbonate is considered a “higher-end” resin. While it does cost more than ABS, it is just a medium-cost resin. It can be very strong, so much so that it is used for bulletproof windows. And while it is often chosen, because of its transparency, for use in lenses and light pipes,

it can also be opaque. Because of its high strength, it is used to make cases and housings which need a stronger material than ABS.

Polycarbonate does have some shortcomings, including a tendency to sink. If a polycarbonate part is not properly designed, the surface of overly thick area can sink significantly during cooling. In some instances, shrinkage may not show on the surface, but internal shrinkage may cause a void inside the part, seriously weakening the finished piece. Proper design and avoiding thick/thin geometries will help prevent such problems. Also, polycarbonate is susceptible to petroleum-based solvents. In some applications, polycarbonate can

©�007 Protomold. All rights reserved. Volume 4 n LiVing in THE mATERiAL WoRLD n

Living in the material world

ABS

Polycarbonate


7©�007 Protomold. All rights reserved. Volume 4 n LiVing in THE mATERiAL WoRLD n

be blended with other resins, like ABS, to achieve a compromise on both properties and cost.

Glass-filled nylon is the strongest of the three resins addressed here. Common glass-filled nylons are medium-cost resins, though some specialized versions of the material can be very costly. The material resists many solvents and hydrocarbons, but is attacked by some acids and

bases. (You should research your application and environment before finalizing your resin choice). And with the addition of glass fiber, nylon is very heat resistant. With up to three times the strength of polycarbonate, this material is used for protective or structural parts that need to withstand a great deal of stress.

On the other hand, glass filled nylon is the most shrink-prone of the three resins being discussed. Nylon itself is very subject to shrinkage as it

cools, and the addition of fiberglass can cause differential shrinkage relative to the direction of resin flow during mold filling and contributing to warp. For this reason, if the strength, heat resistance, and chemical compatibility of this material are needed, good design is critical in preventing distortion of the finished parts.


glass-filled nylon

When you’re an injection molded part, however, “what you’re made of” literally determines a great deal of what you can do or withstand.



8

Fillets, in certain geometries, can be a problem. Take, for example, the part shown in Figure 1. As produced by the designer’s CAD package, the green faces of this part (including the left and far sides that don’t show in the diagram) are created by the A-side mold and drafted toward the A-side. In other words, they taper slightly toward the bottom of the diagram. The surfaces shown in blue are created by the B-side of the mold and are drafted toward the B-side, which means they taper toward the top of the diagram. The problem is with the fillet.

Figure 1 actually shows the 3D CAD diagram as evaluated by Protomold’s ProtoQuote® quoting and analysis software. The area noted in red, the fillet, is where ProtoQuote® has identified a problem. The customer’s CAD software, recognizing that the fillet connects to both an A-side face and a B-side face has tried, unsuccessfully, to resolve the conflict between two opposite draft directions. The reason it has been unsuccessful is that, in reality, this fillet can not be part of either side. In other words, this feature is an orphan.

To be clear, this is a fillet which connects an A-side drafted face to a B-side drafted face, and is over (or could be under) a flat surface, which creates undercut geometry.

If this fillet were created by the B-side of the mold, it would have to taper in the same direction as the adjoining blue face, that is, toward the top of the part. The problem is that the adjoining green face, which is part of the A-side, is tapering in the opposite direction, toward the bottom on the part. The result would be a misalignment — a step — along the line where the fillet (red) meets the A-side face (green).

If, on the other hand, this fillet were created by the A-side of the mold, there would be a problem in the area that appears, in Figure 1, as a small red triangle on the foot of the part at the base of the vertical tower. The part of the A-side of the mold that created the fillet would trap the plastic part under it at that triangle when the mold opens. (Figure � shows the A-side mold itself and in red the projecting feature that would trap the part.)

In Figure 1, Protomold’s analysis software has attempted, unsuccessfully, to resolve the conflict by dividing the fillet between the two mold halves. The red half has been assigned to the A-side, the blue half assigned to the B-side. The bright blue lines indicate the undercut area. Unfortunately, the problem of mold entrapment remains, as can be seen in both Figure 1 and Figure �. Figure � also shows a secondary problem. The area of the mold that is supposed to create part of the fillet comes to a “razor” edge. Such an edge would be subject to extreme wear and, as a result, allow the formation of undesirable flash.

There are three possible solutions to the problem of the orphan fillet:

The designer could redesign the part so that everything was drafted toward the top. In that case, the entire part, fillets included, could be molded in the B-side mold, with the A-side just forming the base of the part.

The designer could avoid vertical fillets that connect A-side and B-side drafted faces. This would prevent the problem in the first place.

This part could be manufactured as designed with the addition of a side-action cam. Protomold can include up to four such cams in a mold, but this would increase the cost of the mold.


©�007 Protomold. All rights reserved. Volume 4 n THE oRPHAn FiLLET n

The orphan fillet

Figure 1

Figure 2

1

2

3



9©�007 Protomold. All rights reserved. Volume 4 n WHAT yoU Don’T “C” CAn’T HURT yoU n

Start with the simple fact that plastic resins shrink as they cool. Some shrink more than others, but they all do it. If the shrinkage were perfectly even, we could simply make the mold slightly oversize and count on shrinkage to reduce them to the desired size. Unfortunately, shrinkage is a more complicated process. As a result, certain shapes that are otherwise perfectly acceptable can be difficult to mold because they tend to warp as

they cool. Anything with a “C” shape, like the part shown in Figure 1, can be particularly problematic.

Of course your choice of resin can contribute to the problem in two ways. The first is variation in the tendency of the resin to shrink as it cools. For example:

Acrylic shrinks very little

HDPE shrinks quite a bit

Nylon 6/6 falls somewhere between the two

The second materials issue is specific to filled materials. As they are injected into the mold, the fiber filler in these materials tends to align with the direction of resin flow. The resulting “grain” causes uneven shrinkage between dimensions that run with the grain and those running across the grain. The result is an increased tendency of parts made of filled resin to warp as they cool.

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As far as shapes are concerned, the problem with “C” is actually a problem with its two right angle“L” corners.

Figure � is a close-up view of the angle of one of the L’s. You can see that the distance along the inside of the angle (from A to B) is shorter than the distance around the outside of the angle (from C to D). As a result, the surface on the outside of the angle is larger than that on the inside. More area means faster radiation of heat. As a result, the C-D side of the angle hardens before the A-B side. As A-B continues to cool, it also continues

What you don’t “C” can’t hurt you

Figure 1

Figure 2

As far as shapes are concerned, the problem with “C” is actually a problem with its two right angle “L” corners.


10©�007 Protomold. All rights reserved. Volume 4 n WHAT yoU Don’T “C” CAn’T HURT yoU n

to shrink, pulling what was designed to be a right angle to something less than 90 degrees.

The solution? If you radius the corner on the inside and outside as shown in Figure 3 (using the formula shown maintains constant wall thickness), you will minimize the warping effect. Using a larger radius (but maintaining constant thickness) will reduce warp more as it reduces the difference in mold metal to cool the inside and outside of the wall.

Of course, everything that’s true of an L is doubly true of a C, which increases the magnitude of the problem because there is more curve, hence more difference between the length of the outside and inside surfaces. Whether made up of angles or curves, the inside of the C will be shorter than the outside and, as a result, will still be cooling and shrinking after the outside has hardened, pulling the “jaws” of the C closer together.

There are a number of ways to address the problem. Turning the C into an “O” eliminates the opening and prevents the ends of the C from being pulled toward one another. In essence, the added part of the circle acts as a brace to help the part hold its shape. Putting a removable brace across one of the open sides can also help counteract the forces trying to close the jaws of the “C” until the part has cooled and stabilized.

If none of these is possible, the best way to reduce the problem is to choose one of the more

“shrink-resistant” resins. These would include: ABS, Polycarbonate, PC/ABS, PETG Polyester, Polystyrene, and K-resin Polystyrene butadiene. And, of course, where shrinkage could distort your part it is particularly important to pay close attention to geometry and avoid filled resins.


Figure 3 Figure 4



11

We’ve spent so much time reminding designers that parts must be drafted to facilitate ejection from molds that it seems strange to talk about how to avoid having to draft parts, but it can sometimes be done when absolutely necessary. Keep in mind, however, that drafting is still the key to simplicity of design, ease of molding, and cost control.

As we’ve said in previous Design Tips, when a surface is parallel to the direction of mold opening, it should be slightly tapered toward the mold; otherwise the mold surface will drag across the surface as the mold opens, damaging the surface. Drafting causes the part face to move away from the mold face as the part is ejected, preventing

damage. The slight change in face angle usually makes no difference in either the functionality or appearance of the part. But what if it does?

Probably the most common reason not to draft a surface is to make it fit with other parts of a finished product. Figure 1 is a bracket which bolts to a machine. If the mating face is drafted, the top face tilts at an angle that is unacceptable for this application

For a specific requirement, like this one, Protomold can incorporate cam-driven side actions into a mold. These are typically used to create undercuts that could not be molded in a simple two-part mold. But, because cams move perpendicular to the direction of primary mold opening they can also be used to produce surfaces that are undrafted in relation to the A- and B-side mold halves.

Imagine a part with a surface parallel to the direction of mold opening. Let’s assume that we cannot draft the problem surface and must find some other way to protect it during ejection (see Figure 1). Protomold would normally require draft on this face as shown in Figure �.

If the surface cannot be drafted, so as to move away from the mold as the part is ejected, an alternate solution is to have the mold move away from the surface. This is achieved using a side-action cam (brown face in Figure 3).

©�007 Protomold. All rights reserved. Volume 4 n WHEn yoU REALLy nEED To DoDgE THE DRAFT n

When you really need to dodge the draft

Figure 1 Figure 2

Drafting is still the key to simplicity of design, ease of molding, and cost control.


1�

In essence, a side action works like the moving wall in the Death Star trash compactor in Star Wars. As the A- and B-side mold halves open or close along the X-axis, the cam (or cams) move along the Y/Z axes. Before ejection, the cam will withdraw leaving no mold wall next to the problem face to cause problems as the part is ejected.

Aside from undrafted faces and the obvious undercut features, there are several other applications for side actions. Raised lettering on a face parallel

to the direction of mold opening presents a problem even if the face is drafted; side actions solve that problem (brown face in Figure 4).

Similarly, texture on a low-draft face, which might not be reproducible in a straight-pull mold, can be produced in a mold with side actions.

There is additional cost for each side action, and there may be some flash between the side action face and the rest of the mold. Therefore they

should be considered an option with tradeoffs, not a panacea for all undercuts or zero-draft faces.

One more application is the production of decal recesses. These are shallow undercuts, but they can simplify the placement of decals and, if they fall in faces that are parallel to mold opening, are made possible using side actions. ProtoQuote® now points out areas that can be produced using side actions, giving users the option of redesigning their parts for standard straight-pull molds or using this more advanced capability.


Figure 3

Figure 4

©�007 Protomold. All rights reserved. Volume 4 n WHEn yoU REALLy nEED To DoDgE THE DRAFT n



13

Night of the living hinge

Take a look at a door and the hinges on which it swings. There are probably three or four hinges, each of which consists of three separate parts and four to six screws. Do the math, and you’ll see that the hinge you take for granted in your daily comings and goings consists of at least �1 separate components. Life would be so much simpler if the number of parts needed for a hinge could be reduced. The good news is that, at least in the design of plastic parts, it often can.

A reduction to three or two or just one part for a hinge would be notable. A reduction to zero is truly impressive, and that’s exactly how many additional parts a living hinge requires. Quite simply, a living hinge is a thin strip molded into a plastic part to create a line along which the part can bend. Properly designed and executed, it can be closed and opened over the life of the part with little or no loss of function. But simple though it may be in concept, a living hinge must follow certain guidelines if it is to work properly.

First, only certain resins are flexible enough to support the degree and frequency of bending required of a hinge. The best resins for parts with living hinges are polyethylene and polypropylene.

When a hinge bends, tensile forces are transmitted to the material along the outside of the bend. The thicker the hinge, the greater the stress in the outside surface, so the hinge should not be too thick or it may crack when it is bent. On the other hand, if the hinge is too thin it will not be strong enough to withstand any tearing forces, especially at the ends. The following geometry (from efunda.com) works well for hinges made of either of the two resins mentioned above.

Also, be careful that, when the hinge is bent fully, there won’t be interference from thick edges along the hinge.

Finally, a thin spot in a part (which is what a hinge is) can be challenging to fill during resin

injection. Success depends on proper gate placement. A single gate that forces resin through the hinge area in a mold increases the strength of the hinge; however, this approach can lead to sink in areas downstream from the hinge.

On the other hand, multiple gates may eliminate the problem of sink, but if resin flows meet at the hinge (which they will tend to do), they will usually cause cracking. When you order a mold, Protomold will propose gate location(s) to optimize filling of the part including any living hinges.

If this all seems like a lot of trouble, keep in mind that experts suggest that a well designed living hinge can be flexed millions of times. That’s more times than most of us will walk through doors in a lifetime.


©�007 Protomold. All rights reserved. Volume 4 n nigHT oF THE LiVing HingE n

Figure 1

Figure 2



14

There’s no shortage of ways to join plastic parts. There are bolts and screws, molded-in clips and snaps, and a variety of adhesives. But for simplicity and permanence, nothing beats sonic welding.

The ultrasonic weld joint is a method for joining two parts. They are designed with a small amount of extra plastic in the area you want to weld. The two parts are placed together and in contact with an ultrasonic generator which causes them to vibrate many thousands of times a second. Friction at the joint liquefies the extra plastic and small adjoining areas on both parts. As the

melted material on the mating parts cools, the two parts become essentially one. If you can live with its permanence, ultrasonic welding is the best of all possible solutions. It eliminates the loose parts and painstaking insertion of threaded connectors. It avoids the geometric complexity of molded-in plastic snaps, and it does away with the chemical problems and mess of adhesives.

The weld is ideal for permanently sealing maintenance-free devices like batteries or assembling a non elastic cup seal to a piston. It is a solution to encapsulating something within plastic when overmolding is not allowable.

Sonic welding is also useful for preventing tampering that could void a warranty.

Protomold does not actually perform the sonic welding of parts, but is often called upon to mold parts that will be joined using that process. Weld interfaces have different configurations from simple to complex. Below are three that work well within the Protomold process and ultrasonic welding in general.


©�007 Protomold. All rights reserved. Volume 4 n gooD ViBRATionS — ULTRASoniC WELDS n

good vibrations — ultrasonic welds

The Shear Joint is a very strong, self-aligning joint that is particularly useful for creating hermetic seals and right-angle joints. It is ideal for crystaline materials such as Nylon, PPS, and PPO, and can also be used with larger parts made of amorphous materials. Note that this joint can leave flash when parts are joined.

The Step Joint is a stronger, self-aligning joint that provides an excellent appearance. It is suitable for use with amorphous materials.

The Tongue and groove Joint eliminates flash caused by the welding process as the weld occurs between two walls and is an excellent choice for hermetic seals. Not recommended for thin walled parts.



15

With a few notable exceptions (H�O for one) most materials shrink as they cool and solidify. This is true, to a greater or lesser extent, of virtually all plastic resins. Uniform, predictable shrinkage would be easy to account for in making a mold; we could simply design the mold slightly larger than its desired size and the part would shrink to a perfect fit. In reality, however, shrinkage is rarely that simple.

Some resins shrink most in the direction of resin flow within the mold, while others shrink least in that direction. And, depending on the shape of the part, shrinkage of later-cooling areas of the part can pull against areas that have already solidified, causing sink or warping. To the extent that these problems can be anticipated, they can be minimized.

There are four primary factors that contribute to sink and warping:

Shrink characteristics of the resin — resins differ in both tendency to shrink and shrinkage relative to direction of resin flow. For example, an alloy of polycarbonate and ABS is very resistant to shrinkage, while glass filled nylon not only shrinks, but shrinks less in the direction of resin flow within the mold than perpendicular to resin flow.

Shape of the part — thick areas are particularly prone to sink when the surfaces closest to the mold solidify and are then pulled inward as the underlying resin cools and shrinks. Potential problem areas can be obvious, e.g., a thick wall. Or they can be subtle, e.g., a boss nestled in an inside corner.

Sudden transitions from thick areas to thin — these can result in stress and warping at the point of transition.

Poor placement of gates — pattern of resin flow can lead to warping of the finished part.

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Fortunately there are ways to address all of these issues and eliminate or reduce distortion of the finished part.

Know the shrink characteristics of your chosen resin These can be found at www.ides.com. As a very general rule, shrink under 0.010 inches/inch (or mm/mm) is more forgiving, higher shrink demands a well designed part. If the material is too shrink-prone for the application, consider another material.

Put thick parts on a diet Unnecessarily thick parts can sometimes be “put on a diet” to prevent sink. If the function of the part requires the larger shape, coring out the thick section can produce a hollow shape with thin walls, which will serve the same function (see Figure 1).

©�007 Protomold. All rights reserved. Volume 4 n RESiST THAT SinKing FEELing n

Resist that sinking feeling1

With a few notable exceptions (H2o for one) most materials shrink as they cool and solidify.

2

Figure 1

http://www.ides.com/


16©�007 Protomold. All rights reserved. Volume 4 n RESiST THAT SinKing FEELing n

Redesign and relocate If placement of a feature, such as a boss, results in excess localized thickness, consider redesigning or relocating the feature (see Figure �).

Ramp transitions and unsupported geometry Transitions from thick to thin can be ramped to reduce stress and eliminate warp. Whenever possible, gussets or some other 3D structure bracing such corners can prevent warping (see Figure 3).

Use Radiused Corners Un-radiused inside corners can overheat and stress the resin flow, causing distortion of the angles between walls. A radius in the corner is always good practice (see Figure 4).

Place gates strategically Gate placement can help control warp when using resins characterized by differential shrink in the direction of resin flow. The disk shown here resulted from a gate in the center of the disk. Placing gates at the edge of the disk reduced the warp (see Figure 5).


Figure 2

Figure 5

Figure 3

4

5

Figure 4

Depending on the shape of the part, shrinkage of later-cooling areas of the part can pull against areas that have already solidified, causing sink or warping.

6

3



17

As every engineer worth his or her sodium chloride knows, one of the most fundamental mechanical devices is the inclined plane, and a spirally-threaded cylinder or screw may be its most commonly used form.

If you are a plastic part designer, you’ve probably incorporated threads into a design or will in the future. We’d like to pass along some suggestions concerning the geometry of outside threads and the limitations of the rapid injection molding process to keep in mind when the time comes.

Most threads have undercut areas. It’s just a fact of the geometry as the surfaces of the screw wrap around, regardless of the

orientation of the screw body. There are several ways to deal with these undercuts.

The first method is driven by the primary rule of engineering: KISS: keep it super simple. Fortunately, for some threads, we can ignore the undercuts, machine what we can, and get a functional thread. For example, Figure 1 illustrates a thread design that cannot be machined exactly as it’s designed. The blue faces are assigned to the B-side of the mold, and the green faces are assigned to the A-side of the mold. The thread faces were split at vertical draft. Unfortunately, some faces overhang others (shown in red as undercut faces), creating a mold that (even if we could machine it) will interlock and can’t open.

Figure � shows the solution. We split the screw at a horizontal plane that passes through the axis. Faces above the plane are B-side, faces below are A-side regardless of whether they

have reverse draft (shown by the dark blue faces). When we design a mold for this thread and machine it, we will leave a little extra metal at the undercuts. When we mold parts, there will be a little plastic missing in these areas. The threads will be a little thinner than the CAD model in those areas, but in most cases you can’t tell the difference without a close examination.

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The inside scoop on outside threads

Figure 1

Most threads have undercut areas. It’s just a fact of the geometry …

Figure 2


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If the first approach doesn’t work, perhaps with an acme thread or on a large screw, a second method would be to modify your design to eliminate the undercut areas from your thread. We call this a “half-thread” design. It involves cutting the threads off the sides of your screw (see Figure 3).

The disadvantages of this are additional design, less thread strength and intermittent threads which might be difficult to screw in.

Lastly, if you need the full thread, the way to go might be to use cams. With a cam (side action) on each side of the part, the undercuts can be pulled and you get the full strength of the thread. Figures 4 and 5 illustrate this approach. Disadvantages of this method include four parting lines instead of two on your thread and the additional cost for the mold.

Can we do outside threads? You bet! We have a whole toolbox of methods for creating external threads on your plastic part. To learn more, submit a 3D CAD model for a quote or call Protomold at 763-479-3680.


Figure 4: Using side actions (cams) to produce undercuts

Figure 5: Faces assigned on thread using side actions

Figure 3: Half threaded part — no undercuts

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We’ve mentioned sliding shutoffs before, but they are both important enough and tricky enough to deserve closer attention. Done right, they can give you a lot of design flexibility; done wrong, they can easily destroy a mold. See Figure 1.

The bottom of the clip’s “hook” and the blue face of the clip’s shaft will be formed by an extension (shown by yellow lines) of the A-side mold half, which protrudes through a hole in the base of the part. The rest of the clip is formed by the B-side mold half. See Figure �.

In this �D diagram, red indicates sliding contact between metal surfaces from the two mold halves. (In the actual mold, there would be three flat faces of the extension from the A-side mold half making sliding contact with the

B-side mold half.) This is called a sliding shutoff, telescoping shutoff or a pass-through shutoff.

As you can imagine, if these surfaces are parallel to the direction of mold closing, they will rub against one another along their entire length as the mold closes. Since the fit of the two mold halves must be tight to prevent “flash,” there will be considerable friction and wear along these faces as the mold opens and closes, quickly ruining the mold. This causes flash on the plastic parts under the clip head, interfering with the operation of your clip.

The solution is to draft the faces by at least three degrees, so the faces approach one another as the mold closes but do not actually touch until the mold is fully closed. See Figure 3.

Sliding Shutoff Demo If you’d like to see sliding shutoffs, both poorly and well designed, in action, click here.


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Sliding shutoffs (again)Figure 1 shows the feature we are molding: a clip rising from a flat surface.

Figure 2 shows a section view of the feature in the closed mold.

Figure 3

Done right, they can give you a lot of design flexibility; done wrong, they can easily destroy a mold.


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Texture on a plastic part serves a variety of purposes from purely esthetic to purely practical. Whatever the goal, there are a few things to remember to ensure that you get the texture you want and that Protomold can effectively produce what you specify. On a surface lying perpendicular to the direction of mold opening, texture is relatively simple. (For features created by a side-action, the same is true for a surface perpendicular to the direction of side-action opening.)

Surfaces parallel to the direction of mold opening are more challenging. Consider what happens when you drag your knuckles across the surface of a brick. That’s pretty much what happens to a plastic part when its surface is dragged across the textured surface of an opening mold. The solution, of course, is to draft the surface so the mold surface moves away from the part surface as the mold opens. That’s true of any

surface parallel to mold opening, but even more critical as the degree of texture increases.

Here are some guidelines for drafting textured surfaces:

A 1-inch high rib with a smooth finish requires 1° of draft.

The same rib with a PM-T1 finish requires 3° of draft.

The same rib with a PM-T� finish requires 5° of draft.

These requirements can impact other aspects of your design as well. Take, for example, the “scoop” shown in Figures 1 and �. In Figure 1, the sides of the scoop are ribs formed in grooves cut into the B-side mold half. The two walls of the groove must be drafted in opposite directions to allow the part to be ejected. As a result, the side walls get thicker toward the back wall of the scoop.

In Figure �, the side walls of the same scoop are formed between a cavity in the A-side mold half and a core in the B-side. In this case, the two mold surfaces that form the side walls of the scoop are drafted in the same direction, resulting in side walls that are of even thickness from end to end.

Both versions of the part have the same side-wall draft, but the one shown in Figure � is the better design since it maintains even wall thickness.

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There are several other points to remember when designing textured surfaces:

Because texture is typically created by processes like bead blasting the mold faces, it may be impossible to texture ribs formed in deep, narrow grooves in a mold. This is one more reason to use the core/cavity approach rather than a deep-groove rib approach for forming walls.

Very thick walls may shrink significantly as they cool, pulling the surface away from the mold face before it has fully cooled, thus failing to properly texture the surface. If your part has mixed thick and thin areas there may be ugly variations in texture. This is a function of part design and can’t be processed out.

Very thin, textured walls may adhere too aggressively to the mold face and be damaged during ejection.

Note that the design guidelines pertaining to textured surfaces are similar to those for any part, except that, when you add texture, you must increase draft and pay more attention to wall thickness.


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When things get rough …

Figure 1

Figure 2


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