Print. Bind and save.giecdn.blob.core.windows.net/Fileuploads/File/Pm_designtipsvol1_v5-1.pdfDesign Tips for Rapid Injection Molding As illustrated in ﬁgure 1, draft is the angle

Design Tips

NOBODY’S FASTER IN THE SHORT RUN.®

for Rapid Injection MoldingVolume 1

The Protomold Company, Inc. 1757 Halgren Road, Maple Plain, MN 55359 (763) 479-3680

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Design Tips for Rapid Injection Molding

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n DESIGN MATRIX n 2

Design Tips categorized by topicPage Material

selectionDesign

guidelinesQuality

assuranceUnderstand the process

3 Draft your way to a better part ñ ñ4 Don’t cut corners! Foci on your radii. ñ ñ5 Start with the right finish! ñ ñ6 Thick section shrinking causes warping and sinking ñ ñ ñ7 The right resin is a material choice ñ ñ8 Expect to eject ñ ñ9 Be open to shut-offs ñ ñ

10 Get a feel for texture ñ ñ11 Living with hinges ñ ñ12 The hole picture ñ ñ13 Consider warp factors ñ ñ ñ ñ14 Know more knit lines ñ ñ ñ ñ

TABLE OF CONTENTS

External link to more information


As illustrated in figure 1, draft is the angle between the direction of ejection of a part from the mold and the surface of the part. Its traditional function is to facilitate the removal of the part from the mold, but in Protomold’s Rapid Injection Molding process, it also permits deeper geometries to be milled while reducing

cost and insuring trouble free molding. Drafts of 1.0 degree and larger may result in lower costs with the Rapid Injection Molding process.

It is important that textured surfaces have adequate draft to prevent the part from sticking in the mold and to prevent “drag” marks. Protomold requires that lightly textured

surfaces (T-1) have a minimum of 3 degrees of draft and that more heavily textured surfaces (T-2) have a minimum of 5 degrees of draft.

Another example of the importance of draft in the manufacturability of your design is illustrated in figure 2. In this case the draft not only helps the part eject from the mold more easily, but it also minimizes the amount of sliding required between the mold’s telescoping shutoff surfaces. A minimum of 3.0 degrees of draft is required for telescoping shutoff features.

Draft your way to a better part

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n DRAFT YOUR WAY TO A BETTER PART n 3

It is important that textured surfaces have adequate draft to prevent the part from sticking in the mold and to prevent “drag” marks.

Figure 1: Draft definition.

Undrafted Drafted

Figure 2: Draft can also improve mold life.

No draft results in sliding parallel mold surfaces

Draft results in improved mold shutoff surfaces

Top view Top view


Whenever possible, a part to be injection molded should be designed with generously radiused corners to enhance its quality and moldability.

As illustrated in figure 1, corners designed without radii can cause stress concentrations. These in turn may reduce the ability of the part to withstand load and/or cause warping in its geometry.

Figure 2 illustrates how sharp corners might adversely affect the flow of resin during molding, potentially causing incomplete fill. They also tend to cause the part to stick to the mold during ejection, which can cause a variety of problems.

And in figure 3 we show how the judicious use of fillets can also help to improve mold life by helping to minimize corner stresses at the bottom of tall, thin cores in the mold (at the entrance to deep thin holes in the plastic part). These fillets also help to enhance the ability of the mold to fill and further reduce internal part stresses.

See the Protomold Design Guide for more details.

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n DON’T CUT CORNERS! FOCI ON YOUR RADII. n 4

Don’t cut corners! Foci on your radii.

Figure 1: Corner radii enhance part quality.

Figure 2: Corner radii enhance moldability.

Figure 3: Corner radii can enhance mold life.


When designing a part for injection molding, it is important to keep in mind the relationships between surface finish, moldability, cost, and lead time.

Table 1 contains the list of standard surface finishes available through Protomold’s rapid injection molding service, listed in order from lowest to highest cost.

“PM” in the table signifies a surface finish adjusted to fit the rapid injection molding

process where SPI (The Society of the Plastics Industry) denotes an industry-standard finish.

The photographs shown in figure 1 illustrate the difference in cosmetic appearance for a few of these options on some example parts.

If the part will not be visible to the end user, you will probably choose to specify either PM-F0 or PM-F1 using the drop-down menus on your ProtoQuote (see sample). But many times your design will require a more cosmetic surface finish. In these cases there are two key things to keep in mind:

Polishing: Smoother part surfaces are achieved using manual mold polishing techniques. Consider a part with tall, thin, and curved ribs which needs to have an SPI-A2 finish. In this case you should expect a significant cost increase, because it is very time consuming to polish deep, narrow

slots in molds. And such lengthy polishing times may also affect the lead time for your parts, potentially making it impossible for Protomold to accept your order for our famous 3-day turn.

Texturing: Given the line-of-sight nature of bead blasting, it may not be possible to texture the sides of minimally drafted ribs on a part because the mold surfaces may be inaccessible. In addition, if the walls of your part are textured, it may have an adverse effect on the ability of the part to release from the mold, potentially resulting in unsightly “drag marks”. For these reasons we can only apply texture to areas of the part that are drafted at least 3 degrees for a T1, and 5 degrees for a T2 texture.

See the Protomold Design Guide for other helpful design information.

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n START WITH THE RIGHT FINISH! n 5

Start with the right finish!

Figure 1: Surface finish examples.

Table 1: Protomold’s standard surface finishes.

Higher Cost

SPI-A2: Grade #2 Diamond Buff, 1-2 Ra

PM-T2: Protomold texture, SPI-C1 followed by medium bead blast PM-T1: Protomold texture, SPI-C1 followed by light bead blast

SPI-B1: 600 grit paper, 2-3 Ra SPI-C1: 600 grit stone, 10-12 Ra

PM-F1: Low-cosmetic — most toolmarks removed PM-F0: Non-cosmetic — finish to Protomold discretion


As the plastic solidifies in the injection mold, it freezes from the outside (near the mold surface) toward the inside. In thick sections this results in inward pulling stresses (due to contraction) that can cause sink marks in the outer surfaces of the part (see figure 1).

In addition, because thinner sections will freeze faster than thicker sections, there is also the possibility of stresses building up between thick

and thin sections, resulting in part warpage. So in the design of parts to be injection molded, it is a good idea to maintain consistent wall thickness and avoid thick areas whenever possible.

Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above (see figure 2). The use of thinner, uniform wall

thicknesses helps to avoid sink (see figure 3).

Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp (see figure 4). The use of gussets can be helpful to provide support in corners to avoid warping.

See the Protomold Design Guide for other helpful design information.

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n THICK SECTION SHRINKING CAUSES WARPING AND SINKING n 6

Thick section shrinking causes warping and sinking

As Designed As Molded

Figure 1

Figure 2

Thick walls cause sink, warp & excess shrink.

Thinner walls give accurate parts

Figure 3

High stress concentrations

Reduced stress concentrations

Figure 4

Thinner wall results in warpage during cooling

Gussets provide additional support to reduce warpage

Bad Bosses

Good Bosses


Application-specific requirements will always drive the need for particular material properties like tensile strength, impact resistance, or ductility.

But successful designs for injection molded parts are also built on an understanding of process-related issues such as the ability to fill the mold, tendency to flash, ease of part ejection, and the potential for warp, sink, or void creation. As noted in prior Design Tips, part geometry can be used to help address some of these issues; but just as material properties are an important factor in meeting the requirements of a given application, they should also be considered to ensure the moldability of the part.

Table 1 lists some commonly used resins along with their brand names and a high-level summary of their material properties, moldability characteristics, and relative costs.

The complete list of resins is available on the Protomold Web site , and you can visit the Protomold Design Guide for other helpful design information.

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n THE RIGHT RESIN IS A MATERIAL CHOICE n 7

The right resin is a material choice

Table 1: Resin Selection Guide.

Mechanical Properties Moldability Characteristics

Some brand names Strength Impact

High temp-strength

Warp and dimensional

accuracy, molded

Fills small

featuresVoids

in thick Sink in thick Flash

High temp on mold & ejectors

Relative cost

Acetal Delrin, Celcon Medium Medium Medium-Low Fair Fair Poor Good Good Fair Medium

Nylon 6/6 Zytel Medium High Low Fair Excellent Good Fair Poor Fair Medium

Nylon 6/6, glass filled Zytel High Medium High Fair Good Good Fair Fair Medium

Polypropylene Maxxam, Profax Low High Low Fair Excellent Poor Poor Poor Good Low

High density Polyethylene (HDPE)

Dow HDPE, Chevron HDPE Low High Low Fair Excellent Poor Poor Good Low

Polycarbonate Lexan, Makrolon Medium High Medium-high Good Fair Fair to

good Fax Good Good Medium-high

Acrylonitrile Butadiene Styrene (ABS) Lustran, Cycolac Medium-

Low High Low Good Fair Good Fair Good Good Low

Polycarbonate/ABS Alloy Cycoloy, Bayblend Medium High Medium Good-excellent Fair Good Fair Good Good Medium

Plybutylene Terephthalate Valox, Crastin Medium High Low Fair Fair Fair Fair Good Medium-

high

Polystyrene Styron Medium-low Low Low Good Good Fair Fair Good Low

Thermoplastic Elastomer Isoplast, Santoprene Low High Low Poor Excellent Good Poor Excellent Low-

medium

Acrylic Plexiglass-Acrylite Medium Low Low Good Fair Good Good Good Medium


The Protomold Rapid Injection Molding process uses ejector pins of various sizes to push the plastic part out of the mold after it has solidified. The sizes and arrangement of these pins are selected to minimize the impact on your part design.

Figure 1 is an example of the illustration Protomold provides early in the process of designing the mold so that the location and size of both the gate(s) and ejector pins can be approved.

Sometimes it is appropriate to adjust the design of the part to accommodate the need for ejection pins to push the part out of the mold. For example, figure 2 illustrates how an injector pin “landing pad” has been integrated into the wall of a part design in order to provide sufficient material for the full diameter of the pin to meet the part. Landing pads may also be used to provide additional support for the ejection of thin curved walls, and in some cases the pins themselves can be contoured to fit the shape of the part.

Of course, always remember to provide as much draft as possible so that the ejector pins can do their job, especially for applications where it isn’t possible to use mold-release lubricants to help the part eject more easily.

Each situation is different, but a good understanding of the use of ejector pins is important when designing a part to be Rapid Injection Molded.

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

©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n EXPECT TO EJECT n 8

Expect to eject

Figure 1 : A typical Protomold-supplied gate and ejector pin layout submitted for customer approval.

Figure 2 : The wall of this part has been increased to support the full impact of the ejector pin without damaging the part.


This month’s Design Tip combines elements of the prior July and November tips to address the important relationship between the draft designed into the part, the resulting parting line of the mold, and the final quality of the parts.

As we reviewed in prior Design Tips, draft is a critical feature of almost all part designs to be injection molded because it helps the part eject from the mold as easily as possible. But when you are deciding exactly how to design the draft, it is helpful to understand how your decisions will effect the mold’s parting line geometry and shut-off surfaces.

Figure 1 illustrates how the decision to draft the walls of a simple part can have a major effect on how the mold will be designed. In “Design #1,” the walls of the part are drafted so that they can be ejected from within deep, thin mold cavities. The issue with this approach is that deep, narrow slots are a challenge to mill and polish, so you may not be able to achieve the desired geometry or final surface quality.

On the other hand, if the walls of the part are drafted as shown in part “Design #2,” the mold

becomes a core/cavity design that is much easier to mill and polish. And the end result is a part that can have significantly improved surface finishes as shown in the photographs in figure 2.


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n BE OPEN TO SHUT-OFFS n 9

Be open to shut-offs

Figure 1 : Draft design affects mold design.

Initial Undrafted

Design

Draft Design #1

Draft Design #2

Design Decision

Mold Design Approach #1: “Deep Ribs”

Mold Design Approach #2: “Core/Cavity”

Figure 2 : Mold design affects part quality.

Part from mold with deep, thin ribs.

Part from core/cavity mold.


As illustrated in the September 2003 Design Tip, Protomold offers the following two texture options:

PM-T1: SPI-C1 (600 grit stone, 10-12 Ra) followed by light bead blast

PM-T2: SPI-C1 followed by medium bead blast

If you plan to specify either of these textures via the drop-down menus of your ProtoQuote®, you need to be aware of the fact that Rapid Injection Molding requires a different draft angle on vertical faces for each of these texture options: 3 degrees for PM-T1 and 5 degrees for PM-T2.

And also keep in mind that due to the line of sight nature of the mold texturing process, it may not be possible to texture rib-shaped areas of the part design such as those illustrated in figure 1.

Another thing to remember is the effect part geometry may have on the quality of the desired texture, even if the mold itself is textured perfectly. For example, a wall with greater than nominal thickness will pull away from the textured mold surface during solidification, resulting in an untextured area on the surface of the part. And a wall with less than nominal thickness will tend to adhere more intimately to the textured surface, which can often result in a flat, chalky appearance on the part. Figure 2 illustrates examples.

So there are even more reasons to pay attention to the guidance in the October 2003 Design Tip about using consistent wall thicknesses.


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n GET A FEEL FOR TEXTURE n 10

Get a feel for texture

Figure 1 : Example “rib” geometries that cannot be textured.

Figure 2 : Examples of texture problems caused by variations in part wall thicknesses.

Sinkage Thickness

Back side geometry

issues

Front side texture

variations

Rib features poorly suited to texturing


At Protomold we don’t design parts - that’s your job. But if you happen to be unfamiliar with the technique called “living hinges”, this Design Tip may come in handy someday.

As described in detail by Dr. Glenn Beall in his August 2002 Injection Molding Magazine article, “By Design: Polypropylene part design, Part 2 — Living hinges”, in the late 1950s it was discovered that below a certain thickness, polypropylene molecules oriented in the direction of flow. And repeated bending perpendicular to that orientation was possible without breakage due to the increased strength that resulted.

The name “living hinge” was given to this technique and has been used ever since.

Living hinges are very useful in certain designs for injection molded parts because you can combine two or three parts into one. And as noted on efunda.com (an excellent online engineering fundamentals resource) in a page dedicated specifically to living hinges, a well designed hinge in these materials can last for millions of cycles. Additional materials with somewhat less of a life expectancy are Nylon and Acetal.

Check out both of these excellent articles for additional technical design details and illustrations.

See figure 3 for a recent example designed by our customers and manufactured via Rapid Injection Molding:


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n LIVING WITH HINGES n 11

Living with hinges

Figure 1A, 1B. Without a living hinge, this box would require two molds, two molding operations, and assembly.

Figure 3 : Example designed by our customers and manufactured via Rapid Injection Molding.

Figure 2 : Box with living hinge.


You may have heard the grade school riddle, “If you dig a four foot hole and fill in two feet of it, how much of a hole do you have left? Answer: All of it, because you can’t have half a hole.” Holes are just one of those things that are defined by the absence of something else.

When we want to incorporate holes into injection molded part designs, it can sometimes be complicated just getting them to happen in the right place with the right shape. One approach often used in moldmaking is to use “core pins” which essentially refers to the use of a cylindrical

piece of metal separately inserted into one side of the mold to provide the inner surface of the desired hole, especially if the hole is deep. In conventional injection molding these pins are typically made from hardened steel.

However, in rapid injection molding these features are milled directly into the mold geometry from a block of aluminum. And because aluminum has less mechanical strength than steel, it is desirable to take this into account when designing your part. Some general guidelines are provided in the following table and figure.

Through-hole Guidelines for Rapid Injection Molding:

• The diameter of the hole should be no smaller than .060” (1.5mm).

• The length of the hole should be less than eight times its diameter.

• The more draft on the hole - the better (minimum ½ degree in most cases).

• A fillet at the entrance to the hole greatly increases the strength of the core.

• A deep hole can sometimes be split and drafted to each side.

Figure 2 illustrates an instance of a through-hole design implemented using Protomold’s rapid injection molding process.


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n THE HOLE PICTURE n 12

The hole picture

Figure 2: Example of a part with through-hole features created with the standard rapid injection molding process.

Part Design Mold Core Design Mold with Core Pin Mold with Part

Draft

Fillet at entrance

Diameter


We have another riddle for you to introduce this Design Tip:

“When is circumference less than 2πr and when is it greater than 2πr?”

The answer is when you’re talking about a cone or a potato chip shape, respectively (see illustrations). What’s the point? Well, if your injection molded part is filled from the center and the resin shrinks less in the flow direction than in the transverse (as with any glass or carbon filled resin), the part will want to warp into the shape of a cone. But if the material has the opposite shrink characteristic, as with unfilled nylon, then it will tend to warp into a potato chip shape. Either way, you may not get the final geometry you’re expecting.

Which brings us to this month’s discussion about warp - something hard to predict with precision but usually manageable with the knowledge of a few fundamentals. We’ve covered part geometry-based techniques for managing warp in our October 2003 Design Tip, but two additional considerations are the characteristics of the resin and the nature of the gate(s) to be used.

As illustrated in the Protomold Design Guide resin properties table, the tendency for resins to warp varies significantly. Good or excellent dimensional behavior can be expected from a polycarbonate/ABS alloy (e.g. Cycoloy), but only fair or poor results should be expected from materials like thermoplastic elastomer (e.g. Isoplast) or glass filled nylon (e.g. Zytel). Of course, there are other considerations to take into account when selecting a resin, such as additional mechanical properties and cost.

In addition to looking at alternative materials, sometimes warp can be reduced by changing the nature and/or location of the gate(s). For example, in the case of the disk-shaped part illustrated above, rather than locating a single gate in the center of the part, it may be advantageous to have several equally spaced gates around the circumference of the part as illustrated in the figure to the right. Although this may result in knit lines where the resin flows meet, the multi-gate approach may cause the overall stresses to balance and help to avoid the cone or potato chip effect.

Very seldom can you get everything you want in one part, which is what engineering trade-offs are all about. But as a well-informed designer, you can make a big difference in our ability to make your parts, and more importantly, the success of your project.


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n CONSIDER WARP FACTORS n 13

Consider warp factors

Figure 1

Figure 2

Gate

GateGate

Cone Shape

Potato Chip Shape


Know about knit lines? If knot — we hope this Design Tip will be useful for you.

A “knit line” in a plastic injection molded part (see figure) is created when two separate plastic flows meet within the mold and resolidify along their interface. Depending on the resin, resin temperature, mold temperature and filling speed, knit lines can vary from virtually invisible to something that looks like cracks in the plastic. And in some cases (e.g. long thin features with resins like LCP) the knit lines can have reduced mechanical properties and be a cause of part breakage. So for reasons ranging from cosmetics to functionality, it is important to know as

much as possible about why they happen and how to reduce their impact on your design.

As noted above, the size and shape of the knit line is affected by the molding parameters, but its location will be primarily governed by the geometry of the part. The primary cause of knit lines is the way the plastic flow rejoins after it goes around a metal core in the mold. So for this reason there is a knit line (visible or not) downstream from every hole that goes through your part. And for similar reasons there is a knit line between every two gates on the part.

Protomold mold technicians try to minimize the appearance of any knit lines, but they must balance this with other challenges like avoiding sink or blush, achieving the desired surface texture, etc. So anything you can do to help avoid knit lines when designing the part would be a benefit.

Here are a few things to consider. First, thicker walls will slow down the cooling rate of the resin and thereby help to improve the appearance and strength of any knit lines. Second, the resin you select may make a difference. For example,

unfilled materials will tend to have stronger knit lines than filled materials. In fact, knit line strength will decrease with higher filler content as well as with longer fibers. In addition, materials that that tend to outgas a lot (e.g. Santoprene) or contain additives like flame retardants, lubricants, and mold releases can further exacerbate the problem. Third, it may be possible to improve the situation by working with Protomold to optimally place the gate(s) so that the knit lines are minimized or moved to a less critical area.


©2007 The Protomold Company, Inc. All rights reserved. Volume 1 n KNOW MORE KNIT LINES n 14

Know more knit lines

Figure 1: Example of a knit line occurring on the backside of a hole.

Documents

Print. Bind and save.giecdn.blob.core.windows.net/Fileuploads/File/Pm_designtipsvol1_v5-1.pdfDesign Tips for Rapid Injection Molding As illustrated in ﬁgure 1, draft is the angle