Protomold PL DT Comps Vol 5

Design Tips for Rapid Injection Molding Volume 5

Real Parts. Really Fast. Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877-479-3680

WWW.PROTOMOLD.COM/PARTS

Design Tips for Rapid Injection Molding

Design Tips categorized by topic Page

TABLE OF CONTENTS

3

Hot tip solves gating problems

5

Playing it safe

6

The plane truth about rotational draft

7

Eliminating side-actions for fun and profit

8

Protomold: it isn’t just for prototypes anymore

9

Design guidelines for bigger parts

10

Put your parts on a diet

11

Avoid knitting around your boss

13

You’ve got to check this!

16

Don’t be square!

17

Cams: they’re not just for undercuts

19

Look sharp

©2009 Proto Labs, Inc. All rights reserved.

Material selection

• •

Design guidelines

• • • • • • • • • • • •

Quality assurance

•

• •

Understand the process

• • • • • • • • •

Volume 5

DESIGN MATRIX

2

Design Tips for Rapid Injection Molding

Hot tip solves gating problems Consider how much simpler life would be if we could simply teleport liquid resin into a mold. Of course, that wouldn’t be injection molding. And since we presume that the developers of practical teleportation will come to us for their plastic prototypes (and they haven’t), we’re prepared to state categorically that teleportation molding is still some time off. That leaves us with gates as a means of getting liquid resin from the barrel and screw of the molding press into the mold. Unfortunately, gates interrupt the mold’s surface, and that interruption, unavoidably, produces a cosmetic defect on the surface of the part. Depending on the function of the part and the location of the gate, this may or may not be a problem. For example, if the part or surface will be hidden from view, any vestiges left by the gate probably won’t be a problem. But if the gate is located on a visible surface, you’ll want to consider its cosmetic impact in designing the part. Tab gates are effective and most common, but not necessarily pretty. (See Figure 1.)

Fig. 1: Tab Gate n

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©2009 Proto Labs, Inc. All rights reserved.

While they are a simple way of getting resin into the mold cavity, tab gates have several drawbacks. First, they carry resin to the mold via a runner. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. This, in turn, leaves a large tab to be trimmed, a process that can mar the finished surface. Second, the runner leading to a tab gate takes up real estate within the allowable mold footprint. This can be a problem if the part pushes the limits of allowable mold size. Third, because resin cools somewhat on its way to the gate, there are potential mold fill considerations like uniformity, concentricity, knit line formation, thin feature challenges, etc. Finally, tab gates must be located at the parting line of the mold.

In many cases, the solution may be a “hot tip” gate. A hot tip gate has a small circular gate opening in the “A” side of the mold that lets plastic into the cavity. It’s called a hot tip gate because there is a thermostatically-controlled heater bolted to the back of the mold to keep the resin hot enough (and thus fluid enough) to pass through the small gate hole. The hot tip can be thought of as a direct extension of the molding press’s barrel and screw. Resin is hotter at the point of injection, so the opening can be smaller. Because no runner is required, the part can use virtually all of the allowable mold X-Y real estate. Hotter resin also means material may be pushed further into a thin feature. Hot tip gates are typically located at the top center of a part (as opposed to on the parting line, as is the case with a tab gate) and are ideal for round or conical shapes where uniform flow can improve concentricity. The hot tip gate leaves a small raised nub on the surface of the part. Adding a hot tip dimple to your design may help shift the nub below the surface of the part, which might allow something like a decal to be applied over it with little or no need for trimming beforehand. Continued on next page…

Volume 5

HOT TIP SOLVES GATING PROBLEMS

3

Design Tips for Rapid Injection Molding

Hot tip dimples are usually 0.125 to 0.375 inches in diameter and 0.010 to 0.030 inches in depth. This can take the shape of a spherical or cylindrical depression. (See Figures 2 and 3). To maintain the always desirable uniform wall thickness, you could add material to the opposite side of the part so the material is not restricted in flow.

Also be aware that some materials such as acetal and glass filled resins are not compatible with hot tip gates, and that small volume parts may be problematic because of the tendency of resin to “cook” in the hot tip longer, potentially degrading its properties. When asking for a hot tip gate, you should also consider the extra expense of installing a hot tip gate, such as the features that need to be machined into the back of the “A” side mold half. In addition, once a hot tip gate is used, it is much more costly to modify the mold. For example, there is no simple way to move a hot tip without completely re-making the “A” side of a mold. Fig. 3: Cylindrical Depression Hot Tip Dimple

Fig. 2: Spherical Depression Hot Tip Dimple

©2009 Proto Labs, Inc. All rights reserved.

Be sure to consider that the “A” side of a part is usually the externally-facing cosmetic side, which means if you use a hot tip, the gate vestige (“nub”) is likely to be visible in an assembly. As noted above, a hot tip dimple is a common method of trying to help hide the nub and should be considered when selecting decal locations on high cosmetic parts.

As with any plastic part, design and resin both have a great impact on the success of your molded part. A well-designed part with a carefully chosen gate type and location that are compatible with the resin you select will allow us to achieve the best possible results.

Volume 5

HOT TIP SOLVES GATING PROBLEMS

4

Design Tips for Rapid Injection Molding

Playing it safe The whole purpose of prototyping is to allow yourself the option of tweaking your model— thickening a wall, adding a rib, placing text or a logo—before locking in a final design for production. For injection molded plastic prototypes, tweaks can entail the creation of a whole new mold. A new mold may be quite affordable if your prototypes are being made by Protomold, but if the change can be made by modifying the existing mold your cost will be lower still. There’s one critical fact to remember if you want to be able to modify your part by modifying the original mold. It is relatively easy to remove metal from an existing metal mold. Adding metal, on the other hand, can be difficult or, for all practical purposes, impossible with rapid injection molding. To look at this from the part perspective, you can add plastic, but you can’t take it away. Designing with this in mind is called “steel safe” or “metal safe,” and doing so can save you both money and time when you have to modify your design. For example:

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You may be able to thicken a wall, but making it thinner requires remaking the mold.

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You can add features—bosses, raised text, ribs, pins etc.—but you can’t remove them.

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You can reduce the diameter of a hole by adding plastic around the perimeter, but you can’t increase the hole size.

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Similarly, you can eliminate holes, but you can’t add them.

The rule to remember when initially designing your part is: “Maximize metal and minimize plastic.” Figure 1, for example includes a post (properly drafted, of course) rising from a base. Figure 2 shows the same part after the designer has decided that the post needs to be thicker. (Added plastic is shown in red.) This change was easy to execute. If the change had gone in the other direction, however, it would have required the milling of a new mold.

Fig. 1

©2009 Proto Labs, Inc. All rights reserved.

If you aren’t entirely sure whether a feature is needed or whether a feature is the right size, you might want to review your part feature by feature, asking yourself which ones may need to be changed in later iterations before committing your design for prototyping; then start with the “less plastic” option. Not sure how thick a wall should be? Start thin and thicken it later. (ProtoQuote® will warn you if your wall is too thin for effective mold filling.) Unsure whether you’ll need a rib to strengthen your part or a brace to prevent warp? Leave it off and add it later if it’s needed. Have two mating parts that might (or might not) need alignment pins? Put the holes in and leave the pins off. If you need them, you can add the pins in the next iteration. If you don’t, you can eliminate the holes. One final thing to keep in mind when you plan for changes: at Protomold, we can make parts to tolerances of ±.003” + the shrink tolerance of the resin.

Fig. 2

Volume 5

PLAYING IT SAFE

5

Design Tips for Rapid Injection Molding

The plane truth about rotational draft Let’s say you’re designing a plastic part with rotational symmetry; for simplicity, we’ll make it a dowel. In your CAD software you would create the shape of half the cross section—in this case, a rectangle (see Figure 1)—and rotate that shape through 360° to create the solid. So far, so good! However, knowing that your part is going to be injection molded and that the parting line of the mold will run along the length of your dowel, you realize that, unless you do something, the end faces of the dowel will be parallel to the direction of mold opening. In other words, those ends need to be drafted. There are two ways to draft those ends, one of which works better than the other.

The problematic method is also the most obvious: when laying out the cross section, you tilt the ends slightly (See Figure 2). This way, when you rotate the shape it makes the ends of the finished design shallow cones instead of flat disks (See Figures 3a and 3b). This is “rotational drafting”, and it combines the drafting step with the rotation that creates the 3D shape.

Sometimes a flat world is just a lot easier to navigate.

Fig. 3a: Part Design Resulting from Rotational Draft Method

Fig. 1: Undrafted Dowel Half Cross Section

Why is this an important topic for rapid injection molding? Protomold’s 3-axis milling process plunges in the z-axis only. This makes a planar draft a simpler, more reliable cut than a rotational draft, due to the latter’s varying cut angle. For this reason, the ProtoQuote design analysis will show these rotational draft issues as required changes and ask for increased draft, wall thickness or both. The easiest fix is to replace the rotational draft with a planar draft.

Fig. 3b: Part Design Resulting from Rotational Draft Method

The preferred method is “planar drafting” and it is a separate step from the rotation that creates the basic part. In this case, each half of the end surface is drafted separately in a plane angled away from the parting line (See Figures 4a and 4b). The key difference between these two approaches is what happens as the drafted surface approaches the parting line.

Fig. 4a: Part Design Resulting from Planar Draft Method

Fig. 4b: Part Design Resulting from Planar Draft Method

Fig. 2: Pre-drafted Dowel Half Cross Section

©2009 Proto Labs, Inc. All rights reserved.

Volume 5

THE PLANE TRUTH ABOUT ROTATIONAL DRAFT

6

Design Tips for Rapid Injection Molding

Eliminating side-actions for fun and profit Quick, how would you make the part shown in Figure 1? Well, since by now you’re aware of the fact that Protomold supports up to four side actions per mold, the obvious answer is to use a mold with a side-action to create the bottomless box with the window. After all, without a side-action, a mold feature that protrudes inward from the A-Side of the mold (or outward from the B-Side) would be entrapped in the window when the mold opened, right?

works for one simple reason: the side walls are drafted. That means that the mold surface will begin to pull away from the part surface as the mold begins to open. So if your design can support the draft and you’d like to save a little money and skip the side action altogether, it’s worth taking note. Take a look at Figure 2 to see how this can be applied.

Fig. 2

Figure 2 shows a cross section of the window area in the closed mold in which: Fig. 1

Wrong! This part can be made in a two-part straight pull mold without side actions. It

©2009 Proto Labs, Inc. All rights reserved.

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The gray areas are the mold walls

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The window is being formed partially by an extension of the mold’s A-Side and partially by an extension of the B-Side Volume 5

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The dotted line represents the plane of the outside surface of the wall

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The red line is the shutoff where the metal faces of the two mold halves meet when the mold is closed

Note that the shutoff runs at an angle across the window, dividing the parts of the window that will be produced by the A and B mold halves. Because the wall is drafted, the two mold halves move away from the part (and from one another at the shutoff) as the mold opens and the part is ejected. No part of either mold half is entrapped in the window; hence, no side action is required. There is one important consideration when designing a part using this technique: the draft angle of the shutoff. To avoid damage to the mold the shutoff must be drafted a minimum of 3°. Because the shutoff is angled slightly relative to the wall itself, the draft of the wall must be greater than 3° to allow a 3° draft of the shutoff. The required amount of wall draft will vary directly with wall thickness and inversely with window height (“shutoff height,” as shown in Figure 2). Most CAD programs can help determine the proper degree of wall draft to create a minimum of 3° draft at the shutoff.

ELIMINATING SIDE-ACTIONS FOR FUN AND PROFIT

7

Design Tips for Rapid Injection Molding

Protomold: it isn’t just for prototypes anymore Protomold has long been the place to go for fast, affordable, injection molded prototypes. But for an increasing number of customers, Protomold is also a source of production parts in quantities of up to 25,000. Depending on the number of parts you need, Protomold can be significantly less expensive than a traditional molder using steel tooling. And regardless of quantity, you still get Protomold’s speedy turnaround for unmatched speed to market. There are several ways that Protomold can help control cost and slash production time on large orders. Multi-cavity molds, which produce up to eight copies of a part in each press cycle, increase tooling cost somewhat, but can more than offset that increase by reducing press cycle time and cost. The ideal number of parts per mold depends on various factors. Obviously, part size can be a limiting factor, but even for small parts the ideal number of cavities per mold can vary. For example, a customer needing 24,000 identical small parts might maximize savings by using an 8-cavity mold. For a customer needing fewer parts, say 2,000, the lower tooling cost of a 4-cavity mold might be the better choice. “What-if” scenarios using the “cavities” option of the ProtoQuote® (see fig. 1) or Protomold’s sales or customer representatives can help you choose the most cost-effective solution.

©2009 Proto Labs, Inc. All rights reserved.

In addition, until the part has been thoroughly prototyped and tested, you should probably stick to single cavity molds. Once your design has been proven, you may elect to convert to a multi-cavity mold for production.

Fig. 1: ProtoQuote® Cavity Specification

There are a couple of limitations to keep in mind as you consider the use of multi-cavity molds: 1. Side actions: Because they tend to use so much of the mold’s maximum “footprint”, multi-cavity molds don’t leave room for side action cams and must be simple straight-pull molds. 2. Hot tips: Due to mold complexity issues, multi-cavity molds cannot use hot tip gates.

Volume 5

Fig. 2: Multi-Cavity Mold

If you have questions regarding the use of multi-cavity molds for production parts, check with your Protomold sales representative, who will be happy to help you find the best and most cost-effective solution.

PROTOMOLD: IT ISN’T JUST FOR PROTOTYPES ANYMORE

8

Design Tips for Rapid Injection Molding

Design guidelines for bigger parts With Protomold’s expanded capability to produce larger parts, there are a few important considerations to keep in mind when designing parts to fit the Protomold process.

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For starters, our current increased size capabilities are as follows:

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Side action cams consume space, reducing maximum mold size.

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To see how these limitations specifically affect your part, upload a 3D CAD model for a free ProtoQuote® moldability analysis and quote.

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Maximum part outline is approximately 8.9 inches by 29.6 inches (the maximum outline applies only to shallower parts).

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Increasing part depth reduces maximum outline. At 1-inch depth, maximum outline is 7.6 x 16.9; at 2-inch depth maximum outline is 5.6 x 14.9; at 3-inch depth, maximum outline is 23.6 x 12.9. Why is this? It’s because plastic is injected at pressures as high as 10,000 PSI, and we need enough mold material surrounding a deeper part to keep the mold sides from bending out.

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Outline notwithstanding, maximum projected mold area is 175 square inches.

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Maximum part volume is approximately 59 cubic inches.

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Maximum mold depth either side of the parting line is 3 inches, which means a part can be up to 6 inches tall if the parting line passes through the middle of the part (inside and out).

©2009 Proto Labs, Inc. All rights reserved.

Parts should have approximately 1° of draft per inch of depth from parting line. Minimum draft, regardless of depth, is 1/2°. This allows the part to eject from the mold without drag marks.

Recommended wall thickness varies by resin, but larger parts should have thicker walls. Large parts should have walls at or near the manufacturer’s maximum recommended thickness for the specific resin. See the recommended wall thickness by resin type chart at protomold.com/ designguidelines.

3. Avoid unsupported geometry (see fig. 2). 4. Use core-cavity construction rather than ribs wherever possible (see fig. 3). For detailed guidelines, examples, and illustrations, see Protomold’s Design Guidelines.

Fig. 1: Radius Corner

ProtoQuote can provide feedback on the proposed wall thickness of your parts. Other guidelines are similar to those for smaller parts: Fig. 2: Unsupported Geometry

1. Try not to mix thick and thin sections; keep wall thickness as constant as possible. 2. Radius corners to minimize flow restrictions and prevent stress concentrations (see fig. 1).

Fig. 3: Rib vs. Core-Cavity Volume 5

DESIGN GUIDELINES FOR BIGGER PARTS

9

Design Tips for Rapid Injection Molding

Put your parts on a diet They say “Less is More,” and while that may be a dubious premise in some areas — your next salary review, for example — it is often true of molded plastic parts. We’ve written in prior Design Tips about the consequences of overly thick features. As liquid resin cools, it can lead to all sorts of problems — sink at the part surface, hidden voids within the feature, or warp as areas of different thickness solidify and shrink at different rates, resulting in undesirable bending. In addition, unnecessary thickness can throw off part dimensions, reduce strength and necessitate post-process machining. In the past, we’ve suggested that large features can be “cored,” hollowed out from the bottom, leaving the original surface shape while reducing wall thickness. Think of a solid square cube being turned into a hollow box. If the part doesn’t have a “back side” from which coring can be done, there’s always the option of splitting the part in two, molding two hollow halves and then joining them; but that can be complicated and expensive. What we’re suggesting here is a form of coring done from the outside. As materials get stronger, you can see this technique being used in many places. The holes in cinder blocks reduce their weight and the cost of concrete without significantly reducing their effective strength. Sporting equipment made of high-tech materials — knives and bicycle parts for example — are increasingly sporting ©2009 Proto Labs, Inc. All rights reserved.

cutouts of various shapes and sizes that leave the part strong enough for its intended use but far lighter than a solid part would be. A common term for this is “skeletonizing.” Our example from the plastics world is a screwdriver handle. The shape shown in Figure 1 is designed to fit the human hand, but in doing so, utilizes far more resin than is necessary to transmit the force exerted by the hand to the shaft of the screwdriver. Coring out the handle in the traditional sense, besides complicating manufacturing, would defeat the handle’s purpose by eliminating material along its central axis, where it connects to the metal shaft.

producing a design that can still be made in a two-part mold. The overly thick shape has been converted in two ways. The first is a series of relatively thin walls, creating a rib cage structure. The second is to core out the mass with a number of pockets. Both examples retain the shape of the handle while reducing excess material and the potential for injection molding process issues. We may also improve the user’s grip through the addition of the ridges.

Fig. 2: Externally cored screwdriver handle

Fig. 1: Solid screwdriver handle

A more practical solution is “external coring” as shown in Figure 2. Here we significantly reduce the amount of material required, while

This kind of external coring can be easily accomplished in most 3D CAD programs. Of course, the surfaces of the individual walls now have to be drafted in the direction of mold opening to facilitate ejection, but this is also easy using 3D CAD. And, last, but not least, your part now has a cool, modern look.

Volume 5

PUT YOUR PARTS ON A DIET 10

Design Tips for Rapid Injection Molding

Avoid knitting around your boss Start with a simple fact: resin cools as it is injected into a mold. This is why the leading edge of the resin flow within a mold is always the coolest area of the resin, and, therefore, the closest to solidifying. In a well-designed mold, this is generally not a problem. The exception may occur when the resin flow is divided by an obstacle and then meets again on the other side of the obstruction, for example, the core that creates a rectangular hole in a cover plate (see fig. 1). When this happens, you have two surfaces meeting downstream from the obstruction. Ideally, they will meld together to form a solid joint, but if they have cooled too much to meld completely, the result is a knit line.

A knit line is any line, visible or not, where two resin flows meet (see fig. 2). Depending on the design of the mold and the material being injected, a knit line may present no problem at all, may be a cosmetic issue, or can cause a potentially serious structural problem. One of the deciding factors is the resin being injected since resins vary in their tendency to form knit lines. Among the most likely to show lines is ABS. In many cases, a knit line in ABS is solid enough that it will not significantly weaken the part, but it may appear to be a crack in the finished part.

One area in which knit lines can cause structural problems is behind a boss. A boss, of course, is a feature with a hole designed to accommodate a threaded fastener. (Protomold doesn’t create internal threads; those would be cut by a self-threading screw, machined in a separate operation, or added as an insert.) The boss is created by a raised core pin inside the mold around which resin flows. When the resin faces meet on the back side of the pin, they form a knit line. Two factors can makes this particularly problematic. If the boss is near the edge of the part, the knit line will be very short, leaving relatively little surface holding the two faces together. When you add the “wedge” effect of a screw being driven into the boss, a knit line can turn into a crack. Knit lines will also occur between gates in a part. Gates are the areas where resin is injected into your part. When you get your gate and ejector layout, check it. We don’t often use multiple gates, but if we do, check to see if you have any critical cosmetic or strength requirements about halfway between every two gates. Continued on next page…

Fig. 1

©2009 Proto Labs, Inc. All rights reserved.

Fig. 2

Volume 5

AVOID KNITTING AROUND YOUR BOSS

11

Design Tips for Rapid Injection Molding

There is one more factor that can contribute to problems with knit lines, and that is the use of filled resins. Picture the flow of a liquid resin filled with, for example, glass fiber. Obviously, as the resin front moves through the mold, the fill material will always be behind the front. So when two fronts meet and solidify, there is little or no fiber crossing the meeting line. This doesn’t necessarily mean that the knit line will be weak, but it will not have the benefit of fiber reinforcement.

©2009 Proto Labs, Inc. All rights reserved.

What can you do to prevent problematic knit lines? You probably can’t eliminate features like bosses, but you can choose resins that are less susceptible to knit line formation. Specifically, you can avoid filled resins in parts that will have features like through-holes. You can thicken part walls to slow resin cooling, being careful not to thicken them enough to cause sink. And you can place knit-linecausing features farther from the edges of parts when the design allows.

Of course, finding knit lines in your prototypes is better than finding them in your production parts, and that’s what prototyping is for. If you have critical requirements for strength of knit lines, please call a customer service engineer at (877) 479-3680 to discuss.

Volume 5

AVOID KNITTING AROUND YOUR BOSS 12

Design Tips for Rapid Injection Molding

You’ve got to check this! “Flaps 10 degrees………check, cabin air intake………check, landing light………check.” Ever dream about being a pilot running through your preflight checklist with your copilot? Well, now as a designer, you can have almost as much fun when you design plastic parts using the Protomold Plastic Part Design Checklist. You too can be confident you won’t miss a single aspect of parts design when you use this handy resource tool. Like a pilot’s preflight routine, our checklist below is a handy reminder of some of the considerations to keep in mind when you’re designing injection molded plastic parts. And, in addition to our checklist, we’ve catalogued dozens of tips that can aid in making your parts easier to manufacture, lighter, stronger and offer improved performance. Each tip is chock full of great manufacturability pointers, including coring out, preventing sink, drafting for easy ejection, adding text, and more. Some of these useful design tips are referenced in the designer checklist; or you can download our complete Design Tip Compilation Volumes at protomold.com/designtips.

Also, you can access ProtoQuote® automated design analyses, which can help to identify potential design problems once a 3D CAD model has been submitted. These free services allow you to quickly see issues that are easy to overlook. Our suggestions and recommended design modifications are returned to you within one day of submitting your part. Try our Checklist! It’s a practical tool that allows you, the designer, to spot potential issues and correct them beforehand.

PLASTIC PART DESIGN CHECKLIST: Designing with Protomold in mind

Design Considerations Are the cores and holes drafted toward the ejector pin side (low cosmetic side) of my part?

YES

Does my part design include sufficient draft for part removal from mold? YES — We strongly advise using at least 0.5 degrees on all “vertical” faces. Do I need to add draft to my design? MAYBE — For aluminum molds, increments of .5 degrees (shallow features) to 3 degrees (deep features) should be added. A good rule of thumb is 1 degree per inch depth including cam/side pull cores.

Continued on next page…

©2009 Proto Labs, Inc. All rights reserved.

Volume 5

YOU’VE GOT TO CHECK THIS 13

Design Tips for Rapid Injection Molding

Does my part have a wall thickness that is greater than .040” and under .180”?

YES

NO — Parts under .040” may produce a part that has shorts, voids, weak knit lines; parts over .180” may have excessive sink, internal voids, warp, poor texture pick-up.

Dimensions

Volume

Does my part have two dimensions that are larger than .25” and less than one of the following? (Dimensions (in mold) = X x Y x Z (depth of part)

Does my part have a volume greater than .005 in3?

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Selection of the proper material is crucial to part production. Designers should consider the mechanical characteristics, molding properties, and cost of the resin used.

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Uniform wall thickness for all resins is the best place to start.

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Glass filled resins are more likely to warp.

YES — proceed

30.5” X 13.5” X .25” 28.5” X 11.5” X 1” 26.5” X 9.5” X 2” 24.5” X 7.5” X 3”

NO — If no, consider the possibility of redesigning the part so Protomold may be able to mold your part. Let us know if we can assist with this.



YES — proceed to Volume

Does my part have a volume less than 59 in3?



NO — My part is over 3”.



* * * *

Materials What do I need to consider when selecting a material that is best for my design?



My part is over 3” deep, is my parting line in the middle of the part?

YES

NO — If no, consider the possibility of redesigning the part so Protomold may be able to mold your part. Let us know if we can assist with this.

YES — proceed

NO — If no, consider the possibility of redesigning the part so Protomold may be able to mold your part. Let us know if we can assist with this.

Surface Area Does my part have a surface area of less than 175 square inches?

YES — proceed

NO — If no, consider the possibility of redesigning the part so Protomold may be able to mold your part. Let us know if we can assist with this. Continued on next page…

©2009 Proto Labs, Inc. All rights reserved.

Volume 5

YOU’VE GOT TO CHECK THIS 14

Design Tips for Rapid Injection Molding

Geometry Is it a straight pull mold?

Is the undercut on my part less than one 8.419” x 2.377” x 2.900”? (Dimensions (in mold) = Horizontal x Vertical x Depth of cam pull).

YES — proceed

NO — Off angle geometry may need to be modified or cams/side actions need to be applied

YES — Protomold will use cam/side action to mold the feature

NO — Can you use sliding shut offs?

I DON’T KNOW — Call Protomold Customer Service Engineers (CSE’s) at 877-479-3680.

Undercuts Are the undercuts located toward the outside of my part? YES — Proceed to next question. Protomold may be able to use cams/side actions to mold the undercut. NO — Pass-through cores or filling in the undercut geometry will be required. Protomold can certainly mold your prototype parts, however may not be able to mold your production parts. You could cut the undercut geometry with a secondary operation for testing or low volume production.

©2009 Proto Labs, Inc. All rights reserved.

Volume 5

YOU’VE GOT TO CHECK THIS 15

Design Tips for Rapid Injection Molding

Don’t be square! We’ve talked in the past about corners and reasons to make them rounded (radiused) instead of sharp. Let’s talk about this in a little more detail and help distinguish between inside and outside corners, both of which should be radiused but for slightly different reasons.

Example of Sharp Corners vs. Radiused Corner

If you look at Figure 1, you see the parting line where A- and B-Side mold halves meet to form the sharp edge of the part. In machining molds, there is always some tolerance, but slight movement of the parting line to the left or right will not change the geometry of the edge.

Outside corners The first thing to keep in mind is that an outside corner of your part is created by an inside corner of a mold, and vice versa. One reason we don’t make parts with sharp outside corners is because our molds are made by a vertical milling process that cannot cut a sharp inside corner. The radius of our inside corner (your outside corner) cannot be smaller than the radius of the cutter, which will vary somewhat with the depth of the cut.

Inside corners Our milling process can produce sharp outside corners when making a mold, so you can have sharp inside corners. The problem is that sharp inside corners can create serious stresses in a part as it cools. The reason is simple. The rate of resin cooling is proportional to surface area. Any corner will have more surface area on the outside of the curve than on the inside. (Think about the advantage of the proverbial “inside track.”) On a radiused corner, there is always a difference between the two surface areas, but if the inside of the corner is square, it essentially has a surface area of zero, which maximizes the difference between the inside and outside surface areas. ©2009 Proto Labs, Inc. All rights reserved.

If the part consisted of two walls meeting to form an “L” shape, the part may tend to warp as it cools, reducing the angle between the two walls. If, however, the corner is in, say, a box whose shape keeps the walls from moving in relation to one another, instead of warping, they’d merely become stressed. The result could be cosmetic problems; a fracture or buckled floor. In addition, because they are sharp, the outside corners of a mold half can “grab” the part within which they are forming a core, either making ejection difficult or risking damage to the part or mold. And finally, sharp corners can contribute to sink and weakened knit lines. So radius those corners! Now that we’ve hopefully convinced you to radius corners, let us describe one situation where you should not radius a corner: at the parting line. See next page…

Fig. 1

In Figure 2, on the other hand, both the mold halves form the parting line edge, so any mismatch in the mold will leave a ledge, changing the shape of the part at the parting line. That’s one reason we recommend leaving the parting line sharp.

Fig. 2 Volume 5

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Design Tips for Rapid Injection Molding

Cams: they’re not just for undercuts By moving in directions perpendicular to the direction of mold opening, side action cams allow the production of parts with “undercuts” that could not be successfully made in two-part, straight-pull molds. But there are parts without undercuts that can also benefit from the use of cams. The Protomold process (vertical milling) requires increased wall thickness and draft as the part depth increases. Using cams, we can reduce the need for draft and wall thickness in some instances. Imagine, for example, a thimble, essentially a cup with tapered sides (see figure 1). The obvious way to mold such a part would be in a two-part mold in which the outside is formed by the A-Side mold half and the inside core is formed by the B-Side mold half (see figure 2). Resin would be injected through a tab gate placed along the parting line at the rim of the cup. If the walls of the part were thick enough, this might work. In a thin-walled design, however, there could be problems.

Fig. 1

Fig. 2

Continued on next page…

©2009 Proto Labs, Inc. All rights reserved.

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Design Tips for Rapid Injection Molding

So how do you identify parts that can benefit from the use of cams? Tall, thin parts with a core, like our thimble, are prime candidates. Another candidate might be a part that, in a straight-pull mold, would require more draft than the designer is able to provide. In that case, side-action cams may completely eliminate the need for draft as the outside face is pulled perpendicular to the part instead of being pushed or pulled from the mold. And, finally, cams can allow production of parts with texture on faces parallel to the direction of mold opening. In simple straight-pull molds, such texture acts essentially as a field of undercuts that can prevent the clean ejection of parts. If there is a design need and we can get a cam on that face, why not? It just may be the face that makes your part.

First, in a thin-walled design, the resin could cool quickly enough to result in a “short shot,” that is, incomplete filling of the cavity. Obviously, this would not be acceptable. Even if the cavity did fill completely there would be two flow fronts meeting on the side of the core opposite of the gate, creating knit lines that could substantially weaken the resulting part. One possible solution would be to use a hot-tip gate and inject resin at the closed end of the cylinder—the bottom of the cup or top of the thimble. This may not be possible due to the height of the part, resin compatibility with the hot tip or other unforeseen issues. A better solution might be to lay the design on its side and use a cam to create the core (see figure 3). In this case, the core of the thimble is formed by a retractable cam while the outside of the part is formed by two straight pull mold halves. As shown in the diagram, the parting line runs across the closed end of the cylinder and down the sides. For the sake of symmetry, a tab gate is located along the parting line at the closed end of the cylinder. Resin is injected through the gate and the resin flows down the length of the cavity uniformly.

©2009 Proto Labs, Inc. All rights reserved.

Fig. 3

Basically, cams aren’t just for undercuts anymore. Need a flat face to bolt up against a mating part? How about texture? How about your company’s logo or part numbers? Adding a cam to the mold may be just what you need.

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CAMS: THEY’RE NOT JUST FOR UNDERCUTS 18

Design Tips for Rapid Injection Molding

Look sharp For several years Protomold has been expanding the range of prototypes and production parts we can make by increasing our mold bases and press sizes. At the same time we’ve been aware of the demand for thinner features and Protomold has begun to incorporate additional technology in our mold making process to allow your parts to be taller and narrower.

Once you have received your ProtoQuote, you can discuss your project with our Customer Service Engineers (CSE’s) by calling us at (877) 479-3680. Our CSE’s can help you understand how your part is going to be affected by our process and if there are alternatives within the Protomold process to manufacture your part. Bottom line: if you want thinner, less drafted features on relatively small parts, give us a try. As always, you can upload 3D CAD models to ProtoQuote at www.protomold.com/ PartUpload.aspx.

For example, we now offer: n

Thinner ribs with less draft

n

Smaller, more complex features atop tall walls

n

Speaking of “bottom line,” this technology does add some cost to the mold. We do add a mold advisory to your ProtoQuote under our “Other Info” column. You can still design to the “old” Protomold rules for draft, and avoid the added expense. Give our CSE’s a call at (877) 479-3680, and they can help you out.

V-ribs for ultrasonic welding

In the past, taller ribs have required more draft than shorter ones, but with the new technology, this is no longer always the case. On small parts, we now have the ability to consider less draft on deep ribs and sharp corners on outside edges.

Fig. 1

These new capabilities do not apply to all parts. As with many of our capabilities, the applicability will vary based on a number of factors. The best way to know whether they apply to your parts is to submit your design to ProtoQuote®, which will evaluate your model against our most current capabilities.

©2009 Proto Labs, Inc. All rights reserved.

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