Design Tips 6

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Design Tipsfor Rapid Injection MoldingVolume 6

Real Parts. Really Fast.

WWW.PROTOMOLD.COM

Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877.479.3680

Design Tips for Rapid Injection Molding

Design Tips categorized by topicPage 3 4 6 7 9 10 12 13 15 16 17 18 TABLE OF CONTENTS Word processing Rapid Injection Molding, an overview Building better bosses Temporary attachments Corn, its not just for breakfast anymore First impressions Things weve learned in the last decade We reexamine cammin Automated and live help: The best of both worlds Sometimes you need to be square When thin is not in Out of many, one Material selection Design guidelines Quality assurance Understand the process

2010 Proto Labs, Inc. All rights reserved.

Volume 6

DESIGN MATRIX

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Design Tips for Rapid Injection Molding

Word ProcessingThere are lots of reasons to add text to a part. It could be an assembly instruction, a part number, a legally-advisable warning, or simply a logo (see gure 1). Whatever the reason, text characters tend to be the smallest features of a part and, as such, deserve the designers careful attention. primary lines of the letter itself, making them too small to mill. Instead, use a sans-serif (non-serif) font like Century Gothic Bold, (the default font in SolidWorks). Other common sans-serif fonts are Arial and Verdana. In general, remember that while most 3D CAD programs allow you to use standard Windows fonts, you should resist the temptation to get cute without a good reason. The third issue is the size of the letters themselves (see gure 2). Text doesnt need to stand very tall above the surface of a part .015 works best but even so, the rules for thin ribs apply. You dont need to measure the thickness of every line of each letter; just stick to font sizes of 20 points or more and use the Bold version of the font and odds are excellent it can be milled (see gure 3). In some cases, we can mill smaller fonts. If you need to do so, submit the part with the smaller text for a quote and well let you know, or you can contact a Protomold customer service engineer at 763.479.3680 to discuss.

Figure 3: The L is a larger font size and allows room for the tool to mill

Finally, if text is located at the top of a tall feature a tall rib, for example there is a tradeoff. If we mill, the text may have to be larger. If we EDM, we can do small features at the top of the rib, but there is an additional charge to make the mold. In summary, for best results when incorporating text, design your parts:

Figure 1: Off/On switch instructions on a part

The rst thing to keep in mind is that it works much better if text on a plastic part is raised above, rather than recessed into the part (which means it will be milled into the mold). Raised letters on a part are easier to read, and recessed text in a mold allows for polishing, whereas raised letters in a mold make it difcult to achieve a good nish. The second issue is consistency of wall size in your lettering. Avoid serif fonts, the ones with the little squiggles at the ends of uprights. The serifs are typically narrower than the2010 Proto Labs, Inc. All rights reserved.

with raised text using a bold sans-serif font of 20 points or more and stay away from the tops of tall features

If you are wondering whether youve designed your text properly, simply upload your 3D CAD model for a free ProtoQuote. If there are any problems youll know by the next day.Figure 2: The O is too small to mill Volume 6 WORD PROCESSING 3

Design Tips for Rapid Injection Molding

Rapid Injection Molding, an overviewOver the last several years, our Design Tips have addressed many details of design for rapid injection molding, but weve never really looked at the overall process. In its simplest form, the injection molding process works as follows: 1. Injection molding resin in pellet form, is loaded into the hopper. 2. The pellets ow into the heated barrel, where the material is melted. 3. A ram-driven screw injects the molten material into the closed mold. 4. After the material cools and solidies, the mold opens and the part is ejected.

Figure 1: The Injection Molding Process

Add side-actions and the process gets a little more complicated. If youve already received one of our free demo molds, youve seen how a two-part mold with a side-action cam works. If you havent received one yet, go to our website to order one. Meanwhile, heres a 2D version:

Figure 2 shows the closed mold. Note that the colors of the mold halves correspond to those in an online ProtoQuote: green for the cavity or A-side, blue for the core or B-side, and red for side actions.

Continued on next page

Figure 2: Protomolds Demo Mold

2010 Proto Labs, Inc. All rights reserved.

Volume 6

RAPID INJECTION MOLDING, AN OVERVIEW

4

Design Tips for Rapid Injection Molding

Lift the green A-side mold half (gure 3, a), and youll see the yellow injected part with its runner, sprue, and edge gate (all of which will be trimmed off) and the red side-action. Note the features indicated on the face of the green A-side mold half, particularly the gate, where resin enters the mold cavity, and the drafted sliding shutoffs, which form the outside of the hook on the part (gure 3, b). Withdraw the red side action (gure 3, c) and note the hole and raised lettering it forms on the side of the part. These are undercuts that can not be formed in a simple two-part (straight-pull) mold.Figure 3: Demo Mold Exploded View

Once the mold is fully open the white ejector will rise to push the part off the core on the blue B-side mold half (gure 3, d). Since the mold halves are normally mounted horizontally in a press, this will cause the part to tumble free from the mold. Note the features indicated on the face of the blue B-side mold half, particularly the core, which forms the hollow center of the part. For more information on Protomolds processes, visit our web site. To request your own Protomold Demo Mold, visit www.protomold.com/demomold.aspx.

2010 Proto Labs, Inc. All rights reserved.

Volume 6

RAPID INJECTION MOLDING, AN OVERVIEW

5

Design Tips for Rapid Injection Molding

Building better bossesPrincetons WordNet denes emboss as raise in relief, and thats exactly what a boss is: a feature raised above a surface. In plastic parts, bosses are typically used to assist in assembly, as a receptacle for a screw or threaded insert or as the locator for a mating pin on another part. Because of its function, a boss must have sufcient strength to do its job. This dictates a minimum size for the feature. At the same time, because a boss rises from a surface, it thickens the surface at that point raising the risk of sink or development of voids as the part cools. The challenge: bosses should be big enough to do their job but not big enough to cause avoidable sink in the surface from where they rise. A typical boss is an open-topped cylinder, essentially a round rib (see gure 1a). Standard guidelines suggest that its wall thickness be between 40 and 60 percent of the thickness of the wall from which the boss rises. If your design requires more strength than this guideline would provide, you should consider ways to strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls (see gure 1b). If a boss is part of a vertical wall, it should not create a thick area in the wall. Figure 1c shows an example of how this can be done. Similarly, if a boss is located close to a vertical wall, it may be tempting to tie the boss to the wall by lling the space between the boss and the wall, resulting in a thick area (see gure 1d). A better way to tie the boss to the wall is with one or more ribs (see gure 1e). Sometimes, the wall of the boss may be too thin to mill with Protomolds conventional process. In some of these cases, we can use a steel core pin to form the inside of the boss. To aid this process, the end of the boss should be square, rather than having llets or chamfers. There are two additional points to consider in designing bosses. As mentioned earlier, a boss is a circular rib, and like any rib, its walls both inside and out must be drafted toFigure 1 The Protomold Sample Cube

facilitate ejection. Depending on the height of the boss, this draft can be anywhere from .5 to 3 degrees. If we use a steel core to form the inside, it will be un-drafted. If we dont use a steel core, because a boss is formed by a blind hole in the mold body, we may have to add vent pins to the rim of the boss to allow the escape of trapped gas during mold lling. Otherwise shorts or burns may form on the rim.

Protomolds free 3D sample cube shows various boss congurations along with many other design capabilities and features of our molding process. If you dont already have yours, you can request one at http://www.protomold.com/SampleCube.aspx.

2010 Proto Labs, Inc. All rights reserved.

Volume 6

BUILDING BETTER BOSSES

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Design Tips for Rapid Injection Molding

T emporary attachmentsThere are plenty of ways to attach plastic parts to one another. They can be permanently welded or cemented. They can be semi-permanently screwed together, with or without inserts. And, for attachments designed to be easily and/or frequently opened, they can be connected using clips or latches. Clips and latches take many forms. Examples include: 1. The back cover on a cell phone (see gure 1), which slides open to reveal the battery. A small notch at the end of the cover engages a protrusion on the underside of the handset shell to latch the cover in place. The elasticity that allows the notc