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.

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

• Maximum part outline is approximately 30.5 inches by 13.5 inches (the maximum outline applies only to shallower parts).
• Increasing part depth reduces maximum outline. At 1 inch depth, maximum outline is 28.5 x 11.5; at 2 inch depth maximum outline is 26.5 x 9.5; at 3 inch depth, maximum outline is 24.5 x 7.5. 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.
• Outline notwithstanding, maximum projected mold area is 175 square inches.
• Maximum part volume is approximately 59 cubic inches.
• 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).
• Parts should have approximately 1° of draft per inch of depth from parting line. Minimum draft, regardless of depth, is ½°. This allows the part to eject from the mold without drag marks.
• Side action cams consume space, reducing maximum mold size.
• To see how these limitations specifically affect your part, upload a 3D CAD model for a free ProtoQuote® moldability analysis and quote.

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 as listed in the chart below (the chart applies to all parts, not just larger parts).

Resin	Recommended Wall Thickness (inches)
ABS	0.045 - 0.140
Acetal	0.030 - 0.120
Acrylic	0.025 - 0.150
Liquid crystal polymer	0.030 - 0.120
Long-fiber reinforced plastics	0.075 - 1.000
Nylon	0.030 - 0.115
Polycarbonate	0.040 - 0.150
Polyester	0.025 - 0.125
Polyethylene	0.030 - 0.200
Polyphenylene sulfide	0.020 - 0.180
Polypropylene	0.025 - 0.150
Polystyrene	0.035 - 0.150
Polyurethane	0.080 - 0.750

ProtoQuote can provide feedback on the proposed wall thickness of your parts.

Other guidelines are similar to those for smaller parts:

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. 1: Radius Corners

3. Avoid unsupported geometry (see fig. 2).

Fig. 2: Unsupported Geometry

4. Use core-cavity construction rather than ribs wherever possible (see fig. 3).

Fig. 3: Rib vs. Core-Cavity

For detailed guidelines, examples, and illustrations, see Protomold's Design Guidelines.

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Design Guidelines

As always, you can visit the Protomold Design Guide for helpful Rapid Injection Molding design information.

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Plastics Trivia Question

The world's first plastic magnet, announced at University of Nebraska-Lincoln in 2001:

 A. proved unstable at temperatures above 10° Kelvin.

 B. only works in a hyper-oxygenated environment.

 C. is being commercially developed for data storage applications.

 D. has been licensed by LEGO for use in the company’s 2009 toy line.

 E. turned out to be a hoax.

(Honor System: No Googling, Yahooing, or Dogpiling until after you've submitted your guess.)

Last month's question/results:
So-called "plastic blood":

A. was first developed at Israel's Ma'ariv University.

B. is made up of plastic molecules with an iron atom that bonds to oxygen.

C. has been described as being "the color and consistency of honey."

D. is twice as effective as natural blood at conveying oxygen to tissues.

E. has not yet been made stable at room temperature.

The correct answer is
B. is made up of plastic molecules with an iron atom that bonds to oxygen.

The responses are represented in the following chart:

Upcoming Tradeshows

Want to talk in person? Stop by to discuss your design.

 Atlantic Design & Manufacturing
June 3-5, 2008
Jacob K. Javits Convention Center
New York, NY

For more information, visit our web site at Protomold.com.

Suggestions

Please e-mail suggested topics for future Design Tips, questions for future Designer Surveys, and obscure bits of Plastics Trivia to news@protomold.com.