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:

• You may be able to thicken a wall, but making it thinner requires remaking the mold.
• You can add features—bosses, raised text, ribs, pins etc.—but you can't remove them.
• You can reduce the diameter of a hole by adding plastic around the perimeter, but you can't increase the hole size.
• Similarly, you can eliminate holes, but you can't add them.

The rule to remember when initially designing you 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

Fig. 2

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.

ProtoQuote®
Interactive Quotes

ProtoQuote is new and improved! Try it out today:

Upload your 3D CAD model and we will e-mail you an informative quote.

ProtoFlow® Analysis
Select ProtoQuotes include a ProtoFlow analysis to predict potential mold fill issues.
Watch a video demo.

Design Guidelines

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

Design Tips Volumes

Download Design Tips compilation volumes and catch up on what you've missed.

Plastics Trivia Question

The journal Nature has reported that the plastic polyethylenedioxythiophene (PEDOT) may prove useful as:

 A. a low-friction bearing material.

 B. a photosensitive lens coating.

 C. thermal insulation.

 D. high-density, low-cost data storage.

 E. an ultra-strong fiber.

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

Last month's question/results:
In 1992, a storm at sea caused 29,000 Friendly Floatees™ plastic bath toys to be washed overboard from the deck of a container ship in the North Pacific:

A. causing an unprecedented "feeding frenzy" among local sharks.

B. providing oceanographers with a means of tracking Pacific ocean currents.

C. forming a bright yellow "spill" that could be seen from space.

D. disrupting U.S. Navy activities in the Pacific for over six months.

E. eventually washing up by the thousands on Antarctic beaches.

The correct answer is
B. providing oceanographers with a means of tracking Pacific ocean currents.

The responses are represented in the following chart:

Upcoming Tradeshows

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

 Pacific Design & Manufacturing
January 29-31, 2008
Anaheim Convention Center
Anaheim, CA
Booth #3714

 Design &
Manufacturing South
March 19-20, 2008
Charlotte Convention Center
Charlotte, NC
Booth #309

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.