Brad Cleveland
President and CEO
Proto Labs Inc.
Maple Plain, Minn.

Selecting a resin for a plastic part can be as important — and challenging — as designing the part itself. Engineers can spend considerable time poring over information in books or online, but making and testing prototypes are often necessary to confirm a resin is right for the job.

Of course there are times when a special requirement overshadows all others and only one product will do, making the choice of resin obvious. On the other hand, some applications have requirements that aren’t very rigorous and a number of resins meet the need.

More often, however, choosing a resin involves balancing the competing demands of cost, cosmetics, moldability, and performance. In such cases there may be several serious contenders. The challenge is determining the best all-around material for the application before committing to full production.

Properties on a resin data sheet can help narrow the options, but in today’s competitive market engineers need more data. The difference between just the right resin and the second best might be small, but when multiplied by a production run of 100,000 pieces, the time and energy invested in finding the right product can bring substantial returns. The goal is to effectively compare the “finalists” to determine the best resin for a particular part.

The traditional way to decide is to select a resin, make prototypes, evaluate the resulting parts, and decide whether or not to try another resin. The problem is the first resin might seem perfectly adequate until it is compared with something better. And with deadlines looming, as they usually do, “adequate” can be tempting, especially when further exploration costs time and money. There is, however, an alternative that won’t squeeze budgets or derail schedules.

Instead of trying resins sequentially, make several versions of the prototype concurrently with different resins and test them side by side. This can be done by machining or molding prototypes.

To begin, first make sure the design can be injection molded before moving forward with prototyping. One way to do this is uploading 3D CAD models to obtain a ProtoQuote (www.protomold.com/PartUpload.aspx), which includes a design analysis that helps identify potential moldability problems before one invests in prototypes.

To compare resins with significantly different shrink coefficients, have prototypes machined from solid resin, for instance by Proto Lab’s First Cut service (www.firstcut.com). All the parts will have the same dimensions for easy comparison, testing, and fitting into assemblies. This should at least let users reduce the resin choices to a set with similar shrink coefficients.

After narrowing resins to those with comparable shrinkage (or if tolerances are not particularly tight), then have a vendor such as Protomold injection mold the prototypes. Keep in mind that each mold is manufactured to accommodate shrinkage. For this reason, when using resins with different shrink coefficients, new molds may have to be cut. (Thus, it makes sense to consider CNC machining first.)

The use of CNC machining followed by injection molding helps designers fully explore resin options while avoiding excessive tooling costs. Also, the additional production time for molding parts in more than one resin is negligible — probably just a matter of hours. The resulting information can be invaluable. Specifications cannot tell precisely how a resin will function in a particular design, which is why companies make prototypes. For a small incremental cost, engineers can answer questions such as:

How does the part look in apple red versus lemon yellow?

How opaque is each resin at a particular point in the design?

How does it stand up to a 3-foot fall, a sharp rap with a ball-peen hammer, or an hour in a closed car on a sunny afternoon?

How much glass fill does it need to meet strength goals?

Will it warp or sink?

Making parts in several resins permits comparative testing and helps keep designs on schedule. It also lets customers see and handle parts, check mechanical properties, evaluate fit, and test them in different settings. This approach compares resins against one another, instead of simply against expectations, to see which delivers the best combination of attributes. The approach often produces designs that actually exceed expectations, and that’s always good.

In short, multiresin prototyping has a lot of advantages and small incremental costs. The only stipulation, as mentioned earlier, is that truly equivalent parts from a single mold require resins with similar shrink coefficients. If, on the other hand, comparing parts made of resins with different shrink rates, do so using automated machining. md

Proto Labs Inc. (www.protolabs.com) provides CNC machined and injection-molded parts for prototypes and short-run production in as little as one business day. Parts are based on 3D CAD designs.

Edited by Kenneth Korane

© 2010 Penton Media, Inc.