May 2010 Design Tip

Making Choices at the Resin Buffet

Choosing a resin for a plastic part can be as important (and challenging) as designing the part itself. You can spend all the time in the world poring over information in books or on the web, but you may not know for certain that the resin you’ve chosen is the right one for the job until you make and test some prototypes.

Of course there are times when the choice of resins is obvious, when there is some special requirement that overshadows all others and only one product will do. On the other hand, there are applications in which the requirements aren’t very challenging and any of a number of resins would meet the need. More often, however, the choice of resins is a matter of balancing competing demands and finding just the right combination of cost, cosmetics, mouldability, and performance. In such cases there may be several serious contenders when you’re ready to start making prototypes. The challenge is choosing the best all-around material for the application before you commit to full-on production.

Using the same 3D CAD model, multiple material prototype part samples were injection moulded for product testing.

Using the same 3D CAD model, multiple material prototype part samples were injection moulded for product testing.

Statistics on a resin data sheet can help narrow the choices, but in a competitive market there’s really no such thing as “good enough.” And while the difference between just the right resin and the second-best choice may be small, when multiplied by a production run of 100,000 pieces, time and energy invested in finding the right product can bring substantial returns. The challenge is to find a way of effectively comparing the “finalists” to find the one best resin for your particular part.

The traditional way to make this decision is to choose a resin, make some prototypes, evaluate the resulting parts, and decide whether to stay with that choice or try another. The problem is that your first choice may seem perfectly adequate until you compare it with something better. And with deadlines looming, as they usually do, “perfectly adequate” can be tempting, especially when further exploration costs time and money. There is, however, an alternative that won’t squeeze your budget or put you off schedule.

Instead of trying resins sequentially, have several versions of your prototype made at the same time and test them side-by-side. This can be done either by machining or moulding prototypes.

  1. To begin, first make sure that your design is injection mouldable before you move forward with prototyping. You can do this by uploading a 3D CAD model to receive a ProtoQuote, which includes a design analysis that can help identify potential mouldability problems before you commit to the cost of prototyping.

  2. If you want to compare resins with significantly different shrink coefficients, you can have prototypes machined from solid resin by First Cut. These will all have the required dimensions so they can be easily compared, fitted into assemblies, etc. This should allow you to at least reduce the resin choices to a set with similar shrink coefficients.

If you’ve narrowed down your resins to a set with similar shrink coefficients or if your tolerances are not particularly tight, you can then use Protomold to injection mould your prototypes. Keep in mind that each mould is manufactured with a shrink coefficient. For this reason, if you need to use a different resin and its shrink coefficient is different, a new mould may have to be cut (hence the above advice to consider First Cut CNC machining first).

The sequential use of First Cut CNC machining followed by Protomold injection moulding will help you fully explore your resin options while avoiding excessive tooling costs, and the additional production time for moulding parts in more than one resin will be negligible—probably just a matter of hours. The information you’ll get can be invaluable. Specifications cannot tell you precisely how a resin will function in your particular design, which is why we make prototypes. For a small incremental cost you can answer questions such as: how does your 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 three-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 really need to meet your strength goals? Will it warp or sink?

With parts in several resins you can get right down to serious comparative testing and stay on schedule while you let your customers take a look at them, check their mechanical properties, evaluate their fit with other parts, test them in different environments, etc. This approach lets you test resins against one another, instead of simply against expectations, and see which one really delivers the best combination of characteristics. You may find that you can actually exceed expectations, and that’s always good.

In short, multi-resin prototyping has a lot of advantages and very small incremental costs. The only stipulation, as mentioned earlier, is that truly equivalent parts from a single mould require the use of resins with similar shrink coefficients. If, on the other hand, you want to compare parts made of resins with different shrink coefficients you can easily do so using First Cut’s automated machining.

If you have questions about how to effectively explore multi-resin moulding, feel free to speak with Proto Labs customer service engineers at +44(0) 1952 607447.