December 2011 Machining Tip

Material Choice for Machining

In a previous Protomold Design Tip, we discussed rapid injection moulding as an inexpensive way of trying out different materials before committing to a resin for full-scale production. In that tip we suggested using material spec sheets to narrow the list of options before actually moulding parts in the candidate materials. But spec sheets alone can’t always tell you all you want to know. In such cases, CNC machining a real part often can.

While rapid injection moulding can be cost effective in quantities of two dozen or more, Firstcut machined prototypes are quick and inexpensive to make in quantities as small as one. This makes it a viable option for testing real thermoplastics and metal materials in the earliest stages of the development process, when designs are sometimes still very much in flux. Automated quoting and machining easily accommodates changes in design, because a new prototype can be re-quoted and made directly from a modified 3D CAD model without the cost of making a new mould.

Machined prototypes can be produced in many of the same resins used in injection moulding. This makes them ideal for early functional testing of materials before the design is finalized and lets you take full advantage of material characteristics in designing your parts. For example, early testing of a stronger resin may let you use less material without sacrificing structural integrity. If, on the other hand, the stronger resin turns out to be unsuitable, you can alter your design and/or material choices accordingly. Machined parts can also be used to test a wide variety of characteristics like heat and chemical resistance and electromagnetic shielding capabilities.

Recently, a customer who was developing parts in an opaque material requested prototypes made of clear resin. The purpose of the prototype test was to see what was happening inside an assembly during functional testing. The cost of machining parts in a clear material was significantly lower than that of moulding parts and made this unusual approach feasible.

Our Firstcut process can also be used to machine metals, like aluminium and brass. This can be useful for testing parts that will be manufactured in metal, or for testing parts in which the choice of metal or plastic has not been finalized. Machined plastic prototypes can also help in the process of replacing metal parts with plastic. This can significantly reduce production costs as long as it does not impair the performance of the finished part or assembly. Because the characteristics of metals and plastics are so different, this conversion may entail significant changes in structure, and various resins may have to be tested to find the right combination of design and material. On the other hand, if size or shape cannot be changed significantly, the testing of resin options becomes even more critical.

Another significant factor in choosing between moulding and machining of prototypes is the shrinkage that occurs as moulded parts cool. While each resin’s rate of shrinkage is predictable and can be compensated for in mould design, the amount of dimensional change varies among resins. If you are test-moulding resins with different rates of shrink, a single mould will not produce dimensionally identical parts. Machining, on the other hand, can produce the same dimensions in any material, so it can be preferable when comparing fit in multiple resins. Once the material is chosen and you move on to moulding (either for further prototyping or for production), a mould can be milled that accounts for that resin’s shrink rate.