Plastics have been around for more than 150 years. Before then, metal and wood were the typical raw materials for mechanical parts. Today, machined plastics can provide a solid alternative to machined metals, or even molded thermoplastics. But they can be temperamental when milled and require different treatment than metals to get good results and usable parts. Here are a few things to keep in mind.
Machined Plastics vs. Metals
It’s almost a guarantee that your machinists have scrapped a lot of pricey plastic to ensure high quality parts during milling. There’s a constant balance of feed and speed: manufacturing quickly without compromising the plastic due to heat stresses. Mill too slowly and you get too much contact between the cutter and the part. The mill will translate its heat to the plastic, potentially melting it. Go too fast and you could end up with large chips, clogging the mill and affecting tolerances. This can happen even though special end mills exist specifically for plastics. Conversely, metals tend to dissipate heat more readily without warping and retain their form when exposed to temperature extremes. High-performance plastics such as PEEK, PEI (Ultem), and acetal (Delrin), however, are well suited to machining and can hold tight tolerances.
Plastics also tend to weigh less than metals but can be surprisingly rugged. They will always be free of corrosion and rust and many resist harsh chemical compounds. Plastics typically cost less, too, which makes them ideal for prototyping, especially considering the typically shorter lead times for machining.
Machined Plastics vs. Molded Plastics
Machining is a subtractive process—it requires removal of material from a block to create the designed part. Because injection molding is a formative process, using a mold to manufacture parts, additional lead time is needed as a new metal tool (the mold) needs to be milled for each revision.
Certainly, injection molding offers some significant benefits in certain situations. If you need larger part quantities, it allows you to manufacture them more quickly than via machining. Injection molding also offers a greater variety of materials and surface finish options.
Machined plastics are great when you need to test and revise to get the part you want. No mold revisions means that updating your part design is as easy as editing your CAD file.
Caveats of Machined Plastics
There are certainly benefits to machining plastics, but material selection is important. For strength and resistance, the best options for machining are often more expensive. While you have a variety of plastics available, they don’t all behave the same. Some are reinforced with glass fibers, adding strength, but also cost. Another cost factor is the time your raw materials will be on a CNC machine. Your manufacturer may need to create special fixtures and jigs to hold the material firmly in place during machining. Working with a company that has experience with materials across multiple manufacturing technologies is critical to your part’s success.
Related Design Tips
For more helpful machining tips, download our CNC Machining Design Guide.