The basic principles behind machining engineering plastics are the same as those behind machining metals, but there are some key characteristics of plastics that make machining them to tolerance an interesting challenge.
In this blog post, we will take a look at what those characteristics are and how to minimize their effects on the finished product.
On average, the melting point of polymers is about 1/10 that of metals, making heat build-up a very critical issue in machining plastics. Because plastics do not conduct heat nearly as well as metals, there is a tendency for heat to build up at the surface. This results in highly elevated temperatures that can easily reach the softening point of the plastic or, if it is a thermoset plastic, scorch the surface.
Polymers can be quite sensitive to changes in temperature. Contraction or expansion due to a change in temperature, either during machining or after machining, can cause a part that was assumed to be in tolerance to be out of toleration.
We already mentioned the potential for the workpiece to reach the softening temperature of the plastic, which in turn can cause the surface of the workpiece to deflect rather than be cut by the cutting tool (making the already mentioned deflection issue far worse).
Deflection Instead of Cutting
Plastics are usually far more elastic than metals, especially at elevated working temperatures. This can make machining a challenge because the stock might deflect under the pressure of cutting rather than be cut.
Dealing with the Challenges
Proper lubrication and use of water or oil coolants can help alleviate these issues, which primarily stem from the thermal characteristics of plastic. Adjusting the speed and feed of the tooling, as well as the tooling itself, can also help.
For issues involving deflection and cutting, special attention needs to be paid to adequately supporting the workpiece and choosing appropriate tooling. For machining plastics, tools should have positive rake angle (this should provide a smooth, clean, accurate cut) and a positive relief angle (this will minimize tool wear). Of course, the tools should be sharp with polished upper surfaces. Keep in mind that sharper, polished tools will help to minimize heat build-up.
Harder tools are also recommended for working with plastics for the same reasons discussed. For filled polymers (like glass or carbon filled) the most highly recommended type of tool for high volume runs is diamond because of its superior hardness.
Finally, to minimize residual stresses that can affect part performance and tolerances you should machine the part symmetrically with shallow cuts. Sometimes a polymer part needs to be annealed post machining and then perform finishing cuts to achieve the closest possible tolerance.
While machining plastic parts has its own set of challenges, these challenges can be met so that high quality parts within very tight tolerances can be produced at both low and high volume production runs.