In recent years, we’ve seen more and more customers begin to utilize plastics for their machined components. Plastics offer unique advantages relative to other materials and often cost substantially less than their metal counterparts. That said, plastics come in an incredibly wide variety, and many can pose challenges when it comes to machining. The information below will provide a helpful overview of things to consider when machining plastics

Basics Of Manufacturing Plastic Parts

Selecting the right plastics for the job

When it comes to plastic, there is such a wide variety available that it can become a challenge trying to select the proper material type for the job. It is important to take into consideration material price relative to performance to ensure the best bang for your buck. 

Ultimately, material selection should be based on the parts function and environment in which it will be employed. Common factors to consider when choosing a plastic are heat resistance, durability, moisture absorption, electrical conductivity, noise elimination, corrosion/wear performance, etc. 

A conversation with your manufacturer or plastics vendor can be incredibly valuable in helping select the most cost-effective and high performing plastics for your job. A quick google search will often spit out common “name-brand” plastics that cost an arm and a leg, but a helpful plastics rep can often steer you toward a comparable alternative that comes at a significantly lower price point.

Surface Finishing Plastic

In the majority of machining applications, heat will play a factor, but these effects become more pronounced when machining plastics. While each plastic is unique, and different material types will be more or less resistant to heat, it is always important to understand how the plastic will react as temperatures increase.

Thermal expansion is a primary concern while machining plastics. As the material gets hotter, it will have a tendency to expand. In certain instances, plastics could grow by as much as 5x-10x relative to the same part made in steel or aluminum. This becomes a significant factor when shop temperatures swing dramatically, or when a part is to be shipped from a cold-weather climate to a warm one (or vice versa.) An operator could inspect a machine plastic component in a chilly shop one day, but the end-user may find it out of tolerance when they receive it on a hot sunny afternoon. In some instances, operators will machine components and let them sit overnight, or outside to see how they expand/relax and then make adjustments accordingly.

It is also important to take into account tooling as well as feeds and speeds. If these numbers are not dialed in for the specific type of plastic you’re machining, it can be easy to create excess friction on the material. This will introduce unwanted heat and can potentially start melting plastic away as opposed to cutting.  This is the same reason why certain machining applications will be limited or unavailable for plastics of a certain type/thickness (i.e. laser, plasma, etc.)

Selecting The Right Material

It is important to remember that plastics are not metals. While plastics are considered “easily-machinable”, they behave quite differently than other common materials with which most shops are familiar. 

Achieving a high-quality surface finish free of burrs and other imperfections will require different procedures than when machining metals. This could include workholding, tooling, feeds/speeds and more. In many cases, especially when machining an unfamiliar plastic, it may require a bit of trial and error until one method proves more effective than another. Burrs are also a common problem to contend with when machining plastics. Adding machined chamfers along sharp edges will help remove some of the trouble spots and save the operator time after the fact. It is likely there will still be a bit of finishing work required, and this can be achieved in a tumbler, on a sanding/polishing wheel or by hand using deburring tools and/or abrasive sheets. 

Manufacturing Plastic By Machine

As we’ve discussed above, plastics can behave unpredictably when machining. That said, there are some best practices that can serve as a jumping-off point for a number of common machining operations.

Cutting

Reducing friction is key when cutting plastics. Depending on the type of cut (curved/straight,) band saws and table saws are an excellent option for plastics. Ensure that the cutting blade is sharp to eliminate tearing or melting. Water jet cuttings is another highly effective option for plastics, as it is remarkably accurate and does not introduce heat into the material. Lasers will do an effective job of cutting certain types of thinner plastics, but this option becomes problematic on denser/thicker materials due to too much heat being introduced which leads to melting.

Milling

Consider soft jaws for workholding. Plastics do not require the same force as metals and will have a tendency to flex/warp under too much pressure. Climb milling with higher spindle speeds would be advised for milling operations but feeds and speeds should be adjusted throughout the machining process to limit chip burrs and chip build-up. Certain plastic types will produce long stringy chips that can bind up tooling and scuff up part surfaces.

Turning

Maintaining proper heat levels when turning is the key to producing a quality part. Fresh inserts with positive geometries can help limit chip build-up on the cutting tool. Excess material on/around the inserts will decrease surface finish on your part. Peck drilling or plunge cutting will help limit heat, as will limiting dwelling at termination which may cause unwanted friction.

Drilling/Tapping

More heat is produced while drilling than in any other machining operation. Reducing these elevated temperatures when drilling/tapping is extremely important when working with plastics. This will help reduce material expansion and ensure that holes are machined on size. Sharp cutters and proper feeds/speeds will be key when drilling and tapping. Experiment with slower RPMs to reduce friction, and rely on peck drilling to help evacuate chips and reduce material build-up. When tapping, keep the cutter free of chip build-up and use coolant where possible.