Challenges of Machining Plastic

Machining plastics can pose many difficulties as opposed to metals.

Plastics offer many advantages in today’s manufacturing.  Plastics are available in an incredibly wide variety, with different characteristics to meet a multitude of needs.  Many types of plastic are highly machinable, durable and formable, and are relatively lightweight. When compared with metal, plastics often offer a cost advantage as well.  Plastic materials are sought for their insulating and corrosion resistant properties in many products.  However, plastic is softer and has a lower thermal resistance than metal and as such, poses some problems for machining.  Below are some considerations for addressing the unique challenges of machining plastics.

Inherent Stress

Stress is more likely to be preexisting in plastic raw material and as such, additional methods need to be employed. Annealing is often needed to remove stress prior to downstream processing.

Raw Material Distortion

Plastic sheets are often not perfectly flat and plastic rods are often not straight. This can present challenges with fixturing and bar feeding. It is therefore important to qualify raw plastic materials prior to machining. Some issues can be resolved by the machinist using techniques such as fly-cutting.

Fixturing and Tooling

Work holding can be a challenge with plastic materials/parts that are brittle or too soft. These parts can become damaged or warped by over-tightening the fixture. Plastics typically cause cutters to wear more quickly, some much more quickly. Therefore, cutter selection is important, and frequent monitoring is needed. Uncoated tools tend to offer a smoother finish when machining plastics, but they sustain more wear and tear than coated tools.

Warping

Parts machined from plastic sheet stock are often subject to warping. This is particularly more prevalent with certain types of plastics or when the sheet is thin. The programmer can make some modifications to the tool paths to minimize the effects of the stress that causes warping.

Cracking

Brittle plastic materials are subject to chipping and cracking when machined. Thin cross sections and sharp corners can pose an increased likelihood of chipping or cracking.  Parts with these features must be machined more slowly. A proper tool path and appropriate cutter selection can help alleviate this issue.

Burring

Burrs are more likely to be present when machining plastic due to the softness of the material. Some burrs can be removed by extra machining passes, others are best handled with subsequent de-burr operations.

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