Plastics encompass a large and varied group of materials consisting of different combinations or formulations of carbon, oxygen, hydrogen, nitrogen and other elements. Most plastics are a solid in finished form; however, at some stage of their existence, they are made to flow and may be formed into various shapes. The forming is usually done through the application, either singly or together, of heat and pressure. There are over 50 different, unique families of plastics in commercial use today and each family may have dozens of variations.

These guidelines are for machinists not familiar with the machining characteristics of plastics. They are guidelines only and might not represent the best conditions for all parts.

When Machining Plastic remember:

1. Thermal expansion is up to 10 times greater with plastics than metals.
2. Plastics loose heat more slowly than metals, so avoid localized overheating.
3. Softening (and melting) temperatures of plastics are much lower than metals.
4. Plastics are much more elastic than metals.
   

Drilling

Small Diameter Holes

High speed twist drills are generally sufficient for small holes. To improve swarf removal, frequent pull out (peck drilling) is suggested. A slow spiral.

Large Diameter Holes

A slow spiral (low Helix) drill or general purpose drill bit ground to a 118 degree point angle with 9 to 15 degree lip clearance is recommended. The lip rake should be ground (dubbed off) and the web thinned.

It is generally best to drill a pilot hole, no larger than a 1/2 " diameter, using 600 to 1000 rpm's and a positive feed of 0.005" to 0.015" per revelation. Avoid hand feeding because the drill can grab and cause micro-cracks. To make the hole larger, use 400 to 500 RPM's at 0.008 to 0.020 per rev.

Milling

Sufficient fixturing allows fast table travel and High spindle speeds when end milling plastics. When face milling, use either high positive or high shear geometry cutter bodies.

Sawing

Band sawing is versatile for straight, continuous curves or irregular cuts. Table saws are convenient for straight cuts and can be used to cut multiple thicknesses and thicker cross sections up to 4" with adequate horsepower. Saw blades should be selected based upon material thickness and surface finish desired.

Rip and combination blades with a 0 degree tooth rake and a 3 to 10 degree tooth set are best for general sawing in order to reduce frictional heat.

Hollow ground circular saw blades without set will yield smooth cuts up to 3/4" thickness.

Tungsten carbide blades wear well and provide optimize surface finishes.