Guidelines for Machining
Performance Plastics
by IAPD Education Committee

tock shapes are rarely used in their standard shape. Most of the time, the shape must be changed for the end use. Most of these end use shapes come from a drawing, a blueprint or a sketch. The print will show what the part is supposed to look like for use, not how it is going to get that way. That’s where machining and fabrication comes into play.

When machining or fabricating performance plastics, you must consider the properties of the material: Is it hard or soft, pliable or firm? Does it need to meet a certain temperature to be fabricated? These details will help you select the correct machinery and tooling to produce the end use part. If an incompatible method of fabrication is used with the material, it could lead to damaged machinery and tooling or a substandard part. Always refer to supplier guidelines for details about your selected material(s) because there can be limitations that are easy to overlook.

Sawing is usually the first step in any plastics fabrication — either cutting a stock shape to a particular length and/or width for distribution OR cutting blanks that will be further fabricated to produce a specified part.

A band saw is useful for trimming formed parts or cutting irregular shapes. Usually hand operated, the operator moves the part in and out of the saw blade to trim off unused material.

Cut-to-size is the process of taking a stock shape piece of plastic and cutting it to a specific length and/or width. In this case, sawing is the only fabrication process used in producing the final part.

CNC cutting is used for high-volume, intricate or precise parts. The part is designed on a computer-aided design (CAD) system and geometrically programmed directly into the CNC saw for cutting.

Although most saws were designed to cut in straight lines, a jigsaw was designed for cutting curves and other arbitrary patterns. A jigsaw uses an electric motor and a reciprocating saw blade to perform its cuts. The operator moves the saw around the part to cut the pattern desired.

A panel saw is usually used for trimming or blanking a sheet for further fabrication in small volumes. Panel saws can be horizontal or vertical. These saws will only cut straight lines all the way across a sheet of plastic. Panel saws are usually tables or vertical stands that hold the sheet of plastic while the saw blade moves across it.

A saber saw uses a reciprocating, toothed blade to cut plastic. It is used for cutting in tight spaces or crevices and allows for cuts to be made close to the blade where a hindrance would otherwise touch the saw frame. The operator holds this saw and moves it through the plastic.

A table saw consists of a circular saw blade mounted on an arbor driven by an electric motor. The blade protrudes through the surface of a table that provides support for the material. The depth and angle of the cut is varied by moving the blade up/down and tilting the blade to one side or the other. Here the operator pushes the plastic across the table, usually with a guide to hold the part straight.

Machining methods
Plastic stock shapes can be machined easily. However, you must adjust the tool geometry and speed for optimum performance with a specific material. The tolerances for machining performance plastics should be wider than those applied to metals, because of the thermal expansion and shape changing from the relaxation of internal stresses within the material. In critical applications, it may be necessary to pre-machine the part so it’s slightly oversized and stress relieve or anneal the part before taking the final cuts.

Annealing is the baking of the material without melting or distorting the part to relax the internal stresses. Uneven cooling usually causes the internal stresses; that is, the outside of the part cools faster than the inside of the part. The uneven cooling can also cause variations in the properties from the outside to the inside.

The poor thermal conductivity of plastics requires that you take care to prevent the area being machined from getting too hot. You may need to adjust the type of tool, depth of the cut, rate of feed and coolant flow. If using coolant, ensure that it does not chemically react with the plastic.

Computer numerical control (CNC) milling uses computer controls to cut different materials. CNCs are able to translate programs consisting of specific numbers and letters to move the spindle to various locations and depths. CNC mills have many functions, including face milling, shoulder milling, tapping, drilling and some even offer turning. Standard CNC mills have three axes (X, Y and Z) but others may also have one or more rotational axes.

CNC lathes are able to make fast, precision cuts, generally using index-able tools and drills. They are particularly effective for complicated programs to make parts that would be more difficult to make on manual lathes. CNC lathes have control specifications similar to CNC mills. CNC lathes generally have two axes (X and Z) but newer models have more axes, allowing the machining of more advanced jobs.

The difference between milling and turning: During milling, the part is held down while the tool rotates and cuts into the part. When turning, the part rotates, while the tool is held steady and cuts into the part.

Grinding machines create a good surface finish and tight tolerances. The part rotates while the grinding surface is held steady to the part.

The lathe turning method rotates an object on its axis alongside a tool; the machine defines the position of the tool. It is useful for fabricating different kinds of objects that feature a round cross section. One element of a lathe that rotates is a spindle.

Small- to medium-sized cam-operated automatic lathes are usually called screw machines or automatic screw machines. Screw machines are used in the high-volume manufacture of a variety of turned components.

chart for Machinability and Fabrication Techniques for Stock Shapes
CNC routers are extremely popular among plastic fabricators with 2-axis, 3-axis and 5-axis machines fulfilling a variety of needs. Flat sheet fabricators use a 3-axis machine. These machines have a spindle capable of turning a tool (router bit) up to 18K RPM. This is the speed of the machine. Then, the spindle draws across the machine at a certain rate, called feed rate or simply feed. This machine is control by numbers and letters fed into the computer running the machine.

Hobbyists use 2-axis routers for basic household items. These types of CNC routers do not use a true Z-axis, making them simple for industrial applications.

The most common CNC routers are 3-axis. They use a long X-axis, a shorter Y-axis and a vertical Z-axis. The Z-axis is mainly used for different depths of cut, and incapable of giving a true third dimension to the sheet.

The 5-axis CNC routers are able to rotate on two additional axes (compared to the 3-axis routers) for cutting three-dimensional objects without having to reposition the part. These are effective on thicker gauge materials to give a profile to a part. This will give the part a true third dimension compared with a 3-axis machine.

The versatile pin routing machines use a horizontal column to support a pneumatically activated guide pin. The pin set directly over a spindle holds the router bit below the worktable. Both the pin and router bit are fed to a preset stop and are activated by a foot pedal. This is usually not a CNC operated machine; rather, stencils are used to contour the parts produced. Here, the tool, while rotating, does not move. The part is then moved across the tool.

Adding value
Instead of asking you customers “How many sheets would you like?” ask them “What are you doing with the plastic?” Ninety-nine percent of the time, the plastic stock shape is being sold to a company that is fabricating something with it. This is a great way to add value to your customers as well as increase your bottom line. Ask your customer:

  • What are you doing with the plastic?
  • What will be the end use? How is the plastic being used?
  • Is there a certain material that needs to be used?
  • What are the specifics (tolerances, thicknesses, dimensions, strengths, etc.) needed?
  • Are you sending it to a machine shop, OEM or fabricator? Can we save you a step by doing the fabrication ourselves?
  • Is this repeat business?
  • How many parts do you need? How quickly do you need this turned around?
  • Have you experienced any problems with having this part fabricated?
  • Do you have a drawing related to this part?
  • Is there any criteria that needs to be addressed?
More online
See the digital edition of this issue for a helpful chart showing which machinability and fabrication techniques can be used with which families of performance plastics.
This article is excerpted from the Introduction to Performance Plastics, 13th Edition. For more information and to get your copy, contact IAPD at IAPD, 6734 West 121st Street, Overland Park, KS 66209 USA; phone (913) 345-1005, info@iapd.org or www.iapd.org.