Tips for Machined Plastic Component Design

MatWeb Featured Article
By Tom Rohlfs
PEP Connecticut Plastics

The following are guidelines for good machined plastic component design. These are not rigid rules but suggestions to maintain reasonable pricing. Exceeding the guidelines will usually result in higher effort and cost for the component. Each part configuration and application is unique.

• Flatness: From the extrusion and cooling, plastics have stress gradients through their cross section which are not relieved by post processing at the manufacturer. Plastics’ flatness stability is substantially lower than metals. Consider the following to improve:
  • Balanced design. Dish shaped plastic machined components are notoriously difficult to avoid warping.
  • 0.003” per a 4” square, unrestrained, is typical. Tighter tolerances require better fixturing and more machining.
  • Consider specifying parallelism instead of flatness. If the plastic part is attached to another (especially a metal part), unrestrained flatness is unimportant. The plastic part will follow the contour.
  • Consider restrained flatness. Again if attached to another component, the plastic part will conform to the contour.
  • Pick a stable material: Low performance materials will easily warp. For tight flatness requirements, harder plastics are better. Try Ultem® PEI, PPS, PEEK and Acrylic
  • Hire a plastic machining company. Flatness can change over time while sitting on a shelf in the stock room. Advanced processing really works for flat critical components.
• Threaded Inserts: For single time assembly, threaded inserts are not necessary given proper torque levels are utilized. Inserts do not add appreciable additional holding power. Applications requiring repeated disassembly (usually for cleaning) do benefit to avoid thread stretch. Heat staked inserts are a better solution than Helicoils by giving higher pullout power. Helicoils impart high stress levels into a threaded hole resulting in crazing at the thread root in sensitive materials. Helicoils are acceptable in materials that do not heat stake well and have good toughness like UHMW.
• Polishing: Typically applies to clear plastics like Acrylic, Polycarbonate, Polysulfone and Ultem PEI but is important for finish improvement in opaque materials as well. Polishing methods are direct machine polish, vapor polishing, flame polishing, and buffing. The most professional looking finishes are accomplished by machining and polishing with one vendor. Polishing only services are available but the finish usually is not as good. Heavy polishing can never make up for and overcome poor machining. Melted surfaces in the machining process will invariably result in stress cracking after polishing.
• Finishes: There is continual demand for better finishes to improve product performance. Given the capabilities of modern CNC equipment, do not accept surface finishes above 63 micro inches from a machining vendor.
  • A 32 to 40 microinch is considered a standard machined plastic finish for a +/- 0.005 toleranced component and should not incur any additional cost to produce.
  • Finishes below 32 are reasonable and to be expected on close tolerance parts (+/- 0.0005).
  • Finishes between 10 to 20 micro inches are typical for a polished plastic component especially as a turned component.
  • Inside milled pocket finishes always have higher/worse finishes. The action of the nose of the endmill does not facilitate clean cutting and chip removal.
  • Exterior flat milled areas can have a very good surface finish
  • In 3D profile milling a good finish requires a large number of passes from a ball endmill. Carefully specify the maximum allowable finish to avoid excessive machine time.
  • Finishes below 10 are difficult to achieve and may require lapping or similar with limited application to part geometry.
  • Some plastics like Teflon cannot have a surface finish below 20 via any method because of the porosity of the material.
• Tolerances: Dimensional tolerance holding is very much configuration and material dependant; however some general rules do apply. +/- 0.005” is an easy to reach tolerance for all but very large dimensions in unstable material. For parts with a volume of 1” cubed and below, +/- 0.003 is readily held. For small components, even in soft materials like Teflon®, +/- .001 is reasonable. +/- 0.0005” and +/- 0.0002” are manufacturable with small dimensions in stable materials. The corresponding surface finish must be improved as tolerances are tightened to avoid measurement repeatability errors.
• Material Selection: There are many good references for plastic material selection by the major manufacturers of both resin and stock shapes. Select the material carefully, as plastics are capable of long term change. Field failures do occur when a plastic component reacts with a chemical in service long after the part has been approved for use by inspection. Be aware that plastic choices are not limited to unfilled materials only. Fillers such as glass fiber can enhance particular characteristics.
• Dimensional Stability: Like flatness above, machined plastics do not have near the same stability as an identical metal component. Plastic components are affected over the long term by many factors
  • Thermal change: Plastics’ coefficient of thermal expansion (CTE) is higher than most metals. Some plastics have a measurable change simply though contact with body heat.
  • Moisture absorption: Nylon can soak up to 9% of its weight at saturation having a dramatic affect on dimensional change. Try Noryl PPO for hot water applications.
  • Creep: Materials like Teflon® PTFE and UHMW when placed under high pressure will change shape and extrude from a joint.
  • Off-gassing: Many machined plastics off gas as a reaction to the extrusion and machining process. Notably, Delrin acetal components shrink over time from the effect.
  • Stress relaxation: Both the extrusion and machining process impart stress into a machined plastic component. Post processing can help, however part configuration determines which direction the plastic will relax and change size.
  • UV/aging: Plastics age and embrittle especially from exposure to UV or chemicals.

Learn more about machined plastic materials selection.