The following are guidelines for good machined plastic component design. These are not hard 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. For specific design support, contact Connecticut Plastics’ engineering at 203-265-3299 or on the web at pepconnecticutplastics.com.
From the extrusion and cooling, plastics have stress gradients through their cross section which are not relieved by post processing at the manufacturer. Plastic’s flatness stability is substantially lower than metals. Consider the following to improve.
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.
Typically applies to clear plastics like Acrylic, Polycarbonate, Polysulfone and Ultem 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.
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.
Dimensional tolerance holding is very much configuration and material dependant. However some general rules do apply. +/- .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, +/- .003 is readily held. For small components, even in soft materials like Teflon, +/- .001 is reasonable. +/- .0005” and +/- .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.
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.
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.