Machining plastics with manual or CNC controlled milling machines is effectively the same as machining ferrous or non-ferrous materials for the most part. However, though the technique and tools are similar, machining plastic is not the same as cutting metal. Moreover, cutting different plastic material can require different manufacturing cutting tools, cutting speeds and tool compensation to achieve desired surface roughness and tolerances. Engineering, design and manufacturing should investigate a particular plastics manufacturing characteristics before assigning tolerance or attempting to manufacture.
Most plastics and thermal plastics are physically affected by temperature and ambient humidity when machining. Skilled machinist and programmers can make a huge difference in getting one's part manufactured on time and dimensionally correct. Plastic materials can perform very differently when a machinist is removing material and may require experimentation by the machinist. Some plastics have a tendency to flow or stress relieve into different geoemtries long after the machining is completed . It is always a good ideal to retain the services of a machine shop that has experience machining the particular material you have specified. Additionally, it is also good practice to request design feedback from the desired machine shop regarding your mechanical tolerances, finishes and geometry stability following machining.
When issuing an RFQ (request for quote) be sure to include a request for alternative materials. Often manufacturing folks are well aware of alternative materials that other products or OEM's utilize. Keep in mind that the manufacturer will initially look at the over size of the part to estimate raw stock thickness and length requirements. Typically, metals are available by the pound, but plastics are sold by the sheet or round size. If your product overall length or width exceeds a smaller sized sheet or round, manufacturing will need to purchase the next larger size to manufacture your part. This could be a cost driver as the manufacturing organization will quote for the entire raw stock versus only the material that is actually used. Plastics materials whether raw or in a preformed shape can vary widely. Cost per square foot can vary from as low as $1 per square foot or $800 per square foot or more. Some exotic plastics material cost can almost be the exclusive cost driver on your part.
One machinability indicator of plastics, as well as the tolerance stability is available on the material specification or data sheets. The plastic's Durometer hardness value is a great way to estimate if your tolerances or finishes are achievable. The lower the reading the softer and more difficult a plastic is to machine and hold finish or tolerances because the material tends to be pushed by the cutting tool rather than cut free. Sometimes a manufacturer will freeze a plastic material to machine features with good surface roughness.
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