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Plastic Parts Design and CNC Machining



Plastic Parts Design and CNC Machining

There are many applications where it makes sense to design a machined part to be made from plastic rather than metal.
There are many types of plastic each with their unique strengths and applications. There are also some details that are good to know about how plastic responds to machining.
Here are a few of the most common plastics we work with. 
ABS comes in both natural and black and with various levels of glass fill. It is a relatively low-cost plastic that is easy to machine. It holds tolerances reasonably easily and sands and paints well. It has great impact strength and abrasion resistance. The tensile strength is approximately 6 KSI and it is generally available in round bar up to 4" diameter and plate up to 3" thick.
Acetal(or Delrin) is one of the best plastics to use for machined parts. It is a medium-cost plastic with good dimensional stability and excellent machinability. It has very low water absorption which improves dimensional stability. It is available in white, black, various levels of glass fill, or as Delrin AF which has Teflon fibers for increased wear characteristics. It has up to 10 KSI tensile strength and is generally available in round bar up to 6" diameter and plate up to 4" thick.
Acrylic (or PMMA). It is a low-cost plastic that has decent machining characteristics. With the right cutter geometry, very fine finishes can be achieved. It is a relatively hard and rigid plastic which makes it susceptible to chipping; avoid designing thin sections and sharp edges. Model radii and chamfers on outside edges to help reduce the chance of chipping. The main reason to design with acrylic is its excellent light transmission and optical properties. It has better dimensional stability than many of the softer plastics although it is still susceptible to changing size with temperature fluctuations. It is also slightly hygroscopic but much less than most.Acrylic responds quite well to vapor polishing or flame polishing in applications where machining marks cannot be tolerated. It has about 9 KSI tensile strength and is available in round bar up to 6" diameter and plate up to 2" thick.
Nylon has a lot of great properties but comes with several disadvantages for machining. It is a low- (Nylon 6) to medium- (Nylon 6/6) cost plastic. It is pretty strong with tensile strength of about 11-12 KSI, but it is softer than acetal
and much more hygroscopic. It tends to warp easily and it seems to move around when you machine it. It is terrible to deburr as it is very stringy and leaves behind fuzz unless cutters are razor sharp. It does have great toughness, wear,and abrasion resistance which is probably why it is harder to machine. Unless there is a specific property that is needed with Nylon, we generally advise acetal be used.
Polycarbonate has superior impact resistance. It is a medium-cost plastic. It comes in clear and black grades as well as myriad filled grades. It machines well, although like acrylic, can also be susceptible to chipping. It is a pretty stable material with very low water absorption and holds higher tolerances well. It has pretty good thermal resistance and resists deformation up to 265 degrees F. It also vapor polishes very well and can give excellent finishes. It has tensile strength of about 10 KSI and is available in round bar up to 6" and plate up to 2" thick.
All plastics are less stable than metals. They have much higher thermal expansion and are affected by humidity if they are hygroscopic. These factors need to be taken in to account when designing and tolerancing your part. 
It is not uncommon to have a machine shop machine and verify a part is in tolerance. A few weeks later, when the parts are inspected at the customer, the results are different, which may result in an out-of-tolerance condition. Care should be taken to reduce the chance of this happening. The best solution is to change the geometry to be more stable or increase the tolerance to allow natural variations to occur without becoming out of tolerance. Suggestions for improving the dimensional stability include designing thicker sections, adding ribs, allowing large fillets and adding corner radii. In contrast to injection molding recommendations, it isn't important to have the wall thicknesses be uniform and thin. Unless weight is a big factor, thick solid sections will be more stable and less costly to machine. The length to diameter recommendations for vertical cutting tools are similar to that for aluminum, if not a little less stringent. 
In general we recommend allowing approximately +/- 0.02mm of tolerance per inch of part size. There are some lower performance plastics that would need approximately double that much to be consistently easy to process and stay in tolerance. Because of the lower thermal (up to 10x greater than metal) and geometric stability of plastics it can be more costly to achieve higher tolerances.
Many plastics are available with glass fill. This can add significant cost to the material itself as well as the machining cost; the glass fibers are very abrasive and tool wear becomes a significant factor. Depending on the amount of machining involved and tolerances required, some cutting tools may last less than one part, which adds considerable complexity to the manufacturing process. The higher the percentage of glass, the stronger, stiffer, and more dimensionally stable the parts will be. However, they will likewise be more expensive as well. 
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