Pro Tips - September 2009
Designing and Machining Plastic Parts
Last month's newsletter discussed a situation where both our customer and Pro CNC missed some details on an ambiguous drawing, which was an expensive mistake. This month we are going to introduce the topic of designing and machining plastic parts. This is a vast topic so this month's newsletter will just be an overview which we will elaborate on in future newsletters.
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 (off white) 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. Be aware it is also hygroscopic which means it will absorb moisture from the air affecting dimensional stability. 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 (widely known by the brand name 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 is also known as 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 good impact strength but not as good as polycarbonate. 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 can be purchased in a MIL-P-5425 grade which is preshrunk to improve it's dimensional stability. In our experience, if acrylic is to be painted after machining, then an additional annealing step is required to ensure it doesn't shrink further when the paint is cured. If threaded inserts are to be installed, it is advisable to rough-machine the material, install the inserts, anneal and then finish machine to reduce the chance of cracking induced by stress of the inserts being installed. Acrylic responds quite well to vapor polishing or flame polishing in applications where machining marks can not 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. (Shameless plug: Pro CNC is particularly good at turn-key acrylic parts!)
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. There is also a grade of Nylon called MD or MDS. This grade has molybdenum disulfide in it which makes it more wear resistant than regular nylon and improves the machinabilty .
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.
Ultem is a translucent amber color and is a very high performance engineering thermoplastic. It is an expensive material but offers lots of great properties. It can handle very high temperatures: up to 340 degrees F. It is very stable dimensionally and has very low moisture absorption. It is also rigid but this rigidity contributes to the tendency to chip which is one of its drawbacks for machining. It is also slightly more abrasive than some plastics which increases tool costs. Ultem 1000 is the basic unfilled variety, with 2300 being the 30% glass version. It is extremely strong with about 17 KSI tensile strength and is available in round bar up to 4" and plate up to 2" thick.
Nearly all the materials above also come in more exotic flavors, such as carbon filled, stainless steel fiber filled, blends of different plastics, abrasion resistant, static-dissipative, tinted colors and even aramid fiber and glass bead filled. There are also many other common machining grades of plastic such as UHMW, HDPE, and PVC. We will discuss these in future issues.
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 +/- 0.001" 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. This is more true as parts get larger and sections get thinner. Sometimes the measurements themselves can cause deflection in the part which leads to erroneous readings. An example would be measuring a large ring with a caliper where the pressure from the caliper will elongate the ring causing it to appear out of tolerance. Non-contact measurement methods can be employed to reduce this problem.
If you are designing a round part that is a bearing, sleeve, or some type of part that will mate with a metal component, consider the application of an average diameter callout. It is common for round parts with thin wall thicknesses to ovalize which may not matter at all to the function of the part but may cause a big hassle for the part inspector. Alternatively, you can specify the type of fit you desire and provide a representative mating part for inspection.
As mentioned above 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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Part of the Month
Every month we feature a really cool part that we have made. September's Part of the Month is this machined acetal housing for a marine application; you may recognize it from the animation in the upper-right corner of each newsletter. It required complex surfacing to create the external geometry that was desired. The windows were all on separate angles so it required machining from several orientations. It was then bead blasted for a smooth and even finish.
See more cool parts here. >>
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