The equipment is essentially identical and, for the most part, so are the tools and techniques. But even with the similarities, cutting plastic is not the same as cutting metal. For that matter, cutting one plastic is often not the same as cutting another plastic, and shop operators need to do their homework if they want to avoid costly mistakes.
"The decision to get into machining plastics should not be taken lightly," said Kevin Delk, president of Delmar Company, a plastics machining and fabrication business in Lakeville, Minnesota (www.delmarcompany.com). "You need to get the wildcards out of the process. Since most of the plastics are sensitive to changes in temperature and humidity, keeping humidity low and temperatures stable in your facility is important. You also need good people. Most runs are low-volume so you need operators who can do set up and think on their feet. Speeds and feeds are just as dependent on the operator as they are on the material.
"The material is often inconsistent and you have to play with it. We've put a lot of rejects in our local landfill learning how to do it right. If you've never worked with a particular plastic, you need to get some and experiment with it before you make any quotes."
Quoting on a plastics machining job is often more difficult than quoting on a metal machining job because of the size of the raw stock. While metals are usually sold by the pound, plastics are sold by the sheet and come in several different sheet sizes. Yield can vary substantially from one sheet size to the next depending on part size and layout.
Cost per square foot can vary dramatically—a specified plastic could cost $8 per square foot or $800 per square foot. And just because the thickness specified falls between two standard thicknesses doesn't mean the cost will—a nonstandard thickness can carry a very heavy price premium. So the only way to be sure about material cost is to get a firm quote from a plastics supplier.
Larry Phippen, owner of Adapt Plastics in Loves Park, Ill., (www.adaptplastics.com) agrees with Delk about the difficulties: "We've been machining and selling bulk plastics for more than 30 years," said Phippen, and it's still just as much an art as a science. Most plastics have a high coefficient of thermal expansion and if your shop is not airconditioned a plastic will not cut the same way in summer that it does in winter. "Most of the plastic we get is annealed or double annealed but we still get variations sometimes. You have to watch it and be able to outguess it. The data sheets usually have information on machining and that's a good place to start, but the best way to really learn the material is to use it and experiment with it."
One machinability indicator on data sheets is a plastic's Durometer hardness value. The lower the reading the softer and more difficult a plastic is to machine because the material will tend to be pushed out of the way rather than cut cleanly. A very soft plastic may have to be frozen in order to machine cleanly.
Another important factor is if the plastic contains glass or ceramic fibers. Both cause heavy wear on tools. While most plastics can be easily machined with high-speed steel tools, fiber reinforced plastics usually are machined with carbide or hardened steel tools because of the higher wear factor. In all cases, the tools need to be very sharp. Experienced operators liken machining plastics to machining aluminum where a sharp tool is much more important than a hard-edged tool.
Machining fiber reinforced plastics also is more difficult than machining other types of plastics because the fibers create excessive, and possibly harmful dust when machined. Operators must wear safety masks, and the excessive dust tends to clog coolant and airline nozzles. The fibers are usually hydroscopic and absorb some of the coolant. This requires longer drying time and may even require heating to drive the coolant out of the fibers.
Absorbing moisture from coolant or from the air is a common property of many plastics. Some plastics will only absorb so much moisture on an exposed surface and then stabilize, so when thicker material is machined the plastic absorbs moisture and swells on the newly exposed surfaces after the machining. If tight tolerances are required, the parts must be rough machined, allowed to stabilize, then finish machined to final tolerance.
Other plastics will absorb moisture and swell while they are being machined ( usually from the coolant), then shrink as they dry out. Where tight tolerances must be held on these plastics, air, rather than water or other liquids, should be used for cooling.
When it comes to determining the feeds and speeds to use, the only consistent rule is that every operator has to find what works for him on his equipment. Some shops use high spindle speeds while others use low spindle speeds. Some use high feed rates and others go slower.
"You can't have a lot of rubbing action," said Larry Phippen, "because that will build up heat and cause the plastic to melt and stick to the tool face. We find that at low speed we get too much heat buildup so we usually run our spindles at 15,000 rpm to 20,000 rpm. This speed also gets chips out of the way and prevents long chips from forming and wrapping around the tool. We also try to go through the material as fast as possible so that the tool doesn't sit there heating up the material."
"We run at slower spindle speeds and greater feed rates than steel," said Bogdan Falat, Vice-president of BEM Mold, Inc. in Bensenville, Ill., (www.bemcnc.com). "At higher speeds the plastics will melt because of the higher heat. If we go too slow and the drill sits there too long it builds up too much heat. If we go too fast we get hard chips that can melt and stick to the drill or the part."
Shops like BEM Mold that machine both metal and plastic have an additional factor to deal with — contamination of the plastic parts by metal chips. Metal chips that are of little concern when machining metal parts can easily contaminate and destroy a run of plastic parts. This means that operators must do a much more thorough job of cleaning their machines when switching from metal to plastic than they would if they were switching from one metal to another. Staging and handling areas and equipment must also be thoroughly cleaned.
It is certainly possible to improve a shop's bottom line by taking on plastics machining work, but care and forethought are needed before the first production block of plastic goes into a chuck. Failing to do so can easily become a recipe for disaster.
