Metal parts, unlike plastic ones, typically require the use of oil-based cutting fluids. And no matter how conscientious the metal machinist is, it is nearly impossible to clean every bit of oil from a machine before using it to make a plastic part. Because of this, equipment used to manufacture metal parts, even if used for metal only, can occasionally contaminate plastic parts with those oil-based cutting fluids. Many plastics are highly sensitive to petroleum-based cutting fluids and will degrade if they come into contact with them. Also, many plastics are hydroscopic and will absorb the cutting oils. The result is a part that may pass initial inspections, but will degrade over time and fail in the field.
It is not just cleaning oils off a machine that is difficult for a metal machinist, but also clearing every tiny metal fragment that may remain on a machine used to make metal parts. If the plastic material being machined is soft, residual metal fragments can become embedded in the plastic machined parts. Again, the metal fragment may not cause a problem initially, but over time it can cause the plastic to degrade and stop performing properly.
Problems can also arise in something as simple as how a metal machinist holds a plastic part. With plastic machined parts, the plastic is usually held with vices. Machinists who don’t specialize in plastic have a tendency to hold the plastic the same way they do metal—but this can be too tight for a plastic. As a consequence, when a drill goes into the plastic, the material flexes a bit, which can put stresses on the part that might not surface until later. A plastics machining company would know that, and be able to prevent that from happening. It is a subtle difference in manufacturing, but one that can turn into a huge problem down the road.
Of course, a plastics expert should be more knowledgeable than a metal machinist about the variety of plastics materials available and what uses they’re best suited for. This level of knowledge is especially important in fields like medical device technology, where manufacturers often use plastics that are less common than those used for other purposes. A plastics expert can help designers and manufacturers sort through materials according to factors such as sensitivity to humidity, abrasion resistance and thermo-sensitivity.