Overcoming the Challenges of Engineered Plastics in CNC Machining

CNC machining summons images of strangely shaped aluminum plates or steel gears being formed by spinning bits. The machines are huge and complex. But precision production has a softer side, too with plastic machining.

We’ve all seen metal parts replaced with engineered plastics to save money or weight. It’s easy to think that they just shoot molten plastic into a mold and out pop a gear. True enough, a lot of plastics are cast in molds. However, some plastic pieces are machined just like a metal part would be.

Engineering Plastic to Get It Right

Sometimes plastics are used in place of metal for testing purposes. A particularly unusual design might be milled in plastic just to ensure it can be produced by the machine. This prototype often isn’t functional but serves as a proof of concept. Machined plastic parts like this can be used to test the fit with other components. Plastics are cheaper than metal, and they can be machined much more quickly since the CNC mills can take much larger cuts in comparatively soft plastic.

There is, however, a sector of the manufacturing industry that deals specifically with engineering plastics. These parts may be milled from thermoplastics like Nylon or Delrin, or thermoset materials like printed circuit boards.

Plastic Machining is Harder Than It Looks

Engineering plastics is actually more difficult than engineering metal. Plastics melt at very low temperatures, by comparison, making the heat of machining a huge problem. Oil-based coolants are typically off the table because oils can have serious effects on plastics. Chips from machining plastics also pose unique problems. They tend to adhere to everything due to static electricity, so collect in the work area. If chips get into the bit, they can cause gouges in the part or partially melt and fuse to the part. Water-based coolants are used, but strong air jets are more common.

Many plastics used today are infused with glass or fiberglass. This is particularly hard on bits, and diamond-coated bits are often used. Thermosets like G7 are commonly used for electrical insulation, especially in high-temperature settings. This plastic is a silicone/glass cloth laminate, which accounts for its high dielectric properties. It’s not unheard of for CNC machine shops to designate mills for plastic milling only. This ensures that metal dust from previous jobs can’t make its way into the plastic parts and compromise their performance as insulators. Plastics laminated with glass cloth are also prone to delamination during milling -another problem metal fabrication doesn’t have to deal with.

Then there’s the issue of tolerance. Plastics are generally designed to a tolerance of five-thousandths of an inch, as opposed to two thousandths for metal working. Some designers, though, will specify plastic parts with much tighter metal tolerances. We have worked out ways to achieve those tighter tolerances in many circumstances, but it does slow down machining and drives up costs.

The Future of Engineered Plastics is Bright

Macfab handles a lot of plastic machining. Our work in the medical industry requires it. Today’s medical care is replete with single-use medical devices, as well as pumps and sensors that benefit from plastic components. Engineered plastics are also finding a home in prosthetics, saving weight that can then be devoted to robotic functions in the limb.

If anything, the field of plastics is growing faster than metallurgy. Plastics are a perfect complement to metal components in spacecraft. Metal parts will often fuse together in the vacuum of space. Plastics are used as bearings to prevent metal parts from touching, as well as serving a host of other functions. Plastics also provide tremendous weight savings, which is critical for spacecraft. No matter where you look, advancements in engineered plastics are growing, and MacFab is growing right along with it. Look to us when your next machining tasks come up, no matter what materials you intend to use.


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