Machining plastics: POM, PA and PEEK in the price

Is plastic just light aluminium?

Engineering plastics such as POM machine quickly, so many spreadsheets and quoting portals price plastic as light aluminium: a price per kilo, a machine hour, done. With POM that often holds. With PEEK the price comes out too low, because the rest, the extra setup and the risk of scrap appear nowhere. The money sits in what happens around the cut.

Plastic behaves differently from metal. The machining guide from Mitsubishi Chemical Group puts its thermal expansion at up to ten times that of metals, and notes that it sheds heat more slowly and is far more elastic. The part stays warm, gives under the clamp and springs back once released.

A shop that machines plastic well plans more than cutting time: roughing, resting, re-clamping, finishing and measuring at an agreed temperature. An ERP system keeps last time's price, not why that PEEK bush needed an extra setup. That knowledge belongs in the estimate lines, where anyone can check it.

POM, PA and PEEK: where is the difference?

POM is the easiest: its machinability is good, it holds its size and takes up little moisture. PA machines easily too, but swells with moisture. PEEK machines well, if less easily, and costs enough that every chip counts. PTFE is soft and yields to the cut: expect a rougher surface and burrs, so deburring is often a separate step. Fibre-reinforced grades wear tools down.

What does the price per kilo do to the material line?

Plastic is bought as semi-finished stock: sheet, round bar or block, in fixed sizes. So the material line starts from the stock, not the net weight: the next sheet thickness or bar diameter up, plus saw allowance and a clamping margin.

With POM and PA, material is rarely the biggest line: a generous size costs little and leaves room to take material off both sides evenly. With PEEK every extra millimetre of bar is money, and an offcut is worth keeping. Choosing the size is covered in sizing the blank.

Price with your own purchase price per grade: unfilled, glass-filled and medical grades of one polymer are three lines with three prices. How that differs from a market reference is in purchase price or market price.

How do you keep a plastic part to size?

In plastic, much of the scrap comes from residual stress and distortion. Producers anneal stock shapes to relieve stress, but heavy machining from one side puts new stress in, a cause Mitsubishi Chemical Group lists alongside dull tools and excess heat.

The standard method is roughing and finishing with a pause between: down to an even allowance, cool and rest, re-clamp, then a light final cut. Tight flatness may need an intermediate anneal; Ensinger and Mitsubishi Chemical Group both advise finishing to size only after it.

Resting is lead time, not cycle time. The part may sit on the shelf overnight while the machine runs other work. It costs no machine hour, only a second setup. Put it into cycle time and you overprice; leave it out and you deliver late.

Inspection happens at an agreed temperature. ISO 1 sets 20 °C as the reference temperature for length measurement, and with plastic a few degrees matter. With PA, so does moisture: Ensinger states that 3 percent moisture uptake changes dimensions by about 0.5 percent, half a millimetre on a 100 mm bush. Agree the condition for the tolerances.

How do you clamp plastic without distorting it?

Plastic gives under the clamp: clamped too hard, it measures right in the machine and wrong on the inspection table. Ensinger advises the lowest possible clamping pressure on a flat, well-supported workpiece. Mitsubishi Chemical Group warns that a part clamped too tightly deforms or springs back.

All workholding costs setup time or tooling, not machining time. Put it on its own setup line, divided by the batch size. That is why a small plastic batch comes out dear per part.

• Vacuum table for flat sheet: the whole face carries the load, but the underside must be flat and seal well.
• Soft jaws, milled or turned to the shape of the part: the clamping force spreads, making them costs setup time.
• A collet instead of a jaw chuck for thin bushes: even pressure all round, less risk of an oval bore.
• A fixture for a batch: more cost up front, faster per part.

Worked example: one bush in POM and in PEEK

Take a bush of Ø 40 by 60 mm, 20 parts, turned from bar. Both go in two stages: roughing, re-clamping and finishing. PEEK gets slightly longer roughing, a rest before finishing and a larger scrap reserve.

The difference sits in two lines. In the example, material costs fifteen times as much in PEEK, so the risk allowance for scrap weighs more too: one scrapped bush loses the block and every hour in it. The machining differs by two euros per part.

Blake will ground that reserve in your own post-calculation per material, in what was actually scrapped. Until then it is a line you set yourself and can defend.

Which questions go back to the customer?

A drawing for a plastic part often leaves open what metal takes for granted. Ask these before you price; the rest of a complete request is in the complete request.

• Which grade: unfilled, filled, medical or food contact?
• Which colour, and may it vary slightly between batches?
• At which temperature and, for PA, which moisture condition do the tolerances apply?
• Will the part be used outdoors or in sunlight?
• Is a material certificate required?
• Are there requirements for cleanliness, deburring or packaging?
• Do inserts, bonding or marking follow after machining?