What is polycarbonate?

Polycarbonate, PC, is an amorphous thermoplastic available glass-clear or coloured, with very high impact strength. You find it in machine guards, sight windows, covers and housings. It machines well but is sensitive to stress cracking: internal stress combined with certain chemicals can cause fine cracks, sometimes only days or weeks later. Its surface is softer than glass and scratches more easily.

What causes stress cracking in polycarbonate?

Stress cracking needs stress and an aggressive medium together. The stress comes from the stock, from heat during machining, from heavy clamping and later from assembly, such as an overtightened screw. Media that can cause cracking include certain coolants, solvents, cleaners and adhesives. Machining is therefore often done dry, with compressed air or with water, and with sharp tools. Annealing after machining lowers the risk, and residual stress can be made visible under polarised light.

How does polycarbonate work in practice?

On a clear part much of the time goes into everything around the machining: protective film left on as long as possible, scratch-free handling, polishing edges that must be clear and packing each piece. Scrap from a scratch or a fine crack often shows up late, after machining or at the customer, and then the whole operation is lost. So ask which faces and edges must be optically clear, and which media the part will meet in service.

What is polycarbonate often confused with?

Polycarbonate is not PMMA, also called acrylic: PMMA resists scratches better, is optically clearer and polishes to a better clarity, but breaks far more easily. Nor is PC glass: it is lighter and impact-resistant, but softer at the surface and less resistant to solvents.