Polycarbonate Rod, Machine Grade

Machine-grade polycarbonate is the thick, stress-relieved stock a shop actually machines - supplied already relieved so parts hold tolerance instead of moving after the first cut. Tensile strength at yield is 8,400 to 10,000 psi on a tensile modulus of 320,000 to 340,000 psi, flexural strength is 13,100 to 14,100 psi on a flexural modulus of 320,000 to 345,000 psi, and compressive strength is 12,500 psi. Hardness is Rockwell M70 to M77, R118. Water absorption is 0.15% in 24 hours. Specific gravity is 1.20.

On impact, be careful which number you take. Published notched Izod for nominally identical unfilled polycarbonate runs from 10 to 18 ft-lb/in, because ASTM D256 does not fix the specimen geometry and polycarbonate is sharply notch-sensitive: 18 for extruded sheet at 0.125 inch, 16 for machine and window grade plate, 10 for one supplier's machine grade. A figure near 2 ft-lb/in belongs to a filled or flame-retardant grade, not to unfilled stock, and you will see it quoted wrongly. For a real part, unnotched and instrumented impact predict behaviour better: unnotched Izod does not break, and instrumented impact by ASTM D3763 is 47 ft-lb at 0.125 inch.

Temperature, stated honestly. Heat deflection temperature for plate and rod is published at 275F at 264 psi and 280F at 66 psi by ASTM D648 - the extruded sheet figure is 270F at 264 psi, so do not mix them - but that is a short-duration softening test and must not be used as a service limit. Published maximum continuous service temperature in air is 240 to 250F depending on the publisher, with intermittent exposure to 275F published. Parts under sustained load should be derated well below that, and anything load-bearing near the top of the range needs creep data.

It is machined into manifolds and sight glasses, machine guards, pump and valve components, fixtures and jigs, bezels and housings, and any transparent part that has to take a knock. Natural stock is transparent with a mill surface rather than a polished one - if appearance matters, window grade is the optically selected version at the same mechanical properties.

Machining and annealing. Polycarbonate machines cleanly with sharp tools and good chip clearance, and it is notch-sensitive, so avoid sharp internal corners, scribed lines and deep tool marks - that sensitivity is the same effect that makes published notched impact figures scatter so widely. Machine-grade plate and rod are supplied stress-relieved and most machined parts need no annealing. Consider annealing when you remove a large proportion of the stock, when you remove it mostly from one face, or when the part will see chemical exposure, because residual machining stress plus a marginal chemical is the main cause of stress crazing. The cycle we work to for unfilled polycarbonate is a ramp to 275F over about four hours, a hold of 30 minutes per 1/4 inch of thickness and a cool-down at 50F per hour, fixtured to prevent distortion, with critical dimensions finish-machined afterwards. Glass-filled grades anneal at 290F.

Chemical compatibility and cleaning, which is where most polycarbonate failures start. Clean it with mild diluted dish soap and a soft microfibre cloth, or an ammonia-free glass cleaner. Do NOT use ammoniated glass cleaner - the common blue kind - because ammonia attacks polycarbonate, and the resin maker's own testing shows 0.1% ammonia is enough to do damage in the presence of water. Also avoid abrasive or high-alkaline cleaners, scrapers and squeegees, dry rubbing, and solvent in direct sun. Isopropyl alcohol diluted 50/50 with water handles oil and tar. Acetone, MEK, methylene chloride, chloroform, toluene, xylene and aromatic fuels all attack it, and so do amines and the plasticisers that migrate out of flexible PVC - which is why PVC gasket, tape and cable tie contact is a real failure mode. The important subtlety is that chemical resistance is not a fixed property: stress cracking only develops where the part is in tension, so a cold-bent, over-torqued or tightly framed part will fail with chemicals a compatibility chart calls safe.