Polycarbonate Plate, 20% Glass Filled

Twenty percent glass fibre roughly doubles the stiffness of polycarbonate and takes the dimensional movement out of it. Flexural modulus rises to 600,000 to 620,000 psi against about 340,000 unfilled, tensile strength is published from 11,000 to 16,000 psi, flexural strength from 16,700 to 19,000 psi, and thermal expansion drops to 1.5 x 10^-5 in/in/F. Heat deflection temperature climbs to 295F at 264 psi and long-term service to 266F. Hardness is Rockwell M87. Specific gravity is 1.33, so it is about 11% heavier than unfilled stock.

Two things to understand before specifying it, because the trade copy usually leaves both out. First, glass filling collapses notched impact: notched Izod falls to about 2.0 ft-lb/in, from 12 to 18 for unfilled. That is roughly an eightfold loss, so this is a stiff, stable, dimensionally tight grade rather than a tough one, and if you chose polycarbonate for impact resistance this is not the grade. Second, it is opaque - no light transmission claim applies. Published tensile strength also disagrees sharply between suppliers, 11,000 against 16,000 psi, so size to the lower figure unless you have a certificate.

It is used for close-tolerance machined parts, structural brackets and housings, fixtures that must not move with temperature, and parts replacing metal where stiffness rather than toughness is the requirement.

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.