Polycarbonate

Polycarbonate (PC) is an amorphous thermoplastic, most often made from bisphenol A. It combines transparency (light transmission of about 88 to 90% at a thickness of a few millimeters) with very high impact strength and heat resistance above 120 °C, which makes it one of the most widely used engineering plastics. A familiar example is the multiwall sheet used to roof pergolas and greenhouses.

PC has a glass transition temperature of about 145 to 150 °C and an HDT at 1.8 MPa of typically 125 to 140 °C. Tensile modulus is 2.2 to 2.5 GPa, tensile strength around 60 to 65 MPa, elongation at break about 100% or more and density about 1.2 g/cm³. In thin-walled parts, notched impact strength is much higher than in ABS and an order of magnitude higher than in PMMA (acrylic): on impact, PC tends to dent rather than crack. It stays tough below freezing, and molding shrinkage is only about 0.5 to 0.7%.

Polycarbonate is processed by injection molding (melt temperature usually 280 to 320 °C, mold 80 to 120 °C), by extrusion of sheet, film and profiles, and by thermoforming. Pellets and sheet must be dried before hot processing, or bubbles and silver streaks form. PC machines and polishes well, but use sharp tools and compatible coolants. For 3D printing there are PC and PC/ABS filaments (nozzle usually 260 to 310 °C, bed 100 to 120 °C, heated chamber). Photopolymer resins sold as "PC-like" only imitate polycarbonate.

The closest alternatives are PMMA and PETG. PMMA is clearer and harder, resists scratching and UV radiation better and cuts cleanly with a laser, but it is brittle. PETG is cheaper and easier to form, but it softens at about 70 °C and is less tough. PC/ABS blends balance the properties of PC against the easier processing and lower cost of ABS, and PC/PBT blends add better chemical resistance.

Choose polycarbonate where transparency must come with resistance to impact and breakage: machine guards and windows, face shields and visors, safety glasses, light fixture covers and LED diffusers, headlamp lenses, clear lids and instrument housings. Opaque grades and PC/ABS blends are used for electronics enclosures, electrical boxes and automotive interior parts where ABS lacks the heat resistance or toughness. In prototyping, machined and polished PC suits clear functional parts.

The main weakness of PC is environmental stress cracking. A part with molded-in stress, a press-fit insert or an overtightened screw can crack after contact with solvents, some cleaning agents, oils, plasticizers from PVC, adhesives or threadlockers, so check compatibility in advance. PC scratches easily and yellows in sunlight without UV protection, so use UV-protected, hard-coated grades outdoors and for optical parts. Long-term contact with hot water, steam or alkalis causes hydrolysis.

In design, avoid sharp internal corners, notches and unnecessarily thick walls, because notched impact strength falls sharply as notches get sharper and walls thicker. For food-contact parts, check the current rules on bisphenol A in your target market, including EU legislation. In your RFQ, specify the grade (transparency, UV stabilization, flame retardancy, hard coating) and, for machined parts, whether to anneal them to relieve stress.