Polycarbonate is a thermoplastic resin containing -[ORO-CO]-links in main chain of molecule. According to different ester groups in molecular structure, it can be divided into aliphatic, cycloaliphatic, and aliphatic-aromatic types. Among them, aromatic polycarbonates are of practical value, and bisphenol A polycarbonate is important, molecular weight is usually 30,000-100,000.

 

Polycarbonate, English name Polycarbonate, PC for short. PC is an amorphous, odorless, non-toxic, highly transparent, colorless or slightly yellow thermoplastic engineering plastic. It has excellent physical and mechanical properties, especially excellent impact resistance, high tensile strength, flexural strength, and high compressive strength; low creep and dimensional stability; good heat resistance and low temperature resistance, stable mechanical properties, dimensional stability, electrical properties and flame retardancy in a wide temperature range, which can be used for a long time at -60~120℃; no obvious melting point, in a molten state at 220-230℃; due to high rigidity of molecular chain, resin melt viscosity is high;
Low water absorption, small shrinkage, high dimensional accuracy, good dimensional stability, low permeability of film; self-extinguishing material; stable to light, but not resistant to ultraviolet light, good weather resistance; 0il-resistant, acid-resistant, not resistant to strong alkalis, oxidizing acids and amines, ketones, soluble in chlorinated hydrocarbons and aromatic solvents, prone to hydrolysis and cracking in water for a long time. Disadvantage is that due to poor fatigue strength, stress cracking is easy to occur, poor solvent resistance, and wear resistance is poor.
PC can be injection molded, extruded, molded, blow molded, thermoformed, printed, bonded, coated, and machined. An important processing method is injection molding.
It must be pre-dried before molding, and moisture content should be less than 0.02%. Addition of trace moisture at high temperature will cause product to produce white turbid color, silver wire and bubbles. PC has considerable forced high elastic deformation ability at room temperature. With high impact toughness, it can be cold-formed, cold-drawn, cold-rolled, etc. Molecular weight of PC for extrusion should be greater than 30,000. Gradually compressed screws should be used, with a length-to-diameter ratio of 1:18~24, and a compression ratio of 1:2.5. Extrusion blow molding, injection-blowing, injection-stretch-blow molding can be used to form high-quality, highly transparent bottles.
There are many kinds of PC alloys, which can improve defects of PC melt viscosity (processability) and easy stress cracking of products. PC forms alloys or blends with different polymers to improve material properties. Specifically, there are PC/ABS alloys, PC/ASA alloys, PC/PBT alloys, PC/PET alloys, PC/PET/elastomer blends, PC/MBS blends, PC/PTFE alloys, PC/PA alloys, etc. Advantages have two material performance advantages and reduce costs. For example, in PC/ABS alloys, PC mainly contributes high heat resistance, better toughness and impact strength, high strength, flame retardancy, and ABS can improve formability, surface quality, and reduce density.
Three major application areas of PC are glass assembly industry, automobile industry, electronics and electrical industry, followed by industrial machinery parts, optical discs, packaging, computers and other office equipment, medical and health care, films, leisure and protective equipment.
PC can be used as door and window glass. PC laminates are widely used for protective windows in banks, embassies, detention centers and public places, as well as aircraft cabin covers, lighting equipment, industrial safety baffles and bulletproof glass.
PC board can be used for various signs, such as gasoline pump dials, car dashboards, warehouse and outdoor commercial signs, point sliding indicators. PC resin is used in car camera systems, instrument panel systems and interior decoration systems, used as headlight cover, front and rear baffle of reinforced car, reflective mirror frame, door frame cover, operating rod sheath, spoiler.
PC is used as junction boxes, sockets, plugs and sleeves, gaskets, TV conversion devices, connection parts of communication cables under telephone line brackets, switch boxes, telephone exchanges, switchboard components, relay shells. PC can be used as low load, parts, used in household appliances motors, vacuum cleaners, shampoos, coffee makers, toasters, power tools handles, various gears, worm gear bushings, guide gauges, refrigerator shelves. PC is an ideal material for CD storage media, PC bottles (containers) are transparent, light in weight, good impact resistance, resistant to washing with certain high temperatures and corrosive solutions, are used as recyclable bottles (containers).

 

PC and PC alloy can be used as computer racks, casings and auxiliary machines, printer parts.
Modified PC is resistant to high-energy radiation sterilization, cooking and baking sterilization, can be used for blood sampling equipment, blood oxygenators, surgical instruments, kidney dialyzers, etc. PC can be used as helmets and safety helmets, protective masks, sunglasses and sports goggles.
PC film is widely used in printing charts, medical packaging, and membrane commutators.
PC-based plastics will hydrolyze and break bonds, reduce molecular weight, and decrease physical strength even when they encounter very low moisture. Therefore, before molding process, moisture content of polycarbonate should be strictly controlled below 0.02% to avoid reduction of mechanical strength of molded product or abnormal appearance of bubbles and silver streaks on the surface. In order to avoid abnormalities caused by moisture, polycarbonate should be dried by a hot air dryer for three to five hours at a temperature of 120℃ before processing, or moisture should be treated by a desuperheating dryer, but dehumidified air should have a dew point of -30℃ at the entrance of funnel.
In order to meet needs of various injection molding processes, polycarbonate has different melt index specifications. Usually melt index is between 5 and 25g/10min, which can be applied to injection molding. However, optimal processing conditions vary considerably depending on type of injection molding machine, shape of molded product, specifications of polycarbonate, and should be adjusted according to actual situation.

Mold clamping pressure:
Multiply 0.47 to 0.78 tons per square centimeter of projected area of finished product (or 3 to 5 tons per square inch).
Machine size:
Weight of finished product is about 40-60% of capacity of injection molding machine. For example, when capacity (ounce) of machine is expressed in polystyrene, it needs to be reduced by 10%, 1 ounce = 28.3 grams.
Screw:
Screw length should be at least 15 diameters long, and L/D of 20:1 is the best. Compression ratio should be 1.5:1 to 30:1. Stop valve at the front end of screw should adopt a sliding ring type, and resin flow gap should be at least 3.2mm.
Nozzle:
Tip opening should be at least 4.5mm (diameter). If weight of finished product is 5.5KG or more, nozzle diameter should be 9.5mm or more. In addition, tip opening needs to be 0.5 to 1mm smaller than gate diameter, and the shorter segment, the better, about 5mm.

Melting temperature and mold temperature:
The best molding temperature setting is related to many factors, such as size of injection molding machine, screw configuration, mold and molded product design, and molding cycle time. Generally speaking, in order to gradually melt plastic, a lower temperature is set in rear section/feeding zone of material pipe, and a higher temperature is set in the front section of material pipe. However, if screw design is improper or L/D value is too small, reverse temperature setting is also possible.
In terms of mold temperature, high-temperature molds can provide better surface appearance, with less residual stress, and can fill thinner or longer molded products. Low mold temperature can shorten molding cycle.

 

Screw rotation speed:
40 to 70 rpm is recommended, but it depends on machine and screw design.
Injection pressure: In order to fill mold as quickly as possible, injection pressure is as large as possible, generally about 850 to 1400KG/CM², up to 2400KG/CM².
Back pressure:
Generally, the lower setting, the better, but for uniform feeding, it is recommended to use 3 to 14KG/CM².
Injection speed:
Rate of fire has a lot to do with gate design. When using direct gates or edge gates, a slower rate of fire should be used to prevent sun glare and wave flow marks. In addition, if thickness of finished product is more than 5mm, in order to avoid bubbles or dents, slow injection will help. Generally speaking, principle of fire rate is that thinner is fast, and thicker is slow.
Switch from injection molding to pressure holding. Holding pressure should be as low as possible to avoid residual stress in molded product. Residual stress can be removed or reduced by annealing; condition is 120 to 130℃ for about 30 minutes to one hour.
Barrel cleaning
(1) Under molding temperature of polycarbonate, add cleaning material (PP or PE is often used), and shoot 20 to 30 times continuously.
(2) Move machine back and continue to empty cleaning material until injected cleaning material starts to expand and foam.
(3) Reset temperature of barrel to 200 to 230℃.
(4) Continue to spray cleaning material until melting temperature of cleaning material reaches 260℃ and surface looks very clean.