What issues should be paid attention to when producing PC polycarbonate products?

Time:2022-04-24 08:33:24 / Popularity: / Source:

PC has excellent performance, high transparency, good impact toughness, creep resistance, and wide operating temperature range. Process characteristics of PC: melt viscosity is less sensitive to shear rate, but is sensitive to temperature, has no obvious melting point, high melt viscosity, resin is easy to hydrolyze at high temperature, and product is easy to crack.
For these characteristics, we must pay special attention to different treatments:
1. To increase fluidity of melt, not by increasing injection pressure but by increasing injection temperature;
2. Runner and gate of mold are required to be short and thick to reduce pressure loss of fluid, and at the same time to have a higher injection pressure;
3. Resin needs to be fully dried before molding to control its moisture content below 0.02%;
4. In addition, insulation measures should be taken for resin during processing to prevent re-absorption of moisture;
5. Not only a reasonable product design is needed, but also molding process should be properly mastered. For example, increasing mold temperature and post-processing product can reduce or eliminate internal stress. According to different conditions of product, adjust process parameters in time.
PC polycarbonate products 

Injection molding process of polycarbonate PC

Raw material drying

PC plastics, even in the presence of very low moisture, will cause hydrolysis and bond breakage, decrease in molecular weight and physical strength. 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.
PC is extremely sensitive to water, so it must be fully dried before injection to reduce its water content to less than 0.02%. PC general drying conditions: 100~120℃, at least 4 hours;

Injection temperature

Injection temperature must be integrated with shape, size, and mold structure of product. Product performance, requirements and other aspects can only be made after consideration.
Generally, temperature used in molding is between 270~320℃. If material temperature is too high, if it exceeds 340℃, PC will decompose, color of product will become darker, defects such as silver wire, dark stripes, black spots, and bubbles will appear on the surface. At the same time, physical and mechanical properties are also significantly reduced.
PC is also very sensitive to temperature. Melt viscosity decreases significantly with increase of temperature. Barrel temperature is 250~320℃ (preferably not more than 350℃). Properly increasing temperature of barrel is good for PC plasticization. If necessary, internal stress annealing, oven temperature 125~135℃, time 2 hours, natural cooling to normal temperature.

Injection pressure

It has a certain impact on physical and mechanical properties of PC products, internal stress, molding shrinkage, etc. It has a greater impact on appearance and mold release of product. Too low or too high injection pressure will cause certain defects in product. Generally injection pressure is controlled between 80-120MPa. For products with thin walls, long processes, complex shapes, and smaller gates, in order to overcome resistance of melt flow and fill cavity in time, higher injection pressure (120-145MPa) is selected to obtain a complete and smooth surface product.
Poor fluidity, high-pressure injection is required, but large residual internal stress of rubber parts (which may cause cracking) should be taken into account. Injection speed: Medium speed is used for wall thickness, high speed is used for thin wall.
PC polycarbonate products 

Holding pressure and holding time

Size of holding pressure and length of holding time have a greater impact on internal stress of PC products. If holding pressure is too small and feeding effect is small, it is easy to cause vacuum bubbles or surface shrinkage. If holding pressure is too large, large internal stress is likely to occur around gate. In actual processing, it is often solved by high material temperature and low holding pressure.
Selection of holding time should depend on thickness of product, gate size, mold temperature, etc. Generally, small and thin products do not need a long holding time. On the contrary, holding time of large and thick products should be longer. Length of holding time can be determined by test of gate sealing time.

Injection speed

It has no obvious influence on performance of PC products. Except for thin-walled, small gates, deep holes, and long-flow products, medium-speed or slow-speed processing is generally used, and multi-stage injection is best. Generally, a slow-fast-slow multi-stage injection method is used.

Mold temperature

Mold temperature control: 85~120℃, generally controlled at 80-100℃. For products with complex shapes, thinner and higher requirements, it can also be increased to 100-120℃, but heat deformation temperature of mould cannot be exceeded. Mold temperature is high to reduce difference between mold temperature and PC material temperature, which can reduce internal stress of part.

Screw speed and back pressure

Because of high viscosity of PC melt, it is beneficial to plasticization, exhaust, and maintenance of plastic machine, to prevent screw from being overloaded, rotation speed of screw should not be too high. Generally, it is appropriate to control it at 30-60r/min. It is advisable to control back pressure between 10-15% of injection pressure.

Use of additives

In injection molding process of PC, use of release agent should be strictly controlled, use of recycled materials should not exceed three times, and use amount should be about 20%.
PC injection molding has higher requirements for molds:
1) Design runners as thick and short as possible, with fewer bends, use circular cross-section runners for grinding and polishing to reduce flow resistance of melt.
2) Injection gate can adopt any form of gate, but diameter of inlet water level is not less than 1.5mm.
Requirements for plastic machine for production of PC products:
It is required that maximum injection volume of product (including runners, gates, etc.) should not be greater than 70-80% of nominal injection volume;
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 machine uses polystyrene to express its capacity (ounces), 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, resin flow gap should be at least 3.2mm.

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