When producing PC polycarbonate products what should be paid attention to ?#PC#PC extrusion#PC material

Raw material drying

For PC plastics,  even when encountering very low moisture, they will be hydrolyzed to break bonds, reduce molecular weight and reduce physical strength. Therefore, before molding, the water content of polycarbonate should be strictly controlled below 0.02%, so as to avoid the reduction of the mechanical strength of the molded product or the abnormal appearance of bubbles and silver streaks on the surface.
PC is extremely sensitive to water, so it must be fully dried before injection molding to reduce its water content to less than 0.02%. PC general drying conditions: 100~120°C, at least 4 hours;

Injection temperature

The injection temperature must consider the shape, size and mold structure of the product. It can only be made after considering the product performance, requirements and other aspects.

Generally, the temperature used in molding is between 270 and 320°C. If the material temperature is too high, if it exceeds 340°C, PC will decompose, the color of the product will become darker, and defects such as silver threads, dark stripes, black spots, and bubbles will appear on the surface. , and the physical and mechanical properties are also significantly reduced. the

PC is also very sensitive to temperature. The melt viscosity decreases significantly with the increase of temperature. The barrel temperature is 250~320°C (preferably not more than 350°C). Properly increasing the temperature of the barrel temperature is good for PC plasticization. If necessary, internal stress annealing, oven temperature 125~135 ℃, time 2 hours, natural cooling to room temperature.

Injection pressure

It has a certain influence on the physical and mechanical properties of PC products, internal stress, molding shrinkage, etc., and has a greater impact on the appearance and demoulding of the product. Too low or too high injection pressure will cause some defects in the product. The injection pressure is controlled between 80-120MPa. For products with thin walls, long processes, complex shapes and small gates, a higher injection pressure (120 -145MPa). Thus a complete and smooth product is obtained.

Poor fluidity, high-pressure injection is required, but it is necessary to take into account the residual internal stress of the plastic part (may cause cracking), injection speed: medium speed for wall thickness, high speed for thin wall.

Holding pressure and holding time

The size of the holding pressure and the length of the holding time have a great influence on the internal stress of the PC product.  It is easy to generate large internal stress around the mouth. In actual processing, it is often solved by high material temperature and low pressure holding method.

The choice of holding time should depend on the thickness of the product, gate size, mold temperature, etc. Generally, small and thin products do not need a long holding time, on the contrary, the holding time for large and thick products should be longer. The length of the holding time can be determined by the test of the gate sealing time.

Injection speed

It has no obvious impact on the 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, preferably multi-stage injection, and slow-fast-slow is generally used. multi-stage injection method.

Mold temperature

Mold temperature control: 85~120°C, generally controlled at 80-100°C, for products with complex shapes, thinner, and higher requirements, it can also be increased to 100-120°C, but it should not exceed the heat deformation temperature of the mold. The mold temperature is high to reduce the difference between mold temperature and PC material temperature, which can reduce the internal stress of the part.

Screw speed and back pressure

Due to the high viscosity of the PC melt, it is beneficial for plasticization, exhaust, and maintenance of the plastic machine, and prevents the screw from being overloaded. It is advisable to control the back pressure between 10-15% of the injection pressure. 

Use of additives

In the injection molding process of PC, the use of release agent should be strictly controlled. At the same time, the use of recycled materials should not exceed three times, and the usage amount should be about 20%.

PC injection molding has higher requirements for molds:

1) Design a flow channel that is as thick and short as possible, with less bending, and use circular cross-section runners and runner grinding and polishing to reduce the flow resistance of the melt.
2) The injection gate can adopt any form of gate, but the diameter of the water inlet should not be less than 1.5mm.

Requirements for plastic machines producing PC products:

It is required that the maximum injection volume of the product (including runners, gates, etc.) should not be greater than 70-80% of the nominal injection volume;

Clamping pressure: 0.47 to 0.78 tons per square centimeter of projected area of the finished product (or 3 to 5 tons per square inch);

Machine size: The weight of the finished product is about 40 to 60% of the capacity of the injection molding machine. For example, if the capacity of the machine is expressed in polystyrene (ounces), it needs to be reduced by 10%. 1 ounce = 28.3 grams.

Screw: The length of the screw should be at least 15 diameters long, and its L/D is 20:1. The compression ratio is preferably 1.5:1 to 30:1. The stop valve at the front end of the screw should be a sliding ring type, and the resin flow gap should be at least 3.2MM.