Here are ten problems to pay attention to when making PC polycarbonate products.
01 drying of raw materials
PC plastics, even in the presence of very low water will hydrolyze and break bonds, molecular weight and physical strength. Therefore, before the molding process, should strictly control the polycarbonate moisture in 0.02% below, in order to avoid the formation of the mechanical strength or the surface of bubbles, silver and other abnormal appearance.
PC is extremely sensitive to water, so before injection, it must be fully dried to reduce its water content to 0.02% below, PC general drying conditions: 100~120℃, time at least 4 hours.
02 injection temperature
The injection temperature can only be made after considering the shape, size, mold structure, product performance and requirements of the product.
Generally used in the molding temperature between 270~320℃, too high material temperature such as more than 340℃, PC will decompose, the product color becomes dark, the surface appeared silver wire, dark strip, black spots, bubbles and other defects, at the same time the physical and mechanical properties also significantly decreased.
PC is also very sensitive to temperature, melt viscosity with the temperature increase and decreased significantly, the barrel temperature 250~320℃(preferably not more than 350℃), appropriately raising the temperature of the barrel temperature is good for PC plasticizing. When necessary, internal stress annealing, oven temperature 125~135℃, time 2 hours, natural cooling to room temperature.
03 injection pressure
Physical and mechanical properties of PC products, molding shrinkage internal stress has a certain influence on products such as appearance and demoulding of sex has great influence, too low or too high injection pressure can make products appear some defects, the injection pressure control between 80-120 mpa, the thin and long process, complex shape, gate smaller products, in order to overcome the resistance of the melt flow, so that full of cavity in time, to choose high injection pressure (mpa), 120-145, to obtain complete and smooth the surface of the products.
Poor fluidity, high pressure injection is required, but the residual internal stress of the rubber parts should be taken into account (may lead to cracking), injection speed: the wall thickness to medium speed, the wall thin to high speed.
Holding pressure and holding time
The size of the holding pressure and the holding time have a greater impact on the internal stress of PC products: the holding pressure is too small, the small shrinkage effect is easy to appear vacuum bubble or surface shrinkage, the holding pressure is too large, the gate around the easy to produce greater internal stress. In the actual processing, often with high temperature, low pressure method to solve.
The choice of holding time should depend on the thickness of the product, gate size, mold temperature and other conditions, generally small and thin products do not need a long holding time, on the contrary, large and thick products should be a long holding time. The length of holding time can be determined by the test of gate sealing time.
05 injection rate
There is no obvious effect on the performance of PC products. In addition to thin wall, small gate, deep hole, long process products, generally use medium or slow speed processing, the best is multi-stage injection, generally use slow - fast - slow multi-stage injection.
06 mold temperature
Mold temperature control: 85~120℃, generally controlled in 80-100℃ can be, for the shape of complex, thin, higher requirements of the product, can also be raised to 100-120℃, but can not exceed the mold thermal deformation temperature. Die temperature to reduce the difference between die temperature and PC material temperature, can reduce the internal stress of parts.
Screw speed and back pressure
Due to the larger PC melt viscosity, from the favorable plasticizing, favorable exhaust, favorable maintenance of the plastic machine, to prevent the screw load is too large, the screw speed requirements can not be too high, generally controlled in 30-60r/min is appropriate, and the back pressure control in the injection pressure of 10%-15% between the appropriate.
08 use of primary auxiliaries
PC in the process of injection molding to strictly control the use of the release agent, at the same time the use of raw materials can not more than three times, the usage should be about 20%.
PC injection molding, requirements for molds
1) the flow channel should be as thick and short as possible with less bending potential. The flow channel with circular cross section should be used for the purpose of reducing the flow resistance of molten materials.
2) the injection gate can be of any type, but the diameter of the water level shall not be less than 1.5mm.
10 to produce PC products of the molding machine requirements
The maximum injection volume of the required product (including runner, gate, etc.) shall not be greater than 70-80% of the nominal injection volume.
Clamping pressure: 0.47 to 0.78 tons (or 3 to 5 tons per square inch) per square centimeter of finished product projected area.
Machine size: the weight of the finished product is about 40 to 60% of the capacity of the injection molding machine. If the capacity of the machine is expressed by polystyrene (oz), it should be reduced by 10%, 1 oz =28.3 g.
Screw length: the screw length should be at least 15 diameters, with L/D of 20:1 being the best. The compression ratio should be 1.5:1 to 30:1. The stop valve at the front end of the screw shall adopt sliding ring type, and the resin clearance shall be at least 3.2mm.
(credit: vzhusu)
