Even when PC-based plastics encounter very low moisture, they will break bonds due to hydrolysis, decrease molecular weight and decrease physical strength. Therefore, before molding, the moisture content of polycarbonate should be strictly controlled below 0.02%, so as to avoid the reduction of mechanical strength of molded products or abnormal appearance such as bubbles and silver lines on the surface.
In order to avoid the abnormal situation caused by moisture, polycarbonate should be dried by a hot air dryer for more than three to five hours before processing, and the temperature should be set at 120℃, or the moisture should be treated by a desuperheating dryer, but the dehumidified air should have a dew point of -30℃ at the funnel entrance.
In order to meet the needs of various injection molding processes, polycarbonate has different melt index specifications. Generally, the melt index between 5 and 25g/10min is suitable for injection molding. However, the optimum processing conditions are quite different due to the different types of injection molding machines, shapes of molded products and specifications of polycarbonate, which should be adjusted according to the actual situation.
Selection points of injection molding machine
Clamping pressure:
The projected area of the finished product is multiplied by 0.47 to 0.78 tons per square centimeter (or multiplied by 3 to 5 tons per square inch).
Machine size:
The weight of the finished product is about 40-60% of the capacity of the injection molding machine. If the capacity (oz) of the machine is expressed by polystyrene, it needs to be reduced by 10%, and 1 oz =28.3 grams.
Screw:
The length of screw should be at least 15 diameters long, and its L/D ratio is 20: 1. The compression ratio should be 1.5: 1 to 30: 1. The stop valve at the front end of the screw should adopt sliding ring type, and the resin flowable clearance should be at least 3.2 mm.
Nozzle:
The tip opening should be at least 4.5MM (diameter), and if the finished product weight is more than 5.5KG, the nozzle diameter should be more than 9.5 mm. In addition, the tip opening should be 0.5 to 1MM less than the gate diameter, and the shorter the section, the better, about 5 mm.
Key points of molding conditions
Melting temperature and mold temperature:
The optimum setting of molding temperature is related to many factors, such as the size of injection molding machine, screw configuration, design of mold and molded product, and molding cycle time. Generally speaking, in order to make the plastic melt gradually, a lower temperature is set in the rear section/feeding area of the feed pipe, while a higher temperature is set in the front section of the feed pipe. However, if the screw design is improper or the L/D value is too small, the reverse temperature setting is also acceptable.
In terms of mold temperature, the high temperature mold can provide better surface appearance, and the residual stress will be smaller, and the thinner or longer molded products will also be filled. While low mold temperature can shorten the molding cycle.
Screw rotation speed:
40 to 70rpm is recommended, but it needs to be adjusted depending on the machine and screw design.
Injection pressure: in order to fill the mold as quickly as possible, the higher the injection pressure, the better, generally about 850 to 1400 kg/cm, up to 2400 kg/cm.
Back pressure:
Generally, the lower the setting, the better, but for uniform feeding, it is recommended to use 3 to 14 kg/cm.
Injection speed:
The firing rate has a great relationship with the gate design. When using direct gate or edge gate, in order to prevent the sun glow phenomenon and wave mark phenomenon, the slower firing rate should be used. In addition, if the thickness of the finished product is more than 5MM, it is helpful to shoot at a slow speed to avoid bubbles or depressions. Generally speaking, the principle of firing rate is fast for thin people and slow for thick people.
Switching from injection molding to pressure keeping, the pressure keeping should be as low as possible to avoid residual stress of molded products. The residual stress can be removed or alleviated by annealing. Under conditions of 120 to 130.d egree. C. for about 30 minutes to one hour.
Barrel cleaning
(1) at the molding temperature of polycarbonate, add a cleaning material (PP or PE is often selected) and continuously spray it for 20 to 30 times.
(2) Move the machine back, and shoot the cleaning materials in the air until the shot cleaning materials start to expand and bubble.
(3) Reset the barrel temperature to 200-230℃.
(4) Continue to empty the cleaning material until the melting temperature of the cleaning material reaches 260℃ and the appearance looks clean and transparent
