Due to the high light transmittance of transparent plastics, the requirement for the surface quality of plastic products must be strict. And there must not be any markings, pores or whitening, haze, black spots, discoloration, poor gloss and other defects. Therefore, we should pay attention to the design of raw materials, equipments, molds, and even products in the entire injection molding process. What’s more, we should stick to the strict and special requirements.
Secondly, Most transparent plastics have the defects of high melting point and poor fluidity. It is often necessary to make fine adjustments in the process parameters such as high temperature, injection pressure, injection speed, etc. to ensure the surface quality of products. Also, when the plastic is injected, the mold can be filled without internal stress which can cause product deformation and cracking.
Notes for the preparation raw material
Raw material preparation and drying
Any impurities contained in the plastic may affect the transparency of the product. Therefore, during storage, transportation and feeding, we must pay attention to the sealed package to ensure the clean raw materials.
In particular, the raw material contains moisture, which will cause the raw material to deteriorate after heating. Thus, it must be dry, and when injection molding, a dry hopper must be used for feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified so that it will not pollute the raw materials.
Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and the existence of old materials or impurities in the recesses of the screw and accessories, especially the resin with poor thermal stability, so before use and after shutdown, all parts should be cleaned with a screw cleaning agent to prevent impurities from sticking. When there is no screw cleaning agent, you can use PE, PS and other resins to clean the screw.
When the machine temporarily shut down, to prevent the raw material staying at high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be reduced. For example, the barrel temperatures for pc, pmma and other should be lower than 160 ℃. (The hopper temperature should be lower than 100℃ for pc)
Mold design (including product design)
To prevent problems like poor return flow, or poor plastic molding causing by uneven cooling, surface defects and deterioration, generally we should pay attention to the following points when designing molds.
- The wall thickness should be as uniform as possible and the draft angle should be large enough.
- The transition part should be gradually smooth to prevent sharp corners and sharp edges. PC products must not have gaps.
- Gate and flow channel should be as wide and thick as possible. Besides, the gate position should be set according to the shrinking and condensing process. And add a cold material well if necessary.
- The surface of the mold should be smooth and low in roughness (preferably less than 0.8).
- The vent groove must be able to exhaust air and melt gas timely.
- Except PET, the wall thickness should not be too thin, generally not less than 1 mm.
Injection molding process and injection molding machine
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process:
- Use special screw, injection molding machine with separate temperature control nozzle.
- The injection temperature should be higher if the plastic resin does not decompose.
- Injection pressure: generally higher to overcome the defect of high melt viscosity. But overly high pressure will produce internal stress and causemold releasing difficulty and deformation.
- Injection speed: in the case of the mold filling, it is generally low and is best to adopt slow-fast-slow-multistage injection.
- Pressure holding time and molding cycle: If the product mold filling doesn’t generate depressions or bubbles. It should be as short as possible to minimize the residence time of the melt in the barrel.
- Screw speed and back pressure: Under the premise of satisfying the quality of plasticization, it should be as low as possible to prevent the possibility of solution drop.
- Mold temperature: The cooling of the product has a great impact on the quality. Therefore, the mold temperature must be able to accurately control its process. If possible, the mold temperature should be higher.
To prevent the deterioration of the surface quality, we should use the mold release agent as little as possible when injection molding. And when using recycled materials, it should not be greater than 20%.
Except PET, products should be post-treated to eliminate internal stress. PMMA should be dried in a hot air cycle of 70-80t for 4 hours. While PC should be in clean air, glycerin and liquid paraffin etc. heating 110-135 ℃. The time depends on the product, the maximum need more than 10 hours. The PET must go through the bidirectional stretching process so that it can get good mechanical properties.