1. Transparent plastics must first have high transparency, and secondly, they must have certain strength and wear resistance, be able to resist impact, have good heat resistance, excellent chemical resistance, and low water absorption. The need for transparency remains unchanged for a long time. Because of its expensive raw materials and difficult injection molding process, PC still mainly chooses PMMA (for products with general requirements), while PET has to be stretched to obtain good mechanical properties, so it is mostly used in packaging and containers. .
Second, the process characteristics of transparent plastic
1. Process characteristics of PMMA PMMA has high viscosity and slightly poor fluidity, so high material temperature and high injection pressure are required for injection molding. The influence of injection temperature is greater than that of injection pressure, but an increase in injection pressure is conducive to improving the shrinkage of the product. The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature is 270°C. Therefore, the material temperature can be adjusted in a wide range and the processability is good. Therefore, the injection temperature can be used to improve fluidity. However, because acrylic injection molding products have poor wear resistance, are easy to scratch, and are easy to be brittle, the mold temperature should be increased and the condensation process should be improved to overcome these defects.
2. Process characteristics of PC PC has high viscosity, high melting temperature and poor fluidity, so it must be injected at a higher temperature (between 270-320°C). Relatively speaking, the temperature adjustment range of the material is narrow, and the processability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, higher injection pressure is still required. Correspondingly, in order to prevent internal stress, the holding time should be as short as possible. The shrinkage rate is large and the size is stable, but the internal stress of the product is large and it is easy to crack, so it is better to improve the fluidity by increasing the temperature instead of the pressure, and reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-processing. When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.
3. Process characteristics of PET PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ℃), but after melting, the fluidity is good, so the processability is poor, and an anti-dripping device is often added in the nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through the stretching process and modification. Accurate control of the mold temperature is an important factor to prevent warpage and deformation, so it is recommended to use a hot runner mold. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding.
