How to improve the transparency of plastic products during injection molding?
Date: October 22, 2024
Due to its light weight, good toughness, and ease of molding. Due to its low cost advantages, plastic is increasingly being used instead of glass in modern industry and daily products, especially in the fields of optical instruments and packaging industry, with rapid development. However, due to the requirements for good transparency, high wear resistance, and good impact toughness, the composition of the plastic, the entire injection molding process, and the equipment are crucial. A lot of work needs to be done on molds and other materials to ensure that the plastic used to replace glass (hereinafter referred to as transparent plastic) has good surface quality and meets the requirements for use.
At present, the commonly used transparent plastics in the market include polymethyl methacrylate (commonly known as acrylic or organic glass, code named PMMA) and polycarbonate (code named PC). Polyethylene terephthalate (PET), transparent nylon. AS (acrylonitrile styrene copolymer), polysulfone (code PSF), etc., among which PMMA is the one we come into contact with more. Due to limited space, we will use PC and PET as examples to discuss the characteristics and injection molding process of transparent plastics.
1、 Performance of transparent plastics
Transparent plastics must first have high transparency, followed by certain strength and wear resistance, be able to resist impact, have good heat resistance, excellent chemical resistance, and low water absorption rate. Only in this way can they meet the requirements of transparency and remain unchanged for a long time during use. PC is an ideal choice, but due to its high raw material cost and difficult injection molding process, PMMA is still the main choice (for products with general requirements), while PPT is often used in packaging and containers because it needs to be stretched to obtain good mechanical properties.
2、 Common issues to be noted during the injection molding process of transparent plastics
Transparent plastics require strict surface quality requirements due to their high light transmittance, and must not have any patterns, pores, or whitening. Defects such as haze, black spots, discoloration, and poor gloss affect the raw materials and equipment throughout the entire injection molding process. The design of molds and even products requires great attention and strict or even special requirements.
Secondly, due to the high melting point and poor flowability of transparent plastics, in order to ensure the surface quality of the product, it is often necessary to make subtle adjustments to process parameters such as barrel temperature, injection pressure, and injection speed, so that the plastic injection can fill the mold without generating internal stress that may cause product deformation and cracking.
Regarding equipment and mold requirements, injection molding processes, and raw material handling of products, please discuss the precautions to be taken:
● Dry
The preparation and drying of raw materials, due to the presence of any impurities in plastics, may affect the transparency of the product, thus affecting storage and transportation.
During the feeding process, it is necessary to pay attention to sealing to ensure that the raw materials are clean. Especially since the raw materials contain moisture, heating them can cause them to deteriorate, so they must be dried, and during injection molding, a drying hopper must be used for feeding. It should also be noted that during the drying process, the input air should be filtered and dehumidified to ensure that it does not contaminate the raw materials.
● Cleaning of the barrel, screw and its accessories
To prevent raw material contamination and the presence of old materials or impurities in the screw and accessory depressions, especially resins with poor thermal stability, screw cleaning agents should be used to clean all parts before and after use to prevent impurities from sticking. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.
When the machine is temporarily shut down, in order to prevent the raw materials from staying at high temperatures for a long time and causing degradation, the temperature of the dryer and the barrel should be lowered. For example, the barrel temperature of PC, PMMA, etc. should be lowered to below 160 ℃. (The temperature of the hopper should be reduced to below 100 ℃ for PC)
