Due to the advantages of light weight, good toughness, easy forming and low cost, plastics are increasingly used to replace glass in modern industry and daily products, especially in the application of optical instruments and packaging industry. However, it is often required that the plastic used must have the characteristics of good transparency, high wear resistance and good impact toughness. Therefore, high requirements are put forward for the composition of plastic, the process, equipment and mold of the whole process of injection molding, so as to ensure that the plastic used to replace glass (hereinafter referred to as transparent plastic) has good surface quality, so as to meet the requirements of use.
At present, the transparent plastics commonly used in the market include polymethylmethacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), transparent nylon, as (acrylonitrile styrene copolymer), polysulfone (PSF), etc, Among them, PMMA, PC and pet are the three plastics that we are most exposed to. Due to the limited space, we will take these three plastics as an example to discuss the characteristics and injection molding process of transparent plastics.
1、 Properties of transparent plastics
Transparent plastics must have high transparency first, then certain strength and wear resistance, impact resistance, good heat resistance, excellent chemical resistance and small water absorption. Only in this way can they meet the requirements of transparency and have stable performance. The performance comparison of PMMA, PC and pet is shown in the table below.
Note: because there are many varieties, this is only the average value. The actual data of different varieties are different. PET data (Mechanical) is the data after stretching.
It can be seen from the data in Table 1 that PC is an ideal choice, but because of its expensive raw materials and difficult injection molding process, PMMA is still used more (for products with general requirements), while pet is mostly used in packaging and containers because it needs to be stretched to get good mechanical properties.
2、 How to choose transparent materials
Commodity materials
1. Transparent films: PE, PP, PS, PVC and pet for packaging, PE, PVC and pet for agriculture, etc
2. Transparent plates: PP, PVC, pet, PMMA, PC, etc
3. Transparent pipes: PVC, PA, etc
4. Transparent bottles: PVC, pet, PP, PS, PC, etc.
Lighting materials
Mainly used as lampshade, commonly used PS, modified PS, as, PMMA, PC, etc.
Optical instrument material
1. Hard mirror body: mainly CR-39 and J.D
2. Contact lenses: commonly used HEMA.
Glass material
1. Traffic glass: commonly used PMMA and PC
2. Architectural glass: commonly used PVF and pet.
Solar materials
Commonly used PMMA, PC, gf-up, FEP, PVF, Si, etc.
Optical fiber material
The core layer is PMMA or PC, and the cladding layer is fluorine-containing olefin polymer and fluorine-containing methyl methacrylate.
CD material
Commonly used PC and PMMA.
PMMA, FEP, EVA, EMA, PVB, etc.
3、 Common problems in transparent plastic injection
Transparent plastics, due to its high light transmittance, must also have strict requirements on the surface quality of plastic products, without any defects such as mottling, air hole, whitening, fog halo, black spot, discoloration, poor gloss, etc., so in the whole process of injection molding, we should pay great attention to and put forward strict or even special requirements for the design of raw materials, equipment, molds and even products. Secondly, due to the high melting point and poor fluidity of most transparent plastics, in order to ensure the surface quality of products, injection molding is often carried out at a higher temperature, and the injection pressure, injection speed and other process parameters also need to be slightly adjusted, so that the injection molding can not only fill the mold, but also generate internal stress and cause deformation and cracking of products.
The following is about the preparation of raw materials, the requirements of equipment and mold, injection molding process and raw material processing of products.
1. Preparation and drying of raw materials
As any impurity in the plastic may affect the transparency of the product, it is necessary to pay attention to sealing during storage, transportation and feeding to ensure the cleanness of raw materials. In particular, the raw material contains moisture, which will cause deterioration of the raw material after heating, so it must be dried, and when injection molding, the feeding must use a drying hopper. It should also be noted that in the drying process, the input air should be filtered and dehumidified to ensure that the raw materials will not be polluted. The drying process is as follows:
2. Cleaning of barrel, screw and its accessories
In order to prevent the raw material pollution and the existence of old materials or impurities in the screw and its accessories depression, especially the resin with poor thermal stability, the accessories shall be cleaned with screw cleaner before use and after shutdown, so as to prevent the impurities from sticking. When there is no screw cleaner, the screw can be cleaned with PE, PS and other resins. In case of temporary shutdown, in order to prevent the raw materials from staying too long under high temperature and causing degradation, the temperature of dryer and barrel shall be reduced. For example, the temperature of barrel such as PC and PMMA shall be reduced to below 160 ℃ (the hopper temperature shall be reduced to below 100 ℃ for PC).
3. Problems in mold design (including product design)
In order to prevent backflow or uneven cooling from causing poor plastic forming, surface defects and deterioration, the following points should be paid attention to in mold design.
