IAs transparent plastics, first of all, we should ensure high transparency, second, strength, impact resistance, wear resistance, heat resistance, chemical resistance, water absorption should be small, only in this way can we meet the requirements of transparency in use.
For the material selection of transparent plastics, PC is an ideal choice, but its price is high and injection molding process is difficult, so PMMA is often selected, while PPE is often used in packaging and containers because it needs to be stretched to get good mechanical properties.
Injection process characteristics of three common transparent plastics
I.PMMA
The viscosity of PMMA is large and its fluidity is slightly poor, so it must be high temperature and injection pressure, in which the effect of injection temperature is greater than that of injection pressure. The increase of injection pressure is beneficial to the improvement of product shrinkage.
The injection temperature range is wide, the melting temperature is 160 ℃, the decomposition temperature is 270 ℃, so the material temperature adjustment range is wide, and the process performance is good. Therefore, to improve the fluidity, we can start from the injection temperature.
Poor impact, poor wear resistance, easy to scratch, easy to crack, so we should improve the mold temperature, improve the condensation process, to overcome these defects.
Ii.PC
PC has high viscosity, high melt temperature and poor fluidity, so it must be injected at a high temperature (between 270 ~ 320 ℃). Relatively speaking, the range of material temperature adjustment is narrow, and the processability is inferior to PMMA.
The injection pressure has little effect on the fluidity, but because of the high viscosity, the injection pressure should still be high. In order to prevent the internal stress, the pressure 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 easy to crack, so it is better to improve the fluidity by increasing the temperature rather than the pressure, and the possibility of cracking can be reduced by increasing the temperature of the mold, improving the structure of the mold and post-treatment.
When the injection speed is low, the gate is prone to corrugation and other defects. The temperature of the radiation nozzle should be controlled separately, the mold temperature should be high, and the resistance of the runner and gate should be small.
III. process characteristics of pet
Pet has a high molding temperature and a narrow range of material temperature adjustment (260-300 ℃), but after melting, it has a good fluidity, so its processability is poor, and it is often necessary to add a delay prevention device in the nozzle.
The mechanical strength and properties are not high after injection, and the properties can only be improved by drawing process and modification.
Accurate temperature control is an important factor to prevent warpage. It is suggested to use hot runner mold.
How to choose transparent materials
1, 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.
2, Lighting materials
Mainly used as lampshade, commonly used PS, modified PS, as, PMMA, PC, etc.
3, Optical instrument material
1. Hard mirror body: mainly CR-39 and J.D;
2. Contact lenses: commonly used HEMA.
4, Glass material
1. Traffic glass: commonly used PMMA and PC;
2. Architectural glass: commonly used PVF and pet.
5, Solar materials
Commonly used PMMA, PC, gf-up, FEP, PVF, Si, etc.
6, Optical fiber material
The core layer is PMMA or PC, and the cladding layer is fluorine-containing olefin polymer and fluorine-containing methyl methacrylate.
7, CD material
Commonly used PC and PMMA.
8, Transparent packaging material
Surface hardened PMMA, FEP, EVA, EMA, PVB, etc.
Precautions in the injection process of transparent plastics
Due to the high transmittance of transparent plastics, the surface quality of plastic products is strictly required, and there shall be no speckles, pores, and whitening. Because of the defects of haze, black spot, discoloration and poor gloss, there are strict and even special requirements for the design of raw materials, equipment, molds and even products in the whole injection molding process.
(1) Preparation and drying of raw materials
In the process of feeding, pay attention to sealing to ensure that the raw materials are clean, especially the moisture in the raw materials. After heating, the raw materials will deteriorate, so it must be dried. During injection molding, the dry hopper must be used for feeding. Also pay attention to the drying process, the input air is best filtered, dehumidified, to ensure that raw materials will not be polluted.
(2) Cleaning of barrel, screw and its accessories
In order to prevent the raw material pollution and the existence of old materials or impurities (especially in the case of resin with poor thermal stability) in the screw and its accessories depression, all components shall be cleaned before use and after shutdown, so that they are free of impurities. When there is no screw cleaner, PE, PS and other resins can be used to clean the screw.
In case of temporary shutdown, in order to prevent the raw materials from falling due to long residence time under high temperature, the temperature of dryer and barrel should be reduced, such as PC, PMMA and other barrel temperature should be reduced below 160 ℃.
(3) Problems in mold design (including product design)
In order to prevent poor forming, surface defects and deterioration of products due to poor flow and uneven cooling, the following points should be paid attention to in mold design:
a) The wall thickness shall be uniform as far as possible, and the angle of demoulding shall be large enough;
b) Smooth transition, to prevent sharp corners, sharp edges, especially PC products must not have gaps;
c) The gate flow channel shall be as wide and short as possible. The gate position shall be set according to the contraction and condensation process, and the cold well shall be added if necessary;
d) The mould surface should be smooth and low roughness (preferably less than 0.8);
e) Vent holes and grooves must be sufficient to discharge air and melt gas in time;
f) Except pet, the wall thickness should not be too thin, generally not small
