As a
transparent plastic , first of all, to ensure high transparency, secondly, strength, impact resistance, wear resistance, heat resistance, chemical resistance, water absorption is small, in order to meet the transparency requirements in use. For the selection of transparent plastics, PC is an ideal choice, but its high price and difficult injection molding process, so PMMA is often used, and PPE can be used in packaging and containers because it has to be stretched to get good mechanical properties. .
Injection molding process characteristics of three commonly used transparent plastics
1.PMMA
PMMA has high viscosity and slightly poor fluidity. It must have high material temperature and high injection pressure, and the injection temperature has a greater influence than the injection pressure. Increased injection pressure helps to improve product shrinkage.
The injection temperature range is wide, the melting temperature is 160 ° C, and the decomposition temperature is 270 ° C. Therefore, the material temperature adjustment range is wide and the processability is good. Therefore, improving fluidity can be started from the injection temperature.
Poor impact, poor wear resistance, easy to scratch, easy to brittle, it should improve the mold temperature, improve the condensation process, to overcome these defects.
2.PC
PC has high viscosity, high melting temperature and poor fluidity. It must be injection molded at a higher temperature (between 270 and 320 °C). Relatively speaking, the temperature adjustment range is narrower and the processability is not as good as PMMA.
Injection pressure has little effect on fluidity, but due to the high viscosity, it still requires a large injection pressure. In order to prevent internal stress, the pressure holding time should be as short as possible.
The shrinkage is large and the size is stable, but the internal stress of the product is large and easy to crack. Therefore, it is better to increase the temperature instead of the pressure to improve the fluidity, and the possibility of cracking can be reduced by increasing the mold temperature, improving the mold structure and post-treatment.
When the injection speed is low, the gate is prone to ripples and other defects, the temperature of the radiation nozzle should be controlled separately, the mold temperature should be high, and the flow path and gate resistance should be small.
3. Process characteristics of PET
PET molding temperature is high, the material temperature adjustment range is narrow (260-300 ° C), but after melting, the fluidity is good, so the processability is poor, and the anti-duction device is often added in the nozzle.
Mechanical strength and performance are not high after injection, and performance must be improved by stretching and modification.
The accurate control of the mold temperature is an important factor to prevent warpage. It is recommended to use a hot runner mold.
How to choose transparent materials
First, daily necessities materials
1. Transparent film: PE, PP, PS, PVC and PET for packaging, agricultural PE, PVC and PET;
2, transparent sheet: use PP, PVC, PET, PMMA and PC;
3, transparent tube: use PVC, PA, etc.;
4, transparent bottles: PVC, PET, PP, PS and PC.
Second, lighting equipment materials
Mainly used as lampshade, commonly used PS, modified PS, AS, PMMA and PC.
Third, optical instrument materials
1. Hard mirror body: mainly use CR-39 and JD;
2, contact lenses: commonly used HEMA.
Fourth, glass materials
1. Traffic glass: commonly used PMMA and PC;
2, architectural glass: commonly used PVF and PET.
Five, solar materials
Commonly used PMMA, PC, GF-UP, FEP, PVF and SI.
Sixth, fiber materials
The core layer is made of PMMA or PC, and the coating layer is a fluorine-containing olefin polymer or a fluorine-containing methyl methacrylate.
Seven, CD material
Commonly used PC, PMMA.
Eight, transparent packaging materials
Surface hardened PMMA, FEP, EVA, EMA, PVB, etc.
Precautions in the process of transparent plastic injection molding
Due to the high light transmittance of transparent plastics, the surface quality of plastic products is strict, and there must be no markings, pores or whitening. Fog, black spots, discoloration, poor gloss and other defects, so there are strict or even special requirements for the design of raw materials, equipment, molds and even products throughout the injection molding process.
(1) Preparation and drying of raw materials
During the feeding process, pay attention to the sealing to ensure that the raw materials are clean, especially the moisture in the raw materials, which will cause the raw materials to deteriorate after heating, so it must be dried. In the case of injection molding, the feed must use a drying hopper. Also pay attention to the drying process, the input air is preferably filtered and dehumidified to ensure that the raw materials are not contaminated.
(2) Cleaning of the barrel, screw and its accessories
In order to prevent contamination of raw materials and the presence of old materials or impurities in the recesses of screws and accessories (except in the case of resins with poor thermal stability), the components should be cleaned before use and after shutdown so that they do not stick to impurities. When there is no screw cleaning agent, the screw can be cleaned with a resin such as PE or PS.
When temporarily stopping, in order to prevent the raw material from staying at high temperature for a long time, the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel such as PC and PMMA should be reduced to below 160 °C.
(3) Problems that should be paid attention to in mold design (including product design)
In order to prevent poor molding, uneven cooling, etc., resulting in poor molding of the product, surface defects and deterioration, generally in the mold design, should pay attention to the following points:
a) the wall thickness should be as uniform as possible, and the draft angle should be large enough;
b) Smooth transition to prevent sharp corners and sharp edges, especially PC products must not have gaps;
c) The gate flow path is as wide and thick as possible, and the gate position should be set according to the shrinkage condensation process, and a cold well should be added if necessary;
d) the surface of the mold should be smooth and the roughness is low (preferably less than 0.8);
e) the venting holes and slots must be sufficient to discharge the air and the gas in the melt in time;
f) Except for PET, the wall thickness should not be too thin, generally not less than 1mm.
(4) Problems that should be paid attention to in the injection molding process (including requirements for injection molding machines)
In order to reduce internal stress and surface quality defects,
In the injection molding process, we should pay attention to the following aspects:
a) An injection molding machine with a dedicated screw and a separate temperature-controlled nozzle should be used;
b) The injection temperature should be higher than the injection temperature on the premise that the plastic resin does not decompose;
c) The injection pressure is generally high to overcome the defect of large viscosity of the melt, but if the pressure is too high, internal stress will cause internal deformation and deformation and deformation;
d) The injection speed should generally be low in the case of filling the mold, and it is best to use [slow-fast-slow] multi-stage injection;
e) The holding time and molding cycle should be as short as possible to satisfy the product filling, no dents or bubbles, to reduce the residence time of the melt in the barrel;
f) screw speed and back pressure should be as low as possible to meet the plasticizing quality, to prevent the possibility of depreciation;
g) The cooling quality of the mold temperature product has great influence on the quality, so the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.
(5) Other issues
In order to prevent the deterioration of the upper surface quality, the mold release agent should be used as little as possible during injection molding, and should not be greater than 20% when using the recycled material.
For products other than PET, the product should be post-treated to eliminate internal stress:
PMMA should be dried in hot air at 70-80 °C for 4 hours;
PC application of clean air, glycerin, liquid paraffin and other heating 110-135 ° C, the time depends on the product, the most need more than 10 hours;
PET must undergo a biaxial stretching process in order to obtain good mechanical properties.
