Because the plastic has light weight, good toughness, easy to form. The advantages of low cost, so in modern industry and daily products, more and more use of plastics instead of glass, especially in the optical instruments and packaging industry, is developing particularly rapidly. However, due to the requirement of good transparency, high wear resistance, and good impact toughness, the composition of the plastic, injection molding process, equipment. Mould, etc., have to do a lot of work to ensure that these plastic(hereinafter referred to as transparent plastic) used to replace glass, the surface quality is good, so as to meet the requirements of use.
The transparent plastics commonly used in the market are polymethyl methacrylate(commonly known as methacrylate or organic glass, code PMMA) and polycarbonate(code PC). Polyethylene terephthalate(code PET), transparent nylon. AS(acrylene-styrene copolymer), polysulfone(code name PSF), etc., of which we are most exposed to PMMA. Due to the limited space of PC and PET three plastics, the following takes these three plastics as an example to discuss the characteristics of transparent plastics and injection molding processes.
Performance of transparent plastics
Transparent plastics must have high transparency first, followed by a certain degree of strength and wear resistance, can resist shocks, heat resistant parts are good, chemical resistance is excellent, and water absorption is small. Only in this way can it be used to meet the requirements of transparency. Long-term change. PC is an ideal choice, but mainly due to the high cost of its raw materials and the difficulty of injection molding, it still uses PMMA as the main choice(for commonly required products), and PPT has to be stretched to obtain good mechanical properties. Therefore, it is mostly used in packaging and containers.
Common problems that should be noticed during the injection of transparent plastics
Due to the high light permeability of transparent plastics, it is inevitable that the surface quality of plastic products must be strict, and there must be no markings, stomata, and whitening. Fog Halo, black spots, discoloration, poor luster and other defects, so throughout the injection molding process on raw materials, equipment. Mould, even the design of products, should be very careful and put forward strict or even special requirements.
Secondly, because transparent plastics have a high melting point and poor liquidity, in order to ensure the surface quality of the product, it is often necessary to make minor adjustments in the process parameters such as barrel temperature, injection pressure, and injection speed, so that the plastic can be filled with molds. It does not produce internal stress and cause product deformation and cracking.
To the equipment and mold requirements, injection molding process and raw material processing of the product, to discuss the matters that should be noted:
The preparation and drying of raw materials due to the presence of any trace of impurities in the plastic may affect the transparency of the product, and therefore storage and transportation.
During the feeding process, attention must be paid to sealing and ensuring that the raw materials are clean. In particular, the raw material contains moisture, which causes the raw material to deteriorate after heating. Therefore, it must be dried, and when molding, the drying Hopper must be used. It is also important to note that during the drying process, the air input should preferably be filtered and dehumidified to ensure that it will not pollute the raw materials.
Cleaning of tubes, screws and accessories
In order to prevent the contamination of raw materials and the presence of old materials or impurities in the screw and accessories depressions, the resin with poor thermal stability is particularly present. Therefore, screw cleaning agents are used to clean the pieces before use and after shutdown, so that they must not stick to impurities., When there is no screw cleaning agent, PE, PS and other resin can be used to clean the screw.
When temporary shutdown, in order to prevent the raw material from staying at a high temperature for a long time and causing a drop, the temperature of the dryer and the barrel should be reduced, such as the temperature of the PC, PMMA and other tubes should be reduced below 160 °C. (Hopper temperature should be below 100 °C for PC)
Problems in die design(including product design)
In order to prevent the appearance of poor back flow, or uneven cooling resulting in poor plastic forming, resulting in surface defects and deterioration;
Generally in the mold design, should pay attention to the following points:
Wall thickness should be as uniform as possible, demoulding slope should be large enough;
The transitional component should be gradual. Smooth transition to prevent sharp corners. Sharp edge generation, especially PC products must not have gaps;
The gate. The channel should be as wide and short as possible, and the gate position should be set according to the shrinkage condensation process. If necessary, cold well should be added.
The surface of the mold should be smooth and low roughness(preferably less than 0.8);
Exhaust. The tank must be sufficient to discharge air and gas in the melt in a timely manner;
Except PET, wall thickness should not be too thin, generally not less than lmm.
Issues to Note in Injection Process(including requirements for injection molding machines)
In order to reduce internal stress and surface quality defects, attention should be paid to the following aspects in the injection process:
The injection molding machine with special screw and separate temperature control nozzle should be selected;
The injection temperature should be higher under the premise that the plastic resin does not decompose;
Injection pressure: Generally higher, to overcome the defect of large melt viscosity, but the pressure is too high will produce internal stress resulting in demolding difficulties and deformation;
Injection speed: In the case of satisfying filling mode, generally low, preferably slow-fast-slow multi-stage injection;
Pressure holding time and forming period: In the case of satisfying product filling, no depression or bubbles are generated; It should be as short as possible to minimize the time spent on the fuse;
Screw speed and back pressure: under the premise of satisfying the plasticized quality, it should be as low as possible to prevent the possibility of decompression;
Die temperature: The cooling of the product is good or bad, and it has a great impact on the quality. Therefore, the die temperature must be able to accurately control the process. If possible, the mold temperature should be higher.
In order to prevent the deterioration of the upper surface quality, the use of demoulding agents is as little as possible when molding; When used back materials must not be greater than 20 <UNK>.
For products other than PET, reprocessing should be carried out to eliminate internal stress, PMMA should be dry at 70-80 °C for 4 hours; PC should be in clean air, glycerin. Liquid paraffin is heated at 110-135 °C, depending on the product, and takes up to 10 hours. PET must go through a two-way stretching process to obtain good mechanical performance.
III. Injection molding process of transparent plastics
Process characteristics of transparent plastics
In addition to the above common problems, transparent plastics also have some process characteristics, which are described below:
1, PMMA process characteristics
PMMA has large viscosity and slightly poor liquidity. Therefore, it must be injected with high material temperature and high injection pressure. The effect of injection temperature is greater than that of injection pressure, but the injection pressure is increased, which is conducive to improving the shrinkage rate 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 regulation range is wide and the process is good. Therefore, improving liquidity can start with injection temperature. The impact is poor, wear resistance is not good, easy to cut flowers, easy to crack, so should raise the mold temperature, improve the condensation process, to overcome these defects.
2, PC process characteristics
The PC has large viscosity, high melting temperature, and poor fluidity. Therefore, it must be molded at a higher temperature(between 270 and 320 °C). The material temperature regulation range is relatively narrow and the process is not as good as PMMA. Injection pressure has less effect on fluidity, but due to the large viscosity, it is still necessary to inject pressure. In order to prevent internal stress, the holding time must 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. Therefore, it is advisable to improve the fluidity by increasing the temperature rather than the pressure, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the dips are prone to ripples and other defects. The radiation mouth temperature must be controlled separately, the mold temperature should be high, and the flow channel and the gate resistance should be small.
3, PET process characteristics
The PET molding temperature is high, and the material temperature regulation range is narrow(260-300 °C), but after melting, the fluidity is good, so the process is poor, and the anti-ductile device is often added to the nozzle. Mechanical strength and performance after injection are not high, must be through the tensile process and modification to improve performance.
Die temperature control is accurate, is to prevent warping. Therefore, it is recommended to use hot channel die. The temperature of the mould should be high, otherwise it will cause the surface gloss difference and the difficulty of demoulding.
Defects and solutions for transparent plastic parts
There are probably the following defects:
Due to the influence of the Anisotropy of internal stress during filling and condensation, the stress generated in the vertical direction causes the resin to flow in orientation, while the non-flow orientation produces different refractive index and produces flash silk lines. When it expands, May cause cracks in the product. In addition to the injection process and mold attention, the best product for annealing treatment. If the PC material can be heated above 160 °C for 3-5 minutes, it can be cooled naturally.
The water gas and other gases that are mainly in the resin can not be discharged,(in the process of die condensation) or due to insufficient filling, the condensation surface is too fast and condenses to form a vacuum bubble.
Poor surface gloss
The main reason is that the mold roughness is large, and on the other hand, the condensation is too early to make the resin unable to copy the surface of the mold. All of these make the surface of the mold slightly uneven and cause the product to lose luster.
It refers to the dense ripples formed from the direct gate. The reason is that due to the excessive viscosity of the melt, the front end material has been condensed in the cavity, and later the material broke through this condensation surface, causing the surface to appear.
White fog Halo
It is mainly caused by dust falling into the raw material in the air or the content of the raw material is too large.
White smoke black spots
Mainly due to the plastic in the barrel, due to local overheating caused by the decomposition or deterioration of the barrel resin and formed