Because of its light weight, good toughness and easy molding. Low cost and other advantages, so in the modern industry and daily products, more and more plastics instead of glass, especially in the optical instrument and packaging industry, the development is particularly rapid. However, due to the requirement for transparency, the wear resistance is high, and the impact toughness is better, so the composition of the plastic, the process of the injection molding process, equipment. Molds, etc., must do a lot of work to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics), the surface quality is good, so as to meet the requirements of use.
Transparent plastics currently used on the market are polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed PMMa) and polycarbonate (codenamed PC). Polyethylene terephthalate (code name PET), transparent nylon. aS (acrylonitrile-styrene copolymer), polysulfone (codename PSF), etc., of which we are most exposed is PMMa. PC and PET plastics, due to limited width, the following three plastics as an example to discuss the characteristics of transparent plastics and injection molding process.
First, the performance of transparent plastic
Transparent plastic must first have high transparency, secondly, it has certain strength and wear resistance, can resist impact, heat-resistant parts are better, chemical resistance is better, water absorption rate is small, and only in this way can it meet transparency. Requires and remains unchanged for a long time.
Comparing PCs is an ideal choice, but it is mainly because of the high price of raw materials and the injection molding process. Therefore, PMMa is still the main choice, (for general requirements), and PPT can get good because of stretching. Mechanical properties, so it is mostly used in packaging and containers.
Second, the common problems that should be paid attention to during the process of transparent plastic injection molding
Due to the high light transmittance of transparent plastics, the surface quality of plastic products must be strict, and there must be no markings, pores or whitening. Fog, black spots, discoloration, poor gloss and other defects, so the raw materials, equipment throughout the injection process. Molds and even product designs must pay great attention to and impose strict or even special requirements. Secondly, because transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of the products, the process parameters such as barrel temperature, injection pressure and injection speed are often finely adjusted so that the plastic can be filled with mold without Internal stress is generated to cause deformation and cracking of the product.
Below it is prepared in the raw materials. Talk about the equipment and mold requirements, the injection molding process and the raw material handling of the product.
(1) Preparation and drying of raw materials Due to the inclusion of any impurities in the plastic, it may affect the transparency of the product, and therefore, storage and transportation. During the feeding process, attention must be paid to the sealing to ensure that the raw materials are clean. In particular, the raw material contains moisture, which causes deterioration of the raw material after heating, so it must be dried, and in the injection molding, the drying hopper must be used for the feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the raw materials are not contaminated. The drying process is shown in Table 2.
(2) Cleaning of the barrel, screw and its accessories
In order to prevent contamination of raw materials and existing materials or impurities in the recesses of screws and accessories, resins with poor thermal stability are present. Therefore, the screws are cleaned before use and after shutdown to prevent impurities from sticking. 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, causing the solution to fall, 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. (The hopper temperature should be reduced to below 100 °C for PC)
(3) Problems that should be paid attention to in mold design (including product design)
In order to prevent the occurrence of poor back flow, or uneven cooling, resulting in poor plastic molding, 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) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;
c) Gate. The flow channel should be 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) Venting holes. The tank must be sufficient to vent the air and the gas in the melt;
f) Except for PET, the wall thickness should not be too thin, generally not less than lmm.
(4) Problems that should be paid attention to in the injection molding process (including the requirements of injection molding machines)
In order to reduce internal stress and surface quality defects, the following aspects should be noted in the injection molding process.
a) Special screw and injection molding machine with separate temperature control 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) Injection pressure: generally higher 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) injection speed: in the case of filling the mold, generally should be low, it is best to use slow-fast-slow multi-stage injection;
e) holding time and molding cycle: in the case of satisfying product filling, no dents or bubbles; should be as short as possible to minimize the residence time of the melt in the barrel;
f) screw speed and back pressure: under the premise of satisfying the plasticizing quality, it should be as low as possible to prevent the possibility of depreciation;
g) Mold temperature: The cooling of the product is good or bad, which has great influence on the quality. Therefore, 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; when it is used, it should not be greater than 20%.
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 should be cleaned with air and glycerin. Liquid paraffin and the like are heated at 110-135 ° C, depending on the product, and it takes up to 10 hours. PET must undergo a biaxial stretching process in order to obtain good mechanical properties.
Third, the transparent plastic injection molding process
(1) Process characteristics of transparent plastics: In addition to the above common problems, transparent plastics also have some process characteristics, which are described as follows:
1. Process characteristics of PMMa PMMa has high viscosity and poor fluidity. Therefore, it must be filled with high material temperature and high injection pressure. The influence of injection temperature is greater than the injection pressure, but the injection pressure is increased, which is beneficial to improve 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, so 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 process characteristics
PC has high viscosity, high melt temperature and poor fluidity. Therefore, 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. The injection pressure has little effect on the fluidity, but due to the large viscosity, the injection pressure is still large, and the pressure holding time should be as short as possible in order to prevent the internal stress. The shrinkage rate 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 to improve the mold temperature and improve the mold structure and post-treatment to reduce the possibility of cracking. When the injection speed is low, the gate is prone to ripples and other defects, the temperature of the nozzle is controlled separately, the mold temperature is high, and the runner and gate resistance are small.
3, PET process characteristics
PET molding temperature is high, and 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-ducting 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 mold temperature is accurately controlled to prevent warpage. An important factor in deformation is therefore recommended for hot runner molds. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding.
Fourth, the defects and solutions of transparent plastics
Due to the length of the relationship, only the defects affecting the transparency of the product will be discussed here. For other defects, please refer to the company's product manual or other materials. The defects are probably the following:
(1) Silver streak: During the filling and condensing process, the internal stress anisotropy affects the stress generated in the vertical direction, causing the resin to flow upwardly, and the non-flowing orientation produces a different refractive index and produces a flashing silk thread. When it is expanded, it may cause cracks in the product. In addition to the attention in the injection molding process and mold (the best product for annealing treatment. If the PC material can be heated to 160 ° C or more for 3-5 minutes, then naturally cool.
(2) Bubbles: The water vapor and other gases in the resin are not discharged, (during the mold condensation process) or due to insufficient filling, the condensation surface is too fast to condense to form a "vacuum bubble". It overcomes the method.
(3) Poor surface gloss: mainly due to the large roughness of the mold, on the other hand, the condensation is too early, so that the resin can not copy the surface of the mold, all of which causes the surface to have a slight unevenness, which makes the product tarnish. It overcomes the method.
(4) Seismic pattern: refers to the dense corrugation formed from the sprue center. The reason is that the melt viscosity is too large, the front end material has been condensed in the cavity, and then the material is broken through the condensation surface, causing the surface to appear. Grain. It overcomes the method.
(5) Whitening. Haze: mainly caused by dust falling into the raw material in the air or the moisture content of the raw material is too large. It overcomes the method.
(6) White smoke. Black spots: mainly due to plastics in the barrel, due to local overheating, the barrel resin is decomposed or deteriorated.
