The characteristics of commonly used plastic transparent raw materials and injection molding process

2021-03-23 12:51:48 admin

The characteristics of commonly used plastic transparent raw materials and injection molding process






Because of its light weight, good toughness and easy forming. The advantages of low cost, so in modern industry and daily products, more and more use to replace glass, especially in optical instruments and packaging industries, the development is particularly rapid. However, due to the requirements for good transparency, high wear resistance parts, and good impact resistance toughness parts, a lot of work must be done to ensure that the correct ingredients, the entire process of injection molding, equipment, injection molding molds, etc. Instead of glass (hereinafter referred to as transparent), the surface quality is good, so as to meet the requirements of use. At present, the transparent ones generally used in the market are polymethyl methacrylate (commonly known as acrylic or plexiglass, code PMMA), polycarbonate (code PC), polyethylene terephthalate (code PET), Transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code-named PSF), etc. Among them, we have the most contact with PMMA, PC and PET. Due to the limited space, the following three examples are used to discuss transparent Characteristics and process of injection molded parts.


1. Transparent performance. Transparency must first have high transparency, and secondly, it must have a certain strength and abrasion resistance, impact resistance, heat resistance, chemical resistance, and water absorption. Only in this way can it be used. It can meet the requirements of transparency and remain unchanged for a long time. Table 1 is listed below to compare the performance of PMMA, PC and PET. Table 1: Comparison of transparency performance Note: (1) Due to the variety of varieties, this is just an average value, and the actual data of different varieties are different. (2) PET data (mechanical aspect) is the data after stretching. From the data in Table 1, it can be seen that PC is an ideal choice, but it is mainly due to its high raw material price and difficult injection molding process, so PMMA is still the main choice (for products with general requirements), and RET has to be stretched. It has good mechanical properties, so it is mostly used in packaging and containers. 2. Common problems that should be paid attention to in the process of transparent injection molded parts. Because the light transmittance is high, the surface quality of the product must be strict, and there should be no markings, pores, whitening, haze, black spots, discoloration, poor gloss, etc. Defects, so in the entire injection molding process, the design of raw materials, equipment, injection molds, and even products must be paid attention to and strict or even special requirements must be put forward. Secondly, because transparency is mostly high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments at higher temperature, injection pressure, injection speed and other process parameters, so that the injection can be filled with the mold without producing Internal stress causes product deformation and cracking. Let's talk about the matters that should be paid attention to in terms of raw material preparation, equipment and injection mold requirements, injection molding process and product raw material processing. (1) Preparation and drying of raw materials Since any impurity contained in it may affect the transparency of the product, it is necessary to pay attention to sealing during storage, transportation and feeding to ensure that the raw materials are clean. In particular, the raw material contains water, which will cause the raw material to deteriorate after heating, so it must be dried, and when the injection molded parts, the material must be fed with a drying hopper. 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 will not be polluted. The drying process is shown in Table 2. Table 2, Transparent drying process (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 the screw and accessories, especially the presence of resin with poor thermal stability, so before use, After the machine is shut down, use the screw cleaner to clean the parts so that no impurities are stuck. When there is no screw cleaner, the screw can be cleaned with resin such as PE, PS, etc. When temporarily shutting down, in order to prevent the raw materials from staying at high temperature for a long time and cause degrading, the temperature of the dryer and barrel should be lowered. For example, the temperature of the barrels of PC, PMMA, etc. should be lowered to below 160°C. (The temperature of the hopper should be lowered to below 100℃ for PC) (3) Issues that should be paid attention to in the design of injection moulds (including product design) In order to prevent poor reflow, or uneven cooling, resulting in poor molding and surface defects In general, the following points should be paid attention to when designing injection molds. a) The wall thickness should be as uniform as possible, and the draft angle should be large enough; b) The transition part should be gradually. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps; c) Gate. The runner should be as wide and short 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 injection mold should be smooth and low in roughness (preferably lower than 0.8); e) Vent. The tank must be sufficient to discharge air and gas in the melt in time; f) Except for PET, the wall thickness should not be too thin, generally not less than 1mm.


(4) Issues that should be paid attention to in the process of injection molded parts (including the requirements of the injection molding machine) In order to reduce internal stress and surface quality defects, the following issues should be paid attention to in the process of injection molded parts. a) A special screw and an injection molding machine with a separate temperature control nozzle should be used; b) The injection temperature should be higher when the resin does not decompose; c) The injection pressure: generally higher to overcome the melt Defects with large viscosity, but too high pressure will cause internal stress to cause difficult demolding and deformation; d) Injection speed: In the case of satisfying mold filling, generally it should be low, and it is best to use slow-fast-slow multi-stage injection e) Holding pressure time and molding cycle: Under the condition of satisfying product filling without dents and bubbles; it should be as short as possible to minimize the residence time of melt in the barrel; f) Screw speed and back pressure: Under the premise of the quality of plasticization, it should be as low as possible to prevent the possibility of degrading; g) Mold temperature of injection molded parts: The cooling of the product has a great influence on the quality, so the mold temperature must be able to accurately control the process, it is possible If it is, the mold temperature should be higher. (5) Other problems. In order to prevent the deterioration of the upper surface quality, generally use less release agent when injection molded parts; when using recycled materials, it should not be more than 20%. Except for PET, all products should be post-treated to eliminate internal stress. PMMA should be dried in 70-80T hot air circulation for 4 hours; PC should be in clean air and glycerin. Liquid paraffin, etc. are heated at 110-135°C, and the time depends on the product, and it takes more than 10 hours at most. However, PET must undergo a biaxial stretching process to obtain good mechanical properties. 3. Transparent injection molding process (1) Transparent process characteristics: In addition to the above common problems, transparency also has some process characteristics, which are described as follows:


1. Process characteristics of PMMA PMMA has high viscosity and poor fluidity, so high material temperature and high injection pressure must be used for injection molding. The influence of injection temperature is greater than injection pressure, but the increase in injection pressure will help 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 manufacturability is better. Therefore, to improve fluidity, start from the injection temperature. The impact resistance is poor, the wear resistance is not good, it is easy to scratch, and it is easy to be brittle. Therefore, it is necessary to increase the mold temperature and improve the condensation process to overcome these defects.


2. Process characteristics of PC PC has high viscosity, high melting temperature and poor fluidity. It is necessary to inject the parts at a higher temperature (between 270-320T). Relatively speaking, the material temperature adjustment range is narrow, and the manufacturability is not as good as PMMA. The injection pressure has little effect on the fluidity, but due to the high viscosity, the injection pressure is still large. Correspondingly, in order to prevent the generation of internal stress, the pressure holding time should be as short as possible. The shrinkage rate is large, the size is stable, but the internal stress of the product is large, and the product is easy to crack. Therefore, it is advisable to increase the temperature instead of the pressure to improve the fluidity, and increase the injection mold temperature, improve the injection mold structure and post-processing to reduce the possibility of cracking . When the injection speed is low, defects such as ripples are prone to occur at the gate, the temperature of the radiating nozzle should be controlled separately, the temperature of the injection mold should be high, and the resistance of the runner and gate should be small.


3. Process characteristics of PET PET molding temperature is high, and the material temperature adjustment range is narrow (260-300℃), but after melting, the fluidity is good, so the manufacturability is poor, and it is often necessary to add anti-spreading devices in the nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through the stretching process and modification. Accurate control of the temperature of the injection mold is to prevent warpage. It is an important factor of deformation, so it is recommended to use hot runner injection molds. The temperature of the injection mold is high, otherwise it will cause poor surface gloss and difficulty in demolding. In summary, we can list the in-plastic process parameter table (table 3) for your reference when injecting. Table 3 Transparent Injection Molding Process Parameters Table IV. Defects and Solutions of Transparent Parts Due to space limitations, only the defects that affect product transparency are discussed here. For other defects, please refer to the product manual or other materials. The shortcomings are as follows:


(1) Silver streak: Affected by the anisotropy of internal stress during the filling and condensation process, the stress generated in the vertical direction causes the resin to undergo flow-up orientation, and the refractive index is different from the non-flow orientation to produce flashing silk patterns. After its expansion, it may cause cracks in the product. In addition to paying attention to the injection molding process and injection molding mold (see Table 4, the product is best to be annealed. For example, the PC material can be heated to 160℃ or higher for 3-5 minutes, and then cooled naturally.


(2) Bubbles: The water vapor and other gases in the resin cannot be discharged (during the condensation process of the injection mold), or due to insufficient mold filling, the condensation surface condenses too quickly to form a "vacuum bubble". See Table 4 for overcoming methods.


(3) Poor surface gloss: Mainly due to the roughness of the injection mold, on the other hand, premature condensation prevents the resin from copying the surface of the injection mold, all of which make the surface produce small irregularities and make the product lose its luster . See Table 4 for overcoming methods. (4) Shock lines: refers to the dense ripples formed from the straight gate as the center. The reason is that the melt viscosity is too large, and the front end material has condensed in the cavity, and then the material breaks through the condensing surface, causing the surface to appear shock lines . See Table 4 for overcoming methods. (5) Whitening and haze: It is mainly caused by dust falling into the raw materials in the air or the raw materials have too much water content. See Table 4 for overcoming methods. Table 4: Defects and overcoming methods of transparent products (6) White smoke and black spots: mainly due to decomposition or deterioration of the barrel resin due to local overheating in the barrel. See Table 4 for overcoming methods. In order to clearly explain the measures taken to overcome these deficiencies, Table 4 is listed for your reference.