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| Introduction of IC tray performance and injection problems |
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| Source£ºBoye International Trade Co., Ltd. Release time£º2020-05-06 |
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Because the IC tray has light weight, good toughness and easy molding. Low cost and other advantages, so in modern industry and IC trays, more and more plastics are used to replace other materials, especially in optical instruments and packaging industries, the development is particularly rapid. However, due to the requirement of good plasticity, high wear-resistant parts, and good impact-resistant tough parts, a lot of work must be done on the composition of the IC tray, the entire injection molding process, equipment, molds, etc., to ensure that these are used to replace Glass IC trays (hereinafter referred to as plastic IC trays) have good surface quality to meet the requirements for use.
At present, the plastic IC trays commonly used in the market include polymethyl methacrylate (commonly known as acrylic or plexiglass, code-named PMMA), polycarbonate (code-named PC), and polyethylene terephthalate (code-named PET) ), Plastic nylon. AS (acrylic eye-styrene copolymer), polysulfone (codenamed PSF), etc., among which we are most in contact with three kinds of IC trays PMMA, PC and PET, due to limited space, the following three IC trays as an example To discuss the characteristics and injection molding process of plastic IC trays.
1. Performance of plastic IC tray
Plastic IC trays must first have a high degree of plasticity, followed by a certain strength and abrasion resistance, impact resistance, heat resistance, chemical resistance, and low water absorption rate. Only in this way can they be used in The requirements for plasticity remain unchanged for a long time. Table 1 below lists the performance of PMMA, PC and PET.
Plastic IC tray performance comparison
Note: (1) Due to the variety of varieties, this is just an average value, the actual data for different varieties are different.
(2) PET data (mechanical) is the data after stretching.
PC is an ideal choice, but mainly because of its expensive raw materials and difficult injection molding process, PMMA is still the main choice (for IC trays that are generally required), and PET has to be stretched to get good mechanical properties. , So it is mostly used in packaging and containers.
Second, the common problems that should be paid attention to in the injection process of plastic IC trays
Due to the high light transmittance of plastic IC trays, it is inevitable that the surface quality of IC trays must be strict. There must be no defects such as streaks, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, the entire injection molding process For the design of raw materials, equipment, molds, and even IC trays, we must pay great attention and put forward strict and even special requirements. Secondly, plastic IC trays mostly have high melting points and poor fluidity. Therefore, in order to ensure the surface quality of IC trays, it is often necessary to make fine adjustments at higher temperatures, injection pressure, injection speed and other process parameters, so that the IC tray can be filled with molds. , It will not cause internal stress and cause IC tray deformation and cracking.
Now talk about the matters that should be paid attention to in terms of raw material preparation, equipment and mold requirements, injection molding process and raw material handling of IC trays.
(1) Preparation and drying of raw materials Because any impurities in the IC tray may affect the plasticity of the IC tray, it must be sealed during storage, transportation, and feeding to ensure that the raw materials are clean. In particular, the raw material contains moisture, which will cause the raw material to deteriorate after heating, so it must be dried, and when injection molding, a dry hopper must be used for feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not pollute the raw materials.
Plastic IC tray drying process
(2) Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and the existence of old materials or impurities in the recesses of the screw and accessories, especially the resin with poor thermal stability exists, so before use and after shutdown, all parts should be cleaned with a screw cleaning agent to prevent impurities from sticking. When there is no screw cleaning agent, you can use PE, PS and other resins to clean the screw. When temporarily shut down, in order to prevent the raw material staying at high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be reduced, such as PC, PMMA and other barrel temperatures should be lower than 160 ¡æ.
(3) Problems that should be paid attention to in mold design (including IC tray design)
In order to prevent poor backflow or uneven cooling, IC tray molding is not good, surface defects and deterioration occur. Generally, the following points should be noted when designing the mold.
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, especially PCIC trays, 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 shrinking and condensing process. If necessary, a cold material well should be added.
D) The surface of the mold should be smooth and low in roughness (preferably less than 0.8).
E) Vent hole. The tank must be sufficient to exhaust air and melt gas in a timely manner.
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 the requirements of the injection molding machine)
In order to reduce internal stress and surface quality defects, attention should be paid to the following issues in the injection molding process.
A) Injection molding machine with special screw and separate temperature control nozzle should be selected.
B) The injection temperature should be higher if the IC tray resin is not decomposed.
C) Injection pressure: generally higher to overcome the defect of high melt viscosity, but too high pressure will cause internal stress and cause difficulty in mold release and deformation.
D) Injection speed: Under the condition of satisfying mold filling, generally it should be low. It is best to use slow-fast-slow multi-stage injection.
E) Pressure holding time and molding cycle: when the IC tray filling mold is satisfied and no dents or bubbles are generated. It 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 quality of plasticization, it should be as low as possible to prevent the possibility of solution drop.
G) Mold temperature: The cooling of the IC tray is good or bad, which has a great impact on the quality, so the mold temperature must be able to accurately control its process. If possible, the mold temperature should be higher.
(5) Other issues
In order to prevent the deterioration of the quality of the upper surface, the mold release agent is generally used as little as possible. When using recycled materials, it should not be greater than 20%.
Except for PET, IC trays should be post-processed to eliminate internal stress. PMMA should be dried in a hot air cycle of 70-80T for 4 hours. PC should be clean air, glycerin. Liquid paraffin and other heating 110-135 ¡æ, the time depends on the IC tray, the maximum need more than 10 hours. And PET must go through the process of bidirectional stretching to get good mechanical properties.
3. Injection molding process of plastic IC tray
(1) Process characteristics of plastic IC trays: In addition to the above common problems, plastic IC trays also have some process characteristics, which are now described as follows:
1. Process characteristics of PMMA PMMA has high viscosity and slightly poor fluidity, so it must be injection molded with high material temperature and high injection pressure. The impact of injection temperature is greater than the injection pressure, but the increase in injection pressure is conducive to improving the shrinkage rate of IC tray . The injection temperature range is wide, the melting temperature is 160 ¡æ, and the decomposition temperature reaches 270 ¡æ, so the material temperature adjustment range is wide, and the processability is better. Therefore, to improve the fluidity, we can start from the injection temperature. The impact is poor, the wear resistance is not good, and it is easy to scratch and brittle. Therefore, the mold temperature should be increased and the condensation process should be improved to overcome these defects.
2. Process characteristics of PC PC has high viscosity, high melting temperature and poor fluidity. In this case, it must be injection molded at a higher temperature (between 270 and 320T). The range of material temperature adjustment is relatively narrow, and the processability 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 greater. 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 and the size is stable, but the IC tray has a large stress and is easy to crack, so it is better to increase the temperature rather than the pressure to improve the fluidity, and increase the mold temperature, improve the mold structure and post-treatment to reduce the possibility of cracking. When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the radiation nozzle should be controlled separately, the temperature of the 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 range of material temperature adjustment is narrow (260-300 ¡æ), but after melting, the fluidity is good, so the processability is poor, and the anti-rolling device is often added 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. The mold temperature is accurately controlled to prevent warpage. An important factor of deformation, it is recommended to use hot runner molds. The mold temperature is high, otherwise it will cause poor surface gloss and difficult demoulding.
Fourth, the defects and solutions of plastic IC trays
Due to space limitations, only the defects that affect the plasticity of the IC tray are discussed here. For other defects, please refer to the IC tray manual or other information. The defects may have the following items:
(1) Silver streak: due to the anisotropy of internal stress during mold filling and condensation, the stress generated in the vertical direction makes the resin flow upwardly oriented, and the refractive index is different from the non-flowing direction to produce a flash of silk, when After its expansion, the IC tray may crack. In addition to the injection molding process and mold attention (see Table 4, it is best to anneal the IC tray. For example, the PC material can be heated to above 160 ¡æ for 3 to 5 minutes, and then cooled naturally.
(2) Bubbles: Water vapor and other gases mainly in the resin cannot be discharged (during mold condensation) or due to insufficient mold filling, the condensation surface condenses too quickly to form a "vacuum bubble".
(3) Poor surface gloss: Mainly due to the large roughness of the mold, on the other hand, the premature condensation prevents the resin from copying the surface of the mold. All of these cause tiny irregularities on the surface, which makes the IC tray tarnish.
(4) Shock pattern: refers to the dense corrugation formed from the sprue gate. The reason is that the melt viscosity is too large, the front end material has condensed in the cavity, and then the material breaks through the condensation surface, and the surface has shock marks. .
(5) Whitening, haze: Mainly caused by dust falling into the raw materials in the air or the raw material moisture content is too large.
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