Because plastics have the advantages of light weight, good toughness, easy molding, high cost, etc., in modern industrial and daily-use products, more and more plastics are used instead of glass, especially in the optical instrument and packaging industry, and the development is particularly rapid. However, because it requires better transparency, high wear resistance, and good impact resistance, so a lot of mission is to be done on the composition of plastics, the process, configuration, and mold of the whole process of injection molding to ensure that these are used instead of glass. The plastic (hereinafter referred to as transparent plastic) has excellent surface quality to meet the requirements of use. Compared to reinforced nylon.
The transparent plastics currently used on the market are polymethyl methacrylate (PMMA, commonly known as acrylic or plexiglass), polycarbonate (PC), polyethylene terephthalate (PET), and transparent. Nylon, acrylonitrile-styrene copolymer (AS), polysulfone (PSF), etc., among which we are most exposed to PMMA, PC and PET plastics. Let's take these three plastics as an example to talk about transparent plastics. Characteristics and injection molding process.
First, the performance of transparent plastic
Transparent plastics must first have high transparency. They mainly have certain strength and wear resistance, can withstand impact, have good heat resistance, and strengthen nylon. It has excellent chemical resistance and low water absorption. Only in this way can it meet the requirements of transparency and remain unchanged for a long time.
Injection molding process for transparent plastics. However, 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 radiation nozzle should be controlled separately, the mold temperature is high, and the flow path and gate resistance are small.
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 defects such as streaks, pores, whitening, halo, black spots, discoloration, poor gloss, etc. Configuration, mold, and even product design must pay great attention to and put forward 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 higher temperature, injection pressure and injection speed are often finely adjusted so that the plastic can be filled with mold and reinforced nylon. . There is no internal stress that causes product deformation and cracking. In the following, in terms of raw material preparation, configuration and mold requirements, injection molding process and raw material handling of products, transparent nylon, talk about matters that should be noted.
(1) Preparation and drying of raw materials Because of the inclusion of any impurities in the plastic, it may affect the transparency of the product. Therefore, during the accumulation, transportation and feeding process, it is necessary to pay attention 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 and enhances the nylon.
So be sure to dry, and when filling, the feed must use a dry 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 are not purified.
Drying process, drying process of transparent plastic: process drying temperature (°C) drying time (h) material layer thickness (mm) Remarks PMMA70～802～430～40 using hot air circulation drying PC120～130 is greater than 6 less than 30 by hot air circulation drying PET140～1803～4 is better to use continuous drying and feeding device.
(2) Cleaning of the barrel, screw and its accessories to prevent the purification of raw materials and the presence of old materials or impurities in the recesses of the screw and accessories.
Specially, the resin with poor thermal stability exists. Therefore, the screw cleaning agent is used to clean the parts before use and after shutdown, so that they do not stick to impurities. When there is no screw cleaning agent, the screws can be cleaned with PE, PS and other resins. When the machine is temporarily shut down, in order to prevent the raw material from stopping at a high temperature for a long time, it causes a drop in the plastic. 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 the design of the mold (including the design of the product) 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 point.
a) The wall thickness should be as uniform as possible. 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 if necessary, a cold well should be added;
d) The surface of the mold should be smooth and nylon 66. Low roughness (preferably below 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 the injection molding machine) 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 plastic resin without decomposition;
c) Injection pressure: generally higher to overcome the drawback of high viscosity of the melt, nylon 66. However, if the pressure is too high, internal stress will cause the demoulding to be difficult and deformed;
d) injection speed: in the case of filling the mold, it is generally low, preferably slow-fast-slow multi-stage injection;
e) Holding time and molding cycle: in the case of satisfying product filling, no dents or bubbles; it should be as short as possible to minimize the stop time of the melt in the barrel;
f) Screw speed and back pressure: Under the premise of satisfying the plasticizing quality, the nylon should be reinforced as low as possible. Prevent the possibility of a drop;
g) Mold temperature: The cooling of the product is good or bad, nylon 66. It has a 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.
(V) Other problems Because 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 20% larger. For products other than PET, the product should be post-treated to eliminate internal stress. PMMA should be dried in 70-80T hot air for 4 hours; PC should be cleaned with air and 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 excellent mechanical properties. Third, the transparent plastic injection molding process of transparent plastic process characteristics: In addition to the above common problems, transparent plastic also has some process characteristics, nylon 66.
It is now described as follows:
1. Process characteristics of PMMA PMMA has high viscosity and slightly poor fluidity, and reinforced nylon. Therefore, it is necessary to have high material temperature and high injection pressure injection molding, wherein the influence of the 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 temperature adjustment range is wide.
Good process. 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. Compare
Third, the transparent plastic injection molding process
Process characteristics of transparent plastics: In addition to the above common problems, transparent plastics also have some process characteristics, nylon 66. It is now described as follows:
1. Process characteristics of PMMA PMMA has high viscosity and slightly poor fluidity, and reinforced nylon. Therefore, it is necessary to have high material temperature and high injection pressure injection molding, wherein the influence of the 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 temperature adjustment range is wide, the process. Good process. 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 viscosity, high melting temperature, poor fluidity, back to this must be injected at a higher temperature (between 270-320T), relatively speaking, the material temperature adjustment range is narrower, the process 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, and the nylon is reinforced in order to prevent the internal stress. Keep the pressure as short as possible.
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 process is poor, and often in the nozzle to add anti-duction device. Mechanical strength and performance are not high after injection, in fact reinforced nylon. Performance must be improved by stretching procedures and modification. The mold temperature is accurately controlled to prevent warpage. The important identity of the deformation, so it is recommended to use hot runner mold. The mold temperature is high, otherwise it will cause poor surface gloss and difficulty in demolding.
Fourth, the defects of transparent plastic parts
Here are only the defects that affect the transparency of the product. The defects are probably the following:
(1) Silver streak: In the process of filling and condensing, the internal stress anisotropy is affected, and it is said to be transparent. The stress generated in the vertical direction causes the resin to be oriented on the flow, and the non-flow orientation produces a difference in refractive index to produce a glittering filament. When it is expanded, the product may be cracked. In addition to paying attention to the injection molding process and mold
(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". Injection molding. The method of overcoming is shown in the table.
(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 makes the surface of the cattle produce small unevenness, which makes the product tarnish.
(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 the condensation surface is broken through the back material to strengthen the nylon. And the surface appears to have a shock pattern.
(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.
(6) White smoke. Black spots: mainly due to plastics in the barrel, due to local overheating, the barrel resin is decomposed or deteriorated.
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