1. Problems that should be paid attention to in the process of transparent plastic injection molding
Since transparent plastics are mostly materials with high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to process parameters such as barrel temperature, injection pressure, and injection speed, so that the mold can be filled during injection molding without Generate internal stress and cause product deformation and cracking.
1). Preparation and Drying of Raw Materials
Since any impurity in the plastic 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 moisture, which will cause the raw material to deteriorate after heating, so it must be dried, and a drying hopper should be used when feeding the injection molding. 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.
2). Cleaning of barrel, screw and its accessories
Prevent raw material contamination and old materials or impurities in the recesses of the screw and accessories, especially resins with poor thermal stability. Therefore, before use and after shutdown, all parts should be cleaned with a screw cleaner to prevent impurities from sticking. When there is no screw cleaner, the screw can be cleaned with resins such as PE and PS.
When temporarily shutting down, in order to prevent the raw materials from staying at high temperature for a long time and causing degradation, the temperature of the dryer and the barrel should be lowered. For example, the temperature of the barrel of PC and PMMA should be lowered below 160°C (the temperature of the hopper should be lowered for PC. to below 100°C)
3). Mold design and product design
In order to prevent poor backflow or uneven cooling, resulting in poor plastic molding, surface defects and deterioration; generally, the following points should be paid attention to when designing molds:
① The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
② The transition part should be gradual. Smooth transition to prevent sharp corners and burrs, especially PC products must not have gaps;
③ 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 cooling well should be added if necessary;
④ The surface of the mold should be smooth and clean, with low roughness (preferably less than 0.8);
⑤ Exhaust holes. The groove must be sufficient to discharge air and gas in the melt in time;
⑥ Except for PET, the wall thickness should not be too thin, generally not less than 1mm.
4). Injection molding process
In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:
① An injection molding machine with a special screw and a separate temperature-controlled nozzle should be selected.
② Injection temperature. Under the premise that the plastic resin does not decompose, a higher injection temperature should be used.
③ Injection pressure. Generally higher to overcome the defect of high melt viscosity, but if the pressure is too high, internal stress will be generated, resulting in difficulty in demoulding and deformation.
④ Injection speed. In the case of satisfying mold filling, it should generally be low, and it is best to use slow-fast-slow multi-stage injection.
⑤ Holding time and molding cycle. In the case of satisfying product filling without causing sags and bubbles, it should be as short as possible to minimize the residence time of the molten material in the barrel.
⑥ 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 degradation.
⑦ Mold temperature. The cooling of the product has a great impact on the quality, so 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 surface quality, the release agent should be used as little as possible during injection molding, and it should not exceed 20% when using recycled materials.
Products other than PET should be post-treated to eliminate internal stress. PMMA should be dried at 70~80°C with hot air circulation for 4 hours; PC should be heated at 110~135°C in clean air, glycerin, liquid paraffin, etc. The time depends on the product Depending on the time, it takes up to 10 hours; and PET must go through the process of biaxial stretching in order to obtain good mechanical properties.
2. Injection molding process characteristics of various transparent plastics
Due to the high light transmittance of transparent plastics, there must be strict requirements on the surface quality of plastic products, and there must be no defects such as streaks, pores, whitening, halos, black spots, discoloration, and poor gloss. Raw materials, equipment, molds, and even product design must pay great attention and put forward strict or even special requirements.
1). Process characteristics of PMMA
PMMA has high viscosity and slightly poor fluidity, so high material temperature and high injection pressure are required for injection molding. The influence of injection temperature is greater than that of injection pressure, but increasing injection pressure is conducive to improving the shrinkage of the product. The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature reaches 270°C, so the temperature adjustment range of the material is wide and the processability is good. Therefore, to improve the fluidity, we can start from the injection temperature. Poor impact, poor wear resistance, easy to scratch, easy to brittle, so should increase the mold temperature, improve the condensation process, to overcome these defects.
2). Process characteristics of PC
PC has high viscosity, high melt temperature, and poor fluidity, so it must be injected at a higher temperature (between 270-320°C). Relatively speaking, the temperature adjustment range of the material is 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 still needs to be relatively high. Correspondingly, in order to prevent internal stress, the pressure holding time should 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, so it is better to improve the fluidity by increasing the temperature instead of the pressure, and reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-processing. When the injection speed is low, defects such as ripples are likely to occur at the gate, the temperature of the nozzle should be controlled separately, the mold temperature 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 ° C), but after melting, the fluidity is good, so the processability is poor, and an anti-dripping device is often added to 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 warping. An important factor for deformation, so it is recommended to use a hot runner mold. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding.
4). Defects and solutions of transparent plastic parts
① Silver Streak
Due to the influence of internal stress anisotropy in the process of mold filling and condensation, the stress generated in the vertical direction makes the resin flow upward orientation, and the difference in refractive index between the non-flow orientation and the non-flow orientation produces shiny silk lines. When it expands, it may crack the product.
In addition to paying attention to the injection molding process and mold, it is best to anneal the product. For example, PC material can be heated above 160°C for 3-5 minutes, and then cooled naturally.
Because the water vapor and other gases in the resin cannot be discharged, (during the mold condensation process) or due to insufficient mold filling, the condensation surface condenses too quickly to form vacuum bubbles.
③ Poor Surface Gloss
Due to the large roughness of the mold, on the other hand, the condensation is too early, so that the resin cannot copy the state of the surface of the mold, all of which make the surface slightly uneven and make the product lose its luster
④ Vibration Pattern
It refers to the dense corrugation formed from the sprue as the center. The reason is that due to the excessive viscosity of the melt, the front-end material has condensed in the cavity, and the later material breaks through the condensation surface, causing vibration lines on the surface.
⑤ White Halo
It is mainly caused by the dust falling into the raw material in the air or the water content of the raw material is too large.
⑥ White Smoke Black Spots
Mainly due to the plastic in the barrel, the barrel resin is decomposed or deteriorated due to local overheating.
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