Characteristics and injection molding process of transparent plastic raw materials

Medical injection molding products Because it is required to have good transparency, high wear resistance, and good impact toughness, a lot of work needs to be done on the composition of plastics, the process of the entire injection molding process, equipment, molds, etc., to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality and meet the requirements of use. At present, the commonly used transparent plastics on the market are polymethyl methacrylate (commonly known as acrylic or organic glass, code name PMMA), polycarbonate (code name PC). Polyethylene terephthalate (code name PET), transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code name PSF), etc. Among them, PMMA is the most commonly used. PC and PET are three kinds of plastics. Due to limited space, these three plastics are used as examples to discuss the characteristics and injection molding process of transparent plastics.

1. Performance of transparent plastics

Transparent plastics must first have high transparency, and secondly, have certain strength and wear resistance, impact resistance, good heat resistance, excellent chemical resistance, and low water absorption. Only in this way can they meet the transparency requirements and remain unchanged for a long time during use. The following table compares the performance of PMMA, PC and PET. (1) Due to the wide variety of varieties, this is just an average value, and the actual data of different varieties are different. (2) PET data (mechanical aspects) is the data after stretching.

2. Common problems that should

be paid attention to during the injection molding of transparent plastics Due to the high light transmittance of transparent plastics, strict surface quality requirements of plastic products are inevitable. There must not be any spots, pores, whitening, halo, black spots, discoloration, poor gloss and other defects. Therefore, in the entire injection molding process, the raw materials, equipment, molds, and even product design must be paid great attention to and strict or even special requirements must be put forward. Secondly, since transparent plastics mostly have high melting points and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make subtle adjustments to the process parameters such as machine temperature, injection pressure, and injection speed, so that the injection plastic can fill the mold without generating internal stress and causing product deformation and cracking. The following is about the preparation of raw materials. The requirements for equipment and molds, injection molding process and raw material processing of products, and discuss the matters that should be paid attention to. (I) Preparation and drying of raw materials Since any impurities in the plastic may affect the transparency of the product, it is necessary to store and transport it. During the feeding process, it is necessary to pay attention to sealing to ensure that the raw materials are clean. In particular, if the raw materials contain moisture, heating will cause the raw materials to deteriorate, so they must be dried, and during injection molding, the feeding must use a drying hopper. Another point to note is that during the drying process, the input air should preferably be filtered and dehumidified to ensure that the raw materials will not be contaminated. (ii) Cleaning of barrel, screw and accessories To prevent raw material contamination and old materials or impurities in the depressions of screw and accessories, especially resins with poor thermal stability, screw cleaning agents should be used to clean all parts before use and after shutdown to prevent impurities from sticking. When there is no screw cleaning agent, the screw can be cleaned with resins such as PE and PS. When the machine is temporarily shut 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 barrel should be reduced. For example, the barrel temperature of PC, PMMA, etc. should be reduced to below 160°C. (The hopper temperature for PC should be reduced to below 100°C)

(iii) Issues that should be paid attention to in mold design

(including product design) In order to prevent poor reflux or uneven cooling from causing poor plastic molding, surface defects and deterioration, the following points should be paid attention to in mold design. a) The wall thickness should be as uniform as possible, and the demoulding slope should be large enough; b) The transition part should be gradual. Smooth transition to prevent sharp corners. Sharp edges are produced, 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 and condensation process. If necessary, a cold well should be added; d) The mold surface should be smooth and low in roughness (preferably less than 0.8); e) Vent. The groove must be sufficient to exhaust 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.

(IV) Issues 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 issues should be paid attention to in the injection molding process. a) A special screw and an injection molding machine with a separate temperature-controlled nozzle should be selected; b) Under the premise that the plastic resin does not decompose, a higher injection humidity should be used; c) Injection pressure: Generally higher to overcome the defect of high viscosity of the melt, but too high pressure will produce internal stress, causing demolding difficulties and deformation; d) Injection speed: Under the condition of filling the mold, it should generally be low, and it is best to use slow-fast-slow multi-stage injection; e) Holding time and molding cycle: Under the condition of filling the product without producing dents and bubbles; 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 meeting the plasticization quality, it should be as low as possible to prevent the possibility of decompression; g) Mold temperature: The cooling quality of the product 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.

(V) Other issues In order to prevent the deterioration of the upper surface quality

generally use as little release agent as possible during injection molding; when using recycled materials, it should not exceed 20%. Except for PET, all products should be post-processed to eliminate internal stress. PMMA should be dried in 70-80t hot air circulation for 4 hours; PC should be heated in clean air, glycerin, liquid paraffin, etc. at 110-135℃. The time depends on the product, and it takes more than 10 hours at most. PET must go through a biaxial stretching process to obtain good mechanical properties.

3. Injection molding process of transparent plastics 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 PM-MA has high viscosity and slightly poor fluidity, so it must be injected at high material temperature and high injection pressure. The influence of injection temperature is greater than injection pressure, but increasing injection pressure is conducive to improving the shrinkage rate of the product. The injection temperature range is wide, the melting temperature is 160℃, and the decomposition temperature is 270℃, so the material temperature adjustment range is wide and the processability is good. Therefore, to improve fluidity, you can start with injection temperature. Poor impact resistance, poor wear resistance, easy to scratch, easy to crack, so 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 melt temperature and poor fluidity, so it must be injected at a higher temperature (between 270-320t). Relatively speaking, the material temperature adjustment range is narrow, and the processability is not as good as PMMA. The injection pressure has little effect on fluidity, but due to the high viscosity, a large injection pressure is still required. Correspondingly, in order to prevent the generation of internal stress, the holding time should be as short as possible. The shrinkage rate is large and the size is stable, but the product has large internal stress and is easy to crack, so it is advisable to increase the temperature rather than the pressure to improve the fluidity, and to increase the mold temperature, improve the mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, the gate is prone to defects such as ripples. The temperature of the radial nozzle should be controlled separately, the mold temperature should be high, and the runner and gate resistance should be small. 3. Process characteristics of PET The molding temperature of PET is high, and the material temperature adjustment range is narrow (260-300℃), but after melting, the fluidity is good, so the processability is poor, and an anti-draft device is often added to the nozzle. The mechanical strength and performance after injection are not high, and the performance must be improved through the stretching process and modification. Accurate control of mold temperature is an important factor to prevent warping and deformation. Therefore, it is recommended to use a hot runner mold. The mold temperature is too high, otherwise it will cause poor surface gloss and difficulty in demolding.

IV. Defects and solutions of transparent plastic parts Due to space constraints

only defects that affect the transparency of the product are discussed here. For other defects, please refer to our company’s product manual or other materials?. The defects are roughly the following: (I) Silver streaks: Due to the anisotropy of internal stress during mold filling and condensation, the stress generated in the vertical direction causes the resin to flow upward, and the refractive index is different from the non-flow orientation, resulting in flash silk streaks. When it expands, it may cause cracks in 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 to above 160℃ and maintained for 3-5 minutes, and then cooled naturally. (ii) Bubbles: mainly due to 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”. (iii) 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 cannot copy the state of the mold surface. All of these make the surface produce tiny bumps and unevenness, and the product loses its gloss. (iv) Chatter marks: refers to the dense ripples formed from the sprue as the center. The reason is that the melt viscosity is too high, the front end material has condensed in the cavity, and the later material breaks through this condensation surface, causing chatter marks on the surface. (v) Whitening. Halo: mainly caused by dust falling into the raw material in the air or too much water content in the raw material. (VI) White smoke. Black spots: mainly due to the decomposition or deterioration of barrel resin caused by local overheating of plastic in the barrel. In order to clearly explain the measures taken to overcome these deficiencies.