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Properties of common transparent plastics and injection molding process

  • release time: 2017-07-03 09:30:36
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Because the plastic is light weight, good toughness, easy molding, low cost advantages, so in the modern industrial and household products, more and more plastic instead of glass, especially used in optical instruments and packaging industry, particularly the rapid development. But due to the requirements of better transparency, high wear resistance, impact on the firmware is better, so the plastic components, the whole process of injection molding process, equipment and die, have to do a lot of work, in order to ensure that used to replace glass plastic (hereinafter referred to as the transparent plastic), good surface quality, so as to achieve the use of request. 
 At present, the general use of transparent plastic on the market with polymethyl methacrylate (PMMA, commonly known as acrylic or organic glass), polycarbonate (PC), polyethylene glycol terephthalate (PET), transparent nylon, acrylonitrile styrene copolymer (AS), polysulfone (PSF), we contact the most is PMMA, PC three kinds of plastic and PET, following by the three kinds of plastics as an example, discusses the characteristics and molding process of transparent plastic. 
  
1, the performance of transparent plastic 
  Transparent plastic must first have high transparency, it must have a certain strength and wear resistance, shock resistance, heat resistance is better, better chemical resistance, water absorption rate is small, this is the only way in use, can satisfy the requirements of transparency and unchanged for a long time, are listed below in Table L, compare the performance of PMMA and PC and PET. 
    Figure 1: comparison of transparent plastic properties 
name density(g/am2) tensile strength(MPa) Notch impact(J/m2) transparency(%) deformation temperature (℃) Allowable moisture content Shrinkage ratio Wear resistance Chemical resistance 
PMMA 1.18 75 1200 92 95 0.04 0.5 poor fair  
PC 1.20 66 1900 90 137 0.02 0.6 fair good 
PET 1.37 165 1030 86 120 0.03 2 fair good 

Note: (1) because of the variety, this is only the average value, and the actual data of different varieties are different. 
  (2) PET data (mechanical aspects) are data after stretching. 
   From the data in Table 1 shows that PC is an ideal choice, but the price of your main raw materials and molding process is more difficult, so still choose PMMA (mainly on the general requirements of the product), and RET due to stretching through to get good mechanical properties, so use in packaging, container. 
  
2, common problems should be paid attention to in the process of transparent plastic injection 
   Due to the high light transmittance of transparent plastic, plastic products requires strict demand for surface quality, without any markings, pores, whitening, fog halo, black spots, color, gloss poor, so in the injection molding process of raw material, equipment, tooling, and product design, have to pay great attention to and put forward the strict even special requirements. Secondly, because the transparent plastic is high melting point, poor liquidity, so as to ensure the surface quality of the products, often have to make minor adjustments in the process parameters of high temperature, injection pressure, injection speed, injection plastic can fill the die, and does not produce internal stress caused by deformation and cracking products. This article from China hot die network 
   The following aspects of the preparation of raw materials, equipment and mold requirements, injection molding process and product processing of raw materials should pay attention to several aspects. 
  (1) raw material preparation and drying due to the presence of any impurities in plastic, may affect the transparency of products, and therefore storage, transportation, feeding process, must pay attention to sealing, ensure the material is clean. In particular, raw materials contain moisture, heating will lead to deterioration of raw materials, so be sure to dry, and in the injection molding, feeding must use a dry hopper. Also note that the drying process, the input air should be filtered and dehumidification, in order to ensure that no pollution of raw materials. The drying process is shown in table 2. 
  Figure 2: drying process for transparent plastics: 
name drying temperature(℃) drying time(h) Thickness of material layer(mm) Remarks 
PMMA 70~80 2~4 30~40 Use hot air circulation drying 
PC 120~130 >6 <30 Use hot air circulation drying 
PET 140~180 3~4 It is better to use continuous drying and feeding device 
  (2) cleaning of cylinder, screw and its accessories; 
  In order to prevent the pollution of raw materials and being old materials or impurities in the screw recess and accessories, there are special resin and poor thermal stability, therefore before use, stop after application of screw cleaning agent to clean each piece, which shall not be stuck with impurities, when no screw cleaning agent, PE and PS resin available cleaning screw. When the temporary shutdown, in order to prevent raw materials at high temperatures for long time, causing drop, the dryer and cylinder temperature should be reduced, such as PC, PMMA and other machine cylinder temperature should be reduced to below 160 degrees celsius. (the hopper temperature should be below 100 degrees for PC) hot mould net blog 
  (3) attention should be paid to the die design (including the design of the product) 
  In order to prevent backflow, or uneven cooling, resulting in plastic molding, surface defects and deterioration, usually in the mold design, should pay attention to the following points. 
  A) the wall thickness should be as uniform as possible, and the taper of the stripping should be large enough; 
  B) the transition should be gradual. Smooth transition prevents sharp corners. Sharp edges, especially PC products, must not be chipped; 
  C) gate. The runner shall be as wide and short as possible, and the gate position shall be set according to the shrinkage and condensation process, and a cold well shall be added when necessary; 
  D) the mold surface should be smooth and low roughness (preferably less than 0.8); 
  E) vent. The tank must be adequate for timely discharge of air and gas in the melt; 
  F) in addition to PET, the wall thickness should not be too thin, generally not less than LMM。 
   (4) attention should be paid to 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 paid attention to in the process of injection molding. 
  A) should use special screw, with a separate temperature control nozzle injection molding machine; 
  B) injection temperature should be higher injection humidity under the premise that the plastic resin is not decomposed; 
  C) injection pressure: generally higher, in order to overcome the viscosity of molten material defects, but the pressure is too high will produce internal stress, resulting in difficult to release and deformation; 
  D) injection speed: in case of mold filling, it is generally advisable to use low - speed - slow multistage injection; 
  E) holding time and molding cycle: in case of product filling, no dent or air bubbles, should be as short as possible in order to minimize the residence time of the melt in the cylinder; 
  F) screw speed and back pressure: to meet the requirements of plasticizing quality, should be as low as possible, to prevent the possibility of falling down; 
  G) die temperature: the cooling of the product is good or bad, and it has great influence on the quality, so the mould temperature must be able to control the process accurately. If possible, the mold temperature should be higher。 
  (5) other issues 
  In order to prevent the deterioration of the surface quality, the general injection molding as much as possible less release agent; when used back materials, shall not be greater than 20%.
  For except PET, products should be postprocessing, to eliminate the internal stress, PMMA should be in the 7080T hot air circulation drying for 4 hours; the PC should be in clean air, glycerol. Liquid stone, such as heating 110-135 degrees, the time depends on the product, the highest need more than 10 hours. The PET must undergo a biaxial stretching process to achieve good mechanical properties. 
 
 3, transparent plastic injection molding process 
  (1) the technical characteristics of transparent plastics: in addition to the above common problems, transparent plastics also have some technical characteristics, which are described as follows: 
1、PMMA Process characteristics
PMMA has high viscosity and poor flowability, so it must be injected at high temperature and high injection pressure. The influence of injection temperature is greater than injection pressure, but the injection pressure is improved, which is beneficial to improve the shrinkage of the product. The injection temperature range is wide, the melting temperature is 160 DEG C, and the decomposition temperature is up to 270 DEG C, so the range of material temperature regulation is wide and the process is better. Therefore, improving liquidity can proceed from the injection temperature. Poor impact, poor wear resistance, easy to flower, easy to brittle crack, it should improve the mold temperature, improve the condensation process, to overcome these defects. Hot mold net blog 
   2、Process characteristics of PC 
  PC high viscosity, high melting temperature, poor liquidity, back to this must be injected at higher temperatures (270 - 320T), relatively speaking, the temperature range is narrow, the process is not as good as PMMA. The injection pressure has little influence on the fluidity, but because of the large viscosity, the injection pressure still needs to be greater. Accordingly, in order to prevent the internal stress from occurring, the holding time should be as short as possible. High shrinkage rate, stable size, but the product of large internal stress, easy to crack, so should be used instead of increasing the temperature pressure to improve liquidity and increase from the mold temperature, improve the mold structure and postprocessing to reduce the possible cracking. When the injection speed is low, the gate is prone to ripple and other defects, the temperature of the nozzle should be controlled separately, the mold temperature should be high, the runner and gate resistance should be small. 
   3、Process characteristics of PET 
  PET molding temperature is high, and the material temperature range is narrow (260-300 degrees Celsius), but after melting, the flow is good, so the process is poor, and often in the nozzle to add anti flow device. Mechanical strength and properties are not high after injection, and the properties must be improved by drawing and modifying. Mold temperature is accurately controlled to prevent warpage. An important factor in deformation is therefore recommended for use in hot runner molds. The mold temperature is high, otherwise it will cause surface gloss difference and demoulding difficulties。 
  4. Defects of transparent plastic parts
   Because of space, we only discuss defects that affect product transparency. Please refer to product specifications or other materials for other defects. The defects may be as follows: 
  Defects and overcoming methods of transparent products:
(1): by filling crazing and condensation process, influence of stress anisotropy, stress perpendicular to the direction of the resin flow on the orientation, and different refractive index and non flow orientation and flash silk pattern, when the expansion, it is possible to make the product appear crack. In addition to the injection molding process and tooling, (see table), the best products are annealed. As Pcliao can be heated to 160 degrees above 3 to 5 minutes, then natural cooling can be.
  (2) bubbles: the water vapor and other gases which are mainly in the resin can not be discharged. During the process of condensation of the mold, or because of insufficient mold, the condensation surface is too quickly condensed to form a vacuum bubble". The ways to overcome it are shown in table.
  (3): the main surface gloss difference in mold roughness, on the other hand, condensation prematurely, the resin can not copy the state of the mold surface, all of which are on the surface of small cattle is uneven, which makes the product lose luster. The ways to overcome it are shown in table.
  (4) shock mark: it is the dense ripple formed from the sprue gate as the center. The reason is that the melt is too large, and the front material has been condensed in the cavity. Later, the material breaks through the condensing surface, which causes the surface to appear earthquake mark. The ways to overcome it are shown in table.
  (5) white. Fog: mainly caused by the fall of dust in the air or the excessive moisture in the raw material. The ways to overcome it are shown in table.
  (6) white smoke. Black spots: mainly due to plastic in the cylinder, due to local overheating and make the cylinder resin decomposition or deterioration of the formation.