How much do you know about the injection molding process of transparent products?

Because plastic has light weight, good toughness, easy to form. The advantages of low cost, so in modern industry and daily products, more and more plastic instead of glass, especially used in optical instruments and packaging industry, development is particularly rapid.

However, due to the requirements of better transparency, high wear resistance, and better impact toughness, the composition of the plastic, the process of injection molding, and the equipment are required. Molds, etc., must do a lot of work to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality, so as to meet the requirements of use.

At present, the transparent plastics commonly used on the market are polymethyl methacrylate (commonly known as adress or plexiglass, code PMMA) and polycarbonate (code PC). Polyethylene terephthalate (code PET), transparent nylon. AS (acrylonitrile - styrene copolymer), polysulfone (code PSF), etc., of which we are most exposed to PMMA.
   
First, the performance of transparent plastics

Transparent plastic must first have high transparency, followed by a certain strength and wear resistance, impact resistance, heat resistance, chemical resistance to be excellent, water absorption to be small, only in this way in use, can meet the requirements of transparency and long-term unchanged, Table l is listed below, compare the performance of PMMA, PC and PET.
Note:
(1) Due to a wide variety of varieties, this is only an average value, and the actual data of different varieties are different. (2) PET data (mechanical) is the data after stretching.

It can be seen from the data in Table 1 that PC is the ideal choice, but mainly due to its raw material price and injection molding process is difficult, so it is still mainly to choose PMMA, (for the general requirements of the product), and PPT because it has to be stretched to get good mechanical properties, so it is used in packaging and containers. Second, the common problems that should be paid attention to in the process of transparent plastic injection molding

Transparent plastics due to high light transmittance, it is necessary to require strict surface quality requirements of plastic products, can not have any markings, pores, white. Fog halo, black spots, discoloration, poor luster and other defects, so in the entire injection process to raw materials, equipment. Mold, and even product design, should pay great attention to and put forward strict and even special requirements. Secondly, because transparent plastics are mostly high melting point and poor fluidity, in order to ensure the surface quality of the product, it is often necessary to make subtle adjustments in the process parameters such as the temperature of the barrel, the injection pressure, and the injection speed, so that the injection of plastic can be filled with mold, and will not produce internal stress and cause product deformation and cracking.
The following is its raw material preparation. On the equipment and mold requirements, injection molding process and raw material treatment of products, talk about the matters that should be paid attention to.
(1) Preparation and drying of raw materials
Due to any impurities contained in the plastic, it may affect the transparency of the product, so storage and transportation. During the feeding process, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw material contains water, which will cause the deterioration of the raw material after heating, so it must be dry, and in the injection molding, the feeding must use the drying hopper. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it will not pollute the raw materials. Its drying process is shown in Table 2.
(2) Cleaning of barrel, screw and accessories
In order to prevent raw material pollution and old materials or impurities in the depression of the screw and accessories, especially the existence of resin with poor thermal stability, so before use and after shutdown, screw cleaning agent is used to clean each part, so that it is not stuck with impurities, when there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw. When the temporary shutdown, in order to prevent the raw material from staying at high temperature for a long time, causing degradation, the temperature of the dryer and the barrel should be reduced, such as PC, PMMA and other barrel temperatures should be reduced to below 160 ° C. (Hopper temperature should be reduced to below 100℃ for PC)
(3) Problems that should be paid attention to in mold design (including product design)
In order to prevent poor back flow, or uneven cooling caused by poor plastic molding, resulting in surface defects and deterioration, generally in mold design, the following points should be paid attention to.
a) The wall thickness should be as uniform as possible, and the demoulding slope should be large enough;
b) The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edge generation, 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 condensation process, and the cooling well should be added if necessary;
d) Mold surface should be smooth, low roughness (preferably less than 0.8); e) Exhaust holes. The tank must be sufficient to discharge air and gas in the melt in a timely manner; f) Except for PET, the wall thickness should not be too thin, generally not less than lmm.
(4) Problems that should be noted 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 paid attention to in the injection molding process.
a) The injection molding machine with a special screw and a separate temperature control nozzle should be selected;
b) Under the premise that the plastic resin does not decompose, higher injection temperature should be used;
c) Injection pressure: generally higher to overcome the defect of large viscosity of molten material, but too high pressure will produce internal stress resulting in difficult demording and deformation;
d) Injection speed: In the case of mold filling, it is generally low, and it is best to use slow-fast-slow multi-stage injection;
e) Pressure holding time and molding cycle: under the condition that the product is filled without dents and bubbles; It should be as short as possible to minimize the residence time of molten material in the barrel;
f) Screw speed and back pressure: under the premise of meeting the plasticizing quality, it should be as low as possible to prevent the possibility of degradation;
g) 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 quality of the upper surface, the release agent is generally used as little as possible during injection molding. Not more than 20% when using recycled materials.
In addition to PET, products should be post-treated to eliminate internal stress, PMMA should be dried at 70-80℃ hot air circulation for 4 hours; PC should be glycerin in clean air. Liquid paraffin is heated at 110-135 ° C, the time depends on the product, and the maximum need is more than 10 hours. PET must undergo a bidirectional drawing process in order to obtain good mechanical properties.
Third, the injection molding process of transparent plastics

(1) 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, PMMA process characteristics PMMA viscosity is large, the fluidity is slightly poor, so it must be high material temperature, high injection pressure injection molding, of which the injection temperature influence is greater than the injection pressure, but the injection pressure increase, is conducive to improving the shrinkage 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 process is good. Therefore, the fluidity can be improved from the injection temperature. The impact is poor, the wear resistance is not good, the scratching is easy, and the cracking is easy, so the mold temperature should be increased and the condensation process improved to overcome these defects.
2, PC process characteristics
PC viscosity, high melt temperature, poor fluidity, so it must be injected at a higher temperature (270-320 ° C), relatively speaking, the material temperature adjustment range is narrow, the process is not as good as PMMA. The injection pressure has little effect on the fluidity, but because of the large viscosity, the injection pressure is still larger, and the corresponding pressure holding time should be as short as possible in order to prevent the internal stress. The shrinkage rate is large and the size is stable, but the internal stress of the product is large and easy to crack, so it is appropriate to increase the temperature rather than pressure to improve the fluidity, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-processing. When the injection speed is low, the gate is prone to corrugation and other defects, the nozzle temperature should be controlled separately, the mold temperature should be high, and the flow path and gate resistance should be small. 3, PET process characteristics
PET molding temperature is high, and the material temperature adjustment range is narrow (260-300℃), but after melting, the flow is good, so the process is poor, and often in the nozzle to add anti-casting device. The mechanical strength and properties are not high after injection, and must be improved by stretching process and modification. Accurate control of mold temperature is to prevent warping. Deformation is an important factor, so it is recommended to use hot runner mold. Mold temperature should be high, otherwise it will cause poor surface luster and demoulding difficulties.
Fourth, transparent plastic common defects and solutions

(a) Silver grain: by the mold filling and condensation process, the influence of internal stress anisotropy, the stress generated in the vertical direction, so that the resin has a flow orientation, and the non-flow orientation produces a different refractive index and flash silk, when it expands, it may cause cracks in the product. In addition to the injection molding process and mold attention (see Table 4), the best product for annealing treatment. For example, PC material can be heated to more than 160 ° C for 3-5 minutes, and then cooled naturally.
(2) Bubbles: the water and other gases in the resin can not be discharged, (in the mold condensation process) or due to insufficient mold filling, the condensation surface is too fast to condense and form a "vacuum bubble". The overcoming methods are shown in Table 4.
(3) Poor surface luster: mainly due to the roughness of the mold, on the other hand, premature condensation, so that the resin can not copy the state of the mold surface, all of which make its surface produce small uneven, and make the product tarnished. The overcoming methods are shown in Table 4.
(4) Seismic marks: refers to the dense ripples formed from the center of the straight gate, the reason is that due to the excessive viscosity of the melt, the front end material has been condensed in the cavity, and the incoming material has broken through the condensing surface, and the surface has seismic marks. Its overcoming method
(5) Whitening. Fog halo: mainly caused by dust falling into the raw material in the air or the water content of the raw material is too large. Its overcoming method
(6) White smoke. Black spots: mainly due to the plastic in the barrel, due to local overheating and the formation of the barrel resin decomposition or deterioration.