Introduction of the most popular transparent produ

2022-09-23
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Introduction to transparent product materials and injection molding process

plastic has light weight, good toughness and easy molding. Because of the advantages of low cost, more and more plastics are used to replace glass in modern industry and daily-use products, especially in optical instruments and packaging industry. However, due to the requirements of good transparency, high wear resistance and good impact toughness, the composition of plastic, the process and equipment of the whole injection molding process are required. A lot of work must be done 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 in the market are polymethylmethacrylate (commonly known as sub afterburner or plexiglass, code named PMMA) and polycarbonate (code named PC). Polyethylene terephthalate (PET), transparent nylon. As (acrylonitrile styrene copolymer), polysulfone (instead of the "guidance" appendix "selection of enclosure thermal insulation materials and thermal performance indicators" No. PSF), among which PMMA is the one we contact most. Due to the limited weaving width of PC and pet plastics, the following will take these three plastics as examples to discuss the characteristics and injection molding process of transparent plastics

I. properties of transparent plastics

transparent plastics must first have high transparency, and then have a certain strength and wear resistance, can resist impact, have good heat-resistant parts, excellent chemical resistance, and small water absorption. Only in this way can they meet the requirements of transparency in use and remain unchanged for a long time. Table 1 below compares the properties of PMMA, PC, and pet

note: (1) due to various varieties, this is only the average value, and the actual data of different varieties are different

(2) PET data (Mechanical) is the data after stretching

according to the data in Table 1, PC is an ideal choice, but mainly due to its expensive raw materials and difficult injection molding process, PMMA is still the main choice (for products with general requirements), while PPT is mostly used in packaging and containers because it needs to be stretched to obtain good mechanical properties

II. Common problems that should be paid attention to in the injection molding process of transparent plastics

due to the high light transmittance of transparent plastics, it is inevitable that the surface quality of plastic products is strictly required, and there should be no stripes, pores, and whitening. Fog halo, black spots, discoloration, poor gloss and other defects, so in the whole injection molding process, the raw materials and equipment. Mold and even product design should pay great attention to and put forward strict or even special requirements. Secondly, because most transparent plastics have high melting point and poor fluidity, in order to ensure the surface quality of products, it is often necessary to make subtle adjustments in the barrel temperature, injection pressure, injection speed and other process parameters, so that when injecting plastics, the mold can be filled without internal stress causing product deformation and cracking

the following is the preparation of raw materials. Talk about matters needing attention in terms of equipment and mold requirements, injection molding process and raw material treatment of products

(I) the preparation and drying of raw materials may affect the transparency of products due to the inclusion of any impurities in plastics, so it is difficult to store and transport. During the feeding process, attention must be paid to sealing to ensure that the raw materials are clean. In particular, the raw materials contain moisture, which will cause the deterioration of the raw materials after heating, so it must be dried, and during injection molding, the drying hopper must be used for feeding. It should also be noted that during the drying process, the input air should be filtered and dehumidified to ensure that it will not pollute the raw materials. The drying process is shown in Table 2,

(II) cleaning of barrel, screw and its accessories

in order to prevent raw material pollution and the existence of old materials or impurities in the depression of screw and accessories, especially the existence of resins with poor thermal stability, screw cleaner should be used to clean all parts before use and after shutdown, so that they cannot be adhered with impurities. When there is no screw cleaner, PE, PS and other resins can be used to clean the screw. In case of temporary shutdown, 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, such as PC, PMMA and other barrel temperatures should be reduced to below 160 ℃. (the hopper temperature should be reduced to below 100 ℃ for PC)

(III) problems that should be paid attention to in mold design (including product design)

in order to prevent poor plastic molding, surface defects and deterioration caused by poor backflow or uneven cooling, the following points should be paid attention to when setting the mold

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 edges, especially PC products, must not have gaps

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 the cold charging well shall be added if necessary

d) the mold surface should be smooth and low roughness (preferably less than 0.8)

e) vent. The tank must be sufficient to discharge air and gas in the melt in time

f) except pet, the wall thickness should not be too thin, generally not less than 1mm

(IV) problems that should be paid attention to in injection molding process (including the requirements of injection molding machine)

in order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in injection molding process

a) special screw and injection molding machine with separate temperature control nozzle shall be selected

b) the injection temperature should be higher on the premise that the plastic resin does not decompose

c) injection pressure: generally high to overcome the defect of high melt viscosity, but too high pressure will produce internal stress and cause difficult demoulding and deformation

d) injection speed: generally, it should be low under the condition of satisfying mold filling, and it is best to adopt slow fast slow multi-stage injection

e) pressure holding time and forming cycle: under the condition of meeting the product mold filling without depression and bubble; It should be as short as possible to minimize the residence time of molten material in the barrel

f) screw speed and back pressure: on 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 products 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

(V) other problems

in order to prevent the deterioration of the upper surface quality, mold release agents are generally used as little as possible during injection molding; When using recycled materials, it shall not be greater than 20%

for products except pet, post-treatment should be carried out to eliminate internal stress. PMMA should be dried by hot air circulation at 70-80 ℃ for 4 hours; PC should be in clean air, glycerin. Liquid paraffin is heated at 110-135 ℃, and the tensile test of time tensile machine is one of the most widely used methods to study the mechanical strength of materials. It depends on the product, and it takes more than 10 hours at most. Pet must undergo biaxial stretching process to obtain good mechanical properties

III. injection molding process of transparent plastics

(I) 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: PMMA has high viscosity and poor fluidity, so it must be injected with high material temperature and high injection pressure. The influence of injection temperature is greater than injection pressure, but the injection pressure increases, It is beneficial to improve 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 processability is good. Therefore, to improve the fluidity, we can start with the injection temperature. Poor impact resistance, poor wear resistance, easy to scratch, easy to crack, so we should improve the mold temperature and 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 ℃). Relatively speaking, the range of material temperature adjustment 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 is still large. Accordingly, in order to prevent internal stress, the pressure holding time should be as short as possible. The shrinkage 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 improve the fluidity by increasing the temperature rather than the pressure, and reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the gate is prone to ripple and other defects. The temperature of the radiation 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 has high molding temperature and narrow adjustment range of material temperature (260-300 ℃), but after melting, it has good fluidity, so it has poor processability, and it is often necessary to add an anti delaying device in the nozzle. The mechanical strength and performance are not high after injection, and the performance must be improved through stretching and modification. Mold temperature is accurately controlled to prevent warpage. Therefore, it is recommended to use hot runner die. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding

IV. defects of transparent plastics and solutions

due to space constraints, only defects that affect the transparency of products are discussed here. For other defects, please refer to our company's extended plan, so we must track them to * Dali before taking down the company's product manual or other materials. Its defects probably include the following:

(I) crazing: 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, while the refractive index is different from the non flow orientation, resulting in flash filaments. When it expands, cracks may appear in the product. In addition to paying attention to the injection molding process and mold (see Table 4, the best product should be annealed. For example, PC material can be heated to above 160 ℃ for 3-5 minutes, and then cooled naturally.

(II) bubbles: water vapor and other gases mainly in the resin cannot be discharged, (in the process of mold cooling) or due to insufficient mold filling, the condensing surface condenses too fast to form a "vacuum bubble". See Table 4 for the overcoming methods

(III) poor surface gloss: with its excellent performance, it can be widely used in many fields such as new energy vehicles, aerospace, etc., mainly due to the large roughness of the mold. On the other hand, the resin can not copy the state of the mold surface because of premature condensation. All these make the surface slightly uneven, which makes the product lose luster. See Table 4 for the overcoming methods

(IV) ripple: refers to the dense ripple formed from the straight gate as the center. The reason is that the front-end material has condensed in the mold cavity due to the excessive viscosity of the melt, and then the material broke through the condensing surface, resulting in the appearance of ripple on the surface. See Table 4 for the overcoming methods

(V) whitening. Fog halo: mainly caused by dust falling into raw materials in the air or the water content of raw materials is too large. See Table 4 for the overcoming methods

(VI) white smoke. Black spot: it is mainly caused by the decomposition or deterioration of the barrel resin due to local overheating of the plastic in the barrel

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