The commonly used transparent plastics on the market today are polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed PMMA) and polycarbonate (codenamed PC). Polyethylene terephthalate (PET), transparent nylon. AS (acrylonitrile-styrene copolymer), polysulfone (code PSF), etc., among which we are most exposed to PMMA. PC and PET are three kinds of plastics. Due to limited knitting, the following three plastics are taken as an example to discuss the characteristics of transparent plastics and the injection molding process.
First, the common problems that should be paid attention to during the injection molding of transparent plastics
Because of the high light transmittance of transparent plastics, the surface quality of plastic products must be required to be strict, and there must be no spots, air holes, and whitening. Fog haze, black spots, discoloration, poor luster and other defects, and therefore in the entire injection molding process for raw materials, equipment, molds, and even product design, must pay great attention and put forward strict or even special requirements.
Second, because transparent plastics are mostly of high melting point and poor flowability, in order to ensure the surface quality of products, it is often necessary to make fine adjustments in the process parameters such as barrel temperature, injection pressure, injection speed, etc., so that the plastic injection can be filled with molds, but not Internal stress can cause deformation and cracking of the product.
Below it is prepared in raw materials. For equipment and mold requirements, injection molding process and product raw material handling aspects, talk about matters that should be noted.
1. Preparation and Drying of Raw Materials Because of the presence of any impurities in the plastic, the transparency of the product may be affected, and therefore stored and transported. During the feeding process, we must pay attention to the seal to ensure that the raw materials are clean. In particular, water is contained in the raw materials, and the raw materials are deteriorated when heated. Therefore, the raw materials must be dried and the dry hopper must be used for feeding.
It is also important to note that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the material.
2. Cleaning of barrels, screws and accessories
In order to prevent the contamination of raw materials and the presence of old materials or impurities in the hollows of the screw and accessories, there are special resins with poor thermal stability. Therefore, screw cleaning agents are used to clean all parts before and after use, so that they must not be contaminated with impurities. When there is no screw cleaning agent, the screw can be cleaned with resins such as PE and PS.
When temporarily shut down, in order to prevent the raw materials from staying in the high temperature for a long time, causing the solution to drop, the temperature of the dryer and the barrel should be reduced. For example, the temperature of the barrels of the PC, PMMA, etc. should be reduced to below 160°C. (The hopper temperature should be less than 100°C for PC)
3. Problems to be noted in the design of the mold (including the design of the product)
In order to prevent back flow, or uneven cooling caused by poor plastic molding, resulting in surface defects and deterioration, generally in the mold design, should pay attention to the following points:
The wall thickness should be as uniform as possible, and the draft should be large enough;
The transition should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially PC products must not have gaps;
Gate. The runners should be as wide and short as possible, and the gate location should be set according to the contraction condensation process. If necessary, cold material wells should be added.
The mold surface should be smooth, with low roughness (preferably less than 0.8);
Vent. The tank must be sufficient to discharge the air and gases in the melt.
Except for PET, the wall thickness should not be too thin, generally not less than lmm.
4. Problems to be noted in the injection molding process (including injection molding machine requirements)
In order to reduce the internal stress and surface quality defects, the following aspects should be noted in the injection molding process:
Should use special screw, injection molding machine with separate temperature control nozzle;
The injection temperature should be higher than the injection temperature if the plastic resin does not decompose.
Injection pressure: generally high to overcome the defects of large melt viscosity, but the pressure is too high will cause internal stress caused by demoulding and deformation;
Injection speed: In the case of filling the mold, it should be generally low, it is best to use slow-fast-slow multi-stage injection;
Pressure holding time and molding cycle: in the case of filling the product to ensure that no depressions or bubbles occur; it should be as short as possible to minimize the residence time of the melt in the barrel;
Screw speed and back pressure: In the premise of satisfying the quality of plasticization, it should be as low as possible to prevent the possibility of solution.
Mold temperature: The cooling of the product is good or bad. It has great influence on the quality. Therefore, the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.
5. Other issues
As it is necessary to prevent the quality of the upper surface from being degraded, the release agent should be used as little as possible during injection molding; when using recycled materials, it should not be greater than 20%.
In addition to PET, the products should be post-treated to eliminate internal stress. PMMA should be dried in hot air at 70-80°C for 4 hours; PC should be cleaned of air and glycerin. Liquid paraffin and other heating 110-135 °C, time is determined by the product, the highest need more than 10 hours. PET must undergo a biaxial stretching process to obtain good mechanical properties.
Second, the performance of transparent plastic
Transparent plastic must first have high transparency, followed by a certain degree of strength and wear resistance, impact resistance, heat-resistant parts is better, chemical resistance should be superior, the water absorption rate should be small, only in this way can be in use, to meet the transparency The demand will remain unchanged for a long time.
Comparatively available PC is an ideal choice, but it is mainly difficult to use expensive raw materials and injection molding process, so PMMA is still the main choice (for general requirements of products), and PPT can only get good due to stretching. Because of its mechanical properties, it is often used in packaging and containers.
Third, transparent plastic injection molding process
Technological characteristics of transparent plastics: In addition to the above common problems, transparent plastics also have some technological characteristics, which are described as follows:
1. PMMA process characteristics PMMA viscosity, fluidity is somewhat poor, so it must be high material temperature, high injection pressure injection only, in which the impact of injection temperature is greater than the injection pressure, but the injection pressure increases, help to improve the product shrinkage.
The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature reaches 270°C, so the material temperature adjustment range is wide and the processability is good. Therefore, the fluidity can be improved by starting from the injection temperature. Poor impact, poor wear resistance, easy to draw flowers, easy to embrittlement, it should increase the mold temperature, improve the condensation process, to overcome these defects.
2. PC has high viscosity, high melting temperature and poor fluidity, so it must be injected at a relatively high temperature (between 270-320°C). Relatively speaking, the material temperature adjustment range is narrow and the processability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to the large viscosity, it still requires a large injection pressure. In order to prevent internal stress, the holding time should be as short as possible.
High shrinkage rate, dimensional stability, but the internal stress of the product, easy to crack, so it is appropriate to improve the temperature rather than pressure to improve the fluidity, and from the mold temperature, improve the mold structure and post-processing to reduce the possibility of cracking. When the injection speed is low, defects such as corrugation are easily generated at the gate, the temperature of the radiation nozzle is controlled individually, the temperature of the mold is high, and the flow path and the gate resistance are small.
3. PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 °C), but after melting, fluidity is good, so the process is poor, and often in the nozzle to prevent the flow device. Mechanical strength and properties are not high after injection and must be improved by stretching procedures and modification.
Mold temperature is accurately controlled to prevent warping. The deformation is an important factor and therefore hot runner molds are recommended. The mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demoulding.
Fourth, transparent plastic defects and solutions
Due to the space, only the defects that affect the transparency of the product are discussed here. The defects are as follows:
1. Craze: from the filling and condensation process, the internal stress anisotropy, the stress generated in the vertical direction, so that the resin flow orientation, and non-flow orientation produces a different refractive index and flash silk, when After expansion, the product may crack.
2. Bubbles: Water vapor and other gases that are mainly contained in the resin cannot be discharged (in the process of mold condensing) or due to insufficient filling, the condensing surface condenses too quickly to form a “vacuum bubble”.
3. The difference in surface gloss: the roughness of the main mold is large, and on the other hand, the condensation is too early, so that the resin cannot copy the state of the mold surface, all of which causes the surface to have a slight unevenness, which causes the product to lose luster.
4. Whitewash. Fog halo: Mainly due to dust in the air falling into the raw material or raw material moisture content caused by too much.
5. White smoke. Black point: Mainly due to plastic in the barrel, due to local overheating, the resin of the barrel may be decomposed or deteriorated.
6. Seismic pattern: refers to the dense ripple formed from the center of the sprue. The reason is that because the viscosity of the melt is too large, the front material has condensed in the cavity, and later the material breaks through the condensation surface, causing the surface to have a shock pattern. .