1. What is the shrinkage of plastics and what are the basic factors affecting the shrinkage of plastics?
After the plastic is taken out from the mold and cooled to room temperature, the characteristic of dimensional shrinkage is called shrinkage. Because this shrinkage is not only caused by the thermal expansion and cold shrinkage of the resin itself, but also related to various molding factors, the shrinkage of plastic parts after molding is called molding shrinkage. The main factors affecting the shrinkage rate include: (1) plastic variety; (2) Plastic structure; (3) Die structure; (4) Molding process.2. What is the fluidity of plastics? What are the basic factors affecting the fluidity of plastics?
The ability of plastic melt to fill the mold cavity under certain temperature and pressure is called plastic fluidity. The main factors affecting the fluidity of plastics are: (1) material temperature; (2) Injection pressure; (3) Die structure.
3. What is stress cracking? What are the measures to prevent stress cracking?
Some plastics are sensitive to stress and are easy to produce internal stress during molding. They are brittle and easy to crack. When plastic parts are easy to crack under the action of external force or solvent, it is called stress cracking. In order to prevent this defect, on the one hand, reinforcing materials can be added to the plastic for modification. On the other hand, attention should be paid to the rational design of the molding process and mold, such as preheating and drying before material molding, correctly specifying the molding process conditions, avoiding the setting of inserts as far as possible, post-treatment of plastic parts, rational design of gating system and ejector, etc. Attention should also be paid to improving the structural manufacturability of plastic parts.
4. What is the curing characteristic of thermosetting plastics and what factors are related to it?
Curing characteristic is the unique property of thermosetting plastics, which refers to the process of crosslinking reaction when thermosetting plastics are formed. The curing speed is not only related to the variety of plastic, but also related to the shape, wall thickness, mold temperature and molding process conditions of plastic parts. The curing speed can be accelerated by using preloaded ingot, preheating, increasing molding temperature and increasing pressurization time. In addition, the curing speed should also meet the requirements of the forming method.
5. Polyethylene can be divided into several types according to the pressure used in polymerization. What aspects can it be applied to?
Polyethylene can be divided into high pressure, medium pressure and low pressure polyethylene according to the different pressure used in polymerization. High pressure polyethylene, also known as low-density polyethylene, is commonly used to make plastic film (ideal packaging material), hose, plastic bottle, insulating parts and coated cables in the electrical industry. Medium pressure polyethylene the most suitable methods of medium pressure polyethylene include high-speed blow molding, manufacturing bottles, packaging films, various injection molding products and rotary molding products, which can also be used on wires and cables. Low pressure polyethylene can be used to manufacture plastic pipes, plastic plates, plastic ropes and parts with low bearing capacity, such as gears, bearings, etc.
6. What are the properties and applications of polystyrene?
The main properties of polystyrene are: (1) it is the most ideal high-frequency insulating material at present; (2) Its chemical stability is good; (3) It has low heat resistance and can only be used at low temperature. Its texture is hard and brittle, and the plastic parts are easy to crack due to internal stress; (4) Polystyrene has good transparency. Polystyrene can be used in industry as instrument shell, lampshade, chemical instrument parts, transparent model, etc; Used as good insulation material, junction box, battery box, etc. in electrical aspect; In terms of daily necessities, it is widely used in packaging materials, various containers, toys, etc.
7. What are the properties and applications of ABS?
ABS has (1) good surface hardness, heat resistance and chemical corrosion resistance; (2) Its tenacity; (3) Excellent molding processability and coloring performance; 4) The thermal deformation temperature is higher than that of polystyrene, polyvinyl chloride and nylon. It has good dimensional stability, certain chemical stability and good dielectric properties. Its disadvantage is poor heat resistance and climate resistance. ABS is widely used in the mechanical industry to manufacture gears, pump impellers, bearings, handles, pipes, motor housings, instrument housings, instrument panels, water tank housings, battery tanks, refrigerators and refrigerator linings; In the automotive industry, ABS is used to make car fenders, handrails, hot air conditioning ducts, heaters, etc., and ABS sandwich panels are used to make car bodies; ABS can also be used to make water meter shells, textile accessories, electrical appliances, stationery, sports goods, toys, electronic organs and cassette recorder shells, food packaging containers, pesticide sprayers and furniture.
8. What are the properties and applications of phenolic plastics?
Compared with ordinary thermoplastics, phenolic plastics have good rigidity, small deformation, heat resistance and wear resistance, and can be used for a long time in the temperature range of 150 ~ 200 ℃. Under the condition of water lubrication, it has very low friction coefficient and excellent electrical insulation performance. The disadvantages of phenolic plastics are brittle and poor impact strength. Phenolic resin can be used to make gears, bearing shells, guide wheels, silent gears, bearings, electrical structural materials and electrical insulation materials. It can also be used to make all kinds of coil frames, wiring boards, power tool housings, fan leaves, acid resistant pump impellers, gears and cams.
9. Characteristics of injection molding?
Injection molding is characterized by short molding cycle, which can form plastic parts with complex shape, precise size and embedded parts at one time; Strong adaptability to various plastics; It is easy to realize automatic production with high temperature and high production efficiency. Therefore, it is widely used in the production of plastic parts, but the manufacturing cost of injection molding equipment and molds is high, which is not suitable for the production of single and small batch plastic parts.
10. The principle of injection molding is briefly described.
Granular or powdery plastic is sent from the hopper of the injection machine into the heated barrel, which is heated, melted and plasticized into viscous flow melt. Driven by the high pressure of the plunger or screw of the injection machine, it passes through the nozzle at a large flow rate and is injected into the mold cavity. After a certain period of pressure maintaining cooling and shaping, the shape given by the mold cavity can be maintained, and then the mold is opened for parting to obtain the molded plastic parts. This completes an injection cycle.
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