1. What is the shrinkage of plastics, and what are the basic factors affecting the shrinkage of plastics?
Shrinkage refers to the dimensional shrinkage of plastic after it is taken out of the mold and cooled to room temperature. Because this shrinkage is not only caused by the thermal expansion and cold contraction 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 varieties; (2) Plastic structure; (3) Mold structure; (4) Forming 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 at a certain temperature and pressure is called plastic fluidity. The main factors affecting the fluidity of plastics are: (1) material temperature; (2) Injection pressure; (3) Mold structure.
3. What is stress cracking? What are the measures to prevent stress cracking?
Some plastics are sensitive to stress, easy to produce internal stress during molding, brittle and easy to crack. When plastic parts are subject to external force or solvent, they are easy to crack, which is called stress cracking. In order to prevent this defect, on the one hand, reinforcing materials can be added into the plastic to modify it; on the other hand, attention should be paid to reasonable design of the molding process and mold, such as preheating and drying of materials before molding, correct specification of molding process conditions, as far as possible not to set inserts, post-treatment of plastic parts, reasonable design of the gating system and ejection device. Attention should also be paid to improving the structural processability of plastic parts.
4. What are the curing characteristics of thermosetting plastics and what factors are relevant?
Curing property is a special property of thermosetting plastics, which refers to the process of completing crosslinking reaction when thermosetting plastics are formed. The curing speed is not only related to the plastic varieties, but also to the shape, wall thickness, mold temperature and molding process conditions of the plastic parts. The curing speed can be accelerated by using pre pressed ingots, preheating, increasing the molding temperature and increasing the pressurization time. In addition, the curing speed should also meet the requirements of the molding method.
5. Polyethylene can be divided into several types according to the pressure used in polymerization, and in which aspects can it be applied?
Polyethylene can be divided into high pressure, medium pressure and low pressure polyethylene according to the different pressures used in polymerization. High pressure polyethylene, also known as low-density polyethylene, is commonly used to make plastic films (ideal packaging materials), hoses, plastic bottles, insulating parts and coated cables in the electrical industry. Medium pressure polyethylene The most suitable methods for medium pressure polyethylene are high-speed blow molding, bottle manufacturing, film for packaging, various injection molding products and rotary molding products, and 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 load capacity, such as gears, bearings, etc.
6. What are the properties and applications of polystyrene?
The main properties of polystyrene include: (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 temperatures. It 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 insulating materials, junction boxes, battery boxes, etc. in electrical aspects; 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) It has excellent molding processability and coloring performance; 4) The thermal deformation temperature is higher than that of polystyrene, polyvinyl chloride, nylon, etc., with good dimensional stability, chemical stability and good dielectric properties. Its disadvantage is poor heat resistance and weather resistance. ABS is widely used in the machinery industry to manufacture gears, pump impellers, bearings, handles, pipes, motor casings, instrument casings, instrument panels, water tank casings, battery tanks, refrigerators and refrigerator linings; In the automobile industry, ABS is used to manufacture automobile 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 equipment, electrical parts, cultural and educational sports goods, toys, electronic piano and recorder shells, food packaging containers, pesticide spray and furniture.
8. What are the properties and applications of phenolic plastics?
Compared with general thermoplastics, phenolic plastic has 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 extremely low friction coefficient and excellent electrical insulation performance. The disadvantage of phenolic plastic is its brittleness and poor impact strength. Phenolic resin can be used to manufacture gears, bearing shells, guide wheels, silent gears, bearings, electrical structural materials and electrical insulation materials, as well as various coil racks, terminal blocks, electric tool housings, fan leaves, acid resistant pump impellers, gears and cams.
9. What are the characteristics of injection molding?
Injection molding is characterized by short molding cycle, and it can form plastic parts with complex shape, precise size and embedded parts at one time; Strong adaptability to various plastics; High production efficiency, product quality temperature, easy to realize automatic production. Therefore, it is widely used in the production of plastic parts, but the manufacturing cost of injection molding equipment and mold is high, which is not suitable for the production of single piece and small batch of plastic parts.
10. Briefly describe the principle of injection molding.
Granular or powdered plastics are sent into the heated barrel from the hopper of the injection machine, which is heated, melted and plasticized into a viscous melt. Driven by the high pressure of the plunger or screw of the injection machine, they are injected into the mold cavity at a large flow rate through the nozzle. 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.