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Engineering plastics are better than general plastics in mechanical strength, impact resistance, heat resistance, hardness, corrosion resistance, wear resistance and aging resistance, so they are used as industrial parts or shell materials or as some mechanical parts instead of metals. The five common engineering plastics are polyamide, polyformaldehyde, polycarbonate, polyphenylene oxide and polybutylene terephthalate. Let' s learn about them mainly from the nature, use and key aspects of processing.

I. polyamide PA (nylon)

Properties and uses: nylon is tough, wear-resistant, oil-resistant, water-resistant, anti enzyme bacteria, but it absorbs water greatly; Nylon 6 has good elasticity, high impact strength and large water absorption; Nylon 66 has better function than nylon 6, high strength and good wear resistance; Nylon 610 is similar to nylon 66, but it has small water absorption and low stiffness; Nylon 1010 is translucent, with small water absorption and good cold resistance. It is commonly used to manufacture general mechanical parts, wear-resistant parts, transmission parts, and chemical, electrical, instrument and other parts.

Processing key:

Forming shortening rate: pa6-0.8-2.5%, pa66-1.5-2.2%.

Forming temperature: 220-300 ℃.

Drying condition: 100-110 ℃ for 12 hours.

　　1. The crystalline material has high melting point, narrow melting temperature scale and poor thermal stability. It will differentiate when the material temperature exceeds 300 ℃ and the residence time exceeds 30min. It is easy to absorb moisture and needs to be dried. The water content shall not exceed 0.3%.

　　2. Good mobility, easy to overflow. Self locking nozzle should be used for injection and should be heated.

　　3. The forming shortening scale and shortening rate are large, and the directionality is obvious, so shrinkage and deformation are easy to occur.

　　4. The mold temperature shall be selected according to the wall thickness of the plastic part within the range of 20-90 ℃, the injection pressure shall be selected according to the type of injection machine, material temperature, shape and dimension of the plastic part and mold pouring system, and the forming cycle shall be selected according to the wall thickness of the plastic part. When the resin viscosity is small, the injection and cooling time shall be longer, and white oil shall be used as the release agent.

　　5. The shrinkage water can be reduced by increasing the size of runner and gate.

II. POM

Properties and uses: polyoxymethylene is a tough and elastic material, which has good creep resistance, stability and impact resistance even at low temperature. POM has both homopolymer and copolymer materials. Homopolymer materials have high tensile strength and fatigue strength, but are difficult to be processed and formed; The copolymer material has high thermal and chemical stability and is easy to be processed. No matter homopolymer or copolymer materials, polyoxymethylene belongs to crystalline materials and is not easy to absorb water. Its high crystallinity results in an appropriately high shortening rate, which can be as high as 2-3.5%. There are different shortening rates for different reinforced materials. Its low friction coefficient and high stability make it especially suitable for manufacturing gears and bearings. Because of its high temperature resistance, it is also used to manufacture lawn equipment and pipeline equipment, such as pipeline valves, pump housings, etc.

Processing key:

Drying treatment: if the material is stored in a dry environment, it generally does not need drying treatment.

Melting temperature: homopolymer material is 190-230 ℃; The copolymer material is 190-210 ℃.

Mold temperature: 80-105 ℃; In order to reduce the shortening rate after forming, a slightly higher mold temperature can be selected.

Injection pressure: 700-1200bar.

Injection speed: medium or high injection speed.

Runner and gate: can use any type of gate. If tunnel gates are used, it is better to use shorter types. It is suggested that homopolymer materials use hot nozzle runner, and copolymer materials can use both internal and external warm runner.

III. polycarbonate PC

Properties and uses: polycarbonate is an amorphous engineering material, which has particularly good impact strength, thermal stability, gloss, bacteriostasis, flame retardancy and pollution resistance; The notch impact strength of polycarbonate is very high, and the shortening rate is very low, generally 0.1-0.2%; Polycarbonate has good mechanical function, but its mobility is poor, so the injection molding process is difficult. When selecting PC materials of any quality, the final expectation of the product shall be taken as the benchmark. If plastic parts are required to have high impact resistance, PC materials with low activity rate shall be used; Conversely, high activity PC materials can be used to optimize the injection molding process. It is mainly used for electrical and commercial equipment such as computer components and connectors, appliances such as food processing machines, refrigerator drawers, and transportation industries such as front and rear lights and instrument panels of vehicles.

Processing key:

Drying treatment: PC material is hygroscopic, and drying before processing is essential. It is suggested that the drying condition should be 100-200 ℃, 3-4 hours, and the humidity before processing should be less than 0.02%.

Melting temperature: 260-340 ℃.

Mold temperature: 70-120 ℃.

Injection pressure: try to use high injection pressure.

Injection speed: low speed injection is used for smaller gates, and high speed injection is used for other types of gates.

IV. polyphenylene oxide PPO

Properties and uses: polyphenylene oxide is a white particle with excellent inductive function. It can be used in 120 ℃ steam. It has good electrical insulation. Its dielectric function ranks first among plastics. It has small water absorption, excellent water resistance and good scale stability, but has the tendency of stress cracking. Modified polyphenylene oxide can eliminate stress cracking; It has high heat resistance, with a glass transition temperature of 211 ℃, a melting point of 268 ℃, and a tendency of differentiation when heated to 330 ℃. The higher the content of PPO, the better the heat resistance, and the thermal deformation temperature can reach 190 ℃; Excellent flame retardancy, self extinguishing; Light weight, non-toxic, can be used in food and drug industries; Poor optical rotation resistance. It will change color when used in the sun for a long time. It can be used to manufacture heat-resistant parts, insulating parts, wear-resistant parts, transmission parts, medical and electronic parts; It can be used as gear, fan blade, valve and other parts under high temperature, and can replace stainless steel; Can manufacture screws, fasteners and connectors; Electrical parts of motor, rotor, casing and transformer.

Processing key:

Forming shortening rate: 0.3-0.8%.

Forming temperature: 260-290 ℃.

Drying condition: 130 ℃ for 4 hours.

　　1. Non crystalline material with low moisture absorption;

　　2. It is a Newtonian fluid with poor mobility. Its viscosity is sensitive to temperature. The thickness of the product is generally more than 0.8mm. It is easy to differentiate, and corrosive gas occurs during differentiation. The forming temperature should be strictly controlled, the mold should be heated, and the pouring system should have little resistance to material flow;

　　3. The water absorption of polyphenylene oxide is very low, about 0.06%, but a small amount of water will also lead to unsmooth phenomena such as silver wire on the product surface. It is best to dry it at a temperature not higher than 150 ℃, otherwise the color will change;

　　4. The forming temperature of polyphenylene oxide is 280-330 degrees, and that of modified polyphenylene oxide is 260-285 degrees;

V. polybutylene terephthalate PBT

Properties and applications: PBT is one of the toughest engineering thermoplastic materials. It is a semi crystalline material with excellent chemical stability, mechanical strength, electrical insulation characteristics and thermal stability. It has good stability under a variety of environmental conditions; PBT has weak hygroscopic properties; It crystallizes rapidly, so it will be twisted and deformed due to uneven cooling in the processing process; For materials with glass additives, the shortening rate in the process direction can be reduced, but it is basically no different from that in the straight direction of the process. The general shortening rate is between 1.5-2.8%, and the shortening rate for materials with 30% glass additives is between 0.3-1.6%; The melting point (225 ℃) and high temperature deformation temperature are lower than those of pet; The glass transition temperature is between 22 ℃ and 43 ℃; Because PBT has a high crystallization rate, its viscosity is very low, and the cycle time of plastic parts processing is generally low; It is commonly used to manufacture household appliances such as food processing blades, vacuum cleaner components, electric fans, hair dryer shells, coffee utensils, etc., electrical components such as switches, motor shells, fuse boxes, computer keyboard keys, etc., radiator grids, body panels, wheel covers, door and window components in the automotive industry.

Processing key:

Drying treatment: this material is easy to hydrolyze at high temperature, so the drying treatment before processing is very important. The drying condition in air is 120 ℃, 6-8 hours, maybe 150 ℃, 2-4 hours. It is necessary that the humidity be less than 0.03%. If it is dried with a hygroscopic dryer, the claim condition is 150 ℃ for 2.5 hours.

Melting temperature: 225-275 ℃, claim temperature: 250 ℃.

Mold temperature: 40-60 ℃ for unreinforced materials. The cooling cavity of the mold should be well planned to reduce the tortuosity of the plastic parts. The loss of heat must be fast and uniform. It is proposed that the diameter of the mold cooling cavity is 12mm.

Injection pressure: medium (up to 1500bar).

Injection speed: the injection speed should be as fast as possible (because PBT coagulates quickly).

Runner and gate: it is advocated to use circular runner to add pressure transmission (experience formula: runner diameter = plastic thickness +1.5mm). Be able to use various types of gates. Warm runner can also be used, but attention should be paid to avoid material leakage and degradation. The gate diameter should be between 0.8-1.0*t, where t is the thickness of the plastic part. In case of submerged gate, the minimum diameter is 0.75mm.

This article is from Jiangmen Engineering Plastics:http://xrtsm.com.cn/