PP plastic (the worst quality plastic snap button)
Features: cheap, easy to break, poor gloss, etc.
PP plastic
(polypropylene)
English name: Polypropylene
Specific gravity: 0.9-0.91 g / cm 3 Molding shrinkage: 1.0-2.5% Molding temperature: 160-220 ° C Drying conditions: ---
The material has low performance density, strength stiffness, hardness and heat resistance, and is superior to low-pressure polyethylene. It can be used at around 100 degrees. It has good electrical properties and high-frequency insulation, which is not affected by humidity, but becomes brittle at low temperature and is not resistant to mold. Easy to age. Suitable for making general mechanical parts, corrosion resistant parts and insulating parts
Molding performance 1. Crystallized material, low hygroscopicity, easy to melt fracture, long-term contact with hot metal is easy to decompose.
2. Good fluidity, but the shrinkage range and shrinkage value are large, and shrinkage, dent and deformation are easy to occur.
3. The cooling rate is fast, the pouring system and the cooling system should be slowly dissipated, and the molding temperature should be controlled. The direction of the material is low and the direction is obvious. The low temperature and high pressure are especially obvious. When the mold temperature is lower than 50 degrees, the plastic parts are not smooth and easy to produce. Poor welding, leaving marks, warping deformation is easy to occur above 90 degrees
4. The plastic wall thickness should be uniform to avoid lack of glue and sharp corners to prevent stress concentration.
Polyoxymethylene plastic (POM) (conventional four-button component)
Features: high price, stable nature, good gloss, good quality, and large difference in raw material prices.
Polyoxymethylene refers to a type of polymer containing a repeating structural unit of oxymethylene in a macromolecular chain. The scientific name is "polyoxymethylene", which is abbreviated as POM. POM is the third largest general engineering plastic.
POM can be divided into two types: "homogeneous POM" and "copolymerized POM" depending on the structure. Due to the different structures, there are some differences in the performance of the two POMs. For example, the density, crystallinity and mechanical properties of the homopolymerized POM are slightly higher, while the thermal stability, chemical stability and processability of the copolymerized POM are better. The use of copolymerized POM is more extensive than that of homopolymerized POM.
POM's outstanding performance is: mechanical properties and rigidity, close to metal materials, is an ideal material to replace copper, cast zinc, aluminum and other metal materials; fatigue resistance and creep resistance is excellent; wear resistance, self-lubricity and It has good friction and is called four wear-resistant plastic materials together with UHMWPE, PA and F4. It has high thermal stability and chemical stability and excellent electrical insulation.
The disadvantages of POM are high density, poor acid and flame resistance, large shrinkage and instability, poor dimensional stability, and low weather resistance.
POM is widely used in the fields of electronics, machinery, automobiles, instrumentation, construction and daily necessities. 40% of Japan is used in electronics and 27% in cars; 45% in the United States is used in electronics and 17.5% in cars; and 39% in Western Europe is used in cars.
1, the performance of polyoxymethylene plastic
(1) General properties The appearance of POM is light yellow or white translucent or opaque powder or granules. The surface of the product is smooth and shiny, hard and dense, similar to ivory. The molding shrinkage rate is as high as 3.5%. It is flammable and its oxygen index is only 14-16. The upper end of the flame is yellow and the lower end is blue. It melts and drops, and has a pungent smell of formaldehyde and fishy smell. POM has a low gas permeability and is only a fraction of that of PE.
(2) Mechanical properties POM has excellent mechanical properties, the specific strength can reach 50.5Mpa, the specific stiffness is 2650Mpa, and it is very close to metal. The mechanical properties of POM vary little with temperature, and the copolymerization ratio is slightly larger.
The impact strength of POM is higher, but the conventional impact is less than ABS and PC; POM is sensitive to the gap, and the gap can reduce the impact strength by 90%.
The fatigue strength of POM is very prominent. After 104 alternating load, the fatigue strength can reach 35MPa, while PA and PC are only 28MPa. The creep resistance of POM is similar to that of PA, which is only 2.3% at 20 °C, 21 MPa, and 3000 h, and is less affected by temperature.
POM has small friction coefficient and good wear resistance (POM>PA66>PA6>PC>ABS>HPVC>PS). It has a large PV value and good self-lubrication. It is suitable for the production of friction products such as gears and bearings.
(3) Thermal properties The long-term heat resistance of POM is not high, but it can withstand 160 °C in the short term, and the short-term heat-resistant ratio of homopolymerized POM is higher than 10 °C, but the long-term heat-resistant copolymerized POM is about 10 °C higher than the homopolymerization.
(4) Electrical properties POM has good electrical insulation and is almost independent of temperature and humidity; dielectric constant and dielectric loss tangent have little variation over a wide range of temperature, humidity and frequency; arc resistance Excellent and can be maintained at high temperatures. The dielectric strength of the POM is related to the thickness, which is 82.7 Kv/mm at a thickness of 0.127 mm and 23.6 Kv/mm at a thickness of 1.88 mm.
(5) Environmental performance POM is not resistant to strong acids and oxidants, and has certain stability to dilute acids and weak acids. POM has good solvent resistance and is resistant to hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oils and weak bases, and maintains considerable stability at high temperatures. The weather resistance of POM is not good. Under the action of ultraviolet light for a long time, the mechanical properties are degraded, and the surface is pulverized and cracked.
2. Forming processing of polyoxymethylene plastic
(1) Processing characteristics
The rheology of the POM melt is non-Newtonian, and the viscosity of the melt is not sensitive to temperature. For injection molding, to increase the fluidity, it is possible to increase the injection rate and reduce the nozzle size.
POM has a large crystallinity, a narrow melting range and a large molding shrinkage (up to 3.5%). For injection molded thick products, it is necessary to pay attention to the pressure holding and feeding, so as to avoid the shrinkage hole being too large and scrapped.
POM has poor thermal stability and can cause decomposition if the temperature is too high or too long. In particular, when the temperature exceeds 250 ° C, the decomposition speed will be accelerated, and the formaldehyde gas which strongly irritates the eyes will be overflowed. In severe cases, bubbles or discoloration may occur in the product, and even an explosion may occur. Therefore, the processing temperature and residence time must be strictly controlled, and an antioxidant and a dicyandiamide formaldehyde absorbent are also required.
POM has a fast condensation rate, and the product is prone to surface defects such as wrinkles, streaks and weld lines. This can be solved by increasing the injection speed and increasing the mold temperature.
POM products are prone to internal stress and post-contraction is also large, requiring post-treatment. Post-treatment conditions are: thickness
6mm or less, temperature 100 ° C, time 0.25 ~ 1h; thickness 6mm or more, temperature 120 ~ 130 ° C, 4 ~ 6h.
POM water absorption is not high, but drying treatment can improve the gloss of the surface of the product. The drying conditions are: temperature 110 to 120 ° C, time 3 to 5 h.
(2) POM can be processed by injection molding, extrusion, blow molding and secondary molding, and is mainly processed by injection molding.
3. Modified varieties of polyoxymethylene plastic
(1) Enhanced POM
The main reinforcing materials are glass fiber, glass microbeads and carbon fiber, and the most commonly used glass fiber. The enhanced mechanical properties can be increased by 2 to 3 times, and the heat distortion temperature is increased by 50 ° C or more.
(2) Highly lubricated POM
Adding F4, graphite, molybdenum disulfide, lubricating oil and low molecular weight PE to POM can improve its lubricating performance. For example, adding 5 parts of F4 to POM can reduce the friction coefficient by 60% and improve the wear resistance by 1 to 2 times. Adding liquid lubricating oil to POM can greatly improve the wear resistance and the limit PV value. The wear resistance of POM added with 5% oil is increased by 72%, the ultimate PV value is up to 3.9 MPa·m/s, (pure POM is 0.213 MPa·m/s), which is 3 to 20 times that of other engineering plastics.
4, the application range of polyoxymethylene plastic
(1) The mechanical industry utilizes POM with high strength, wear resistance, fatigue resistance, high impact strength and good self-lubricating properties. It can be used to manufacture gears, bearings, pulleys, cams, pulleys, pump bodies, housings, valves, faucets and Pipe joints, etc. Its advantages over high strength, in the vehicle instead of metal copper, aluminum, zinc, etc., used as a tank valve, scattered
(2) Automotive industry The use of heat box covers, fans, levers, switches, gearbox housings and bearing brackets.
(3) The electronic/electrical appliance utilizes the advantages of high dielectric strength, small dielectric loss tangent, and good arc resistance, and is used for power tool housings, switch handles, and accessories for televisions, computers, and fax machines, and timers. Parts, tape recorders, tape recorders, etc.
(4) Others Second-generation zipper materials, water tanks, wash basins and toys.
PA plastic (mainly used for transparent four-button)
PA plastic (nylon)
(polyamide)
English name: Polyamide
Specific gravity: PA6-1.14g/cm3 PA66-1.15g/cm3 PA1010-1.05g/cm3 Molding shrinkage: PA6-0.8-2.5% PA66-1.5-2.2% Molding temperature: 220-300°C Drying conditions: 100 -110 ° C 12 hours
Material properties Tough, wear-resistant, oil-resistant, water-resistant, anti-enzyme, but water absorption.
Nylon 6 has good elasticity, high impact strength and high water absorption.
Nylon 66 has better performance than nylon 6, high strength and good wear resistance
Nylon 610 is similar to nylon 66 but has low water absorption and low stiffness
Nylon 1010 is translucent, with low water absorption and good cold resistance. It is suitable for making general mechanical parts, reducing wear parts, transmission parts, and parts such as chemicals, electrical appliances, instruments, etc.
Molding performance 1. Crystalline material, high melting point, narrow melting temperature range, poor thermal stability, decomposition temperature when the material temperature exceeds 300 degrees and residence time exceeds 30 minutes. It is easier to absorb moisture and needs to be dried. The water content should not exceed 0.3%.
2. Good fluidity and easy to overflow. It is advisable to use a self-locking nozzle and it should be heated.
3. The molding shrinkage range and shrinkage rate are large, the directionality is obvious, and shrinkage and deformation are likely to occur.
4. The mold temperature is selected according to the wall thickness of the plastic part in the range of 20-90 degrees. The injection pressure is selected according to the type of the injection machine, the material temperature, the shape and size of the plastic part, and the mold casting system. The molding cycle is selected according to the wall thickness of the plastic part. When the viscosity of the resin is small, the injection and cooling time should be long, and white oil is used as a release agent.
5. The form and size of the mold casting system, increasing the size of the runner and gate can reduce shrinkage.
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