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PP plastic (the worst quality plastic snap button)
Features: Cheap, easy to crack, poor luster, etc.
PP plastic
(Polypropylene)
English name:Polypropylene
Specific gravity: 0.9-0.91g/cm3 Molding shrinkage: 1.0-2.5% Molding temperature: 160-220°C Drying condition:---
Material performance density, strength, stiffness, hardness, heat resistance are better than low-pressure polyethylene, can be used in about 100 degrees. Has good electrical properties and high frequency insulation resistance from humidity, but at low temperatures becomes brittle, not resistant to mold Easy to age. Suitable for making general mechanical parts, corrosion-resistant parts and insulating parts
Molding performance 1. Crystalline material, small moisture absorption, prone to melt fracture, long-term contact with hot metal easy to decompose.
2. The fluidity is good, but the shrinkage range and the shrinkage value are large, and the shrinkage holes, dents and deformations are easy to occur.
3. Cooling speed, pouring system and cooling system should be slowly cooling, and pay attention to control the molding temperature. The material temperature is low direction direction is obvious. Especially low temperature and high pressure, mold temperature is less than 50 degrees, the plastic parts are not smooth, easy to produce Poor welding, leaving marks, warpage easily occur above 90 degrees
4. The plastic wall thickness must be uniform to avoid lack of glue and sharp corners to prevent stress concentration.
Polyoxymethylene Plastic (POM) (General Snap Button Component)
Features: high price, stable nature, good glossiness, good quality, great difference in raw material price.
Polyoxymethylene is a type of polymer containing oxymethylene repeat units in the macromolecular chain. The scientific name is "polyoxymethylene", and is abbreviated as POM in English. POM is the third largest general engineering plastic.
According to different structures, POM can be divided into two types: homopolymer POM and copolymer POM. Due to different structures, there are some differences in the performance of the two POMs, such as the density, crystallinity, and mechanical properties of homopolymerized POM, which are higher, while the thermal stability, chemical stability, and processability of the copolymerized POM are better. The use of copolymerized POM is more extensive than POM.
The outstanding properties of POM are: good mechanical properties and rigidity, close to metal materials, it is an ideal material to replace copper, zinc, aluminum and other metal materials; excellent fatigue resistance and creep resistance; wear resistance, self-lubricating and Good friction, together with UHMWPE, PA, and F4, they are called four wear-resistant plastic materials; they have high thermal stability and chemical stability and excellent electrical insulation.
The disadvantages of POM are high density, poor acid and flame resistance, large post-shrinkage and instability, poor dimensional stability, and low weatherability.
POM is widely used in electronic appliances, machinery, automobiles, instrumentation, construction and daily necessities. 40% of Japan is used for electronic appliances and 27% for automobiles; 45% of the US is for electronics and electronics, 17.5% for automobiles; and 39% of Western Europe is used for automobiles.
1, the performance of polyacetal plastic
(1) General properties The appearance of POM is light yellow or white translucent or opaque powder or granular. The surface of the product is smooth and glossy, hard and dense, similar to ivory. Molding shrinkage up to 3.5%. Flammable, its oxygen index is only 14 to 16, the top of the flame is yellow, the bottom is blue, melting dripping, there is a stimulating formaldehyde and fishy flavor. POM has a small air permeability, only a fraction of PE.
(2) Mechanical properties The mechanical properties of POM are excellent, the specific strength is up to 50.5Mpa, and the specific rigidity is up to 2650Mpa, which is very close to the 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 that of ABS and PC; POM is sensitive to the notch, and the gap can reduce the impact strength by 90%.
The fatigue strength of POM is very prominent. After 104 alternating loads, the fatigue strength can reach 35MPa, while PA and PC are only 28MPa. The creep resistance of POM is similar to that of PA, and it is only 2.3% at 20°C, 21MPa, and 3000h, and is less affected by temperature.
POM has a low friction coefficient and good wear resistance (POM>PA66>PA6>PC>ABS>HPVC>PS), extreme PV value is great, self-lubricating is good, and it is suitable for production of frictional products such as gears and bearings.
(3) Thermal properties The long-term heat resistance of POM is not high, but the short-term heat resistance is 160°C. The homopolymerization POM short-term heat resistance is more than 10°C higher than the copolymerization, but the long-term heat-resistant copolymerization POM is about 10°C higher than homopolymerization.
(4) Electrical properties POM has good electrical insulation and is hardly affected by temperature and humidity; the dielectric constant and dielectric loss tangent vary little over a wide range of temperatures, humidity and frequency; arc resistance Excellent and can be kept 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 a certain degree of stability for dilute and weak acids. POM has good solvent resistance, can be resistant to hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oils, and weak bases, and can maintain considerable stability at high temperatures. POM weatherability is not good, long-term under the action of ultraviolet light, mechanical properties decline, the surface of powdering and cracking.
2. Polyformaldehyde plastic molding processing
(1) Processing characteristics
The rheological properties of POM melts are non-Newtonian, and their melt viscosity 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 degree of crystallinity, a narrow melting range, and large molding shrinkage (up to 3.5%). For injection molding thick products, pay attention to pressure and feed, so as not to cause the shrink hole is too large and scrapped.
Poor thermal stability of the POM causes decomposition when the temperature is too high or for a long time. In particular, when the temperature exceeds 250°C, the decomposition rate will increase, and formaldehyde gas that irritates the eyes will be emitted. In severe cases, the product will generate bubbles or discoloration, and may even cause an explosion. Therefore, the processing temperature and residence time must be strictly controlled. In addition, antioxidants and dicyandiamide formaldehyde absorbers must also be added.
The POM has a rapid condensation rate and the products are prone to surface defects such as wrinkles, markings and weld marks. This can be solved by increasing the injection speed and increasing the mold temperature.
POM products are prone to internal stress and have a large post-shrinkage, requiring post-treatment. Post-processing conditions are: thickness
Below 6mm, temperature 100 °C, time 0.25 ~ 1h; thickness more than 6mm, temperature 120 ~ 130 °C, 4 ~ 6h.
POM water absorption rate is not high, but the drying process can improve the surface gloss of the product. The drying conditions were: temperature 110-120°C, time 3-5 h.
(2) POM can be processed by injection molding, extrusion, blow molding and secondary molding, and injection molding is the main method.
3, polyoxymethylene plastic modified varieties
(1) Enhanced POM
The main reinforcing materials are glass fibers, glass beads and carbon fibers, and glass fibers are the most commonly used. The enhanced mechanical properties can be increased by 2 to 3 times, and the heat distortion temperature can be increased by more than 50°C.
(2) Highly lubricated POM
Adding F4, graphite, molybdenum disulfide, lubricating oil, and low molecular weight PE to POM can improve its lubricating properties. For example, adding 5 parts of F4 to POM can reduce the friction coefficient by 60% and increase the wear resistance by 1 to 2 times. Adding liquid lubricating oil to POM can significantly increase the wear resistance and limit PV value. Adding 5% oil, the wear resistance of POM 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 scope of application of polyacetal plastic
(1) The machinery industry uses POM strength, wear resistance, fatigue resistance, high impact strength, good self-lubrication, can be used to manufacture gears, bearings, pulleys, cams, pulleys, pumps, housings, valves, faucets and Fittings, etc. Its advantage of high specific strength, in the transport of metal instead of copper, aluminum, zinc, etc., as water tank valve, scattered
(2) Automotive Industry Heater covers, fans, control levers, switches, gearbox housings, and bearing supports are used.
(3) Electronic/electrical appliances utilize their advantages of high dielectric strength, small dielectric loss tangent, and good arc resistance, and are used in power tool housings, switch handles, and accessories for televisions, computers, and fax machines, and timers. Parts, tape recorders, video tape decks, etc.
(4) Other Second-generation zipper materials, water tanks, washbasins, and toys.
PA plastic (mainly used for transparent snap closures)
PA plastic (nylon)
(polyamide)
English name: Polyamide
Specific Gravity: PA6-1.14 g/cc PA66-1.15 g/cc PA1010-1.05 g/cc Molding shrinkage: PA6-0.8-2.5% PA66-1.5-2.2% Molding temperature: 220-300°C Drying conditions: 100 -110°C for 12 hours
Material properties tough, wear-resistant, oil-resistant, water-resistant, anti-bacteria, but large water absorption.
Nylon 6 has good elasticity, high impact strength and large 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 translucent, low water absorption, good cold resistance Suitable for the production of general mechanical parts, wear reduction parts, transmission parts, and chemical, electrical, instrumentation and other parts
Moldability 1. Crystalline material, high melting point, narrow melting temperature range, poor thermal stability, material temperature exceeds 300 degrees, residence time exceeds 30 minutes, decomposition. More hygroscopic, need to be dry, the water content must not exceed 0.3%.
2. Good liquidity, easy to overflow. The nozzle should be self-locking and should be heated.
3. Molding shrinkage range and shrinkage rate, obvious direction, easy to shrink hole, deformation and so on.
4 mold temperature according to the thickness of plastic parts in the range of 20-90 degrees selection, injection pressure according to the type of injection machine, material temperature, plastic parts shape size, mold casting system selection, molding cycle by plastic wall thickness selected. 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 runners and gates can reduce shrinkage.
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