What we are talking about is mainly the processing of pellets. The main characteristics of the process performance are:
1. Shrinkage rate The production of plastic products from powders or pellets is usually carried out in moulds at high temperatures and in the molten state. When the product is cooled to room temperature, its dimensions will shrink. In order to ensure the accuracy of the product size, an appropriate magnification factor should be determined in conjunction with the shrinkage rate when the dimension of the die cavity is specified. Generally, products with too high shrinkage rate are susceptible to warping and cracking.
2. Fluidity. The ability of plastic to fill the entire cavity under heat and pressure is called fluidity.
3. Moisture, plastic often contains more or less moisture. When there is too much moisture in the plastic, the fluidity increases, the molding cycle increases, the shrinkage of the product increases, the porous and prone to warping, and the surface has ripples and stuffiness, which reduces the mechanical properties and dielectric properties of the product.
In the following, we discuss the commonly used plastics (HDPE, PP, PET) separately:
l. PE has a regular molecular space arrangement, and the chain links are small, which is the inherent condition of the crystalline polymer. Due to the different polymerization methods, there is a difference in the length and length of the branches in the PE main chain. HDPE nearly linear molecules (an average of 5 methyl groups per 1,000 carbon atoms in the chain 0.5 ethyl), the molecular chain can be arranged regularly, closely packed, easy to form a crystalline structure, and therefore high density (0.94-0.96 / g, /cc), high crystallinity (80% - 95%) melting temperature (126 - 136 °C), hardness, strength, rigidity, toughness are also relatively high. Also excellent heat resistance (softening temperature 124 - 127 °C, continuous heat temperature 120 °C, decomposition temperature of 350 °C), better solvent resistance, vapor permeability. In general, the degree of crystallinity decreases with increasing temperature, and HDPE quenches the crystallinity by 40%, thereby changing its performance. Melt flow rate refers to the amount of molten material extruded within a certain period of time under specified conditions. It reflects the melt viscosity and is a measure of process fluidity. For HDPE, due to the larger molecular weight (relative molecular mass from tens of thousands to hundreds of thousands), high density, high viscosity, so the melt flow rate is low (0.1-4/g/10min), more expressed by the reduced viscosity . The HDPE water absorption rate is very low (0.03%), so it does not require drying during processing. HDPE extrusion temperature of 165 - 260 °C pressure of 35-140MPa. At the time of injection, the molding temperature was 180-250°C, the mold temperature was 50--70°C, and the injection pressure was 80-100 MPa. Molding line shrinkage is 2%-5%
2. PP, polypropylene and polyethylene belong to the same polyolefin, and they must have certain commonalities. They are all linear polymer compounds and almost do not contain double bonds. Still saturated aliphatic long-chain polymers and also non-polar crystalline polymers. In particular, it is similar in terms of swelling properties, solution properties (resistance, acid, alkali, organic solvent, and absorption and expansion of moisture, etc.), and electrical properties. However, PP and PE have some differences in the main chain structure. There are methyl groups alternately on the carbon atoms of PP main chain. The existence of methyl group makes the main chain slightly stiff, also reduces the symmetry of the molecule, and activates the carbon atoms. Oxygen is more sensitive and less stable. Overheating will lead to the breakage of the PP molecular chain, not glued. (Usually add a combination of antioxidants, antioxidants). The relative molecular weight of general industrial PP is higher than the corresponding value of PE, between 220,000 and 700,000. Another important factor that affects PP performance is isotacticity. That is, the order of methyl groups on the molecular chain. (Atactic PP is an amorphous viscous material that cannot crystallize and has no strength.)
Based on the above factors, the strength, rigidity and hardness of isotactic PP are higher than those of PE under normal conditions. Thermal oxidation stability is worse than PE. The heat resistance of PP is higher than that of PE. The general industrial PP (isotacticity 90%--95%) has a melting temperature of 165--170°C. The long-term use temperature can be 100--120°C, and the heat distortion temperature/°C. (1.86 MPa) is 56--67°C. The melt flow rate of PP (2401 Yanshan brand) is 2-3.5/g/10min, which is larger than PE and has good fluidity. When processing, it is easier to mold. Due to the high melting temperature of PP, thermal oxidation is more sensitive - so its heating time during the molding process should be minimized. PP injection molding, injection temperature 200--230 °C mold temperature 20--60 °C, injection pressure 70--100 °C PP density 0.9-0.91g / cc, water absorption 0.02-0.03% (processing, do not have to dry) , Molding shrinkage 1.0-2.5%. Generally HDPE high-density polyethylene injection molding process, if the plastic parts are not full, may increase his temperature, pressure, pressure holding time. On the contrary, if the plastic parts fly, then the above parameters are reduced accordingly. PP polypropylene is usually used to regulate pressure because of its temperature sensitivity.
3. PET polyethylene terephthalate
1 PET resin molecules contain ester groups, which have a certain degree of water absorption (water absorption rate is greater than 0.08%), and the performance of the products after water absorption changes little. However, even if a small amount of water is present after the pellets absorb moisture, melting of the pellets will lead to degradation of the resin, resulting in breakage of the molecular chains and decrease in strength. Therefore, it must be dried before molding. It is recommended to dry at 130 °C to 150 °C for 3 to 5 hours and 150 to 170 °C for 2 hours. After drying, it should be placed in the dryer for backup to prevent it from absorbing water again.
2 PET is a crystalline polymer with a distinct melting point (258°C). After melting, it has good fluidity. However, when the range of the processing temperature is narrow (270--290°C) exceeds 304°C, the resin degrades and the product becomes black.
3 PET molding shrinkage rate of 1.5-2%.
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