1. The performance of PE PE is the plastic with the largest output among plastics. It is characterized by soft, non-toxic, inexpensive, easy to process, good chemical resistance, not easy to corrode, and difficult to print. PE is a typical crystalline polymer. There are many types of it. Commonly used are LDPE (low density polyethylene) and HDPE (high density polyethylene), which are translucent plastics with low strength and a specific gravity of 0.94g/cm3 (less than water); very low density LLDPE resin (The density is lower than 0.910g/cc, and the density of LLDPE and LDPE is between 0.91 and 0.925). LDPE is relatively soft, (commonly known as soft rubber) HDPE is commonly known as hard soft rubber. It is harder than LDPE and is a semi-crystalline material. It has a higher shrinkage rate after molding. It has poor light transmission between 1.5 percent and 4 percent , and has a large crystallinity. It is easy Environmental stress cracking occurs. It is possible to use materials with very low flow characteristics to reduce internal stress and thereby reduce cracking. When the temperature is higher than 60 degree , it is easy to dissolve in hydrocarbon solvents, but its solubility resistance is better than LDPE. The high crystallinity of HDPE leads to its high density, tensile strength, high temperature distortion temperature, viscosity and chemical stability. It has stronger resistance to penetration than LDPE. The impact strength of PE-HD is low. The characteristics are mainly controlled by density and molecular weight distribution. The molecular weight distribution of HDPE suitable for injection molding is very narrow. For the density of 0.910.925g/cm3, we call it the first type of PE-HD; for the density of 0.9260.94g/cm3, we call it the second type of HDPE; for the density of 0.940.965g/cm3, we call it It is the third type of HDPE. The material has very good flow characteristics, with an MFR between 0.1 and 28. The higher the molecular weight, the worse the flow characteristics of LDPE, but it has better impact strength. HDPE is prone to environmental stress cracking. It is possible to reduce internal stress by using materials with very low flow characteristics, thereby reducing cracking. HDPE is easily dissolved in hydrocarbon solvents when the temperature is higher than 60C, but its solubility resistance is better than LDPE.
LDPE is a semi-crystalline material with a high shrinkage rate after molding, ranging from 1.5 percent to 4 percent . LLDPE (Linear Low Density Polyethylene) has higher stretch resistance, penetration resistance, impact resistance and tear resistance properties that make LLDPE suitable for film. Its excellent resistance to environmental stress cracking, low temperature impact resistance and warpage resistance make LLDPE attractive for pipe, sheet extrusion and all molding applications. The latest application of LLDPE is as a mulch film for waste landfill and waste liquid pond lining.
2. Process characteristics of PE The most notable feature of PE parts is the large molding shrinkage rate, which is easy to shrink and deform. PE material has low water absorption, so it does not need to be dried. PE has a wide processing temperature range and is not easy to decompose (the decomposition temperature is 320 degree ). If the pressure is high, the density of the part will be high and the shrinkage rate will be small. PE has medium fluidity, so the processing conditions should be strictly controlled and the mold temperature should be kept constant (40-60 degree ). The degree of crystallization of PE is related to the molding process conditions. It has a higher cold-setting temperature, and the mold temperature is low, and the crystallinity is low. During the crystallization process, due to the anisotropy of shrinkage, internal stress is concentrated, and PE parts are prone to deformation and cracking. The product is placed in a water bath in hot water at 80 degree to relax the pressure to a certain extent. During the molding process, the material temperature and mold temperature should be higher. The injection pressure should be lower under the premise of ensuring the quality of the part. The cooling of the mold is particularly required to be rapid and uniform, and the product is hot when it is demolded. HDPE drying: no need to dry if stored properly. The melting temperature is 220260C. For materials with larger molecules, the recommended melting temperature range is between 200 and 250C.
Mold temperature: 5095C. Plastic parts with a wall thickness of 6mm or less should use a higher mold temperature, and plastic parts with a wall thickness of 6mm or more should use a lower mold temperature. The cooling temperature of the plastic parts should be uniform to reduce the difference in shrinkage. For the optimal processing cycle time, the diameter of the cooling channel should not be less than 8mm, and the distance from the mold surface should be within 1.3d (where "d" is the diameter of the cooling channel). Injection pressure: 7001050bar. Injection speed: High-speed injection is recommended. Runner and gate: The diameter of the runner is between 4 and 7.5mm, and the length of the runner should be as short as possible. Various types of gates can be used, and the gate length should not exceed 0.75mm. Especially suitable for the use of hot runner molds. The "soft when stretched" feature of LLDPE is a disadvantage in the blown film process. The blown film bubble of LLDPE is not as stable as that of LDPE. The die gap must be widened to avoid lowering production due to high back pressure and melt fracture. The general die gap sizes of LDPE and LLDPE are 0.024-0.040 in and 0.060-0.10in, respectively.
3. Typical application range: LLDPE has penetrated into most traditional polyethylene markets, including films, molding, pipes, wires and cables. Anti-leakage mulch is a newly developed LLDPE market. Mulch, a large extruded sheet, is used as a waste landfill and waste pool liner to prevent leakage or pollution of the surrounding area. For example, the production of bags, garbage bags, elastic packaging, industrial liners, towel liners and shopping bags, these are the advantages of this resin after the use of improved strength and toughness. Transparent films, such as bread bags, have always been dominated by LDPE because of its better turbidity. However, a blend of LLDPE and LDPE will improve strength. Penetration resistance and rigidity of the LDPE film without significantly affecting the transparency of the film. HDPE application range: refrigerator containers, storage containers, household kitchen utensils, sealed lids, etc.
