PE is one of the most produced plastics, characterized by soft, non-toxic, inexpensive, easy processing, good chemical resistance, not easy to corrode, and difficult to print.
Properties of PE plastics
It has many types, commonly used LDPE (low density polyethylene) and HDPE (high density polyethylene), translucent plastic, low strength, specific gravity of 0.94g/cm3 (smaller than water); very low density LLDPE resin (density below 0.910g/cc, LLDPE and LDPE density are in 0.91-0.925 (between).
LDPE is softer, (commonly known as soft glue) HDPE is commonly known as hard soft glue, it is harder than LDPE, is a semi-crystalline material, after molding shrinkage is high, between 1.5% to 4% poor light transmission, crystallinity is large, it is easy to environmental stress cracking phenomenon. The cracking phenomenon can be reduced by using materials with very low flow characteristics to reduce the internal stress. It is easily dissolved in hydrocarbon solvents when the temperature is higher than 60°C, but its resistance to dissolution is somewhat better than LDPE.
The high crystallinity of HDPE results in high density, tensile strength, high temperature twist temperature, adhesion, and chemical stability. It has greater resistance to permeation than LDPE. PE-HD has a lower impact strength. Properties are mainly controlled by density and molecular weight distribution.
The molecular weight distribution of HDPE suitable for injection molding is very narrow. For density of 0.91~0.925g/cm3, we call it the first type PE-HD; for density of 0.926~0.94g/cm3, it is called the second type HDPE; for density of 0.94~0.965g/cm3, it is called the third type HDPE.
The material has good flow characteristics with MFR between 0.1 and 28. HDPE is susceptible to environmental stress cracking. HDPE is easily dissolved in hydrocarbon solvents when the temperature is above 60C, but its resistance to dissolution is better than LDPE.
LDPE is a semi-crystalline material with a high shrinkage rate between 1.5% and 4% after molding.
LLDPE's (linear low density polyethylene) higher resistance to elongation, penetration, impact and tearing makes LLDPE suitable for use as a 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 ground film for landfill and waste liquid pond liners.
Process characteristics of PE
The most significant feature of PE parts is the molding shrinkage rate, easy to produce shrinkage and deformation. PE material is small water absorption, can be used without drying. PE processing temperature range is wide, not easy to decompose (decomposition temperature of 320 ℃), if the pressure is high, the parts of high density, shrinkage rate is small.
PE liquidity is medium, to strictly control the processing conditions, and keep the mold temperature constant (40-60 ℃). the degree of crystallization of PE is related to the molding process conditions, it has a high cold solidification temperature, mold temperature is low, the degree of crystallization is low. In the crystallization process, due to the anisotropy of shrinkage, resulting in internal stress concentration, PE parts are prone to deformation and cracking.
The product is put in a water bath of 80℃ hot water, which can make the pressure get some relaxation. During the molding process, the material temperature and mold temperature is higher than appropriate, the injection pressure is low under the premise of ensuring the quality of the parts, the cooling of the mold especially requires rapid and uniform, and the product is hotter when it is demolded.
HDPE drying: no need to dry if stored properly. Melting temperature 220~260C. For materials with larger molecules, it is recommended that the melting temperature range is between 200~250C.
Mold temperature: 50~95C. Higher mold temperature should be used for plastic parts with wall thickness below 6mm, and lower mold temperature for plastic parts with wall thickness above 6mm. The cooling temperature of plastic parts should be uniform to minimize the difference of shrinkage. For optimal cycle time, the cooling cavity diameter should be not less than 8mm and the distance from the mold surface should be within 1.3d (where "d" is the diameter of the cooling cavity).
Injection pressure: 700~1050 bar. injection speed: high speed injection is recommended. Runners and gates: The runner diameter should be between 4 and 7.5 mm and the runner length should be as short as possible. Various types of gates can be used, the gate length should not exceed 0.75mm. especially suitable for molds with hot runners.
LLDPE's "soft on stretch" characteristic is a drawback in the blown film process, as LLDPE blown film bubbles are not as stable as LDPE. The die gap must be widened to avoid lower yields due to high back pressure and melt breakage.
Typical mold gap dimensions for LDPE and LLDPE are 0.024-0.040 in and 0.060-0.10 in, respectively.
Typical application areas
LLDPE has penetrated most traditional markets for polyethylene, including film, molding, tubing and wire and cable. Leak-proof ground membranes are a newly developed LLDPE market. Mulch, a large extruded sheet, is used as a landfill and waste pond liner to prevent leaks or contamination of surrounding areas.
LDPE is mainly used for transparent films, such as bread bags, because of its better turbidity.
HDPE applications are mainly refrigerator containers, storage containers, household kitchenware, sealing lids, etc.
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