Polyethylene is a typical thermoplastic, is odorless, tasteless, non-toxic flammable white powder. PE resin molding process are extruded granulated waxy particles, the appearance was milky white. The molecular weight of 10,000 l0 million range.
The molecular weight of more than 100,000 while the ultrahigh molecular weight polyethylene.However, the higher the molecular weight, the better the physical and mechanical properties, the closer the level of engineering materials.However, the higher the molecular weight, the more difficult it is to process.Polyethylene melting point 100-130C, its low temperature performance is excellent at -60 ℃ can still maintain good mechanical properties, but the use of temperature at 80 ~ 110 ℃.
Polyethylene under the action of the atmosphere, sunlight and oxygen, aging, discoloration, cracking, brittle or pulverized, loss of its mechanical properties.
In the molding temperature, but also due to oxidation, the melt degree of decline, the occurrence of discoloration, streaks, and therefore in the molding process and the use of process or material selection should be noted that, because polyethylene has the above characteristics, easy processing Forming, therefore, polyethylene recycling has a very far-reaching value.
The main forming conditions of polyethylene
Cylinder temperature: Cylinder temperature is mainly related to the density of PE and the size of the melt flow rate, in addition to the type and performance of the injection molding machine, the shape of a plastic parts.
As PE is a crystalline polymer, the crystal should absorb a certain amount of heat during melting, so the temperature of the cylinder should be higher than its melting point by 10 ° C. For LDPE, the temperature of the cylinder is controlled at 140-200 ° C, Cylinder temperature control at 220 ℃, take the smallest cylinder back, take the maximum front.
Mold Temperature: Mold temperature has a greater impact on the crystallization of plastic parts, high mold temperature, high melt crystallinity, high strength, but the shrinkage rate will increase.LDPE mold temperature is usually controlled at 30 ℃ ~ 45 ℃, The corresponding high temperature HDPE 10 ~ 20 ℃.
Injection pressure: to improve the injection pressure is conducive to the filling of the melt, due to the mobility of PE is very good, so in addition to the thin slender products, the injection pressure should be well selected lower pressure, the general injection pressure of 50 ~ 100MPa. Simple shape After the larger plastic parts behind the wall, injection pressure can be lower, and vice versa.
Drying: No drying if stored properly.
Melting temperature: 220 ~ 260 C. For larger molecular materials, the proposed melting temperature range between 200 ~ 250C.
Mold temperature: 50 ~ 95 C. 6mm thick plastic parts should use a higher mold temperature, more than 6mm thick plastic parts use a lower mold temperature.
The cooling temperature of the plastic parts should be uniform to reduce the difference in shrinkage rate. For optimum machining cycle time, the diameter of the cooling channels should be not less than 8mm and the distance from the mold surface should be within 1.3d (where 'd' is the cooling The diameter of the tunnel).