The barrel screw has several critical parameters that […]
The barrel screw has several critical parameters that affect the quality of plasticization. Generally, the barrel screw is selected according to the following principles:
1. Screw diameter (D)
1. Generally speaking, the screw diameter D is inversely proportional to the maximum injection pressure and directly proportional to the plasticizing power.
2, related to the required injection volume: injection volume=1/4*π*D2*S (injection stroke)*0.85;
2. Measurement section
1. Heat-sensitive plastics such as PVC should not stay for too long to avoid thermal differentiation. A shorter metering section or no metering section can be used.
2. Long metering section will result in good mixing results, too long will make the melt stay too long and thermal differentiation will occur, and too short will make the temperature uneven;
3. Generally, 20'25% of the screw working length is used to ensure that all plastics are melted and the temperature is average, and the mixing is average;
Three, compression section
1. Responsible for the mixing, compression and pressurization of plastics. The raw materials passing through this section have almost all melted, but they will be mixed evenly;
2. Generally it accounts for more than 25% of the working length of the screw, but the compression section of the nylon (crystalline material) screw accounts for about 15% of the working length of the screw, and the plastic screw with high viscosity, fire resistance, low conductivity and high additives accounts for 40% '50% of the working length of the screw, PVC screw can account for 100% of the working length of the screw to avoid violent shear heat.
3. In this area, the plastic gradually melts, and the volume of the screw groove must be reduced accordingly to correspond to the decrease in the volume of the plastic, otherwise the material is not compacted, the heat transfer is slow, and the exhaust is poor;
4. Feeding screw groove depth, measuring screw groove depth
1. The deeper the feed groove depth, the greater the conveying capacity, but the strength of the screw needs to be considered. The shallower the metering groove depth, the higher the plasticizing fever and the higher the inclusion function index, but the too shallow the metering groove depth will result in shearing. Heat increase, self-generated heat increases, and temperature rise is too high, causing plastics to discolor or burn, especially for heat-sensitive plastics;
2. Measuring screw groove depth=KD=(0.03'0.07)*D, if D increases, select a small value for K.
5. Conveying section
1. Responsible for the transportation, pushing and preheating of plastics, and preheating to the melting point should be guaranteed;
2. Long crystalline plastics (such as POM, PA) should be followed by amorphous materials (such as PS, PU, ABS), and the shortest heat sensitivity (such as PVC).