The plasticizing screw of the injection molding machine […]
The plasticizing screw of the injection molding machine has the functions of conveying, melting, mixing, compression, metering and exhaust, which play an important role in the quality of plasticization.
1. Several important geometric parameters of the screw:
1, screw diameter (D)
A. Generally speaking, the screw diameter D is inversely proportional to the maximum injection pressure and proportional to the plasticizing capacity.
2, conveying section
A. Responsible for the transportation, pushing and preheating of the plastic, and ensure that the preheating reaches the melting point;
B, crystalline plastics should be long (such as POM, PA), followed by amorphous materials (such as PS, PU, ABS), and the shortest heat sensitivity (such as PVC).
3, compression section
A. Responsible for the mixing, compression and pressurization of plastics. The raw materials passing through this section have almost all melted, but they may not be evenly mixed;
B. In this area, the plastic gradually melts, and the volume of the screw groove must decrease accordingly to correspond to the decrease in the geometric volume of the plastic, otherwise the material will not be compacted, heat transfer will be slow, and exhaust will be poor;
c. 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% screw working length, PVC screw can account for 100% screw working length to avoid intense shearing heat.
4, measurement section
A. Generally 20-25% of the working length of the screw is used to ensure that all plastics are melted and the temperature is uniform, and the mixing is uniform;
B. Long metering section will result in better mixing effect, too long will make the melt stay too long and cause thermal decomposition, too short will make temperature uneven;
C, PVC and other heat-sensitive plastics should not stay for too long to avoid thermal decomposition. A shorter metering section or no metering section can be used.
5. Feeding screw groove depth, measuring screw groove depth
a. The deeper the feed groove depth, the greater the conveying capacity, but the strength of the screw must be considered. The shallower the metering groove depth, the higher the plasticizing heat and mixing performance index, but the shallow metering groove depth means shearing Heat increases, self-generated heat increases, and the temperature rise is too high, causing plastic discoloration or burning, especially not conducive to heat-sensitive plastics;
B. Measuring screw groove depth=KD=(0.03-0.07)D, if D increases, select a small value for K.