Screw Diameter (D)
The screw diameter is inversely proportional to high injection pressure and it is proportional to plasticizing power.
The correlation with the required injection volume = 1/4 x π x D2 x S (injection stroke) x 0.85
Measurement Section
The PVC and other heat-sensitive plastics should not stay long for avoiding thermal differentiation. There should be a shorter metering section or no metering section.
The length of the metering section is very important as if it is too long the heat will be differentiated and if it is too short the temperature will irregular.
There is 20 to 25% of screw working length for ensuring the melting of plastic and temperature and mixing is average.
Compression Section
The mixing, compression, and pressurized exhaust take place in the compression section. Raw material in the compression section is melted completely and also mixed evenly.
It is generally 25% of screw working length but the compression section of nylon (crystalline material) screw is about 15% of screw working length, high viscosity, fire resistance, low conductivity, high additives. Some of the plastic screws are accounted for 40 to 50% of screw working length. The PVC screw is accounted for 100% of the screw working length for avoiding severe shear heat.
The plastic is melted gradually in the compression section and the volume of the screw groove must reduce to correspond to the reduction of the volume of the plastic. If not, the material pressure is not real the heat transfer is slow and exhaust gas is poor.
Feed Screw Groove Depth and Metering Groove Depth
If the deeper the depth of the feeding groove the larger the conveying volume and the screw strength should consider. The shallow the measuring groove depth the plasticizing fever and inclusion function index is higher. But the metering groove depth is too shallow. The increased heat, self-generated hear increases and temperature rise is to high cause the plastic discoloration of charring especially helium in sensitive plastics.
The groove depth = KD = (0.03 x 0.07) x D, if D increases, select the small value of K
Conveying Section
The plastic is transported, pushed, and preheat in the conveying section. Also, it is preheated to the melting point
The crystalline plastics should be long like POM, PA amorphous materials like PS, PU, ABS, heat-sensitive zui short like PVC.
