Extruder screw parameters and influence First, several […]
Extruder screw parameters and influence
First, several important geometric parameters of the screw 1. Screw diameter (D)
a, related to the required injection volume:
Injection volume = 1/4 * π * D2 * S (injection stroke) * 0.85;
b. In general, the screw diameter D is inversely proportional to the highest injection pressure and proportional to the plasticizing capacity.
2. Conveying section
a, responsible for the transportation of plastic, pushing and preheating, should ensure preheating to the melting point;
b, crystalline plastics should be long (such as: POM, PA) amorphous materials (such as: PS, PU, ABS), the heat sensitivity is the shortest (such as: PVC).
3, the compression section
a. Responsible for the mixing, compression and pressurization of plastics. The raw materials through this section have been almost completely melted, but not necessarily uniformly mixed;
b. In this area, the plastic gradually melts, and the volume of the screw groove must be correspondingly decreased to correspond to the decrease of the geometric volume of the plastic. Otherwise, the material pressure is not real, the heat transfer is slow, and the exhaust gas is poor;
c, generally accounted for more than 25% of the screw working length, but the compression section of nylon (crystalline material) screw accounts for about 15% of the screw working length, high viscosity, fire resistance, low conductivity, high additives and other plastic screws, accounting for 40% -50% screw working length, PVC screw can account for 100% of the screw working length, so as not to produce intense shear heat.
4, metering section
a, generally accounted for 20-25% of the screw working length, to ensure that the plastic is fully melted and the temperature is uniform, evenly mixed; b, the metering section length is good mixing effect, too long is easy to make the melt stay too long to produce thermal decomposition, Too short is easy to make the temperature uneven;
c, PVC and other heat-sensitive plastics should not stay too long, so as to avoid thermal decomposition, you can use a shorter metering section or do not meter the section.
5, feed screw groove depth, metering groove depth
a. The deeper the depth of the feed screw groove, the larger the conveying volume, but the screw strength should be considered. The shallower the metering groove depth, the plasticizing heat and the higher the mixing performance index, but the metering groove depth is too shallow. The heat increases, the self-generated heat increases, and the temperature rise is too high, causing plastic discoloration or charring, especially for heat-sensitive plastics;
b. Metering groove depth = KD = (0.03-0.07) * D, D increases, then K selects a small value.
Second, the main factors affecting the quality of plasticization
The main factors affecting the quality of plasticization are: aspect ratio, compression ratio, back pressure, screw speed, barrel heating temperature, etc.