Introduction to screw knowledge parameters of single/twin screw extruder

September 17, 2019

Introduction to screw knowledge parameters of single/twin screw extruder
 

First, there are everal important geometric parameters of a screw
 

1, screw diameter (D)
a). Relevant 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 pressurized exhaust of plastics. Almost all of the raw materials through this section have been 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 length of the metering section is good for mixing. If it is too long, the melt will stay too long and cause thermal decomposition. If it is too short, the temperature will be 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 groove, the larger the conveying capacity, 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.

 

1. Length to diameter ratio: the ratio of the effective working length of the screw to the diameter of the screw.
a), the long diameter ratio is large, the feeding material is easy to be uniform;
b). The plastic with better thermal stability can be used with a longer screw to improve the kneading property without scorching. The plastic with poor thermal stability can be used without a screw or a screw end. Considering the plastic characteristics, the general flow length is as follows: thermoset is 14'16, hard PVC, high viscosity PU and other heat sensitivity is 17'18, general plastic is 18'22, PC, POM and other high temperature stability plastic is 22'24 .

 

2. Compression ratio: the ratio of the last groove depth of the feed section to the first groove depth of the metering section.
a). Considering the compressibility, filling degree, reflow, etc. of the material, the product should be dense, heat transfer and exhaust;
b). Appropriate compression ratio can increase the density of plastics, make the combination of molecules and molecules more tight, help to reduce the absorption of air, reduce the temperature rise caused by pressure, and affect the difference of output, improper compression Than will destroy the physical properties of the plastic;
c). The higher the compression ratio, the higher the temperature rise generated during the plasticization process of the plastic in the material tube, and the better the uniformity of the plasticization in the plasticized plastic, and the relative discharge amount is greatly reduced.
d). High compression ratio is suitable for non-melting plastics, especially low melt viscosity, heat stable plastic; low compression ratio is suitable for fusible plastics, especially high melt viscosity, heat sensitive plastics.

 

3, back pressure
a). Increasing the back pressure can increase the work done by the screw on the molten resin, eliminate the unmelted plastic particles, and increase the density and uniformity of the raw materials in the material tube;
b), back pressure is used to increase the temperature of the barrel, the effect is most significant;
c). The back pressure is too large, and the plastic with high heat sensitivity is easy to be decomposed. For the low viscosity plastic, there may be a flow phenomenon, and the back pressure is too small, and the finished product may have air bubbles.

 

4, screw speed
a), the rotation speed of the screw directly affects the shear of the plastic in the spiral groove;
b). The shallow screw groove absorbs the heat source quickly, which is enough to soften the plastic during the compression period. The frictional heat energy between the screw and the wall of the barrel is low, suitable for high-speed rotation and increase the plasticizing ability;
c). Large screws should not be rotated quickly to avoid uneven plasticization and excessive friction heat;
d). For plastics with high heat sensitivity, if the screw speed is too large, the plastic will be easily decomposed;
e). Generally, the screw of each size has a certain speed range, and the general speed is 100'150 rpm; if it is too low, the plastic cannot be melted, and if it is too high, the plastic is burnt.

 

5, Electric temperature setting
A. Melting the chilled plastic retained in the barrel and the screw to facilitate the rotation of the screw, providing a part of the heat required for the plastic to melt;
B. The setting is 5'10 ° C lower than the melting temperature (partially provided by frictional heat);
C,The adjustment of the temperature of the nozzle can also be used to control problems such as flow, condensate (plug) and wire drawing;
D, Crystalline plastics generally temperature control.

 

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