Summary of screw knowledge parameters of twin screw granulator

1. Several important geometric parameters of the screw

1, screw diameter (D)

A is related to the amount of injection required: ejection volume =1/4* PI *D2*S (firing stroke) *0.85;

B, generally speaking, the screw diameter D is inversely proportional to the maximum injection pressure and is directly proportional to the plasticizing capacity.

2. Transport section

A, responsible for transporting plastics, pushing and preheating, should ensure preheating to melting point.

B, crystalline plastics suitable for long (e.g. POM, PA) amorphous materials (such as PS, PU, ABS), heat sensitivity is the shortest (e.g. PVC).

3. Compression section

A is responsible for mixing, compressing and pressurizing the exhaust of plastics. The raw materials of this section have almost all melted, but they do not necessarily mix evenly.

B, in this area, the plastic gradually melt, the volume of the slot must be reduced to correspond to the drop of the plastic volume, otherwise the material pressure is not true, the heat transfer is slow, and the exhaust is bad.

C, generally accounts for more than 25% screw working length, but the compression section of nylon (crystalline material) screw accounts for about 15% screw working lengths, high viscosity, fire resistance, low conductivity, high additives and other plastic screws, which account for the working length of 40%'50% screw, and PVC screw can account for *** screw working lengths, so as to avoid intense shear heat.

4. Measurement section

A, generally account for 20'25% screw working length, to ensure that all plastics melt and temperature uniformity, mixing evenly;

B, the length of the metering section is good, and too long is easy to cause the melt to stay too long to produce thermal decomposition. Too short is easy to make the temperature uneven.

C, PVC and other thermosensitive plastics should not stay too long, so as to avoid thermal decomposition.

5, the depth of the feed slot, measuring the depth of the snail groove

The deeper the depth of a, the deeper the depth of the feed slot, the greater the transport, but the greater the strength of the screw, the higher the shallowness of the screw slot, the higher the mixing performance index, but the higher the depth of the grooves, the increase of the shear heat, the increase of the autogenic heat and the high temperature rise, which causes the discoloration or scorching of the plastic, especially the heat sensitive plastic.

B, the depth of the metering grooves =KD= (0.03'0.07) *D and D increase, and K selects a small value.

Two. The main factors affecting the plasticizing quality

The main factors affecting plasticizing quality are length diameter ratio, compression ratio, back pressure, screw speed and heating temperature.

1. The ratio of length to diameter is the ratio of the effective working length of screw to the diameter of screw.

A and large length and diameter ratio are easy to eat.

B, thermal stability of the plastic can be used for a long screw to improve the mixing but not burnt, the thermal stability of the plastic can be a shorter screw or screw thread at the end of the screw. In consideration of the plastic characteristics, the general flow length ratio is as follows: thermosetting is 14'16, hard PVC, high viscosity PU and other thermal sensitivity is 17'18. The general plastic is 18'22, PC, POM and other high temperature stability plastics are 22'24.

2, the compression ratio is the ratio of the last groove depth of the feed section to the first depth of the groove of the metering section.

A, considering the compressibility, loading degree and backflow of the material, the product should be compact, heat transfer and exhaust.

B, suitable compression ratio can increase the density of plastic, make the combination of molecules and molecules more closely, help to reduce the absorption of air, reduce the temperature rise caused by pressure, and affect the difference of output, the improper compression ratio will destroy the physical property of plastic.

The higher the ratio of C, the higher the compression ratio, the higher the temperature of plastic in the plasticizing process in the tube, the better uniformity of mixing in the plastic plastic and the decrease of the relative amount of the material.

D, high compression ratio suitable for non melt plastic, especially low melting viscosity, thermal stability plastic; low compression ratio suitable for melt plastic, especially high melting viscosity, thermal sensitive plastic.

3, back pressure

A, increasing back pressure can increase the work done by the screw to melt resin, eliminate the unmelted plastic particles, and increase the density and uniformity of raw materials.

B and back pressure are used to increase the barrel temperature, and the effect is most significant.

C, too big back pressure, easy to decompose the high heat sensitive plastic, the low viscosity plastic may produce the saliva phenomenon, the back pressure is too small, the finished product may have the bubble.

4. Screw speed

The rotation speed of a and screw affects the shear of plastic in spiral groove directly.

B and small screw slot are faster than shallow absorption heat sources, which can help the plastics to soften in the compression period, and the friction heat between the screw and the wall of the material is low, so it is suitable for high speed rotation and the plasticizing ability is increased.

C, large screw is not suitable for rapid rotation, so as to avoid unevenness of plasticization and cause excessive friction heat.

D, for high thermal sensitive plastics, if the screw speed is too large, the plastics will be easily decomposed.

E, usually the size of the screw has a certain speed range, the general speed 100'150rpm; too low can not melt the plastic, too high will burn the plastic.

5. Temperature setting of electric heating

A, so that the cold plastic stuck in the barrel and screw is melted to facilitate screw rotation, providing a part of the heat needed to melt the plastic.

B, setting temperature is lower than that of melts at 5'10 degrees (partly supplied by frictional heat energy).

The adjustment of C and nozzle temperature can also be used to control salivation, condensation material (Sai Ju) and wire drawing.

General temperature control of D and crystalline plastics