Cause and solution of shearing heat of screw in injection molding machine

1、 Role of screw segments
1. Screw conveying section function
The screw conveying section of the injection molding machine is responsible for the transportation, extrusion and preheating of plastics. The materials should reach the melting point temperature after passing through the conveying section. Generally, the screw conveying section accounts for about 50% of the total length of the screw.
The deeper the screw groove in the conveying section is, the greater the conveying capacity is, but the actual design needs to consider the strength of the screw.
2. Screw compression section function
The screw compression section is responsible for plastic compression, pressurization and exhaust. After passing through this section, the materials have almost been dissolved, but not uniformly mixed. Generally, the compression section accounts for 20% - 25% of the total length of the screw.
The plastic gradually melts in this area, and the volume of screw groove must be gradually reduced, otherwise the material pressure is not solid, the heat transfer is slow, and the exhaust is poor, which will also cause the formation of workpiece pores.
The screw compression ratio is the ratio of the depth of the last screw groove in the feeding section to the depth of the first screw groove in the metering section. When the compression ratio is fixed, the longer the screw compression section is, the smaller the shear heat is. Similarly, if the compression section length is fixed, the smaller the compression ratio is, the smaller the shear heat is. Therefore, the compression section structure is the main part that determines the shear heat.
Generally, screw compression ratio of low shear heat is 1.8-2.2, and h2 (screw groove depth of metering section) is 0.09-0.1D (screw diameter)
3. Screw metering section function
The screw metering section generally accounts for 20% - 25% of the total length of the screw, and is mainly responsible for the uniform mixing of plastics to ensure the complete melting of plastics.
If the metering section is too long, the plastic mixing effect will be good. If it is too long, the melt will stay too long and cause thermal decomposition. If it is too short, the material temperature will be uneven.
Generally, when we select the injection molding machine to process the product, the best product weight is 70% - 80% of the injection volume. If the injection volume is too large, the melt will stay in the metering section for a long time, resulting in material thermal decomposition, and the final product will have pores, and even affect the mechanical properties of the product.
2、 Influence of process parameters on plastic processing
1. Melt glue back pressure
Increasing the back pressure of the sol can increase the work done by the screw on the molten resin, eliminate the unmelted plastic particles, and increase the density and uniformity of raw materials in the tube;
The increase of back pressure can increase the shear heat of plastic in the barrel, and then increase the barrel temperature. Therefore, the size of back pressure plays a certain role in the shear heat
If the back pressure of the sol is too large, the plastic with high thermal sensitivity is easy to decompose, and the plastic with low viscosity may be taped. On the contrary, if the back pressure is too small, the plastic will not be compacted, and then the workpiece will have pores.
2. Sol velocity
The rotation speed of the screw directly affects the shear of plastic in the spiral groove. The small screw groove absorbs heat faster than the shallow screw groove, which is enough to soften the plastic in the compression section. The friction between the screw and the barrel is low, which is suitable for high-speed rotation, improving the shear heat and increasing the plasticizing capacity.
Large screws should not be rotated quickly to avoid uneven plasticization, resulting in excessive friction heat. Generally, screws of all sizes have a certain rotational speed range. Generally, if the rotational speed is 100-150rpm, the plastic cannot be melted. If the rotational speed is too low, the material will be scorched.
3. Temperature setting of heating coil
The temperature generated by the heating ring will melt the cold and hard plastic trapped between the barrel and the rod to facilitate the rotation of the screw and provide a part of the heat required for plastic melting.
To sum up, there are several ways to reduce the shear heat of injection molding:
Reduce the melt back pressure.
Negative effect: it may cause the melt density to decrease and the workpiece is easy to form pores.
Reduce the speed of sol.
Negative effect: reduce production efficiency.
Reduce the compression ratio or lengthen the compression section length.
Negative effect: replace the screw and increase the cost.
Add h2 (screw groove depth of metering section).
Negative effect: It is not easy to process and requires surface treatment after processing.
Increase the temperature setting of compression section and metering section.
Negative effect: may cause material delay.
A cooling fan is installed at the metering section of the injection molding machine.
Negative effects: affect the appearance of equipment and increase the cost of electricity.