How to Solve the Shear Heat of Injection Molding Machine Screw?
Injection molding machine screw is generally divided into three parts: feed section, compression section, metering section.
To process different materials, you should select the material of corresponding structure to obtain good workpiece processing quality.
This article analyzes the structure of the injection molding machine screw and the processing technology to solve the problem of shear heat of injection molding machine.
1. The role of each screw section
(1) The role of screw feed section
The screw feed section of the injection molding machine is responsible for the conveying, pushing and preheating of the plastic. The material should reach the melting point temperature through the feed section. Generally, the screw feed section accounts for about 50% of the total length of the screw.
The deeper the groove depth of the feed section, the greater the conveying capacity. But the actual design needs to consider the strength of the screw.
(2) The role of screw compression section
The screw compression section is responsible for plastic compression, pressurization and air exhaust. The resin have almost completely dissolved after passing through this section, but they are not uniformly mixed. Generally, the compression section accounts for 20%-25% of the total length of the screw.
In this area, the resin gradually melts, and the volume of the screw groove must be gradually reduced. Otherwise, the melt plastic will not be compacted, the heat transfer will be slow. And the air exhaust will be poor. Besides, it will also cause the formation of pores in the plastic product.
The screw compression ratio is the ratio of the depth of the last groove in the feeding section to the depth of the first groove in the metering section. When the compression ratio is constant, the longer the screw compression section, the smaller the shear heat. Similarly, if the compression section length is constant, the smaller the compression ratio, the smaller the shear heat. Therefore, the compression section structure is the main part that determines the magnitude of the shear heat.
Three-stage combined screw
Generally, the compression ratio of the screw with low shear heat is 1.8-2.2, and the h2 (the depth of the groove in the metering section) is 0.09-0.1D (the screw diameter).
(3) The role of the screw metering section
The metering section of the screw generally occupies 20%-25% of the total length of the screw. It is mainly responsible for the uniform mixing of the plastic and ensuring that the plasticization is complete.
If the metering section is long, the plastic mixing effect will be good. If it is too long, the melt will stay for too long and cause thermal decomposition. If it is too short, the temperature of the material will be uneven.
Generally, when we select the injection molding machine to process products, the best product weight is 70%-80% of the machine injection volume. If the injection volume is too large, the melt plastic will stay in the metering section for a long time. And that will result in thermal decomposition of the material and pores in the final product. What’s worse, it even affects the mechanical properties of the product.
- Oil separator
- Heat recoverer
- Screw compressor
- Control board
- Eletrical control box
- Evaporator
- Sight glass
- Angle valve
- Dry filter
- Condenser
2. The influence of machine parameters setting on injection molding process
(1) Screw back pressure
Increasing the back pressure can increase the work done by the screw on the molten resin, eliminate unmelted plastic particles. Besides, it also increases the density and uniformity of the raw materials in the barrel.
The increase of the back pressure can increase the shear heat of the plastic in the barrel, thereby increasing the barrel temperature. So the strong of the back pressure has a certain effect on the shear heat.
If the back pressure too strong, the plastic with higher heat sensitivity will be easy to decompose. It may cause overflow in the nozzle to the low viscosity plastic. On the contrary, if the back pressure is too small, the plastic will not be compressed, and the plastic product will have pores.
(2) Melting speed
The rotation speed of the screw directly affects the shear of the plastic in the spiral groove. The shallower screw groove of the small screw groove absorbs the heat source quickly, which is enough to soften the plastic in the compression section. The friction between the screw and the barrel is low. So it’s suitable to adopt high-speed rotation and increase the shear heat and the plasticizing capacity.
Large screws should not be rotated quickly to avoid uneven plasticization, resulting in transitional frictional heat. Generally, screws of various sizes have a certain speed range. Generally, if the speed of 100-150rpm is too low, the plastic cannot be melted. And if the speed is too high, the material will be burned.
(3) Heating brand temperature setting
The temperature generated by the heating brand melts the chilled plastic that remains between the barrel and the screw. That can facilitate the rotation of the screw and provide part of the heat required for the melting of the plastic.
In summary, there are several ways to reduce the shear heat of injection molding:
- Reduce melt back pressure.
Negative effect: It may decrease the melt density, and make the plastic product easy to form pores.
- Reduce the screw rotation speed.
Negative effect: reduce production efficiency.
- Reduce the compression ratio or increase the length of the compression section.
Negative effect: Replace the screw and increase the cost.
- Increase h2 (the depth of the groove in the metering section) of the screw.
Negative effect: It is not easy to process and requires surface treatment for the screw after processing.
- Increase the temperature settings of the compression section and the compression section.
Negative effect: It may cause material to overflow.
- Install a cooling fan at the metering section of the injection molding machine.
Negative effects: It will affect the appearance of the equipment and increase the cost of electricity.
