How to Extend the Service Life of the Injection Molding Machine Screw?
1. Brief Introduction of Injection molding machine screw
The screw is an important part of the injection molding machine. Its function is to transport, compact, melt, stir and apply pressure to plastics. Those are accomplished by the rotation of the screw in the barrel. When the screw rotates, the plastic will produce friction and mutual movement on the inner wall of the barrel, the bottom surface of the screw groove, and between the plastics.
The advancement of plastic is the result of this combination of movements. And the heat generated by friction is also absorbed to increase the temperature of the plastic and melt the plastic. The design structure of the screw will directly affect how these processes work.
The screw of the injection molding machine itself has undergone various surface treatments, from quenching to nitriding, boronizing to spraying, and then to double alloy spraying. Its quality has been improved a lot. But in actual production, why is there such a big difference in the life span f screw? The same injection molding machine screw can be used for 8 years in some factories. While some can only be used for 2 months.
2. Screw segment description
In general, the screw of the injection molding machine can be divided into a feeding section, a compression section, and a homogenization section (also called a metering section).
Note: Different screws have different ratio of the three sections, different depth of the screw groove, different transition form of the screw bottom diameter.
2.1 Feeding section
This section has a fixed depth of screw grooves. Its function is to be responsible for preheating and plastic solid transportation and pushing. It must be ensured that the plastic begins to melt at the end of the feed section.
2.2 Compression section
This section is the depth of the screw groove of the tapered screw. Its function is the melting, mixing, shear compression and pressurized exhaust of plastic raw materials. The plastic will be completely dissolved in this section and the volume will be reduced. In this section, the design of the compression ratio is very important.
2.3 Homogenization section
This section is the fixed groove depth of the screw thread. Its main function is mixing, melt transportation, and metering. It must also provide sufficient pressure to maintain a uniform temperature of the melt and stabilize the flow of the molten plastic.
3. Maintenance of injection molding machine screw
The key to the life of the injection molding machine screw is use and maintenance. The following use and maintenance principles are for reference:
3.1 Fully preheat the injection molding machine.
When the injection molding machine is started, it must be fully preheated. After the barrel reaches the preset temperature, keep heat preservation for 15 minutes below Φ60mm. After that, 30 minutes later, start the motor and the related actions of screw.
3.2 Material and screw must match.
Especially for the corrosive materials and hard materials, such as PVC, flame-retardant materials, and glass fiber-added materials. We should use the professional screws instead of ordinary screws.
If the corrosive materials are used for an ordinary machine, then it must be cleaned and replaced with stable materials such as HDPE, HIPS, etc. immediately after use.
If the glass fiber materials are used in the ordinary machine occasionally, remember to set high material temperature, low screw speed, and low back pressure.
Regardless of the material, try not to produce at the lower temperature limit. For example, PC material can barely be produced at 250 degrees, and ABS can be barely produced at 185 degrees. However, production at these temperatures will be bad for the screw because it requires a large torque and will be severely worn. At the same time, if the lower limit must be used, use a low screw speed.
If the machine is shut down for more than half an hour each time, the blanking port should be closed. Besides, the material in the barrel should be cleaned, and heat preservation should be provided.
Avoid foreign matter falling into the barrel and damage to the screw and barrel. Prevent metal fragments and debris from falling into the hopper. If processing recycled materials, a magnetic hopper must be added to prevent iron filings from entering the hopper.
Make sure that the plastic in the barrel is completely melted, so as not to damage the transmission system parts when the screw retreats.
Avoid the phenomenon of screw idling and slipping.
When using new plastic, the remaining material in the barrel should be cleaned. When using POM, PVC, PA+GF and other materials, try to minimize the degradation of the raw materials. And then rinse them off with ABS and other nozzle materials in time after shutdown.
Avoid mixing POM and PVC into the barrel at the same time. Because it will react at the melting temperature and cause serious industry accident.
If the temperature of the molten plastic is normal but black spots or discoloration of the molten plastic are constantly found, check whether the rubber screw non-return ring seat (apron, meson) is damaged.