Screw is an important part of injection molding machine. Its function is to transport, compact, melt, stir and apply pressure to plastics. All of this is accomplished by rotation of screw in barrel. When screw rotates, plastic will generate friction and mutual movement on inner wall of barrel, bottom surface of screw groove, screw edge advancement surface, plastic and plastic.
Advancement of plastic is result of this combination of movements, heat generated by friction is also absorbed to increase temperature of plastic and melt plastic. Design structure of screw will directly affect extent of these effects.
Screw of injection molding machine works under high temperature, high pressure, high mechanical torque and high friction environment for a long time, and loss is inevitable. Even if screw of injection molding machine has undergone various surface treatments, from quenching to nitriding, boronizing to spraying, to double alloy spraying, it seems that quality of screw itself has been improved a lot.
But in actual production, why is there such a big difference in life span? Same injection molding machine screw can be used in some factories for 8 years, and some can only be used for 2 months. Same is difference between injection molding machine screw and iron.

 

In general, screw of injection molding machine can be divided into a feeding section, a compression section, and a homogenization section (also called a metering section).
Note: Ratio of three sections of different screws is different, depth of screw groove is different, and transition form of screw bottom diameter is different)
(1) Description of feeding section: depth of screw groove in this section is fixed, its function is to be responsible for preheating, plastic solid transportation and pushing. It must be ensured that plastic begins to melt at the end of feed section.
(2) Description of compression section: This section is depth of screw groove of tapered screw, its function is melting, mixing, shear compression and pressurized exhaust of plastic raw materials. Plastic will be completely dissolved in this section and volume will be reduced. Design of compression ratio is very important.
(3) Description of homogenization section: This section is fixed groove depth of screw groove. Its main function is mixing, melt conveying, and metering. It must also provide sufficient pressure to maintain a uniform temperature of melt and stabilize flow of molten plastic.

 

1) When injection molding machine is started, injection molding must be fully preheated. After barrel reaches preset temperature, keep temperature for temperature below Φ60mm for another 15 minutes. After above 30 minutes, start motor and start screw related actions.
2) Material and screw must match. Especially corrosive materials and hard materials, such as PVC, flame-retardant materials, and glass fiber-added materials should use professional screws instead of ordinary screws.
3) If corrosive materials used for ordinary machines must be cleaned and replaced with stable materials such as HDPE, HIPS, etc. immediately after use.
4) If ordinary machine occasionally uses glass fiber material, be sure to use high material temperature, low screw speed, low back pressure, remember to remember.
5) Regardless of material, try not to produce at lower temperature limit. For example, PC material can be produced at 250 degrees, and ABS can barely be made at 185 degrees, but it is not good for screw because it requires large torque and severe wear. At the same time, if you must use lower limit problem, use a low screw speed.
6) If machine is shut down for more than half an hour each time, blanking port should be closed, material in barrel should be cleaned, and heat preservation should be provided.
7) Avoid foreign matter falling into barrel, damage screw and barrel. Prevent metal fragments and debris from falling into hopper. If processing recycled materials, a magnetic hopper must be added to prevent iron filings from entering hopper.
8) When using anti-salvation, make sure that plastic in barrel is completely melted, so as not to damage transmission system parts when screw retreats.

 

9) Avoid phenomenon of screw idling and slipping.
10) When using new plastic, remaining material in barrel should be cleaned. When using POM, PVC, PA+GF and other materials, try to minimize degradation of raw materials, rinse them off with ABS and other nozzle materials in time after shutdown.
11) Avoid mixing POM and PVC into barrel at the same time, which will react at melting temperature and cause serious industrial accidents.
12) When temperature of the molten plastic is normal but black spots or discoloration of molten plastic are constantly found, check whether rubber screw non-return ring seat (apron, meson) is damaged.