Principle analysis of screw use and maintenance of injection molding machine

Screw is an important part of injection molding machine. Its function is to transport, compact, melt, stir and pressure plastics. All this is 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, the screw edge propulsion surface, and between the plastic and the plastic. The forward movement of plastic is the result of this combination of motion, and the heat generated by friction is also absorbed to raise the temperature of plastic and melt plastic. The design and structure of screw will directly affect the degree of these functions.

The screw of injection molding machine works under high temperature, high pressure, high mechanical torque and high friction environment for a long time, and the loss is inevitable. Even the screw of the injection molding machine has undergone various surface treatments, from quenching to nitriding, boronizing to spraying, to double alloy splashing.

But in actual production, why is there such a big difference in service life? For the same injection molding machine screw, some factories can use it for 8 years, and some can only use it for 2 months. Why is the gap between injection molding machine screws so large.

Screw segment description

Generally, the screw of injection molding machine can be divided into feeding section, compression section and homogenization section (also known as metering section). (Note: the ratio of three sections of different screws is different, the screw groove depth is different, and the transition form of screw bottom diameter is different)

(1) Description of feeding section: the depth of screw groove in this section is fixed, and its function is responsible for preheating, plastic solid transportation and extrusion. It must be ensured that the plastic begins to melt at the end of the feed section.

(2) Description of compression section: this section is a tapered screw with deep screw groove. Its functions are melting, mixing, shear compression and pressurized exhaust of plastic raw materials. The plastic will completely dissolve in this section and the volume will be reduced. The design of compression ratio is very important.

Description of homogenization section: this section is the fixed depth of screw groove. Its main functions are mixing, glue conveying and metering. It must also provide sufficient pressure to maintain the uniform temperature of glue and stabilize the flow of molten plastic.

The key to the life of injection molding machine screw is use and maintenance. The following use and maintenance principles can be used for reference:

1) When the injection molding machine is started, the injection molding shall be fully preheated. After the barrel reaches the preset temperature, Φ Keep the temperature below 60mm for another 15 minutes. After the above 30 minutes, start the motor and start the screw related action.

2) The material shall be matched with the screw. Especially for corrosive materials and hard materials, such as PVC, flame retardant materials and glass fiber, screws should be used, and ordinary screws should not be used.

3) If ordinary machine corrosive materials must be cleaned and replaced with stable materials such as HDPE and hips after use.

4) If the ordinary machine occasionally uses glass fiber material, be sure to use high material temperature, low screw speed and low back pressure.

5) No matter what kind of material, try not to use the lower temperature limit for production. For example, PC material can be produced at 250 degrees, and ABS can be done at 185 degrees, but it is not good for the screw because it needs large torque and serious wear. At the same time, if the lower limit problem must be used, use a low screw speed.

6) If the machine is shut down for more than half an hour each time, the blanking port shall be closed, the materials in the barrel shall be cleaned, and thermal insulation shall be set. 7) Prevent foreign matters from falling into the barrel and damaging the screw and barrel. Prevent metal fragments and sundries from falling into the hopper. If recycled materials are processed, a magnetic hopper needs to be added to prevent iron filings from entering the barrel.

8) When using anti salivation, make sure that the plastic in the barrel is completely melted to avoid damaging the parts of the transmission system when the screw retreats.

9) Avoid screw idling, slipping, etc.

10) When using new plastics, clean the remaining materials of the barrel. When using POM, PVC, PA + GF and other materials, try to reduce the degradation of raw materials. After shutdown, wash them with ABS and other nozzle materials in time.

11) Avoid mixing POM and PVC into the barrel at the same time, which will react at the melting temperature and cause serious industrial accidents.

12) When the temperature of the molten plastic is normal, but black spots or discoloration of the molten plastic are constantly found, check whether the check ring seat of the rubber screw (passing through the rubber ring and meson) is damaged.