Injection Molding Machine Screws

The screw of injection molding machine is the heart part of one machine. Through the reciprocating action, the machine melts the plastic resin and injects the molten material into the mold cavity.

The screw is a part inside the barrel, which is installed on the injection carriage. The injection carriage is fixed on the rail or bar.

Injection Molding Machine Screws

Movements of the injection screw

The action of screw in the injection molding machine is reciprocating, so sometimes it is called reciprocating screw as well.

There are 2 kinds of horizontal reciprocating actions:

1. The reciprocating action of each work of the injection carriage

At the beginning of the work, it is necessary to move the nozzle close to and contact the mold gate. And then tighten the injection carriage to resist the injection pressure during the pressure-holding stage, so that the nozzle is always close to the mold gate without loosening.

When the work is done, the injection carriage will move to the right side and injection nozzle will leave the mold gate.

In this action, the screw is inside the barrel and moves with the injection carriage definitely. So it is one action of the screw, but with no direction. Normally this kind of action is for every time the work starts. That is, one switch, mold changing, or machine preparation etc.

2. The reciprocating of the screw of each cycle.

2.1 Plasticizing: melt the plastic. This is an action of the screw back. Usually the screw motor rotates, so that the plastic inside the barrel moves forward. Then the force of the plastic moving forward will react in the groove of the screw, pushing the screw back. Further make it reach the set position, that is, enough material in the space between the screw head and the nozzle. And that is generally controlled by the shooting stroke.

2.2 Injection: The injection cylinder pushes the screw forward to achieve injection. What is reflected here is the injection speed and injection pressure, both of which are very important parameters. Then comes the pressure holding stage, which is the stage with the highest pressure in the entire production cycle. After the plastic in the mold cavity is cooled, the screw starts the plasticizing process and enters next cycle.

# Injection molding screw Types

As for the types of screw, we will talk about these two issues: Screw materials and Screw finishing.

For screw material, it’s about what kind of steel has used in the screw. It determined the hardness of screw and difficult degrees for the process.

The screw finishing drives more attention, because the screw is in close contact with the molten plastic all the time while it is working. Each kind of plastic has its own characteristics, so there are basically two functions on the surface of the screw:

  • Prevent viscous plastic from sticking to the surface of the screw for a long time, which is generally realized by a process of electroplating.
  • Wear-resistant.Because some plastics or plastic mixtures have high hardness, such as plastics with glass fibers. This requires the surface of the screw to be very wear-resistant. In terms of the process, it is made of bimetal. That is, it’s added with a wear-resistant metal layer on the surface.

# Injection molding screw zone

The structure of the screw is the main factor affecting the working effect of the screw. The structure here refers to the three segment of the screw: feed zone, transition zone, metering zone. Each segment has its own function. Through different groove depths and groove angles, the plastic is cut, melted, and sent to the front of the barrel for injection.

Three segments of screw

# Injection molding screw functions

The plastic is melted and sent to the space between the nozzle and the screw head to prepare the injection. It is to realize the measurement of the injection amount and the final good mixing of the molten material.

# Injection molding screw parameters

The diameter and aspect ratio of the screw are the main parameters of the screw.

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