Injection Moulding: Plunger and Screw Machines

Definition

Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould. The two primary mechanisms used to transport and inject this material are the reciprocating plunger (ram) system and the reciprocating screw system.

Main Content

1. Plunger (Ram) Injection Machines

These are the older generation of injection moulding machines.
A hydraulic plunger pushes cold plastic pellets through a heated cylinder, where they are melted by contact with the walls before being forced into the mould.

2. Reciprocating Screw Machines

This is the industry standard for modern moulding.
It uses a rotating screw inside a heated barrel. The screw acts as a conveyor to melt, mix, and transport plastic pellets to the front of the barrel before the entire screw moves forward like a piston to inject the material.

3. Comparison of Plasticization

Plunger machines rely on heat transfer from the barrel walls (conduction), which is slow.
Screw machines use both conduction and mechanical shear (friction), which allows for faster, more uniform melting and mixing of additives or pigments.

Working / Process

1. Material Feeding and Melting

In plunger machines, cold material is forced into a "torpedo" or spreader to increase contact with the heated walls.
In screw machines, the rotating screw conveys pellets forward, compressing and heating them through friction and external heaters.

[Screw Mechanism Process]
Hopper (Feed) -> [ Rotation & Shear ] -> Melted Plastic
       |                |                     |
   Pellets      Rotating Screw          Molten Material

2. The Injection Phase

The plunger (or the entire screw) moves forward linearly.
High pressure is applied to force the molten plastic through the nozzle and into the cold mould cavity.

[Injection Stroke]
Pressure Applied -> [ Plunger/Screw Moves Forward ] -> Melt into Mould

3. Holding and Cooling

Once the mould is full, pressure is maintained (holding phase) to compensate for material shrinkage.
The plastic solidifies in the mould cavity until it is cool enough to be ejected as a finished component.

Advantages / Applications

Screw Machines: Provide superior homogeneity and are ideal for high-volume production of complex parts with color additives.
Plunger Machines: Historically used for simple parts or materials that are sensitive to the degradation caused by long residence times in the screw.
Versatility: These machines are the backbone of the automotive, packaging, and medical device manufacturing industries for producing components like caps, containers, and housing covers.

Summary

Injection moulding machines transform solid plastic pellets into finished products by melting and injecting them into a mould. While plunger machines use a simple ram system, modern screw machines use a rotating screw to achieve better material mixing and faster cycle times. Key terms to remember: Plasticization, Shear heating, Injection pressure, Cycle time, and Reciprocating screw.