Injection molding machines sit at the foundation of modern plastics processing. The operating principle is straightforward — plastic pellets are heated until molten, forced under high pressure into a mold cavity, then cooled and solidified — but the precision of the machine’s internal mechanics shapes product quality and production efficiency.

At the center of that process is the screw, supported by coordinated injection, mold and clamping systems. Machine selection then comes down to two core specifications: clamping force and injection volume.

From pellets to a finished part

The process begins when plastic pellets are fed into a heated barrel and melted. Once the material reaches a molten state, it is injected at high pressure into a closed mold cavity.

After the melt fills the cavity, the material is cooled and solidified. The mold then opens and the finished part is ejected. Each stage depends on tightly coordinated mechanical and hydraulic systems.

The screw drives plasticizing and injection

The screw is among the most important components in an injection molding machine. It performs two central functions: it conveys material through the barrel, and it injects molten plastic into the mold with precision.

Injection and material conveyance

During injection, the screw acts like a plunger, applying force to push molten plastic into the mold cavity. As the process consumes material, the screw rotates and its helical flights move raw material forward for continuous feeding.

Check valve and backflow prevention

The screw head is fitted with a check valve to help ensure injection accuracy. When the screw advances, the valve moves to the front cone and its slots allow material to flow toward the mold. When the screw retracts or is under pressure, the valve moves to the rear cone and forms a sealing surface that prevents molten plastic from flowing backward.

Three-zone screw design

Screw length is typically divided into three functional zones: the feed zone, the compression or transition zone, and the metering zone. Groove depth decreases gradually along the screw, with the most significant change in the compression zone, helping ensure thorough plasticization.

Three systems work together

A standard injection molding machine relies on three main assemblies operating in concert.

• Injection unit: includes the screw, barrel, hopper and drive system, and handles plasticizing and injection.
• Mold assembly: includes the ejection cylinder and hydraulic circuit, and handles mold opening and closing and part ejection.
• Clamping unit: presses the mold halves tightly together; common designs include toggle mechanisms and direct hydraulic clamping.

These systems must remain synchronized throughout the cycle to maintain repeatability and consistent part quality.

Clamping force and injection volume

When evaluating an injection molding machine, clamping force in tons and injection volume in grams are two primary specifications. Both are important, but they do not follow a simple linear relationship. Selection should therefore refer to industry standard tables and specific production requirements.

In practice, clamping force must keep the mold securely closed against injection pressure, while injection volume sets the upper limit for the weight of a single molded part.

Frequently Asked Questions