Injection Molding

Process Definition & Working Principle

Polymer injection molding is a cyclical manufacturing process designed to convert thermoplastic or thermoset polymers into finished products with intricate geometries and high dimensional accuracy. The thermodynamic cycle relies on transforming a solid polymer granulate into a pressurized, highly viscous non-Newtonian melt via thermal energy and viscous shear heating, injecting this melt into a temperature-controlled mold tool cavity, solidifying it through conduction, and ejecting the resulting rigid component.

The injection molding cycle consists of five discrete phases:

Kinematics, Axes, & Machine Configuration

Modern injection molding machine configurations coordinate multiple linear and rotary axes of motion, categorized into clamping, injection, and auxiliary systems, driven by either hydraulic cylinders, electric servomotors with ball screws, or hybrid drivetrains.

Clamping Kinematics

The clamping unit secures the mold halves against high cavity pressures. The two dominant kinematic layouts are:

1. Toggle Clamping System: Comprises a mechanical linkage system actuated by a hydraulic cylinder or an electric servo drive. Toggle clamping delivers high mechanical advantage as the link reaches its fully extended, locked position. The kinematic velocity profile is highly non-linear, providing high travel speed during the open stroke and natural deceleration before mold touch-down, protecting tooling faces. Mold thickness changes require moving the entire rear platen along the tie-bars via tie-bar nut gears.
2. Direct Hydraulic Clamping System: Utilizes a double-acting hydraulic ram connected directly to the moving platen. Clamping force is directly proportional to hydraulic pressure and platen area. This configuration allows flexible mold-height adjustment and direct, real-time clamp force tuning, but exhibits higher power consumption and larger footprints compared to toggle systems.

Injection Unit & Screw Mechanics

The inline reciprocating screw design serves as a plasticator (extruder) and an injection plunger. Its performance is defined by the following metrics: