The Injection Molding Machine

Machine Overview

An injection molding machine consists of two main units:

• Injection Unit — Melts and injects the plastic
• Clamping Unit — Holds the mold closed during injection

These units work together in a precisely timed sequence to produce parts.

The Injection Unit

Hopper

The hopper is a funnel-shaped container that holds plastic pellets (granules). Features include:

• Capacity: 25-100+ kg depending on machine size
• Drying: Often integrated with a dryer for hygroscopic materials (PA, PC, PET)
• Material handling: Vacuum loaders for automatic refilling

Barrel

The barrel is a hardened steel cylinder that contains the screw and heater bands:

• Length: Typically 20-25 times the screw diameter (L/D ratio)
• Heating zones: 3-6 zones with independent temperature control
• Lining: Bimetallic or tool steel for wear resistance

Reciprocating Screw

The heart of the injection unit—it performs three functions:

• Conveying: Moves pellets forward from hopper
• Melting: Generates heat through shear (friction) and conduction
• Injecting: Acts as a plunger to push melt into the mold

• Diameter: 20-150 mm typical
• L/D Ratio: 20:1 to 25:1
• Compression Ratio: 2:1 to 3:1

Nozzle

The nozzle connects the barrel to the mold sprue:

• Types: Open, shut-off (valve), or filter nozzles
• Heating: Independent heater band to prevent freeze-off
• Radius: Must match the mold sprue bushing radius

The Clamping Unit

Platens

Two large steel plates that support the mold halves:

• Stationary platen: Fixed, holds the injection side of the mold
• Moving platen: Moves to open/close the mold
• Platen size: Determines maximum mold dimensions

Clamping Mechanism

Generates the force to keep the mold closed during injection.

• Mechanical advantage through linkage system
• Fast, energy-efficient
• Common in machines <500 tons

• Direct hydraulic cylinder actuation
• Uniform force distribution
• Preferred for large machines >1000 tons

Clamping Force = Projected Area × Cavity Pressure

• Projected area: Part footprint in cm²
• Cavity pressure: 300-800 bar typical
• Safety factor: 1.1-1.2x

Ejection System

Pushes the part out of the mold after cooling:

• Ejector pins: Push against the part surface
• Ejector plate: Moves all pins simultaneously
• Stroke: Must exceed part depth + safety margin

Machine Types

Hydraulic Machines

• Lower initial cost
• High clamping force capability
• Robust and reliable

• Higher energy consumption
• Oil contamination risk
• More maintenance required

All-Electric Machines

• 30-50% energy savings
• Higher precision (±0.01 mm)
• Clean operation (medical, electronics)
• Faster response time

• Higher initial cost
• Limited to ~500 tons (economics)

Hybrid Machines

Combine electric injection with hydraulic clamping:

• Best of both worlds for medium-large machines
• Electric precision for injection
• Hydraulic force for clamping

Key Machine Specifications

Machine Selection Guidelines

• Calculate required clamping force:

• Determine shot size:

- Should be 30-80% of machine capacity

• Check physical fit:

- Mold height vs. daylight opening

• Consider cycle time requirements:

- Plasticizing capacity for recovery

Key Takeaways

• The injection unit melts and injects plastic via screw, barrel, and nozzle
• The clamping unit holds the mold closed with sufficient force
• Electric machines offer precision and efficiency; hydraulic offers force
• Key specs: clamping force, shot size, injection pressure, platen dimensions
• Machine selection starts with clamping force and shot size calculations

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