The Molding Cycle

Cycle Overview

The injection molding cycle consists of four main phases that repeat continuously:

• Fill — Inject molten plastic into the cavity
• Pack — Add more material to compensate for shrinkage
• Hold — Maintain pressure until gate freezes
• Cool — Solidify the part before ejection

Understanding each phase is essential for process optimization and troubleshooting.

Phase 1: Fill (Injection)

During the fill phase, the screw moves forward rapidly to inject molten plastic into the mold cavity.

• Velocity-controlled (not pressure-controlled)
• Cavity fills to 95-98% capacity
• Duration: 0.5-5 seconds typically
• Injection speed: 50-500 mm/s

• Screw acts as a plunger, pushing melt through nozzle
• Melt flows through sprue, runner, and gate into cavity
• Material contacts cold mold walls and begins to freeze
• A frozen layer forms while the core remains molten

Modern machines use multi-stage injection:

• Slow-fast-slow profiles reduce jetting and burn marks
• Start slow through gate, accelerate for fill, slow before pack

Phase 2: Pack

The pack phase adds additional material into the cavity to compensate for volumetric shrinkage as the plastic cools.

• Pressure-controlled
• Adds 2-5% more material
• Duration: 1-5 seconds
• Pressure: 50-80% of injection pressure

• Cavity is nearly full; injection slows dramatically
• Machine switches from velocity to pressure control
• Additional melt is pushed in against increasing resistance
• Compensates for shrinkage in thick sections

Phase 3: Hold

The hold phase maintains pressure on the melt until the gate freezes (solidifies), preventing backflow.

• Pressure-controlled (often stepped down)
• Continues until gate freeze
• Duration: Gate freeze time (material-dependent)
• Critical for part weight consistency

• Gate area continues to cool
• Pressure prevents melt from flowing back
• Once gate freezes, cavity is sealed
• Further pressure has no effect

Determined by:

• Gate dimensions (smaller = faster freeze)
• Material properties (crystalline freezes faster)
• Mold temperature

Common approach: Step down pressure
- Stage 1: 80% pack pressure for 2 sec
- Stage 2: 60% pack pressure for 3 sec
- Stage 3: 40% pack pressure until gate freeze

Phase 4: Cool

The cooling phase solidifies the part while the screw retracts and prepares the next shot.

• Time-controlled
• Longest phase (50-80% of total cycle)
• Screw recovery occurs simultaneously
• Part must reach ejection temperature

• Heat transfers from plastic to mold (conduction)
• Coolant carries heat away from mold
• Part shrinks as it solidifies
• Screw rotates backward, conveying and melting new material

t_cool = (s²/π²α) × ln[(4/π) × (T_melt - T_mold)/(T_eject - T_mold)]

Where:

• s = wall thickness (mm)
• α = thermal diffusivity (mm²/s)
• Simplified: t ≈ s² × constant

Cooling time increases with the square of wall thickness:

• 2 mm wall → ~8 seconds
• 4 mm wall → ~32 seconds (4× thickness = 16× time!)

Cycle Time Breakdown

V/P Transfer (Switchover)

The transition from fill (velocity) to pack (pressure) is critical:

• Position-based: Switch at screw position (most common)
• Pressure-based: Switch when cavity pressure reaches threshold
• Time-based: Switch after fill time (least accurate)

• Cavity 95-98% full
• Before pressure spike
• Consistent shot-to-shot

Screw Recovery

While the part cools, the screw prepares the next shot:

• Rotation: Screw turns, conveying pellets forward
• Melting: Shear and heater bands melt the plastic
• Metering: Molten plastic accumulates in front of screw
• Decompression: Small retraction to relieve pressure (prevent drool)

Key Takeaways

• Fill phase: Velocity-controlled, fills cavity to ~95%
• Pack phase: Pressure-controlled, compensates for shrinkage
• Hold phase: Maintains pressure until gate freezes
• Cool phase: Dominates cycle time, scales with wall thickness²
• V/P transfer is critical for quality and consistency
• Recovery must complete within cooling time

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