Gas?assisted injection molding is widely used for thick?wall, long?strip parts in automotive, home appliances, electronics, and furniture. It reduces clamping force, weight, sink marks, and warpage. However, holding time settings differ greatly from conventional molding and directly affect quality, dimensions, and stability. Improper settings lead to channel collapse, sink marks, flash, bubbles, and poor structure.

Basic Logic: Melt Holding and Gas Holding

Gas?assisted holding includes two stages: melt holding and gas holding. Melt holding fills the cavity and forms a stable layer before gas injection. Gas holding uses nitrogen to support the wall, compensate shrinkage, and prevent sinking.

Short melt holding causes blow?through, piercing, or channel leakage. Excessively long melt holding prevents gas penetration and channel formation. Insufficient gas holding causes collapse and sink marks. Too long gas holding reduces efficiency and increases stress. The key is to seal the cavity first with melt holding, then support with gas holding, and release pressure before gate freeze?off.

Melt Holding Time Setup

Melt holding time is typically 0.5–2.5 seconds, based on part thickness, gate size, and material.

Small, thin parts with small gates: 0.5–1.0 second

Medium parts: 1.0–1.8 seconds

Large, thick parts with large gates: 1.8–2.5 seconds

The ideal time is when the cavity is sealed but not fully frozen, allowing stable gas penetration without blow?out.

Gas Holding Time Setup

Gas holding time is usually 3–10 seconds, longer for thick parts. It maintains internal pressure to compensate cooling shrinkage. Sink marks or channel collapse mean gas holding is too short. The optimal time is when no sinks appear and channels remain stable. Crystalline materials such as PP and PE need slightly longer holding than amorphous materials.

Switching and Pressure Relief Control

The sequence is critical: end melt holding → inject gas immediately → hold gas until just before gate sealing → slow pressure relief → cooling.

Sudden pressure release causes dimples, bubbles, and ripples. Two?stage pressure relief is recommended: reduce to 30–50% for 0.5–1 second, then fully release. Holding time must match cooling to ensure rigidity before ejection.

Common Defects and Adjustments

Blow?through: shorten melt holding, lower gas pressure

Sink marks: extend gas holding, increase pressure

Incomplete channels: shorten melt holding

Channel collapse: extend gas holding, delay relief

Long cycle: reduce holding times appropriately

Weld lines: slightly extend melt holding

Summary

Holding time in gas?assisted molding is adjusted dynamically according to part structure, material, mold, and machine. Melt holding ensures sealing without blow?through; gas holding ensures dimension and appearance. Stable, smooth switching and pressure relief guarantee high consistency and productivity.