How to Troubleshoot Gate Vestige Problems in Injection Molding

Defect Diagnosis Table

Problem 1: Gate Stringing

Stringing is the most common pin-point gate defect. A thin thread of resin trails from the vestige, creating a cosmetic defect and potentially interfering with part assembly.

Root causes and fixes:

• Gate diameter too large — A larger gate takes longer to freeze off. Reduce gate diameter by 0.1–0.2 mm and retest. According to the gate freeze-off formula: t ≈ (d/2)² / (4α), reducing diameter from 1.2 mm to 1.0 mm cuts freeze time by 30%.
• Cooling time too short — The mold opens before the gate has fully solidified. Increase cooling time by 1–2 seconds and check for stringing improvement.
• Mold temperature too high at gate — Local mold temperature above the resin's solidification point keeps the gate soft. Add a cooling channel near the gate bushing or reduce coolant temperature.
• Resin type — Some crystalline resins (POM, PA) have a narrow solidification range that makes them prone to stringing. Consider switching to an angle transition tip which creates a sharper gate break point.

Problem 2: High Vestige

Vestige height exceeds specification (typically > 0.3 mm for cosmetic parts). This is usually caused by the gate not shearing cleanly at the intended break point.

• Gate bushing tip protrusion — If the gate bushing tip protrudes beyond the cavity surface, the vestige starts higher. Verify tip is flush ±0.1 mm with the cavity surface.
• Gate diameter too large — Larger gates have more material to shear, producing higher vestiges. Reduce gate diameter.
• Wrong transition geometry — Round transitions generally produce lower vestiges for amorphous resins. Angle transitions produce more consistent (but slightly higher) vestiges for crystalline resins.

Problem 3: Discoloration at Gate

Yellow, brown, or dark marks around the gate area indicate thermal degradation from excessive shear heating. The resin overheats as it accelerates through the narrow gate orifice.

• Gate shear rate check — Calculate gate shear rate: γ ≈ 4Q / (πr³). If γ exceeds 50,000 s⁻¹, increase gate diameter. According to ScienceDirect's shear rate reference, most commodity resins begin degrading above 40,000–60,000 s⁻¹.
• Injection speed — Reduce injection speed by 10–20% to lower resin velocity through the gate.
• Gate diameter — Increase by 0.1–0.2 mm to reduce velocity and shear rate.

Problem 4: Worn Gate Bushing

Over time, the gate orifice enlarges from abrasive wear (especially with filled resins). Symptoms include increasing vestige size, decreasing vestige consistency, and gradual weight increase in molded parts.

• Measurement — Use a pin gauge to check the gate orifice diameter. If it has enlarged by > 0.1 mm from the original specification, replace the bushing.
• Prevention — Switch to carbide gate bushings for filled resins. See the carbide ROI guide for cost justification.

According to ISO 20457, gate vestige dimensions should be inspected at regular intervals during production and documented in the process control plan.

Systematic Troubleshooting Procedure

• Step 1 — Identify the defect type using the diagnosis table above
• Step 2 — Check the gate bushing condition (measure orifice diameter, inspect tip for wear)
• Step 3 — Verify gate bushing tip position (flush with cavity surface?)
• Step 4 — Review process parameters (cooling time, injection speed, mold temperature)
• Step 5 — If process adjustment doesn't resolve the issue, consider gate bushing specification change (smaller diameter, different transition, carbide material)