How to Balance Cavity Filling with Flow Rate Adjustment Pins

Step 1: Diagnose the Imbalance

Before installing flow adjustment components, confirm that cavity fill imbalance exists and quantify its severity:

• Short-shot study — Reduce injection volume to 80–90% of full fill. Photograph or weigh each cavity's short shot. The heaviest (most complete) cavities are receiving the most flow.
• Cavity weight measurement — Run full parts and weigh each cavity's output separately. Calculate the range (heaviest − lightest) and coefficient of variation (CV%). Target CV% below 1% for precision parts, below 3% for general purpose.
• Pressure sensor data — If cavity pressure sensors are installed, compare peak cavity pressure and pressure arrival time across cavities. Differences indicate flow imbalance.

According to ScienceDirect's multi-cavity mold reference, even geometrically balanced (H-pattern) runner systems can exhibit 5–15% fill imbalance due to shear-induced flow variations at runner splits.

Step 2: Identify the Cause

Common causes of fill imbalance in multi-cavity molds:

Step 3: Install Flow Adjustment Pins

Install flow rate adjustment pins in the runner branches feeding the cavities that fill first (fastest). The pins restrict flow to these cavities, diverting more resin to the slower cavities and equalizing fill time.

Installation guidelines:

• Location — Install in the branch runner, upstream of the gate, at a point accessible from the mold exterior
• Orientation — Perpendicular to the runner channel, protruding from the plate back surface
• Starting position — Set pin flush with the runner wall (zero protrusion) as the baseline

Step 4: Iterative Adjustment

Cavity balancing is an iterative process. Follow this procedure:

• Increase pin protrusion on the fastest-filling cavity by 0.2 mm
• Run 10–20 full cycles to stabilize the process
• Weigh cavity outputs and compare to the baseline
• If the target cavity's weight decreased by less than 1%, increase protrusion by another 0.2 mm
• If the target cavity is now under-filling, reduce protrusion by 0.1 mm
• Repeat until all cavities are within specification (typically ±1–3% of target weight)

According to ISO 20457, cavity-to-cavity weight variation tolerances must be specified on the part drawing and verified during mold qualification. Document all pin positions after final adjustment for process records.

Practical Tips

• Never exceed 50% protrusion — Restricting more than half the runner cross-section causes excessive pressure drop, material degradation, and unpredictable flow behavior at the restriction point.
• Adjust one cavity at a time — Changing multiple pins simultaneously makes it impossible to attribute improvements to specific adjustments.
• Lock the pins after final setting — Mold vibration and thermal cycling can gradually shift pin positions. Use lock nuts or set screws to fix the final position.
• Record final pin positions — Document the protrusion depth for each pin as part of the mold setup sheet. This allows fast restoration after mold maintenance.