Three-Plate Mold Runner System: How Locks, Ejectors & Change Pins Work Together

Three-Plate Mold Structure Overview

A three-plate mold has three main plates that separate during mold opening:

• Fixed plate (A plate) — Mounted to the injection machine's fixed platen. Contains the sprue bushing and the runner channels on its parting face.
• Runner plate (B plate) — The middle plate that carries the runner system on one face and part of the cavity on the other. This plate is free to move independently.
• Cavity/core plate (C plate) — Contains the cavity or core geometry and is mounted to the moving platen.

This three-plate structure enables automatic degating: when the mold opens, the runner separates from the parts at the gate points, and both are ejected independently.

The Opening Sequence

Three-plate molds open in two distinct stages, controlled by limit bolts and runner lock mechanisms:

Stage 1: Runner-Cavity Separation

• The cavity plate (C) pulls away from the runner plate (B)
• Pin-point gate bushings shear the gates from the parts as the plates separate
• Runner lock pins grip the solidified runner, keeping it attached to the runner plate
• Parts remain in the cavity/core for subsequent ejection

Stage 2: Runner Ejection

• The runner plate (B) separates from the fixed plate (A)
• Runner lock pins release the runner (undercut or friction grip yields)
• Runner ejector sets push the runner out of the runner plate
• The runner falls free for collection or regrinding

According to ScienceDirect's three-plate mold reference, the opening sequence timing is critical — the first opening must be long enough to fully shear all gates before the second opening releases the runner.

Component Roles in Detail

Runner Lock Pins

Runner lock pins are installed in the runner plate with their tips protruding into the runner channel. When resin solidifies around the pin tip, the undercut geometry creates a mechanical grip that holds the runner during Stage 1 opening.

Runner Ejector Sets

Runner ejector sets are spring-loaded pin assemblies installed in the fixed plate. During Stage 2 opening, the ejector pins push against the solidified runner in the runner plate, forcing it to separate and fall free. The springs return the pins to their seated position before the mold closes for the next cycle.

Runner Change Pins

Runner change pins redirect the runner flow path where it needs to change direction or transfer between plates. Three configurations handle different flow routing needs:

• Z-pin type — Redirects flow through a Z-shaped path between two parallel runner channels on different plates
• Ejector-pin type — Combines flow redirection with runner ejection function
• Fixed-side type — Installed in the fixed plate to redirect flow before it enters the runner plate

Component Sizing Guidelines

According to JIS B 5111 mold component standards, runner lock pin and ejector set dimensions follow standardized series that ensure interchangeability across mold makers.

Common Problems and Solutions

• Runner sticks in runner plate — Lock pin grip too strong, or ejector force too weak. Solution: reduce lock pin undercut depth or add more ejector sets.
• Runner falls during Stage 1 — Lock pin grip too weak, or pins worn. Solution: switch to hard lock tip pins or replace worn pins.
• Runner breaks during ejection — Runner too cold and brittle, or ejector pins unevenly distributed. Solution: optimize cooling time and balance ejector pin placement.
• Gate vestige pulled from part — Stage 1 opening speed too fast, or gate diameter too large. Solution: slow down Stage 1 opening or reduce gate bushing tip diameter.