What Are Runner Ejector Sets and How Do They Work in Three-Plate Molds?

How Runner Ejector Sets Work

In a three-plate mold, the solidified runner must be ejected from the runner plate (B plate) during second-stage opening. Runner ejector sets accomplish this with a spring-loaded push mechanism.

The operating sequence:

• Mold closed — The ejector pin is pushed flush with (or slightly below) the runner channel surface by the fixed plate. The spring is compressed.
• Stage 1 opening — The cavity plate separates from the runner plate. Runner lock pins hold the runner. The ejector pin remains compressed because the runner plate is still against the fixed plate.
• Stage 2 opening — The runner plate separates from the fixed plate. The spring extends, pushing the ejector pin into the runner channel. The pin pushes the solidified runner out of the channel, where it falls free for collection.
• Mold closing — As the runner plate returns to the fixed plate, the fixed plate surface pushes the ejector pin back to its compressed position, re-loading the spring for the next cycle.

Key Specifications

Sizing the Stroke

The ejector stroke must push the runner completely clear of the runner channel. If the stroke is too short, the runner only partially ejects, stays lodged in the channel, and blocks the next injection cycle.

The minimum stroke formula:

• Minimum stroke = Runner depth (D) + Clearance (2 mm) + Safety margin (1 mm)

For a full-round runner with 6 mm diameter (3 mm depth per plate half), the minimum stroke is 3 + 2 + 1 = 6 mm. Select the next available standard stroke size — typically 8 or 10 mm.

According to ScienceDirect's ejection force reference, the force required to push a solidified runner from a steel channel depends on the runner material's shrinkage rate and the channel surface finish. Polished channels (Ra ≤ 0.4) reduce ejection force by 30–50% compared to machined channels (Ra 1.6).

Installation Layout

Distribute ejector sets evenly along the runner path for uniform ejection force:

• At each runner intersection — Where branches meet the primary runner, place an ejector set at the junction point
• At runner ends — Place an ejector set near each dead-end of the runner to prevent the runner from sticking at the extremities
• Avoid gate locations — Do not place ejector sets directly at gate bushing positions — the gate bushing already provides support during gate shear

For an H-pattern runner in an 8-cavity mold, a typical layout uses 4–5 ejector sets: one at the sprue location, one at each primary branch intersection, and one at each secondary branch end. According to JIS B 5111 mold component standards, ejector set spacing should not exceed 100 mm to prevent runner deformation during ejection.

Troubleshooting Common Issues

• Runner not fully ejected — Stroke too short, spring weakened, or channel surface too rough. Verify stroke length exceeds runner depth by ≥2 mm. Replace spring if force has dropped below specification.
• Runner sticks to ejector pin — Pin surface too rough or no draft angle on pin tip. Polish pin contact surface to Ra ≤ 0.4 and add 0.5–1° draft to the pin tip.
• Spring fatigue — Springs lose 10–15% of their force over 500,000–1,000,000 cycles. Include spring replacement in the preventive maintenance schedule at 500,000 cycle intervals.
• Runner breaks during ejection — Ejection force concentrated at too few points. Add more ejector sets to distribute the force, or slow down the Stage 2 opening speed.