A Concise Guide to Ejector Pin Wear Inspection and Replacement

Ejector pins are among the most critical and vulnerable components in plastic injection molds. Their surface condition and dimensional accuracy directly impact part appearance and stability. Operating under high temperature, pressure, and friction, pins are prone to wear, bending, and corrosion. Without timely maintenance, these issues lead to defects such as ejection marks, cracking, sticking, and even mold damage. This guide provides a clear, practical approach to detecting wear and executing replacements.

1. Wear Inspection Methods

Inspection should be systematic, combining visual checks, dimensional measurements, and functional tests to ensure early detection of issues.

Visual Inspection

Visual checks are the fastest initial screening method, using the naked eye or a 5–10x magnifying glass. Key points include:

Surface wear such as scratches, grooves, or pitting. If depth exceeds 0.03mm or covers more than 30% of the tip, further measurement is needed.

Bending or deformation. Testing on a flat surface with a feeler gauge; gaps over 0.02mm/m indicate bending.

Rust or plastic residue, which increases friction and accelerates wear.

Chipping or edge damage, which directly creates defects on the part surface and requires immediate replacement.

Dimensional Measurement

Dimensional checks are the core basis for determining pin failure, using calipers, micrometers, and roughness testers:

Diameter wear. A reduction of ≥0.05mm, or a deviation of ≥0.03mm on the same cross-section, means excessive clearance, risking flash or jamming.

Surface roughness. New pins typically have Ra 0.2–0.4μm; if wear increases this to Ra ≥1.6μm, ejection resistance rises, causing whitening.

Length change. A reduction of ≥0.1mm impairs ejection stroke, preventing full part release.

Functional Testing

Functional checks are performed with the mold assembled, using actual ejection actions to verify performance:

Smooth movement. Listen for sticking or abnormal sounds during ejection, which suggest misalignment or debris.

Ejection quality. Persistent whitening, cracking, or sticking—after ruling out process factors—usually indicates worn pins.

Flash at the pin hole. Plastic leakage means clearance is too large and requires immediate inspection.

2. Standard Replacement Process

Replacing ejector pins must follow a standardized workflow to avoid damaging the mold or affecting production.

Preparation

First, shut down the injection machine and disconnect cooling or heating lines. Once the mold has cooled, remove it and place it on a dedicated maintenance platform. Clean the surface of debris and oil, then remove the ejector system cover to expose the retainer and backing plates, marking their positions to prevent misalignment during reassembly.

Removing Worn Pins

Based on inspection results, mark the worn pins on the retainer plate. Loosen the fixing screws and remove the retainer, then use pliers or a specialized tool to extract the worn pins. If pins are stuck due to rust or residue, apply a small amount of rust remover or release agent and allow it to penetrate; never strike violently, as this damages the guide holes.

Cleaning and Preparing New Pins

Blow out the ejector holes with compressed air to remove chips and oil, then lightly polish the inner walls with a hone or 400–600 grit sandpaper to restore smoothness. Select new pins matching the original specifications, typically SKD61 for high‑temperature applications or SUJ2 for general use, and verify their dimensions and surface quality.

Installation and Testing

Apply a thin layer of high‑temperature lubricant to the new pin and insert it slowly into the guide hole and retainer plate, ensuring a clearance of 0.01–0.02mm for smooth movement. Reinstall the retainer plate, aligning the marks and tightening screws evenly. Replace the cover, remount the mold, and reconnect cooling or heating lines.

Run a dry ejection cycle to confirm smooth operation, then produce a test shot to check for ejection marks or defects. Once confirmed normal, the mold can return to production.

3. Maintenance Best Practices

To extend pin life and reduce failures:

Regular lubrication minimizes friction and wear.

Material matching ensures durability; high‑temperature plastics require heat‑resistant grades like SKD61.

Process optimization avoids excessive ejection speed or pressure.

Spare parts inventory reduces downtime by enabling quick replacements.

Conclusion

Inspecting and replacing worn ejector pins is a fundamental yet vital part of mold maintenance. A systematic approach—combining visual, dimensional, and functional checks—ensures issues are identified early. Following standardized replacement procedures safeguards the mold and ensures stable production. By implementing these practices, manufacturers can improve part quality, extend mold life, and reduce overall costs.