How to Detect and Replace Worn Ejector Pins in Injection Molds

Ejector pins are critical components of the ejection system, operating under high temperature, pressure, and friction. Over time, they wear, bend, or gall, leading to part defects like ejection marks, whitening, or dimensional errors, and potentially causing mold jams. Detection involves both daily visual checks and periodic precise measurements, while replacement must follow a standardized procedure to ensure reliable ejection.

1. Detection Methods for Ejector Pin Wear

1.1 Daily Visual Inspection (Qualitative)

During production, assess pin condition through part quality and mold behavior:

Part Defects: Irregular ejection marks, whitening, or localized damage at the ejection point often indicate a worn or bent pin causing uneven force distribution.

Surface Checks: After mold disassembly, inspect pins for scoring, scratches, oxidation, or rounded ends.

Movement Check: Manually actuate the ejector plate. Sticking, poor return, or wobbling indicates excessive clearance between the pin and its bushing, requiring quantitative measurement.

1.2 Periodic Precision Measurement (Quantitative)

Use measuring tools to check diameter wear, concentricity, and fit:

Removal: Remove the ejector plate and tap pins out gently with a brass rod to avoid damage.

Diameter Check: Use a micrometer at the head, middle, and tail. Replace if wear exceeds 0.03mm at any point or 0.05mm overall. Replace immediately if bent or bulging.

Concentricity Check: Place the pin on V-blocks and measure runout with a dial indicator. Replace if runout exceeds 0.02mm.

Fit Check: Insert a new standard pin into the hole. If the total clearance exceeds 0.04mm (0.02mm radial) or the pin rocks significantly, the hole is worn and may need reaming or a larger pin size.

1.3 Wear Criteria

Replace a pin if it meets any of the following:

Surface scoring ≥ 0.02mm deep or end rounding ≥ 0.1mm.

Diameter wear ≥ 0.03mm or bending ≥ 0.02mm.

Clearance ≥ 0.04mm, causing wobbling or sticking.

Severe oxidation or rust that cannot be polished out.

2. Ejector Pin Replacement Process

2.1 Preparation

Spare Selection: Choose pins matching the original size and material (e.g., SKD61 for general use, SKH51 for abrasive/glass-filled materials).

Tools: Prepare a micrometer, brass rod, screwdrivers, sandpaper, and high-temperature lubricant.

2.2 Step-by-Step Replacement

Mold Removal: Stop the machine, cool the mold, and secure it on a maintenance rack.

Old Pin Removal: Remove the ejector plate stop screws. Separate the plate and tap out worn pins. Use rust remover if seized; never pry violently.

Hole Cleaning: Clear debris from the ejector hole using a brass brush or compressed air. Polish minor scoring with fine sandpaper. Ream severely worn holes if necessary.

New Pin Installation: Apply high-temperature grease to the new pin. Insert it vertically to avoid binding. Reassemble the ejector plate and secure the stop screws.

Testing: Manually cycle the ejection system to ensure smooth movement. Test the mold in the machine to verify proper part ejection and absence of marks.

3. Daily Maintenance Tips

Lubrication: Apply high-temperature grease during routine mold maintenance to reduce friction. Avoid low-temperature oils that carbonize.

Cleanliness: Regularly clear plastic debris from the ejection system to prevent abrasion.

Process Optimization: Adjust ejection speed and pressure to minimize impact forces on the pins.