Every ejector pin leaves a mark. The pin face is a separate piece of steel meeting the cavity surface, so there is always a witness line - the question is only whether it is acceptable on the part. What turns an invisible witness into a reject is nearly always one of five specific faults, and each one points somewhere different.
Why Every Pin Leaves a Mark
The pin sits in a bore through the core, its face forming a small island of the moulding surface. Three things make that island visible:
- A different surface finish from the surrounding cavity steel - the pin face is usually turned or ground, the cavity often polished or textured.
- A different height - a pin standing proud leaves a depression, a pin standing low leaves a raised pad.
- A different thermal history - the pin is a thin steel rod in a bore, poorly connected to the mould's cooling, so it runs hotter than the surrounding steel.
You cannot eliminate the witness line. You can make it flat, matched in finish, and put it where nobody looks.
Diagnosing the Mark
| What you see | Most likely cause | First fix to try |
|---|---|---|
| Glossy ring or circle, part otherwise flat | Pin face finish differs from cavity; pin running hot | Match the pin face finish to the surrounding surface; improve core cooling near the pin |
| Shallow depression at the pin | Pin standing proud of the cavity surface | Re-fit the pin flush, or set it 0.02 - 0.05 mm below the surface so any mark becomes a raised pad |
| Raised pad or "pip" at the pin | Pin set below the surface, or pin has sunk in its bore | Check pin height and shoulder seating in the ejector plate |
| Sink or dimple around the pin | Local overpacking or a thick section behind the pin | Reduce hold pressure or hold time; review wall thickness behind the pin |
| Part pushed through / distorted at the pin | Ejection force concentrated on too little area | Add pins, use larger pins, or move to blade or sleeve ejection |
| Scored line radiating from the pin | Pin galling in its bore; pin bent or misaligned | Replace the pin, check bore alignment and guide pillars |
| Thin fin of plastic at the pin edge | Worn or oversize pin bore - flash into the clearance | Replace the pin with the correct diameter; re-bush the bore |
| White or cloudy patch at the pin | Stress whitening - ejecting too early or too hard | Extend cooling time; reduce ejection speed; increase draft |
Gloss Rings and Witness Marks
A gloss ring with no height difference is a finish and temperature problem, not a mechanical one. The pin face polishes differently from the cavity, and because the pin sits in a bore with little contact area it runs hotter, so the plastic against it cools more slowly and takes a higher gloss.
Fixes, roughly in order of cost:
- Match the pin face finish to the surrounding cavity - including texture, if the surface is textured.
- Move the pin onto a surface that is already textured or hidden. A texture hides a witness mark far better than a polish.
- Improve cooling in the core around the pin so the local temperature difference falls.
- On a cosmetic face, move to sleeve ejection or a stripper plate so no pin touches the visible surface at all.
Sink and Push Marks
Sink around a pin is usually not an ejection fault at all - it is a packing fault that the pin has made visible. The pin is a local hot spot, so the material there is the last to freeze, and any overpacking or thick section behind the pin shows up as a dimple exactly at the pin.
A push mark - the part visibly deformed where the pin pressed - is a different problem: not enough ejection area for the force required. The part is still gripping the core when the pins fire, and all the release force goes through a few small circles.
- Increase the ejection area: more pins, larger pins, or spread them more evenly over the part.
- Reduce the force needed: more draft, better core polish in the draw direction, or a light mould release while you fix the tool.
- Eject later: a slightly longer cooling time gives a stiffer part that resists deformation.
- Change the method: deep ribs and boxy parts want blade or sleeve ejection, not round pins.
A push mark is a tool design message, not a process one. If you are dialling back ejection speed and adding release agent to stop a part deforming, the ejection layout is undersized - and it will get worse as the tool wears and draw friction rises.
Drag, Flash and Stress Whitening
Drag lines radiating from a pin mean the pin is not moving cleanly in its bore. A bent pin, a bore that has picked up, or an ejector plate that is not running square will all score the moulding as the pin travels. Replace the pin, check that it slides freely by hand through its full stroke, and check the ejector plate guidance - a plate running on worn guides cocks and side-loads every pin on it.
Flash at the pin - a thin fin of plastic around the circle - is wear. The clearance between pin and bore has opened up enough for melt to creep in. It rarely gets better on its own: each shot forces a little more material into the gap. Replace the pin with the correct diameter, and if the bore itself has worn oval, bush it.
Stress whitening - a pale, cloudy patch at the pin - is the material yielding under the ejection load. It is most obvious in ABS, polycarbonate and filled grades. It means the part is being pushed before it is stiff enough, or too fast: extend the cooling time, slow the ejection stroke, and address whatever is making the part grip the core.
Designing the Marks Out
- Put pins where the eye does not go - inside surfaces, under bosses, behind ribs, on a textured face.
- Eject from the strongest sections: at rib intersections, under bosses, on flanges. Never in the middle of an unsupported thin wall.
- Balance the layout. Uneven pin placement twists the part off the core and deforms it.
- Choose the right pin type. Straight pins for general use, shouldered pins where you need a small face on a slender pin, blade pins for thin ribs and slots, sleeve ejectors around bosses.
- Set pins fractionally low on non-critical faces - typically 0.02 to 0.05 mm - so the witness is a small raised pad rather than a depression, which reads far better on most surfaces.
Maintenance That Prevents Them
Most pin marks arrive slowly as a tool ages. A short routine catches them early:
- At each service, push every pin through its stroke by hand. Any that binds is about to score a part.
- Check pin height against the cavity surface with a depth gauge - pins sink as the shoulder pocket wears.
- Look at the pin faces for erosion or pitting; a corroded face transfers straight onto the part.
- Replace pins as a set in a given area rather than singly, so heights stay consistent.
- Keep the ejector plate guides and return pins in good order - most pin misalignment starts there, not at the pin.
Ejector pins are consumable tooling. Replacing a worn set is far cheaper than the scrap, sorting and rework generated by a tool that is marking parts.
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Nitrided and through-hardened ejector pins, Ø1 to Ø25 mm, in lengths from 100 to 1000 mm - ex-stock Mumbai.
View Ejector PinsFrequently Asked Questions
Why do my moulded parts have shiny circles where the ejector pins are?
A gloss ring is a finish and temperature difference rather than a mechanical fault. The pin face has a different surface finish from the surrounding cavity, and because a pin sits in a bore with poor thermal contact it runs hotter, so plastic against it cools slower and takes a higher gloss. Match the pin face finish to the cavity and improve local core cooling.
How do I stop ejector pins pushing through or deforming the part?
A push mark means the ejection force is concentrated on too little area. Add pins, use larger diameters, or spread them more evenly across strong sections such as rib intersections and bosses. Also reduce the force needed - more draft and a better core polish - and consider blade or sleeve ejection for deep or boxy parts.
What causes flash around an ejector pin?
Wear. The clearance between the pin and its bore has opened enough for melt to creep into the gap and form a thin fin around the circle. It progresses shot by shot, so replace the pin with the correct diameter, and if the bore has worn oval, bush it rather than fitting an oversize pin.
What is the white patch around my ejector pins?
Stress whitening - the polymer yielding under ejection load, most visible in ABS, polycarbonate and filled grades. The part is being ejected before it is rigid enough, or too quickly. Extend the cooling time, slow the ejection stroke, and reduce the force needed by improving draft and core finish.
Should ejector pins sit flush, proud or below the cavity surface?
Flush is the nominal, but on non-critical surfaces many toolmakers set pins fractionally low - typically 0.02 to 0.05 mm - so the witness appears as a small raised pad rather than a depression. Pads read better than dimples on most surfaces. A pin standing proud always leaves a visible depression and should be corrected.
Can ejector pin marks be removed completely?
Not entirely - the pin face is a separate piece of steel meeting the cavity, so there is always a witness line. What you can do is make it flat, match its finish to the surrounding surface, and place it where it is not seen. For genuinely mark-free cosmetic faces, use sleeve ejection or a stripper plate so no pin touches the visible surface.