Guide
How do you specify where ejector pin marks are allowed on a die casting?
Ejector pin marks are the small round witness marks, slightly raised or sunk and usually ringed with a thin fin of flash, that the pins pushing a die casting out of the die leave on every part. You control where they land by marking on the drawing which faces may carry them and which may not, stating how proud or sunk a mark may be and what happens to its flash, and agreeing pin locations with the die caster before the die is designed.
By Saola Manufacturing engineering team · Published · Updated
The problem this guide solves is a familiar one: first articles arrive and a row of round marks sits in the middle of the face your customer sees, or across the gasket land that is supposed to seal. Nothing was wrong with the casting process. The drawing simply never said where marks were not allowed, and the industry default leaves that choice to the die caster. The fix is cheap on paper and expensive in steel, so it belongs on the drawing and the RFQ, not in a deviation request after the die is built.
On this page
What are ejector pin marks, and when do they become a defect?
Every die casting carries ejector pin marks, because movable pins must push the solidified part off the die. They become a defect only when they land on a face where the drawing or the end use cannot tolerate them, or when they exceed the agreed height, depth or flash condition.
The North American Die Casting Association (NADCA) states in its guideline G-6-4 that movable ejector pins must be used to eject a die casting and will leave a residual mark, and that the pins also keep the casting from bending as it comes out of the die (NADCA Product Specification Standards for Die Castings, 2021, guideline G-6-4-21). So the question is never whether a part will have marks, only where they will be and what they will look like.
On the part, a pin mark looks like a flat circle the size of the pin face. It may stand slightly proud of the surrounding surface, sit slightly below it, or both on different pins, and NADCA notes that it is surrounded by a flash of metal, which forms in the clearance around the pin. You will typically find them on the surfaces formed by the ejector half of the die, often on the inside of a housing, on the tops of ribs and bosses, or on small round pads that look as if they were added for no reason. Those pads are usually there for the pins.
Three situations turn a normal mark into a reject: a mark on a cosmetic face that will be seen, painted or plated; a mark or its flash across a sealing land, an O-ring seat or a mating face that must sit flat; and a mark on a face used as a datum or a fixturing point. A fourth, less obvious one is bumping: NADCA describes how a pin placed on a flat surface can make the side opposite the pin bulge out, so a bump can show on a cosmetic face even though the pin itself is on the hidden side.
Why do ejector pin marks end up on cosmetic or sealing faces?
The most common cause is that nobody told the die caster which faces mattered before the die was designed. After that come part geometry that leaves the cosmetic face on the ejector half, a thin flat wall with nowhere better to push, and flash around worn pins.
Ranked by how often they cause the problem, the usual causes are these:
- No zone information at die design. NADCA’s guideline says ejector pin locations are at the option of the die caster, subject to the customer’s agreement, and that where cast surface cosmetics are important, locations should always be discussed in advance of die design. If the drawing is silent, the die designer places pins where ejection works best, which is often a large flat face.
- Part orientation in the die. NADCA describes the ejector pins as mounted on the ejector plate in the ejector half of the die, so the faces formed by that half are where pins can reach. If the parting line and draw direction put the cosmetic or sealing face on the ejector side, pins will want to push on it. The parting line decision and the pin decision are made together.
- Geometry with no alternative push points. A thin, flat panel with no ribs, bosses or walls near the area to be ejected gives the die designer nowhere else to push. NADCA lists placing pins near walls, on top of ribs and on bosses, together with more wall thickness and more draft, as ways to reduce bumping, and the same features give pins somewhere harmless to land.
- Draft too small for easy release. Low draft generally makes the part harder to strip from the die. NADCA lists increasing draft as one of the ways to reduce bumping, and the less ejection force the part needs, the more freedom the die designer has in where to push.
- Pin wear and die maintenance. Pin flash forms in the clearance around each pin, so as pins and their bores wear, the flash can get heavier. A part that was acceptable at first article can drift over the life of the die, which is why the flash condition belongs in the specification and not only in the first sample approval.
How do you check whether an ejector pin mark is acceptable?
Check three things on each mark: where it is relative to the zones on the drawing, how far it stands proud or sits below the surrounding surface, and whether its flash has been left, crushed or removed as specified. Then check the opposite face for bumping.
Location comes first because it is the cheapest check and the most common failure. Overlay the drawing zones on a sample part, or mark the zone boundaries on a limit sample, and look for any mark that crosses into a no-mark zone. If the pin is on a face that will be machined later, confirm that the machining allowance is deeper than the mark so the cut removes it completely.
Height or depth is measured relative to the surrounding cast surface, not to the nominal CAD surface, because the cast face itself carries its own tolerance. A dial indicator on a surface plate, a height gauge or a depth gauge bridging the mark all work. Per NADCA guideline G-6-4-21, ejector pin marks on most die castings may be raised or depressed .015 in (0.381 mm), that raised marks are preferred for optimum production, and that larger castings may require additional ejector pin tolerances. If your drawing and RFQ say nothing, the caster will normally work to that standard practice.
For flash, look at the ring around each mark under the lighting used for appearance inspection, and run a fingertip or a cotton swab around it on sealing and handling faces. NADCA notes that flash which is left in place or only crushed or flattened may flake off in use. That matters for hydraulic, pneumatic and fluid-handling parts, where a loose flake can become contamination, so those parts usually need removal at the critical locations rather than crushing. Finally, lay a straightedge across the face opposite each pin on thin walls; a local bulge there is bumping, and it is a die design issue rather than a trimming issue.
How do you keep ejector pin marks off cosmetic and sealing faces?
Decide face by face what is allowed, then give the die designer somewhere better to push: ribs, bosses, pads, walls or faces that will be machined anyway. Specify full removal of marks only where the end use justifies the extra operation.
The table below is a decision aid you can apply to each face of the part. It uses the three ejector flash options on the NADCA production checklist, which are numbered so that the lowest number is the most economical basis for production, together with a limited-area (zone) indication on the drawing.
| Face type | Pin marks allowed? | What to specify | How to verify |
|---|---|---|---|
| Hidden, non-functional (inside walls, rib tops) | Yes | NADCA default: raised or depressed up to 0.015 in (0.381 mm); flash not removed (C-8-1 E1) | Spot check height on first article |
| Handling or wire-routing faces | Yes | Flash crushed or flattened (C-8-1 E2) | Visual and touch check for sharp flash |
| Face machined after casting | Yes, if inside the stock to be removed | Machining allowance deeper than the deepest mark; mark removed by the cut | No residual mark visible on the machined face |
| Cosmetic, painted or plated face | No | No-mark zone on the drawing; bumping not visible; pins moved to ribs, bosses or pads | Compare to signed limit sample under agreed lighting |
| Sealing land, gasket or O-ring face | No | No-mark zone, or marks and flash removed at listed locations (C-8-1 E3), or face machined; also line B3 (O-ring seats) | Location check plus leak or flatness test as specified |
| Datum target or fixture contact area | No | No-mark zone around each target | Location check against datum target drawing |
The most effective fixes happen in the part design, before tooling. Add ribs or bosses where pins can push on the ejector side, thicken a wall locally under unavoidable pins, increase draft so less force is needed, and consider whether the part can be turned in the die so that the cosmetic face is formed by the cover half, which in a conventional die carries no ejector pins. Ask the die designer whether some ejection can be taken on the overflows or the runner, which are trimmed off, and on pads placed outside the no-mark zones.
When a mark cannot be moved, choose between removal and machining. NADCA advises that complete removal of marks and flash by machining or hand scraping be specified only when requirements justify the added expense, and hand work on a cosmetic face is variable. If the face has to be machined for flatness or sealing anyway, the cut removes the mark as a by-product; the machining allowance guide covers how much stock to leave.
If the die already exists and marks are landing on the wrong face, work through the options in order of cost. First, check whether the face can simply be machined, or the marks scraped or spot-faced off at the listed locations, as a priced extra operation under line E3. Second, if a mark is out on height rather than location, ask the caster to check that pin: a raised mark means the pin face sits short of the cavity surface and a sunk mark means it stands into the cavity, so the pin length or a worn pin and bore is usually the thing to correct. Third, if neither is acceptable, ask for a die change that moves the pin to a pad, rib or boss outside the zone, and treat it like any tooling change: the caster should confirm that the part still ejects without distortion, and you re-approve first articles from the modified die. Record whichever option you choose on the drawing so the next die or the next supplier does not repeat it.
What should the drawing and RFQ say about ejector pin marks?
Mark the no-mark zones on the drawing, state the allowed height, depth and flash condition for the zones where marks are allowed, and fill in line E of the NADCA production checklist with the RFQ. Ask the caster to propose pin locations on a drawing or model for your approval before die steel is cut.
A zone drawing does not need special symbols. On drawings to ASME practice, a requirement that applies only to a limited area of a surface is normally shown with a chain line and the area located by dimensions, following the limited-area convention of ASME Y14.5-2018; a shaded or hatched area with a note on the view does the same job on simpler drawings. Two further standards cover drawings of cast parts: ASME Y14.8-2022, Castings, Forgings, and Molded Parts, and, for drawings to ISO conventions, ISO 10135:2007, drawing indications for molded parts. Check the edition your drawing cites before relying on a specific symbol from either.
A practical set of notes, adapted to your part, looks like this:
- Zone S (sealing, shaded): “No ejector pin marks, gate or overflow stubs in zone S.”
- Zone A (cosmetic, shaded on view 2): “No ejector pin marks or bumping visible in zone A when compared with the approved limit sample.”
- All other surfaces: “Ejector pin marks permitted; raised or depressed within NADCA G-6-4 limits unless noted; pin flash crushed or flattened.” If you need a tighter limit, write it here, for example “raised ___ max, depressed not permitted.”
- Machined faces: “Ejector pin marks permitted only inside the machining allowance; no residual mark after machining.”
- Approval: “Supplier to submit ejector pin, gate and overflow locations for customer approval before die design release.”
On the RFQ, attach NADCA checklist C-8-1 with line E completed. Its three options are flash not removed, crushed or flattened, and removed from specific locations, with a blank for the locations (NADCA C-8-1-21). Line B of the same checklist lists O-ring seats under its highest cast surface quality (B3), so a sealing face should be flagged in both places. The surface finish classes guide shows how to combine the zone with an as-cast surface grade.
Who handles ejector pin locations on a Saola order?
Die casting on a Saola order is done by a partner plant we manage, so pin locations are agreed with that plant’s die designer through us, against your zone drawing, before the die is built. Any machining that removes marks from sealing or mating faces is quoted separately, and the quote names where each step is done and the country of origin.
When a drawing arrives without zones, we ask which faces are cosmetic, which seal and which are datums before the casting is quoted, because the answer changes the die design and sometimes the part orientation. If you want to approve the proposed ejector, gate and overflow layout before the die is designed, put that note on the RFQ, and agree signed limit samples for any cosmetic zone. See the die casting page for the alloys and part types handled through partner plants.
Where a face has to be machined after casting, that work can be placed at our lead CNC plant, T.K Viet Nam Precision Company Limited, which runs CNC turning and milling, as described on the CNC milling page. Any finishing after that, such as paint or plating, is done by a partner plant and noted on the quote. When the drawing has its zones marked, send it for a quote.
Frequently asked
Can a die casting be made with no ejector pin marks at all?
Not as cast. Pins are needed to push the part off the die, so every casting has marks somewhere. What you can control is which faces carry them, and you can have marks machined or scraped off specific locations at extra cost.
Are raised or sunk ejector pin marks better?
NADCA states that raised marks are preferred for optimum production. A practical reason when marks must be removed: a raised mark can be scraped or spot-machined down to the surrounding surface, while a sunk mark can only be removed by cutting the surrounding face below the bottom of the mark. On a sealing or mating face, neither is acceptable without removal.
Who decides where the ejector pins go?
By default the die caster does, subject to the customer’s agreement, according to NADCA. If you have cosmetic, sealing or datum faces, that agreement should happen on a proposed layout before die design, not at first article.
Can pin locations be changed after the die is built?
Sometimes, but it means die rework, and a pin that moves can change how the part ejects and whether it distorts. It is far cheaper to settle the zones on the drawing before tooling, which is why the approval note belongs on the RFQ.
What is ejector pin bumping?
Bumping is a bulge on the face opposite an ejector pin, which NADCA describes for pins placed on a flat surface. NADCA lists increasing the wall thickness (locally if needed), placing pins near walls or on top of ribs, increasing draft and placing bosses where the pins are located as ways to minimise it.
Sources
- Product Specification Standards for Die Castings, Section 6, Guideline G-6-4-21: Ejector Pins, Pin Marks and Pin Flash, NADCA, 2021 edition, accessed October 2026
- Checklist C-8-1-21, Casting Production Specifications, NADCA Product Specification Standards for Die Castings, 2021
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
- ASME Y14.8-2022, Castings, Forgings, and Molded Parts, ASME, 2022 (stabilized maintenance)
- ISO 10135:2007, Geometrical product specifications (GPS): Drawing indications for molded parts in technical product documentation (TPD), ISO, 2007 (catalog listing)