Last spring an OEM rejected a full lot of 400 ductile iron pump housings at incoming inspection, and the reason was not the casting. The foundry had done its part: every housing carried a raised heat number, legible and correct. The housings then came to us for machining, and the pocket we milled for the seal carrier sat exactly where the foundry had put its characters. The raised heat number went into the chip bin with the rest of the 0.8 mm we took off that face. What arrived at the OEM was a batch of housings with a 3.1 material certificate in the box and no number on the part to match it to. Their quality engineer would not accept the lot, the certificate covers parts we cannot individually point to, they wrote, and they were right. Re-marking all 400 housings, re-issuing certificates, and paying for 100 percent manual verification on the customer side came to about 5,900 dollars. Planning a machined marking pad into the casting would have cost 310 dollars, as a line item inside the same machining quote.
*(Composite example; details changed.)*
That story is what cast component marking is actually about. Not the characters, the plumbing. A casting passes through a foundry, a blast cabinet, a machine shop, a paint line, sometimes a heat treatment furnace, and at every one of those steps somebody can destroy or bury whatever the previous step wrote on the surface. Our job as the machining shop is to make sure the part still says what it needs to say when it reaches the dock. This guide is the checklist we work from: what content a marked casting has to carry, which of the four surface states it can be marked in, which process fits each state, and where in the routing the marking station sits so nobody mills it off afterwards.
What the Mark Has To Carry: Four Layers of Identity
A marked casting is not carrying one number. It is carrying a small database, and the layers stack:
- Foundry identity: the heat number. Which melt the part came from. This is the number that ties the part to its EN 10204 3.1 material certificate, to the charge record, and, if a metallurgical problem surfaces two years later, to every sibling part from the same pour. On safety-relevant castings this layer is not optional; traceability starts here.
- Manufacturing identity: the lot or batch number. Which machining lot, which coating lot. Under IATF 16949, which we hold, a batch has to be defined so that a containment action can bound the affected population. The lot number is what makes a recall a bounded campaign instead of the whole shipment history.
- Individual identity: the serial number. Which specific part this is. Serialized castings are the norm in our automotive and medical work: pumps, brackets, housings, surgical instrument frames. The serial is what the customer scans at assembly and what comes back with a warranty claim.
- Product identity: the part number and revision. What the part is, and which drawing revision it was made to. When a customer revises a rib thickness mid-year, the revision letter in the mark is the only thing standing between a mixed bin and a quiet field failure.
The mistake we see most often on drawings is specifying one layer when the customer actually needs three. A drawing that calls for a part number alone, on a serialized traced component, sends the shop back for a markup and a re-quote. A drawing that asks for all four when the component is a commodity bracket pays for serialization it will never use. When we run DFM on a cast component, which we do free on every RFQ, marking content is one of the first things we flag, because it changes the fixture, the process, and the routing, and it is much cheaper to fix in the DFM stage than on the machine.
The Four Surface States of a Casting
Where you mark depends on what state the surface is in when the mark goes down, and each state has a different process window:
| Surface state | Typical roughness | Dot peen | Fiber laser | Electrochemical |
|---|---|---|---|---|
| As-cast, unmachined | Ra 12.5 to 50 | Works, depth 0.30 mm and up | Engraving only, anneal will not read | Not practical |
| Blast cleaned | Ra 6.3 to 25 | Our default state for deep marks | Engraving, multi-pass | Not practical |
| Machined pad | Ra 1.6 to 3.2 | Works but leaves marks above spec sometimes | Anneal or engrave, machine readable | Works on clean iron |
| Coated / painted | Paint film 40 to 120 µm | Only before coating | Through-marking is unreliable | No |
Two rows deserve comment. The as-cast row is where foundry raised characters live, and it is the cheapest marking real estate in the whole chain because the characters are formed by the mold for free. The machined pad row is where anything machine readable needs to live, and it is the most expensive real estate because it costs a facing pass, but it is the only place a laser annealed or electrochemical mark will both apply cleanly and scan reliably.
We covered the depth numbers that survive blast and paint in a separate article on <a href="../blog/deep-marking-cast-iron-guide">deep marking on cast iron and cast steel</a>. The short version: 0.20 mm is the floor if the code must be read through paint, and our default on iron and cast steel is 0.30 mm.
Raised Cast Characters: Free Marking With Real Limits
The foundry can put characters in the mold, raised or recessed, and they appear on every part with zero per-part cost. Every customer with cast components should know both sides of that trade.
The strengths are real. Raised characters survive blast, paint, and years of corrosion because they are part of the geometry. They cannot be milled off by accident unless someone machines that specific face. For internal parts, date codes on non-traced components, and part numbers on service castings, raised characters are often all a buyer needs, and we regularly tell customers to stop paying us for laser work the pattern can do for free.
The limits are equally real, and they are what pushed our pump housing story into a 5,900 dollar rework:
- Foundries control raised characters, and they control the content poorly. A foundry changes the heat number every pour; that means swapping inserts or rebuilding a portion of the pattern, which they will schedule on their convenience, not yours. Serial numbers are effectively impossible: nobody is re-lettering a sand mold per part.
- Draft angle and demold force set the geometry. Practical raised characters on sand castings run 3 to 6 mm tall with 2 to 5 degrees of draft and generous character spacing. That is fine for a heat number read by a person, useless for a 10 mm Data Matrix read by a scanner.
- Location is hostage to parting line and core prints. The only faces a foundry can mark cheaply are the ones the pattern naturally forms. Those are rarely the faces that stay unmachined on the finished part.
So the working rule we give customers: raised cast characters for the foundry layer, the heat number, and for nothing that a machine has to read. Everything else goes on a machined pad, after machining, in a process we control.
The Machined Pad: Reserving the Real Estate
A marking pad is a small raised boss or a faced flat, called out on the casting drawing, that survives machining untouched. Machining it to Ra 3.2 or better takes one facing pass, and everything a modern traceability system needs can then be done on it.
We specify pads to three rules, learned the hard way:
- The pad belongs to the mark, and to nothing else. No datum targets on the marking pad, no clamping points on it, no gauge points. The pump housing story happened because the pocket and the foundry mark competed for the same real estate; pads prevent that competition by contract.
- The pad is sized for the code plus scanner tolerance. A 10 mm square Data Matrix wants a 14 mm pad as a minimum, giving the scanner edge margin; a serial in 2.5 mm dot peen characters wants 30 to 40 mm of length. Pads are cheap when they are cast in and expensive when they are added by hand finishing later.
- The pad is placed where the part can present it. In the assembly, the code has to be visible to the scanner without disassembling the machine. We have seen perfectly marked components whose pads faced the inside of a welded assembly, which converts a working traceability system into an audit finding.
When a casting arrives with no pad and the customer needs machine readable marking, the fallback is dot peen on the blast cleaned surface, with the depth rules from our <a href="../blog/deep-marking-cast-iron-guide">deep marking guide</a>, or laser engraving with enough power to cut through the roughness. Both work; both cost more per part than a pad that was designed in, and the laser route eats into the surface in a way some coating specs dislike.
Machine Readable Codes on Cast Components
Human readable characters carry the part number and revision fine. The layers that really want a machine readable code are the serial and the lot, because scanning is what makes traceability operational instead of theoretical.
On castings the practical choice is between a laser engraved Data Matrix on a machined pad and a dot peen Data Matrix on a rougher surface:
- Laser engraved DMC, machined pad. Cell size 0.2 to 0.4 mm on a 10 to 12 mm square code. Reads at Ra 3.2 with margin. Fast, permanent, and the first choice whenever the routing gives us a faced surface. Our laser marking service pages cover the equipment side in more detail in the <a href="../blog/laser-marking-service-guide">laser marking service guide</a>.
- Dot peen DMC, blast cleaned surface. Cell size 0.5 to 0.8 mm, pin displacement set deep enough to survive any later coating. Reads reliably off a uniform blasted finish, degrades if the blast media changes mid-lot, so we freeze the media per lot. This is the route for castings that will never see a machining pass on the marked face.
- Both, plus human readable. The default on safety parts in our automotive work: the DMC for the scanner, and the same content in dot peen characters beside it for the person with a flashlight when the scanner is down. Redundancy is cheap compared with a line stop.
One rule that buyers should hold us to: the grade of the code is a real number, and it belongs in the control plan. We verify marked codes to a decode grade at first article and at lot sampling, and that verification step is written into the control plan rather than assumed, the way any other characteristic in our <a href="../blog/control-plan-and-capability-study-guide">control plan and capability study guide</a> is handled.
Where the Marking Station Sits in the Routing
The routing question kills more marking programs than the process question. A casting passes through up to six stages, and marking has exactly one correct position for each content layer:
| Stage | What can be marked here | What destroys the mark afterwards | Verdict |
|---|---|---|---|
| Foundry, in mold | Heat number, raised | Machining on that face, wear | Right place for heat number |
| After blast, before machining | Temporary lot tags, paint pen | Everything downstream | Never for permanent marks |
| After final machining | Serial, DMC, part and rev | Coating only if depth planned | Right place for individual identity |
| After coating | Nothing permanent goes on well | Nothing | Verify only, do not mark |
The row people get wrong is the third one. Marking after final machining is correct for content, but if the part is painted afterwards, the mark depth has to be planned against the paint film, which is exactly the arithmetic in our <a href="../blog/deep-marking-cast-iron-guide">deep marking article</a>: paint fills a shallow groove, blast before paint empties it, and a code that must scan after coating needs to be engraved deep enough that the film cannot bridge it.
There is one variant worth calling out because it recurs in pump and valve work: castings that get pressure tested after machining. The test leaves water in the marking grooves and, on cast iron, a light flash rust that softens laser anneal contrast. On those parts we mark after the test, with a drying pass in between, and we say so in the routing so the test station does not become a marking station by accident.
Process Selection: What Works on Iron, Steel, and Aluminum Castings
The substrate moves the process choice as much as the surface state does:
- Gray and ductile iron. The graphite is both a gift and a nuisance. Dot peen compacts graphite into a bright, high contrast groove, which is why dot peen reads so well on iron. Fiber laser engraving also works well; the graphite vaporizes before the matrix, giving a clean dark groove. Laser annealing is the weak option: the oxide color it relies on is blotchy on graphite-rich zones, and it will not survive any blast.
- Cast steel. Tougher matrix, so stylus wear is real and per-character cost is higher, as we quantify in the deep marking guide. Laser engraving is the economical default on steel once a machined pad exists. Carbon steel will flash rust in a freshly engraved groove in humid storage; a light oil wipe in the routing is the fix, and it costs nothing if it is planned.
- Cast aluminum. The easiest substrate for laser: annealing produces a clean high contrast mark on a machined pad with almost no depth, and engraving is fast. The trap with aluminum castings is porosity: a mark laid across a porosity zone will show pinholes in the groove and scatter a scanner. We route the pad onto sound material, away from gate and riser territory, when we plan the casting.
- Coated castings. Zinc and epoxy coats up to about 60 µm can be marked through with a multi-pass laser, but decode reliability drops and we do not recommend it for anything the customer will scan on a moving line. Deep engraved marks before coating, sized for the film, are the robust route.
For the process comparison across all our substrate families, not just castings, the tables in our <a href="../blog/part-marking-methods-comparison-guide">part marking methods comparison</a> are the reference our quoting engineers use.
The Certificate Has To Match the Part
The mark on a cast component is half of a two-part record. The other half is paper, and the two halves have to close the loop:
- EN 10204 3.1 certificates carry the heat number, the chemical analysis, and the mechanical results for the melt the part came from. The number stamped or cast into the part is what links a specific housing in a crate to a specific certificate in the same crate. Lose the number and the certificate becomes a description of a population the customer cannot pick members out of. That was the whole failure in our pump housing case.
- Under IATF 16949, the traceability chain has to run from the raw melt to the shipped box, with defined batch boundaries. Cast component marking is where that chain is physically anchored. A missing anchor at lot level turns a containment action into a 100 percent sort.
- Under ISO 13485, which we also hold for our medical machining work, device history records require the same discipline, and the marking content is specified and verified, not incidental. Our article on <a href="../blog/cnc-part-traceability-marking-guide">CNC part traceability marking</a> covers the record-keeping side in detail.
What we ask customers to do is simple: tell us in the RFQ which certificate type the part needs and which numbers the mark must carry, and we will quote the marking operation with the verification built in. Finding out after the first shipment that the certificate cannot be matched to the parts is the most expensive possible way to learn this.
What Buyers Actually Ask Us
Over hundreds of cast component quotes, the same questions come back, and the answers are shorter than buyers expect:
Can you mark parts that arrive from another foundry? Yes. Most of our cast component marking work is exactly that: castings cast elsewhere, machined and marked here. We need to know the surface state the parts arrive in, whether the foundry already marked a heat number and where, and what has to survive our machining.
Can you keep the foundry heat number and add your own numbers? Yes, and that is the standard arrangement. The foundry layer stays on the as-cast face, our layer goes on the machined pad, and the two never compete for space once the pad is in the casting drawing.
How much does this add to the part price? On a machined pad with a laser DMC, marking adds about 0.4 to 1.2 percent of a typical machined cast component price at our volumes, including pad machining. Dot peen on a rough surface is similar. The number that matters more is the one it prevents: rework, re-certification, and containment when a mark fails or goes missing.
Do you re-verify marks at shipping? Yes, at first article and by sampling within the lot, with the decode grade and the human readable content both checked against the certificate. The check is in the control plan, and the first article inspection that gates the lot is the same one described in our <a href="../blog/first-article-inspection-fai-guide">first article inspection guide</a>.
We have no marking spec, only a one-line note on the drawing. Can you help? That is half the RFQs we see, and the one-line note is usually where the free DFM review earns its keep. We will propose the pad, the content layers, the process, and the routing, and the customer only has to approve the marking drawing once instead of discovering the gaps at incoming inspection.
Cost Shape: Where the Money Actually Goes
Marking costs are not in the seconds of marking time. They are in the surrounding decisions:
- The pad. One facing pass, a fixture that presents the pad, and a casting drawing that was right the first time. Cheap planned, expensive retrofitted.
- The verification. Decode grade checking, certificate matching, and the record keeping behind them. Not free, and always cheaper than the alternative.
- The rework you never do. The 5,900 dollars in our opening story was not marking cost. It was the cost of not having spent 310 dollars.
That ratio is the entire argument for treating cast component marking as a design input rather than a late production detail, and it is why our quoting engineers raise it in the DFM conversation instead of waiting to be asked. We run more than 200 CNC machines across a 23-year-old shop floor, holding ±0.01 mm on machined features, and the two departments that touch every marked casting are the fixture group, which makes the pad presentable, and the quality lab, which makes the code verifiable. Neither of those is the bottleneck when the marking was planned; both become the schedule when it was not.
FAQ: Questions About Cast Component Marking We Get in Writing
Can the foundry put our serial numbers in the mold?
No, not practically. Mold characters are fixed at pattern build; serials change every part. Foundry characters work for heat numbers and fixed part identifiers, which is why we treat the mold as the right place for the foundry layer only.
What is the minimum mark depth that survives coating on a casting?
Start at 0.20 mm for a code that must be read through paint on a blast cleaned surface; our default on iron and cast steel is 0.30 mm. The full reasoning and the numbers by process are in our deep marking guide for castings.
Should marking happen before or after heat treatment?
After. Scale growth and distortion in the furnace damage or destroy marks made before, and on hardened cast steel the marking process choice narrows anyway. We plan marking as the last permanent operation on the surface, before coating only if the depth is planned for the film.
Do we need a machined pad, or can you mark the as-cast surface?
A machined pad is required for laser annealed and electrochemical marks and for any machine readable code you want to scan reliably. Dot peen and laser engraving can go directly on a blast cleaned as-cast surface, at the deeper settings.
How is the marked code verified?
By decode grade at first article and by sampling within the lot, with human readable content checked against the EN 10204 3.1 certificate. The verification is a line in the control plan, like any other characteristic.
Can you mark castings we machine elsewhere?
Yes, if the parts arrive in a known surface state and the marking face is accessible. We would quote it from photos and a sample in that case, and the free DFM review still applies to the marking side.
Does cast component marking fall under your IATF 16949 scope?
Yes. Traceability, batch definition, and record keeping for marked castings run under the same certified quality management system that covers our machining work, and medical work follows the equivalent requirements of ISO 13485.
Closing: Get the Marking Into the Drawing Early
If your cast component needs to be traced, the marking belongs in the casting drawing, next to the pad that carries it, decided before the pattern is cut and before the first machining op eats the surface. We are glad to look at it early. Send us the casting print with whatever marking note you have, even a rough one, and our engineers will come back within 24 hours with a marking plan in plain language: which numbers go where, which process we would run, what the pad needs to look like, and what it adds to the piece price. That review is free, and the alternative, as the pump housing customer will tell you, runs about nineteen times the cost. Reach us through <a href="../#contact">the contact section</a> and we will start with the drawing.