The short answer
A **mill certificate** — also called a mill test report (MTR) or material test certificate (MTC) — traces the metal or polymer **stock** back to a producing mill or distributor lot and lists the chemistry and mechanical results for that heat or lot. Request one when the alloy and temper on the drawing are load-bearing, regulated, or must match a named specification, and when you later need to prove the blank was that grade rather than a look-alike substitute.
A simple **certificate of conformance (CoC)** only states that the shipment meets the purchase order and drawing. That is often enough for uncritical prototypes, shop fixtures, and cosmetic covers in a common grade such as 6061-T6, where substitution risk is low and you will not have to reconstruct the heat later.
A mill cert answers “what was the stock?” It does **not** replace dimensional inspection, a first-article report, or a finish certificate. Write the document type and marking scope on the PDF you upload to quote-new.php so the quote band matches the paperwork you intend to buy.
What a mill cert is — and what a CoC is
Buyers often mix three paperwork ideas:
1. **Mill test report / material test certificate.** Issued against a specific heat or lot of stock. Typical contents include the producer or distributor, alloy and temper, the material specification, a heat or cast number, chemical analysis, and mechanical results (tensile, yield, elongation) as reported by the mill. In many European supply chains, buyers name an **EN 10204** type: 2.1/2.2 for non-specific reports, **3.1** for specific inspection by the mill, and **3.2** when an independent inspector also signs. North American practice usually calls the same idea an MTR. Treat these as document types you request, not as a promise that every cutoff already has a 3.1 packet attached.
2. **Certificate of conformance.** A supplier statement that the parts, or the stock used, conform to the purchase order and drawing. A CoC does not reprint the mill chemistry or, by itself, tie a heat number to your bag of parts.
3. **Full lot traceability.** The mill cert exists *and* the heat or lot number is marked on the part, the bag, or both, so a later audit can join the physical piece to the PDF. First-article packages often collect this under product-accountability paperwork; see first article inspection reports for the ballooned-drawing side.
Special-process certificates — heat treat, anodize, passivation, plating — travel with the finishing operation, not with the raw blank. Ask for them separately when the drawing calls those processes.
When a mill cert is worth requesting
Ask for a mill cert on the blank when most of these are true:
- The part is **structural or safety-related**, or a grade mix-up would be expensive to find in the field.
- Your quality system, customer, or industry program requires **material traceability**, even if the machining itself is ordinary.
- The alloy is one where a **cheaper look-alike** is tempting: 304 versus 316 versus 17-4PH, 6061 versus 7075, or Ti-6Al-4V versus a commercial pure grade.
- **Temper or condition matters** as much as the alloy name: 17-4PH H900 versus H1025, 7075-T6 versus T73, or a heat-treated steel hardness band. A cert that only says “17-4PH” without the condition is not the document you think you bought.
- A first-article package will need a **material certificate referenced to the lot** anyway.
Naming the alloy without naming the cert scope widens quote bands: the shop cannot tell whether a commercial 6061 drop is acceptable or whether certified plate is required.
When a CoC — or no extra document — is usually enough
Stay with a CoC, or with no extra material paperwork, when:
- The job is an **early prototype** whose geometry will change. Prototype versus production CNC quotes is the broader planning fork.
- The part is a **fixture or shop aid** in everyday 6061 or mild steel, and a grade mix-up would be an inconvenience rather than a safety issue.
- The drawing already allows **“or equivalent”** or lists a family of acceptable alloys.
- Cosmetic covers where the finish specification matters more than the mill chemistry.
- Polymer prototypes in POM, nylon, or similar shop-stock shapes where a resin lot certificate would not change any decision this week.
If your customer’s quality clause requires a mill cert on every metal part, write that clause on the RFQ even for a first article. Silence is what creates clarification loops.
What a useful mill cert typically shows
When the PDF arrives, check that it is usable — not merely that a file was attached:
- **Identity match.** Alloy, temper or condition, and specification on the cert header match the drawing note, or an allowed substitute you already wrote down.
- **Heat or lot number.** Present, readable, and — if you asked for marking — the same number that appears on the bag or part.
- **Chemistry and mechanicals.** Reported against the named specification. Use the certificate minima for design checks; handbook “typical” values are not a substitute.
- **Form and size sanity.** A cert for 25 mm plate does not cover a 100 mm billet cutoff, and a bar cert does not cover plate of the same alloy. See plate versus billet stock for CNC aluminum; the cert must belong to the form you actually cut.
A cert that belongs to a different heat or specification than the parts is a traceability break. Ask for a replacement document or a formal deviation before you treat the lot as certified.
Traceability levels buyers actually write
Pick one level and put it in the title block or notes, instead of writing “full certs”:
1. **Commercial stock, no extra material document** — a common path for uncritical prototypes.
2. **CoC with the shipment** — the supplier affirms conformance; you do not receive mill chemistry.
3. **Mill cert on the blank, archived by lot** — the PDF is filed against the job; parts are not individually marked.
4. **Mill cert plus marking** — heat or lot number on the bag, on a non-cosmetic face, or both. Say the location so marking does not land on a sealing land or a show face.
5. **First-article package** — mill cert and special-process certificates plus full characteristic inspection. See first article inspection reports rather than pasting “FAI + certs” with no format.
Each step up can add sourcing time, because the shop may need a lot that still carries the document. Say which level you are buying so the estimator can plan.
How cert scope shows up on an RFQ
A missing cert note shows up as a wider min–mid–max band when the files say “aerospace aluminum” or “316 stainless, certified” with no document type. Reading a preliminary online quote is easier when the PDF states one of:
1. **Commercial 6061-T6 or T651 plate, CoC only.** Substitution allowed / not allowed.
2. **316L per [your specification], mill test report for the blank** (name EN 10204 3.1 only if that is actually your system). Mark heat number on the bag.
3. **17-4PH H1025, mill cert showing the aged condition**, hardness verification location if required, no substitute.
4. **Quote both** a commercial-stock path and a certified-stock path, and write the rule that wins (for example: “mill cert required on the production lot; prototype lot may ship with CoC only”).
Upload **STEP plus PDF** on quote-new.php with that paragraph visible in the notes or title block. Pair it with the material callout — alloy, temper, and form — not with a vague “use good material.” Related notes live in 6061 versus 7075 aluminum, stainless 304 versus 316 versus 17-4PH, and 7075-T6 versus T73.
If the drawing also calls heat treat, anodize, or passivation, list those certificates on their own lines. A mill cert on the blank does not prove the aging cycle or the anodic coating.
Plastics, powder, and special processes
Resin lot certificates look different from a steel MTR — they may list grade, lot, color, and sometimes moisture data. Ask for them when the polymer grade is functional (see choosing engineering plastics and POM versus PEEK for CNC functional prototypes). Metal powder pedigree and reuse records are not a wrought-plate mill cert; if the part will be printed, see powder reuse notes for SLM buyers rather than pasting an EN 10204 3.1 note from a billet job. Heat-treat, anodize, plating, and passivation certificates belong beside the process callout — see heat treating steel after CNC machining and when to specify heat treat after CNC for sequence, not as a substitute for the process document.
RFQ checklist (material paperwork)
Before you submit on quote-new.php, confirm:
- Alloy, temper or condition, and stock form (plate, bar/billet, extrusion, or polymer shape)
- Paperwork level: none, CoC, mill cert on the blank, mill cert plus marking, or first-article package
- Document type if you truly need one (MTR / EN 10204 3.1 / 3.2) — do not copy “3.1” onto every prototype
- Whether a substitute alloy or temper is allowed
- Marking: bag only, non-cosmetic face, serialized part, or none — and keep-out zones
- Special-process certificates listed separately from the blank mill cert
- Whether the prototype lot and the production lot have different paperwork rules
- STEP and PDF uploaded together, with the cert paragraph visible on the PDF
Common buyer mistakes
- Writing **“full certs”** or **“certified material”** without saying mill cert versus CoC versus a first-article package
- Asking for **EN 10204 3.1 on every prototype** in a common grade
- Calling an **aerospace specification** when a commercial grade plus a mill cert would have been enough — or the reverse
- Accepting a cert for the **wrong heat, form, or thickness**, or treating a mill cert as dimensional approval
- Forgetting whether **parts must be marked** with the heat number
- Mixing **blank pedigree** with **finish-process certificates**, or pasting a wrought note onto an SLM or polymer job
- Leaving the cert requirement only in email instead of on the drawing uploaded to quote-new.php
Bottom line
Request a mill cert when you must prove later that the blank was the named alloy, temper, and specification — especially on load-bearing, regulated, or easily substituted grades, and on any job that will need that document inside a first-article package. Use a CoC, or no extra material paperwork, when the part is an uncritical prototype, fixture, or cosmetic piece in a common grade. Write the paperwork level, marking rule, and substitution rule on the PDF, keep special-process certificates on their own lines, and send STEP plus PDF on quote-new.php so the quote band matches the traceability you actually need. Ready to price a CNC lot with a clear cert note? Start on quote-new.php.