Mold Specification Sheet — How Do You Write a Technical Specification That Eliminates Miscommunication with a Chinese Mold Maker?
Last updated: 2026-09-03 · Reading time: about 6 min
A mold specification sheet is the technical contract for the build — it defines the part, cavity layout, steel, surface finish, gating, cooling, mold base and documentation. If a requirement is in the spec, the mold maker must deliver it; if it is not, nobody owns it — so every parameter that touches cost, quality or performance goes in writing before design starts.
See also related article.
Key facts
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Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.
What sections does a mold specification need — and what goes in each?
Eight sections, each specific enough that the maker builds without asking questions.
| Section | What it specifies | Example |
|---|---|---|
| 1. Part information | Part number, resin, shrinkage, weight, drawing revision | #A1234, ABS Cycolac MG94, shrink 0.005 mm/mm, 45 g, Rev C |
| 2. Cavity layout | Cavity count, spacing, family or identical, runner | 4 cavities (2×2), identical, cold submarine gate |
| 3. Steel selection | Cavity and core steel, hardness, treatment | P20, 30–34 HRC, pre-hardened, no texture |
| 4. Surface finish | SPI class, texture standard, gloss | SPI-A2 (600-grit polish), gloss 60 GU at 60° |
| 5. Mold base | Supplier, size, plate material, guide pins | LKM standard, 350 × 400 mm, P20 plates, ball-cage pins |
| 6. Gating | Gate type, location, dimensions, runner | Submarine gate, location per drawing, 1.0 × 0.8 mm |
| 7. Cooling | Channel diameter, circuit layout, inlet/outlet | 10 mm channels, baffle circuits, rear inlet |
| 8. Testing & documentation | T1 quantity, CMM report, mill certificate, trial video | T1: 20 shots, full CMM report, cavity-insert mill certificate |
Every line must be specific enough to remove interpretation. “P20 steel” alone is not a spec — name the grade, the hardness range (30–34 HRC), the heat treatment (pre-hardened, or hardened and tempered), the supplier if one is required, and the mill certificate that proves composition and hardness. Left vague, the maker may default to a below-spec P20 — hardness under the range, steel with inclusions — and the mold life falls short of what you assumed.
What omissions cause disputes — and how do you close them?
Four gaps that turn into arguments at T1, and the line that prevents each.
- State the mold life in shots. If the spec is silent, the maker builds to a 100,000-shot life (SPI Class 2, P20) while you needed 500,000 (Class 1, H13). When the cavity wears at 150,000 shots, nobody ever agreed on a number. Write the shot count and tie it to the steel, and it becomes a contractual obligation.
- Set the cycle-time target. Cooling designed for 30 s will not hit the 15 s your volume needs — the channels are too small or too few. Name the target cycle so the cooling is sized for it; otherwise the maker meets a spec you never wrote and you absorb the longer cycle.
- Name the machine it must run on. Give the make, model, tonnage, platen and tie-bar spacing and nozzle type. A mold built for a 200-ton press will not fit a 150-ton machine’s tie bars. Stating the machine makes the fit a spec item rather than a surprise at install.
- List the documentation deliverables. 2D and 3D mold drawings, mill certificates, heat-treatment reports, the CMM report, trial video, electrode map and BOM. If they are not listed, the mold can ship without them and you cannot verify the steel or the dimensions. Make the documentation part of acceptance.
A specification is a contract, and silence is a gap you pay for later. Every parameter that touches cost, quality or performance must be written down — mold life, cycle time, machine, documentation — because whatever the spec omits, neither side owns. Ambiguity gets resolved at T1, and at T1 the party with the weaker document loses.
“Most mold disputes I see are not about a maker cutting a corner — they are about a requirement nobody wrote down. We build the spec with the customer before design starts: the shot life, the cycle target, the machine it runs on and the documentation we hand over, all signed and dated, so a change later goes through a change order rather than an argument.”
— Project Engineer, aCatMould
Frequently asked questions
How detailed should the spec be — and who writes it?
Detailed enough that the maker builds without asking questions: every parameter affecting cost, quality or performance gets a value.
A standard mold runs 5–15 pages; a complex one with hot runners, side actions and multi-level parting lines runs 20–30.
The customer’s tooling engineer writes it — they know the resin, tolerances, finish, volume and machine. The maker reviews and suggests changes to steel, gating or cooling from build experience, then both sign. The signed spec is the design baseline; any deviation goes through a formal change request.
What if the customer has no spec — can the mold maker supply one?
Yes. A maker can offer a standard template covering steel, mold base, gating, cooling, ejection, testing and documentation. The customer reviews it and adds their own resin, machine and quality requirements before approving.
The risk is that a maker’s template reflects the maker’s standard practice, not your requirement — a plainer cooling layout, or a mold base you did not choose. Read it against your shot life, cycle time and documentation needs, and add anything missing before you sign.
How do spec changes after design starts get handled — and what do they cost?
Through a formal change order: the customer describes the change and its reason, the maker quotes the design, machining, material and schedule impact, and the customer signs and pays before it is implemented.
Cost scales with timing — $200–1,000 in design, $1,000–5,000 during machining, $5,000–20,000 after assembly — and each change adds 1–4 weeks. A 1–2 week spec review up front costs far less than a change order later.
Key takeaways
- Treat the spec as the contract: whatever it names, the maker builds; whatever it omits, nobody owns — so every parameter touching cost, quality or performance goes in writing before design starts.
- Cover eight sections — part info, cavity layout, steel, surface finish, mold base, gating, cooling, testing/documentation — and make each specific (grade, hardness, SPI class, dimensions, mill certificate) rather than open to interpretation.
- Close the four dispute-causing gaps — mold life in shots, cycle-time target, the machine it runs on, documentation deliverables — and route any later change through a signed change order, since a change after assembly costs $5,000–20,000.
A mold specification is not paperwork — it is the difference between a mold that matches what you designed and a mold that matches what someone assumed. Write every requirement down, make each one specific, sign it before design begins, and route changes through a change order. The hour spent closing a gap in the spec costs far less than the week spent arguing about it at T1.
Sources
MoldMaking Technology — mold build specification and mold design data
Plastics Technology — mold specification and DFM practice
Plastics Industry Association — SPI mold class and surface finish standards
HASCO — mold base and standard component specifications
Tooling & Production — mold scope and tooling documentation practice
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet