Mold Making Services — Design, Steel, Machining & FAT
Manufacturing Guide
Mold Making Services: From Cavity Design to FAT — The Complete Engineering Guide
Table of Contents
- What “mold making services” actually delivers
- The mold design phase: DFM, gate, runner and cooling
- Steel selection: grades, hardness and when each is used
- Mold machining: CNC, EDM and wire EDM (9+4 machines)
- Assembly, fitting and dimensional verification
- Trial molding (tryout) and FAT: what a mold acceptance report contains
- Mold classes 101-105: life, tolerance and lead time benchmarks
- Mold cost and lead time: 25-30 days explained
- Mold maintenance, repair and longevity
- In-house mold making vs. outsourcing
- Frequently asked questions
What mold making services actually delivers
A mold (or tool, or die) is a hardened steel cavity set that shapes molten plastic into a finished part tens of thousands to millions of times. Mold making services cover the entire chain that produces that tool: design and DFM review, steel selection, CNC and EDM machining, heat treatment, assembly, polishing, trial molding and factory acceptance testing (FAT). Buyers who procure molds from China, Mexico or the US are not buying “a cavity” — they are buying a repeatable manufacturing asset whose geometry, steel, cooling and ejection design determine part quality for the life of the program.
This guide is written from the engineering-sourcing side: what specifications belong on a mold RFQ, what mold classes mean in real numbers, which steel grades suit which resins, how EDM and wire EDM shape the cavity, what a FAT should verify, and how to budget a realistic 25–30 day production-tool build.
The commercial reality of mold sourcing
- A production injection mold is typically the single largest capital cost in a molded-part program, commonly US$5,000 to US$100,000+ depending on size, cavity count and steel grade.
- Mold lead time is the critical path for product launch — which is why rapid tooling (7–12 days) exists alongside production tooling (25–30 days for our standard builds).
- The mold maker’s capabilities — wire EDM count, 5-axis CNC, CMM verification, mold-flow analysis — predict whether your tool will hit dimensional targets on the first tryout or after ten.
MOLDITQUICK (Dongguan Guohong Precision, 东莞国宏精密) runs an integrated mold shop: Sodick wire EDM (9 + 4 machines), Sodick injection machines (18 + 3), a 10,000 m² plant, and 280 employees across mold making, injection molding, CNC, die casting, stamping (20 Aida presses), rapid prototyping, two-shot, overmolding, LSR and low-volume production. We are certified to IATF 16949, ISO 13485 and ISO 9001 — relevant because mold sourcing for automotive and medical programs requires documented process control, not just a machined block.
The mold design phase
The mold is designed before any steel is cut. The design package normally includes:
| Deliverable | What it proves | Why buyers care |
|---|---|---|
| DFM (design for manufacturability) report | Part is moldable: draft, wall, gate, ejection | Fewer tryout iterations, lower tool cost |
| Mold-flow analysis (Moldex3D/Moldflow) | Fill, weld lines, sink, cooling balance | Predicts defects before steel is cut |
| 2D mold assembly drawing | Steel sizes, ejector layout, cooling circuits | Clear quotation and buy-off baseline |
| 3D cavity/core models (STEP/IGES) | Machinable geometry | Verifies inserts, slides and lifters |
| Cooling analysis | Cycle time estimate | Predicts per-part cost and throughput |
DFM rules that survive contact with a mold maker
These are the numbers experienced mold shops check first (industry-standard ranges; see our DFM checklist for the full list):
- Wall thickness: 0.5–5 mm typical (RapidDirect publishes 0.5–5 mm as standard guidance; source: rapiddirect.com/services/injection-molding/). Uniform walls prevent sink and warp.
- Draft angle: 0.5–2° per side for texture-free faces; 3°+ for textured faces (RapidDirect guidance, same source).
- Ribs: 0.5–3 mm; bosses: 3–10 mm OD (RapidDirect guidance, same source).
- Undercuts: avoid where possible — each undercut adds a slide, lifter or hand-load core, and each moving component adds cost and maintenance.
- Gate location: should feed thick sections and avoid visible cosmetic faces.
Mold-flow analysis as a procurement tool
A serious mold shop runs fill, cooling and warp analysis before cutting steel. The output is not decorative: fill analysis predicts short shots and weld lines, cooling analysis predicts cycle time (which directly sets your unit cost), and warp analysis predicts dimensional drift. When you receive a DFM from a supplier, ask to see the mold-flow summary page — a shop that cannot show one is quoting geometry, not manufacturability.
Steel selection
Steel choice decides mold life, surface finish capability and cost. These are the standard grades used across the industry, with their typical hardness after heat treatment:
| Steel grade | Type | Typical hardness | Best for | Notes |
|---|---|---|---|---|
| P20 | Pre-hardened tool steel | 28–32 HRC | Low-to-mid volume, prototypes, Class 105–103 | Machinable without post-heat-treat |
| 718H (P20 modified / 40CrMnMo7) | Pre-hardened mold steel | 33–38 HRC | General production molds | Improved polishability and uniformity |
| S136 (420SS / 1.2083) | Stainless mold steel | 48–52 HRC (hardened) | Optical, medical, corrosive resins (PVC, PC) | Excellent corrosion resistance, mirror polish to SPI A1 |
| H13 | Hot-work die steel | 44–52 HRC | Die casting dies, high-temperature molding | Thermal fatigue resistance |
| NAK80 | Pre-hardened maraging-type | 38–42 HRC | High-polish, long-life production molds | Machineable in hardened state, mirror finish |
| S7 / D2 | Shock-resistant / high-carbon tool steels | 54–60 HRC | Inserts with high wear, high cavitation | Used for cores and wear parts |
Selection logic in practice:
- Class 105 prototype molds (under ~500 shots): P20 or aluminum (7075-T6) — cheap, fast, no heat treatment.
- Class 103–102 production molds (100k–1M shots): 718H cores/cavities with hardened inserts, or S136 where corrosion or polish matters.
- Medical devices: S136 stainless cavities support the cleanliness and corrosion requirements of ISO 13485 programs; MOLDITQUICK holds ISO 13485 and runs medical molds under documented control.
- Die casting: H13 (or premium H13 variants) is the standard for aluminum die casting dies because it survives thermal cycling.
One nuance buyers often miss: hardness is a budget line. Pre-hardened P20/718H avoid the cost, time and distortion risk of post-machining heat treatment. If your program is 50,000 parts, a hardened 718H tool with replaceable wear inserts is usually the sweet spot — not a fully hardened S136 tool.
Mold machining
A modern mold shop machines the cavity with three complementary technologies. Our shop uses all three, with Sodick wire EDM (9 + 4 machines) as the backbone for precision steel cutting.
CNC milling (3-axis, 4-axis, 5-axis)
- 3-axis mills pockets, cores and faces. Standard for the bulk of cavity machining.
- 4-axis / 5-axis machines complex cores, angled slides and deep ribs in one setup — reducing both lead time and the number of EDM electrodes needed.
- Tolerances: ±0.01 mm on mold machining, tightening to ±0.005 mm on precision features (industry benchmark published by FirstMold: firstmold.com/cnc-machining-service/).
EDM (sinker EDM)
- Uses a graphite or copper electrode to erode complex shapes — deep ribs, sharp internal corners, textures — that milling cannot reach.
- Corner radii: EDM can hold internal corner radii down to ~0.1 mm where a milling cutter would need a much larger radius.
- Surface finish from roughing (Ra ~3.2 µm) to mirror finish for optical parts.
Wire EDM (WEDM)
- Cuts with a traveling brass/molybdenum wire; used for punch-outs, insert blocks, core pins, and critical shut-off faces.
- Wire EDM holds ±0.005 mm or better and produces near-burn-free, parallel vertical walls — indispensable for precision insert fit and ejector pin holes.
- Why the machine count matters: 9 + 4 Sodick wire EDM machines mean wire work is never the bottleneck in a 25–30 day tool build. In many competitor shops, wire EDM capacity is the single largest constraint on quoting fast tooling.
Machining sequence for a typical production mold
- Rough milling of cavity/core blocks from pre-hardened steel.
- Heat treatment (if the steel is hardened post-machining; avoided for pre-hardened grades).
- Finish milling / 5-axis finishing of cavity geometry.
- EDM for ribs, inserts, textures and tight corners.
- Wire EDM for insert blocks, shut-offs, ejector holes, runner systems.
- Polishing / texturing — SPI grades from A1 (mirror) to D3 (sandblasted); VDI texturing for cosmetic surfaces.
- Fit and assembly.
Assembly and verification
Mold assembly is where tolerances stop being drawings and become fits:
- Core-to-cavity alignment: typically held to ±0.02 mm or better on the mold; cavity details to ±0.005 mm on precision features.
- Ejector system: ejector pins and sleeves must move freely and return consistently; pin clearance and plate parallelism are checked with a CMM.
- Slide and lifter systems: fit and travel verified — sloppy slides cause flash and short shots.
- Cooling circuits: pressure-tested (typically 10–15 bar water) and flow-checked. A leaking cooling line is a cycle-time and quality disaster.
- Hot runner systems (if used): nozzle tip alignment and thermal profile verified.
Verification equipment is part of the offering: a CMM (coordinate measuring machine) check of the cavity and cores against the 3D model, plus fit checks of all moving components, should be standard in the FAT.
Tryout and FAT
Trial molding (试模) is where the mold earns its keep. A structured tryout program covers:
- First shots: check fill, flow marks, weld lines, flash and short shots.
- Dimensional measurement: parts measured on CMM/vision against drawing — typically targeting ±0.01 mm on critical mold dimensions and standard molding tolerances of ±0.1 mm (standard) to ±0.05 mm (precision) on parts (industry benchmark: firstmold.com/plastic-injection-molding-service/).
- Process window check: shot weight, injection pressure, melt temperature and cooling time recorded to establish a repeatable process.
- Cosmetic sign-off: surface finish, gate vestige, ejector marks and texture match.
- Mold acceptance report: documents steel, hardness, dimensions, tryout part measurements and agreed changes. This is your FAT document — insist on it in writing.
What a FAT report should contain
- Mold photos and serial numbers, steel certificates and hardness test values.
- Part measurement report (CMM) against the approved drawing/3D model.
- Tryout part samples and process parameters.
- List of agreed modifications and completion dates.
- Sign-off sheet from both parties.
Mold classes 101-105
Mold classes are the industry shorthand for tool life, tolerance and cost. The commonly published benchmark table (RJCMold: rjcmold.com/services/injection-molding-processes/) is:
| Class | Typical shot life | Typical tolerance | Typical lead time | Use case |
|---|---|---|---|---|
| Class 105 | < 500 shots | ±0.05 mm | 10–18 days | Prototype / low-volume validation |
| Class 104 | < 100,000 shots | ±0.05 mm | — | Short-run production |
| Class 103 | < 500,000 shots | ±0.05 mm | — | Standard production |
| Class 102 | < 1,000,000 shots | ±0.05 mm | — | High-volume production |
| Class 101 | 1,000,000+ shots | Tightest | — | Ultra-high-volume, often multi-cavity |
Xometry publishes the same class ladder — Class 105 (prototype) through Class 101 (ultra-high volume) — with steel vs. aluminum tooling options (xometry.com/capabilities/injection-molding-service/).
How to read this table when sourcing:
- Class 105 tools are aluminum or P20, un-hardened, and cost a fraction of production tooling. They are for design validation and small pilot runs, not mass production.
- Class 103–102 tools (718H/S136 hardened) are the workhorses for 100k–1M part programs — the range most B2B buyers actually need.
- Class 101 tools are for automotive and appliance mega-programs (multi-cavity, hot runner, fully hardened). Automotive programs typically also demand PPAP/IMDS documentation under IATF 16949, which we support.
Rapid tooling compresses the class 105 end of the spectrum: 3ERP publishes rapid-tooling lead times from 24 hours versus 4–8 weeks for conventional tooling (-75%), with 1–10,000 shot capability (3erp.com/blog/rapid-tooling/). See our rapid tooling guide for when that trade-off is right.
Mold cost and lead time
Why 25–30 days is the production-tool benchmark
FirstMold publishes 7–12 days for rapid tooling and 25–30 days for production tooling (45 days for automotive) (firstmold.com/plastic-injection-molding-service/). Our production tool builds sit in the same band. The 25–30 day window assumes:
- DFM approved and steel ordered at project kick-off.
- Cavity/core machining scheduled without machine contention (wire EDM capacity is why this is credible).
- Single tryout round before FAT; complex multi-slide tools push toward 35–45 days.
- Automotive-grade documentation (IMDS, PPAP support) adds days, hence the 45-day automotive benchmark.
Cost drivers
| Cost driver | Impact |
|---|---|
| Cavity count | Doubling cavities roughly 1.5–1.9x the tool cost |
| Steel grade | P20 < 718H < S136/H13 |
| Part complexity (slides, lifters, cores) | Each moving component adds machining + assembly + risk |
| Hot runner vs. cold runner | Hot runner adds significant cost but cuts cycle time and scrap |
| Texture/polish spec | SPI A1 mirror polish costs more than VDI 24 texture |
| Mold standards | DIN 7168-m / ISO 2768-m general tolerances are the norm (firstmold.com/plastic-injection-molding-service/) |
Get a realistic mold-cost estimate methodology from our injection molding cost guide.
Mold maintenance
Mold life is not “until it breaks” — it is “until the cavity wears past print tolerance.” Planned maintenance is the difference between 500k and 1M+ shots:
- Preventive maintenance (PM): every N shots (commonly 10k–50k for production tools), clean vents, polish cavity, check ejector pins, verify cooling flow.
- Wear-item replacement: ejector pins, slides, shut-off inserts are consumables — budget replacement stock.
- Storage: molds stored on standardized bases with anti-corrosion protection; steel rusts faster than most people think.
- Monitoring: track shot count, maintenance history and part measurements per tool so wear is predicted, not discovered.
We also offer mold repair and modification services — adding a slide, changing a gate, re-polishing after corrosion — which is usually far cheaper than a new tool.
In-house vs. outsourcing
The structural advantage of an integrated mold shop is that the people who build the mold also run it on their own Sodick injection machines. That closes the loop on tryout: a mold maker who also molds production parts optimizes the tool for the actual process window instead of handing it over and walking away.
- Single accountable partner for tool + part quality.
- Tryout done on the same machine platforms used for production.
- Faster iteration: design change → steel change → trial, without cross-company handoffs.
- One P.O. for mold, parts, secondary operations (overmolding, insert molding, stamping, surface finishing) and low-volume production.
See how to choose a mold maker for the procurement checklist, and our mold making service page for capabilities.
Mold gating, runner and cooling design details
The internal geometry of the mold is where cycle time and part quality are really decided.
Gate types and when to use them
| Gate type | Best for | Notes |
|---|---|---|
| Edge gate | General parts | Simple, easy to trim; leaves a witness mark |
| Pin gate | Cosmetic parts | Small vestige, three-plate mold |
| Tunnel (submarine) gate | Automated production | Gate self-trims on ejection |
| Fan gate | Large flat parts | Even fill, low stress |
| Sprue gate | Single-cavity large parts | Cheap but leaves large vestige |
| Hot runner valve gate | Cosmetic high-volume | No cold runner scrap, zero vestige |
Cooling circuit design
Cooling controls cycle time — and cycle time controls unit cost. A well-designed cooling layout typically uses:
- Baffles and bubblers for cores and thin ribs where straight lines can’t reach.
- Conformal cooling (via machined or 3D-printed inserts) that follows the cavity contour, cutting cycle time by 10–30% versus straight-drilled lines on complex parts.
- Turbulent flow at 10–15 bar water pressure to maximize heat transfer.
- Balanced circuits so every cavity in a multi-cavity tool cools at the same rate — otherwise cavities drift dimensionally from each other.
Ask any mold shop how many cooling circuits a quoted tool has and where they’re located. A tool with thin, single-line cooling on a thick part will run slow forever; you pay that cycle penalty on every shot.
Ejection system design
- Ejector pins are the default: placed on ribs, bosses and flat areas, away from thin walls.
- Stripper plates / sleeves for thin-wall and cup-shaped parts that would distort under pins.
- Air poppets for parts that cling to the core.
- Lifter and slide actuation — either angle pins or hydraulic cylinders, depending on travel and locking force.
Ejector mark and witness placement is a cosmetic decision as much as a mechanical one; a good DFM puts ejector points where the customer will never see them.
Quality control and measurement in mold making
A mold is only as good as its verification. The QC chain for a production tool:
- Incoming steel check — certificate of heat treatment and hardness (Rockwell C) on receipt.
- In-process inspection — CMM spot checks during machining, not just at the end; dimensional drift caught early costs nothing to fix.
- Fit-up checks — core/cavity alignment, slide travel, ejector return, gate and runner seating.
- Trial part measurement — molded parts measured on CMM/vision against the drawing; typically targeting the ±0.1 mm standard / ±0.05 mm precision band on parts (benchmark: firstmold.com/plastic-injection-molding-service/).
- Capability (CpK) review for automotive programs — the mold must not just hit nominal, it must hold the process stable shot after shot under IATF 16949.
Measurement equipment a serious mold shop keeps in-house: CMM, surface roughness tester (for SPI/VDI finish verification), hardness tester, and vision system for small features.
Common mold defects and the DFM fixes that prevent them
| Mold-stage symptom | Root cause | DFM/preventive fix |
|---|---|---|
| Flash at parting line | Poor shut-off, inadequate clamp force, soft steel | Proper shut-off design, hardened inserts, adequate tonnage |
| Short shots | Poor venting, thin ribs, unbalanced fill | Add vents, rebalance runners, adjust wall/rib proportions |
| Sink marks on thick sections | Uneven wall, poor packing, weak cooling | Uniform walls 0.5–5 mm (RapidDirect guidance: rapiddirect.com/services/injection-molding/), strong cooling under bosses/ribs |
| Weld lines visible | Melt fronts meeting at a feature | Relocate gate, add overflow wells |
| Part sticks / ejection marks | Insufficient draft, pin placement | 0.5–2° draft, proper pin layout |
| Mold corrosion after storage | Wrong steel for resin, poor storage | S136 for PVC/corrosive resins, anti-corrosion storage |
Every one of these is cheaper to fix in DFM than in steel. That is why we run mold-flow analysis and a DFM review before cutting any material — and why buyers should treat “we’ll fix it in tryout” as a yellow flag.
How to buy a mold: RFQ checklist and commercial terms
What to send with a mold RFQ:
- Part 3D model (STEP preferred) and 2D drawing with GD&T.
- Annual volume and expected program life — this sets the mold class (105→101).
- Material grade (or let us recommend from our material selection guide).
- Cosmetic requirements (SPI/VDI finish, texture area).
- Target cycle time, if you have one.
- Documentation needs (PPAP, IMDS, mold inspection reports).
Commercial terms worth negotiating:
- Milestone payments — typically 50% to start, 40% at tryout, 10% at FAT/shipment.
- Mold warranty — 1 year or a shot count (e.g., 100k shots) is common; covers workmanship, not normal wear.
- Trial shots included — one structured tryout round should be in the price; extra rounds billed at cost.
- Mold ownership and shipping — confirm export packing, fumigation (wooden crates), and incoterms.
- After-sales support — who repairs the mold, at what rate, with what turnaround.
FAQ
1. What does mold making service include? Design/DFM, steel procurement, CNC/EDM/wire EDM machining, heat treatment, assembly, polishing, tryout, FAT and typically a warranty period. Confirm the scope in writing before ordering.
2. How long does it take to make an injection mold? Rapid tooling: 7–12 days; production tooling: 25–30 days standard, ~45 days for automotive-grade documentation (industry benchmarks: firstmold.com).
3. What tolerance can a mold achieve? Cavity/core details to ±0.005 mm; critical mold dimensions ±0.01 mm; molded parts typically ±0.1 mm standard, ±0.05 mm precision (benchmarks: firstmold.com, firstmold.com).
4. What is the difference between Class 101 and Class 105 molds? Class 105 is a prototype tool for under ~500 shots; Class 101 is ultra-high-volume (1M+ shots), fully hardened, often multi-cavity with hot runners (rjcmold.com).
5. Which steel is best for injection molds? 718H for general production, S136 for corrosion/medical/optical, P20 or 7075 aluminum for prototypes, H13 for die casting dies.
6. What is a mold tryout and why does it matter? Trial molding on the actual machine to verify fill, dimensions, cosmetics and process window before the tool ships. One structured tryout round should be included in every mold quote.
7. What is FAT in mold making? Factory acceptance testing: the documented verification (CMM measurement, tryout parts, steel certificates, agreed changes) that the tool meets spec before payment/shipment.
8. How many shots does a production mold last? Class 105 < 500; Class 104 < 100k; Class 103 < 500k; Class 102 < 1M; Class 101 1M+ (rjcmold.com).
9. Can you make molds for medical devices? Yes — MOLDITQUICK is ISO 13485 certified, uses S136 stainless cavities where required and documents process control for medical programs.
10. Is mold making cheaper in China? Mold cost in China is typically 30–50% below US/EU quotes for the same class, with the trade-off of shipping and communication. Insist on the same DFM/FAT discipline regardless of geography.
11. What is wire EDM used for in mold making? Precision insert blocks, shut-off faces, ejector pin holes and any geometry requiring ±0.005 mm vertical-wall accuracy — our shop runs 9 + 4 Sodick wire EDM machines.
12. Do you provide mold maintenance or repair? Yes: preventive maintenance plans, wear-item replacement, and mold modification/repair are part of our services.
13. What documentation comes with a mold? Steel certificates, hardness reports, CMM part measurements, tryout samples, process parameters and the mold acceptance report (FAT).
14. What is the largest part size you can mold? Mold design scales with part size; our injection machines accommodate a wide envelope (industry reference: Protolabs lists a maximum part of 480×751×203 mm — protolabs.com/services/injection-molding/). Send your envelope for a feasibility check.
15. What standards apply to mold tolerances? Common drawing standards: DIN 7168-m and ISO 2768-m general tolerances; molded-part tolerance standards include DIN 16901 and ISO 20457 (boyanmfg.com).