A mold is priced by three numbers
Every injection mold quotation reduces to cavitation, steel grade and mechanism. Cavitation sets the cycle output and the tool cost. Steel grade sets the life and the maintenance interval. Mechanism — sliders, lifters, hot runner, unscrewing — sets both the engineering hours and the risk.
Buyers who negotiate the total price without separating those three usually win a discount on the wrong line and pay for it later in tool repairs.
Cavitation economics
Tool cost does not scale linearly with cavities. A 4-cavity mold typically costs about 1.8–2.2 times a 1-cavity, and a 16-cavity about 4–5 times, because the base, the ejector system and the machine interface are shared while the cavity set grows.
| Cavitation | Relative tool cost | Typical machine | Best fit |
|---|---|---|---|
| 1 | 1.0 | 80–150 t | Prototypes, large parts, low volume |
| 2 | 1.4 | 150–250 t | Pilot runs, moderate volume |
| 4 | 1.9 | 250–350 t | The default workhorse for small parts |
| 8 | 2.8 | 350–500 t | Volume parts with simple geometry |
| 16 | 4.2 | 500–650 t | Commodity parts, caps, closures |
| 32–64 | 6–9 | 650 t+ | Very high volume, tight part uniformity |
The break-even calculation is straightforward: compare tool cost per part at annual volume, then check whether the larger mold fits the machine tonnage available. A 16-cavity tool that requires a machine the supplier does not have is not cheaper, it is a delay.
Cavity balance is the hidden variable. An 8-cavity tool with 10% fill imbalance produces parts that vary in shrink and weight, which shows up later as assembly rejects. Ask for a cavity balance study on the first shot.
Steel selection
| Steel | Common designation | Hardness (HRC) | Typical life | Notes |
|---|---|---|---|---|
| P20 / 1.2311 | 718 | 28–32 | 300k–500k | Pre-hardened, general purpose, easy to repair |
| 718H / 1.2738 | 738 | 32–36 | 500k–800k | Better polish, thicker sections stay hard |
| NAK80 | — | 38–42 | 500k–1M | Mirror polish, good for optical and cosmetic |
| S136 / 1.2083 | 420 stainless | 48–52 | 1M+ | Corrosion resistant; PVC, flame-retardant, medical |
| H13 / 1.2344 | 8407 | 48–52 | 1M+ | High-temperature resins, glass-filled, hardened inserts |
| BeCu (beryllium copper) | — | 36–42 | Insert only | Thermal conductivity 3–4× steel; used at hot spots |
| Aluminum 7075 | — | 15–20 | 5k–50k | Prototype and bridge tooling only |
Three practical rules. First, glass-filled resin above 30% glass accelerates gate and cavity wear, so gates belong in hardened inserts even on a P20 base. Second, PVC and flame-retardant grades generate corrosive by-products, so cavity surfaces need S136 or a proven coating. Third, beryllium copper inserts pay for themselves wherever a thick boss or a long core otherwise controls the cycle.
Runner system
| Option | Cost | Material waste | When it pays |
|---|---|---|---|
| Cold runner, 2-plate | Baseline | High — sprue and runners are scrap | Low volume, large parts, colour changes |
| Cold runner, 3-plate | +15–25% | Lower | Multi-point gating on flat parts |
| Hot runner, open gate | +25–40% | Near zero | Volume parts, engineering resins |
| Hot runner, valve gate | +40–60% | Near zero, best gate cosmetics | Visible surfaces, sequential fill, family tools |
Hot runner adds 5–10 days to the schedule and a controller to the machine, but it removes regrind from the value stream and shortens the cycle. On glass-filled resins, hot runner manifolds need hardened flow bores and careful thermal zoning, otherwise the material degrades in dead spots.
Sequence and lead time
| Stage | Days | Output |
|---|---|---|
| DFM review and mold flow | 3–7 | Wall thickness, gate location, warp prediction, draft report |
| 3D mold design and buyer approval | 5–10 | Full assembly drawing, cooling layout, ejector plan |
| Steel and standard parts procurement | 7–15 | Material certs, hot runner lead time |
| CNC roughing and semi-finish | 10–20 | Cavity and core blocks, inserts |
| EDM, wire cut, deep hole drilling | 5–15 | Sharp corners, ribs, water lines |
| Polishing, texturing, assembly | 5–10 | Surface finish, fit check |
| T1 trial and sample report | 2–3 | First samples, dimensions, defects |
| Correction and T2 | 7–15 | Steel changes, re-trial |
A straightforward 2-plate tool with no sliders lands around 30–40 days from design freeze. Add sliders, lifters or an unscrewing core and expect 45–60 days. A valve-gate hot runner with a textured cosmetic surface can run 60–75 days. Quotations that promise 15 days are usually quoting a mold built in parallel with an unfinished part design — which is where most disputes begin.
What T1 should deliver
A trial shot is a data event, not a photo opportunity. The report should include:
- Sample quantity per cavity, with cavity number marked on each part
- Dimensional report against the drawing, showing which dimensions are in and out of tolerance
- Cavity-to-cavity weight spread
- Cycle time and the process window used
- Gate, weld line, sink, flash and short-shot documentation with photographs
- Mold temperature map at steady state
- Warp measurement on a fixture, for flatness-critical parts
Acceptance is normally CPK ≥ 1.33 on critical dimensions with production tooling and production resin, not with a surrogate grade.
Tool ownership and handover
Mold payment schedules in China commonly run 30% deposit, 40% at T1 approval, 30% before shipment, though 50/50 is negotiable for small tools. The commercial terms matter less than the documentation package:
- Mold marking: part number, cavity count, steel grade and date stamped on the mold base
- Spare parts: one set of ejector pins, springs, and a spare cavity insert for high-wear tools
- Cooling layout drawing and water fitting specification
- 2D part drawings, 3D mold design files, and the mold flow report
- Material certificates for cavity and core steel
- Tool life statement and the recommended preventive maintenance interval — typically every 50,000 shots for a P20 tool
Ownership should be explicit. A mold paid for in full belongs to the buyer, and the agreement should state that it can be transferred to another molder with the design files and the trial report.
Maintenance and the real cost of a cheap tool
A P20 tool run with abrasive resin and no maintenance can lose its gate in 100,000 shots, then need welding, re-polishing and re-qualification. A hardened insert costing a few hundred dollars at build time avoids that. Budget 2–4% of tool cost per year in maintenance for a production tool, and expect flash growth, ejector wear and cooling channel scale as the three routine issues.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
Standards referenced
- ISO 294 — injection moulding of test specimens, used for shrink and warp trials
- ISO 8407 — removal of corrosion products, for mold storage practice
- NAADM / AISI steel designations referenced alongside DIN 1.xxxx equivalents
- ISO 20457 — plastic moulded part tolerances, for dimension acceptance
