Tooling PO is where OEM budgets leak — not because mold steel is mysterious, but because buyers accept a lump sum without steel grade, cavitation, or iteration scope. A clear injection mold design brief before quote prevents six-figure surprises at T1, and the rework loops that push launch dates into the next fiscal year.
Procurement teams often treat mold design as a supplier-side detail. That works until the first tryout shows weld lines on a Class A bezel, ejector pin marks on a visible lid, or a parting line flash line the industrial design team rejects. By then, steel is cut, schedule is burned, and every correction is a change order.
This guide explains what mold design must resolve before cut, what inputs your engineering team should supply, how gate and runner choices interact with cosmetic requirements, and why design decisions affect piece price for the entire program — not just the tooling line on the PO.
What mold design must answer before steel
Every production mold is a stack of irreversible decisions. The mold designer is not “making a negative of your CAD.” They are choosing how plastic enters the cavity, how it cools, how the part ejects without damage, and how the tool survives years of clamp cycles.
- Parting line — cosmetic impact, degating path, flash risk on visible surfaces, and whether the line crosses a customer-visible edge
- Gate type and location — flow balance, weld line placement on Class A faces, shear at the gate, and vestige the customer will see or feel
- Cooling layout — cycle time, warp on flat lids and long walls, differential shrink across thick sections
- Ejection — mark-free Class A, stripper plate vs ejector pins, blade ejectors on thin ribs
- Side actions — lifters, slides, hydraulic vs mechanical sequencing, maintenance cost and failure modes
- Steel and cavitation — P20 vs H13, single vs multi-cavity, hot vs cold runner, expected shot life
- Venting and shut-off — burn marks at last fill, flash at shut-off land, tolerance stack on mating features
Each decision should appear in the written quote — not as verbal assumptions after award. If the proposal is one total with no gate sketch, no steel callout, and no cavitation drawing, you are comparing fiction.
Parting line: the decision industrial design forgets
The parting line is where the two mold halves meet. Plastic can flash across that interface if clamp force, shut-off angle, or wear is wrong. More importantly for OEM programs, the parting line is often visible on the finished part.
Common buyer pain: marketing approves a render, but nobody mapped where the mold will split. The tool designer places the parting line on the only feasible shut-off — through a curved cosmetic surface — and the first T0 samples fail aesthetic review.
Fix this in DFM, not at tryout. Provide a cosmetic class map (Class A / B / C) and ask for two parting-line options with trade-offs before steel. A small CAD change — a hidden parting on an internal rib land — can save weeks.
Gate types and when each fits
| Gate type | Typical use | OEM watch-out |
|---|---|---|
| Edge / tab gate | Non-cosmetic edges, easy trim | Trim scar on visible edge if placed wrong |
| Submarine (tunnel) | Automatic degate, hidden on underside | Not suited to all materials or thin walls |
| Direct sprue | Thick sections, simple tools | Gate mark and stress at entry |
| Hot tip / valve gate | Class A surfaces, multi-cavity | Tip wear, heater failure, maintenance |
| Fan or diaphragm | Large flat parts | Weld line pattern across face |
Gate location drives weld lines, orientation of fiber-filled resins, and whether you need post-mold trimming labor in your total cost model. A gate moved 8 mm can move a weld line off a logo zone — but only if someone reviews flow before cut.
Mold design inputs from the OEM
| Input | Why mold designer needs it |
|---|---|
| Approved STEP + revision block | Shrink compensation and shut-off geometry |
| Material grade or performance spec | Shrink, venting, corrosion on hot runners |
| Annual volume bands (year 1 and steady state) | Cavitation and runner economics |
| CTQ list and cosmetic class map | Gate placement, polish level, texture pull direction |
| Press preference / tonnage cap | Tool size, side-action feasibility |
| Inspection plan (FAI, SPC) | Cavity correlation requirements |
| Target cycle time (if known) | Cooling channel density and cavitation |
| Secondary ops (inserts, pads, assembly) | Fixturing and ejection orientation |
Package these using our DFM review checklist before requesting mold design feedback. Incomplete RFQs produce quotes that assume single-cavity P20, cold runner, and unlimited engineering — then reality arrives at T1.
Cooling and warp: where cycle time is won or lost
Under-cooled tools run longer cycles and produce more warp. Over-cooled thin sections can show sink on the opposite side. Mold designers balance channel diameter, baffle vs straight drill, and proximity to thick bosses.
OEM teams feel this as “the supplier keeps tweaking process but dimensions drift week to week.” Often the root cause is cooling fouling or channels that never matched the part geometry — not operator error.
Ask for a cooling concept in the mold design proposal, especially on flat enclosures and large lids. For programs with tight flatness CTQs, cooling is not a detail — it is the process.
Design decisions that affect piece price for years
Multi-cavity, hot runner, and automated degating raise tooling cost but cut cycle time and labor per part. Under-specified cooling saves on the mold quote and costs on every shot through longer cycle and higher scrap. Side actions that could be eliminated with a small CAD change often pay back in weeks at volume.
Example logic procurement can use: if a hot runner adds $8,000 to tooling but saves 12 grams of runner scrap per shot at $4/kg resin and 120,000 shots per year, scrap savings alone can exceed $5,700 annually — before counting cycle time. Model decisions with total cost of ownership, not tooling alone.
A mold that runs 5 seconds faster per cycle can recover hot-runner investment within months at 100k+ parts per year. A mold that needs manual degating adds labor every shift — invisible in piece price until you audit the supplier’s cost build.
Mold design approval workflow (recommended)
- Receive written DFM + mold concept (gate, parting, cavitation, steel)
- Cross-functional review: engineering, quality, procurement, industrial design
- Sign off on items that affect cosmetics and CTQs — in writing
- Release steel only after sign-off; track revision on mold drawings
- Require updated mold design summary if product CAD changes post-sign-off
Skipping step 3 is how programs arrive at T0 with “we assumed you would accept the gate there.”
Red flags in mold design proposals
- No gate location sketch or flow comment on your geometry
- Steel grade omitted or listed as “standard” without P20/H13 spec
- Cooling described generically with no channel approach
- Zero included T1 iteration scope in writing
- Parting line placed through a cosmetic surface without discussion
- No mention of shrink model or material data source
- Multi-cavity quote with no balance or fill approach
How Deuchi approaches mold design
We integrate mold design with DFM review and mold build — not as a black box line item. Quotes include steel, cavitation, runner type, gate concept, and included engineering loops so procurement can compare scope across suppliers.
When geometry allows, we return mold design notes with the quote — not a price alone. Programs that continue into contract manufacturing use the same engineering thread from DFM through production release, so design intent is not lost between tooling and molding vendors.
FAQ
Who owns mold design — OEM or supplier?
Supplier designs to your part geometry and process requirements; OEM owns product requirements and must approve gate, parting line, and ejection proposals before steel is cut. Ownership of mold IP and CAD files should be defined in the tooling agreement.
Can we reuse mold design from a prior supplier?
Rarely drop-in — review steel spec, shrink model, press compatibility, and hot half wiring. See mold ownership before assuming files transfer cleanly. Even native mold files may not match your new molder’s press and maintenance standards.
When should mold design start relative to design freeze?
Preliminary mold concept at ~70% CAD is useful for cost and lead time planning; binding design for steel cut typically follows DFM sign-off at ~80–90% freeze. Starting too early wastes rework; starting too late compresses launch.
Do we need mold flow simulation?
Not every tool requires formal simulation, but geometry with thin ribs, long flow lengths, or multiple gates benefits from flow analysis before steel. Ask whether simulation is included or charged separately.
Next step: Send CAD for mold design review with volume bands, CTQ list, and cosmetic class map.