Oct 19, 2026Sourcing Guides
Injection Molding and Sheet Metal Under One Roof: What One-Stop Manufacturing in China Actually Saves
Each factory can pass its own inspection while the assembly still fails between them. The interface rules, drawing freezes, tooling terms, claim path and payment split that one contract has to settle.
A molded housing and a stamped chassis each pass their own inspection. Then they meet, and the door sits proud, a screw hole lands off its boss, or the powder coat reads a shade away from the plastic. Neither factory is necessarily at fault. The failure lives between them: the interface, the drawing revision nobody owns, the claim that bounces. The buyer's ledger covers one contract and two processes, from the DFM sign-off and mold ownership to the cross-process claim, the five-category rule, and the payment split. We are a Shenzhen team grown out of a family sheet metal shop, with three decades of manufacturing behind us, mold work at the center.
One Contract, Two Processes: What the Roof Really Covers
The phrase under one roof gets used for three different arrangements: one building, one contract, one quality owner. We claim the second and third. Our mold team can take a tool from DFM through try-out, and our metal shop stamps, deep-draws, bends, welds and finishes parts within the same operation. CNC work, PCBA where the BOM calls for it, sub-assembly and full assembly sit on the same route. Two-plate, hot runner, insert and overmolding tools are all built here, with steel chosen for surface, heat and abrasion. The wider BOM, from a display bezel to a cable assembly, can still reach partner factories outside that core. So the roof is a management line, not a postal address. A buyer who expects every step in one building will be disappointed by any supplier, including us. What travels with the parts is one standard, not two. When we say one roof, we mean one contract, one quality owner and one fixed route between processes. One contract should name one project lead for the handoffs, the inspection records and the paperwork that travels with the parts. Two separate contracts mean two schedules, two standards and two people to call when the assembly fails. That is the useful part for a buyer: fewer places for a problem to hide. A single machine has its own process story; a two-process build has a handoff. The mold side of this work, from DFM through try-out, has its own page (/industry/custom-plastic-injection-molds-tooling).
The Interface Is the Part Nobody Quotes
Two suppliers can each hold their print and still fail the assembly. The gap does not appear on either quote. It appears at the interface. A molded boss meets a stamped bracket, and the two materials move at different rates. A metal insert sits in plastic, and the plastic shrinks around it. Powder coat or anodizing adds thickness to a mating gap that was drawn before finishing. Flatness, datum choice and assembly sequence decide whether a panel closes clean. Grounding, EMI and screw connections decide whether the product works and whether it can be serviced. Powder coat color sits next to molded color under the same light, and the eye catches the mismatch before the spec sheet does.
None of these is a defect in one process. Each is a decision about the pair. That is why the buyer pays for the interface, not the piece. We see it most often when one supplier owns the housing and another owns the chassis: both can point to a conforming part, and the assembly still needs a shim, a re-drill or a color waiver. First article reports and salt spray results describe the part, not the pair.
A workable split is simple. The one-stop lead owns the interface: the shared drawing, the datums, the fit checks, the finish callouts. The buyer owns the fit, form, function and cosmetic standard, because those are brand decisions, not factory preferences. Each process owner owns its own capability: the molder signs what it can hold, and the metal shop signs what it can form and finish. Put that split in writing before the first tool is cut. A supplier can be capable and still lose the interface. Capability is what a process can hold. The interface is what two processes must agree on. The point is not to assign blame after the fact. It is to assign a signature before the fact.
The stamping, forming and finishing side sits here (/industry/metal-stamping-precision-forming).
Who Signs the DFM, and When Does the Drawing Freeze?
DFM is not one meeting and one email. It is a set of signatures, and each one covers a different risk. The buyer signs the cosmetic standard, the functional standard, the assembly standard and the acceptable sample. That signature is the commercial one: what the product must look like and do, and which sample governs a dispute. The mold side signs gate location, draft, wall thickness, material, cooling and ejection feasibility. The metal side signs blank layout, bend radius, springback, die design, weld sequence and finish feasibility. The one-stop lead signs the interface and the freeze date.
When does the drawing freeze? Before steel or dies are cut. A freeze after the first tool exists is not a freeze; it is a negotiation with the tool already losing. After the freeze, any change is a change order. It gets a price, a schedule and a new revision, and it applies to both processes at once. A change that touches the housing and the chassis together is exactly where separate suppliers lose time.
Our flow follows that order: DFM review before steel, try-out on production-grade machines, sample approval against the signed standard, then a CMM report covering each cavity and core. The report is not a trophy. It is the record that shows whether the tool cut the dimensions the sample promised.
The signature list is project discipline, not a warranty. Put each signature in the project file. When the drawing is silent, the loudest supplier in the room wins, and that is rarely the buyer.
Who Owns the Mold After the Invoice
A mold is not a one-time cost. It is the ticket to the second order, and the terms around it decide whether you can leave. The invoice usually names a price and a delivery date. It rarely names the owner. That is the first question to fix in the contract, and it should be answered in writing before the tool is cut.
Work through the ownership questions with your supplier, one by one. Who owns the tool? Where is it stored? Who pays for maintenance, and who approves the maintenance records? Who owns modifications, and what happens to the old cavity when a change is approved? What spare parts are kept, and for how long? How does the tool transfer if you move the program? How do you inspect the tool before you accept it? None of these has a universal answer. The answer is whatever your contract says, and a buyer who leaves it silent has decided by default.
A tool that cannot be maintained or moved locks the buyer to one supplier. That lock shows up the day you want a second source or a new finish. The second order tests whether the tool, the drawing and the process still match.
We design and build molds in-house rather than buying them through brokers, and we keep spare parts and maintenance support going after delivery. That does not answer the ownership questions for you, but it means the people who built the tool are still reachable when it needs work. Ask for the tooling file, the maintenance log and the transfer procedure before you pay the last installment. If a supplier cannot produce them, the mold may still be good. The exit is a separate question.
One Quality Gate, One Claim
When a molded part and a metal part fail together, the buyer should not become the router between two suppliers. One quality gate means one place where the goods are judged, and one claim path when they are not. The gate starts with a registry check on the supplier's business scope, shareholders, equity freezes and litigation. Then the physical references: two signed golden samples, one kept at the factory and one at our office. On the line, IPQC patrols are logged. Before shipping, every unit is checked against the sealed sample. Loading supervision checks carton condition, pallets, marks, piece counts, container condition and the seal number, with photos and video on file.
Then the claim. Attribute in this order: incoming material, molding, stamping, finishing, assembly. Each stage should leave a record. The metal side has first article reports with full dimensional data, SPC logs on critical dimensions, salt spray results and material traceability back to the mill. The mold side has the DFM record, try-out data, sample approval and the CMM report. Assembly sits last in that order. When a failure appears, the record trail points to the stage. That is not a compensation formula. It is a way to stop the argument from starting at the wrong end.
One quality gate does not mean every defect disappears. It means defects have one owner and one file. The factory-side QC stages and the golden-sample protocol have their own write-ups; across two processes, the join is what matters. A claim here has a route, a record and a name. A clean run is never automatic. The aim is simpler: a claim that does not land on your desk as a mystery.
What the Invoice Misses: Freight, Categories, and the 30/70 Split
The invoice lists the piece price, the tooling and the freight. Those are the visible lines. The expensive lines are the ones that never get printed: coordination time, stock sitting in transit, repeat inspection, freight between two suppliers, rework, and the line-down hours after a failure. A one-stop supplier can remove some of those lines. It cannot remove all of them, and it may not win on unit price alone. What it changes is the number of handoffs and the cost of one failure.
Consolidation is where the freight line gets honest. Goods from several suppliers arrive at our warehouses, get repacked and volume-compressed, then leave as one FCL or LCL load. The standard program covers up to five product categories. More categories, and the extra handling, are quoted separately. That limit is about consolidation, not about the processes we can run. A BOM with more than five categories is quoted as its own scope.
Payment follows the same logic. A thirty percent deposit, seventy percent on a passed inspection, and no credit terms. The final seventy percent keeps the buyer's hand on the gate. Inspection is the trigger, not a promise of a shipment date. This is not a financing arrangement; it is a milestone that keeps the final payment tied to the quality gate.
Put the split on one page: on-invoice lines, off-invoice lines, and who carries each. The supplier who can show that page is easier to compare than the one who only quotes a piece price. Two separate suppliers also mean two pickup windows and two invoices to reconcile, even when both quote the same unit cost. Freight and consolidation mechanics have their own write-up.
Put Your Two-Supplier Split on the Table
The most useful thing you can share is the current split: which supplier molds the housing, which one forms the chassis, and which drawing each of them works from. Add the BOM, the key dimensions and your annual volume. If the two drawings disagree, say so. That disagreement is often the first thing a review finds. Send it here (/contact-us). We come back with a cross-process review, one consolidated quote and the quality plan behind it: interfaces, DFM signatures, mold ownership, the quality gate and the consolidation categories. The review comes as a working document, not a sales deck. We work with importers, distributors and brands buying USD 10,000 or more a year. Contracts sit with Dore Technology Co., Ltd.
