Buyers used to place a finished product with one supplier and let that supplier arrange everything. A growing share of programmes now stop at the moulding gate: the buyer takes delivery of parts still on runners, or bagged and kitted, and does final assembly in-house or at a third party. This note looks at why the split happens and what it means for tooling, cost and lead time.
What is driving the split
Three forces push programmes toward semi-finished supply. The first is assembly localisation: brands that assemble close to their end market want moulded parts shipped in bulk rather than finished goods, because the freight is denser and the duty position is simpler. The second is tooling ownership: buying parts rather than products keeps the tool in the moulder's press but under the buyer's name, which protects the design without operating a press. The third is cost transparency: an unassembled part price separates resin, machine time and overhead from assembly labour, which makes year-on-year cost discussions factual rather than negotiable.
What the buyer takes on
The split moves work from the supplier to the buyer. Sorting, de-gating, bagging and kitting either happen at the moulder as a paid service or move downstream entirely. Quality becomes a shared responsibility: the moulder controls part-level quality, and the buyer controls assembly-level quality. Programmes that split moulding and assembly without agreeing the inspection boundary end up with arguments about which side owns a defect that only appears after assembly.
| Model | Supplier scope | Buyer scope | Best for |
|---|---|---|---|
| Parts on runners | Mould, pack by cavity count | De-gate, sort, assemble | High volume, buyer has own assembly |
| Bagged and sorted | Mould, sort, count, bag | Assembly | Mid volume, buyer assembles |
| Kitted modules | Mould, sub-assemble, kit to BOM | Final assembly | Complex BOM, short final line |
| Finished goods | Full build and pack | Brand and distribution | Classic OEM, single market |
Inspection: PPAP and the part-level boundary
When assembly moves away from the moulder, part-level documentation matters more. PPAP-style packages, dimensional reports per cavity, material certificates and a control plan give the buyer what they need to assemble confidently. The moulder's obligation ends at the agreed inspection boundary; the buyer's starts there. Agreeing that boundary in writing before tooling kick-off is the single most useful thing a split programme can do.
Where the split does not work
Not every product suits semi-finished supply. Parts with critical fits that are only achieved in assembly, for example snap fits tuned on the line, usually stay with the assembler. Products where the assembly process itself protects the part, such as ultrasonically welded housings that gain strength from the weld, lose that protection once the parts are shipped loose. And low-volume programmes rarely justify the extra logistics: the freight saving on a few thousand parts does not cover a second inbound stream.
Lead time and inventory effects
Semi-finished supply changes the lead time structure. The moulder runs economic batch quantities instead of order quantities, which shortens the response to a demand spike because parts are often in stock; but the buyer now carries component inventory instead of finished inventory. For seasonal categories the trade is usually worth it, because component stock is fungible across finished variants while finished stock is not.
Cost structure: what the split actually saves
The savings come from three places. Freight, because unassembled parts pack denser than assembled products with void fill. Labour arbitrage disappears, which is often the point: buyers who assemble locally pay their own labour rate and stop exposing themselves to wage inflation in the moulding country. And overhead, because the buyer stops paying a supplier margin on assembly content the buyer no longer buys. Against that sit real costs: a second inbound stream, sorting and kitting either paid for or performed, and a quality function that now has to cover two sites.
For most programmes the break-even sits somewhere between 50,000 and 200,000 pieces per year. Below that range, finished-goods supply is cheaper on total cost; above it, the split usually pays for itself, and the documentation boundary becomes the main risk to manage rather than the price.
Related reading
Our unassembled moulded components programme, PET preform line and custom extrusion profiles cover the semi-finished families, and the semi-finished category lists the full range including kitting services.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
