How many film-build points does an incoming lot need?

A: 5 cases, 5 points each, 3 reads per point, 75 readings total. One point under 60 μm returns the lot; more than 3 points in the 60–65 μm band means concession plus doubled sampling.

Can a 5–7 day cycle be compressed to 3 days?

A: Yes, split as 2 days pretreatment and coating, 1 day print, with a dedicated van between. Freight rises about CNY 320 per trip and defects run 1.8 points higher; reserve it for spares.

Is ΔE ≤1.5 too strict for small orders?

A: Small orders are where standards slip: only one plaque gets made. Keep 3 from the same batch and hold the first lot to 1.2.

Can we use a vendor with no pretreatment line?

A: Not for visible surfaces. Adhesion swings between grade 3 and grade 4, and blistering after 300 h of salt spray becomes likely, so restrict such a shop to hidden faces.

What is the minimum ledger for outsourcing control?

A: 7 columns. Under 5 you cannot produce rework data at price review; past 9 the shop stops filling it in.

Outsourcing Control: Outsourcing Spray and Print

Quick answer: Outsourcing control turns on volume and colour count, not unit price: under 3,000 pcs the coating goes out, over 8,000 it comes back in-house.

Where Outsourcing Control Begins: 3,000 and 8,000 Units
Incoming Inspection: Four Numbers That Gate Every Lot
Three Checkpoints on the Vendor Floor
Who Pays When a Batch Comes Back
The Ledger, WIP Turns and Two Gate Criteria
FAQ

1. Where Outsourcing Control Begins: 3,000 and 8,000 Units

A 620 × 420 × 220 mm case at 2,600 pcs a month looks like a simple make-or-buy sum: the coater quotes CNY 8.6, in-house variable cost is CNY 6.1, a book saving of CNY 6,500 a month. The sum ignores two costs. A colour change means stripping the jig, 40 min, plus 35 min for the oven to climb from 160 ℃ to 185 ℃, so six colours a month is 7.5 h of dead time. At a jig pitch of 150 mm, a larger case drops from 12 pieces to 8 and throughput falls a third.

So the first test is volume against colour count, never unit price. Below 3,000 pcs a month, one line cannot absorb its fixed cost and the work goes out. Above 8,000 pcs, two-way freight over a 40 km radius eats the gap and it comes back. In between, read the line: under 65% load keep it in-house, over 85% send it out.

Left to the shop floor, the switch condition drifts with whoever is on shift.

Process	Monthly volume	Make or buy test	Switch trigger
Spray coating	Under 3,000 pcs	Outsource, no in-house line	2 months above 8,000 pcs
Screen print	3,000–8,000 pcs	Outsource above 85% press load	More than 40 screens a month
Laser marking	Under 200 pcs	Outsource for sampling	Single run above 500 pcs
Hardware assembly	Above 8,000 pcs	In-house, same takt as final line	Vendor distance beyond 80 km

Laser marking is the row people skip: above 500 pcs in a run, set-up pushes the price CNY 1.2 past in-house.

2. Incoming Inspection: Four Numbers That Gate Every Lot

Control sits at your goods-in door, not the vendor's plant. Appearance parts argue over colour and feel, so the door checks four measurable items and leaves the rest to first-piece and patrol checks.

Outsourcing control in practice: six olive-grey cases stood upright in two rows on cardboard over a green workshop floor, each with a white label

Film build is judged at 60–90 μm. Pull 5 cases per lot, take 5 points on each — four corners plus centre — and average 3 reads per point, 75 readings in total. Any point below 60 μm returns the lot. Corners and boss roots run thinnest: a 15° change in gun angle costs 10 μm.

Adhesion uses a cross-cut, 1 mm spacing, 6 × 6 squares, graded by area lifted; grade 3, meaning 5%–15%, is the floor. Colour is read against 3 plaques under D65, ΔE ≤1.5 accepted. Print abrasion runs 500 g over 200 strokes.

Control item	Limit	Sampling	Action on failure
Film build	60–90 μm	5 cases × 5 points × 3 reads	Any point under 60 μm, lot returned
Cross-cut adhesion	Grade 3 or better	2 cases per lot	Grade 4 returned, grade 3 plus 5 cases
Colour difference	ΔE ≤1.5	3 cases, D65 light	1.5–2.5 concession, stored apart
Print abrasion	500 g / 200 strokes	3 cases per lot	Missing stroke, part rejected

Colour is the item that gets waved through: one batch reads as one shade under shop lighting and splits into three near a window, usually because cure ran 8 ℃ low or film ran 12 μm thin. Judge it in a light booth and renew the plaques every 12 months.

3. Three Checkpoints on the Vendor Floor

The vendor side needs 3 actions, no more.

First-piece approval: 2 pieces before each run, with film and colour records attached, signed before volume starts. An unsigned first piece turns every later patrol check into a rescue.

Patrol every 2 h: read film build at the top, middle and bottom of the jig. When cure is pushed from 180 ℃ / 20 min, film barely moves while adhesion drops a grade, so watch the oven profile as well as the gauge.

Tail-piece retention: 3 pieces per lot, held 6 months. Arguments surface two months later; without a retained piece you argue from photographs.

4. Who Pays When a Batch Comes Back

Settle liability before the purchase order, not after the reject.

Outsourcing control at the bench: an open black case beside black-and-yellow parts trays, a worker leaning over it, wrapped red cases stacked behind

Outsourcing control here runs on 3 tiers. Vendor liability covers thin film, ΔE above 2.5 and grade 4 adhesion: rework and both freight legs, closed in 5 working days. Shared liability covers a drawing that said dark grey instead of a colour reference: split 5:5, agreed in 7 working days. Buyer liability covers claims raised after 15 days and handling damage: internal work order, confirmed in 3 working days.

Shared liability is the tier that burns time. If colour reference, film window and print tolerance are not all on the drawing, the cost lands on purchasing.

5. The Ledger, WIP Turns and Two Gate Criteria

Log 7 columns: lot number, process, vendor, receipt date, inspection result, rework quantity, closure date. Only rework quantity gets read twice; it is the hard number at price review.

Work in process turns twice a week; the outside cycle runs 5–7 days including 2 days of freight, on a 40 km radius and 2 runs a week. Past 80 km, move to 3 runs, or 3 days of WIP sit at each end.

Two gate criteria: an independent pretreatment line, shot blast or phosphating, and 15,000 pcs a month of coating capacity. A shop with booths but no pretreatment hits 90 μm film and sits at grade 4 adhesion, with a rework rate 2.4 times our own.

Conclusion

Outsourcing control is less a process problem than a matter of whether the tests are written down: volume, colour count and line load decide make or buy; film, adhesion, colour and abrasion decide accept or reject; first piece, patrol and tail piece decide whether the process holds. Put those ten items on one page and most disputes disappear.

This article covers case-side specifications only and does not address any handling, loading or use of the contents.