At what volume does a Case Assembly Workstation justify a conveyor?

Answer: 30,000 cases. Below that, depreciation never reaches unit cost; between 30,000 and 80,000 variant count decides; above 80,000 the line has little competition.

How far apart are cells and lines on changeover?

Answer: 12 minutes against 90, a factor of seven, mostly from fixtures and recipes. Quick-change plates reach 25 minutes; below that, carts must arrive pre-kitted.

Can a 30 kg case still run on a line?

Answer: It can, but it should not. Roller capacity is fine; turning needs assistance and pitch grows to 1.5 m. Most projects switch at 40 kg.

Is buying finished cases cheaper than building a station?

Answer: Under 1,500 units, usually. The 46 yuan price includes tooling amortization, while half the 58 yuan in-house figure is fixed cost. The balance flips near 20,000 units.

One Changeover Night: When a Case Assembly Workstation Loses

Quick answer: A Case Assembly Workstation pays back above roughly 30,000 units a year with under ten variants. Below 1,500 units, or above 40 variants, buying finished cases or running U-cells costs less.

Three numbers to compare first
High volume: the line wins on buffer
High mix: cells win on changeover
Heavy cases: benches and cranes
Three cases where you should not build one
FAQ

1. Three numbers to compare first

On a November night shift in Ningbo, a workshop swapped a straight line for a U-cell. Removing 18 metres of conveyor took six hours; the shift fell from seven operators to four, and output rose from 860 cases to 940. The hardware was fine, but the mix had gone from six variants to forty-two in two years.

Three numbers settle the comparison: seconds per case, minutes per changeover, full cost per unit. The first caps capacity, the second caps variant range, the third decides payback. Quotations tend to price labour and bench tops only.

Layout	Cycle	Changeover	Footprint	Fits
Straight line	45 s	90 min	18 m, 6 m² per station	Above 30,000
U-cell	78 s	12 min	32 m² per cell	3,000 to 20,000
Fixed bench	210 s	5 min	9 m² per bench	Below 2,000
Bought cases	n/a	n/a	40 m² of racking	Below 1,500

The last column is a field boundary. A 260,000 yuan line depreciated over five years loads 26 yuan on each case at 2,000 a year, more than the moulding; at 30,000 it loads 1.7 yuan.

2. High volume: the line wins on buffer

Above 50,000 cases a year with eight variants or fewer, a straight line earns its keep through buffer, not pace: six to eight pieces between stations let an upstream cell stop for 40 minutes while packing ships.

Case Assembly Workstation: black base and milky lid side by side, white hinge pins in a row, black latch at each lid corner

At 1,200 mm pitch and 0.12 m/s belt speed, nobody waits for transport. Six people over eight hours give 640 cases in theory; at 97.5% first-pass yield the count lands near 620.

Rigidity is the price. A new variant means a 90-minute stoppage, and twelve changeovers a month burn 18 hours, or 1.5 shifts.

Floor-side stock is the quiet cost: a two-hour buffer ties up 46,000 yuan, while a 20-minute cell needs a fraction.

3. High mix: cells win on changeover

With 42 variants at 200 pieces a batch, a straight line cannot compete, and changeover is why. A U-cell reaches 12 minutes on quick-change fixtures, parameter recipes and pre-kitted carts.

Bench height sits at 900 mm, material on three sides, one-hand reach under 4.5 m, which is 50th-percentile male elbow height minus 100 mm.

Volume	Variants	Layout	Cost per case	What it costs
80,000	6	Straight line	8.4 yuan	90-minute changeovers
12,000	42	U-cell	11.7 yuan	78 s cycle
3,000	15	U-cell	16.3 yuan	55% utilization
800	30	Bought cases	46 yuan	35-day lead time

At 800 a year, in-house costs 58 yuan against 46 bought, and the 12 yuan gap is tooling amortization.

The cell has a ceiling: at 78 s per case two shifts top out near 700 pieces, and each extra cell adds 32 m² and four people.

4. Heavy cases: benches and cranes

At 32 kg empty and 58 kg loaded, a conveyor stops being sensible. Turning 58 kg by hand puts over 30 N·m through the wrist, 200 times a day.

Case Assembly Workstation: black base left, frosted lid swung open right, moulded ribs and stepped rim inside, six white hinge pins, two black latches

A fixed bench hands turning to a 500 kg single-girder hoist on a 1.5 m span, corners locked at 3.2 N·m, two turns replacing four. Bench height drops to 750 mm.

Cycle time is 210 s, slow until rework is counted: one knock deforms a latch by 0.8 mm and the seal is gone, which at 2% rework costs 19,000 yuan a year.

Forty kilograms is the second boundary, past which most safety regimes demand mechanical assistance.

5. Three cases where you should not build one

First, demand under 1,500 cases a year. Tooling and fixtures never amortize: 58 yuan in-house against 46 bought, a gap worth three years of freight.

Second, more than 30 changeovers a month. Fixtures still need five minutes, so 30 changeovers eat 2.5 hours before first-article inspection.

Third, cases longer than 1.6 m. Conveyor radii and aisle widths both fail, the station collapses into a single point, and cycle time falls past 300 s.

Situation	What it looks like	Hidden cost	Better fit
800 cases, 30 variants	In-house feels controllable	12 yuan tooling per case	Buy, MOQ 500
30 changeovers a month	Changeover is five minutes	2% yield loss on first articles	Subcontract line
Cases over 1.6 m	Just widen the bench	1.2 m more aisle, 180,000 yuan	Bench plus hoist

None of this argues against the layout, only against pushing it past stable variants, real volume and 1.6 m.

Conclusion

Twelve minutes against 90 minutes of changeover outweighs 45 s against 38 s of cycle. Match layout to order book: cells for high mix, a buffered line for stable volume, a bench and hoist for heavy cases. Settle volume, variants and unit cost before signing for automation, because that order decides payback more than the hardware, and it is why a Case Assembly Workstation deserves a second calculation.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.