Mass, finishing area and bought-in parts

Once the blank is fixed, three quantities follow from it that the run’s plan depends on: the mass of each tray, which drives handling, packing and freight; the area to be coated, which drives the largest bought-in cost; and the bought-in parts, which must be ordered in time and in the right quantity. Each is a simple calculation, and each is easy to get wrong by a factor that matters: one face instead of two, a net count with no allowance for losses, a pallet plan that ignores how the product actually stacks.

This chapter works those quantities out for the running case’s 500 trays: mass and shipping mass, powder coat area and cost, racking for the coater, rivets with an allowance, and the pallets the run will leave on. It ends with a coating estimate made on one face of the blank.

Mass

From Chapter 3, the tray’s blank is about 0.579 m² of 1.2 mm steel, so each tray weighs 0.579 × 0.0012 × 7,850 = 5.45 kg before coating. The eight rivets add a few grams. The powder coat adds a little more: this is an illustration, and at an average film of about 80 µm on each face and a cured powder density of about 1.5 g/cm³, the coat on 1.158 m² of surface weighs about 0.14 kg.

So a finished tray weighs about 5.59 kg, and the run of 500 about 2,796 kg, before pallets and packing. That is the number the freight company needs, and it is also a check: the shop’s estimate of steel used, 2,727 kg in the blanks, plus coating, should agree with the shipping mass, and it does.

A full tray, at 25 kg of contents plus its own 5.6 kg, weighs about 30.6 kg. That is the load the distributor’s staff will lift by the two handle slots, and it is worth putting on the tray’s label beside the capacity, as Chapter 2 suggested, because a person lifting the tray lifts both.

The area to be coated

Powder coating covers every exposed surface: inside and outside, both faces of every wall, the base, the rim and the hem. The area to be coated is therefore about twice the blank’s area, 2 × 0.579 = 1.158 m² per tray, plus the cut edges, which are small enough to ignore in an estimate.

This is an illustration. The shop’s powder coater charges A$18.00 per square metre of coated area, excluding GST, for a standard colour in batches of this size. The tray’s coating therefore costs 1.158 × 18.00 = A$20.84 excluding GST, and the run’s 500 trays A$10,420.20 excluding GST, or A$11,462.22 including GST. That is the second-largest cost per tray after the steel.

Figure 1What a finished tray weighs

1. Base and walls, 1.2 mm steel5.45 kg · 97%
2. Powder coat, both faces0.14 kg · 3%
3. Rivets, eight0.01 kg · 0%

Racking for the coater

A powder coater hangs each part on a hook or rack as it passes through pre-treatment, the spray booth and the curing oven, and how a part hangs decides how many fit on a line and whether every surface is coated evenly. The running case’s tray has a natural hanging point: either handle slot. Hung from one slot, the tray hangs with its open top to one side, so pre-treatment liquids drain out and powder reaches inside.

Hanging from a slot also leaves a small contact mark, where the hook touches, inside the slot’s top edge. That is the place the specification can accept a mark, because hands will cover it. A tray hung from its rim instead would carry a mark on the rim’s visible top. The shop tells the coater which slot to use and confirms it on the first batch.

The coater’s price may depend on how many trays fit on a rack or a line length, not only on area, and a flared tray hung by one slot takes more space than a flat panel of the same area. The A$18.00 per square metre here is an illustration; a coater quoting by the piece or by the rack load would give a different number for the same tray, and the shop asks for a quote on the actual part, with the hanging method agreed, before the run is costed.

What the coater needs from the shop

Powder coat adheres only to clean steel. Cold-rolled sheet arrives with a light oil film, laser-cut edges carry a thin oxide, and folding and riveting add fingerprints and lubricant. The coater’s pre-treatment line removes most of it, but the shop’s part in a good finish is to deliver trays that the pre-treatment can clean: no marker pen on visible surfaces, no grease from the press brake, no tape residue, and laser edges deburred so that the coat covers them rather than thinning to nothing over a sharp lip.

That is why the etched marks from Chapter 3, the fill line and the lug positions, are etched and not drawn. An etch survives pre-treatment and shows through the coat; a marker line either washes off, leaving nothing, or does not, leaving a stain under the coat.

The coat’s thickness also enters the tray’s fit in three small places. It narrows the handle slots by twice the film, a fraction of a millimetre, which does not matter. It coats the rivet heads, which were set before coating, sealing the corners. And it adds to the walls’ thickness in the nested stack, which at the 3.78 mm clearance of Chapter 4 makes no difference. The shop confirms all three on the first coated batch.

Weighing as a check

Mass is not only a freight number; it is a quick test of the product. A finished tray that weighs noticeably less than 5.59 kg is missing something or made from thinner sheet; one that weighs noticeably more has extra material or a heavy coat. This is an illustration. The shop weighs the first ten finished trays and every fiftieth after that on a bench scale, and treats a tray more than about 2% from the expected mass as a reason to look.

The check is cheap and catches errors that dimensional checks can miss. A batch cut from 1.0 mm sheet by mistake, for instance, would look identical and measure almost identically, but each tray would weigh about 4.69 kg finished, nearly a kilogram light, and the scale would show it at once.

Bought-in parts

The tray’s only bought-in parts are its blind rivets: eight per tray, 4,000 for the run. A run never uses exactly the net number. Rivets are dropped, misfed or set crooked and drilled out, and the first articles and setup trays use some. This is an illustration. The shop’s allowance for blind rivets on a run like this is 2%, so it orders 4,080 rivets, about A$204.00 excluding GST at A$0.05 each.

The allowance is small in money and large in consequence. A run that runs out of rivets with 20 trays to go stops until the next delivery; an extra 80 rivets cost A$4.00. The same reasoning applies to every consumable on the run: the shop’s purchase list gives the net quantity, the allowance and the reason, so that the buyer does not trim the allowance to save a few dollars.

Figure 2The run's bought-in items

Item	Net quantity	Allowance	Order	Cost, excluding GST
Blind rivets, 4 mm	4,000	2%	4,080	A$204.00
Powder coating	579 m²	none, charged on actual parts	500 trays	A$10,420.20
Labels, capacity and load	500	2%	510	as quoted
Pallets and stretch wrap	7 pallets	| 7	as quoted

Lead times work backwards

Every bought-in item has a lead time, and the run’s schedule is built backwards from the delivery date through them. This is an illustration. The distributor wants the trays in five weeks. The coater needs the trays for one week, and its special colour must be ordered two weeks before coating. The shop’s own work, cutting, pressing, folding and riveting 500 trays, takes about two weeks of the shop’s available capacity once Chapter 7’s routing is timed. The steel arrives three working days after ordering; the rivets, a week.

Working backwards from week five: the trays go to the coater at the start of week four, so the colour is ordered in week two at the latest, and the shop’s work runs through weeks two and three, so the steel must be on the floor by the start of week two and ordered in week one, with the rivets ordered at the same time. There is no slack in that chain. A first article that fails in week two, or a colour ordered a week late, moves the delivery.

The shop therefore orders the steel, rivets and colour on the day the order is confirmed, makes the first article from the first sheets, and sends the coater a single first-article tray for a trial coat while the rest of the run is being made, so that any finishing problem is found in week two, not week four.

Packing and pallets

The trays leave nested in stacks of twenty, each stack 680 mm high, as Chapter 4 set out. Five hundred trays make 25 stacks. This is an illustration. On a 1,200 × 800 mm pallet, the 600 × 400 mm stacks sit four to a layer in one layer, so the run needs 25 ÷ 4 = 6.25, so 7 pallets, the last carrying a single stack.

Each full pallet carries 80 trays, about 447 kg plus the pallet itself, and stands about 680 mm plus the pallet’s height. The shop could stack two layers to use fewer pallets, but a second layer on nested trays rests its weight on the bottom stack’s rims and lugs. The specification does not cover that load, and the shop chooses one layer rather than test it.

The pallet plan feeds back into the run’s schedule: coated trays are packed as they come back from the coater, in stacks of twenty, so the coater’s batches are best sent in multiples of twenty too.

Freight

Seven pallets, the last nearly empty, carry about 2.8 tonnes of trays plus the pallets’ own mass. This is an illustration. That fits comfortably on one small rigid truck by mass, and its floor space, seven pallets of 1,200 × 800 mm, decides the vehicle more than its weight does. Nested trays are light for their volume: a pallet of 80 trays weighs under half a tonne but fills a pallet space to 680 mm high.

The freight booking needs the mass, the number and size of pallets, their height and whether they can be double-stacked, which they cannot under this plan. The shop’s dispatch note also records the trays’ count by stack, so that the distributor can check the delivery against the order without unstacking.

The error: coating estimated on one face

This is an illustration. The shop’s first cost estimate for the trays took the coated area as the blank’s area, 0.579 m², and costed the coating at 0.579 × 18.00 = A$10.42 per tray. The estimate went into the shop’s first price to the distributor.

When the coater’s quotation for the actual trays arrived, it was for about A$20.84 per tray, twice the estimate. The coater was not overcharging: the estimate had counted one face of the sheet, and a tray is coated inside and out. Across 500 trays, the gap was about A$5,210 excluding GST.

A one-face estimate is an easy slip because the blank’s area is the number already on the estimating sheet, calculated for the steel, and it is natural to reuse it for anything charged by area. The steel is bought by the face; the coating is applied to both.

The shop found it before the order was confirmed and revised its price. Its estimating sheet now has a line for the coated area that reads “blank area × 2 for both faces, plus edges if significant”, and the coater’s quote on the actual part is required before any price is given, not after.

Figure 3Mass, finish and bought-in checks

• Do: Tray mass from the blank's area and thickness5.45 kg steel, 5.59 kg finished
• Do: Coated area counted on both faces1.158 m² per tray
• Do: Coater's quote on the actual part with the hanging method agreedhung by a handle slot
• Do: Bought-in items ordered with allowancesrivets 4,080 for 4,000
• Do: Pallet plan matched to the nested stacks25 stacks, 7 pallets
• Avoid: Coated area taken as the blank's areacoating estimated at half its cost

Exercise: the half-size tray

The half-size tray’s blank is about 0.370 m² of the same 1.2 mm steel, with eight rivets. Find its mass, its coated area and coating cost at A$18.00 per square metre excluding GST, and the rivets to order for a run of 300 with a 2% allowance.

Worked answer.

• Mass: 0.370 × 0.0012 × 7,850 = about 3.48 kg of steel.
• Coated area: 2 × 0.370 = about 0.739 m², so coating costs 0.739 × 18.00 = A$13.31 per tray excluding GST, or A$14.64 including GST.
• Rivets: 300 × 8 = 2,400, plus 2% = 2,448.

The half-size tray has 64% of the full tray’s area and costs 64% as much to coat, but it holds only half as much, so its coating cost per litre of capacity is higher. Small products carry proportionally more surface, which matters whenever the finish is charged by area.