Design for transport: packing and stacking sheet metal

A coated panel that is flat on the inspection table and bowed at the customer's assembly station has usually not been dropped. It has been packed. The stack was built from habit — foam at 400 mm, a strap at each end, twelve high — and habit is what produces a rework run every few months, a claim on a painted face, or a line stoppage while somebody argues about who owns the damage.

Design for transport treats the journey as a manufacturing operation with its own loads. A rigid truck bed delivers vibration at 3–15 Hz with peaks of 0.5–1.5 g, a container sitting in the sun reaches 60 °C inside, and a stack of panels is held together by strap tension that nobody measured. A part detailed for a static rack and a forklift will not survive that envelope unless the drawing says how it is packed.

Three numbers decide the pack, and all three belong on the part drawing rather than in a conversation at dispatch: the largest rigid dimension, the mass one pack can carry, and the support pitch under the flattest part. This guide works the pitch calculation, names the damage mechanisms that follow a bad pack, and gives the note block that carries the answer to the shop floor. The wider set of design rules sits in the design and CAD hub.

What does the transport envelope decide?

The envelope decides the largest rigid dimension of the part, and it is smaller than most drawings assume. A road truck bed is about 2,350 mm wide internally between the side rails. A 40 ft high-cube container gives 2,340 mm of door width, 2,280 mm of door height and 12.03 m of internal length, with the door frame proud of the walls by about 50 mm.

A part 2,400 mm wide therefore cannot pass a container door upright, however carefully it is packed. A 2,600 mm-long panel that comes off a 4,450 mm press brake in one piece must be carried on an A-frame at about 30° from vertical, and that tilt adds 1,300 mm to the pack height: 2,600 × cos 30° becomes a 2,250 mm vertical footprint plus the rack base. The pack gets taller than the part is wide, and the container height becomes the constraint instead of the width.

The second number is pack mass. ISO 11228-1 evaluates manual lifting against a reference mass of 25 kg close to the body, and that reference drops quickly as the load moves away from the waist or the lift is repeated. A pack of twelve 1.5 × 900 × 1,400 mm CRCA panels weighs 178 kg. Nobody lifts it by hand at either end, so the customer needs a pallet truck at goods-in — or the pack should be four packs of three panels, which costs four times the handling and four times the dunnage.

The third number is the support pitch, and it is the one that gets guessed.

Design for transport: the numbers that set the pack

The values below are the ones that change a drawing. They assume road transport in India and a sea container for the longer legs, CRCA or aluminium sheet from 0.8 mm to 3.0 mm, and a powder-coated finish unless stated.

How a flat stack bows: the dunnage pitch calculation

Take a stack of twelve CRCA panels at 1.5 × 900 × 1,400 mm. One panel is 1.5 × 900 × 1,400 = 1.89 × 10⁶ mm³, which at 7.85 g/cm³ is 14.8 kg. Eleven panels rest on the bottom one: 163 kg, or 1.60 kN, spread along the 1,400 mm length as 1.14 N/mm.

The bottom panel acts as a beam 900 mm wide and 1.5 mm thick. Its second moment is I = 900 × 1.5³ / 12 = 253 mm⁴, and EI = 210,000 × 253 = 5.31 × 10⁷ N·mm². Deflection between dunnage strips at pitch L is 5wL⁴/384EI, which gives 7.2 mm at 400 mm, 2.3 mm at 300 mm, 1.1 mm at 250 mm and 0.45 mm at 200 mm.

At 400 mm the bottom panel bows 7.2 mm, nearly five thicknesses. The load is elastic and the panel springs back, so the damage is not the bow itself. The damage is what happens while it is bowed: the stack loses contact through the middle, the straps hold only at the ends, and every panel above is free to move against its neighbour at whatever frequency the road supplies. At 250 mm the deflection is 1.1 mm, the panels stay in contact through 3 mm foam, and the pack behaves as one block.

The calculation is conservative in one direction and optimistic in another. Friction between panels shares load, so 7.2 mm is an upper bound for a single panel. But it also assumes even strap tension. A 500 N strap pulled hand-tight on one side of the pallet and left slack on the other doubles the local deflection at the slack corner, which is exactly where the paint rub appears.

Fretting, abrasion and edge rust: three mechanisms, one pack

Fretting. Two coated faces in contact with 0.1–0.3 mm of relative movement will wear through a 60–80 µm powder film in a stripe, not over the whole face. The stripe sits where the dunnage is, because that is where the contact pressure is highest. Over 500 km of mixed road the panel sees roughly 10⁵ load cycles at 3–15 Hz. The film fails when the displacement amplitude is large enough to break the polymer surface repeatedly; below about 50 µm the contact shakes down and the wear rate falls away. Tight packing is the fix, not thicker paint.

Abrasion from edges. A laser-cut edge carries a burr of 0.1–0.3 mm depending on the cut quality and the gas. Strapped against a coated face with 160 kg behind it, that burr is a scraping tool, and the damage appears as a straight score along the edge of the panel underneath. Any cut edge that faces a coated face is deburred to 0.1 mm, and the rule applies to the inside of a return flange as much as to an outside profile. The mechanisms behind cut quality are set out in deburring and edge quality after laser cutting.

Edge rust. A powder-coated face at 60–80 µm is a barrier. The cut edge is not coated at all, and a 1.5 mm edge on a 900 mm panel is 1.8 m of unprotected steel per panel. In a container that heats to 55 °C by day and drops to 20 °C at night, the pack crosses the dew point twice a day and water condenses on the coldest surface, which is usually the steel. Rust starts at the edge and creeps under the film. A 30 µm VCI film plus a desiccant sachet per cubic metre holds it back for the length of a normal sea leg; a coat of primer on the edge does the same job for a domestic route.

Bending and handling damage is a separate problem with a separate cause: a long panel handled at its ends sags under its own weight, and a 2,600 mm panel of 1.5 mm at 30 kg sags visibly at the middle. The support rules for that are in bending long panels without distortion.

The gap at the end of the container

A pack that is right in the middle of a container can still fail at the last 300 mm. Cargo that does not fill the length is restrained by the lashing alone, and a 500 kg pallet of panels bearing on two straps will move forward under braking at 0.8 g until it finds the door end. The load on the straps is then the pack mass times the deceleration, not a share of it: 500 kg at 0.8 g is 3.9 kN through two straps, which is why the lashing points are rated to the pack mass and the pack is blocked against a bulkhead or filled to the wall.

The same arithmetic applies inside a truck on an Indian state highway, where a hard brake from 60 km/h is a routine event and a pothole at 40 km/h produces a vertical acceleration of 2–3 g for a few milliseconds. A pack designed only for static stacking is designed for the easy half of the journey.

When the part is the fixture

Returnable racks solve most of this, and they cost money that a one-off order cannot carry. Where the volume justifies a rack, design it with the part: the contact points should sit on flanges or stiffened areas rather than on a flat face, the part should be retained by gravity in a slot rather than by a strap across a coated surface, and the rack should stack with the load carried through its corner posts rather than through the panels. A rack that carries load through the parts is a rack that damages them at the bottom of every stack.

What Ganesh runs for parts that travel

Panels are cut on the 12 kW fibre laser with its 40 ft × 16 ft bed, which means a 4 m part is cut in one piece rather than spliced — and then has to be packed for a 2,340 mm container door, so the drawing either splits it or the pack goes on an A-frame. Edges are deburred before coating, and parts that will be handled repeatedly get a 0.1 mm burr limit called out rather than implied.

Coated parts run through the seven-tank pretreatment and the 18 ft × 14 ft powder booth, with DFT and cross-hatch adhesion checked on the line. Packing is agreed at quotation and recorded against the part number: rack or pallet, panels per pack, dunnage pitch, interleave, strap positions and the ISO 780 symbols on the label. Customers including Mahindra, Godrej & Boyce and TKE receive parts packed to a written specification, and the first pack for a new part is photographed and kept with the job card so a repeat order is packed the same way.

That recorded pack is also the argument used when a claim arrives. If the pack was built to the agreed specification and the damage is in the middle of a face rather than at a contact line, the cause is handling after dispatch, and the photograph settles it faster than an inspection visit.

What to put on the drawing

• The maximum part envelope in three dimensions, and a note if it must be split because of it.
• The finish system, the film thickness and the faces that must stay uncoated, including threads and earth studs.
• A burr limit per edge class: 0.1 mm on handled and coated-contact edges, 0.3 mm elsewhere, with the edge class named on the view.
• The packing specification by number and revision, not by description in prose.
• Panels per pack, dunnage pitch and interleave material, referenced to that specification.
• Strap positions and strap type, with corner protection at every line.
• The rack or pallet interface, including the fork entry height and the anchor points if it is a returnable.
• The handling symbols required by ISO 780, and where they go on the pack.

The DXF and RFQ checklist covers the drawing fields that a fabricator needs before a pack can be quoted, and the finished coated assemblies capability describes how the coated and packed part is handed over. A part that has been through this list arrives as designed; the alternative is a conversation about who pays for a repaint.

When the pack cannot protect the part

Three cases. A panel thinner than 1.0 mm and longer than 2,000 mm has so little section that no practical dunnage pitch holds it flat; it travels on edge in a slotted rack or in a purpose-built returnable, and the rack becomes part of the tooling cost. A mirror or brushed finish on stainless or aluminium shows every contact mark, so the pack needs a full-face interleave and a no-touch handling rule, which usually means one part per sleeve. And a part with a machined face or a welded boss has a datum that cannot tolerate a strap line, so the pack has to carry the part on a fixture rather than on its own faces.

For everything else, the pitch calculation above is the whole design for transport argument in one line: 250 mm keeps a twelve-panel stack in contact, 400 mm does not, and the difference shows up as a stripe of bare metal on a panel that was never dropped.