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A cast iron housing does not damage a tray by impact. It sits there, day after day, on four small contact points, with edges that were never deburred to packaging standards. Designing for that is a load problem rather than a packaging problem, and it is solved in the geometry before the sheet is chosen.
The first calculation is contact pressure, not total weight. A twelve-kilogram casting resting on four pads of forty by forty millimetres loads each pad far harder than the same mass spread over a full base. We take the part's own datum faces and calculate the pressure at each support, then size the material and rib spacing from that number.
Gauge follows the result. Trays for iron castings typically run from 3 mm to 6 mm in PP or HDPE, with heavy variants using a composite build that bonds a reinforcing layer into the base. Thin-gauge blister material that works for a plastic trim clip will deform permanently under the same edge load.
Base structure carries what gauge alone cannot. Formed ribs run between the support pads rather than around them, bosses sit under each contact point to spread the load into the panel, and where a span is long we add a second formed layer that acts as a box section. The tray then behaves like a shallow structural panel rather than a folded sheet.
Deflection is the figure we control and report. A tray that sags two millimetres in the middle is still a tray; a tray that sags ten millimetres no longer stacks squarely, and the stack above it starts to lean. We hold mid-span deflection to a stated limit under full rated load, measured on a sample from the production tool.
Protecting the tray from the part matters as much as the reverse. Sharp casting edges are given thicker contact sections, and where a part has a consistently abrasive corner we fit a replaceable wear pad instead of thickening the whole tray. That keeps replacement cost at the pad rather than the tray.
Thermoforming earns its place here for a practical reason: tooling is affordable at low and medium volumes, and a geometry change after a casting revision means an edited tool rather than a new mould. For a plant handling a family of castings in modest quantities, that flexibility is worth more than the material efficiency of injection moulding.
Stacking is designed as a separate system. Corner posts transfer vertical load from tray rim to tray rim, so the weight of a stack never passes through the parts, and the posts are tall enough to keep the casting clear of the tray above.
Foundry dust and swarf need somewhere to go. Drain and clearance holes let shot, sand and coolant fall through during wash-down, and the cavities are open enough to be inspected without removing the part.
Send us the part weight, its contact faces and the stack height you plan, and we will return a tray design with gauge, rib layout and a deflection figure at rated load.
