Does the cavity wall collapse after the grid is torn out?

Answer: Depth and density decide it. A 70% to 80% cut in 30 kg/m³ foam tears flat and stands up; a 90% cut leaves a wall one skin thick that folds under a 2 kg part.

When should I choose pluck foam over a CNC insert?

Answer: Look at volume and change frequency. A fixed outline above a few hundred units is cheaper as a CNC cavity; below 50 units, tearing wins because no new die is needed.

How much weight can one cavity carry?

Answer: About 5 kg. Above that, bond a 3 mm PVC plate to the cavity floor so the load spreads into the shell, or the foam creeps within six months.

Can a 3 kPa air test replace an IP67 certificate?

Answer: No. The air test is in-process control. IP67 needs a 1 m immersion for 30 minutes under IEC 60529, reported by a third party.

Hard plastic equipment case with pick and pluck foam

Quick answer: A hard plastic equipment case with pick and pluck foam is decided where shell and liner meet. The body gives a 3.5 mm wall and a 12 mm sealing land; the liner closes six faces with 30 kg/m³ grid foam. A 15 mm grid cut to 70% of board thickness tears flat, and the air test passes at 3 kPa with a drop under 0.3 kPa in 60 seconds.

Shell Structure and the 3.5 mm Wall
Grid Pitch and Pre-Cut Depth
Three Foam Layers
Gasket Compression and the Pressure Valve
Pass Lines for Air, Drop and Stack Tests
FAQ

1. Shell Structure and the 3.5 mm Wall

The shell sets the floor: put 30 kg/m³ foam behind a 2.5 mm wall and a 1.2 m drop bulges the side before the foam travels 20% of its stroke.

Wall thickness comes from outside length. Under 400 mm, mould 2.8 to 3.2 mm. From 400 to 600 mm, use 3.5 mm. Above 600 mm, use 4 mm plus an 8 mm rib along the long side, spaced no wider than 80 mm. Empty weights land near 1.2, 2.0 and 3.4 kg.

Ribs fail at the root: twice the wall thickness, slow to cool, and a sink mark lands on the sealing land where the gasket cannot seat. Hold rib thickness at 0.6 of the wall and the mark stays under 0.05 mm.

The sealing land is the flat ring where lid and body meet: 10 to 14 mm wide, flat within 0.2 mm. Narrower and the gasket rides onto the corner radius; wider and closing force spreads thin. Two latches carry a 400 mm case, four carry 600 mm, each on 3 mm of solid plastic.

Outside length	Wall	Rib height	Empty weight	Inner depth
Under 400 mm	2.8–3.2 mm	6 mm	1.2 kg	60–90 mm
400–600 mm	3.5 mm	8 mm	2.0 kg	90–140 mm
Over 600 mm	4.0 mm	8 mm	3.4 kg	140–200 mm

2. Grid Pitch and Pre-Cut Depth

Pick and pluck foam is properly pre-cut grid foam: a die cuts a square array and leaves the bottom 20% to 30% intact. Bend along a seam and the bridge tears away flat.

Hard plastic equipment case with pick and pluck foam open, black shell with two latches and a handle, grid foam cut into seven cavities

Pre-cut depth has no room to negotiate. At 60% the blocks skew instead of separating; at 90% the remaining skin splits under road vibration and cavities collapse before shipping. Thick boards take the low end: 38 mm on a 50 mm board, 23 mm on a 30 mm board.

Pitch follows the outline: a 10 mm pitch suits parts under 50 mm but leaves a wall one cell wide that frays at the corners. A 15 mm pitch covers instruments from 50 to 120 mm; above 150 mm, step to 20 mm and keep two cells unbroken at each end.

Interference sits at 1 to 2 mm: one lets gravity seat the part, two buy zero movement at the cost of hand pressure. Die tolerance is ±0.3 mm, and torn walls stay vertical above 30 kg/m³.

Pitch	Board	Pre-cut	Part outline	Torn wall
10 mm	20 mm	16 mm	Under 50 mm	One cell, frayed
15 mm	30 mm	23 mm	50–120 mm	Two cells left
20 mm	40 mm	30 mm	120–200 mm	Two cells required
25 mm	50 mm	38 mm	Over 200 mm	Pair with 2 mm base

3. Three Foam Layers

Liner thickness is subtraction: at a 120 mm inner depth with 15 mm lid foam, 10 mm base foam and a 2 mm allowance, the pluck layer is 93 mm, rounded to 90 mm so the lid preloads by 3 mm.

Lid foam: 10 to 15 mm, flat for machined faces, convoluted for curved loads, compressed 15% to 25%.
Pluck layer: 30 to 90 mm for lateral location; unbroken end cells act as the shear zone.
Base foam: 8 to 10 mm for the landing, 5 kg/m³ lighter than the pluck layer.

The lid compresses 3 to 5 mm, the pluck walls cut movement to 0.5 mm, and the base foam drops peak acceleration from 40 g to below 25 g.

4. Gasket Compression and the Pressure Valve

Sealing comes from compression, not latch force. An O-ring forms a continuous band only when flattened 20% to 30%; under 20% the band breaks and 3 kPa escapes, over 35% the rubber takes a permanent set.

Hard plastic equipment case with pick and pluck foam workshop, green mold frame beside an injection press, kraft material bags on a pallet, yellow aisle

Size the groove from the cord: width 1.3 times, depth 0.75 times, fill 75% to 85%. A 4 mm cord needs a 5.2 mm wide, 3.0 mm deep groove for 25% compression. Past 90% fill, closing force climbs from 60 N to 110 N.

Compound follows temperature: EPDM covers -40 to 120 °C and resists UV, so it suits outdoor storage; silicone covers -60 to 200 °C with better rebound for cold stores and air freight.

A pressure valve solves differential pressure, not water: it opens at 3 to 5 kPa with about 0.5 L/min of flow for aircraft holds and high-altitude trucking. A diaphragm inside stays closed to water; replace the valve when it ages.

5. Pass Lines for Air, Drop and Stack Tests

Run the air test before the drop test, or micro-cracks in the wall will fail it and hide whether the fault came from design or the fall.

Test	Condition	Pass line
Air	3 kPa for 60 s	Drop under 0.3 kPa
Immersion	1 m for 30 min	No visible water inside
Drop	1.2 m, six faces, -20 °C for 4 h	No cracks, latches closed
Stack	500 kg for 24 h at 40 °C	Under 3 mm, 80% recovery
Salt spray	48 h neutral	No red rust

Test the first article on every item, then sample air at AQL 2.5, pulling three units per batch for drop. Tested units never return to the packing line; a drop leaves invisible craze that understates the next result.

Conclusion

Choose in four steps: fix pitch and interference from outline and weight, subtract layers from inner depth, match the gasket to service temperature, then write air, drop and stack lines into the drawing. Skip a step and the cost returns as rework.

This article covers container-side specifications and design implications only.