How is Custom Foam Insert thickness decided?

A: Work back from the interior: subtract base and lid pad to get pocket depth, then add compression to reach part thickness.

What tolerance goes on a Custom Foam Insert drawing?

A: The accumulated value, usually plus or minus 2.8 mm; pockets take minus, the outer profile plus.

Can a Custom Foam Insert be quoted from outside dimensions?

A: No. Pocket depth and compression would be guesses and the insert comes back loose.

How many Custom Foam Insert samples are needed?

A: One where all dimensions are given, two where pockets are stepped: the first for outline, the second for compression. ---

Custom Foam Insert: Thickness, Compression and Tolerance

Q: What compression ratio should a Custom Foam Insert use?

A: Under 2 kg takes 5 to 8 percent, 2 to 10 kg takes 8 to 12, above 10 kg takes 12 to 15.

Quick answer: Four figures on a Custom Foam Insert sheet decide whether it assembles: base thickness, pocket depth, compression ratio and tolerance. The first two set how deep a part sits, the third takes up movement, the fourth makes unit twenty match unit one.

Four figures that matter
Thickness in three segments
Compression ratio
Tolerance accumulation
Measuring before ordering
FAQ

1. Four figures that matter

A sheet usually lists material, colour, density, thickness and tolerance, which looks complete. Only four decide whether the insert fits: base thickness, pocket depth, compression and tolerance. Start with material and the usual ending is a pocket too shallow to use.

Line	Typical value	What it governs
Base thickness	15 mm	travel on a corner drop
Pocket depth	55 mm	how deep the part sits
Compression	8–12%	movement and lid force
Tolerance	±0.5 mm	batch interchangeability

Base thickness is the distance from pocket floor to case floor; pocket depth against part thickness sets compression. Read all four together: base plus pocket plus lid pad equals interior height, and the millimetres left over are the compression.

Custom Foam Insert: an open case holding a meter in a cut pocket, three long pads in front, a steel rule and knife nearby

2. Thickness in three segments

Interior height covers base, pocket and lid pad. Base takes 10 to 15 mm, pocket depth is part thickness minus compression, and the pad takes the rest. Too much and the lid will not close; too little and the part rattles.

At 120 mm interior and a 60 mm part: base 15, pocket 55, pad 50. The part is held under 5 mm, about 8 percent compression. Thicker is not better, since 5 mm onto the base takes 5 mm off the pocket and eats the grip; heavy parts ride on the base, light ones on the pad.

Custom Foam Insert: an aluminium case open on a bench, convoluted foam in the lid, a full pad in the base, two slotted pads nearby

3. Compression ratio

Compression is part thickness minus pocket depth, divided by part thickness, and usually runs 5 to 15 percent. Below 5 the part moves and hits the wall in a drop; above 15 the lid needs force and the latches stay loaded.

By weight: under 2 kg takes 5 to 8 percent, 2 to 10 kg takes 8 to 12, above 10 kg takes 12 to 15. Frequent handling takes the low end, a single shipment the high end.

Compression and hardness are a pair. Soft foam compresses but recovers slowly, hard foam refuses and pushes back, so a sheet quoting compression without hardness has quoted half a specification. Check it by closing the case for 24 hours and looking for an even ring on the pocket wall.

Custom Foam Insert: a closed black case seen from the rear corner, ribs across lid and sides, two latches and a valve at the end

4. Tolerance accumulation

Tolerance is not cutting alone. Three sources add up: interior moulding at plus or minus 1.5 mm, pad cutting at 0.5, lamination at 0.8, reaching 2.8 mm when they run the same way.

Step	Single tolerance	Note
Case interior	±1.5 mm	moulding variation
Pad cutting	±0.5 mm	die or routing
Lamination	±0.8 mm	glue and press shift
Accumulated	±2.8 mm	all in one direction

Direction matters as much as size. Pockets take the minus side so interference grips the part; the outer profile takes the plus side, or the insert will not enter the case.

Custom Foam Insert: dark grey cases stacked three high in a workshop, corners nesting, the bottom row inside a timber crate

5. Measuring before ordering

Measure the interior at three points and take the smallest; moulded walls bulge by a few tenths and an average makes the insert arch. Measure the part at its largest outline, bosses and connectors included, and use one instrument for both. Where an antistatic bag is used, add 1 to 2 mm all round.

Six lines complete a drawing: interior size, base thickness, pocket depth, pocket outline, compression and accumulated tolerance.

Conclusion

A Custom Foam Insert sheet does not need length, it needs closure: base plus pocket plus lid pad equals interior height, compression sits between 5 and 15 percent, tolerance is stated as an accumulated figure. Fill those four lines and the Custom Foam Insert assembles on the first try.

This article covers container-side specifications and design implications only, not the handling, loading or use of the contents.