Is higher density always better in a foam-lined protective case?

Answer: No. Density raises strength but shortens travel; above 90 kg per cubic metre the foam acts like a board. Band it by unit weight.

Can one case hold two instruments of different weight?

Answer: Yes, in separate cavities. Cut the heavy item into a dense sheet and the light one softer, leaving 10 mm of solid foam at the joint.

Can the egg-crate foam in the lid be left out?

Answer: Not advisable. It holds the top face down; without it the item moves vertically, worst in deep cavities.

What changes for a case sent by sea?

Answer: Adhesive and clearance. Deck heat reaches 70 °C and hot melt creeps; specify a vented valve and 2 mm of expansion allowance.

How often should the insert be replaced?

Answer: By cycle count. Once a day gives about three years; ten times a day shows slack within a year.

Foam-Lined Protective Case: 5 Selection Criteria

Quick answer: Five things decide a foam-lined protective case: foam family, density, layer count, compression and rebound, and the fit between insert and shell. Unit weight, step height and shipping mode settle all five in one pass.

Which Foam Family Sits Inside the Foam-Lined Protective Case
Density Follows Unit Weight, Not Feel
Layer Count Follows Step Count
Compression and Rebound Decide the Grip
Five Criteria on One Sheet
FAQ

1. Which Foam Family Sits Inside the Foam-Lined Protective Case

Most of the difference between two foam-lined protective cases sits in the cut block, not the shell. The same shell with different foam may carry twice the weight. Start with the item: weight, steps, handling frequency.

Three families cover nearly every order. Expanded polyethylene is closed cell, light, slow to recover and cheapest. Ethylene vinyl acetate is closed cell too, but dense and clean-cutting. Polyurethane is open cell, softest and fastest, best against curved faces.

Foam	Density	Best cavity use
Expanded polyethylene	25 to 35 kg per cubic metre	Light parts, one-way transit
Ethylene vinyl acetate	60 to 90 kg per cubic metre	Precision parts, daily handling
Polyurethane	25 to 40 kg per cubic metre	Lid pressure, curved faces

2. Density Follows Unit Weight, Not Feel

Density is kilograms per cubic metre. It sets how hard a cavity wall pushes back and how heavy the loaded case becomes. Fingers tell soft from firm, not density: two foams that feel alike can differ by thirty percent.

Foam-Lined Protective Case: dark grey case closed on grey gradient, centred handle, a latch each side, round pressure valve, ribs around the lid

Band it at 30 for parts under one kilogram, 45 for one to five, 60 to 75 above. Mixed loads follow the heaviest item, because the wall fails there first. Check with a sample: 200 by 200 by 50 mm is 0.002 cubic metres, so 90 grams reads 45 kg per cubic metre. Cut from the middle, not the densified edge. Too low collapses cavities, too high lets shock bypass them.

3. Layer Count Follows Step Count

Layers give every height a surface to rest on. A flat part needs one sheet; a flange or connector needs its own, or it hangs free and takes the shock at its root.

Two layers cover most builds, three cover instruments with accessories. Use 10, 15 and 20 mm sheet, keep the stack under sixty percent of internal depth; at 45 mm, a case under 75 mm deep leaves no finger room.

Layer build	Step height	Minimum depth
Single sheet	Under 5 mm	60 mm
Two layers	5 to 25 mm	70 mm
Three layers	25 to 50 mm	75 mm

4. Compression and Rebound Decide the Grip

Compression ratio is the interference measured against foam thickness. A foam-lined protective case wants twenty to thirty percent: less and the item shifts, more and the lid needs force while the latch tongue wears.

Foam-Lined Protective Case: dark grey case split open on pale grey, flat foam in lid, square and long cavities in base, handle and two latches

Polyurethane recovers within 1.5 seconds, expanded polyethylene in about 3, ethylene vinyl acetate almost instantly with little travel. Split interference: 2 mm sideways, 3 mm in depth. Judge replacement by permanent set; after 2000 cycles a wall down eight percent is due.

5. Five Criteria on One Sheet

The order matters. Family sets direction, density sets load, layers shape, compression feel, and the shell interface comes last. On a foam-lined protective case, latch count follows weight: two to 20 kg, four above, and the vent valve must clear the handle swing. Hot melt is rated to 60 °C, while a ship deck reaches 70 °C and lets the glue creep.

Criterion	Decided by	Check on site
Foam family	Handling frequency	Thumbnail press, watch crumbs
Density	Unit weight band	Cut, weigh, convert
Layer count	Steps on the item	Lay down, count outlines
Compression	Interference, recovery	Close, shake, listen
Shell interface	Total weight	Lift by handle, watch seats

Four lines on the enquiry sheet: unit weight, step height, daily handling count, shipping mode and peak temperature.

Conclusion

Family first, then density by unit weight, layers by step count, compression by handling rhythm, shell interface last. Skip one and trial samples settle it. Put all five numbers on the sheet and the foam-lined protective case is properly specified.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.