How thick should instrument case foam be?

Answer: Work back from pressure. A 3 kg instrument dropped 1.2 m needs 20 mm of base foam at 0.19 MPa; above 8 kg, use 30 mm base and 50 mm mid.

Is EVA or EPE better in a drop?

Answer: At equal thickness EPE cuts peak acceleration by about 10% on the first drop, but loses 8% after 50,000 closes. Specify 80 kg/m³ EVA for both.

Can one layer be replaced when the foam chalks?

Answer: Yes, the base layer: peel the old adhesive, clean the residue, bond a new 20 mm sheet for about CNY 35. A chalked mid layer is cut to the instrument outline and must be recut whole.

Does instrument case foam need a flame rating?

Answer: Only in some rooms. Laboratories do not; substations, petrochemical units and ship compartments specify UL94 HF-1 PU or FR EVA, which self-extinguishes in 2 seconds.

Foam for Instrument Cases: EVA vs EPE vs PU

Quick answer: Thickness comes from contact pressure, not density. A 3.2 kg instrument dropped 1.2 m pulls about 55 g, which spreads to 0.19 MPa over 90 cm², so 20 mm of 70 kg/m³ EVA compressing 15% is enough; EPE suits instruments under 1 kg, and PU is for curved shapes only.

Pressure First, Density Second
EVA at 15% Compression
EPE After 50,000 Closes
PU: Curves Now, Chalk Later
Three Layers and Their Jobs
Half a Millimetre a Side
FAQ

1. Pressure First, Density Second

Instrument case foam is chosen by the pressure an instrument can survive: weight times peak acceleration, divided by contact area. A 3.2 kg instrument dropped from 1.2 m onto 20 mm of EVA sees about 55 g and 0.19 MPa across 90 cm².

What the housing tolerates is a question for the mechanical engineer. A sheet-metal chassis takes 0.8 MPa, an extruded aluminium frame 0.3 MPa, a glass panel only 0.1 MPa. Once that ceiling is known, thickness stops being a matter of taste.

Halve the base from 20 to 10 mm and peak acceleration climbs to 95 g, pushing pressure to 0.33 MPa. The 10 mm saved is about CNY 4 of material; the dented aluminium it causes is not.

2. EVA at 15% Compression

EVA is closed-cell, usually 60 to 90 kg/m³. At 70 kg/m³ its 25% compression stress is 0.18 MPa with 48% rebound, so a 3.2 kg instrument sinks only 15% into a 20 mm base layer, leaving 17 mm of travel for the drop.

Foam recess close-up, black high-density foam cut into a square pocket, fine cell walls, a metal instrument half a millimetre away

Water uptake is where EVA beats PU: 0.8% after 24 hours of immersion. The same closed cells trap air, so an instrument with a battery needs a 5 mm vent channel.

Parameter	EVA	EPE	PU
Density	70 kg/m³	30 kg/m³	28 kg/m³
25% compression stress	0.18 MPa	0.09 MPa	3.5 kPa
Water uptake, 24 h	0.8%	0.4%	220%
Price	CNY 26/kg	CNY 15/kg	CNY 34/kg

Heat is the weak point. Above 60 °C the modulus falls, and a closed van reaches 65 °C, where compression set rises from 4% to 9% and the pocket loosens by 0.6 mm in one season. For southern projects, specify 80 kg/m³ EVA.

3. EPE After 50,000 Closes

EPE is expanded polyethylene at 30 kg/m³, with 0.09 MPa at 25% compression and 52% rebound. Softer than EVA and travelling further, it suits handheld terminals and thickness gauges under 1 kg.

Its weakness is fatigue. Twenty closes a day for seven years is 50,000 cycles, and a 20 mm EPE mid layer ends at 18.4 mm, an 8% loss that turns a snug pocket into a loose one; EVA loses 3% on the same test. A die cut 40 mm deep also leaves a 0.5 mm burr.

What keeps EPE in the catalogue is price: CNY 15 a kilo, about CNY 40 less per full interior than EVA, and a 3-day sample against 5.

4. PU: Curves Now, Chalk Later

PU is open-cell, 28 kg/m³, with only 3.5 kPa at 25% compression: the softest of the three and the least useful as a shock absorber. Its value is conformity. A curved handle, an odd probe, a 20-piece batch where a die makes no sense, all hand cut in about 2 hours.

The cost is life. Stored at 30 °C and 70% RH, PU chalks after 24 months: 0.2 mm of surface turns to powder, compression set reaches 18%, and a pocket that once gripped now rattles. Open cells take up 220% water, so a wet season puts the interior at 85% RH.

Instrument weight	Base	Mid	Lid	Material
under 1 kg	15 mm	25 mm	20 mm	EPE
1 to 3 kg	20 mm	30 mm	25 mm	EVA 70
3 to 8 kg	25 mm	40 mm	25 mm	EVA 80
above 8 kg	30 mm	50 mm	30 mm	EVA 90 with a timber frame

One niche remains: flame retardancy. Treated PU reaches UL94 HF-1 and self-extinguishes in 2 seconds, at 10% of its rebound, so power and petrochemical clients specify it.

5. Three Layers and Their Jobs

A full interior is three layers plus two accessories, each doing one job.

Lid foam: 25 mm egg-crate, 12 mm peak height, holding the top face down in transit
Mid foam: 40 mm die-cut, pockets 0.5 mm larger per side, fixing the instrument laterally
Base foam: 20 mm plain sheet, absorbing vertical impact, and the only layer replaced alone
Side strips: 15 mm, filling the gap to the case wall
Separators: 0.3 mm PET, so a shifted adhesive cannot glue all three together

In a 380 × 260 × 95 mm interior, 20 plus 40 plus 25 leaves 10 mm for the handle and any protrusion; below 2 mm the lid will not close. Layers are bonded with tape rated at 12 N per 25 mm, since below 8 N two weeks at 65 °C opens the joint.

6. Half a Millimetre a Side

Pocket clearance decides whether the interior works. Half a millimetre a side is the EVA default: under 0.3 mm the instrument will not go in, over 0.8 mm it moves, and 2 mm of movement works a connector loose.

Foam interior in workshop, black case open on a wooden bench, egg-crate lid, four cut compartments, wide latches with metal pins

Method	Tolerance	Minimum radius	Cost per set	Batch
Steel die	±1.0 mm	3 mm	CNY 8	above 300
CNC routing	±0.5 mm	1 mm	CNY 22	10 to 300
Waterjet	±0.3 mm	2 mm	CNY 30	5 to 50

A pocket deeper than 40 mm is cut in two passes: one pass lets the die drag the wall on ejection and widens tolerance from ±1 mm to ±3 mm, while two 20 mm passes hold ±0.5 mm. Die clearance follows the material: 0.3 mm for EVA, 0.8 mm for EPE, none for PU. A 600 × 400 mm die costs about CNY 1,800 and runs 30,000 strokes.

Conclusion

The order is pressure, travel, material, cutting: work out the 0.19 MPa class from weight and drop height, set 20 mm of base foam at 15% compression, pick the polymer by cycle count and curvature, then hold half a millimetre a side.

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