Which one ships instruments better?

Answer: Set the deflection budget first. At 2.4 mm of sag aluminium still holds 25% compression; if 5 mm is fine and air freight is frequent, a 4 mm moulded box saves 1.4 kg.

Can an aluminium box damage the instrument?

Answer: Its own way. A 6 mm dent pushes the wall onto the payload; moulded PE springs back to 1.5 mm and shifts the hinge seat 0.4 mm.

Will a plastic box replace aluminium for precision gear?

Answer: On span. At 400 mm and 200 N a ribbed moulded box reads 3.1 mm; past 600 mm it needs a cross rib or an aluminium frame.

How is condensation handled?

Answer: Put 100 g of molecular sieve and a humidity card inside, and leave the lid shut until the shell warms, so water forms outside.

How far apart are repair costs?

Answer: About 20 CNY per event. Patching runs 45 CNY and restores 70%; a weld costs 25 CNY and restores 85%, 30% cheaper across 200 trips.

Aluminium vs Plastic Case: Weight and Rigidity

Quick answer: Aluminium vs plastic case comes down to three measured gaps: 1.4 kg of tare at 40 L, 2.7 mm of lid sag at 400 mm, and 22 min of condensation delay.

Tare on a 40 L Cavity
Rigidity: 1.5 mm Sheet Against 4 mm Moulded Wall
Drops and Dents: Two Failure Modes
Sealing, Conductivity and Condensation
Repair and Service Life
FAQ

1. Tare on a 40 L Cavity

Both boxes carry the same 40 L of instruments by different routes. The aluminium shell is 1.5 mm sheet folded into shape, framed with 20×20 mm extrusion on four sides and riveted through eight corner caps that hold the thin panel at its edges. The plastic route is either rotational moulding at 4 mm with ribs grown from the same wall, or injection moulding at 2.5 mm in ABS with separate latch seats.

Item	Aluminium 1.5 mm	Moulded PE 4 mm	Injection ABS 2.5 mm
Density, g/cm³	2.70	0.95	1.04
Modulus, GPa	70	0.9	2.3
Conductivity, W/(m·K)	205	0.40	0.17
Expansion, 10⁻⁶/K	23	150	80
Tare at 40 L, kg	6.2	4.8	3.9
Price, CNY	780	320	260

Per box the gap is 1.4 kg; per batch it grows. Twenty boxes weigh 124 kg against 96 kg, and 28 kg by air at 30 CNY/kg costs 840 CNY a trip, or 10,000 CNY over 12 trips, against a 9,200 CNY purchase gap. Where air freight is routine, tare is a recurring cost.

Road freight inverts it: 84 kg against a 12 t payload is nothing, so only rigidity is left to pay for.

2. Rigidity: 1.5 mm Sheet Against 4 mm Moulded Wall

Deflection is measured across a 400 mm span with 200 N on the centre of the lid. The aluminium box reads 2.4 mm, the moulded box 5.1 mm between ribs, and 3.1 mm directly over a rib. Ribs recover most of a 78-fold modulus gap, which is why rigidity is a layout question as much as a material one.

Aluminium vs plastic case stack: white rotomoulded case half open on a galvanised shelf, two latches and an end handle, black trays inside

Stiffness scales with the cube of thickness: 70 GPa times 1.5³ against 0.9 GPa times 4³ gives 236 against 58, so the sheet is four times stiffer before ribs are counted. Strip the ribs and the same 4 mm wall deflects 6.2 mm.

Sag decides the seal: a 3 mm dip takes compression from 28% to 18% and decay from 0.5 kPa to 2.4 kPa, ending IP67 on the spot. At 2.4 mm the aluminium lid still holds 25%.

3. Drops and Dents: Two Failure Modes

The drop is 1.2 m on a corner, three times per face, with 15 kg of ballast. Aluminium fails by yielding: a 6 mm dent at the corner, rivet holes going oval by 0.3 mm, and a crack starting at the hole edge on the second impact. Moulded PE fails by springing back: 1.5 mm of permanent set that the eye barely catches, but hinge seats shift 0.4 mm and the lid needs a push to close.

Box	Corner set	Cracking	Decay after drop	Verdict
Aluminium 1.5 mm	6 mm dent	None	1.2 kPa	Pass, needs straightening
Moulded PE 4 mm	1.5 mm	None	1.8 kPa	Marginal
ABS 2.5 mm at -10 °C	12 mm crack	Yes	Fails	Reject

Cold widens the spread. ABS loses two thirds of its notched impact at -10 °C and cracks 12 mm on the same drop; aluminium only dents deeper and PE loses 30% of its rebound.

Repairability belongs to the failure mode: a dent in aluminium can be straightened, a dent in polyethylene waits on its own memory.

4. Sealing, Conductivity and Condensation

Conductivity of 205 against 0.40 W/(m·K) is a 500-fold gap, and condensation shows it first. Take a box from -10 °C overnight into a room at 25 °C and 70% RH: the aluminium skin fogs in 3 min, runs water at 8 min, and holds about 8 g/m². The moulded box fogs at 25 min with under 2 g/m².

Aluminium vs plastic case test: white rotomoulded case, lid fully open on a steel bench, black gasket in the rim, grey egg-crate foam, ribbed walls

Water matters once it reaches connectors, vents or optical windows. Boxes crossing temperature bands need desiccant in aluminium; plastic buys time by holding the cold back until the lid opens.

Steady-state work uses conductivity the other way: a sample tracking ambient settles in 20 min in aluminium and over 2 h in PE. Sealing differs too, an extrusion channel with a hollow cord at 30% against a moulded groove with a solid cord at 25%.

5. Repair and Service Life

Defect	Aluminium route	Plastic route	Strength back	Cost each
3 mm puncture	Rivet a 1.5 mm patch, sealant	Hot-air weld at 270 °C, same-resin rod	70% / 85%	45 / 25 CNY
Corner dent	Hammer and flat plate	Heat to recover, cannot true	90% / 95%	15 / 10 CNY
Aged gasket	New hollow cord	New solid cord	—	40 / 18 CNY
Worn hinge pin	New stainless pin	New pin, hole reamed	—	20 / 12 CNY

Repair shapes the life curve. Aluminium steps down, since every patch costs strength and after a third one little undisturbed sheet is left. Plastic slopes: a weld reaches 85% of parent strength, but the 10 mm either side takes a second heat cycle and fades first outdoors.

Surface treatment counts too. Anodising at 15 μm holds white rust off for 500 h of salt spray, while a bare shell pits within three months on a coast; moulded pigment with hindered-amine stabiliser stays inside ΔE 3 for eight years. Over five years, patch count and recoating settle the decision, not list price.

Conclusion

Three numbers settle it: 1.4 kg of tare, 2.7 mm of deflection, 22 min of condensation delay. Air freight, small temperature swings and gear needing a stiff base point to 1.5 mm sheet in an extrusion frame; road batches, condensation and low weight point to a 4 mm moulded wall. The 460 CNY gap is the smallest part of the five-year account.

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.