Why do Small Plastic Case cavity bands step by 40 mm?

Inserts are cut on a 40 mm module, so one frame with swapped inserts yields four bands. A non-standard size doubles the tooling.

How high can a 1.5 mm Small Plastic Case stack?

Three high at 8 kg, about 0.9 MPa. Four high needs 2.0 mm, five high 2.5 mm and a 3 mm lip.

Is a living hinge more durable than a metal one?

By count yes, 100,000 folds against 30,000, but it cracks after 2000 folds at minus 10 C.

Why does lid depth rise with case size?

A taller hold-down, set 2 to 3 mm below the rim by the 18 to 22 percent rule.

How much does 0.6 percent shrinkage move a cavity?

1.6 mm over 260 mm, so the mould is cut at 261.6 mm; below 150 mm the loss is 0.9 mm.

Small Plastic Case Selection: Size Bands, Wall and Stacking

Quick answer: Size a Small Plastic Case backwards from three floors. Cavity ratio 1.5 to 1.6, shell 6 to 8 mm larger per side. A 1.5 mm wall carries 1.2 kg, 2.0 mm carries 3 kg, 2.5 mm carries 5 kg. Three-high stacking works at 1.5 mm, five high needs 2.5 mm plus a 1.2 mm base rib.

Sizing a Small Plastic Case by Cavity
Wall Thickness from 1.5 to 2.5 mm
Three-High Stacking Is the Floor
Latch and Hinge Life
Cycle Time Against Unit Cost
FAQ

1. Sizing a Small Plastic Case by Cavity

Bands start at the cavity; the shell only confirms them. A Small Plastic Case runs a length-to-width ratio of 1.5 to 1.6, with the shell 6 to 8 mm larger per side to share wall, rib and lip.

Four bands do most work: 120 by 90 by 45, 160 by 120 by 60, 210 by 160 by 75 and 260 by 200 by 90 mm. Each step adds 15 mm of height, lid depth runs 18 to 22 percent, and the lid overlaps 8 to 10 mm at 0.3 mm per side.

Cavity L×W×H	Shell edge	Lid depth	Typical fit	Cost of missing it
120×90×45 mm	132×102 mm	18%	Single cell, hand carry	Lip under 1.5 mm
160×120×60 mm	174×134 mm	20%	Two or three cells	Cell under 45 mm
210×160×75 mm	226×176 mm	21%	Six cells, label area	Ratio over 1.6 lifts lid
260×200×90 mm	278×218 mm	22%	Full spare kit	Weight passes 300 g

2. Wall Thickness from 1.5 to 2.5 mm

On a Small Plastic Case, thickness is unit load times stack count. At 1.5 mm over a 210 by 160 mm base, 1.2 kg sags the centre 0.8 mm; at 2 kg it reaches 1.9 mm.

Stiffness rises with the cube: 2.37 times from 1.5 to 2.0 mm, 4.6 times to 2.5 mm. Above 2.5 mm a 0.2 mm sink appears, so stiffen with ribs at 0.6 of the wall.

Wall	Unit load	Sag over 210×160	Cavity ceiling	Rib
1.5 mm	1.2 kg	0.8 mm	210 mm	0.9 mm
2.0 mm	3 kg	0.5 mm	260 mm	1.2 mm
2.5 mm	5 kg	0.3 mm	300 mm	1.5 mm

3. Three-High Stacking Is the Floor

Stacking a Small Plastic Case is side-wall compression, not base strength: weight passes down the wall ring and a thin rim flares first.

Three high is the floor for a Small Plastic Case: three 60 mm cases stand 180 mm, and 8 kg on top gives 0.9 MPa in a 1.5 mm wall. Four high reaches 1.2 MPa and the rim spreads 1 mm in a week.

A 2.5 mm lip holds offset to 1.5 mm; a 0.5 mm crown absorbs 0.4 mm of rim flatness.

Stack	Top load	Min wall	Rim stress	Lip
3 high	8 kg	1.5 mm	0.9 MPa	2.5 mm
4 high	8 kg	2.0 mm	1.2 MPa	2.5 mm
5 high	12 kg	2.5 mm	1.5 MPa	3 mm

4. Latch and Hinge Life

Most failures sit in the latch. A snap latch is rated 8000 cycles, 3.6 years at six opens a day; a living hinge lasts 100,000 folds, 45 years.

Hinge: 1.2 mm thick, 60 percent of case width, 0.6 mm radius. Retention runs 25 to 35 N with a 0.5 mm root groove. At minus 10 C PP impact falls to 40 percent, so cold work takes a 1.8 mm tongue.

5. Cycle Time Against Unit Cost

Cooling drives the cycle: a 2.0 mm wall needs 18 s, 56 percent of a 32 s cycle; at 2.5 mm the cycle stretches to 45 s and output falls from 5400 to 3800 a day.

Shrinkage of 0.5 to 0.7 percent costs a 260 mm cavity 1.6 mm, so the mould is cut oversize. Material is 55 percent of cost, and four seconds off the cycle trims 11 percent of processing.

When band, wall and stack disagree, a low price moves rework to the floor.

Conclusion

Three numbers decide it: cavity ratio 1.5 to 1.6, wall 1.5 to 2.5 mm, stacking from three high. The band sets the cells, the wall the load, the stack whether the wall needs thickening. Line those up and the Small Plastic Case is right.

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.