Why does the cavity of a custom box include the dividers?

Answer: Dividers take cavity space. A 180 mm cavity with two 1.5 mm dividers leaves 58 mm per pocket.

Is ±0.15 mm on pocket pitch too tight?

Answer: No. A 0.3 mm shift puts the divider out of line with the lid groove.

Can the 2.2 mm wall be thinner?

Answer: Few pockets allow 1.8 mm; a dense grid short-shots instead.

Is a 0.4 mm lid gap nominal or a limit?

Answer: A nominal, inside a 0.3 to 0.5 mm window; gasketed versions go to 0.2 to 0.3 mm.

How is warpage of 0.4 mm per 100 mm measured?

Answer: Rest the box on a plate, measure the largest gap under the lid, and scale the gap.

Custom Box Specifications: Dimensions, Wall Thickness and Load

Quick answer: A custom box is specified from the inside out: cavity, height, pocket pitch, lip, lid gap, then wall thickness and single-unit load.

Setting the Cavity of a Custom Box
Compartments, Dividers and the Lip Fit
Wall Thickness and Single-Unit Load
Lid Gap and Closing Features
Shrinkage, Warpage and Tolerances
FAQ

1. Setting the Cavity of a Custom Box

A cavity is an addition, not a copy: the largest part, plus take-out clearance, plus the divider. At this scale 0.8 mm in 180 mm is four parts in a thousand.

Level	Definition	Example	Tolerance
Cavity	Floor to lid face	180 × 120 × 45 mm	±0.2 mm
Clear cavity	Minus dividers and lid boss	176 × 116 × 39 mm	±0.25 mm
Outline	Clips and lid rim included	196 × 136 × 58 mm	±0.3 mm
Packed unit	Plus bag and stacking clearance	200 × 140 × 62 mm	—

The 8 mm per side between 180 and 196 splits into 2.2 mm of wall, 2.3 mm of rib zone and 3.5 mm of radius relief, set by load, moulding and the shape inside.

Depth is the trap: a 45 mm cavity loses 6 mm to the lid boss, leaving 39 mm usable.

2. Compartments, Dividers and the Lip Fit

In a one-part-per-pocket custom box the pocket matters more than the cavity: 42 × 38 mm on 1.5 mm dividers, ±0.15 mm pitch.

Custom box: closed white body flat on a warehouse steel shelf, one recessed groove across the lid, a plastic latch and a folding handle

The lip is the feature most often drawn wrong: 2.5 mm high, 1.2 mm wide, 0.15 mm clearance per side. At 3.5 mm the rim lifts before the clips seat; at 1.5 mm side load lands on the dividers.

Grid count drags wall thickness with it: six pockets to twelve doubles divider length and drops fill pressure at the far end by 15 percent, short-shotting 1.5 mm dividers; the fix is a gate change or 1.8 mm dividers.

3. Wall Thickness and Single-Unit Load

Walls are graded by zone: 2.2 mm sides, 2.6 mm base, 2.0 mm lid, ribs 1.6 mm at the root tapering to 1.0 mm at the crest, since a 0.4 mm step unbalances flow.

Judge load by base deflection: a 2.4 L custom box carrying 8 kg sits on a 3 × 4 rib grid with 4 mm ribs and holds under 0.5 mm; without the centre rib the same load sinks 1.4 mm.

Carrying is separate: the handle root runs 2.8 mm and holds 8 kg for 24 hours without whitening, below R3 the stress concentration doubles.

4. Lid Gap and Closing Features

Design the lid gap at 0.4 mm inside a 0.3 to 0.5 mm window: under 0.3 mm the rim binds, over 0.5 mm the body rocks and dust walks in.

Custom box: worker in a pink top pressing a black gasket into a black body at a workshop bench, cartons of coiled gasket behind

Clips carry force and life: 60 N holding, 25 to 35 N to open, a 2.0 mm root, under 15 percent loss after 5,000 cycles. A living hinge runs a 0.8 mm film, and at 1.2 mm it whitens within thirty folds.

Sealing decides the gap: a plain lip uses 0.4 mm as a labyrinth, while a gasket needs a 2.0 by 1.5 mm groove and a 0.2 to 0.3 mm gap.

5. Shrinkage, Warpage and Tolerances

Tolerances run one grade tighter than on a case: ±0.2 mm on the cavity, ±0.3 mm on the outline, ±0.15 mm on pocket pitch.

Feature	Nominal	Tolerance or range	Method
Cavity	180 × 120 mm	±0.2 mm	CMM
Outline	196 × 136 mm	±0.3 mm	Calipers
Pocket pitch	42 × 38 mm	±0.15 mm	Projector
Lid gap	0.4 mm	0.3 to 0.5 mm	Feeler gauge
Warpage	—	0.4 mm per 100 mm	Plate and shim

Polypropylene shrinks 1.5 to 2.0 percent, so the 196 mm length takes 3.5 mm at 1.8 percent, and 0.2 of a point in crystallinity moves that edge 0.4 mm, the whole tolerance.

Warpage follows temperature and packing: mould halves within 3 °C, 3 seconds of pack, and a 2.0 mm lid over a 2.6 mm base stays inside 0.4 mm per 100 mm on a 350 tonne press.

Conclusion

A custom box carries more specification per litre than any large case: cavity, clear height, pocket pitch, lip, gap, wall grades and a shrinkage allowance that eats the outline tolerance when guessed. One drawing with a method beside each makes the custom box specification hold.

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.