How is the slat thickness of a Case Internal Divider chosen?

Start with width: up to 120 mm use 4 mm, to 180 mm use 6 mm, above that 8 mm at 100 mm pitch.

Why keep 2 mm of interference?

It is total: 1 mm per side at the root, mouth 2 mm wider as a lead-in. Wear hits the mouth, not the grip.

Can slats be flush with the cavity?

No. Flush leaves no compression, so the foam bottoms out and pushes the rim off its seal.

Does a Case Internal Divider loosen in the cold?

It tightens. At minus 10 C, PP shrinks 0.3 percent, interference grows and insertion force rises by a third.

Why half-lap the crosses?

Each slat loses half its height to interlock, notch 0.1 mm wider than the slat. Full depth cuts the section.

Case Internal Divider: Slat Thickness, Slots and Key Numbers

Quick answer: A Case Internal Divider is laid out in three steps: compartment width, slat thickness, then height. Slats run 4, 6 or 8 mm; a cell wider than 150 mm moves up one step. The slot mouth is 2 mm wider than the slat, with 1 mm of interference per side at the root.

Laying Out a Case Internal Divider
Slat Thickness from 4 to 8 mm
Slot Pitch, Slot Width and 2 mm Interference
Slat Height Against Cavity Depth
Bottom Seating, Lid Foam and Wobble
FAQ

1. Laying Out a Case Internal Divider

Four contacts hold a Case Internal Divider: top edge against lid foam, bottom in a base groove, ends in wall slots, slats at half-lap crosses. One out of tolerance and the rest go loose.

Width comes first, thickness second, height last, after foam compression. Every extra 2 mm of thickness costs one cell in a fixed length.

2. Slat Thickness from 4 to 8 mm

Four-millimetre PP twin-wall is two skins over a rib core. Its limit is a 120 mm cell: wider, with 3 kg inside, the middle bows about 4 mm. Six-millimetre sheet stiffens with the cube of thickness, so a 150 mm span deflects 1.2 mm instead of 4. Past 180 mm, a Case Internal Divider needs an 8 mm slat and a 10 mm slot, leaving 3 mm of wall.

Slat	Max cell	Load	Slot
4 mm twin-wall	120 mm	2 kg	6 mm
6 mm twin-wall	180 mm	4 kg	8 mm
8 mm twin-wall	220 mm	6 kg	10 mm
8 mm solid	260 mm	10 kg	10.5 mm

3. Slot Pitch, Slot Width and 2 mm Interference

Pitch is cell width plus slat thickness: a 100 mm cell with a 6 mm slat is 106 mm. Production runs 50 to 120 mm in 5 mm steps.

The mouth is cut 2 mm wider as a lead-in; at the root, two 1 mm ribs make 2 mm of total interference. Interference sits at the root because the mouth takes the shear.

One millimetre per side over 12 mm is about 8 percent strain, elastic in PP, holding 30 to 50 N after ten insertions. Add a 0.5 mm chamfer, 1 degree of draft and an R0.5 root. Shrinkage of 0.4 to 0.6 percent shortens a 300 mm cavity by 1.2 to 1.8 mm, so slots run from the centre outward.

4. Slat Height Against Cavity Depth

Height is cavity depth minus foam compression: at 0.72 to 0.78, a 120 mm cavity takes an 88 to 94 mm slat, leaving 26 to 32 mm for peaks compressed 30 to 40 percent, lid fit and margin. Below 0.7 the slats miss the foam and jump; above 0.82 the lid is pried off its seal.

Draft widens the cavity 1.7 mm per side over 100 mm, so height on a Case Internal Divider is taken at the deepest point.

Cavity	Slat	Top gap	Peaks	Squash
80 mm	58 to 62 mm	18 to 22 mm	12 mm	4 mm
120 mm	88 to 94 mm	26 to 32 mm	18 mm	6 mm
160 mm	118 to 125 mm	35 to 42 mm	20 mm	7 mm
200 mm	148 to 156 mm	44 to 52 mm	20 mm	8 mm

5. Bottom Seating, Lid Foam and Wobble

The base groove is 4 mm deep and 0.3 mm wider than the slat: it locates in plan and leaves clamping to the foam. Wall slots are 3 mm deep and blind; through-cutting breaks the load path.

Peaks 18 mm tall compress 6 mm, about 12 N over a 100 mm cell, enough to stop a Case Internal Divider jumping. Wobble stacks 0.2 mm pitch, 0.15 mm thickness and 0.5 mm shrinkage into 1 to 2 mm per cell.

Source	Value	Control
Pitch	0.2 mm	Lay out from centre
Slat thickness	0.15 mm	Match by lot
Shrinkage	0.5 mm	One cavity number
Base gap	0.3 mm	Slat plus 0.3
Per cell	1 to 2 mm	Invert and shake

Conclusion

Width sets thickness, thickness sets slot width, foam sets height. Four, six, eight millimetres; 2 mm of interference; 0.72 to 0.78. Once they agree, the wobble is calculated, not discovered. That is a Case Internal Divider.

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.