Do I need more compartments in a plastic case with dividers than part families?

One extra pocket is the margin. Fewer and two families share a compartment; many more and empty pockets waste volume.

Is a 2.5 mm divider enough?

Up to a 120 mm cavity with 110 mm clear width, yes. Past 200 mm clear width, use 3.0 mm with a root fillet.

Drop-in sheets or single-shot moulding?

Under 500 units a year with revisions likely, use sheets. Past 3,000 units with a frozen layout, mould it.

Can the cases be stacked on a shelf straight away?

Yes, once the bottom unit is checked for stack load. Three tiers need ribs at 60 mm centres; four need load pillars.

Does a 75 percent divider spoil the gasket?

No, provided the top stays 15 mm below the rim. Sealing depends on the liner meeting the rim plane.

How to Choose a Plastic Case with Dividers: Five Criteria

Quick answer: Judge a plastic case with dividers on five points: compartment size, divider height, forming route, floor joint, and lid compression with stack load. The first two follow from the contents, the last three from tooling. On a 340×240×120 mm cavity with six compartments, drop-in sheets cost CNY 16 a unit in 7 days, single-shot moulding CNY 28 plus a CNY 28,000 tool.

Five Criteria for a Plastic Case with Dividers
Criterion One: Size Compartments to the Longest Item
Criterion Two: Divider Height as a Share of Cavity Depth
Criterion Three: How the Dividers Are Formed
Criteria Four and Five: Joints, Compression and Stacking
FAQ

1. Five Criteria for a Plastic Case with Dividers

Two cases both marked 400×300×150 mm can differ by 75 percent in unit cost and sixfold in tolerance, on divider thickness and forming route alone. Size and height follow from the contents; route, joint and stacking from the tooling.

Criterion	Driven by	Typical range
Compartment clear size	Longest item, finger access	45 to 200 mm
Divider height	Item height, reach-in	55 to 75 percent of cavity
Forming route	Volume, revision rate	Single-shot, drop-in, welded
Floor joint	Vibration, drops	Weld, channel, friction
Lid compression	Stacking, transit	2 to 8 mm liner

Settle the first two before asking prices, or the quote has to be redone.

2. Criterion One: Size Compartments to the Longest Item

Clear size is the cavity minus dividers, not the cavity divided by compartment count. A 340 mm cavity with two 2.5 mm dividers leaves 335 mm, or three columns of about 110 mm. Test the longest item against it; if it will not fit, cut a compartment rather than thin the divider.

plastic case with dividers: a white open case on a steel bench, black foam dividers making four pockets, dark boxes, kraft bags and grey foam

The lower limit comes from the hand: bare fingers 45 mm, a cotton glove 60 mm, a cut-resistant glove 75 mm. A plastic case with dividers needs part families plus one pocket; ten pockets for five families wastes a third of the volume.

3. Criterion Two: Divider Height as a Share of Cavity Depth

Three bands cover most work: 55 percent for flat parts, 65 for bars on end, 75 for small parts. In a 120 mm cavity a plastic case with dividers lands at 66, 78 or 90 mm of divider height, needing 2 to 4, 4 to 6 and 6 to 8 mm of lid liner.

Keep the divider top 15 to 25 mm below the rim, since that gap is the liner's compression space. Under 10 mm the lid cannot seat; over 30 mm the dividers restrain nothing.

4. Criterion Three: How the Dividers Are Formed

Single-shot moulding holds ±0.3 mm with a 1.5 mm root fillet; tooling starts near CNY 28,000, lead time 25 days. Drop-in sheets need no tooling, cost CNY 16, ship in 7 days.

plastic case with dividers: a white case on a steel shelf, lid half open over black compartment trays and a kraft bag, three moulded latches

Welded walls sit between the two: ±0.5 mm at roughly 80 percent of parent strength, about CNY 34.

Forming route	Tooling, CNY	Unit, CNY	Tolerance, mm
Single-shot moulded	28,000 and up	28	±0.3
Drop-in divider sheet	0	16	±2.0
Ultrasonic welded walls	6,000 to 12,000	34	±0.5

Revision cost sits in the total: CNY 8,000 on a mould, zero on sheets, CNY 3,000 on a jig. Below 500 units a year sheets win; past 3,000 moulding pays back.

5. Criteria Four and Five: Joints, Compression and Stacking

Full welds barely move, but a channel joint shifts up to 1.5 mm after a 1.2 m drop on six faces. Freight vibration is continuous, so channel joints need a floor retainer.

A 5 mm EVA liner compressed 15 percent gives 0.75 mm of travel, enough to bear on the divider tops; a thicker liner lifts the lid and cracks the latch base. Stack load in a plastic case with dividers is read at the bottom unit: three tiers at 30 kg put 60 kg on it, so ribs go at 60 mm centres. At -20 to 60 °C PP turns brittle, so repeat a warm 1.2 m drop at 0.9 m cold.

Conclusion

Take them in order: compartment size and divider height first, then forming route, floor joint and stack load. A supplier given all five quotes a realistic route, not the most conservative one. Get the arithmetic and the stack check right, and a plastic case with dividers runs three years without a revision.

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.