What is the thinnest wall a small plastic case can take?

Answer: 0.8 mm, only under a 120 mm span, no stacking and 0.3 kg per unit.

How much clearance should the cavity leave?

Answer: 2 mm at the sides and 4 mm above the part; below 1.5 mm the part cannot be gripped.

Is 0.5 degree of draft enough for a small plastic case?

Answer: For plain cavities 30 mm and deeper, yes. Textured faces need 1 degree or the rim drags.

Does a 1.6 mm wall make a small plastic case stronger?

Answer: Not automatically. Weight rises 26 percent, stack load rises with it, and thick walls sink.

Which tolerances belong on the spec sheet?

Answer: Outside 0.3 mm, cavity 0.2 mm, wall 0.08 mm, flatness within 0.6 mm, closed gap within 0.8 mm.

0.8 or 1.6 mm? How a Small Plastic Case Gets Its Wall

Quick answer: A small plastic case is sized inside out: part dimensions set the cavity, span and stacking set the wall, draft and radii close the chain. Two parts of 96 by 62 by 38 mm give a 198 by 66 by 44 mm cavity, a 1.2 mm wall and a 201 by 69 by 53 mm outside.

Cavity Size Starts From the Part
Picking Wall Thickness
Draft Angle That Still Releases
Ribs and Radii Restore Stiffness
Turning the Chain Into a Spec Sheet
FAQ

1. Cavity Size Starts From the Part

The load is 96 by 62 by 38 mm, two pieces side by side at 0.35 kg each. The outside of a small plastic case follows from those numbers, it is not a guess.

Length comes from three gaps and two pieces: 96 times 2 plus 2 times 3, or 198 mm. Width holds one row, so 62 plus 2 twice gives 66 mm. Depth is 38 plus 4 mm on top plus 2 mm of base rib, or 44 mm.

The allowance is not padding: 2 mm at the sides leaves finger room, under 1.5 mm the part cannot be gripped with a glove.

Step	Basis	Result
Part envelope	Measured	96 by 62 by 38 mm
Cavity length	96 x 2 plus 2 x 3	198 mm
Cavity width	62 plus 2 x 2	66 mm
Cavity depth	38 plus 4 plus 2	44 mm

Running the chain backwards costs money: size the outside first and the cavity lands 3 to 5 mm short, made up by thicker walls that add weight, not space.

2. Picking Wall Thickness

Two figures decide the wall: a 198 mm base span and a stack four high.

Small Plastic Case: a white case standing open on a workshop bench, black foam compartments, four trays and two kraft paper packs

At four high the bottom case carries 1.86 kg over 198 by 66 mm, under 1.5 kPa, so stiffness decides rather than strength.

A 0.8 mm wall holds only below 120 mm span unstacked; at 198 mm, 45 degrees Celsius for 24 hours sags the base 1.6 mm.

Wall	Max span	Stack	Unit weight	Use
0.8 mm	120 mm	1 high	Under 0.3 kg	Disposable inserts
1.0 mm	160 mm	2 high	Under 0.8 kg	Spare part packs
1.2 mm	220 mm	4 high	Under 1.5 kg	General parts
1.6 mm	300 mm	6 high	Under 3 kg	Heavy parts

Thicker is not stronger. At 1.6 mm unit weight is 0.78 kg, so the bottom case carries 2.34 kg at four high. Cooling time grows, sections above 2.2 mm sink, base flatness falls from 0.4 to 0.9 mm, and a sink mark on a sealing face widens the closed gap from 0.5 to 0.8 mm. PP shrink runs at 1.6 percent, so the thicker the wall, the harder the tolerance. Fix span and stack, step up one grade, stop.

3. Draft Angle That Still Releases

On a 44 mm cavity, 0.5 degree releases 0.38 mm per side, 0.77 mm across.

That is the floor for ejection: below half a degree the wall rubs the tool and leaves drag marks at the rim.

Plain cavities 30 mm and deeper take 0.5 degree, the minimum.
Textured faces take 1 degree; 0.1 mm grain barely clears at that.
Keep every cavity in one case on one angle, or ejection twists the body.

Draft also reaches the outside numbers: half a degree over 44 mm adds 0.38 mm per side, so the outside runs 0.8 mm larger than wall thickness suggests, and 1 degree doubles that to 1.5 mm.

4. Ribs and Radii Restore Stiffness

With the small plastic case wall at 1.2 mm, the rest of the stiffness comes from ribs and radii.

Small Plastic Case: a white case on a galvanised shelf beam, lid ajar, kraft paper packs and a grey box inside

Base ribs run as a grid at 0.6 times the wall: 0.72 mm thick, 6 mm tall, 45 mm apart, so rib and wall cool together. A thicker rib pulls a sink on the opposite face.

Every rib root needs R0.5: a square root concentrates stress, and a 1.0 m twelve-corner drop cracks it 15 mm along. Inside corners take R3 and outside R5, so the cavity wipes clean and a knock only scuffs.

5. Turning the Chain Into a Spec Sheet

The chain ends as a toolmaker table.

Parameter	Value	Tolerance
Outside	201 by 69 by 53 mm	Plus or minus 0.3 mm
Cavity	198 by 66 by 44 mm	Plus or minus 0.2 mm
Wall	1.2 mm	Plus or minus 0.08 mm
Draft	0.5 degree	Not under 0.4 degree
Base flatness	0.4 mm	Not over 0.6 mm
Closed gap	0.5 mm	Not over 0.8 mm

Base flatness and closed gap get cut most often, yet both decide dust control and stack stability: at 0.6 mm flatness a four-high stack rocks 2 mm.

Three checks settle a sample: cavity size, base flatness, and sink after 24 hours at four high.

Conclusion

The small plastic case spec is a one-way chain: part size sets the cavity, span and stacking set the wall, draft and radii decide what the tool can make. Change one number and three others move, so walk the chain before cutting steel on any small plastic case.

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.