Can a Case Assembly wall be set at 5 mm from experience?

Answer: On a small case, yes. Past a 500 mm span it fails: deflection at 600 mm runs five times that at 400 mm.

Do more ribs always stiffen a Case Assembly?

Answer: No. Below 18 mm spacing the panel dishes between ribs and gains flatten while mass climbs.

Should cavity dimensions include shrinkage allowance?

Answer: The mould does, the drawing does not. Allowance on the drawing pushes assembly dimensions outward and the liner rattles.

Which of the four numbers is easiest to get wrong?

Answer: Wall thickness. Flexural modulus moves with temperature and glass content, shifting the result by 0.5 mm.

Does a thicker boss show on the outer face?

Answer: It can, so bosses sit on the cavity side, with a 12.5 mm recess 1 mm deep on the outside to hold any sink mark.

Case Assembly in Four Numbers: Wall Thickness and Load Path

Quick answer: Case Assembly specifications are not a wall picked first and a load checked later. Four numbers chain backwards: cavity from the loaded envelope plus cushioning, wall from an allowed mid-span deflection, rib proportions from a fixed ratio, boss depth from thread engagement.

Inside Dimensions Come From the Load
Wall Thickness Solved Backwards From Deflection
Ribs and Rim Section in Fixed Proportions
Boss Thickness and Thread Engagement
Four Numbers on a Drawing With Tolerances
FAQ

1. Inside Dimensions Come From the Load

Cavity size looks easy: measure what goes in and copy it. In practice Case Assembly adds three segments - envelope, cushioning, clearance for hands. Drop one and the case swallows the load.

Take a 600 x 400 x 250 mm cavity, 60 litres of clear space. The envelope measures 510 x 310 x 190 mm, cushioning runs 15 mm per side and 20 mm above and below, and 8 mm stays for fingers.

Parameter	Value	Basis
Cavity	600 x 400 x 250 mm	Envelope, cushion, clearance
Nominal wall	4.5 mm	Rounded up from deflection
Rib thickness	2.7 mm	0.6 times nominal wall
Unit mass	7.8 kg	With hardware and liner
Stack height	3 high	120 kg on the top case
Deflection limit	3 mm	1/200 of span

Load comes from two directions: stacking presses the lid, carrying hangs on latches. A 600 mm span at 120 kg is where the thickness line starts.

2. Wall Thickness Solved Backwards From Deflection

Thickness is not set by feel. Fix an allowable deflection, then solve for the wall.

Case Assembly black hard-shell case seen from above at an angle on white, flat lid face, four horizontal ribs at each corner, red grip blocks at both handle ends, lid-to-body seam and side wall visible

The pass line is mid-span deflection under 1/200 of the span: 3 mm across 600 mm, where sag turns visible. Deflection varies with the cube of thickness, so 120 kg on that span at 1300 MPa returns 4.2 mm. Round to 4.5 mm with 7 percent for batch variation.

Going from 3.2 mm to 4.5 mm does not add 40 percent stiffness. It follows the cube: (4.5/3.2)³ is about 2.78 times, so 1.3 mm of wall buys close to triple the bending capacity while mass grows about 20 percent.

Temperature overturns this: at minus 30 degrees Celsius polypropylene reaches 2600 MPa, brittle enough to crack rib roots in a drop; at 60 degrees it falls to 700 MPa and deflection doubles. Verify at 60, impact-check at minus 30.

3. Ribs and Rim Section in Fixed Proportions

Ribs buy section modulus with less material than thickness does, but the ratios are hard limits.

Five rules govern the rib set:

Rib thickness at 0.6 times the wall, 2.7 mm here; past 0.7 sinks appear;
Rib height under 4 times the wall, 18 mm here;
Root fillet R0.5 minimum, or stress concentrates;
Draft 1 degree per side, more as ribs deepen;
Rib spacing at least 4 times the wall, 18 mm.

The rim is the second section: a gasket presses on it, so a latch must not twist it. An inward lip 3 mm thick and 8 mm tall forms a box section with the side wall. Proportions travel: at 5.5 mm, ribs go to 3.3 mm with a 22 mm ceiling.

4. Boss Thickness and Thread Engagement

Bosses are the only locally thickened feature in a Case Assembly: a 4.5 mm wall is too thin for a screw, so the boss is built to 6 mm, below which a thread holds two turns and strips.

Case Assembly black hard-shell case opened flat, two empty cavities facing each other, a row of continuous hinge barrels between them, round mould marks in the corners and a raised rim bead, plain white background

Engagement runs at twice the bolt diameter: an M5 screw takes 10 mm of thread at 3.5 N·m; at 8 mm it yields early. Boss diameter should reach 12.5 mm, or the wall splits.

5. Four Numbers on a Drawing With Tolerances

Item	Drawing callout	Tolerance	Check
Cavity length and width	600 x 400 mm	±1.5 mm	CMM sampling
Cavity height	250 mm	±1.0 mm	Caliper, five points
Rim flatness	Not called out	Within 0.6 mm	Plate and feeler
Wall thickness	4.5 mm	±0.2 mm	Ultrasonic gauge
Boss thickness	6 mm	±0.3 mm	Caliper

Under the set sits shrinkage: polypropylene moulds at 1.4 to 2.0 percent, cavities are cut at 1.6, and a batch at 1.7 grows a 600 mm cavity by 0.6 mm - half the band gone. First articles get measured for that reason.

Trial moulding runs three parts: full dimensions, a 1.0 m drop on six faces, 3-high stacking for 24 hours to read creep.

Machine capacity belongs to the chain: a 600 x 400 mm case in a single cavity needs about 650 tonnes of clamp force, and short of that flash appears on the thinnest parting line and rim flatness slips from 0.6 mm to 1.2 mm.

Conclusion

Case Assembly specifications form a closed loop: cavity fixes the envelope, thickness fixes deflection, ribs fix the section, bosses fix engagement, and slack in one forces three recalculations. Draw the four numbers, attach tolerances, prove them with three trial parts, and an estimate becomes a repeatable Case Assembly specification.

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.