Can a Case Mould just use 8 mm walls from experience?

Answer: Small sizes, yes. Past a one metre cavity it fails, because deflection rises with the cube of the span.

Is 1.5 degrees enough draft on a 1200 mm cavity?

Answer: Yes. On a 220 mm draw it opens about 6 mm, and the wall will not grab.

Are latch counts set by perimeter or weight?

Answer: By perimeter. Four latches cover 1.2 m at 300 mm spacing; past 1.6 m go to six.

Why push tolerance to one side instead of splitting it?

Answer: A symmetric split binds at both ends; one-sided tolerance sends variation where it cannot hurt.

Are three trials excessive for one case?

Answer: Not with this many features. One treats warp, one latch pull, one leakage.

Where does the 0.6 mm per 100 mm shrink figure come from?

Answer: From measured shrinkage between 60 C and room temperature, corrected by section: 0.8 mm thick, 0.4 mm in ribs.

One Trial Run: How a Case Mould Reaches 8 mm Walls

Quick answer: A Case Mould is specced backwards from a 1200 mm cavity and a 42 kg load, which set an 8 mm base and a 6 mm wall; the seal groove chain then pins tolerance to one side.

Deriving a Case Mould Spec from a Blank Drawing
Load Paths Set the Wall Thickness
Split Line and Draft Decide Where Tolerance Sits
The Dimension Chain of Latches Hinges and Seal Groove
What Three Trials Changed in the Spec
FAQ

1. Deriving a Case Mould Spec from a Blank Drawing

The sample job carried three lines: a cavity of 1200 by 480 by 220 mm, a 42 kg load and two-high stacking. The wall thickness column was left empty.

Wall thickness is not a guess: the load path and the dimension chain squeeze it from both ends. Area first, wall last.

Input	Intermediate	Drawing value
Cavity 1200 x 480 x 220 mm	Projected area, stack weight	8 mm base
42 kg, stacked two high	Side edge load, bulge	6 mm wall
1.2 m drop, corner first	Corner shear, rib root	R12, R2 at root
5000 hinge cycles	Boss bearing	10 mm seat
Service -20 to 55 C	Shrink per 100 mm	0.6 mm

2. Load Paths Set the Wall Thickness

This Case Mould runs three gauges: 8 mm base, 6 mm sides, 10 mm hinge seat. The base carries 42 kg over a 480 mm panel and bends; the sides take the edge load of a stacked case; the hinge seat holds eight screws and would strip near cycle five thousand.

Case Mould: stacks of black latched cases on wooden benches, a worker in a striped shirt bending over one shell, blue process signs overhead

Ribs let this Case Mould save material without collapsing. They run the outer face at 4 by 12 mm, first 90 mm above the base, then every 90 mm, three in total. They cross all four corners with an R2 blend, so stress does not pile at the root. Height stops at 12 mm, since taller ribs foul the feet of the case above. Width stops at 4 mm, the moulding floor: thinner will not fill, thicker sinks the face. Three ribs cut the unsupported 220 mm span into four bands and pull bulge from 3 mm to 0.6 mm.

3. Split Line and Draft Decide Where Tolerance Sits

The split line sits on the largest projected outline, the lip below the case mouth, so flash lands on the outer edge and faces inward. Move it to the base and flash sits under the case.

Draft is set per feature, never as one blanket angle:

Main side faces: 1.5 degrees, enough to release
Deep cavity floor: 0.8 degrees, since a 220 mm draw grabs below that
Rib flanks: 0.5 degrees, with ejector pins taking the rest
Corners: R12 outside against R6 inside
Split line flash: 0.1 mm to the outside only

Tolerance is not spread evenly: the outer profile a buyer sees gets plus or minus 0.4 mm, the cavity plus or minus 0.25 mm, and every datum runs off the split line.

Draft is a release minimum, not a comfort setting.

4. The Dimension Chain of Latches Hinges and Seal Groove

The chain starts at the seal groove: 6 mm wide, 4 mm deep, taking a 5 mm O-ring so compression lands between 22 and 25 percent; groove bottom to mouth is 12 mm.

Case Mould: a yellow 28-tonne crane beam spans the moulding shop ceiling, hoppers and ducts along yellow frames, a worker beside a wrapped pallet

Link	Nominal	Own tolerance	Total
Groove depth	4 mm	0.05	0.05
Lid boss	3.2 mm	0.06	0.11
O-ring cord	5 mm	0.08	0.19
Latch travel	2.6 mm	0.10	0.29
Hinge clearance	0.15 mm	0.06	0.35

At plus or minus 0.35 mm the latches still have margin; one more link and they will not pull. Four latches at 180 N give 720 N. A long chain cannot be rescued at the end: groove first, latches second, hinge last.

5. What Three Trials Changed in the Spec

Trial one produced 1.8 mm of base warp, enough to slide paper under a corner. An 8 mm base cooled unevenly on a 650 tonne press at 60 C for 32 seconds, and shrink bowed the face. Radial and ring ribs with a 2.5 mm root brought warp to 0.7 mm.

Trial two failed at the latch: 240 N against an 180 N target. Travel was set at 1.8 mm, 0.8 under drawing, because the lid boss ran to the top of its band; dropping it 0.1 mm returned 190 N.

Trial three leaked at 0.3 bar within 30 seconds, at 18 percent O-ring compression. Deepening the groove to 4.3 mm lifted compression to 23 percent and the retest held.

All three fixes went onto the drawing and the gauge list. A fix that lives only in conversation returns with the next Case Mould.

Conclusion

The spec of this Case Mould is a chain, not a list: cavity and load set projected area, load paths set an 8 mm base, a 6 mm side and a 10 mm seat, and the groove chain pushes tolerance to one side. Each trial only asked which link had gone slack. When the chain closes, a Case Mould drawing is worth signing.

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.