Why can the cavity of a custom size protective case not copy the item length?

Answer: It needs handling clearance and space for ribs and draft. Copying the item yields a case that accepts the part but will not release it.

Can every envelope tolerance be held to ±1.0 mm?

Answer: It can, at the cost of extra cooling and slides, about thirty percent more tooling. Grading by function holds only what affects pallet fit.

Is one fixed shrinkage value good enough?

Answer: No. Shrinkage moves with resin lot, colour and mould temperature, so a batch cut at 1.8 percent and measured at 2.4 percent is recut accordingly.

Must the pallet module always be respected?

Answer: Only while the cavity allows it. A 690 × 490 case reaches 35 percent utilisation, fine at low volume; at high volume move toward 600 × 400.

Why list diagonal difference as its own item?

Answer: Length and width can sit inside tolerance while diagonals differ by 3 mm, and the lid lands on one side with the gasket open.

Custom Size Protective Case Specs: Envelope, Cavity, Tolerance

Quick answer: A custom size protective case is specified as a four-level dimension chain, cavity to envelope, with its own tolerance on each level, a shrinkage allowance behind the mould, and a pallet module that decides where the outside faces land.

How the Size Chain Is Built
Allocating Cavity and Envelope Tolerances
How Thickness and Shrinkage Move Dimensions
Aligning the Envelope with Pallet Modules
Writing the Dimensional Report
FAQ

1. How the Size Chain Is Built

A single dimension cannot be inspected: two millimetres of deviation and nobody can say whether the mould drifted or the note was wrong. A custom size protective case needs a four-level chain, each level carrying a tolerance.

Level	Definition	Example	Tolerance
Cavity clearance	Floor to lid inner face	640 × 440 × 300 mm	±2.0 mm
Usable cavity	After ribs, bosses, draft	618 × 418 × 274 mm	±2.5 mm
Envelope	Outline with latches	690 × 490 × 340 mm	±1.5 mm
Shipping unit	Envelope plus clearance	700 × 500 × 358 mm	reference

The 50 mm gap is 25 mm per side: 6.0 mm wall, 8.0 mm rib crown, 11 mm lid seat and gasket groove. Keep them apart: wall changes move the first, seal the third.

2. Allocating Cavity and Envelope Tolerances

Tolerances on a custom size protective case follow function: the envelope governs pallet fit, the cavity must clear ribs and draft, and depth follows the seat.

Custom size protective case: closed white case lying on a warehouse steel shelf, two black latches and a folding handle, scuffed corners, steel rack behind

Item	Nominal	Tolerance	Method
Envelope length and width	690 × 490 mm	±1.5 mm	callipers
Cavity length and width	640 × 440 mm	±2.0 mm	CMM sampling
Cavity depth	300 mm	±1.2 mm	depth gauge
Lid seat gap	1.8 mm	1.5 to 2.5 mm	go/no-go plug
Diagonal difference	—	3.0 mm max	tape

Each tightening has a price. Holding the envelope to ±1.0 mm needs extra cooling and slides, about thirty percent more tooling; holding the cavity there leaves no room for ribs.

3. How Thickness and Shrinkage Move Dimensions

Polypropylene shrinks about 1.8 percent along flow and 2.1 percent across it. That 0.3 point spread is 2 mm on a 690 mm edge, so the steel is scaled per direction: 12.4 mm on the long edge, 10.3 mm on the short one.

Walls decide whether shrinkage stays even. Main wall 6.0 mm, base 7.0 mm, lid face 6.5 mm, corner posts 9.0 mm: past a 2.5 to 1 ratio the thin zone sets while the thick zone shrinks, and sink marks follow.

Shrinkage is corrected from measurement: mould the first batch at 1.8 percent, measure 686 mm on a 690 mm edge, and the real figure is 2.4 percent.

4. Aligning the Envelope with Pallet Modules

Freight shows as soon as a custom size protective case leaves the pallet module. On a 1200 × 800 mm pallet, a 600 × 400 mm footprint tiles four per layer, twelve cases over three layers; a 690 × 490 mm footprint fits one per layer at 35 percent.

Custom size protective case: showroom shelves lined with orange and black cases, a man in a white polo shirt taking a selfie in front

Layout gives the tolerance meaning. A 600 × 400 case measuring 604 mm jams against the bearers, so it is held to ±1.5 mm; a 690 × 490 case owning a pallet can run to ±2.5 mm.

5. Writing the Dimensional Report

Inspecting a custom size protective case means stating the environment, instrument and number of points. Measure 24 hours after demoulding at 23 degrees Celsius plus or minus 2, since a fresh part still shrinks and reads 1 to 2 mm oversize.

The first article goes to a CMM; production samples two units per 300 and re-checks envelope, depth and diagonals. A trend column matters: an envelope walking from +0.4 to +1.3 mm means mould temperature is drifting.

Conclusion

A custom size protective case is held together by four sets of numbers: the chain that decides whether the item fits, the grades that decide what must be tight, the shrinkage allowance, and the pallet module that fixes where the envelope stops. With a tolerance and a method behind each line, a custom size protective case becomes a specification a supplier can be held to.

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.