With half a day on site, which reading comes first?

Answer: The internal diagonal spread. It reflects tool precision and shrinkage at once, and one tape plus one caliper gives a number in 10 minutes. Above 1.5 mm, stop there.

The press is big enough, so why is the unit price still high?

Answer: Check cavity count and cycle. A single cavity on a 300 mm class case runs past 45 seconds, keeping labour and power per part high; two cavities cut it to 25 seconds.

How should a buyer test the gasket during a case factory tour?

Answer: Close the lid, press for 3 seconds, release and watch the lid lift. A crisp rebound means compression between 25% and 30%; a slow lift means the wrong section.

The liner comes from the plant, so does it still need checking?

Answer: Yes. Density and cut quality decide whether equipment shifts after a 1.2 m drop. If recovery takes over 3 seconds, ask for another foam or a die cut.

A Caliper on the Shop Floor: Judging a Case Factory Tour

Quick answer: Five readings taken on the floor settle fit faster than any brochure: internal diagonal spread, thinnest wall, rim flatness, press tonnage and cavity count.

Three readings before the price talk
Clamping force and cavity count
Latches, hinges and handles
Gaskets and liners: reading a cut edge
A one-page acceptance sheet
FAQ

1. Three readings before the price talk

Last November a recorder maker sent a buyer to a case plant in Dongguan with eight pages of specification and three samples on the table. The group sat 40 minutes without opening a lid; the meeting moved only at the press. Two samples fell out in 20 minutes, one on a 0.8 mm diagonal spread, one on a 2.1 mm thin patch against a 3.0 mm nominal wall. A case factory tour earns its cost by replacing adjectives with numbers.

The first screening covers internal size, wall distribution and rim flatness; those three decide whether equipment fits, whether the case survives a 1.2 m drop and whether the lid seals.

Where to measure	Tool	Pass line on site	What failure looks like
Internal length and diagonals	Tape plus 0.02 mm caliper	Length ±1.0 mm, diagonals within 1.5 mm	Bright rub marks in corners
Side and base wall	Ultrasonic gauge	No point below 2.7 mm at 3.0 mm nominal	Light shows through, wall flexes
Rim flatness	Straight edge plus feeler	Gap under 0.2 mm over 100 mm	A 0.3 mm feeler slides in

Wall thickness is hardest: a 3.0 mm wall often reads 2.8 mm after ejection, and a 2.2 mm patch points to poor venting that cracks first at -20℃.

2. Clamping force and cavity count

For polypropylene, allow about 0.4 tonne of clamping force per square centimetre of projected area. A 1200 square centimetre case needs 480 tonnes, and a 20% margin pushes the requirement past 600 tonnes. A plant quoting a 1.2 m cavity on a 400 tonne press is subcontracting, and consistency pays for it.

Case Factory Tour: close-up of a yellow case corner, black gasket in the lid rim, pale foam with a raised pad and a curved notch

Ask about cavities by the second workstation. One cavity against two differs by 80,000 yuan in tooling, or 16 yuan versus 8 yuan per unit across 5000 pieces. Below 3000 pieces a single cavity usually wins.

Internal length	Suggested clamping force	Usual cavities	Cycle per shot
300 mm class	250 to 350 tonnes	2	45 seconds
450 mm class	450 to 600 tonnes	1 to 2	60 seconds
600 mm and above	800 tonnes plus	1	85 seconds

A small press hides behind cooling: the cycle stretches from 60 to 90 seconds and part cost climbs.

3. Latches, hinges and handles

Hook a spring gauge on the latch nose and pull until it releases. A peak between 60 and 90 N feels right, under 40 N the latch pops open on a 1.2 m drop, and above 120 N it needs two hands.

Hinges come down to cycle count and axial play. After 20,000 cycles the play should stay under 0.3 mm; past that the lid shifts 2 mm to 3 mm at closing and the gasket wears thin on one side. Hang a 40 litre cavity at 25 kg for 30 minutes and check the handle mounts for whitening.

These checks need no instrument. Shake the sample gently to hear latch rattle, then carry it 20 paces by the handle to feel where the grip digs in.

4. Gaskets and liners: reading a cut edge

Cut a piece of gasket and look at the section: a hollow round profile tolerates rim waviness above 0.2 mm, a D profile suits a flat rim, a solid square only pays on small cases. Compression runs 25% to 30% of section height, so an 8 mm gasket sits at 5.6 mm to 6 mm with the lid closed.

Case Factory Tour: a yellow hard case standing open, grey foam panel in the lid, cut-outs holding a yellow handheld unit and a round part

Read a liner at its cut edge. Die cutting and knife cutting differ by 0.3 mm, and a knife cut frays within two weeks in a 40℃ shop. Foam below 30 kg/m³ takes over 3 seconds to recover, so equipment settles 2 mm in transit.

Lift the liner and count adhesive marks: more than five spots means hand gluing, and a glued liner loses 40% peel strength after 24 hours at -20℃.

5. A one-page acceptance sheet

Put the readings on one page and get a signature before leaving. Three columns cover it: what to measure, the pass line, who records it.

Criterion	Pass line	Record
Internal diagonal spread	1.5 mm or less	Caliper reading plus photo
Thinnest wall	2.7 mm or more	Five-point gauge average
Latch release force	60 to 90 N	Peak on spring gauge
Gasket compression	25% to 30%	Before and after closing
Liner cut edge	No fray, no crumbs	Visual plus thumb wipe

A signature separates data from a promise; a verbal assurance deserves a discount of half.

Add a date: a passing sample says little about 5000 pieces, so test 5 units from the first 50 within 3 working days, while the tool is still sharp.

Conclusion

A useful visit produces a signed page of readings, not an impression. Diagonal spread, thinnest wall, latch force and gasket compression carry most of the fit question. Write them down and a case factory tour becomes an audit.

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.