Why classify defects before sampling a rotomolded case?

Answer: A through crack and a pit differ by orders of magnitude. One shared number either passes structure faults or rejects usable lots.

Can incoming inspection replace type testing?

Answer: No. Incoming checks one lot; type tests prove the design, once per new mould or material.

Is a colour shift of 1.5 strict or lenient?

Answer: Strict within a lot, about right across lots. Holding 2.0 on a reorder needs powder batch records.

Why is the thin-wall limit 80 percent of nominal?

Answer: Corners run thin by nature. On a 5.0 mm rotomolded case, 4.0 mm is the floor; below that a 1.1 m drop cracks.

What is the safest way to handle a failed lot?

Answer: Reject class A, downgrade class B to non-protective use in writing, and screen class C with the cost on the supplier.

Rotomolded Case Acceptance: Inspection Items and Verdicts

Quick answer: Acceptance for a rotomolded case runs in three steps: classify defects, fix the sampling rules, then write the pass lines as numbers.

Sorting Defects Before Sampling a Rotomolded Case
Appearance and Colour Verdicts
Re-Measuring Dimensions and Wall Thickness
Type Tests for Sealing and Load
Lot Verdicts, Retests and Records
FAQ

1. Sorting Defects Before Sampling a Rotomolded Case

Inspection rarely fails because nobody looked; it fails because findings cannot be named. A rotomolded case carries three classes: class A breaks protection, such as a through crack; class B affects life, such as wall below the lower limit; class C is cosmetic.

Take a 320-unit lot at level II: 50 units sampled, one class A finding rejects the lot, class B accepts one and rejects two, class C accepts three and rejects four.

Class	Definition	Examples	Verdict
A	Breaks protection	Through crack, latch fault	1 unit rejects the lot
B	Affects life	Thin wall, hinge offset	Sample 50, Ac 1 / Re 2
C	Cosmetic only	Pitting, colour drift	Sample 50, Ac 3 / Re 4

Samples must also span pallet levels, with three first-off and three last-off pieces.

2. Appearance and Colour Verdicts

Appearance needs fixed viewing conditions, or two people reach two verdicts. Use 1000 lx, a 500 mm distance and a 45-degree sight line on visible faces. Pitting: three spots below 1.0 mm per face, one between 1.0 and 2.0 mm, larger is class C.

Rotomolded case: white shell open on a wooden bench, grey foam in the lid and cut cavities in the base, grey trays at left

Colour is judged by instrument. Under D65 and 45/0 geometry the Lab reading stays within 1.5 of the master chip in a lot and 2.0 between lots, because three minutes of extra mould time moves yellowness by 0.8.

3. Re-Measuring Dimensions and Wall Thickness

Dimension checks need fixed points: two readings on each outer dimension, two on each cavity dimension and two diagonals, twelve figures against plus or minus 3 mm outside, 2 mm inside and 3 mm diagonal difference. Lid flatness is checked with a straight edge and feeler, under 1.0 mm.

Wall thickness uses a 5 MHz ultrasonic gauge at the same points. The pass line is spread, not one reading: 0.8 mm maximum across twelve points, and nothing below 80 percent of nominal, which is 4.0 mm on a 5.0 mm rotomolded case.

Record the environment. Polyethylene expands 1.5 x 10⁻⁴ per degree, so a 1150 mm case shifts 4.3 mm between 10 C and 35 C; measure at 23 C.

4. Type Tests for Sealing and Load

Sealing separates a rotomolded case from an ordinary tote. Empty it, close the latches, submerge 100 mm for 5 minutes, dry and reopen: a water film fails as class A. Valved models take 20 kPa for 120 seconds with a 1.5 kPa drop allowed.

Rotomolded case: white shell open on a workbench with grey foam, a tape roll and two wooden-handled tools, bins behind

Test	Condition	Pass line
Immersion	100 mm depth, 5 minutes	No water film inside
Pressure	20 kPa held 120 seconds	Drop under 1.5 kPa
Static load	1.5 times rating, 24 hours	Set under 2 mm
Drop	1.1 m, six faces and four corners	No crack, latches shut

Top load is 800 kg for 24 hours with no collapse. Hardware takes -25 C for four hours and 48 hours of neutral salt spray; service life is 5000 closures, with latch pull checked every 1000.

5. Lot Verdicts, Retests and Records

Verdicts must be fixed first. One class A unit rejects the lot with no retest; two class B units in 50 trigger a sample of 100, and three cumulative findings reject; class C accepts three and rejects four.

Lot size	Sample	Class A	Class B	Class C
51 to 150	20	0 / 1	1 / 2	2 / 3
151 to 500	50	0 / 1	1 / 2	3 / 4
Over 501	80	0 / 1	2 / 3	5 / 6

Records are part of acceptance: lot number, mould number, powder batch, twelve dimensions, twelve wall readings, the Lab value and test results, kept three years, or a crack a year later cannot be traced. Retests use fresh samples only; a failing lot is best handled by concession with a written price adjustment.

Conclusion

Acceptance for a rotomolded case comes down to three habits: class the defects, write the verdicts as numbers, keep the records. Together they make inspection repeatable rather than personal, which is what a rotomolded case acceptance standard delivers.

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.