How many PP Plastic Case units should be sampled?

Code letter K at level II gives 80 pieces for eight hundred units. Put the AQL values in the contract too: zero-accept-one-reject, 1.0, 2.5.

Without a force gauge, how do you judge a latch?

A 200 newton spring scale screens well enough: hook the lip, pull square, take the median of three peaks, expecting 3 to 5 newtons more error.

Does the drop test have to be run cold?

It follows service temperature: at room temperature the case dents, after four hours at minus 18 it breaks brittle. Drop within 30 seconds or re-chill.

How is the concession discount set?

By how much it shortens life: colour 6% to 8%, latch force below line 10% to 12%, anything touching seal or stacking load returned.

PP Plastic Case Acceptance Day: 800 Units, Three Drops

Quick answer: The inspection came down to four moves: pull 80 units, measure cavity and wall, gauge latches, run five thousand open-close cycles, drop from 1.2 metres on six faces. Every limit was printed before the trucks arrived.

Day of arrival: 800 units, two trucks, one checklist
First-article check: five tools, three limits
Latches and hinges on a 5000-cycle rig
Drop and stack: how two lots were failed
Class A rejection, Class B concession, Class C rework
FAQ

1. Day of arrival: 800 units, two trucks, one checklist

Two trucks reached the bay at nine with eight hundred PP Plastic Case units of 590 by 390 by 220 millimetres on four pallets. We had one morning.

Sample size follows lot size: eight hundred units fall under code letter K at general inspection level II, so 80 pieces, zero-accept-one-reject on critical, AQL 1.0 on major.

PP Plastic Case in olive green open on a steel bench, black egg-crate foam in the lid, kraft pouches in the base, warehouse shelving behind

The first look is the seam: an even gap along the long edge means latches and gasket groove line up, a wedge means the rim has distorted. Three units took a 0.35 millimetre feeler at the corner. Cavity readings ran 589.2 to 590.8 against plus or minus 0.6, wall 2.66 to 2.91 against a nominal 2.8.

Item	Basis	Limit
Cavity size	Plus or minus 0.6 mm	Out of band is Class B
Wall	2.8 mm nominal, 5% loss	Below 2.66 is Class B
Latch force	Cold first reading	55 to 95 N
Drop	1.2 m, three per face	No through crack

2. First-article check: five tools, three limits

One PP Plastic Case on the bench, worked top to bottom in forty-five minutes.

Vernier caliper, 0.02 millimetre, for cavity depth;
Ultrasonic gauge, 10 millimetre probe, five points clear of ribs;
Force gauge, 500 newton range, on the latch lip;
Feeler set, 0.05 to 1.0 millimetre, for the closed-lid gap;
Colour card under D65 light, against Delta E 2.

PP Plastic Case in lemon yellow standing closed against a white wall, red latches on both handle ends, two moulded ribs across the lid

Appearance is usually an opinion, so we cut it to 300 millimetres and 500 lux: a sink mark visible only closer is not a defect. White weld lines signal low melt temperature.

Tools first, limits after.

3. Latches and hinges on a 5000-cycle rig

The rig loops every 3 seconds on a PP Plastic Case, pushing the latch, lifting the lid to 100 degrees and closing; the full run takes 4 hours 10 minutes, reading every 500 cycles.

Cycles	Latch force	Hinge end play
0	68 N	0.2 mm
500	66 N	0.3 mm
2000	63 N	0.5 mm
5000	57 N	0.8 mm

A rig turns feel into numbers: hand readings scatter by 6 newtons, and a rig stopping at 500 cycles misses the knee past 2000.

This lot graded Class B: 57 newtons clears the 55 newton floor, yet a 16% loss crosses the 15% internal line. The fix was a thicker rib at the latch root, 1.2 to 1.8 millimetres. Hinge play past 1.0 millimetre lets the lid shift and drops the dust rating.

4. Drop and stack: how two lots were failed

Drops run from 1.2 metres, once per face and once on an edge. This lot sat at minus 18 degrees Celsius for four hours and had to be dropped within 30 seconds of leaving the freezer. Two PP Plastic Case units cracked 12 millimetres at the lower corners, both opposite a latch. The crack did not go through, and the limit graded it Class A.

Stacking: five high, 72 hours, 25 degrees Celsius, the bottom unit carrying four cases plus weights. This lot read 3.1 millimetres against a 2.5 limit.

Both failures pointed at one place: at minus 18 degrees the wall of a PP Plastic Case stiffens, so impact energy crowds into the corner radius and stacking doubles the load there. Two diagonal ribs and a radius opened from R8 to R12 fixed it; the second lot read 1.9 millimetres clean.

5. Class A rejection, Class B concession, Class C rework

Of the 80 sampled, two graded Class A, nine Class B, fourteen Class C.

Class A, corner crack or latch popping open: reject the lot or screen every unit.
Class B, latch force below line or Delta E 3.2: concession at 6% to 8% off, indoors only.
Class C, flash, skewed label, misaligned insert: supplier rework the same day, no credit.

The fourteen Class C units were cleared in forty minutes on a separate pallet. Class A defects require an 8D in seven days, and the next lot is sampled at 125 until two lots run clean. After three rounds, arguments about a PP Plastic Case shipment stay technical.

Conclusion

One checklist, one rig, five tools, four limits. Set sampling and AQL first, then method and instrument, then grading, and acceptance of a PP Plastic Case stops being an argument about feel.

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.