Can ±0.5 mm case dimensional tolerance hold above 600 mm?

Answer: In injection moulding, at a price. Above 600 mm it needs mould temperature at ±2 °C and measurement after 24 hours, taking scrap from 2% to 6% and cost up 12% to 18%.

What makes 4 mm appear in one batch?

Answer: First check whether one tool made both. Different cavities, resin lots or mould temperatures shift case dimensional tolerance by 1 to 2 mm; 4 mm inside one tool means uneven cooling or ejection distortion.

How much clearance should foam have in a ±2 mm case?

Answer: 1.5 mm of interference per side, or a 5 mm EVA pad. At 0.5 mm the foam shifts; at 2.5 mm it will not enter.

What happens if the drawing carries no tolerance?

Answer: Each supplier uses its own capability, so one drawing returns ±0.5 mm and ±2 mm mouldings, and liners get built to the widest batch.

Case Dimensional Tolerance: ±2 vs ±0.5 mm

Quick answer: A 4 mm case dimensional tolerance band on a 450 mm cavity makes foam fit neither end; ±0.5 mm needs moulding control, ±2 mm suits rotomoulding.

Where four millimetres lands
Injection moulding and rotomoulding
Lid joint and sealing face
Liner fit clearance
Inspection and gauge capability
FAQ

1. Where four millimetres lands

Two cases from one batch measure 452 mm and 448 mm along the cavity. Foam cut to 450 mm must be squeezed 2 mm into the small one, so the lid never closes flush, and it rattles 2 mm in the large one.

The same 4 mm means three things: on cavity length it hits the liner, on tongue depth it hits sealing, on hinge boss spacing it hits lid alignment. All three are case dimensional tolerance, yet each needs its own gauge.

A ±2 mm drawing allows 448 to 452 mm; a ±0.5 mm drawing allows 449 to 451 mm. Same nominal, four times the band, so a note reading "±2" tells the liner cell nothing.

2. Injection moulding and rotomoulding

Injection moulding builds tolerance from mould precision plus shrinkage compensation. ABS shrinks 0.4% to 0.7%, so a 500 mm dimension is cut at 502.8 mm; shrinkage scatter of ±0.03% adds ±0.15 mm and mould temperature drift from 60 to 64 °C adds ±0.1 mm, so ±0.5 mm is achievable but not cheap.

Rotomoulding starts elsewhere. A steel mould at 260 to 300 °C expands 6 mm over 2 m, and powder shrinks 2% to 3% while a 6 mm section cools out of step with a 9 mm one, so ±2 mm on a 2 m case is good control.

The choice is a process boundary, not a precision contest: ±0.5 mm means injection moulding at ±2 °C, a 2 m single-piece shell means rotomoulding at ±2 mm.

Case dimensional tolerance verified beside a black open case, a woman holding clear extruded strips against a yellow perforated panel

Process	Long side mm	Achievable mm	Control
Injection ABS	up to 300	±0.3	Mould at ±2 °C
Injection ABS	300–600	±0.5	Mould at ±2 °C, 24 h
Injection PP	600–1000	±1.0	Two-stage packing
Rotomould PE	1000–2000	±2.0	Steel tool, stable shop
Rotomould PE	over 2000	±3.0	Split tool, water cooled

3. Lid joint and sealing face

Sealing runs on compression, and compression is calculated, not assembled. A 4.0 mm silicone cord in a 2.6 mm groove gives 1.4 mm of squeeze, or 35%, the usual IP67 point; below 25% nothing seals and above 45% the cord takes a set.

Case dimensional tolerance subtracts from that. At ±0.5 mm per half the stacked band reaches 1 mm, so groove depth runs 1.6 to 3.6 mm and squeeze 0.4 to 2.4 mm, or 10% to 60%. Assembly size is a sum of parts, so tolerances add rather than average out.

The fix is structural: cut the groove in the lower shell and locate on a tongue, so depth comes from one part and ±0.5 mm maps to 22.5% to 47.5% compression.

Case dimensional tolerance set by injection machines in a row, red material lines and silver hoppers overhead, yellow floor markings

Tolerance case	Groove mm	Squeeze mm	Compression %	Verdict
Nominal	2.6	1.4	35	Pass
Single half ±0.5	2.1–3.1	0.9–1.9	22.5–47.5	Marginal
Both halves ±0.5	1.6–3.6	0.4–2.4	10–60	Leaks or locks
Single half ±2	0.6–4.6	−0.6–3.4	0–85	Unusable

At ±2 mm the worst groove is 4.6 mm, deeper than the cord, so the seal never touches the lid; the opposite leaves 0.6 mm and crushes the cord 85%.

4. Liner fit clearance

Foam allowance and cavity tolerance are a pair: too little lets foam shift, too much forces it in and EVA collapses.

At ±0.5 mm, cut foam to nominal with 0.5 mm of interference per side, and movement stays under 0.5 mm, so holes line up. At ±2 mm the interference must be 1.5 mm, compressing the foam 20% on entry and lifting density from 45 to 56 kg/m³, which shifts the cushion curve upward.

Locating holes differ. A ±0.5 mm case takes 0.2 mm of clearance per side and survives 200 insertions; at ±2 mm the pin needs 0.8 mm and wears the hole oval after 100 km of mountain road.

Temperature adds a layer. PE expands at 1.5×10⁻⁴ per kelvin, so a 1 m cavity grows 6 mm from 20 to 60 °C, more than a ±2 mm band, and hot-service liners must be cut against the 60 °C cavity.

5. Inspection and gauge capability

Measuring ±0.5 mm work with a caliper is close to measuring nothing. A digital caliper resolves 0.01 mm and repeats within 0.03 mm, but 0.05 mm of uncertainty is a tenth of the band.

Choose gauges on a 1:10 ratio of tolerance to uncertainty: ±0.5 mm needs uncertainty under 0.05 mm, so volume work needs a go/no-go fixture at 12 s per part, while ±2 mm tolerates a caliper.

A CMM takes 8 minutes for five points and suits disputes; a laser scan returns a cloud in 3 minutes and reads joint mismatch, which is a continuity error.

One trap sits in the clock: a moulded part keeps shrinking for 30 minutes after ejection, up to 0.3 mm at 500 mm, so judge it after 24 hours.

Gauge	Resolution mm	Repeatability mm	Five points	Applies to
Digital caliper	0.01	0.03	90 s	±0.3 and wider
Micrometer	0.001	0.005	120 s	±0.05 and wider
Go/no-go fixture	0.05	0.02	12 s	±0.5 in volume
CMM	0.001	0.003	8 min	First article
Laser scan	0.02	0.05	3 min	Joint mismatch

Treat the last column as a floor, since narrower bands leave less room for gauge uncertainty.

Conclusion

Run two acceptance schemes. For ±0.5 mm cases, measure 18 points on the first article, then pull 5 parts every 2 hours against cavity length, cavity width and joint mismatch. For ±2 mm cases, sample 8 per batch, or follow GB/T 2828.1 level II at AQL 1.0, where a 500-piece lot maps to 32 samples.

Match gauges to the band: fixtures at ±0.5 mm, calipers at ±2 mm, a CMM when the argument starts. Case dimensional tolerance left off the drawing leaves inspection no decision line.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.