Injection vs blow moulded tool box at the same volume, what decides it?

Answer: Annual volume. Above 12,600 units injection amortises tooling and the ±0.05 mm tolerance removes hand-fitting; below it, a 60,000 CNY blow mould with a 500 piece MOQ is lighter.

Why can't the quoted wall on a blow moulded box be trusted?

Answer: It is an average. Stretch ratio varies and the bottom corner reads 2.3 mm, 34% under nominal. Inspect the corner, not the sidewall.

Can a 3,000 piece order justify injection tooling?

Answer: At 213 CNY against 214 CNY, cost is not the reason; structure is: a moulded-in groove and latch seats.

Can a blow moulded box reach IP67?

Answer: With extra operations: mill the rim, cut a groove, fit eight latches at 120 N total, add 11 CNY a unit. Otherwise 0.8 mm of flatness gives IP65.

Which one for −20 °C service?

Answer: Follow the material. PP cracks 15 mm where HDPE dents 4 mm, but the blow moulded hollow rib root opens 3 mm in the same drop and needs a 0.5 kPa leak retest.

Injection vs Blow Moulded Tool Box

Quick answer: At the same 40 L, an injection moulded tool box holds 2.4 mm of wall within ±0.05 mm, while a blow moulded tool box averages 3.5 mm but thins to 2.3 mm at the bottom corner. Tooling is 450,000 against 60,000 CNY, and the crossover is 12,600 units.

Where the Two Processes Split: Which Thickness Is Quoted
Forming Routes: 80 MPa Melt Against 0.4 MPa Air
Thickness Spread: ±0.05 mm Against 34%
Ribs and Hollow Ribs: Where Stiffness Comes From
Tooling and MOQ: Spreading 450,000 CNY over 3,000 Units
Sealing Face, Hinge and Cold Drop
FAQ

1. Where the Two Processes Split: Which Thickness Is Quoted

Both data sheets say 3 mm but measure different things. An injection moulded tool box quotes minimum wall: 2.4 mm on the drawing, 2.35 to 2.45 mm at four points. A blow moulded tool box quotes average wall, since the parison stretches as it inflates: 3.8 mm mid-wall, 2.3 mm at the bottom corner.

Item	Injection moulded	Blow moulded
Thickness quoted	Minimum, 2.4 mm	Average, 3.5 mm
Measured range	2.35–2.45 mm	2.3–3.8 mm
Tare, 40 L box	3.2 kg	4.1 kg
Outline tolerance, 600 mm edge	±0.15 mm	±0.8 mm
Unit price at 30,000 pcs	78 CNY	96 CNY
Tooling	450,000 CNY	60,000 CNY

Material decides the rest. PP shrinks 1.5%, so the cavity is cut oversize; HDPE takes its shape from a parison pressed against steel. The injection moulded lid closes to 0.3 mm, the blow moulded one needs 1.0 mm.

That 0.7 mm is gasket compression taken out of the seal.

2. Forming Routes: 80 MPa Melt Against 0.4 MPa Air

Injection drives PP at 230 °C into the cavity at 80 to 120 MPa, packs 8 s, cools 18 s, and turns out a 600×400×180 mm lid on a 52 s cycle from a 1,000 t press. Pressure copies every detail of the steel: ribs, grooves, bosses, latch seats.

Injection vs blow moulded tool box set: off-white case fully open on a steel bench, black rim gasket, foam trays, five front latches

Blow moulding drops a 120 mm parison with an 8 mm wall at 190 °C; it sags 3%, the mould closes, and 0.4 MPa of air inflates it against a 25 °C wall in a 120 s cycle. One two-hundredth of the injection pressure buys cheap tooling and large single pieces.

3. Thickness Spread: ±0.05 mm Against 34%

Injection thickness is set by the cavity gap: four points fall within 0.1 mm, a 4% spread. Blow thickness follows parison distribution, and 2.3 mm at the bottom corner is 34% below the 3.5 mm nominal, exactly where a drop lands.

Position	Injection moulded	Blow moulded	Difference
Mid sidewall	2.40 mm	3.80 mm	+1.40 mm
Long edge centre	2.40 mm	3.20 mm	+0.80 mm
Corner	2.40 mm	2.80 mm	+0.40 mm
Bottom corner	2.40 mm	2.30 mm	−0.10 mm

That corner is why the thicker box does not survive better. On a 1.2 m corner drop after 4 h at −20 °C, the PP shell cracks 15 mm, the HDPE shell dents 4 mm. Notched impact at −20 °C is 12 kJ/m² for HDPE against 4 kJ/m² for PP.

The cold-drop line is drawn by material, not by millimetres.

4. Ribs and Hollow Ribs: Where Stiffness Comes From

Injection ribs cost almost nothing: 1.2 mm thick, 6 mm tall, 1° draft, R0.5 mm at the root. The price is sink: a rib heavy enough to matter leaves a 0.05 mm hollow on the show face unless the mould runs at 50 °C.

Injection vs blow moulded tool box pair: closed black case on white, two red latches at the front, moulded handle recess, faint embossed lines

Blow moulded ribs are inflated: the outer wall bulges 8 mm, the inner follows, and the two weld at the root to close a 10 mm section. Bending resistance comes from depth, so a 3.5 mm wall behaves close to a 4 mm solid one.

Span 400 mm, 200 N central: 3.4 mm of deflection over three injection moulded ribs, 2.6 mm over four hollow ones.
A hollow root is a weld holding 60–70% of parent strength, so it opens first on a corner drop.
An injection moulded root is continuous: it dents without cracking, but the dent presses into the cavity.

None of it shows on a sample.

5. Tooling and MOQ: Spreading 450,000 CNY over 3,000 Units

Item	Injection moulded	Blow moulded
Mould type	Steel, single cavity	Aluminium, single cavity
Tooling cost	450,000 CNY	60,000 CNY
Mould life	300,000 shots	100,000 shots
Trial rounds	3	2
Usual MOQ	3,000 pcs	500 pcs

Add tooling and it flips. The 78 CNY injection figure carries 15 CNY of amortisation, leaving 63 CNY of conversion; the 96 CNY blow figure carries 2 CNY, leaving 94 CNY. At 3,000 units injection lands at 213 CNY, at 500 units blow moulding lands at 214 CNY: the early price is level.

The crossover is 12,600 units: conversion differs by 31 CNY, tooling by 390,000 CNY, and 390,000 divided by 31 gives 12,600, where both read 98.7 CNY. Below that, the 60,000 CNY blow mould is lighter.

6. Sealing Face, Hinge and Cold Drop

Sealing starts with flatness. The injection moulded groove forms with the body: 6 mm wide, 4.5 mm deep, a 5 mm cord at 28% compression, rim flatness 0.3 mm, decay 0.5 kPa over 30 s, and IP67 stands. The blow moulded rim is milled after moulding to 0.8 mm, pulling compression to 18%; the same test reads 1.6 kPa, IP65.

IP67 from a blow moulded box means cutting a groove in a second operation and fitting eight latches at 120 N total, for 11 CNY a unit.

Hinges split the same way. Injection boxes run a 0.8 mm living hinge: after 50,000 cycles the fold whitens, and at −10 °C 200 folds leave a 0.3 mm craze. Blow moulded boxes take a 4 mm stainless pin, 0.2 mm oval after 20,000 cycles. Both shift on a 1.2 m drop, 0.4 mm and 0.9 mm.

Conclusion

Four numbers separate the two: ±0.05 mm against ±0.8 mm of tolerance, 0.3 mm against 0.8 mm of rim flatness, 450,000 against 60,000 CNY of tooling, and a 12,600 unit crossover. Sealing class points to injection, low MOQ and cold impact to blow moulding.

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.