Why is a Gun Case rotomoulded rather than injection moulded?

Answer: Rotomoulding gives a seamless hollow body with an adjustable wall and cheap tooling. Injection needs two halves, a gasket and volume to pay the mould off.

Which cost block is worth attacking first?

Answer: Press time and foam, 30 yuan together, and neither weakens the structure. Taking 2 yuan out of resin means a thinner wall and a lower drop rating.

Is 150 N of latch force really necessary?

Answer: It is the floor for a 30 kPa hold over 60 seconds. Below that, a Gun Case weeps after a temperature swing, most visibly on long road legs.

Does the foam have to be die-cut?

Answer: Above 300 sets, yes: 9 yuan a set with a stable pocket. Below that, hand cutting costs 5 yuan more and consistency depends on who is holding the knife.

One Mold Across 3,000 Units: What a Gun Case Really Costs

Quick answer: A Gun Case is built from four cost blocks: resin, press time, hardware and foam. Everything else is rework. Write 4.6 kg of material, 26 minutes of press time and 150 N of latch force into the drawing, and quotes finally compare.

What a Gun Case Costs, in Four Lines
Rotomoulding in Four Moves
Rework Lives at the Latch and the Seam
Foam Sets the Second Cost Block
The Price of One Millimetre
FAQ

1. What a Gun Case Costs, in Four Lines

A Gun Case with a 420 × 300 × 150 mm interior leaves the plant at 76 yuan, with exactly four parts.

Cost block	Per unit	What sets it
Resin	32 yuan	4.6 kg HDPE at 3.2 mm wall
Press time	21 yuan	26 minutes a shot, 630 tonne press, two cavities
Hardware	14 yuan	Two latches, folding handle, pressure valve
Foam	9 yuan	EVA at 30 kg/m³, die-cut in three pockets

Add them and you get 76. The twenty yuan gap between suppliers sits elsewhere.

Tooling at 48,000 yuan across 3,000 pieces adds 16 yuan a unit; across 300 pieces it adds 160. Same case, double the price, and the difference is volume, not craft.

2. Rotomoulding in Four Moves

A Gun Case is rotomoulded rather than injection moulded because the body has to be hollow and seamless with an adjustable wall.

Gun Case: a dark blue case open on a workshop bench, black foam with cut pockets in the lid, wrench and cable beside it

Charging: 4.6 kg of powder in one pass; a 30 g error lands in the wall.
Heating: oven at 280°C, nine minutes inside, powder fusing into shell.
Rocking: two axes at roughly 1:3, which decides whether four faces come out even.
Cooling: six minutes of forced air, demoulding below 60°C to avoid warp.

Shorten any of the four and the saving reappears as scrap. Holding a 3.2 mm wall leaves no slack in heating or cooling.

3. Rework Lives at the Latch and the Seam

Two defects account for most Gun Case rework: latch force under spec, and a seam that sits off-centre.

Defect	Found at	Cost per unit	Cost per batch
Latch under 150 N	Final leak test	6.5 yuan	500 units rechecked, 3,250 yuan
Seam offset 0.8 mm	First-off	18 yuan	Tool down 3 days
Foam pocket off 2 mm	Assembly	4 yuan	3% foam scrap

A weak latch does not leak on day one. It shows up in a 30 kPa hold for 60 seconds, and if that test waits until the dock, the bill is no longer 6.5 yuan.

4. Foam Sets the Second Cost Block

Foam looks like an accessory and behaves like structure: it fixes cushion travel and assembly time.

Gun Case: a dark blue case open on an airport floor, foam pockets holding black electronic modules, a grey tester and coiled cable beside it

Die-cut EVA at 30 kg/m³ needs ±1 mm on the pocket; tightening to ±0.5 mm means a new die and 14 yuan a set instead of 9. A worker seats a module in four seconds when the pocket is clean, nine when it drags, and across two thousand cycles a day that is where labour cost hides.

Foam also eats 30% of the interior, so a Gun Case cannot be sized from the empty body.

5. The Price of One Millimetre

The ±1.5 mm outline tolerance is not a formality. Lose 1 mm of interior and the longest module no longer fits; gain 1 mm outside and a pallet carries two fewer units, forty fewer in a 20 ft container. Drop 0.2 mm of wall and the case sheds 0.3 kg while the drop height falls from 1.5 m to 1.2 m. Move a latch 0.5 mm and clamp force drops 20 N, turning a sampling test into a full inspection. Tool compensation gets dialled in at first-off; a correction afterwards costs three days of press time. Those numbers together, not the 76 yuan, are the real boundary of the design.

Conclusion

The cost of a Gun Case never lives in the total on the quote; it lives in the four blocks, where resin follows wall, press time follows cycle, hardware follows clamp force and foam follows pocket tolerance. Put those four numbers in the drawing and rework has a boundary. Every yuan saved deserves one question: which block did it come from? Read the blocks and a Gun Case quote stops being a single figure.

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.