Below what volume should a case injection molding machine be dropped?

Answer: For a 400 × 300 × 150 mm case, 1,400 pieces; at 600 mm internal, 2,600. Below that, rotomoulding or vacuum forming is cheaper per part.

Is 800 mm internal a hard limit?

Answer: An economic one. Past it, clamp force passes 2,000 tonnes, presses and moulds thin out, and lead time stretches from 55 days to 8 months.

How tight is a rotomoulded wall?

Answer: ±1.5 mm as standard, ±0.8 mm with internal air cooling. Machining 2 mm off a face still beats a bigger press.

Can a hydraulic press hold thin-wall tolerance?

Answer: To ±0.15 mm, with 4 s added to the cycle and flash removed by hand. At ±0.05 mm, all-electric is the only answer.

Can a two-shot press run one colour?

Answer: It can, but the turntable still indexes in 2.6 s, so the cycle runs 10 percent slower and draws 15 percent more power.

Moulding or sheet metal for the case body?

Answer: Under 50 pieces, above 1 m with an access door, sheet metal wins; above 1,000 pieces with a sealed, insulating body, a case injection molding machine is cheaper.

When Not to Buy: 5 Limits for a Case Injection Molding Machine

Quick answer: A case injection molding machine suits batches above a thousand pieces, internal sizes under 800 mm and walls of 2 to 5 mm. Below 300 pieces, past 800 mm or above 6 mm of wall, rotomoulding, sheet metal and vacuum forming cost less.

Volume first: where the cost crossover sits
Past 800 mm internal: why rotomoulding wins
All-electric versus hydraulic: when the premium is wasted
Two-shot presses and second operations: skip the turntable
FAQ

1. Volume first: where the cost crossover sits

For a 400 × 300 × 150 mm case, an injection mould is quoted at 120,000 yuan and the part lands at 62 yuan in material and machine time. A rotational mould costs 28,000 yuan with the part at 145 yuan. The lines cross at 1,400 pieces; below that the moulding route cannot amortise its tool.

Tooling spreads over life: 120,000 yuan across 100,000 parts is 1.2 yuan each; across 300 parts a year, 400 yuan each, more than the material.

Prototyping adds a layer: a simple tool at 22,000 yuan proves the structure over 30 parts, where the rotational route needs 4,000 yuan for the same check. Past three revision rounds, the trial cost closes the tooling gap.

Volume band	Better route	Unit cost	Break-even test
300 to 1,400	Rotomoulding or vacuum forming	145 yuan	20-day lead time against 55
1,400 to 20,000	Single-cavity moulding	62 yuan	A 25-minute changeover absorbs the tool
Over 20,000	Four-cavity moulding	41 yuan	Half the labour per part

2. Past 800 mm internal: why rotomoulding wins

A 900 × 600 × 400 mm cavity has 5,400 cm² of projected area. At 350 kg/cm² that is 1,890 tonnes of clamp force, 2,174 tonnes with a 15 percent margin, on a 28 t mould. Few presses accept it; a 2,500-tonne machine is 8 months away.

Case Injection Molding Machine: a black flat case facing front on white, a curved recess in the lid front, latches left and right

Rotomoulding runs a different sum: a welded steel mould at 30,000 yuan, 20 days, walls of 6 to 12 mm in one shot, 260 yuan for a 1.2 m case. At 200 pieces a year the oven runs four hours; an injection press idles after one day on a 3,000 yuan power bill.

Injection keeps two advantages, ±0.2 mm on dimensions and a 55 s cycle. A case that carries instruments or must survive a 1.5 m drop justifies both; a case for tools does not.

Measure	Case injection molding machine	Rotomoulding
Wall range	1.5 to 5 mm	6 to 12 mm
Economic batch	Above 1,400 pieces	50 to 800 pieces

3. All-electric versus hydraulic: when the premium is wasted

At 1,000 tonnes an all-electric press costs 30 percent more, a 600,000 yuan gap. It runs 8 percent faster and draws 40 percent less power; at 20 hours a day and 0.8 yuan per kWh that is 110,000 yuan a year, so the gap takes five and a half years.

Thick parts with long hold times suit hydraulics better: a 4.5 mm lid holding 18 s keeps the servo working throughout, and by the third hour the motor derates.

The line sits at wall thickness and tolerance: under 2 mm with ±0.05 mm and a clean room, the 600,000 yuan is earned; above 3.5 mm at under 50,000 pieces, hydraulics win.

4. Two-shot presses and second operations: skip the turntable

A 1,300-tonne two-shot press costs 900,000 yuan more than the single-shot version, carries a 3.2 t turntable and needs two moulds. One-shot moulding removes an assembly step worth 8 yuan a part, so the gap needs 112,000 pieces to close; at 5,000 pieces a single-shot press is far cheaper.

Case Injection Molding Machine: a black lid over a white base, angled on white, one latch at the near corner

Colour changes cost time too: 2 hours to purge both barrels against 40 minutes on a single-shot machine, and 12 changes a year throws away 24 hours.

Only three jobs need two-shot: two hardnesses in one part, colour zones that cannot be painted, a soft seal over a hard shell. Elsewhere a single-shot route, then ultrasonic welding or insert moulding, runs 2 points better on yield.

Conclusion

Volume, internal size and wall thickness draw the boundary: above 1,400 pieces, inside 800 mm and between 2 and 5 mm of wall, moulding is cheapest; outside it, rotomoulding, sheet metal and vacuum forming each own a band. Saying why a machine is not chosen, before the quote, beats explaining a tool that sits idle, and that is the first test a case injection molding machine should pass.

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.

When Not to Buy: 5 Limits for a Case Injection Molding Machine

Table of contents

1. Volume first: where the cost crossover sits

2. Past 800 mm internal: why rotomoulding wins

3. All-electric versus hydraulic: when the premium is wasted

4. Two-shot presses and second operations: skip the turntable

Conclusion

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