Does a case mould always need a split construction?

Answer: No. Past roughly 150 mm of depth, or with latches on four sides, a split build pays for itself.

Do more cavities always lower the unit price?

Answer: Only while tonnage keeps up. Four cavities need over twice the clamp force, and a short machine forces lower packing pressure.

Is the gap between 718H and S136 worth paying?

Answer: For PP utility shells, no. For PC windows or flame-retardant compounds, hardened and nitrided S136 separates on life.

Must cooling bores sit on a 60 mm pitch?

Answer: It is the starting point, and deep pockets tighten to 45 mm. Wider pitch lifts the wall between circuits by more than 3 °C.

How long a trial run counts as stable?

Answer: 300 consecutive shots. Below that, mould temperature has not finished a cycle and dimensional drift stays hidden.

How to Choose a Case Mould: Five Selection Criteria

Quick answer: A case mould is chosen by quantifying five criteria, not by comparing prices: cavity count against tonnage, steel against resin, cooling against wall thickness, life against the maintenance record, and delivery against the sign-off you can enforce.

Five Criteria That Define a Case Mould
Criterion One: Cavity Count Against Machine Tonnage
Criterion Two: Frame Steel Paired With Cavity Steel
Criterion Three: Cooling Layout and Ejection
Criteria Four and Five: Life Records and Delivery Sign-Off
FAQ

1. Five Criteria That Define a Case Mould

A case mould serves one shell: one outer profile, one wall thickness, one latch pattern. Quotations a few percent apart rarely differ by steel grade; they differ on cavity count, cooling, life target and delivery.

Each criterion must be measurable. "Two cavities" is not a criterion; "two cavities, 620 cm² projected each, on a 530 t machine or larger" is.

Criterion	What to quantify	Typical range	What breaks first
Cavity count	Shots per cycle, projected area	1 to 4 cavities	Short tonnage, flash
Steel	Frame and cavity grade	45 steel with 718H or S136	Walls collapse early
Cooling	Bore, pitch, depth from cavity	10 mm bore, 60 mm pitch	Warp above 0.5 mm
Life	Design shots	300,000 to 500,000	Early rework
Delivery	Trial dates	T1 samples in 45 days	Window missed

2. Criterion One: Cavity Count Against Machine Tonnage

Cavity count follows annual volume and shell size. A shell over 500 mm needs above 1,000 t for two cavities, and few plants own more than one such press, so scheduling bottlenecks before the mould does. Below 200 mm, four cavities on 250 t is cheapest per part.

Case Mould: red overhead crane over a shop, two workers in red hard hats between mould bases, guide pillars and crates on the floor

Tonnage comes from projected area: multiply outer length by width, apply 0.45 to 0.55 t per cm², add 20 percent. A 560 by 380 mm lid projects 2,128 cm², giving 960 to 1,170 t, where an 800 t press flashes on the parting line opposite the gate. Oversizing costs frame steel and stroke: a 180 mm shell needs 290 mm of travel.

3. Criterion Two: Frame Steel Paired With Cavity Steel

Frames are 45 steel at 28 to 32 HRC, while cavity and core are selected separately. PP shells run 718H at 30 to 34 HRC, since inner walls need no mirror finish; PC windows or flame-retardant grades need S136 at 48 to 52 HRC, nitrided 0.05 to 0.08 mm.

Depth is the difficulty. A 120 to 200 mm pocket leaves 30 mm walls that injection pressure bows outward, so the case mould is built from four side inserts and a separate base, torqued to 80 N·m on M12 bolts, joints within 0.02 mm.

Harder is not better: a 52 HRC pocket takes three times the polishing hours, and one scratch means reworking the block.

4. Criterion Three: Cooling Layout and Ejection

Cooling takes more than 60 percent of a 2.5 to 3.5 mm wall cycle. The standard is a 10 mm bore on a 60 mm pitch, 25 to 30 mm from the cavity, and baffles cut the top-to-bottom spread from 6 °C to under 2 °C.

Ejection follows the undercuts. Latches take lifters at 3 to 5 degrees, the seal groove takes a stripper plate at 0.03 to 0.05 mm clearance, and pins run 3 to 5 mm with stroke at deepest cavity plus 5 to 8 mm.

5. Criteria Four and Five: Life Records and Delivery Sign-Off

Life is counted in shots: 718H reaches about 300,000 on PP, nitrided S136 reaches 500,000, and past that flash grows from 0.05 mm to 0.15 mm while rework frequency doubles. The record tracks accumulated shots and next due dates.

Delivery belongs in the contract: T1 at 45 days, a 300 shot run at T2, drop and stack tests, then the production schedule.

Acceptance item	Method	Pass line
First-off key dimensions	CMM	Cpk at or above 1.33
Parting-line flash	0.05 mm feeler gauge	Gauge does not enter
Cavity temperature spread	Infrared meter	2 °C or less
Cooling circuit seal	6 bar held 10 minutes	Drop of 0.2 bar or less
Continuous run	300 shots	No short shots, no warp

Conclusion

Selecting a case mould turns requirements into numbers. Cavity count, steel, cooling, life and delivery constrain one another, and a loose one surfaces in the third month of production. Written into the enquiry, they give every quotation one baseline, and the case mould a payback period you can calculate.

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.