Core box clamping force on a core making machine is the hydraulic (or magnetic/vacuum, on light cold box tools) load that holds the box halves together against the sand shooting impulse and the curing-gas reaction load [S2][S8]. Unlike injection molding, where clamp tonnage is sized against injection pressure [S5], the core room sizes clamp force against shooting pressure and the projected box area, then layers on a curing safety margin.
The physics differs because the box does not need to resist plastic injection pressure, it needs to resist a brief sand impulse and then a gas reaction. Laempe Reich's L, LL, LFB and LHL lines all specify continuous hydraulic clamping of the corebox, with LFB/LHL shooting volumes of 25 to 200 L and 30 to 200 L respectively on a 4-column table that lets the cylinder area push straight onto the parting line [S4]. The Laempe L Series accepts 1 to 150 L shoot volumes and up to 6-parted tooling, which is where the clamping cylinder has to be matched to a much larger projected area.
What clamping force has to resist in a core machine
The shot impulse is the first load: a core shooting machine fires the sand-resin mix into the box through a sudden expansion of a limited volume of compressed air, which compacts the mold material inside the cavity [S2]. For flowable shell-mix blowing, the inlet pressure is only 0.2 to 0.4 MPa, but the core shooter path used for cold box and hot box mixes drives a much higher peak impulse on a smaller area, which is why the box must be hydraulically clamped and not just latched [S2].
The second load is the curing reaction. In cold box, amine gas is pushed through the sand mass and the box has to stay sealed so gas does not leak past the parting line; the gassing plate in particular has to be sealed well and constructed for even flow [S2]. In hot box and shell, the box walls sit at 250 to 350°C and the resin cures against a heated contour, which adds thermal expansion load on the clamp during the variable hardening times characteristic of these processes [S2]. Sinto America's CB cold box core machine names vacuum or magnetic clamping as an option specifically to speed box change, and pairs that with an ejection slide for core removal and a PLC storing up to 500 recipes [S8].
Sizing rule: projected area × peak pressure, plus a curing margin
For a sand core machine, the working sizing rule is projected box area (cm²) × peak shooting or gassing pressure (bar), converted to kN, then multiplied by a curing/handling safety margin. A 40 cm × 40 cm box at 4 bar shoot peak gives 1600 cm² × 4 bar × 10 N/(cm²·bar) = 64,000 N, or about 64 kN, of static reaction, and the clamp should be sized above that to leave headroom for the curing and handling loads that follow the shot; this falls within the hydraulic cylinder envelope of an LFB-class machine [S4].
The unit choice matters. Cold box and hot box machines in the 25 to 200 L shoot class are hydraulic, with cylinder bore and pressure chosen so the clamp can both hold during shot and actively pull the box halves apart at the end of cycle, since the separating device has to overcome any resin flash or sand bridging on the parting line [S2]. Laempe's LFB uses two decentralized hydraulic lifting cylinders to apply that clamping force evenly across a 4-column table, which is the geometry that prevents the box from rocking when the shoot head fires [S4]. For small or quick-change cold box work, magnetic or vacuum clamping can replace the hydraulic clamp because the projected area and shoot volume are small, with the trade-off that magnetic clamping needs ferromagnetic tooling and vacuum needs a sealed face [S8].
Continuous hydraulic clamping vs latching
Continuous hydraulic clamping keeps a steady holding force on the box across the entire blow, gassing and cure window, rather than just locking a mechanical latch at the start of cycle [S4]. Laempe's L, LL, LFB and LHL model lines all list continuous hydraulic clamping of the corebox as a standard feature, and pair that with pneumatic and optional hydraulic connections for tooling loose pieces so internal slides can be actuated while the box is still clamped [S4].
Latching-only designs are not enough for cold box because the gassing plate seal depends on a steady face pressure; if the box relaxes even a few tenths of a millimetre, amine escapes and the core cures soft at the edges. Foundry Magazine's 2005 profile of an improved coremaker describes a continuous clamping path with no corebox movement throughout the blow and cure cycle, which is the same engineering point that modern hydraulic machines still use today [S7]. ATD's cold box core machines build on that logic with quick-change blow and gassing plates, which only seal reliably if the clamp force is held constant during the changeover [S1].
Tooling split direction and clamp layout
The clamp layout is set by how the box is split, and the machine has to be able to switch between vertical and horizontal parting without re-plumbing the cylinder. Core shooting machines are described as using clamping and separating devices that can be changed so it is possible to work with vertically and horizontally split core boxes [S2]. Laempe's L and LL platforms accept vertical, horizontal and combination tooling up to 6 parts on the same frame, with the hydraulic clamp sized to push along the dominant parting axis [S4].
Multi-part tooling adds a second problem: loose pieces inside the box have to be held in place while the shot fires, otherwise they shift and the core geometry drifts. The LFB line solves this with pneumatic and optional hydraulic connections for automatic tooling loose pieces, so each slide is actively pulled into position before the shot and held there under the same hydraulic clamp envelope [S4]. The L Series caps at the same 6-parted tooling count, which is what determines the maximum cylinder area Laempe ships in the standard platform [S4].
Comparison: machine classes and their clamp envelope
Four machine classes cover most foundry core rooms, and each has a different clamp envelope matched to its shoot volume and cycle rate: [S1]
- Laempe L Series (1 to 150 L shoot, 4 standard platforms, up to 6-parted tooling, continuous hydraulic clamping) [S4], the small-to-mid workhorse, clamp sized for hand-loaded boxes and short cure cycles.
- Laempe LL Series (LL10 and LL20 sizes, 150 cycles per hour, vertical/horizontal/combination tooling) [S4], higher cycle rate, so the clamp has to re-stroke fast between shots without losing holding pressure.
- Laempe LFB Series (25 to 200 L shoot, 4-column table, two decentralized hydraulic lifting cylinders) [S4], heavy industrial core shooter, with clamp sized for the largest projected area the foundry can still load.
- Laempe LHL Series (30 to 200 L shoot, mobile injection unit, fully automatic fast tool change) [S4], top-of-range, where the clamp cylinder is sized for 24-hour continuous operation and rapid tool swap.
- Sinto CB cold box core machine (vacuum or magnetic clamping option, PLC with 500-recipe storage) [S8], small-tool cold box class, where magnetic/vacuum clamping is acceptable because the box area and shoot impulse are small.
- Laempe LVM / LVMD mixers (volumetric sand and binder dosing) [S4], sit upstream of the shooter, do not clamp, but set the sand flow rate that the downstream clamp has to resist.
Who this is for, and where the rules break
This is for process engineers and tool designers sizing a cold box core machine or hot box core machine against a new core box, and for buyers comparing a core machine class against a target cycle rate. It is not for foundries running shell or no-bake exclusively, where the box is not always hydraulically clamped during cure. The break points are: a 6-parted box on the LFB frame where loose-piece actuation starts to compete with main-clamp cylinder area, and any cold box tool where the parting line is not flat, because uneven face pressure lets amine gas leak past the seal even when the total kN is correct [S2][S4].
Two signals to track over the next planning cycle: how Laempe-class LFB/LHL platforms report actual measured clamp force at the parting line (not just nominal cylinder pressure), and whether cold box OEMs such as Sinto and ATD publish magnetic-clamp pull-force ratings on the same data sheet as the box-change time [S1][S4][S8]. For background on how the blow impulse couples to box venting, see the cold box vent to blow hole area ratio spec guide.