In high pressure die casting, squeeze pin diameter for boss porosity work typically lands in the 10-12 mm range, with one published HPDC case study documenting a 10 mm pin and a separate bearing-cap study using a 12 mm pin placed directly over the center of a flange [S5][S6].
Stroke is not a free dimension, it has to be sized against the local shrinkage volume measured at the targeted hot spot, while pin force is translated into metal pressure and coupled with intensification pressure (commonly reaching 600 bar in the late feeding stage for an AlSi9Cu3 alloy modeled in FLOW-3D CAST) to push solidification past the critical solid fraction [S1][S5].
Why Pin Diameter, Stroke, and Force Have to Be Sized Together
Squeeze pin performance is governed by four coupled parameters: pin diameter, line-of-action travel (stroke), applied force, and activation time, and getting any one of them wrong usually relocates the porosity rather than removing it [S1][S2].
A 10 mm pin cylinder modeled in FLOW-3D CAST, defined as metal at 631 °C, can be cut from the casting mesh and set as a moving boundary that advances to compensate shrinkage volume; the same model will let the pin stop automatically when it has either reached maximum travel or hit a solid fraction high enough to block further motion [S1][S5]. Below the critical solid fraction (a default 0.7 in the FLOW-3D database for AlSi9Cu3) interdendritic feeding can still collapse macropores; above that threshold the alloy switches to microporosity formation and a larger pin force, not a larger pin, is what is needed [S5].
Sizing the Diameter: 10 mm vs 12 mm vs Larger
For a bearing-cap boss with a known hot spot at the flange center, a 12 mm pin located directly over the flange center was enough to eliminate shrinkage-driven porosity in a published implementation study on cast aluminum bearing caps [S6]. For a thinner (3 mm wall) hollow exhaust pipe in EN AB 46000 (AlSi9Cu3), 10 mm was the modeled working diameter, set against a 17.5 mm thick connection block that was the source of two large coalescent macropores in the initial (no-pin) simulation [S5].
The general engineering rule, visible in the published cases, is that pin diameter should be small enough to fit the local boss geometry without bridging into adjacent features, but large enough that the projected pin face area multiplied by the available metal pressure produces a force exceeding the feeding resistance at the solidification front [S1][S6]. Squeeze-pin controllers such as the Shibaura Squeeze Master are quoted at more than 3× casting intensification pressure, which is the headroom typically required to feed a thick boss through the final solidification stage [S3].
Sizing the Stroke Against the Local Shrinkage Volume
Stroke is the parameter most often mis-sized in practice. The FLOW-3D CAST Squeeze Pins Model handles this directly: once activated, the pin "senses the amount of shrinkage in the adjacent liquid metal and moves in to exactly compensate for that volume," advancing along a defined line of action until it either reaches its maximum travel or meets a solidified working surface [S1]. In Altair Inspire Cast the same logic is exposed to the user as a workflow, measure the shrinkage size at the porosity location, set the squeeze-pin parameter from that volume, and let the simulation verify whether the chosen stroke and timing actually remove the defect rather than move it [S2].
Where the metal compressibility matters, the late stage of feeding can see metal pressure rise to roughly 600 bar, and the effective liquid density at the liquidus should be increased by about 4% to reflect that, which in turn raises the volumetric shrinkage the pin has to compensate for [S5]. A practical consequence: stroke sized only from the ambient-pressure shrinkage curve will under-feed the boss under intensification, and the pin will stop early against a partially solidified face instead of completing its compensation [S1][S5].
Activation Timing and the Three Control Modes
Pin timing is critical because a pin actuated too early can push molten metal out through an unsealed gate, while a pin actuated too late meets a fully solidified face and does not move at all [S1]. The Squeeze Pins Model exposes three activation modes that map directly onto how stroke and force should be specified [S1]:
User-prescribed time mode fires the pin at a fixed moment, suitable when prior simulation or shop-floor experience has already nailed the window; automatic activation mode fires when the adjacent liquid is isolated from free surfaces and is not connected to another pin by a contiguous liquid path, which avoids two pins fighting over the same feeding volume; and Active Simulation Control mode ties activation to state variables at user-defined locations, replicating the actual PLC logic of the squeeze-pin controller on the die casting machine [S1]. Shibaura Machine's Squeeze Master is a commercial implementation of the same idea, delivering targeted squeeze pressure at the most critical locations and using high-pressure actuation at the end of shot and during intensification to feed shrinkage [S3].
Process Limits, Failure Modes, and What the Pin Cannot Fix
A squeeze pin does not remove gas porosity, it only compensates shrinkage volume, so entrapped air pores will still survive the cycle and need to be addressed by venting, vacuum, or shot-profile changes [S2][S5]. On an AlSi9Cu3 exhaust-pipe part, the no-pin baseline showed two large spheroidal, coalescent macropores in the connection block; with a properly sized 10 mm pin, the post-simulation defect map shifted to a labyrinth of smaller, irregular pores localized on the top of the boss, which is a sign that the stroke was adequate to collapse the bulk macropore but the pin was not given enough force, or was not aligned with the final hot spot, to suppress the residual microporosity [S5].
Two other failure modes show up repeatedly in shop data: pins placed off the thermal center of the boss simply displace the void to one side, and pins fired before the gate freezes produce a flash or a short-shot rather than densification [S1][S2]. For a broader view of how local squeeze-pin pressure relates to global tonnage and intensification choices on the squeeze casting machine, and for a focused write-up on local pressure intensification, see the related coverage on Squeeze Pin Design for Local Pressure Intensification in HPDC and the tonnage-vs-intensification selection spec at Squeeze Casting Tonnage vs Pressure Intensification: Selection Specs.
Selection Criteria: Pin Diameter vs Stroke vs Force vs Timing
The four parameters trade off against each other, and the right combination depends on the boss geometry and the alloy's feeding behavior: diameter is bounded by the available boss footprint, stroke is bounded by the local shrinkage volume, force is bounded by the controller's hydraulic capacity, and timing is bounded by the gate-freeze window and the critical solid fraction [S1][S3][S5]. A common working envelope in published HPDC cases is 10-12 mm pin diameter, a stroke on the order of a few millimeters tuned to the modeled shrinkage volume at the hot spot, an applied pressure above 3× the casting intensification pressure, and activation either automatic (simulation-driven) or PLC-driven to fire inside the feeding window above the critical solid fraction [S1][S3][S5][S6].
For a job-shop foundry choosing between a 10 mm and a 12 mm pin on a similar boss, the 10 mm option is usually preferred when the boss is on a thin (3 mm class) wall section or where the pin must clear an adjacent feature; the 12 mm option is preferred when the boss sits over a thick (15-20 mm class) connection block and the available hydraulic force is the limiting factor, not the geometric clearance [S5][S6].
Verifiable Next Signals for Spec Writing
Two trackable items to pull into a process spec or a simulation brief: the diameter-stroke-force-timing tuple should be quoted with the alloy designation and the critical solid fraction used (for AlSi9Cu3, the published FLOW-3D default is 0.7), and any pin pressure claim should be expressed as a multiple of the machine's intensification pressure, since this is the ratio that Shibaura uses to position the Squeeze Master at "more than 3× that of casting intensification pressure" [S3][S5]. Track also whether the pin is fired in user-prescribed, automatic, or PLC-mapped mode, because the mode dictates which input parameters (time, isolation, or state variable) actually drive activation in simulation and on the floor [S1].
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