Process Deep-Dive · 3D-Printed Sand Molds
American LaFrance T-head cylinder block
A T-head cylinder block for an American LaFrance fire engine, modelled in 3D CAD from an original casting and poured in gray iron from printed sand molds — one proving casting first, then a production run of six.
The process, step by step
From an original head to a crate of new ones
The original reached our design partner in January 2022; the production castings shipped in July 2023. In between: measuring the part, modelling the raw casting and the machined casting as two separate models, designing the mold for printing, and assembling it by hand from ten pieces of sand.
The original is the only drawing there is
No pattern survived and no drawing was ever going to turn up. The part itself is the specification, so it went to our design partner, Hunter Mechanical Design, to be measured and modelled before anything else could happen.
Two models, not one
A raw-casting model and a machined-casting model are built separately and reconciled against each other. The machinist approves the machined version; we approve the raw one. Both are signed off before a mold is ordered.
The mold arrives in pieces
Printed by Hoosier Pattern from the mold model, the package turns up as a flat-packed kit: drag, cope, cylinder cores, gating and the riser sections, each one a solid of bonded sand.
Every pocket already in the sand
The drag comes out of the printer with the combustion pockets, port openings and jacket contours finished. None of this geometry has draft, so none of it could be withdrawn from a pattern.
Cylinder cores set by hand
The two cylinder cores stand into the drag on printed locators. The mold face carries its own identification, printed into the sand: O.K. Foundry — Hoosier, T-Head Mold.
Closing over the cores
The cope goes down over the core package. A core that shifts on the way down puts the water jacket wall in the wrong place, and a block cannot be corrected after it is poured.
The cope, face up
The other half of the cavity — the mirror of the drag, printed to the same tolerance in the same machine.
Closed, banded, ready
The finished stack, weighted and banded for pouring.
Out of the sand
The as-cast head at shakeout, deck face up, all six openings formed. The first was poured as a single proving casting and shipped for machining; molds for the balance were ordered once it machined clean.
Six heads, crated
The production run, cleaned, inspected and crated. Six weeks from order to shipment.
Material
Gray iron, to ASTM A48
We pour gray iron to ASTM A48 across the full class range, certified to the same chemistry tolerances we hold for industrial production — a restoration casting is not held to a lower standard than a production one.
| Grade | Min. tensile (psi) | Hardness (HB) | Typical use |
|---|---|---|---|
| Class 20 | 20,000 | 141 (max) | Intricate, low-stress castings |
| Class 25 | 25,000 | 174 | General-purpose work |
| Class 30 | 30,000 | 201 | Engine blocks, housings |
| Class 35 | 35,000 | 212 | Higher-strength service |
| Class 40 | 40,000 | 235 | Heavy-duty, wear-resistant |
Where a part needs steel-like ductility instead, we cast ductile iron to ASTM A536 (60-40-18 through 100-70-03). Every heat is verified by tensile and Brinell hardness testing in house.
Where the difficulty is
- Thin walls — jacket walls run close to the minimum a gray iron section will fill. Too thin misruns or cold shuts; too thick shrinks into itself.
- Delicate cores — the cylinder and jacket cores are slender, unsupported over much of their length, and have to hold position against the metal front.
- Venting — gas generated at the core surface has to leave the mold rather than the casting. Vent paths are designed into the printed package.
- Undercut geometry — the jacket and ports have no draft in any direction, so the cavity cannot be produced by withdrawing a pattern.
- Low volume, rational cost — printing puts a casting of this complexity within reach at a quantity that would not otherwise support making it at all.
- Repeatable later — the mold model is a file, so a further run does not need the original part back.
Figures
The full sequence