Process Deep-Dive · 3D-Printed Sand Molds
Reproducing a 1912 Velie engine block
A complete antique engine block, reverse-engineered from a single corroded original and poured in gray iron — straight from a printed sand mold, with no hard tooling and no surviving pattern.
The process, step by step
From one corroded original to a poured casting
No pattern survived this part, so there was nothing to mold from directly. Here is the full path we took — reverse-engineering the raw casting, printing the mold and cores, and pouring gray iron.
The original is our only reference
We start from a single antique block — corroded, with detail lost to a century of service. There’s no drawing and no pattern, so the first job is to read the part and decide what the raw casting needs to be.
Reverse-engineer the raw casting in CAD
Our 3D engineer rebuilds the casting in SolidWorks — not the finished part, the raw casting: adding draft, restoring lost features, and laying out the gating and risering (shown in red) that feed clean solidification.
Print the sand mold
The drag half is printed directly in resin-bonded sand on a Matthews additive printer. No hard tooling and no pattern-shop time — the mold geometry comes straight off the CAD model.
Set the printed cores
Printed cores form the internal water-jacket passages. With printed sand, near-any internal geometry is possible; the practical limits become sand removal after casting and trapped gas — both designed for up front.
Close the mold
The cope seats onto the drag over the core package. The urethane binder off-gasses moisture, so the closed mold gets a multi-week dry before it sees iron — a step that protects against blows and porosity.
Pour the iron
Induction-melted gray iron, analyzed every heat for carbon and silicon and adjusted before tapping, is poured at temperature through the printed gating. Because the mold is cheap to reprint, we can pour aggressively and iterate.
Shake out & clean
The casting breaks out of the sand still dark with burnt binder and its gating attached. It’s cut free, cleaned, and blasted back to a uniform as-cast gray.
The reproduction, beside the original
A faithful gray-iron reproduction at 1% shrink — every port, boss and water passage carried over from a single 110-year-old casting, with no surviving tooling to work from.
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. This block is Class 30 — strong enough for an engine, fluid enough to render fine as-cast detail.
| 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 — this casting |
| Class 35 | 35,000 | 212 | Higher-strength service |
| Class 40 | 40,000 | 235 | Heavy-duty, wear-resistant |
When 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 Riehle tensile and Brinell hardness testing.
Designed for casting, not just drawn
- Consistent wall thickness — pour temp and solidification are section-modulus dependent; thick sections beside thin ones warp.
- Minimum wall ~1/8″ — thinner risks cold shuts and misruns.
- Draft on vertical faces — so the mold and cores draw clean.
- Break the sharp corners — round small radii (~1/32″) to kill hot spots.
- Allow 1% shrink — every iron casting comes out ~1% smaller than the pattern.
- Gating & risering are ours — we feed solidification — there are no simple rules, so we detail it.
Figures
The full sequence