Sand Casting

Part of Metalworking

Casting metal objects using sand molds for repeatable production of tools and parts.

Why This Matters

Forging produces excellent blades and tools, but some shapes are impractical or impossible to forge — pot shapes, pipe fittings, gear blanks, machine parts with internal cavities, decorative items, and any object needed in identical multiples. Sand casting solves all of these problems using the most abundant material on Earth: sand.

Unlike lost-wax casting, sand casting preserves the pattern. You carve or shape a wooden pattern once and use it to make hundreds of identical molds. This makes sand casting the production method of choice for a rebuilding civilization — it bridges the gap between one-off craft and repeatable manufacturing. Every industrial revolution in history has relied heavily on sand casting for producing machine components.

The equipment is simple: wooden frames (flasks), sand mixed with clay binder, a pattern, a crucible, and a furnace hot enough to melt your chosen metal. With these, you can produce anything from simple ingot molds to complex multi-part castings with internal passages.

Sand Preparation

Green Sand

The standard casting sand is “green sand” — not green in color but meaning unbaked, moist sand held together by clay binder. The clay gives the sand enough cohesion to hold a shape while remaining permeable enough to let gases escape during the pour.

Basic green sand recipe:

• 85–90% silica sand (sharp, angular grains preferred — not rounded beach sand)
• 8–12% bentonite clay (or any clay with good binding properties)
• 3–5% water (by weight)

Preparation:

1. Screen the sand through a mesh (2–3 mm) to remove stones and debris
2. Mix dry sand and dry, powdered clay thoroughly
3. Add water gradually while mixing — use a shovel or work by hand
4. Test by squeezing a handful: it should hold its shape without crumbling, break cleanly when snapped in half, and not feel wet or sticky

The Squeeze Test

Squeeze a ball of sand in your fist. It should:

• Hold together when you open your hand (enough clay + moisture)
• Break cleanly into two pieces when snapped (not crumbly = good moisture)
• Not stick to your skin (not too wet)
• Show finger detail on the surface (fine enough grain)

If it crumbles: add water or clay. If it sticks: too wet, add dry sand. If it won’t show detail: sand grain too coarse.

Sand Recycling

Used casting sand can be reused almost indefinitely. After each casting:

1. Break up the mold sand
2. Screen out metal fragments and burnt lumite
3. Add fresh clay (1–2% of volume) and water to replace what was lost
4. Re-mix thoroughly

Over time, repeated heating degrades the clay binder. If the sand stops holding shapes well despite adequate moisture, replace 20–30% of the sand with fresh green sand.

Flask Construction

The flask is the frame that holds the sand mold. It consists of two halves:

• Cope (top half)
• Drag (bottom half)

Building a Flask

Materials: hardwood boards (25 mm thick), nails or screws, alignment pins.

1. Build two identical rectangular frames — no top or bottom, just four sides
2. Size: large enough to leave 50–75 mm of sand on all sides of your largest pattern
3. A good general-purpose flask: 400 mm × 300 mm × 100 mm deep (each half)
4. Install two alignment pins on the drag and matching holes on the cope — these ensure the halves align perfectly every time

Multiple Flask Sizes

Build 2–3 flask sizes. A small flask (200 × 150 mm) for buckles and small fittings uses far less sand and is easier to handle than a large flask. Match flask size to pattern size.

The Molding Process

Step 1: Prepare the Drag

1. Place the drag (bottom frame) upside-down on a flat board (the moldboard)
2. Position the pattern (or half-pattern for split patterns) face-down on the moldboard, centered in the flask
3. Dust the pattern and moldboard with parting powder — fine dry clay, talcum, or wood ash. This prevents sand from sticking
4. Sift a thin layer (10–15 mm) of fine-screened sand (facing sand) over the pattern. Pack gently around all details with your fingers
5. Fill the rest of the drag with green sand in layers, ramming each layer firmly with a flat-ended stick or ramming tool
6. Strike off excess sand level with the flask top using a straight edge
7. Place a bottom board on top and flip the entire assembly over

Step 2: Prepare the Cope

1. Remove the moldboard, exposing the pattern sitting in the compacted drag sand
2. Apply parting powder to the exposed sand surface (the parting line)
3. Place the cope frame on top, aligned by the pins
4. Insert the sprue pin (a tapered wooden dowel, 15–20 mm diameter) into the sand next to the pattern — this creates the channel for pouring metal
5. Insert a riser pin (similar to sprue) at the far end of the pattern — this allows gas to escape and acts as a reservoir
6. Pack the cope with green sand as you did the drag — facing sand first, then bulk sand, ram firmly
7. Strike off level

Step 3: Separate and Finish

1. Remove the sprue and riser pins by twisting gently and pulling straight up
2. Lift the cope straight up off the drag — the two halves should separate cleanly at the parting line
3. Remove the pattern from the drag. Tap it gently to release it from the sand. If it sticks, insert a small screw in the pattern surface and pull straight up
4. Cut the pouring basin — a funnel shape at the top of the sprue hole in the cope
5. Cut gates — channels connecting the sprue to the mold cavity, if they are not already formed by the pattern
6. Repair any damage to the mold cavity walls with a small trowel or spoon. Smooth surfaces that will become the casting surface
7. Reassemble — place the cope back on the drag, aligned by pins

Pattern Draft

Pattern sides must taper slightly outward (draft angle of 1–3°) so the pattern can be removed without tearing the sand walls. A straight-sided pattern will damage the mold every time. Add draft when carving your wooden patterns.

Pouring the Metal

Pre-Pour Checklist

• Mold assembled and clamped or weighted (place heavy stones or metal on top of the cope to prevent the cope from lifting when metal enters)
• Crucible at pouring temperature (50–100°C above melting point)
• Pouring area clear of moisture — any water on tools, sand surface, or ground can cause explosive steam
• Vent holes poked through the cope sand with a thin wire (1–2 mm) to help trapped gas escape
• Safety gear: leather gloves, face shield, long sleeves, closed shoes

Pouring Procedure

1. Remove crucible from furnace with tongs or a lifting shank
2. Skim slag from the metal surface with a dry steel rod
3. Pour steadily into the sprue basin, keeping the basin full throughout the pour. Never let the basin empty — air drawn into the mold creates defects
4. Continue until metal appears at the riser hole — the mold is full
5. Let the metal solidify undisturbed. Minimum times:

Metal	Minimum Cooling Time
Aluminum	5–10 minutes
Bronze/brass	15–30 minutes
Cast iron	30–60 minutes
Steel	45–90 minutes

Breaking Out and Finishing

1. Remove clamps/weights from the cope
2. Separate cope from drag and dump the sand out. The casting should be visible, still attached to its sprue and riser
3. Let it cool further if still red — handling red-hot castings is unnecessary risk
4. Cut off the sprue and riser with a hacksaw, chisel, or angle grinder
5. Remove parting line flash — the thin fin of metal that seeps into the parting line. File or grind it smooth
6. Remove sand from the surface with a wire brush
7. Inspect for defects (see table below)

Common Casting Defects

Defect	Description	Cause	Prevention
Misrun	Mold not fully filled	Metal too cold, gates too small	Higher pour temp, larger gates
Cold shut	Visible seam where metal fronts met	Metal too cold, poor gating	Pour faster, redesign gates
Blow holes	Round holes in or below surface	Moisture in sand, inadequate venting	Dry sand properly, add vent holes
Sand inclusions	Sand particles embedded in surface	Loose sand fell into mold	Pack more firmly, use facing sand
Shrinkage	Concave surface or internal void	Thick sections cool and shrink	Add risers at thick sections
Hot tears	Cracks in casting	Casting cannot shrink freely	Add fillets to sharp corners, looser sand
Rat tail	Irregular groove on surface	Sand expansion cracking	Add sea coal or wood flour to sand (3%)

Core Making (Internal Cavities)

To cast hollow objects (pipe fittings, vessels, cylinders):

1. Make a core — a sand shape representing the internal cavity
2. Core sand recipe: Same as green sand but with higher binder content. Add 2–3% linseed oil or flour paste, then bake the core at 200°C until hard
3. Place the core in the mold cavity, supported by core prints — extensions on the pattern that create recesses in the mold to hold the core in position
4. Pour normally — metal flows around the core, forming the outer shape. After cooling, break out the core sand from inside

Core Supports

For long cores that might sag or shift during pouring, insert chaplets — small metal pins or crosses that support the core. They become embedded in the casting and must be the same metal as the pour to avoid weak spots.

Pattern Design Principles

Good casting starts with good pattern design:

1. Uniform wall thickness — thick sections shrink more than thin sections, causing internal stress and cracks. Keep walls as uniform as possible.
2. Fillets at all inside corners — sharp inside corners concentrate stress and cause hot tears. Round them with a radius of at least 3 mm.
3. Draft on all vertical surfaces — 1–3° taper allows clean pattern removal.
4. Shrinkage allowance — metal shrinks as it cools. Make patterns oversized by the shrinkage rate of your metal:

Metal	Shrinkage Rate
Aluminum	1.3% (13 mm per meter)
Bronze	1.5% (15 mm per meter)
Brass	1.5% (15 mm per meter)
Cast iron	1.0% (10 mm per meter)
Steel	2.0% (20 mm per meter)

5. Machine allowance — if the casting will be filed, ground, or turned on a lathe to final dimensions, add 2–3 mm extra material on machined surfaces.