Casting Process: 5 Proven Steps From Melt to Part

• 1
  Key takeaways
• 2
  What the casting process actually does
• 3
  Choosing the alloy before the casting process starts
• 4
  Design rules that keep the casting process stable
• 5
  When the casting process beats CNC machining
• 6
  Checking a casting before it becomes a part
• 7
  Step by step: running a casting job from drawing to part
• 8
  Casting process methods compared

Casting is a liquid metal forming method. You pour molten alloy into a cavity that matches the outside shape of the part, let it freeze, and pull out a blank. The blank is usually not the finished part. It is the near-net shape that a machinist then trims to size, drills, and threads.

The whole route runs in one direction: liquid metal, mold filling, solidification with shrinkage, then the casting itself. Each arrow is a place where the part can fail. Gas trapped during filling becomes porosity. Feeding that stops too early becomes shrinkage voids. A die that is too cold freezes the metal before it reaches the thin sections.

That is why castings are specified with allowances, not just dimensions. You give the foundry a part drawing plus a machining allowance, a draft angle, and a shrinkage factor. The foundry builds the tool from that, and the CNC shop later removes the allowance. Both sides need the same numbers on the print, or the blank arrives 0.8 mm too small on one face.

For low volume work we often skip hard tooling. Die casting needs a steel die and pays off over thousands of parts. Vacuum casting uses a silicone mold and suits 20 to 200 parts. Sand casting handles large, heavy shapes. The casting process itself is the same in all three cases.

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  Near-net, not netCasting holds roughly ±0.1 to ±0.5 mm on as-cast features, depending on alloy and size.
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  Wall thickness mattersKeep sections between 2 mm and 6 mm where possible. Thick-to-thin transitions cause hot tears.
• 3
  One drawing, two shopsMark datum faces and machining stock so the CNC setup matches the casting.

Alloy choice fixes shrinkage, fluidity, and the corrosion you will live with for years. Aluminum ADC12 flows well and is the default for die casting enclosures and housings. It machines cleanly and takes bead blasting or anodizing. Aluminium 6061 and 7075 are common when the part is cast as a near-net blank and then machined heavily.

Zinc alloys freeze fast, hold fine detail, and cast thin walls down to about 0.8 mm. They are heavier than aluminum and less corrosion resistant without plating. Magnesium AZ91D is the lightest common option for housings, but it needs careful handling because chips and dust burn.

Steel and stainless castings are used where strength or wear resistance matters. Stainless 316 and 17-4PH appear in pump bodies and valve parts. Copper alloys such as C36000 and beryllium copper show up in electrical contacts and busbars because of conductivity. Titanium TC4 (Ti-6Al-4V) is cast for aerospace brackets, but the tooling cost is high and the process window is narrow.

Pick the alloy from the function first, then check whether it casts. A part that must be Ra 0.8–1.6 μm on a sealing face will be machined there anyway. The casting only needs to deliver sound metal and the right bulk shape.

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  FluidityAluminum and zinc fill thin walls best. Steel needs thicker sections and hotter molds.
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  Shrinkage rateAluminum about 1.0–1.3%, zinc about 0.4–0.7%, steel up to 2.5%.
• 3
  MachinabilityADC12, 6061, and C36000 cut fast. Titanium and Inconel cut slow and wear tools.

Draft is the first rule. A vertical wall with zero draft will drag, score the die, and tear the part on ejection. Give outside walls 1–2°, inside walls 2–3°, and more on deep pockets. Text and logos on a vertical face need 5° or they will not release cleanly.

Radius every inside corner. Sharp internal corners concentrate stress and block metal flow. A fillet of at least half the wall thickness spreads the load and lets the alloy reach the corner before it freezes. Outside corners can be smaller, roughly one quarter of the wall thickness.

Keep wall sections uniform. A 3 mm wall feeding a 10 mm boss creates a hot spot that shrinks last and pulls a void. Core out thick bosses or step the thickness down over 3 to 5 mm of length. Ribs should be 50–70% of the nominal wall and no taller than five times their thickness.

Plan the parting line early. It decides where flash appears and which faces can be flat. If a cosmetic surface crosses the parting line, expect a visible witness line. Move that surface to a machined face or accept the line in the drawing.

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  DraftOutside 1–2°, inside 2–3°, text faces 5°.
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  FilletsInside radius ≥ half wall thickness. Outside radius about one quarter.
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  Ribs50–70% of wall thickness, height ≤ 5× thickness.
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  BossesCore them out or blend them into the wall with a taper.

Casting wins when the shape is complex and the volume is high. An engine housing with internal ribs, bosses, and cooling passages is expensive to cut from solid and cheap to cast once the die exists. Above roughly 5,000 parts a year, die casting usually pays back the tool cost and then drops the piece price.

CNC machining wins when the tolerance is tight, the volume is low, or the material is hard to cast. A bracket needed in three days, held to ±0.005 mm, is a machining job. Casting it would need tooling, a trial shot, and a second setup for the critical faces. The casting process adds days before the first chip is cut.

A mixed route often makes sense. Cast the bulk shape, then machine the sealing faces, bores, and threads. This keeps the tool simple and puts precision only where it is needed. We run this pattern often: the foundry delivers a blank with 0.5–3 mm stock, and our 3-axis and 5-axis machines finish it to ±0.005 mm and Ra 0.8–1.6 μm.

One caution. Cast skin is not as strong as wrought metal and can hide subsurface porosity. If a face carries a fatigue load, machine through the skin or specify a higher grade of casting with radiographic inspection.

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  Choose castingComplex geometry, volumes above a few thousand, thick sections, internal passages.
• 2
  Choose machiningTight tolerance, low volume, fast turnaround, wrought material properties.
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  Choose bothCast the blank, machine only the critical faces and bores.

Incoming inspection should look at more than dimensions. Check for cold shuts, visible porosity, flash, and incomplete fill on thin ribs. Tap the part lightly and listen; a dull thud often means an internal void. X-ray or dye penetrant finds what the eye cannot.

Measure the as-cast features at the datum faces first. If the first face is off by 0.5 mm, every downstream cut shifts with it. On parts we machine from castings, we check the raw material, monitor in process, and inspect 100% before shipment. Reports are available on request.

Keep one sample from each lot. When a later part fails a fit check, the retained sample tells you whether the casting moved or the machining drifted. That one habit saves days of argument.

Casting suppliers working to ISO 9001:2015 will have a traceable heat number and a dimensional report. Automotive and medical programs add IATF 16949:2016 and ISO 13485:2016 requirements on top, which tighten process control and documentation.

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  VisualCold shuts, flash, short fill, surface porosity on machined faces.
• 2
  DimensionalDatum faces first, then as-cast features, then wall thickness.
• 3
  SubsurfaceX-ray, dye penetrant, or ultrasonic where the part sees load or pressure.