Ductile iron castings, QT400 to QT700, sand cast without a pattern

Three ferrous options land on the same foundry floor. Picking between them is a design decision, not a price decision, so it is worth being precise about what separates them.

Against grey iron. Grey iron is cheaper and damps vibration better, and for a rigid machine base it is still the right answer. What it does not have is elongation. Its flake graphite acts as an internal crack, so it fails without warning; ductile iron's nodules blunt the crack tip and give you 2% to 18% elongation before fracture, plus roughly double the tensile strength. Anything that sees impact, fatigue or a pressure boundary should not be grey iron.

Against cast steel. Steel is stronger and weldable. But steel shrinks far more on solidifying and has no graphite expansion to offset it, so it needs much heavier feeders and a bigger yield loss — and it pours about 100 °C hotter, which is harder on the mould. Ductile iron's graphite expansion during solidification partly feeds the shrinkage, so the rigging is lighter, the yield is better, and it machines considerably more freely. For a thick-walled housing or a pump body, ductile iron usually lands cheaper than the steel equivalent.

Against a printed sand mould in another alloy. The mould-making side is identical across all of them — the same printed sand, the same no-pattern economics. So choose the alloy on the duty, not on the supply route.

A conventional ductile iron job starts with a pattern. Someone cuts a wooden or metal copy of your part, and that pattern is what sets your lead time and your minimum order quantity. It is also the reason a single casting is normally uneconomic.

We delete that step. Your CAD model is printed directly as a sand mould and as sand cores on a binder-jetting sand printer, using furan resin at 1.5–2.5% by weight. The mould arrives at the pouring floor in days rather than weeks, and the mould-making cycle drops by about 70% against the traditional route. Because no pattern exists, there is nothing to amortise: one casting costs what one casting costs.

1. 01 · Print

   Mould and cores printed from CAD

   The outer mould and every internal core are printed in one setup. No pattern, no core box, no draft angle to design around.

2. 02 · Assemble

   Cores set as one assembly

   Printed cores are glued into the mould as a single assembly, so internal passages and water jackets have no joint line and no core print to leak past.

3. 03 · Pour and finish

   Poured, knocked out, machined

   Iron is poured into the mould under gravity, then the casting is knocked out, shot blasted, heat treated and machined to the faces that need it.

Ductile iron is poured on our own iron and steel line, not bought in and resold. Composition, heat treatment and inspection all stay inside the same iron foundry, and the records ship with the casting.

• Melting and composition. Medium-frequency induction melting, with a spectrograph sample taken from every heat before it is poured. The magnesium treatment that turns flake graphite into nodules fades with time, so the window between treatment and pour is kept short — this is the step that decides whether the casting is ductile iron at all.
• Mould and cores from our own printers. The printed sand mould and every core come off the binder-jetting printers on the same site, so the mould does not travel between suppliers before it is filled. Gravity pouring on the iron line, to 2 t per pour.
• Heat treatment on site. Stress relief, normalising, and austempering where an ADI grade is specified — in our own furnace, not at a subcontractor. A hardness or matrix specification is met before the part is machined, not after.
• Testing and records. Tensile and hardness testing per heat, dimensional inspection on a coordinate measuring machine, and non-destructive testing before shipping. The heat number stays with the casting so composition and process records can be traced back, and they are exported with the part.
• Scale. Across the group our iron foundry pours roughly 4,000 tonnes of iron a year. Ductile iron work is quoted from a few kilograms up to 2 t per casting, with no pattern cost at any point in that range.

A ductile iron casting straight out of the mould is rarely the part you install. The as-cast matrix depends on how the section cooled, and the dimensional result depends on residual stress that will move when you cut it. So the chain does not stop at knock-out.

• Heat treatment

  Stress relief before machining so faces stay where you cut them. Normalising where a pearlitic grade or a consistent hardness is specified. Austempering for ADI, where the duty needs wear resistance and toughness together.

• Machining

  CNC machining from 3-axis to 5-axis in the same chain, so a face that has to be flat, concentric or dowelled is cut after the stress relief rather than before it. Ductile iron machines freely, which is one of the reasons it undercuts steel on total part cost.

• Surface treatment

  Shot blasting to clean the as-cast skin, plus the coating or painting your drawing calls for. As-cast surfaces come off a printed mould at around Ra 12.5–25 µm, noticeably better than a traditional sand surface because the mould itself is printed rather than rammed.

• Inspection

  Nine inspection methods in house, including CMM and 3D scanning, plus non-destructive testing and a chemical analysis on the heat. You get the dimensional report and the material certificate with the parts, not on request afterwards.

We build the machines as well as run them, so the equipment on our own iron line is the equipment we sell to foundries. Two models cover the ductile iron work: one for moulds and cores up to about a metre, one for the large housings.

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  3DPTEK-J1800

  The workhorse. Covers most ductile iron moulds and cores, and every one-off pattern you would otherwise have to make.

  Full specifications and case work →

• 

  3DPTEK-J4000

  Large format. For the moulds behind oversized iron castings, printed in one piece instead of assembled from sections.

  Full specifications and case work →

It is more useful to know where a supplier stops than to read a list of what they claim. These are the cases where we are the wrong answer, and we will say so in the first reply rather than after a week of quoting.

• We do not die cast

  High-pressure die casting is a different process with a different cost structure, and it is the right answer above roughly 10,000 parts a year. We do not run it, and we will not quote a die-cast part as if sand casting were equivalent.

• We are not the cheapest route at high annual volume

  A printed mould is economic because there is no pattern. Once a pattern or a die pays back over tens of thousands of parts a year, a traditional foundry or a die caster will beat us on piece price. Our advantage is the first part, not the hundred-thousandth.

• We do not hold walls under about 4 mm in iron

  Sand casting has a minimum practical wall, and iron is less forgiving than aluminium. If your geometry is thin-walled and intricate, investment casting or a machined part is the better route and we will point you at it.

• We do not pour titanium or superalloys as iron

  Titanium, Inconel and the nickel superalloys are real capabilities here, but they run through investment casting or metal 3D printing, not through the iron line. Ask for the right route rather than the same one.

• What is the difference between ductile iron and cast iron?

  "Cast iron" usually means grey iron, where the graphite forms flakes. Ductile iron treats the melt with magnesium so the graphite forms nodules instead. That one change roughly doubles tensile strength and gives the material real elongation, so it bends instead of cracking. Both are still sand cast.

• What does QT400-18 mean?

  It is a GB/T 1348 grade. QT stands for the Chinese term for ductile iron, 400 is minimum tensile strength in MPa, and 18 is minimum elongation in percent. QT400-18 is a ferritic, tough grade. Its nearest ASTM A536 equivalent is 60-40-18.

• Can you cast ductile iron without a pattern?

  Yes, and that is the whole point of this service. The sand mould and the cores are printed directly from your CAD file, so no pattern is made. There is nothing to amortise, which is what makes a single casting economic.

• What is the largest ductile iron casting you can make?

  2 t for a single pour, in an envelope up to 3 m on the largest dimension. If your part is larger, send it anyway and we will tell you whether it splits into sections or goes elsewhere.

• How long does a ductile iron casting take?

  It depends on the part, but the mould-making cycle runs about 70% shorter than the traditional route because there is no pattern. The QT400 engine block case above went from drawing to five finished castings in 45 days.

• Which ductile iron grade should I specify?

  Specify by duty rather than by the biggest number. QT400-18 for impact and low temperature, QT450-10 for general pressure-retaining bodies, QT500-7 and QT600-3 where load and wear dominate. Send us the duty and we will say which one we would pour, and why.

• Do you do ADI?

  Yes, austempered ductile iron is available. It needs a specific austempering treatment after casting rather than a simple normalise, so tell us the wear and toughness requirement on the drawing and we will confirm the grade and the treatment route with the quote.

• Is ductile iron machinable?

  Very. It machines considerably more freely than cast steel because the graphite acts as a lubricant at the cutting edge. That is a large part of why the total part cost is often lower than the steel equivalent, even when the raw casting is not.

• What tolerance can you hold on a ductile iron casting?

  CT8 to CT12 as cast, to ISO 8062, with an as-cast surface around Ra 12.5 to 25 µm. Anything tighter is achieved by machining, which we do in the same chain after stress relief.

• Is there a minimum order quantity?

  One piece. There is no pattern cost to spread across a batch, so a single casting is a legitimate order. Serial production is welcome too, but it is not what the process is optimised for.