Designing for Stainless Steel Investment Casting: Guidelines for Engineers

1. Your part passes through four materials, not one

The habit that helps most is to stop thinking of the casting as a piece of steel and to follow the geometry through the process. Your part is first a wax injected into an aluminium die and ejected from it. It is then a cavity inside a ceramic shell that has to be built up by repeated dipping and must survive dewaxing and firing. It becomes liquid stainless steel that has to reach every corner before it freezes, and then a solid that shrinks about two percent while the shell resists. Finally it is cut from a tree, ground, solution annealed, water quenched, blasted and machined. Each stage has its own constraints and every feature on the drawing has to be feasible for all of them.

2. Wall thickness: thin is possible, uniform is not the whole answer

2.1 How thin

Stainless steel investment castings can be made with walls of about 1.5 mm over short distances and 2.5 to 3 mm as a general design value. Below this the metal freezes before the cavity fills, particularly where a thin wall is far from the gate or is fed only through another thin section. Thin walls cast best when they are attached to a thicker section that supplies metal as the thin wall freezes. The Steel Founders' Society of America (SFSA) gives the same picture: walls of 1.5 mm are common for steel investment castings, with sections tapering to about 0.8 mm achievable and 2 mm as the practical minimum section, against a 6 mm design minimum for conventional sand casting. For a given thickness steel also runs better in a narrow web than in a wide one, so a thin wall that must be long should be kept narrow or broken up by a rib.

2.2 Why "uniform walls" is only half a rule

Every casting design guide says to keep wall thickness uniform, and as far as it goes that is right. The catch is what happens where two walls meet. The classical junction studies give the picture: a thin rib meeting a wall acts as a cooling fin; a rib of half the wall thickness is roughly thermally neutral; a rib equal to the wall thickness creates a hot spot at the junction; and only when the appendage is twice the wall thickness does the balance return, with the wall now chilling the junction. The pointed consequence is that castings of uniform wall thickness necessarily contain 1:1 T-junctions that are hot spots. The foundry-side rule says the same thing: the increase in mass at a joint is estimated by inscribing circles in the section and rises with the square of the ratio of the circle diameters, so ribs and webs should be thinner than the walls they join (but not so thin that they act as cooling fins), and ribs should be staggered so that X-junctions never form.

A hot spot freezes after its surroundings, so the last liquid at the junction shrinks with nothing to feed it and leaves porosity on the inside corner. The practical responses, in order of preference, are: remove metal from the junction with a cored recess or dimple on the far side so that the local mass is no larger than the wall; use a fillet radius on the inside corner but do not add a matching build-up of material on the outside; or accept the junction and place it where the foundry can put a gate or feeder on it. Tell the foundry which option you prefer.

2.3 Section changes and directional solidification

Where sections must change, blend them with a taper rather than a step, and arrange the thicker sections so they lead towards a place where a gate can sit. The foundry's aim is directional solidification: the casting freezes progressively towards the feed metal so that every region that is still shrinking has liquid available. The classical check is to inscribe circles in the section and confirm that their diameters increase progressively towards the feeder, refined in three dimensions to a progressive increase in modulus, the ratio of volume to cooling surface area. A design with a heavy region at each end of a thin span has made this impossible, and the foundry will either gate both ends or ask for a change.

2.4 Isolated heavy sections

A boss, lug or flange that is much thicker than the wall carrying it is an isolated heavy section. It freezes last, cannot be fed through the thin wall, and will contain porosity unless it is gated directly. Keep flanges no thicker than the thread engagement needs, core heavy bosses from the back where function allows, and ask whether a thick section is really carrying load or is a legacy of a machined-from-solid design. Two practical rules follow: a boss or pad should preferably be thinner than the section it sits on, just tall enough for the cutter to clear the wall, and several bosses on one face are better replaced by a single panel or continuous rib of uniform height, which also lets hole positions move later without a die change.

3. Corners, fillets and ribs

Sharp internal corners are bad for every stage: they concentrate stress in the wax during ejection, the shell cracks at them during dewaxing, and the casting tears at them during cooling. The classical rules for preventing hot tears by design read almost like a design standard on their own: no sharp re-entrant corners, no straight member joining two potential hot spots (curve it), curved gates so that deformation can be accommodated, and angled or offset stiffeners and ribs so that the casting can accommodate strain. The point about ribs is often missed. A grid of ribs that meet at right angles in a plane locks the casting against its own contraction; staggering the ribs, or letting them meet the wall at an angle, gives the structure room to move.

Internal radii of 1 to 2 mm and external radii of 0.5 to 1 mm are typical design values for stainless investment castings. A common recommendation is an external corner radius of 0.1 to 0.2 times the section thickness to avoid the steep thermal gradients that crack sharp corners, and for a change of section either a 15 degree taper or a generous radius, with the change made on one side of the thinner member rather than symmetrically about its centre line. Wave construction, curved or slightly waved members such as the spokes of a wheel, is a standard way of letting a casting relieve its own cooling stresses; it is the practical form of the rule to curve any member joining two hot spots. Larger radii are generally better for soundness, with the caveat in section 2.2 that a large fillet that adds mass to a junction can recreate the hot spot it was meant to soften. Standard cast grades such as CF8M and CF3M freeze with some delta ferrite and tolerate constraint reasonably well; fully austenitic grades do not, and for those the rules above are mandatory.

4. Holes, bores, slots and cores

4.1 What a hole costs

A hole in an investment casting is formed one of three ways. If it can be pulled straight out of the die it is a metal core pin, essentially free. If it has undercuts or curves it needs a soluble wax insert, dissolved out of the pattern before shelling, or a preformed ceramic core that is placed in the die and stays through pouring, then is leached or broken out of the casting. Each is a step up in cost and in dimensional risk. Long, thin ceramic cores deflect under the metal and are the main cause of wall thickness variation in valve bodies and manifolds.

Working limits for stainless: blind holes to about one to one and a half times the diameter in depth, through holes to about three times the diameter for small bores, and holes below about 2 mm diameter drilled rather than cast. A cored passage needs an exit large enough to remove the core and, for leached cores, for the caustic to reach it. If a passage has no exit, or its exit is smaller than the core behind it, the drawing needs to change. The same logic applies to sand cores in steel foundry practice: the minimum core diameter rises with the length of the core and with the thickness of metal around it, small openings for core removal drive cleaning cost up sharply, and applications needing smaller cores than normal practice should be discussed with the foundry. Ceramic cores in a shell are stiffer than sand cores but the geometry logic is identical.

4.2 Slots and pockets: think of the shell

The shell is built by dipping. Slurry has to flow into every recess and drain back out, and stucco has to reach the bottom of every pocket. A narrow deep slot fills with shell only partially and the shell that does form there is weak; it may break during dewax and leave a ceramic inclusion or a fin of positive metal in the casting. Slots narrower than about 2 mm, or deeper than three or four times their width, should be reviewed with the foundry. Widening the slot, adding a draft on its walls, or machining it afterwards are the usual answers.

4.3 Undercuts

Undercuts are possible, which is one reason to choose the process, but never free: external undercuts need slides or loose pieces in the die, internal ones need soluble or ceramic cores. Before accepting one, ask whether the feature can be re-oriented to pull with the main die direction.

5. Draft, parting line and the die

Draft on the casting is not needed, since the shell is destroyed to release the part. A nominal 0.5 degree on deep features helps the wax eject cleanly and reduces die wear, but zero draft can be held where a feature demands it. One trap: ISO 8062-3:2023 Table 6 defines default draft angles for investment castings (grade A external draft of 0.8 degree for a feature 16 to 25 mm high, 0.5 degree from 25 to 63 mm, 0.3 degree from 63 to 100 mm and 0.2 degree above that), so a drawing that calls up the general tolerances of ISO 8062-3 without stating "no draft" or "draft grade A" has, in principle, allowed the foundry that taper on every feature. State the draft requirement explicitly. The die still has a parting line, and features that lie across it inherit its mismatch and flash. Place critical dimensions within one die half where you can, and tell the foundry which faces are cosmetically important so that the parting line and gate can be kept off them. The parting line is also where the wax pattern can be handled and where the wax is most likely to distort, which matters for flat and thin parts (section 7).

A part that pulls in two directions with no cores has the cheapest die; each slide, loose piece or core adds cost and a source of variation. For prototypes, printed wax or resin patterns avoid the die at the expense of surface and dimensional control; the shell and metal rules still apply.

6. Give the foundry somewhere to put the gate

Metal enters the casting through one or more ingates, which are cut off and ground flush after casting. The gate has two jobs: it fills the cavity and, in investment casting, it usually acts as the feeder as well. The junction analysis above gives the rule of thumb: a feeder joining the casting at a T needs a modulus about twice that of the section it feeds, which moves the hot spot out of the casting and into the feeder where it does no harm. In practice this means the gate must attach to the thickest section, and that section must be accessible from outside the part.

How far a gate can feed is limited. The classical feeding distance rule for steel plates, that a feeder makes a plate sound for a distance of about 4.5 times the section thickness from its edge, comes from work on carbon steel in greensand at sections of 50 to 200 mm. The data are process-specific, and the definition of feeding distance depends on how much porosity you can detect and tolerate. Do not apply the number to a 4 mm stainless wall in a hot ceramic shell. The reason is that in a cold sand mould an unfed steel section will almost certainly be porous, whereas in an investment casting poured into a hot shell with clean metal the section can feed itself by plastic collapse (so-called "solid feeding") and end up sound without a feed path. Feeding distances in a hot shell are therefore longer and less well defined than the sand casting data suggest, which is a reason to talk to the foundry rather than to design to a formula. Do take the principle: a long thin wall with heavy sections at both ends needs a gate at each end, and each gate leaves a witness.

On the drawing, identify the faces where a gate witness is not acceptable and leave at least one thick, accessible face where it is. Do not put a cosmetic or sealing face on every thick section. A gate-free finish everywhere means the witness is machined off, which you should budget for.

7. Flat, thin and long: the geometry that moves

Three stages each add distortion. The wax pattern is soft, and a large flat plate or a thin ring sags or twists between the die and the tree unless it is jigged. The casting contracts inside a shell that resists it, so a thin section between two heavy ones is stretched, and asymmetric sections bow. Austenitic and duplex stainless castings are then solution annealed and water quenched, which moves flat parts again. Straightening after heat treatment is normal practice, and the specification should allow for it rather than treat it as a deviation.

Design responses: avoid large unsupported flat areas or add a shallow rib or a slight crown; keep the section symmetric about its neutral plane; specify flatness on a machined face rather than an as-cast one; and where a thin ring has to hold roundness, consider a temporary cast-in tie bar or web that is machined out afterwards. Agreed location points for dimensional checking and machining, fixed before tooling is cut, are where all of this is settled between the designer, the foundry and the machinist.

8. Datums, tolerances and machining allowance

8.1 Datums that exist on the casting

A familiar failure is a drawing dimensioned with perfect logic that is nearly unmanufacturable as a result: the datum sits at one end of a long part whose length varies with the process, the only critical feature (a boss) is at the other end, and the datum is defined on a row of machined holes that do not exist when the casting is first inspected. The remedies are simple. Choose three orthogonal datum planes that exist on the as-cast part. Put the primary datum at, or as close as possible to, the feature whose position matters most, so that process variation elsewhere does not accumulate onto it. Where the casting is long, a datum near the middle halves the effect of length variation at the ends. Then dimension everything from those datums, including the machined features, so that the machinist and the foundry are measuring the same part in the same way. Steel foundry drafting practice calls these tooling or target points, recommends that they be marked on the drawing and used identically for the foundry layout and the customer's inspection, and notes that raised datum targets can be cast onto the part for fixturing, provided they are placed where they will not be ground away during cleaning or fettling. The same practice prefers a single combined casting and machining drawing, with cast contours in solid lines and machined outlines in broken lines, so the foundry can see where the stock is and where the gates and datums may sit.

8.2 Tolerances

General dimensional tolerances for investment castings are specified in ISO 8062-3:2023 as dimensional casting tolerance grades (DCTG). The standard's own recommendation for investment castings is DCTG 4 to 6 for parts up to 100 mm in largest dimension, 4 to 8 from 100 to 400 mm and 4 to 9 above 400 mm (Table A.1, note a), with geometrical grades GCTG 4 to 6 for the smallest size band on the same basis (Table A.3). In millimetres, Table 7 gives a total tolerance on a 16 to 25 mm dimension of 0.30 mm at DCTG 4, 0.42 mm at DCTG 5 and 0.58 mm at DCTG 6, disposed symmetrically by default, so roughly ±0.15 to ±0.3 mm on a 25 mm feature; on a 100 mm feature the same grades give 0.40, 0.56 and 0.78 mm total. Measured capability data point the same way: SFSA's long-series model for steel investment castings gives a total tolerance that half of producers can hold of about 0.21 mm plus 0.005 mm per millimetre of feature length plus 0.012 mm per kilogram of casting weight, and roughly 0.72 mm plus the same terms for 90 percent conformance. Tighter figures, down to ±0.1 mm on small features, are what the best foundries hold on developed tooling, not a design assumption. Three further points from the standard: wall thickness is toleranced one grade coarser than the general DCTG unless the drawing says otherwise (clause 9); the standard adds no separate parting line allowance, since surface mismatch is deemed to be contained within the Table 7 tolerance and within the form tolerances for straightness, flatness and roundness (clause 8), which in practice means dimensions across the parting line consume more of their tolerance than dimensions within one die half; and geometric tolerances (flatness, concentricity, position) should be quoted separately and only where needed. The single most useful thing an engineer can do at this stage is to tolerance the few features that matter tightly and everything else to the general grade. A drawing with ±0.05 mm everywhere either gets quoted as a machined part or does not get quoted.

8.3 Machining allowance and surface finish

Any feature tighter than the process capability, any sealing face and any bore that must be round should be shown as a machined feature with an allowance of 0.5 to 1.5 mm depending on size. ISO 8062-3 assigns investment castings required machining allowance grade E, which Table 12 turns into 0.4 mm for parts up to 63 mm largest dimension, 0.7 mm from 63 to 100 mm, 1.1 mm from 100 to 160 mm and 1.4 mm from 160 to 250 mm (Table B.1 and Table 12); state the grade on the drawing as, for example, "ISO 8062-3 DCTG 5, RMA 0.7 (RMAG E)". As-cast surfaces on stainless are typically Ra 3.2 µm, with Ra 1.6 µm achievable on small, well controlled parts, and are usually good enough for non-sealing faces. One reason to keep a machining allowance on wear faces of martensitic and precipitation hardening grades is that steel poured in air into a hot porous shell develops a decarburised skin whose depth grows with mould temperature and section modulus; the hardness you specified is under that skin, not on it. For austenitic grades the effect on corrosion resistance is minor.

9. Using the process well: consolidation and cost

The strongest economic argument for investment casting is consolidation: three welded plates, a manifold machined from a block, or a housing with a bolted cover can often become one casting with less weight and no weld to inspect. Apply the rules above to the new geometry, not the old parts: a welded plate assembly turned into a casting has just acquired a set of 1:1 junctions, and a rib layout that suited welding may lock the casting against contraction.

Cast-in identification (grade, cavity number, logo, part number) is nearly free if it is in the die and supports traceability. Raised characters cast better than recessed ones and should sit on a surface that is neither machined nor a gate face. Mark the location and character size on the drawing, and if heat numbers or serials are to be stamped rather than cast, say where and with what stamp type, since low-stress stamps are sometimes required on pressure parts. A hardness test pad, where hardness is specified, should likewise be located on the drawing.

Cost drivers, roughly in order: metal weight (stainless is priced per kilogram poured, and gates and runners are re-melted but not free); ceramic cores and soluble inserts; die complexity; the number of gates and the finishing they need; straightening; the NDT method and acceptance level; and machining. Almost all of these are set by geometry before the die is cut, which is why a design review with the foundry at the drawing stage is the cheapest engineering hour in the project.

10. Checklist before releasing the die

For grade selection, ferrite control, heat treatment and inspection standards, see our companion article, Stainless Steel Investment Casting: An Engineer's Reference.

Frequently asked questions

What is the minimum wall thickness for a stainless steel investment casting?

About 1.5 mm over short distances and 2.5 to 3 mm as a general design value. Thin walls fill more reliably when they are connected to a thicker section that can feed them, rather than fed from an isolated ingate.

Does an investment casting need draft angles?

Not on the casting itself: the ceramic shell is broken away, so zero draft is possible. A nominal 0.5 degree on deep features helps the wax pattern eject from the metal die and extends die life, but it is not a process requirement. Note that ISO 8062-3 Table 6 defines default draft angles for investment castings, so state "no draft" on the drawing if you rely on zero draft.

Why does uniform wall thickness not guarantee a sound casting?

Because two walls of equal thickness meeting at a T or L junction concentrate metal at the join. That junction freezes last and shrinks with no metal to feed it. The remedy is to reduce the junction mass with a cored recess or a radius that does not add material, or to place the junction where the foundry can feed it.

How should I dimension a casting drawing?

Choose three orthogonal datums that exist on the as-cast part, place the primary datum near the most critical feature, and dimension everything from them. Datums built on machined holes do not exist when the casting is first inspected. Specify a general tolerance to ISO 8062-3 (DCTG 4 to 6 for parts up to 100 mm) and show machined features separately with an allowance (RMAG E).

What drives the cost of a stainless investment casting?

Weight of metal, the number and complexity of ceramic cores or soluble inserts, die complexity (slides and loose pieces), the amount of finishing and straightening, NDT and acceptance level, and machining. Geometry decided before the die is cut sets most of these.

Sources and further reading

Sources: Campbell, J., Complete Casting Handbook, 2nd ed., Butterworth-Heinemann, 2020 (paraphrased, chiefly chapters 5, 7, 8 and 10; the feeding distance data and the datum example are drawn from sand and gravity die casting, the principles are applied here, not the figures). Steel Founders’ Society of America, Steel Castings Handbook, Supplements 1, 3 and 4 (written for sand cast steel; principles only, except the wall thickness and tolerance capability figures quoted). BS EN ISO 8062-3:2023. Always work from the current edition of any standard.

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