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Wall Thickness, Ribs, Fillets and Draft: Design-for-Casting Rules OEM Buyers Should Apply Before Releasing a Drawing
2026-09-22
A Part That Passed Every Drawing Check and Failed Every Casting Trial
An industrial OEM buyer sent a pump housing drawing to three foundries and received three very different quotes and three very different warnings. The geometry was clean, the tolerances were tight, and the part had been machined from solid for years. Every first article came back with shrinkage porosity in the same heavy boss, and the foundries all said the same thing: the drawing is machinable but it is not castable. A 14mm boss sitting against a 5mm wall, with no fillet and no taper, could not feed. Nine weeks and three tooling modifications later, the part still needed a redesign that should have taken two days on a screen. The buyer had optimised for the machining operation that was being eliminated, and carried a solid-metal mindset into a process that fills a cavity with liquid.
Castings Follow Different Physics
Metal shrinks as it solidifies, and it does not shrink evenly. Every design-for-casting rule exists to control where the last liquid metal sits, so that shrinkage ends up in a feeder and not in the part. Machining from solid rewards material where it is needed and no more; casting rewards uniform sections, generous transitions and a clear path for feed metal to travel. Translate the part into casting logic before the quote, not after the first article, because tooling cut to a machinable drawing will produce a castable part only by luck.
A CNC machining operation attached to the casting supply changes how much of this matters: where the supplier also machines, the foundry can leave deliberate machining allowance on heavy sections instead of fighting to cast them sound. Buy Casting And Machining from one source on pressure-tight parts so the allowance and the feed path are designed together.
Rule One: Uniform Wall Thickness
The single most valuable rule is uniform wall thickness. Thin sections freeze first, and where a thin section sits between the feeder and a thick one, the feed path closes and the thick section is left to shrink on its own. Aim for one nominal wall across the whole part, and where thickness must change, taper it gradually rather than stepping it. A sharp step between a 5mm wall and a 12mm wall is the classic origin of a porosity hotspot, and it appears on the radiograph at exactly the point the drawing looks strongest.
Weight reduction is usually the reason thickness varies, and the correct way to remove weight from a casting is with ribs and coring, not with thinner walls. Hold the nominal wall and take material out of the middle, keeping a consistent skin all the way through.
Rule Two: Ribs Instead of Mass
Ribs add stiffness without adding section thickness, and they are the standard answer to a part that flexes. A rib should be thinner than the wall it attaches to, typically around a fraction of that wall, and it should be filleted generously at the root. A rib as thick as the wall creates a new thermal mass, a new shrink point and often a sink mark on the opposite face, which is then machined off and shows as a blemish on the visible surface.
Space ribs so that the spaces between them can be cored or formed without a fragile core, and keep rib depth moderate. Deep, narrow ribs are hard to fill, hard to core and prone to cold shuts, and a rib that cannot be filled consistently will be the first feature to fail at volume.
Rule Three: Fillets Everywhere
Sharp internal corners are the second most common defect source. A sharp corner creates a stress concentration in service, a turbulence point during filling, and a local hot spot during solidification. Put a radius on every internal corner, sized in proportion to the adjoining wall, and apply it consistently across the drawing. A drawing with a general fillet note but no specified radius will get whatever the toolmaker chooses, and that choice is made for tooling convenience rather than for your part.
Where the part is visible, fillet quality also drives appearance. A surface treatment operation applied to the cast part will not hide a sink or a flow line, and in many finishes it makes them more obvious. Decide which faces are cosmetic, mark them on the drawing and agree the acceptance standard before tooling starts.
Rule Four: Draft on Every Vertical Face
Draft is the taper that lets the part leave the die or the mould. Without draft the part drags, the tooling wears early, and the foundry compensates with ejector marks and extra allowance that you then pay to machine away. Specify draft on every face parallel to the draw direction, and increase it on deep pockets and on textured or rough surfaces. Zero draft is achievable only on sacrificial features or with slides and cores that add tooling cost to every shot.
The process choice sets how much draft is realistic. A high pressure die casting application for robot components demands generous draft because the part is ejected from a steel die at speed, whereas gravity and low pressure work with sand or permanent moulds where slightly less draft is workable. Confirm the draft rule with the foundry for the specific process, then design to it.
Rule Five: Design the Boss and the Hole Properly
Bosses for fasteners are where most porosity is found. A solid cylindrical boss is a concentrated mass surrounded by thinner metal, and it will shrink internally almost every time. Cored the boss, or attach it to the wall with webs so it has a feed path, and keep the boss wall close to the nominal wall of the part. A blind boss machined and tapped into an unsound core is how a pressure tight housing leaks in service, and the leak is discovered by the customer, not by the foundry.
A low pressure casting application in electromechanical housings shows the advantage of a controlled fill: bottom filling under low pressure produces a quieter, more directional fill that suits parts with long walls and pressure tightness requirements. Where the part is a housing that must hold pressure, bring the process conversation in at drawing stage rather than at quotation stage.
Rule Six: Machine Allowance Where It Earns Its Cost
Every machined face costs twice: once in casting allowance and once in cycle time. Put allowance only on sealing faces, bearing bores, threaded features and datum surfaces, and leave everything else as cast. Blanket allowance across the whole part adds weight, adds material cost and moves the machined surface into the region where the casting was least sound, which is the opposite of the intent.
A die casting application in a medical field component is a useful reminder that tight cosmetic and dimensional expectations belong on the drawing, in measurable terms, before tooling is cut. "Clean appearance" is not a specification, and it is argued about after delivery rather than before tooling.
Release the Drawing Only After This Pass
Before release, run one disciplined pass: one nominal wall thickness across the part, ribs thinner than the wall they sit on, a stated radius on every internal corner, draft on every vertical face, bosses cored or webbed rather than solid, and machining allowance only where a tolerance or a seal demands it. Send the drawing to the foundry for a castability review before tooling, and treat the review comments as design input rather than as commercial pushback. Nine weeks of first article iteration costs far more than two days of geometry changes, and the part that comes out of the second route is cheaper to cast, cheaper to machine and sound where it matters.