Draft angle in sand casting is the taper given to every pattern wall that runs perpendicular to the parting line, so the pattern can be drawn out of the sand without tearing the mold. On steel sand castings it usually runs from 1° to 2°, set wall by wall: smaller on tall walls, larger on internal surfaces and in hand-molded green sand.
Investment casting needs far less. ISO 8062-3’s Grade A values give 0.2° for an investment-cast feature 100 to 160 mm tall, against 1.0° for the same feature molded by hand.
Why Sand Casting Needs Draft
Sand casting needs draft because a straight-walled pattern stays in contact with the mold walls until it is fully out and drags them with it, while a tapered pattern clears the sand on all its faces as soon as it starts to lift. Sand is held together only by its binder, so a wall dragged that way breaks.
Standard Draft Angle for Sand Casting
For steel sand castings the standard is 1.0° to 1.5° on most features and 1.5° to 2.0° in deep pockets. That is the general recommendation in the Steel Founders’ Society of America’s dimensional capabilities guide, and it is meant as a minimum to meet or exceed on every surface perpendicular to the parting line.
SFSA’s figures are written for steel. ISO 8062-3, the international standard for casting tolerances, sets its default draft angles by molding method, feature height and surface, with no alloy column, and its scope covers all cast metals, gray and ductile iron included.
How to Determine the Draft Angle
Four things set the draft a given wall needs: its height, its surface type, the molding method and the pattern material. The final angle for each wall is agreed with the foundry that will pour the casting.
Wall Height
A taller wall needs a smaller draft angle, even though the offset it produces in millimeters grows. In ISO 8062-3’s Grade A values for hand-molded castings, the external angle falls from 2.2° on a feature 25 to 40 mm tall to 0.5° on one 400 to 630 mm tall. A deep pocket is the exception: its walls face each other, which makes them internal surfaces.
Internal and External Surfaces
An internal surface needs more draft than an external one of the same height. ISO 8062-3 counts a surface as internal when it faces another surface of the part, such as the side walls of a pocket or recess, whether or not a core forms it.
In ISO’s tables for hand and machine molding the internal angle is larger than the external one in every height band, usually by a few tenths of a degree; a hand-molded feature 100 to 160 mm tall gets 1.5° internal and 1.0° external.
Molding Method
Machine molding needs the least draft and hand-molded green sand the most. In SFSA’s steel figures, hand-molded green sand takes half a degree more than automated green sand, no-bake and shell molding, which share the same values. Settle the molding method with the foundry before fixing draft on the drawing.
Pattern Material
A wood pattern needs more draft than a metal or plastic one on short external surfaces; on tall ones the difference disappears. In a widely taught university guideline table, an external surface 1 in tall gets 3.00° on a wood pattern and 1.50° on metal or plastic, while surfaces 8 to 32 in tall get 0.50° on either.
That table’s values sit below SFSA’s steel figures for tall walls, so read it for the direction of the effect and take the number from the foundry.
Zero and Negative Draft
Zero or negative draft is possible only on walls the pattern is not drawn past. Surfaces parallel to the parting line need no draft, and turning the part or moving the parting line can put a wall in that position.
A wall perpendicular to the parting line that must be straight, or that leans back to form an undercut, is formed by a core or a loose piece instead of the pattern, which adds significant cost. For a wall that only has to be straight in service, casting it with draft and machining it straight is usually the cheaper route.
Too Little and Too Much Draft
Too little draft damages the mold, and the damage comes out in the casting; too much adds metal and machining the part did not need.
Too Little Draft
Too little draft tears the mold walls as the pattern is drawn. The molder has to rap the pattern harder to free it, and rapping enlarges the cavity, so the walls come out oversize.
Torn edges are patched by hand, leaving rough or uneven surfaces. Any sand the pattern drags loose can wash into the cavity during pouring and end up as sand inclusions.
Too Much Draft
Excess draft costs metal, and on faces that get machined it costs machining time as well. Even so, SFSA’s guidance is that the angle chosen should be no less than the design can tolerate, so reduce draft only on faces where it affects fit, weight or machining.
Specifying Draft on a Casting Drawing
Specify draft in a general note on the drawing, with the angle agreed with the foundry, and state whether draft is added to or subtracted from the dimensions shown. Without that note the foundry’s pattern shop still adds draft to straight walls, and by default it adds it to the dimensions. Faces that need a different angle get their own callout.
A general tolerance note that invokes ISO 8062-3 applies the standard’s default draft angles to every surface not marked individually. Confirm with the foundry that those defaults suit its molding method.
Dimensional and geometric tolerances on a drafted surface are measured from the drafted surface: draft is applied first, then the tolerance. Folding draft into a tolerance band is done only by agreement with the foundry.
Machining allowance goes in its own general note and sits on top of draft. Before fixing the angle on a wall that mates with another part, convert it into the offset it produces at that wall’s height and check the fit.