Draft angle — the taper applied to vertical surfaces of a casting that are parallel to the die opening direction — is one of the most fundamental design requirements in gravity die casting, and one of the most consistently underspecified in component drawings received from designers who have not been trained in casting process requirements. The consequence of inadequate draft is casting ejection failure — the casting adheres to the die surface and cannot be removed without force that damages either the casting or the die or both. By the time this consequence is discovered, the die has already been machined, the tooling investment has been committed, and correcting the draft requires either redesigning the component to add the missing taper or modifying the die in ways that are expensive, time-consuming, and sometimes metallurgically compromised relative to the original machined surface.
Understanding draft angle requirements — why they exist, how much draft different surfaces need, and where the common design errors originate — is knowledge that should be front-loaded into the casting design process, not discovered through production trial failure.
Why Draft Is Required — The Physics of Casting Ejection
When aluminium solidifies in a die cavity, it contracts. This contraction has two effects that are relevant to draft angle requirements. In the plane perpendicular to the die opening direction — the casting's footprint as seen from above — contraction causes the casting to pull away from the die cavity walls, reducing the contact pressure between casting and die surface and making lateral ejection easier. In the direction parallel to the die opening direction — along the surfaces that the die core or cavity walls form — contraction causes the casting to grip the die surface, because the casting is contracting onto the die surface rather than away from it.
A vertical surface in the die — a wall exactly parallel to the die opening direction — provides no geometric release as the die opens. The casting must slide along this surface during ejection, and the friction between the casting surface and the die surface, combined with the gripping force from thermal contraction, produces an ejection resistance that the ejector pins must overcome. For a surface with zero draft, this resistance is at its maximum and may be sufficient to prevent ejection entirely — the casting effectively locks itself to the die surface through the combination of friction and thermal contraction.
Adding draft — tapering the surface by even a small angle from the vertical — changes the geometry of the casting-die interface during die opening. As the die opens and the casting moves away from the die surface, the taper creates a progressively increasing clearance between the casting surface and the die surface — the casting lifts clear of the die surface rather than sliding along it. This clearance eliminates the friction and gripping force that vertical surfaces generate, and the ejection force required drops to the level needed to overcome the casting's inertia rather than its adhesion to the die.
Draft Angle Values — How Much Is Required and Where
The draft angle required for reliable ejection depends on the surface length in the draft direction, the alloy being cast, the die surface finish, and whether the surface is on a die core — a feature that the casting contracts onto during cooling — or on the die cavity — a feature that the casting contracts away from.
The distinction between core surfaces and cavity surfaces is the most important factor in draft angle determination. A die core is any feature that projects into the die cavity — the core that forms an internal pocket, the boss that forms a hole in the casting, or the pin that forms a through-hole. During solidification, the casting contracts onto the core, increasing the grip that the casting exerts on the core surface. Core surfaces therefore require more draft than cavity surfaces to achieve the same ejection resistance. Standard design practice specifies a minimum of 2 to 3 degrees of draft on external cavity surfaces and 3 to 5 degrees on internal core surfaces — with the higher end of these ranges used for longer surfaces, rougher die surface finishes, and alloys with higher solidification contraction.
Surface length in the draft direction amplifies the effect of any given draft angle. A short surface — 10 millimetres in the draft direction — with 1 degree of draft creates 0.17 millimetres of clearance at the top of the surface when the casting has moved 10 millimetres away from the die bottom. A long surface — 100 millimetres in the draft direction — with the same 1 degree of draft creates only 0.17 millimetres of clearance at the top of the 100 millimetre surface after the casting has moved 10 millimetres — a clearance that is geometrically inadequate for the friction that a 100 millimetre surface generates. Longer surfaces require either more draft angle or they rely on the ejection mechanism to develop the necessary force — and the ejection mechanism approach always carries the risk of casting deformation if the ejection force required exceeds what the casting's mechanical strength at ejection temperature can sustain.
Common Draft Angle Design Errors — Where Problems Originate
The most common draft angle design errors follow predictable patterns that reflect the gap between solid modelling software capabilities and casting process knowledge. Understanding these patterns allows them to be identified and corrected before tooling is committed.
Applying zero draft to surfaces that the designer intends to machine is the most frequent error in component drawings received from non-casting-specialist designers. The logic appears sound: if the surface will be machined to its final dimension after casting, the as-cast dimension does not matter, so draft is not needed on machined surfaces. This logic fails because the machining operation requires the casting to be ejected from the die first — and a zero-draft surface cannot be ejected. The correct approach is to apply standard draft to all surfaces in the die opening direction, including those that will be machined, with the machining allowance sized to remove the taper and produce the required final geometry. The alternative — designing a zero-draft surface that will be "cut square" in the die — requires a complex die feature or an insert that adds cost and complexity to achieve what adequate draft applied from the beginning would provide simply.
Specifying draft angles that are consistent with the nominal direction of the die opening but not with the actual geometry at specific features is a subtler error that requires careful geometric analysis of the complete casting in its die orientation to identify. A boss that projects from an angled casting surface, a rib that runs at an angle to the parting plane, or a pocket whose axis is not parallel to the die opening direction — each of these creates local surface orientations where the effective draft angle differs from the nominal draft applied in the die opening direction. Identifying these features and applying draft in the correct local direction for each is the die design analysis that converts a nominal draft specification into a casting that actually ejects.
Ignoring the draft requirements of deep pockets and blind holes is a third error pattern whose consequences are severe. A deep blind pocket in a casting — formed by a projecting core in the die — requires generous draft on both the pocket walls and the core tip geometry, because the casting grips the core across its full depth during solidification. Inadequate draft on deep pockets is one of the most common causes of core pull-out — the core detaches from the die during ejection and remains embedded in the casting — or pocket wall tearing, where the casting surface tears at the pocket entrance as the casting is forced off the insufficiently drafted core.
Design Review Before Tooling — The Investment That Pays for Itself
A systematic draft angle review of the casting drawing before the die design is committed — conducted by an engineer with practical die casting knowledge examining the drawing from the perspective of die opening direction and ejection mechanics — takes a few hours and costs a fraction of the tooling rework that inadequate draft produces. The review identifies every surface in the die opening direction, assesses whether the specified or implied draft is adequate for that surface's length and orientation, and flags any features whose draft specification requires either design change or special die features to accommodate.
For foundries that receive customer drawings without draft angle specification — drawings that show the finished component geometry without the process-specific additions that casting production requires — conducting this draft angle analysis and communicating the required additions to the customer before die design is a service that adds genuine technical value to the supply relationship. A customer who receives back a drawing with draft angle recommendations, with the commercial and quality rationale for each recommended addition explained, is receiving technical input that improves the castability of their component and reduces the risk of tooling rework delays in their production programme. This is the kind of technical engagement that differentiates a casting supplier who understands the process from one who simply machines what they are given — and it is the foundation of the technical partnership relationships that the most commercially successful foundries build with their customers.