Zero draft angles in die casting

Weight sensitivity through design

Zero draft angles in die casting refer to the practice of casting metal components without incorporating draft on faces that traditionally aid ejection from the die. This technique is believed to enable extremely tight tolerances, which are essential for parts that must fit and align precisely with other features in the final product.

With our proprietary Multi-Slide die casting capabilities, casting with zero draft angle is achievable. The question, then, is whether casting with zero draft angle is necessary to achieve the requisite tolerance for the part to perform.

Functionality of draft angles

Unlike 3-D printed parts or machined parts, die casting engineers must consider how the part will be ejected from the die to avoid damaging the part or the tool. Draft angles are the degree of taper incorporated into the side walls and cores of the die casting tool to aid part removal. Primarily, they reduce friction between the part and the tool during ejection. Without draft angles, parts can be stripped, dented or become stuck in the tool.

In some instances, the net-shape part lends itself to ejection from the tool without additional taper. Take a Coca Cola can, for example. If a Coca Cola can were cast using conventional methods, the two cavities would join at the midpoint (and widest part) of a net-shape, upright can. Since the can is cylindrical and the widest part of the crescent of each cavity is where the die separates, the can is easily ejected when the die opens.

Most parts, however, are far more complex and intricate than a soft-drink can. In these cases, draft angles are often incorporated into the design so that the part is not damaged during ejection.

Why use zero draft angles?

Theoretically, zero draft angles allow for tighter tolerances. Since incorporating a draft angle into the part design can change the shape of the component, it can affect its final assembly. If a draft angle is not incorporated into the die, there is arguably greater control over part precision in relation to other features in the component.

Achieving zero draft angles in die casting

With Dynacast Multi-Slide die casting, zero draft angles are achievable under certain conditions. The part must be cast using zinc, both for its shrinkage characteristics and physical properties. Zinc has a predictable 0.7% shrinkage rate, which is easily compensated for in the tool design. Unlike aluminium, zinc is relatively smooth. For standard aluminium castings, engineers incorporate ± 1-2° of draft angle to accommodate the abrasive nature of the metal. For zinc castings, the standard draft angle is 0.5°. Because of zinc's greater fluidity, it is more easily ejected from the tool and is therefore a more suitable material when casting with zero draft angle.

When casting without draft angle, using a Multi-Slide process is essential. Multi-Slide die casting is less volatile than conventional casting, partly because the Multi-Slide die casting machine's pneumatic slide actuation system allows for a cycle time up to four times faster than conventional die casting. The less time the part spends in the tool, the better. Additionally, Multi-Slide die casting gates and runners are smaller than in other processes, enabling lower shot weights and therefore a less violent process.

Achieving tighter tolerances with Dynacast

While casting with zero draft angle is certainly possible, it is almost always unnecessary with Dynacast. With zinc Multi-Slide die casting and an incorporated draft angle, we can achieve tolerances of just ±0.02mm. This means that consistent, uniform results can be achieved with Multi-Slide die casting, with or without zero draft angle.

We assess tolerances differently. Our quality level both meets and exceeds industry standards and is used as a guideline by designers worldwide. When casting to tight tolerances, we assess the form of the part, the proximity of the feature within the tool and its relationship to other features on the part. More often than not, casting with zero draft angle is not necessary to achieve functional, tight tolerances.