Recently, there has been a lot of buzz surrounding the feasibility and functionality of zero draft angles in die casting, or casting a metal component without conceding draft on faces that traditionally ensure easier ejection from the mold. This practice presumably allows for incredibly tight tolerances, which are critical for parts that need to be fitted and assembled in precise alignment with other features of the finished 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 be able to perform.
Functionality of draft angles
Unlike 3-D printed parts or machined parts, die cast engineers must consider how the part will be ejected from the die so as not to damage the part or the tool. Draft angles are the degree of taper that is incorporated into the side wall and cores of the die cast tool to aid in the removal of a part. Primarily, they function to reduce the friction between the part and the tool as it is ejected. Without draft angles, parts can be stripped, dented, or get stuck in the tool.
In some instances, the net-shape part lends itself to ejection from the tool without additional tapering. Take a Coca Cola can, for example. If one were to cast a Coca Cola can 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, when the die opens, the can is easily ejected.
Most parts, however, are far more complex and intricate than a soda can. In these cases, draft angles are often incorporated into the design so that the part is not damaged during ejection.
So why zero draft angles?
Theoretically, zero draft angles allow for tighter tolerances. Since incorporating draft angle into the design of the part can change the shape of the component, it can affect the end assembly of the component. If you don’t incorporate a draft angle into the die, you arguably have more control over the precision of the part as it relates 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. And unlike aluminum, zinc is relatively smooth. For standard aluminum castings, engineers will incorporate ± 1-2° of draft angle to accommodate for the abrasive nature of the metal. For zinc castings, the standard for draft angles is 0.5°. Because of the more fluid nature of zinc, it is more easily ejected from the tool and, therefore, a better material when aiming to cast 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, partially because the multi-slide die casting machine’s pneumatic slide actuation system allows for a cycle time up to four times as fast 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 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 the tolerances of just ±0.02mm. This means that you can achieve consistent, uniform results with multi-slide die casting, with or without zero draft angle.
We look at tolerances differently. Our level of quality both meets and exceeds industry standards and is used as a guideline for designers worldwide. When casting for tight tolerances, we assess the form of the part, the proximity of the feature within the tool, and its relation to other features on the part. More often than not, it is not necessary to cast with zero draft angle to achieve functional, tight tolerances.