Alloy Properties

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General Remarks

The production engineer has the greatest influence in the production of a die-cast part. This has a critical impact on the cost of the entire assembly, the design, the performance as well as the production process of die casting. An experienced production engineer is characterized by taking into account every aspect of the production and function of the finished assembly. However, it can not be expected that he is an expert in all areas. As a coach, he should normally be moderated by a team of specialists who provide important technical contributions to the project. Representatives of all companies involved in the supply chain should also participate. In the course of the design process, all participants are involved and contribute to the success.

For the production of a zinc die-cast part, the die-casting machine, the toolmaker, if necessary also the experts for post-processing and the surface coating should be involved in the decision-making process, especially in the case of more complex parts.

Normally, these development teams are only justified for large projects. In the design phase, the production engineer will usually trust his own experience as well as that of his professional in-house colleagues. Therefore, a good understanding of the material and the manufacturing process is a prerequisite for the practical implementation of the design requirements.

The weights of a zinc die-casting is usually between 1 gram and about 1 kilogram, the outer dimensions corresponding between 3 mm and 300 mm. The production of larger and heavier castings is possible on larger die casting machines. The cost advantage of zinc die-casting production lies in the fact that a higher production volume per unit of time can be achieved than with other materials. This is due to the heat content, the melting temperature and the extremely low crystallization rate of the zinc die casting alloys. Therefore, even larger zinc die-castings are very efficient to manufacture.

The basic contours that can be achieved with die casting are largely unlimited. In practice, very complex die-casting molds can be produced. However, it is useful to determine within the framework of a cost analysis whether a subsequent mechanical processing is more favorable in a particular case than a costly complex die casting mold.

However, the geometry of a zinc cast part is predominantly determined by its function and less by the production effort. Material-saving designed parts can be produced which are not possible with other cast materials such as aluminum, cast steel, brass.

One of the main features of a component is its mechanical properties with a corresponding safety factor. The cost-effectiveness of the zinc die-casting is also related to the weight reductions that can be achieved through extremely thin wall thicknesses.

The operational loads and the resulting stresses should be taken into account in the design. This is achieved by using numeric component simulation. This allows to visualize the filling process, the heat balance of the die-casting mold and the stress distribution under load.

Elements of a component such as lever arms, joints and headers are often designed according to “eye” or their visual relationship to the entire assembly, which is unnecessarily enlarged. For this reason, it is always advisable to make contact with the foundry in order to obtain a design most appropriate for diecasting. As with all casting and other forming processes, the product designer should, if possible, make all wall sections very uniform and avoid greatly varying wall thicknesses. If this is not possible, the cast part can still be produced. However, the particular properties of the casting must be taken into account during casting. Due to the physically induced volume contraction, porosities and shrinkage holes can possibly lead to increased rejects rates, which in turn leads to an increased unit cost. Computer simulation helps to avoid such errors.

The minimum wall thickness of a diecast part depends on the distance between the sprue and the solidification time of the casting alloys. For short distances, ie, less than 50 mm, a minimum wall thickness of approximately 0.3 mm can be achieved, continuously increasing to 2.0 mm and more at 200.0 mm. The computer-assisted design of the gate and overflow geometry also provides an insight into the achievable minimum wall thicknesses.

The separation plane of the die-casting tool is largely predetermined by the construction of the component. The production of completely burr-free components is determined not only by the quality of the tooling but also by the handling during the casting process and the maintenance of the tool. It is often better to produce a “wanted” ridge in order to obtain a better venting or a better flow behavior during the filling of the mold. These burrs are removed by means of the subsequent press deburring without additional depth. Moreover, these measures can be used to avoid so-called unwanted, difficultly removable burrs in a process-safe manner.

The position of the sprue and the length of the gate should be taken into account when defining the mold separation plane. A more unfavorable position of the inlet and the gate leads to the metal melt not reaching all regions of the mold cavity directly and with the shortest flow path. This can lead to undesirable casting defects such as cold flow points, pores and surface bubbles. This in turn can lead to problems in a subsequent surface treatment or to a loss of strength in the case of porosities at critical positions. The position of the necessary overflow must be considered as well. They absorb the air displaced by the molten metal and direct the flow front to obtain an optimum form filling. These air beans often serve to evenly eject the cast part and avoid ejector marks on the casting part. The positioning of the ejectors falls within the range of tasks of the foundry engineer and the toolmaker, since these specialists can detect in advance the possible danger of sticking and possible warping of the casting part during ejection.

For a reliable, automated production of die-cast components, it is crucial that both the cast parts and the sprue system are firmly fixed to the movable mold half as soon as the die opens. In the case of the slightest sticking or adhesion during ejection, appropriate measures must be taken. This is achieved by means of special design on shaped elements of the movable mold side. In many cases, special ejector rods, which are provided with an undercut at the end face (so-called snatch pins or z pins), are also very helpful. Pre-cast holes in the castings are produced either completely by the movable mold half and by cores, or to 2/3 by the movable mold half and 1/3 by the fixed mold half.

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