Written by Editors EUROGUSS 365
Damian Föllmi: It begins with the conventional high-pressure die casting process. First, we force the molten aluminium into the die cavity. The component then solidifies from the surface layer inwards. While the outer regions form a load-bearing outer shell, the melt in the intended channel region remains liquid or semi-liquid.
Now the actual gas injection begins. At a precisely defined moment, the gas injector pierces the outer shell, which has already solidified, and gains access to the liquid melt inside. At the same time, a shut-off slide opens a secondary cavity that was previously closed and empty. The nitrogen from the gas injector then forces the liquid melt out of the channel region and into the secondary cavity. This step is also known as 'blowing out the melt'. A hollow channel then forms along the predetermined path of the component geometry. Once the component has solidified completely, it can be demoulded and the residual material displaced during the process can be separated from the secondary cavity.
To ensure high reproducibility, the most important factors are the geometries, meaning the component itself, the gas injection channel and the secondary cavity. The injection timing also matters, in interaction with the controlled opening of the secondary cavity. Beyond that, gas pressure, gas volume or rather the pressure curve, die temperature and the state of solidification of the melt all have a decisive influence on process stability.
Damian Föllmi: The basic idea comes from plastics injection moulding, where gas assisted injection moulding has been established for a long time. Aalen University and TiK took up this approach and transferred it to die casting. The real challenge lay in applying the principle to metallic melts. Aluminium high-pressure die casting involves far higher temperatures than plastics injection moulding, and on top of that come very short process times, high pressures, rapidly advancing solidification and a generally harsher production environment.
In order to achieve series readiness, we therefore had to work on several fronts simultaneously. This meant ensuring we had reliable and durable equipment and die components, matching component and die geometries, and stable process parameters to guarantee consistent component quality. The most important elements were the designs of the components and the dies. We quickly realised that the technology could not simply be incorporated into an existing part. The component had to be developed specifically for gas injection, with die geometry and die temperature control factored in from the outset. Only when all influencing variables are consistently matched to one another will you achieve components of the required quality. We have now validated the process on demonstrator and prototype components, and it is ready in principle for series production. The next decisive step is to implement it in a concrete customer project.
Incidentally, the equipment technology and the special die components were developed by TiK (Technologie in Kunststoff) within the EU-funded MAGIT project. MAGIT stands for 'Magnesium and Aluminium Gas Injection Technology', which is also the name under which TiK markets the technology today. The project involved a European consortium comprising Aalen University, TiK, Surtechno and ourselves, ALUWAG.
Damian Föllmi: The gas injection process suits die castings into which you want to integrate hollow channels that cannot be demoulded. Fluid-carrying channels are a particularly important application, for example for integrated cooling or oil channels in housings for power electronics, as well as in components of electric motors and gearboxes, and potentially in battery or thermal management components. Another area of application is the deliberate placement of cavities to reduce weight. This allows thick-walled component regions to be designed to suit their function without filling the entire volume.
In general, the main advantage lies in functional integration. Assemblies that previously consisted of a casting, a cover, seals and fasteners can, under certain conditions, be replaced by a single die casting. This eliminates the need for machining, joining, sealing and assembly operations. At the same time, the process allows hollow channels with changes of direction that conventional slides and core pins in a die casting tool simply cannot demould.
Damian Föllmi: The economic benefits of gas injection become apparent when we can replace previously assembled or joined units. Consider a casting whose cooling channel was previously created using a friction stir welded plate. We expect to see cost savings of around 15 to 20 per cent across the entire life cycle, including tool amortisation.
Damian Föllmi: We have already realised and produced several different cavity geometries, all of which differ in length, cross section, cross section profile and changes of direction. In principle we achieve channel lengths of up to 2.5 metres. We have not yet investigated longer channels.
Ideally, we make the cross sections round, though oval or flat geometries are also possible. We have achieved very good results with channel cross sections that have a clear diameter of between 10 and 50 millimetres. When making changes to the cross section and direction, we always aim for transitions that favour the flow. Provided the changes to the channel geometry are not too abrupt, deflections involving changes in direction of up to 180 degrees are feasible.
As a demonstrator, we even produced a chair base with a centrally located gas injector that clears out five chair legs simultaneously. Today's technology also enables us to form two separate channels within one component or one channel per component in a two-cavity die. However, in every application, the possible cavities always depend on the component and its geometry, and must be adapted accordingly.
Damian Föllmi: It is not physically possible to achieve completely even wall thicknesses across the entire channel length. Therefore, we cannot claim to achieve a geometry comparable to that of a cast or machined bore with defined dimensions and tight tolerances. The already solidified outer shell and the channel's routing form the boundary conditions within which the incoming gas will seek the path of least resistance. Our aim is therefore to achieve a defined residual wall thickness reproducibly within specified tolerances.
We control this process by adjusting the gas injection parameters, such as process times and gas pressure, as well as the die temperature. As the blow-out process is very quick, advancing solidification only has a minor influence on the residual wall thickness during that short period. During production, we check the results using computed tomography, X-ray inspection, and destructive examinations, such as sawn sections. We also monitor the relevant process data, which enables us to detect deviations and ensure that the process remains within a defined window.
Already in the development phase we simulate mould filling and solidification, and by now even the gas injection itself. Based on this, we optimise the component geometry, including the channel geometry, wall thicknesses, gas feed channel, secondary cavity and die temperature control. We design everything to ensure optimal gas injection conditions. We therefore have the whole chain under control, from component design and simulation to inspection in production. The only thing missing is the first concrete customer project, which is exactly what we are working towards.
