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Walther Trowal's THM continuous shot-blast system, with aluminum diecastings supplied by a conveyor belt.

Trying a Little Gentleness in Surface Treatment

Aug. 11, 2017
An automotive aluminum diecaster has adopted a new concept in continuous shot-blasting.

Manufacturing design trends typically focus on higher speed or intensified power, or increased reliability and durability. There is one process technology familiar to foundries and diecasters that has at its focus for improvement … gentleness.

Walther Trowal GmbH, a developer of surface treatment technologies, recently commissioned a troughed-belt continuous shot-blast machine for a Mexican automotive diecasting plant outfitted with a new generation of blast turbines specifically designed for surface treatment of aluminum parts. The THM continuous shot-blast process attains considerably higher blast media throwing speeds, according to the developer, and so the new turbines achieves significantly shorter cycle times than earlier continuous surface treatment processes.

It is the fifth continuous surface-treatment system installed for the diecaster by Walter Trowal, which commented that a rising number of customers are switching to aluminum media for shot-blasting aluminum components. Because of its lower bulk density, aluminum media is much gentler than stainless steel shot, for example. Many automotive components already are formed in aluminum (steering knuckles, swivel bearings), and automakers frequently evaluate aluminum alloys conversions of part and system designs, in the interest of reducing vehicle mass. The aluminum diecastings processed in the THM shot-blast machine include several types of housings, covers, and levers.

Because of its lower density of the media, its impact energy on the workpieces is considerably lower compared to other blast media. To offset this limitation, Walther Trowal developed turbines with curved throwing blades that generate a significantly higher throwing speed for the blast media than straight blades. Combined with the fact that in THM machines the turbines are located very close to the workpieces, and that the media throughput is considerably higher, this results more optimal energy utilization and surprisingly short cycle times.

However, the shot-blast treatment remains very gentle, according to the developer. Despite the relatively low density of aluminum blast media, the shot-blast process is highly effective and at the same time surprisingly mild, preventing any distortions of parts' surfaces.

Another reported benefit of aluminum blast media is that it reduces the wear rate in the turbines and the machine to a fraction of the wear caused by steel shot, resulting in higher uptimes and lower operating costs.

Walther Trowal offers several machine sizes equipped with up to four turbines, so new installations can be optimized to the types and volumes of parts to be treated

To address all the challenges posed by aluminum shot, Trowal engineers redesigned many machine assemblies, among them the media dosing system and the turbines. For example, a rough surface of the throwing blades would quickly destroy the aluminum particles, so by smoothing the blade surface with in-house vibratory finishing equipment the process is able to reduce blast-media consumption, lower dust emissions, and significantly improved turbine uptimes.

THM troughed belt continuous shot-blast machines are equally well suited for treating high-volume bulk parts or large, "delicate" components. In the latter case, the THM concept offers significant advantages: fragile workpieces are evenly distributed through the entire machine length, resulting in a very gentle transport with reduced part-on-part collisions. Because the transport rods are covered with relatively soft polyurethane, the tumbling action is made even more "gentle." This is particularly important for thin-wall components.

In addition to its new system, Walther Trowal noted that operators of previous versions of the THM system could have the new turbines retrofitted to their machines. And, for operators still requiring steel or stainless steel blast media, it will supply the new turbines with curved throwing blades made of tool steel, also offering significantly higher turbine uptimes.