Doncasters Settas is a Belgian division of the British-based foundry and forging group, which produces precision investment and sand castings for aerospace, offshore, and high-performance automotive customers.
The operation is centered on a vacuum arc furnace that melts titanium and titanium alloys, as well as zirconium, serving a range of casting options. This includes high-pressure centrifugal casting and “centrispinning” on a 3-m diameter table, and investment casting products. The castings produced, some up to 400 kg, are destined to be components in aerospace structures, gearbox housings, and critical applications for offshore and undersea structures.
The value of these castings and the nature of the customer’s activities put particular emphasis for Doncasters on high-volume production, in addition to the casting buyers’ high standards for precision and product quality. These are standards that can easily be compromised during demolding.
An air-intake guide ring, an example of the titanium castings produced by Doncasters.
None of these objectives were being met by the previous shot-blasting system. Doncasters Settas also wanted a new system that would be cost-effective, meaning one that would clean high volumes of castings — and because of the product mix there it would have to be a very flexible operation to do so.
“The new machine had to be capable of removing virtually 100% of the entire molding sand sticking to the components with either thin or thick cross sections, without damaging any of the castings, preferably in a shot-blasting time that allowed for a high throughput rate,” details the plant’s managing director Axel Luckow. “Furthermore, we wanted the new system to provide for a certain capacity reserve. Another crucial aspect was an effective media-recovery system that helped to minimize our cost for wear parts.”
They chose to invest in a custom-designed demolding system developed by Rsler Surfacefinish (www. rosler.com). It is flexible enough to handle the variety of different shapes and materials the foundry produces, and it’s effective at removing sand and reclaiming blast media. Most important, it upholds Doncasters’ product quality standards.
PLC-based controls that guide the RHBE 20/25 overhead monorail shot-blasting machine contain casting- specific processing menus. Rsler engineers conceived and developed the system with a fully encased, low-wear vibratory conveyor that feeds the blast media/ sand mixture back into the integrated blast media recovery cycle.
“Rsler’s solution really convinced us, as it not only provides high flexibility but is also more suitable for our requirements than the machines offered by the other companies,” according to Luckow.
The shot blast machine is housed in a 102118106-in. (whl) cabinet made of wear-resistant manganese steel. The areas directly exposed to the blast stream are further protected by wear-resistant, replaceable cast liners.
The shot-blasting chamber is equipped with four “Hurricane H 28” blast wheels, each one with an installed power of 15 kW. The 8-in. diameter, high-performance wheels are vertically mounted on the sidewall of the blast chamber. 3D-animated simulations of the shot blasting process help to determine the wheels’ optimal location and inclination angle. Each blast wheel can be driven individually via the PLC, so the blast wheels can be used in any combination.
Once the metal is poured and the castings are set, a Y-shaped overhead monorail with two shuttle carriages transports the sand molds, which may weigh up to three tons, into the shot blasting chamber in a wire basket. The system’s PLC has various programs stored for processing different castings, including blast time and blast intensity.
|Finished castings, still in the sand molds, are carried into the chamber in a wire basket.|
Aerospace castings, such as this compressor part, are among the complex shapes demolded and cleaned by the system.
Beginning with the top wheel, de-molding takes place with single wheels blasting defined areas on the castings with S-390 steel shot. Throughout this 4-5- minute process the castings rotate in front of the blast wheels, in a technique that Rsler compares to peeling and apple: it prevents damage to the cast parts as the sand mold is removed in a controlled manner and, thus, prevents the titanium components from dropping within the wire basket. Blast media and sand are kept in the system at a fixed ratio, which helps to ensure an effective separation of media and sand.
Once the castings are 80-90% free of sand, all four wheels simultaneously blast the components for five to 10 minutes, ensuring that any residual sand is safely removed from any cavities in the castings.
Nothing in the air
Sand dust generated during shot blasting is extracted continuously. This requires high-velocity streams of air that draw blast media as well as sand, so the structure includes a “dropout” box to receive the all of the debris. Rsler supplied the RHBE 20/25 shot blast machine with a fully enclosed low-wear vibratory conveyor to return the blast media/sand mixture. It contains a screen that automatically separates and discharges sand lumps, coarse dirt particles, and gates and risers from the system.
Quartz dust generated during the process is exhausted at multiple points in the workflow, so that dust doesn’t escape into the work area. A double-filtration dust collector removes airborne particles and recirculates cleaned air back into the plant.
For economic considerations, the shot-blasting machine is equipped with a dual-drum magnetic separator that guarantees a high percentage of sand is separated from the blast media. A thin film of the blast media/ sand mixture is spread evenly across two broad magnetic drums. An electromagnet field from inside the drum separates the ferritic blast media from the sand. In addition, a double-cascade air-wash separator removes microfine particles from the media.
As cast parts are loaded and unloaded, the mixture is cleaned further by a bypass separator: A special valve located at the blast media storage hopper releases a measured quantity of blast media that is fed via the bucket elevator into the magnetic separator, then further on to the double-cascade air-wash separator and back into the blast media hopper. This media reclamation system guarantees that only a trace of sand remains in the blast media, thus reducing both wear and operating costs. An integrated back-feed system transports the recovered sand to a nearby reclaim system.
Treating mixtures of blast media and dust generated from shot-blasting titanium parts usually requires explosion- protection measures similar to what are used in processing of aluminium and magnesium parts. According to Rsler, an analysis of dust generated at Doncasters Settas showed that the dust generated by its de-molding shot blasting process is not explosive — a finding that has added to the economic benefits of its installation.