Save Money with Automated Shot-Blasting Systems as a Cost Save

 

Willingness to accept integrated automation concepts is the key to future manufacturing capabilities. There is still considerable potential for optimization, particularly with shot-blasting finishing. This article presents two specific customer automation solutions for the shot-blasting of aluminium die castings.

Automated Shot-Blasting Systems as a Cost Saver

Willingness to accept integrated automation concepts is the key to future manufacturing capabilities. There is still considerable potential for optimisation particularly with shot-blasting finishing. This article presents two specific customer automation solutions for the shot-blasting of aluminium die castings. 

Today automotive factories are not only designed with maximum built-in flexibility, production and assembly plants must also be planned to adapt to shorter production runs, and the increasing number of special vehicles. This applies in particular to retooling and maintenance in order to ensure that there is fast and flexible reaction to different demand situations. Therefore, an adaptable and easy-to-maintain robot-based plant concept was chosen as part of the automation of shot-blasting and surface-finishing of cylinder heads at Volkswagen de México from the start of production. The Rösler shot-blasting system (Roboblaster RROB 350/1200-4) was designed so that cylinder heads for five-cylinder as well as four-cylinder engine blocks can be processed completely automatically in the shot-blasting machine. This dual use is possible because of the special design of the robot gripper unit. It is an essential component of the system concept and of paramount importance for the fulfilment of customer requirements for high system availability, reduction in downtimes and also time and cost savings. Assurance of maximum process stability, improved and reproducible quality are further decisive characteristics of the system concept.

Cylinder heads produced by low-pressure die-casting are de-sanded, de-burred and subjected to surface finishing. Surface roughness of 50 to 70 µm, primarily in the inlet and outlet channels of these components, is required.

Intelligent Gripper Unit

The cylinder heads are transported from heat treatment as part of the cycle on a roller conveyor first of all to a turning station where they are rotated around the Z axis with the combustion chamber side upwards. From there the robot's gripper arm picks up the component and guides it to the shot-blasting machine. The water channels are hermetically sealed by the special gripper unit - a sealing device made of hardened tool steel as a result of this no shot-blasting media may enter during the shot-blasting process,. It is fitted with a quick-change device in order to reduce to a minimum the resetting time in the process change to different type cylinder heads.

Short, Intensive Shot-Blasting
A cylinder head passes through the entire finishing process in 36 seconds, of which 26 seconds for handling and only 10 seconds for the actual shot-blasting. Four heavy-duty 15 kW shot-blasting wheels are directed at the component centre and achieve full coverage due to their inclined position. The machine is completely lined with manganese steel plates in order to avoid excessive wear on the interior. Stainless steel shot is used as themedia. The shell valves for the shot-blasting media feed are positioned directly in front of the shot-blasting wheels because of the extraordinarily short shot-blasting time. Partitions placed in front of the continuously running shot-blasting wheels upon completion of the shot-blasting process reduce the noise level and prevents media escaping during the loading and unloading process. A special labyrinth seal at the machine chamber aperture prevents the accidental loss of any media.

Loose media is removed from the component surface upon completion of the shot-blasting process by turning the cylinder heads whilst still in the chamber. The gripper unit moves out of the shot-blasting machine and deposits the component on the turntable. The cylinder head is rotated around the X axis and transported to the vibrating station by a short conveyor where the component is rotated again and vibrated to remove the remaining shot-blasting media residues. An injector nozzle vacuums up the collected shot-blasting media at the end point of the machine and feeds it again into the shot-blasting media loop in the machine. In order to remove burrs and other contamination, all the shot-blasting media is fed through a cascade coil connected to the filter. Micro-particles are vacuumed out of the shot-blasting media stream. A bucket-elevator then feeds the cleaned media upwards again into the shot-blasting media reservoir. The shot-blasting media quantity and mix are maintained at a constant level by the continuous addition of new shot-blasting media by automatic top-up with level control. An inspection platform in the vicinity of the elevator and separator makes maintenance and inspection work easy. The whole process is carried out in a closed acoustic chamber - noise is reduced to maximum 75 dB. Sliding doors in the side walls ensure that inspection work can be carried out comfortably. Simple removable components are provided for major operations - such as filter or robot change.

A water-spray separator protected against explosion is provided for the processing of aluminium in accordance with ATEX (protection against explosion under atmospheric pressure). The dust is sprayed with water according to the Venturi principle. It is deposited as sludge in the lower part of the filter and is continually removed by an integrated sludge scraper into a special sludge cart. The clean air with the residual dust content of maximum 20 mg per standard cubic metre is discharged to the outside atmosphere. An afterfilter controlled by differential pressure, which also protects against rough particle escape and a pipe attenuator for noise reduction are fitted downstream of the wet filter.

Proving through Trials
In the tendering phase there was very close collaboration on engineering between the Rösler Engineering Team and the client, "we have worked out an impressive complete solution together with the client," said Ronny Paasche, engineering consultant for Shot-Blasting Systems in Machine Sales at Rösler. Due to the size of the components for processing and the ambitious processing task, the project work started with a shot-blasting system with six shot-blasting wheels. However, early trials in the test centre proved  that a system with four shot-blasting wheels would be adequate - with considerable financial benefits for the client through a significant reduction in shot-blasting media and electricity and also smaller space requirements.

The Challege: A Complex Component

Maximum automation does not always lead to the most economic solution. On the contrary, component types and sizes require increased flexibility. Due to increasing product and component quality requirements with simultaneous need for shorter throughput times, the demand is increasing in shot-blasting technology for processing centres in which variable quantities of complex components can be shot-blasted productively and profitably.

The light-metal foundry, Schulte & Schmidt in Flossenbürg, commissioned Rösler with the design of a suitable system to achieve economic surface finishing of large quantities of diecast aluminium slip-ring bearings. The company already had some Rösler closed-loop conveyor shot-blasting systems which were no longer adequate for the size and special geometry of the slip-ring bearings and the throughput quantities for processing of approx. 450 component parts per hour. Sensitive components and components liable to roll over such as slip-ring bearings with journals and studs require a significantly larger turning radius than the 200 mm available in the previous system in order to prevent any interlocking. Initial processing trials in the RMBD 500 tumble-belt machine in the test centre produced such good results in shot-blasting output and component quality that no other machine was considered.

In a Tenth-of-a-Second Cycle
The continuous tumble-belt machine type RMBD 500 is a proven system solution especially for aluminium die-casting foundries which require a particularly high throughput. It is an ideal option to hanger conveyor shot-blasting systems for die-casting foundries with a large component requirement. With its large trough diameter of 520 mm, it is ideally suitable for gentle but at the same time extremely economic processing of slip-ring bearings. Due to the dual inclination of the trough for component transportation, the machine does not require any helical catches on the trough bars. Sensitive aluminium components with journals, studs and sealing surfaces have more room to manoeuvre and a larger radius for tumbling in the trough. Therefore they can not  interlock even with large throughput quantities. Eight wire-mesh cages with 700 litre capacity for approx. 450 component parts each are processed at Schulte & Schmidt in an hour in the machine, i.e. de-burred and contamination removed from the surfaces. Therefore, one wire-mesh cage can pass through in 7.5 minutes. This equals a cycle time of only one second. The shot-blasting output was sized accordingly with two 15 kW shot-blasting wheels. Optimum efficiency and a high degree of economic viability are achieved by the concentrated shot-blasting of the components.

Special Loading Solution
As the machine is designed as a processing centre and is not directly linked to systems upstream or downstream of production, the slip-ring bearings for processing must be emptied from the wire-mesh cages into the machine. In this respect, the system concept represents a compromise between facility flexibility and automation. The benefits are that components from neighbouring factories can be safely delivered in the wire-mesh cages and fed into the machine for refinishing. Because the sensitive slip-ring bearings may not fall very far or jam, a special loading solution had to be found. The fact that a very small drop is enough to damage the components had to be taken into consideration. A special solution was planned and implemented accordingly. Udo Hampel, engineering consultant for Shot-Blasting and Vibratory Finishing Machines at Rösler said: "We have put forward a very individual solution to Schulte & Schmidt for their special processing application." It is as follows: a loading unit was built instead of a conventional lifting and tipping unit so that the buffer channel, which has to accommodate the whole component volume from the wire-mesh cage, is combined with the reservoir pick-up/tipping device. The wire-mesh cage is raised first and allows the aluminium components at the top swivel point to slide gently without a drop into the swivelling buffer channel. It is swivelled until the wire-mesh cage is standing on its head and therefore completely empty. The loading solution has been completely and individually re-calculated and re-designed for the processing application with regard to the angle of inclination.

Low-Maintenance Design
The diecastings are continuously transported quickly and without delay via a run-in channel into the shot-blasting chamber which is divided into three zones. The rotary movement of the trough together with the dual inclination means that the components are transported further with guidance and without contact in a fast continuous flow.
In order to design the system to be maintained easily, it was fitted with a special trough removal device, i.e. the trough is mounted on rollers and can be completely moved out of the machine. This enables individual bars to be easily changed and the complete trough change handled without difficulty. Large inspection doors enable minor maintenance tasks to be undertaken without moving out the trough. This small detail, which was demonstrated to the client in the Test centre, contributed to the decision by Schulte & Schmidt in favour of this system according to Udo Hampel: "Due to the opportunity of being able to demonstrate the different problems in our well equipped demonstration and test centre, our client was able to form a comprehensive picture even in the planning phase. This had a positive influence on, and simplified, the whole decision-making process for the client and ourselves."

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