Vacuum Clamping for Aluminium Plate Processing

Willich, Germany The core expertise of Alimex GmbH, based in Willich, Germany, is manufacturing low-tension aluminium cast plates with a homogeneous structure. These semi-finished products are used wherever very high demands are placed on the material. The plates are processed on CNC portal milling machines to face mill the surfaces. A vacuum generated by Mink claw vacuum pumps from Busch is used to hold them securely on grid tables. In vacuum generation of this type, Alimex has found a highly reliable, clean and energy-efficient solution.

Alimex was founded in 1968. Initially, it only worked on sawing sections out of rolled plates.

At the start of the 1970s, Alimex specialised in sawing plates out of aluminium rolling ingots. Unlike rolled plates and sheet metal, sawing can be used to produce aluminium plates that have a homogeneous structure and are stress-relieved. These are essential requirements for many customers in the field of aviation technology, the optics industry, and the solar and semi-conductor industry, when processing materials to form high-quality components. Today, Alimex supplies customers worldwide with aluminium case plates in various aluminium alloys with sawn, milled and mirror-finish surface qualities.

Standard sizes measure 3 x 1.5 metres, and special formats of up to 5 x 2.9 metres are also available on request. The alloy EN AW-5083 (AlMg4.5Mn0.7) is most widely used, and produces low-tension precision cast plates with very balanced mechanical properties. The fine, low-porous cast plates are also homogenised by extended heat treatment and undergo thermal stress relief. At the customers' request, the surfaces of these plates are milled with a surface finish of under 0.4 micrometres (µm), mostly on both sides. A special milling technique means that values below 0.2 micrometres (µm) are also available.

Fig. 1: Portal milling machine with processed aluminium cast plate on the grid table

Although the standard plate thicknesses are between 5 and 50 millimetres, much thicker plates are also produced.

Alimex uses rolling ingots obtained from different foundries as its raw material, and these are annealed under tension before processing to eliminate the tensions that were created during the casting and cooling process.

An initial step sees the ingots sawn to size on a vertical band saw, and the plates are then sawn on a horizontal band saw. Some of the aluminium plates are delivered with a saw cut, though the majority are further processed on one of the three CNC portal mills. The milling heads on two of the mills have a diameter of 1.7 metres, meaning that standard-format plates can be milled on one side using a feed motion. The third mill has a milling head diameter that mills a maximum width of 2.5 metres using a feed motion.

Fig. 2: Mink claw vacuum pump for generating retaining vacuum on one of the three CNC portal milling machines

All three portal mills were originally equipped with oil-lubricated rotary vane vacuum pumps, which generated the vacuum to hold the aluminium plates on the grid table. However, Production Manager Najib El Byad was not satisfied with this solution. The maintenance effort for these vacuum pumps was considerable, as the filters for exhaust air and oil had to be changed, and the oil had to be topped up, every three months. In addition, the oil had to be completely replaced once a year. Some of the fine oil mist that escaped from the oil-lubricated rotary vane vacuum pump condensed on the mirror-finish surfaces of the aluminium plates – and this contamination could not be tolerated. Moreover, the level of energy consumption was considered too high as each of the three vacuum pumps was equipped with a 7.5 kW motor – this meant that the energy consumption to produce the vacuum 24 hours a day, five days a week came to 121,500 kilowatt hours a year. 

Busch then suggested to Najib El Byad the use of Mink claw vacuum pumps to hold the aluminium plates when milling. These vacuum pumps generate the retaining vacuum without oil or contact, and as a result the extracted air can be compressed without operating fluids. The contact-free technology in turn prevents wear in the vacuum pump, making the pump almost maintenance-free. Furthermore, contact-free operation enables a high efficiency factor, which again has a positive effect on energy consumption. The benefits could be seen right from the first few months of operation after the new vacuum technology was installed.

Fig. 3: Section of a Mink claw vacuum pump

Oil-free operation means there is no contamination whatsoever from Mink claw vacuum pump exhaust air. Maintenance is limited to the preventive annual gear oil change. A significantly smaller quantity of gear oil (0.85 litres or one litre) is used than with rotary vane vacuum pumps that need replacement, as the latter require an annual oil change with 5 to 6.5 litres of oil in addition to the quarterly top ups.
The output of the new Mink claw vacuum pumps is precisely configured to provide the required clamping force, meaning that a smaller size was installed on each of the milling machines for standard plate sizes than on the mill for special formats. Both of the smaller Mink claw vacuum pumps are driven by a 3-kilowatt IE3 motor, and the larger one has a 6-kilowatt IE3 motor. As a result, the annual energy consumption for vacuum generation for all three milling machines is 64,800 kilowatt hours, which almost halves the energy costs compared to the previous solution. After more than two years of operating time, Production Manager Najib El Byad is very happy with the investment in the new Mink vacuum technology, which has met all expectations, as well as proving that vacuum generation works very reliably. There have been no faults or downtime whatsoever.

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