Low-maintenance polymer bearings keep the nappies flying
09 June 2013
Dry running linear bearing systems installed on Ontex Mayen’s nappy production lines allow 24/7, all year round, lubricant-free operation and speedy changeovers of production systems for the variety of product formats offered by this leading European manufacturer.
Ontex Mayen is Europe’s leading manufacturer of hygienic disposable products for the private label sector, offering a wide range of products for baby, feminine and adult hygiene. At one of its plants, eleven lines manufacture all sizes and variations of babies' nappies, which are sold worldwide.
Each nappy production line is approximately 60m long and produces and packs approximately 1,000 nappies per minute - that's around 17 nappies per second. The throughput time for one nappy is about two seconds and between six and eight million nappies leave the factory every day.
The output of nappy production lines at Ontex Mayen has tripled over the past few years – and this has nothing to do with investment in new machines. Ensuring that the production line runs with virtually no downtime, plus a programme of continual process improvements have contributed towards this achievement. In addition, the production line set-up times have been reduced significantly.
Nappies differ depending on customer requirements and various raw materials, percentages or compositions are used. Some customers want their nappies to be perfectly white, while others want a colourful product.
As the nappy manufacturer serves many customers, line safety and flexibility must be guaranteed for production and packaging, and every order or size changeover results in set-up times or adjustments, as the company’s head of engineering, Thomas Heege explains:
“We have an average of four changes in size per week per line, which adds up to more than 200 changes in size per year. In addition, we have three product changes, which makes at least 30 a day. Our lines run round the clock seven days a week. In other words, they are used almost 365 days a year without interruption.”
High process speeds of up to 600m/min subject the lines to high dynamic loads, and the super-absorber and cellulose used in the manufacturing process result in a relatively high dust yield. The super-absorber behaves in a similar way to sand and contamination occurs through the effects of friction.
The necessary flexibility when changing the lines to different product formats, not to mention a high tolerance of these dusty conditions, is provided by igus drylin linear bearings, used throughout the plant.
Lubricant- and maintenance-free, insensitive to dust and dirt, corrosion-free, durable, quiet and lightweight, they are mainly responsible for making precise, positive adjustments at various stages along the line, such as the adhesive application points.
The adhesive application head must be moved reproducibly according to the product being presented for assembly, a process that has proved to work reliably using linear guides. The drylin linear systems used here are also reported to be performing well in the quality assurance and packaging areas of the production lines.
A variety of lead screw driven linear units or tables of various designs and sizes are used across the production lines. They are often equipped with lead screw clamps to prevent unintentional twisting, position indicators for safe position setting and a hand wheel for manual adjustment.
Both right- and left-hand threaded trapezoidal nuts are used, which, as Thomas Heege explains has proved very useful when they needed a combined left- and right-hand thread on one threaded lead screw. “With this, we can adjust two positions at the same time around a virtual centre point,” he says. According to igus sales consultant, Markus Schwarz, while this opposed linear movement configuration was created for this specific application, igus imposed no minimum order for the system.
Changing production settings
There are special format setting lists for every size or product changeover. When a line is retooled, operators are informed of the respective parameters. Once set-up has been completed, it is 100 percent correct; no readjustments have to be made and production can begin reliably.
In addition, the production lines make use of the igus ‘Easy Tube’ system, a compact linear unit for light adjustment tasks with a simple, effective and robust set up; a complete system can be built using only a few components.
The outer anodised aluminium tube guides the carriage while protecting the trapezoidal threaded lead screw and nut from external contamination. The slides, torque compensator and trapezoidal threaded nut are made from a high-performance polymer which offers lubricant-free operation combined with low friction and very good wear performance.
Along with other components from the drylin W modular system, RJUM linear plain bearings of different design sizes substitute ball bearing guides in a range of different components. They are based on highly wear-resistant polymers specially developed for linear technology. igus says these tough and economical materials are available from stock within 24 hours of order placement.
Other special equipment used on the nappy production lines includes a material handling crane which brings the heavy rolls of raw material to the processing stages. These overhead cranes are 2m high and move over large distances.
Hybrid bearings modified from the drylin W range, which offer a combination of sliding and rolling have also proved themselves in this environment, as Thomas Heege explains. “We mainly use these bearings for ergonomic reasons,” he says. “As friction values are minimised, adjustment and drive force are kept to a minimum and material feeding can easily be done by one operator without physical effort.”
The hygienic process environment required for nappy production means that Ontex Mayen is classified as a food processing company; consequently, lubricants are generally banned throughout the production areas – a requirement adequately met by the igus lubricant-free linear units.
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