The Spanish province of Almería claims to have the largest cultivation area under film. In total, around 36,000 hectares are covered with plastic, primarily in greenhouses. That is how the region manages to provide a large portion of Europe with fresh vegetables year-round.
Sotrafa produces multilayered foils for different applications
This makes it unsurprising that a major foil manufacturer is also headquartered in this region in the city of El Ejido, namely Sotrafa S.A., a company of the Grupo Armando Alvarez. The group is Spainʼs largest foil processor, with group sales of nearly 700 million euros. The subsidiary Sotrafa does not just produce greenhouse foils though. Another pillar of its production is geomembranes, mostly multilayered foils for large-format applications. Potential uses for geomembranes include sealing landfills, in order to collect and drain – often aggressive – leachate. They are also used for creating artificial bodies of water such as firefighting ponds or storm water tanks. And in agriculture, they are used to cover fields in order to both reduce evaporation and enable selective irrigation of crops.
Extrusion system with polishing stack in size XXL
“KraussMaffei Berstorff has built a special extrusion system for manufacturing this type of membrane. It features a polishing calender that differs from others due to its unusual dimensions,” reports Hilmar Heithorst, Division Manager for Sheet and Foam Extrusion Lines at KraussMaffei Berstorff. “Featuring a face width of 8.3 meters, the polishing calender is the largest that KraussMaffei Berstorff has ever built. Not to mention the heaviest: Each of the three rolls weighs a little over 30 tonnes,” Heithorst continues. These massive weights place special requirements on the design. The challenge lies in ensuring that the rolls have a high bending stiffness despite their above-average width. This is necessary in order to ensure an optimal thickness profile for the foils. The solution: The conventional cooling coils are replaced by peripheral drill holes over the entire face length right below the roll surface. Even at a diameter of 20 millimeters, drilling these cooling channels requires maximum precision so that they do not stray from their optimal positions, and so that the roll is cooled securely and uniformly.
In-line compounding and shaping in a single work step
The polishing calender is part of a system used to manufacture multilayered HDPE foils. These foils consist of an intermediate layer made of recyclate with filler, a top layer made of HDPE with UV stabilizer and dye as well as a bottom layer, also made of HDPE. Additional additives are integrated into the top and bottom layers to improve their ability to be fused with the material in the middle. For extruding these layers, three two-screw extruders with an overall capacity of 3,700 kilograms per hour are connected upstream of the polishing calender. The system is designed for foil thicknesses of 0.75 to 3 millimeters, for net foil widths up to a maximum for 7.5 meters and the nominal roll width of 8.3 meters. Using the two-screw technology, all raw materials are compounded in-line.The in-line compounding allows the overall production costs to be optimized. The following individual factors support this optimization:
- The use of more cost-effective raw materials instead of finished compounds,
- Lower raw material costs thanks to complete reincorporation of start-up material, edge strips and scrap webs,
- High throughputs
- Lower operating costs thanks to a reduced need for energy, equipment and maintenance,
- More flexibility and the option of producing on shorter notice, particularly with non-standard formulations and
- Lower recipe production costs with increased amounts of additives.
The polishing calender calibrates the extruded foils for the purpose of further cooling, lamination, trimming and winding based on the material type and thickness. “This system allows us to demonstrate just what is possible with polishing calenders from KraussMaffei Berstorff. For us, the system is a true benchmark,” says Heithorst. Production is scheduled to start in October 2017.