(Munich, October 18, 2017) From a process technology standpoint, the mixing head is front and center of PUR processing. The prerequisite for a high product quality and maximum efficiency is that the mixing heads adapt exactly to the requirements of the respective PUR product. In this area, KraussMaffei presents three new mixing head developments that make it possible to increase flexibility and shorten cycle time.
"KraussMaffei has developed the linear mixing head MK 5-2+1K specially for the ColorForm process, and our product fulfills the requirements for this procedure perfectly," states Sebastian Schmidhuber, head of RPM Research and Development at KraussMaffei. During the ColorForm process the thermoplastic mold base body is flow-coated after the injection molding in the second cycle with polyurethane (PUR) or polyurea (PUA) as a surface material. The use of the new mixing head allows for a very fast dye change – to do so, the dye module mounted to the mixing head need only be replaced. This process is possible in few minutes. Dye carryover can be excluded due to the design of the mixing head because the dye circulates in a closed system in the dye module, and thus not through the mixing head. The dye nozzle opens only when material is discharged, and the dye is added directly in the mixing chamber.
The new ColorForm mixing head is lightweight and compact, which reduces the space requirements and facilitates the installation on molds. Furthermore, the new mixing head series is particularly well suited for reliable and uniform mixing of smaller PUR quantities. "This is a critical factor in the ColorForm process, because the applied PUR layers are very thin,"according to Schmidhuber.
In order to simplify the dye change during the production of semi-rigid foams or rigid, flexible foams, KraussMaffei has developed the transfer mixing head MK 10/15 ULP-2K+4 with multiple dye dosage. With this mixing head it is possible to feed up to four dyes directly and independently in the mixing chamber.
"The mixing head is suitable for all applications in which PUR processors face the challenge of providing components in relatively small quantities and in numerous different colors," according to Schmidhuber. With the new transfer mixing head, it is now possible to change the dye in continuous production from shot to shot. "The result is clearly increased flexibility of manufacturing. Since production no longer has to be interrupted for a dye change, the availability of the machine increases also," explains Schmidhuber.
To reach a higher mixing quality even in unfavorable mixing conditions, the mixing head works according to the principle of T-mixing, which means that the polyol component is split in two substreams and introduced through two nozzles to the mixing chambers. At the newly developed mixing head two different dyes can be added at both polyol nozzles respectively. The metering occurs at the same time between polyol nozzle and mixing chamber, so that no dye carryover into the polyol component takes place. Through this positioning it is also possible to meter the dye with relatively low pressure.
The mixing heads used in Structural Component Spraying (SCS) have also been further developed. In this process technology, layers made up of fiber mats and honeycomb cores are sprayed with unreinforced PUR, inserted into a mold and pressed into shape. SCS permits outer layers with low thickness, so that the manufactured components are particularly lightweight.
Flat-fan nozzles and air-assisted circular jet sprays are available for the spraying process. Flat-fan nozzles produce a fan-like flat jet and are suited for large-format coating of the PUR mixture on relatively level, large-sized components. The air-assisted circular jets produce a finer spray and have a lower output capacity than the flat-fan nozzles. They are used primarily for component areas with challenging geometry.
The newly developed nozzle-changing component by KraussMaffei now makes it possible to equip spray mixing heads with both nozzle varieties. In the manufacturing of the component, both nozzles can go into action alternately. This has the advantage of making the cycle time shorter. "Furthermore the investment costs are reduced because only one mixing head is required and the previously inserted hydraulic switchover unit is not required anymore," adds Schmidhuber.