The mixing head is the heart of the polyurethane foaming system because this is where it is decided whether the polyol and isocyanate components will be homogenized thoroughly and in the desired ratio. If this does not happen, sticky spots appear on the cured foam or it is too hard or too soft. Both raw materials flow through nozzles (throttled by needles) into the mixing chamber, where they are atomized and mixed at 100 to 200 bar. If material is to be discharged, the control piston opens the path to the mixing chamber until the shot weight is reached. For use in open molds, a transfer mixing head is required, in which the resulting foam is deflected at a 90-degree angle into the outlet chamber and completely ejected by a cleaning piston. KraussMaffei has held numerous patents for this technology since the 1980s and the mixing heads are known in the industry for their high performance.
Grammer, the world's leading supplier of seating solutions for commercial vehicles, operates two rotary table systems at its Haselmühl plant (also from KraussMaffei and in operation since 2001), each with over 20 molds for tractor, construction machinery and forklift seats.
A mixing head demonstrates its capabilities with the lowest possible material discharge. Grammer runs at 220 to 250 grams per second in series production. With the new mixing head, the team led by Heinrich Hammer (Head of Process Engineering, Grammer Deutschland GmbH) tested the limits. "We went down to 60 grams and homogeneity and uniformity were still top-notch. The production window has become enormously wide as a result." Another benefit of the new generation of mixing heads is that certain components have been simplified and made interchangeable, increasing the ease of maintenance. For the first time, the mixing chamber, outlet chamber and control piston can be completely replaced if worn. The latter two are now single-piece (previously multi-piece), which in turn saves space and weight.