Perfect PUR Mixing
Mixing heads with proportional valve technology automatically match the metering gap at the outlet from the mixing chamber to varying flow rates during production. This contributes to improved quality and reproducibility when manufacturing flexible PUR foam products with different hardness zones.
Automotive seating of PUR foam is manufactured with up to four hardness zones in order to provide maximum comfort to passengers. For PUR mixing heads, such parts present a special challenge, since the viscosities and flow rates of the PUR mixtures change from shot to shot when producing the different hardness zones.
Special mixing heads can process up to six different components. How well the components mix is critical for product quality. To improve the mixing action, the cleaning piston is positioned such that the cross section of the outlet from the mixing chamber is matched to the flow rate (amount of material dispensed). To date, it has only been possible to position the cleaning piston by means of a screw. With this system, it was not possible to vary the position from shot to shot.
Adjusting the gap from shot to shot
Quite recently, Krauss-Maffei has started to offer mixing heads in which the metering gap can be adjusted quickly and automatically. A servovalve moves the cleaning piston hydraulically to various positions. The cross section of the mixing chamber outlet thus changes with each shot to ensure uniform mixing of the components – even when short shots of different mixtures at different discharge rates follow in quick succession. The cleaning piston can even be actuated during the shot. In this way, the gap can be adjusted exactly even as the discharge rate changes in the course of the shot.
In addition, the control piston is also braked hydraulically in the new generation of mixing heads before reaching reaching the end position. The result is lower mechanical forces than in conventional mixing heads. This extends the service life of the mixing head and prevents spikes in the hydraulic system.