According to KraussMaffei, physical foaming of thermoplastics (MuCell) saves money, energy and resources. After intensive development work, KraussMaffei is presenting the new HPS-Physical Foaming universal screw for MuCell applications.
Lightweight solutions with a smaller CO2 footprint
KraussMaffei says that In the effort to conserve resources, MuCell is being used more and more frequently and is a strong growth market. By adding a physical blowing agent to the thermoplastic (usually nitrogen), it is possible to save significant material weight compared to compactly manufactured components. In addition, longer flow paths are possible for thin-walled components, and foaming results in low-warpage components.
Universal screw with versatile applications
The materials used for MuCell are diverse and often contain different proportions of fibres and fillers. In the HPS-Physical Foaming, KraussMaffei has responded to this by developing a screw that can be used universally and according to the company has a 30 percent higher plasticizing capacity.
For this purpose, all screw types available on the market were compared in extensive laboratory tests. A modular concept made it possible to combine the individual components by means of bolting so that a new screw did not have to be made each time. The focus was on the mixing and gas supply area, the centre backflow barrier and the three-zone area.
More plasticizing performance with less wear at the same time
KraussMaffei says that the three-zone area was able to be enlarged without loss of quality at the expense of the former and is now 17 times the diameter (previously 15D; mixing and gas supply area now 4D). This both increased the plasticizing performance and had a favourable effect on the wear behaviour. Up to now, larger screws (with a correspondingly higher investment) were usually used than the component weight would normally dictate in order to ensure complete homogenization of the melt before injecting the gas.
The middle non-return valve (M-RSP) closes at the end of the metering process, thereby separating the mixing and gas supply area from the three-zone section and preventing the melt from flowing back. According to the company, this is the only way to keep the critical pressure above 33.9 bar and thus prevent foaming in the plasticizing unit. Another task of the M-RSP is to ensure a constant shot weight. The developers' analysis showed that the most effective design of the M-RSP has a ball check valve. Here, too, the various materials used played a major role.
Trials with different fillers
The KraussMaffei team investigated the previously existing and the newly developed screw using PP with different flow indices (MFI 11 and MFI 44), with mineral filling, glass fibre content of 20 and 30 (LGF) percent, and with ABS and PA6 GF 30. According to Krauss Maffei, the plasticizing performance changes depending on the compound selected and the parameters set, such as dynamic pressure.
Since KraussMaffei's MuCell customers have a wide variety of applications worldwide, the universal screw had to reflect this diversity. The three-zone section was therefore designed with one thread (for PA6 GF30, two threads would be even better), and the M-RSP with ball check (for high-viscosity materials, a helical shear section is also possible).
The in-depth research effort yielded additional benefits beyond HPS-Physical Foaming. KraussMaffei says it now has a knowledge toolbox that makes it possible to develop screws specially adapted to a material—for customers who manufacture corresponding products constantly over a long period of time, for example. Here, too, the modular concept is helpful for quick tests.