New MuCell Screw Delivers 30% More Plasticizing Performance

Physical foaming of thermoplastics using the MuCell process saves money, energy, and resources. German injection machine OEM KraussMaffei is taking that to the next level: After intensive development work, the company presents the new HPS-Physical Foaming universal screw for MuCell applications with 30% higher plasticizing performance.

In an effort to conserve resources, MuCell is being used more and more frequently to lower CO₂ emissions. By adding a physical blowing agent — usually nitrogen — to the thermoplastic, it is possible to save significant material weight compared with compactly manufactured components. In addition, longer flow paths are possible for thin-walled components, and foaming reduces warpage.

Universal screw has versatile applications

The molding materials used in the MuCell process are diverse and often contain different proportions of fibers and fillers. KraussMaffei has responded to this by developing a screw that can be used universally. 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 center backflow barrier, and the three-zone area.

More plasticizing performance with less wear

The three-zone area can be enlarged without loss of melt quality and is now 17 times the diameter (previously 15D); the mixing and gas supply area is now 4D. This has increased plasticizing performance and improved wear behavior. Until now, larger screws (with a correspondingly higher investment) were usually used, dictated by the component weight to ensure complete homogenization of the melt before injecting the gas.

The center 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 back-flow of the melt. This is the only way to keep the critical pressure above 33.9 bar and, thus, prevent foaming in the plasticizing unit, according to KraussMaffei. Another task of the M-RSP is to ensure constant shot weight. The developers' analyses showed that the most effective design of the M-RSP has a ball check valve.

Trials with different fillers

The KraussMaffei team investigated existing and newly developed screws using PP with different melt flow indices (MFI 11 and MFI 44) and with mineral filler, glass-fiber content of 20% and 30% (LGF), and 30% glass-fiber-reinforced ABS and polyamide (PA) 6. Plasticizing performance varied depending on the compound and parameters, such as dynamic pressure.

Since KraussMaffei's MuCell customers have a variety of applications worldwide, the universal screw had to reflect this diversity. The three-zone section, therefore, was designed with one thread (for PA 6 GF30, two threads would be even better), and the M-RSP non-return valve included a ball check. For high-viscosity materials, a helical shear section is also possible.

The in-depth research yielded additional benefits beyond HPS-Physical Foaming. KraussMaffei now has a knowledge toolbox that makes it possible to develop screws specially adapted to a material, of value to customers who manufacture corresponding products constantly over a long period of time, for example. Here, too, the modular concept is helpful for quick tests.