How to save energy with optimal screw design
The screw is the heart of the extruder. It conveys the plastic granulate forward in the barrel and transfers the melt to the downstream die. But not all screws are the same: "The design influences the output, the product quality and the efficiency of the process," explains Udo Nuber, who advises customers on this subject at Reifenhäuser Reiloy. "The optimum screw design is therefore important for the economic success of the producer." This also includes energy efficiency - especially in times of rising energy prices.
But getting the design of a screw right is anything but trivial. "Nowadays, there are many designs that look similar but don't have the same performance," says Nuber. For example, the optimal screw design must avoid pressure fluctuations, as these can lead to differences in film thickness or, in the worst case, total extruder failure. The challenge here is that every plastic, filler and additive influences the melting process. "Therefore, everything has to be aligned with the actual process, from the geometry of the screw to the wear protection and the interaction with the other components of the plasticizing unit," says Nuber. Reifenhäuser Reiloy has more than 50 years of experience in screw design for this purpose and offers customers design consulting and optimization.
3 adjusting screws for optimum screw design
1. Screw design adapted to the application
Influencing factors in screw design are, for example, the design of flight depths, shear gap, barrier bar, spiral shear section and mixing section - in other words, a wealth of different factors that have to function in combination. Due to this complexity, Reifenhäuser Reiloy simulates processes digitally before the screws are built. The simulation allows the performance of different designs to be tested and compared with each other. In addition, each new screw design is tested in the company's own laboratory.
2. Suitable wear protection
Wear protection also plays a major role in screw design. If, for example, the gap between the screw and barrel changes due to wear, the process no longer runs stably. Wear-resistant screws, on the other hand, ensure permanently high performance. Lines also have to be stopped less frequently for maintenance work and then restarted in an energy-intensive process. Reifenhäuser Reiloy has different iron-based alloys and nickel-based alloys in its portfolio for various wear protection requirements, which fit a whole range of raw materials. In addition, Reiloy is the only screw and barrel manufacturer that develops and produces its own hard material alloys - and even the powders required for wear protection - in its own materials laboratory.
3. Components matched to each other
However, the screw can only be as efficient as the entire plasticizing unit. For this reason, for example, the material pairing between the screw and barrel must be matched to each other, as must the drive technology, heating bands and fans.
These individual influencing factors work together and thus determine the extruder's efficiency - in other words, how much energy has to be expended to melt one kilogram of plastic. The efficiencies of different screw designs can be compared with each other and the associated energy savings calculated. "Especially in the case of high energy costs and mass-produced products with high competitive pressure, reductions in the decimal places already make a difference here," explains Nuber (see graphic).
Convert screw
The screws of current Reifenhäuser extruders are already designed for an energy-efficient process. However, producers who want to save energy, improve their performance or switch to a different raw material can also replace the screw in existing plasticizing units with an optimized version. "Of course, a retrofit only works if the overall system is still efficient enough," Nuber explains. "It is important to look at the entire process; the screw is only one component."
Available geometries
For this reason, Reifenhäuser Reiloy always first determines the actual condition when receiving inquiries and simulates the process to identify problems. On this basis, the experts develop an optimum design for the extruder screw. "This procedure is also a good way of demonstrating that an optimized screw usually brings a fast return on investment," says Nuber.
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