RKW and Nordson are taking steps to prepare for the future by enabling producers to process blown film with high recycled content

Compensating Material Quality Differences

By using more recycled material, the process needs to handle an additional component with much impact on the final product. „The quality of the recyclate containing dirt particles fluctuates significantly. Thus, we cannot guarantee that the quality of the final product is always the same. We needed to enable ourselves to react flexibly to varying levels of contamination, „says Sven Pastrik, Manager of Extrusion at RKW in Echte.

As a first step, continuous screen changers were implemented. These filters allow operators to change screens without interrupting production and adjust screen fineness for different production batches based on quality requirements. "As a result, we have significantly fewer machine downtimes, and the productivity of our plants has increased significantly," reports Sven Pastrik. "We can compensate quality differences in the material very well and process high proportions of recyclate, for example, in our FFS bag production."

The use of recyclate in RKW ProVent plastic bags, commonly used in the construction and chemical industry for packaging powdery goods, is becoming more prevalent and continues to increase. "Here, it is essential that the protective and barrier properties are guaranteed and that no dust, dirt, or moisture can penetrate," explains Thomas Steffen. "We inspect our films with optical systems before further processing. But for us, finding defects is not a strategy; the goal is to prevent them altogether. We knew that the proportion of recycled material would keep increasing and that we would have to redefine the filtration process when designing a new 5-layer line."

The goal was a system in which high-quality films could be produced in a continuous, automated process. "We needed a plant concept with fast setup and changeover times. We don't want to deal with screen changes and thus line downtimes often and for a long time, but rather ensure an extrusion process that is as uninterrupted as possible."

RKW had a positive experience with Nordson® BKG® screen changers, leading them to approach the filter manufacturer again. Their new five-layer plant is now equipped with continuous BKG screen changers and has been running successfully since 2021. "We are very satisfied with the performance of the system. Thanks to continuous filtration, we produce high-quality films with high recycled content in an uninterrupted process and can cover a wide application window," explains Sven Pastrik.

At RKW and Nordson, they were certain that the development must go further. "The use of recyclates will continue to increase in the future," explains Stefan Wöstmann. "The EU's Green Deal and national packaging laws are massively increasing the pressure and forcing system - and component manufacturers - to act."

The Solution: An Innovative Backflush Screen Changer

To expand the application window and process higher proportions of recyclate and prospectively even highly contaminated materials, e.g., from the yellow bag, without compromising quality, a screen changer with backflush function has now come into play. "We are already seeing that the backflush technology enables the use of recyclate from the PCR area," explains Sven Pastrik."

"Melt filters without a backflush function reach their limits when processing recycled material. The degree of contamination is high, and the screens become clogged quickly," explains Stefan Wöstmann. "Screens are expensive, and changing them is time-consuming. Backflushing repeatedly clears the screens of contaminants and deposits, so manufacturers need fewer filter elements, and operators have to change screens less often."

It was necessary to adjust to the conditions to achieve optimal results in blown film lines. Since the film bubble is sensitive to changes in temperature, viscosity, and pressure, even minor deviations can negatively impact the final product. Adding recyclates to the process can make it even more challenging. Pressure fluctuations during screen changes, backflushing, and deaeration can significantly reduce film quality or cause downtime. To address this issue, Nordson collaborated with equipment manufacturer Windmöller & Hölscher to develop the Nordson® BKG® HiCon™ K-SWE-HD/RS backflush recycling filter. This new filter is specially designed for blown film lines and has a compact design suitable for tight spaces.

Eliminating Even the Slightest Pressure Fluctuations

Maintaining constant pressure is crucial for a successful operation of a blown film line. Any material withdrawal for screen changes or backflushing can result in a balancing act in this area of tension. Hence, it is imperative to avoid pressure fluctuations through intelligent solutions in the high-pressure range. Each screen change in the filtration process is a sensitive step since the empty cavity must be refilled with melt after the change. If this is done too quickly, it can lead to pressure fluctuations and quality issues. "This is a major problem with blown film lines, as the film is usually very thin and sensitive to such changes," explains Stefan Wöstmann. "The BKG HiCon K-SWE-HD/RS is equipped with the patented melt pressure-controlled venting start, which fully automates the filling of the screen cavity after the screen change and thus ensures maximum pressure consistency. In addition, filling is so sensitive that no air pockets endanger the process or the end product. Manufacturers can rely on a stable process and high blown film quality."

The new BKG HiCon K-SWE-HD/RS was provided to RKW as part of a user test and installed in a 3-layer plant. "The start-up was a complete success. We installed the screen changer and produced film successfully immediately; our machine operators were thrilled," reports Sven Pastrik. "The handling of the screen changer is simple, and due to the high degree of automation, we only have to intervene minimally in the process," explains Detlef Nolte, machine operator.  "The fact that a newly developed machine was installed in one of our lines without any complications and then works so well directly is quite remarkable. In addition, it is immediately apparent that the experience gained from the initial joint project has been directly incorporated into the new screen changer.”