Compounds: Research for the best compound quality

HF Mixing Group specializes in the development and supply of machine and automation solutions for the international rubber processing industry. At its German location in Freudenberg, the company uses measuring technology from Pixargus in its R&D lab.

As Head of Technical Center & Process Engineering, Ricarda Kendler is the person in charge of the latest machine developments and activities in process engineering at HF Mixing Group in Freudenberg, Germany. Whether for tyres, sealings, cables or brake pads: “In our Technical Center, we can try out and produce blends for virtually any application in the polymer sector,” says Kendler. “Our Technical Center, which is equipped with two larger lab-scale mixing lines, is the beating heart of our research and development activities.”

With its about 1,000 employees worldwide, of whom about 400 are located in Freudenberg, HF Mixing Group serves the entire market of the rubber processing industry. Mixing room solutions for the entire process chain - from weighing and mixing to the downstream processing plants - and automation solutions from HF Mixing Group, based in the German Siegerland region, are worldwide in use in technological rubber and tyre production facilities.

The development activities at the Technical Center are often performed in close collaboration with the customers. Here, tyre producers and manufacturers of technological rubber products have the opportunity to develop new formulas and test them right away at production scale on the fully automatic mixing lines installed at the Center, as an answer to the ever more exacting requirements placed on the raw materials and the production processes. Transformation has also reached the rubber industry. The entire sector is working to become more sustainable and digital.

“It is essential that we can demonstrate that - in addition to achieving the desired throughput - our solutions also guarantee that the compound meets the customer’s quality requirements. The Center is the perfect environment to demonstrate our products and innovations and convince the customer of their quality,” explains Kendler. For measuring the quality at the Technology Center, HF Mixing Group uses systems from measuring technology specialist Pixargus, based in Würselen, near Aachen in Germany.

Ricarda Kendler studied mechanical engineering with focus on process engineering at Ruhr-Universität Bochum. After earning her doctorate degree in 2018, she joined HF Mixing Group as Head of Technical Center. In 2021, she additionally became Head of Process Engineering. Her passion is analytics: “We constantly strive to enhance our understanding of how things correlate with each other in order to optimize processes.” The Technical Center in Freudenberg provides a hands-on approach to analyzing the performance of mixing room equipment along the entire process chain from material feeding up to the finished compound and downstream processing stages such as the extruder, rolling mill and batch-off unit. “You can’t study all this from a textbook. It rather takes a great deal of experience. We learn something new every day.”

A key subject Kendler and her team are working on is the further reduction of energy use in the mixing process. “We try to optimize the mixing process by reducing the mixing time and/or the number of mixing stages.” Important parameters for the team to study include the time and sequence of material feeding, the rotor speed and other process-inherent aspects. Another ‘actuator’ in this respect is the selection of the best suited equipment. “All these considerations have the potential to save energy and make the process more sustainable as a result.” In a current R&D project, the team is testing a new generation of mixing rotors that use less energy during operation and achieve higher throughput rates. Kendler knows: “It is essential that with the many optimization options available we still get the same high quality in the end.” And this is checked at the Technical Center in Freudenberg with the measuring systems from Pixargus.

We are observing how the biggest of the interior mixers installed at the Technical Center is discharging a black batch weighing about 35 kilograms. The stand-alone single-screw lab extruder is ready and waiting to produce a thin extruded strip from the batch. “Now we let the strip run through the Pixargus system. It’s as simple as that,” explains Kendler. The argus-eyed optical sensors of the system check the dispersion quality of the compound. Have the particles been uniformly comminuted? Has the rubber homogeneously mixed with the fillers, such as carbon black? “We don’t want to see any large particles or agglomerations of particles.”

The Pixargus inspection system ProfilControl 7 Roughness solves this task by measuring the particle size distribution. The system features a special LED unit that illuminates the test strip in such a way that the integrated high-resolution line scan camera can capture every detail of the strip surface. The images taken by the camera are evaluated by a dedicated software in real time at a rate of approximately 100,000 particles per minute. The system triggers an alarm signal as soon as the size distribution is not as specified. Even extremely small polymer specks are reliably detected down to a size of 20 µm.

The HF Mixing Group has been using measuring systems from Pixargus for about 20 years. Just recently, the group ordered another new Pixargus inspection system. “The benefits speak for themselves,” says Kendler. “To start with, the sample preparation is very easy and samples of any size can be continuously inspected 100%.” Not less important is the fact that the Pixargus system can inspect both master batches and final mixtures. “Most of the measuring methods available focus on the inspection of vulcanized samples. However, we want to measure the quality ex the mixer, that is to say in the non-vulcanized state. We know that we can achieve this with the Pixargus system,” emphasizes Kendler.

For the HF Mixing Group, process optimization today is closely related with digitalization. The automation solutions for mixing rooms developed by the mechanical engineering experts at Freudenberg leverage untapped efficiency potential. “As a matter of fact, we want to improve the efficiency of resources. However, we also look for possibilities to substitute the partly quite heavy physical work and ease the consequences of the shortage of skilled labour for our customers. Our big vision is the entirely unmanned mixing room,” says Kendler. The centerpiece of all activities at Freudenberg remains the mixing process because, as Kendler puts it: “The quality of a compound is set in the mixer. The influence of the downstream processes and equipment to improve the quality afterwards is really limited.”

The Pixargus measuring experts know this very well, too. They offer their ProfilControl 7 Roughness system as a highly compact smart version for lab environments and as an inline system for the mixing room. “We want to make it as convenient as absolutely possible for our customers to integrate the system into the line and adapt it to different tasks,” says Pixargus Sales Manager Michael Frohn. The system operates within an integrated network. Its software provides a wide range of analysis functions. By relating the analysis data from the mixing room to the data acquired by the inspection systems that check the quality of the final product at the end of the production chain, it is possible, for example, to tell whether a compound will produce optimum quality or whether it will result in out-of-spec products. “With this knowledge, we can make the production processes even more reliable,” Frohn adds. Pixargus looks back with pride on the longstanding cooperation with HF Mixing Group, which was one of the company’s earliest customers.

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IMG 1: The Technical Center of the HF Mixing Group features two larger lab-scale mixing lines. For quality control, the compound mixing experts use inspection technology from Pixargus. (Photo ©HF MIXING GROUP)
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IMG 2: The Pixargus inspection system ProfilControl 7 Roughness checks the dispersion quality of rubber mixtures by measuring the particle size distribution. Even extremely small polymer specks are thus reliably detected down to a size of 20 µm. (Photo ©HF MIXING GROUP)
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IMG 3: Pixargus proprietary software evaluates the camera images in real time. Highly valuable quality data can be derived from this evaluation process. By relating the analysis data from the mixing room to the data acquired by the inspection systems checking the quality of the final product at the end of the production chain, it is possible, for example, to tell which compounds will produce optimum quality and which ones will result in out-of-spec production. (Photo ©HF MIXING GROUP)