See-through extruder reveals secrets

University of Toronto professor Steve Balke routinely does the impossible: he and his graduate students observe polymers inside an extruder barrel. The technology which makes it possible is a glass-walled extruder built as part of a project sponsored by Mississauga-based Macro Engineering (186), whose object is to better understand the dynamics of polymer and polymer-additive mixing in extruders.

“Our work is directed at finding out what’s going on inside an extruder; says Balke, whose research group operates within U of T’s Departments of Chemical Engineering and Applied Chemistry, exploring polymer processing, in-line monitoring, and off-line analysis. Since the majority of the action is invisible to the naked eye, the team is developing new ways to monitor melt flow processes. Fluorescent tracers (A.R. Monteith Inc., 190) added at the feed throat give a startling look at plastication and mixing when illuminated with ultraviolet lighing equipment supplied by Efos Canada Inc. (192) The images will be digitized and analyzed by computer. In-line particle monitoring is another component of the research. In-line particle monitoring isn’t new, but the Balke team, working with Nikolay Stoev, a senior staff scientist with Photonics Research Ontario (an Ontario Centre of Excellence) has developed a method to scan across the melt stream, rather than just observe the region adjacent to the barrel wall. An “interface” equipped with a CCD camera can be installed behind the nozzle, giving real-time analysis, including live video images.

Microgels and particulates are well-known quality problems, especially in films, and both are clearly visible within the river-like moving image. “There is a lot of interest in both the particle size distribution and the concentration of these particles,” says Balke, “With in-line process monitoring the big advantage is that you get an immediate answer as to whether your material is within specifications. You don’t have to run to the laboratory, and you can also sample a large quantity of product. That opens up the possibility of using process control.” Current research uses an extruder with a single-stage screw design and flat barrel temperature profiles, suggesting many opportunities for further research. With other projects measuring both composition and color in-line, the future may see closed-loop controls based on melt-flow quality, rather than on machine operating parameters. But as Stephen Balke states simply: “before you can control, you’ve got to be able to monitor.”

Professor Balke can be contacted at: balke@chem-eng.utoronto.ca