US based Davis-Standard and Gneuss, in Germany, recently joined forces to supply a non-dried PET sheet system said to combine high outputs with cost saving technology. This system merges Davis-Standard engineering and project management with a patented Multi Rotation System (MRS) from Gneuss.
Processing rates of 900kg/hour for PET thermoformed sheet can be achieved while reducing overhead costs associated with similar non-vented extrusion processes, the companies state.
The conventional method for processing PET, post consumer or industrial flake is via a single screw extruder. Using this method, moisture and volatiles must be removed first to avoid breakdown of the polymer chain during the melt phase of the process. This requires the feedstock to be crystallized so it can withstand elevated drying temperatures. The crystallizing, drying, and time involved is costly, requiring more energy, equipment and plant space for the finished product.
The technology offered by Davis-Standard and Gneuss streamlines this process. On the Davis-Standard system, a blender combines the PET, post consumer or industrial flake with virgin pellet flake from each component’s blender hopper in one step, eliminating the crystallizing and drying processes. Once blended, the resin is vacuum conveyed to the feed throat of the MRS extruder for processing.
The design of the MRS extruder is comparable to the vent section of a single or twin screw extruder where degassing of the melt stream occurs under precise vacuum control. However, the degassing section of the MRS is much larger, surpassing the surface exchange of a twin screw by 25 times and a single screw by 50 times. This makes it possible to remove moisture and volatiles in the melt stream even more efficiently.
The degassing properties of the MRS are also effective when used with a moderate vacuum level, enabling processors to use less expensive and easier to maintain vacuum equipment. Other advantages claimed include low shear stress and thermal stress for melt, excellent pressure build-up (eliminating the need for a gear pump in some applications), low energy consumption and a compact design.
In addition, when processing post-consumer reclaim, the degassing section is said to provide excellent decontamination of the material. FDA approval for processing PET bottle flakes is pending.
The finished sheet is thermoformed into translucent trays for fresh food items such as strawberries and tomatoes.
