Scraper blades for single screw extruder
Angled scraper blades provided on the end of a single screw extruder provide thrust for propelling polymer toward the die opening of the extruder. The scraper blades help prevent trapping of polymer between the shaft and the die plate of the twin screw extruder, which trapping can lead to scorching, burning or break down of the polymer.
Screw type extruders may be used for masticating and mixing polymer compositions such as plastics and rubbers, and in the finishing process, for drying rubber. Raw materials are added in one end of an extruder housing and said materials are broken up and mixed as they are transported by the flight of the screw shaft toward a die opening in the opposite end of the housing. Those skilled in the art carefully watch the viscosity of the composition and screw speed in relation to the physical properties of the composition since the mixing generates heat, and excessive heat may cause breakdown, burning or decomposition of the polymer being mixed.
In some prior art sheet production extruder, the end of the shaft next to the die plate is equipped with a scraper blade, extending beyond the end of the shaft and oriented parallel to the length direction of the shaft (i.e. perpendicular to the flat surface of the end of the shaft) to help scrape the mixed polymer trapped between the end of the shaft and the die plate of the extruder away from the die plate, and to help force the mixed polymer into the die opening in the die plate. Such prior art scrapers push the polymer radially outward and do not provide any forward impetus to the mixed polymer, but rely on a continuous flow of polymer from the extruder to keep extrudate flowing through the die opening.
Sometimes, and more frequently as the scraper becomes worn, portions of the polymer composition become trapped between the die plate and the shaft, and the continuous turning of the shaft against the polymer generates friction and overheating of the polymer until it decomposes. This decomposition may manifest itself in the creation of a gel or oil, or scorching or burning of the composition, which may contaminate the product or create packaging and shipping problems.
The problem is particularly noticeable when there is no scraper blade. In one instance, a Goodyear maintenance supervisor in Korea, where no parts were available improvised a scraper blade by cutting a piece of pipe in four pieces (each section comprising 90° of the 360° circumference of the pipe) and welding the sections to the end of the cap screw at an angle of 90° relative to the face of the cap screw, to function as a scraper blade.
It is an object of this twin screw extruder to provide a more uniform extrudate product by providing more consistent and uniform extrudate progression from a single screw extruder, especially between the extruder shaft and the die opening.