Adjustable Barrier Screws:
Process a Wider Range of Materials on a Single Screw
The R. Dray Removable Dams, patent no. 6,988,821, provides the ability of the operator to change the design of extrusion and injection screws to process a wider range of materials on a single screw, in house.
The wide variability of materials contained within recyclable feedstocks is one of the most challenging processing obstacles for extruders and molders reclaiming scrap plastics. The post-consumer or industrial scrap materials recyclers are processing into pellets isn’t all the same. Customers encountering feedstocks that vary in material composition have to halt production and change the screw to one that processes that material at a desired throughput and quality. Thus, screw changes can happen often - leading to a loss in up-time, and the screw may not process the scrap material perfectly, which can hamper throughput rates and productivity. A processor may also have to set aside factory space to house its growing inventory of screws.
DAM TECHNOLOGY ENHANCES MELTING
Typical screw configurations have zones or sections for material feeding, material melting and for metering/conveying. The melting section of the screw is distinguished from the other two common design sections (feed and metering). With the incorporation of an auxiliary flight a screw becomes a “barrier design.”
The barrier element on the screw in the melt section creates primary and secondary channels; solid, unmelted pellets or powder accumulate in the primary channel while molten, melted material flows in the secondary channel. Barrier screws designed where the primary channel is wider than the secondary channel have a higher melting efficiency. ("How to Compare Barrier Screws")
The melting section containing the barrier screw technology is where big processing problems can occur. Controlling the pressure in the primary channel of a barrier screw is paramount to successful plastics melting and extrudate quality. Forcing material together under pressure makes the product denser and creates higher heat transfer. Processors swap screws because of inefficiencies in controlling the pressure, and hence the flow rate and quality of polymers.
The patented removable dam technology for use in extrusion and injection screws that reprocess these recycled materials, allows a wide range of comingled materials to be better processed on a single screw. The design allows the extruder or molder to make an adjustment to the flow geometry of the screw themselves without having to send the screw out for modification, or purchase a new screw. The removable dam allows the molder to control the pressure in the primary channel of a barrier screw. Increasing or decreasing the dam to barrel ID clearance controls the pressure and flow of the material, forcing more or less material across the dam and barrier flight.
HOW IT WORKS
The removable dam allows for the operator to push the screw out far enough to expose the dam then exchange the existing dam with another - with either more or less clearance.
With the removable dam, technicians can pull the screw, change the height of the dam and re-install the screw in roughly 2 hours, instead of shipping the screw out of the shop for design adjustments that can take weeks. The dam allows processors to use a general purpose screw by altering the compounding ability of the screw by just changing the dam height. This allows companies to run a range of materials on one screw rather than use several screws, each for a different material. Customers using this design achieve maximum throughput while ensuring proper venting. Machines running with the dam technology represents a significant percentage of capacity for the processor, and helps complete job runs more quickly.
WHAT DAM IS BEST?
Lower dam heights work best with higher-viscosity resins and vice versa. The lower dam allows the greatest clearance to the barrel wall, appropriate for resins like a 1-MI LLDPE for blown film. A medium clearance dam would be suitable for something like a talc-filled, 6-MI polypropylene. While the highest dams allow minimum clearance - used for materials such as nylon.
In a coextrusion line for barrier films, removable dams would allow one screw in a satellite extruder to run both EVOH and nylon barrier resins. Or an LLDPE film producer could optimize throughput for formulations with different levels of regrind. LLDPE regrind, for example, has a lower viscosity than the same virgin resin, so a formulation high in regrind would run better with a higher dam.
The economic and environmental potential associated with running recycled resin is attractive for many processors around the world. The shortcomings associated with processing these materials are only dealt with innovation. The R. Dray Removable Dam technology allows resin processors to leverage the benefits of recycled resins.