1879 – First Patented Rubber Extrusion Screw
It is believed that the Kerite Company built the first screw extruder in 1865. The earliest extruders processed rubber for telegraph wire coating. The extruder was belt driven from a line shaft, which was powered by a water wheel. The barrel of the extruder was heated using an external steam jacket, so that the viscous rubber could be pumped through a crosshead die. The first patents for rubber extruder screws like the one displayed at UML were issued to Mathew Grey and Francis Shaw from the UK, and John Royle in the U.S. These new screws were developed for rubber processing. The screws were single flighted, had deep channels of constant depth, and a decreasing lead angle to create compression. The length to diameter (L/D) ratios of these screws was very short, typically 3:1 to 6:1.
1935 – First Thermoplastic Extruder Screws
As thermoplastics became commercial, engineers at companies like John Royle & Sons, NRM, and Union Carbide Corporation (UCC) began to experiment with the extrusion of thermoplastics. Early thermoplastics extruder pioneers included Bill Hemperly of UCC, Ben Davis of Davis-Standard, and Paul Troester in Germany. Most early thermoplastic screw designs were longer versions of the rubber screw, with higher L/D ratio to allow the thermoplastic materials to melt. The screws were similar to rubber screws in that they were single flighted, had deep channels of constant depth, and a decreasing lead angle but had L/D ratios of 12:1 to 16:1.
1953 – First Metering Type Screws
Scientific analysis highlighted by Bruce Maddock’s “screw freezing” experiments are examples of laboratory work with emphasis on fluid mechanical flow in the extruder. This resulted in single flighted meter type screws; with deep channel feed sections; tapered root diameter transition sections; and shallower, constant depth metering Single Screw Extruder (Courtesy A. Routsis Associates) HISTORY OF EXTRUDER SCREW DESIGNsections. The metering screw became the standard with higher L/D ratios of 16:1 to 20:1.
1960 – First Two-Stage Vented Screws
The two-stage vented screw was developed for devolatilizing polymer materials during the plasticizing process. The offgassing occurred at the end of the screw’s “first stage”, which contained regular feed, transition, and metering zones. The polymer melt would then flow over a rapid decompression followed by the extraction zone where the dissolved gasses would escape through the vented barrel into the atmosphere. Typically, the second stage was shorter than the first one to avoid long residence times in the second stage transition zone. The first two-stage screws had typical L/D ratios of 22:1 to 30:1.
1963 – First Melt Barrier Screws
The melt channel theory represented the most revolutionary screw design change in almost one hundred years. Barrier screws were designed to ensure that the polymer being extruded was completely melted. This was accomplished by separating the solid, unmelted polymer from the melted polymer pool using a secondary barrier flight. This barrier flight created two separate channels. As the solids channel decreases the melt channel increases. Once the solid polymer is melted it can pass through the screw clearance and continue along the melt channel. The first barrier screws were designed by Charles Maillefer and Paul Geyer. At that time, screw L/D ratios were usually 18:1 to 24:1, but have increased significantly since then.
1967 – The Union Carbide or Maddock Mixing Screw
Bruce Maddock introduced a novel mixing device that contained a number of longitudinal barriers and channels spaced around the screw circumference somewhere near the end of the extrusion screw. This mixing section had both wiping and mixing lands, which prevented unmelted polymer from exiting the mixing section of the screw. Robert Gregory is credited with a helical version of this type of mixer. Both mixers were commercially successful and are still widely utilized today for the processing of polyolefins. At the time, L/D ratios of 20:1 to 24:1 were standard, but that would be considered short today.
1971-80 – Barrier Screw Improvements
During the 1970’s increased activity in laboratory trials and new designs resulted in a proliferation of barrier or melt channel designs. Barrier screws, such as the one displayed, use a second screw flight to separate the solid pellets from the molten polymer. Robert Dray’s Efficient Screw, Norton Wheeler’s DSB-2, Robert Barr’s Barr 2, Hartig’s MC3, NRM’s Plastiscrew and C.Y. Cheng’s Sterlex were all commercially successful applications of the original Maillefer/Geyer barrier technology where a melt channel was used to improve melting efficiency. The screw shown also features Bob Gregory’s Spiral Maddock mixing element in the metering zone.
1981 – First Dissipative or Distributive Mix-Melt Screw
Several attempts have been made to improve the barrier concept and take it to the next generation of screws. A combination of interruptions to barrier flights that allow melted polymer to mix with the unmelted material, along with an eccentric root configuration, provided low shear melting and effective mixing. George Kruder’s Wave and Double Wave, Chan Chung and Robert Barr’s Energy Transfer, Jon Kuhman’s DM2, and Tim Womer’s Fusion are all commercially successful applications of distributive mix/melt type screw. These design have typical L/D ratios of at least 24:1 to 26:1.