European processors have turned to high-speed single-screw extrusion. More than 100 of the small hyper-drive machines whose screws turn at up to 1500 rpm, about 8-10 times faster than standard extruders are commercially established in Europe. At least two German machine builders are working on machines that will go to 2000 rpm and even higher. The goal is to raise output without increasing extruder size. These machines are long, thin and cool to the touch. These extrusion systems have so far processed a limited range of materials�mainly PE, PP, PS and ABS all of them typically unfilled or lightly filled. Machine wear at these hyper speeds is a potentially important unknown, since the phenomenon is so new.
European processors bought the first high-speed extruders five years ago to make PS and PP sheet for in-line thermoforming. Their main goals were energy and space savings, neither of which was comparable concerns for US based processors. The high-speed machines provide energy savings of 10-15%. The savings come from not heating or cooling the mass of metal in the extruder barrel and not losing energy in gear reduction. High-speed extruders use heater bands sparingly or not at all. Most high-speed extrusion is done in-line with thermoforming. Those extruders run at 400 to 800 rpm and typically feather the heater bands on and off. Machines producing sheet off-line run at 900 to 1500 rpm and use heater bands only on start-up. Thereafter, plastic is melted entirely with frictional heat. In both cases, the barrel stays cool to the touch, so no cooling is needed, except perhaps in the feed zone. High-speed extruders also save energy by dispensing with gear reducers. These machines either have motors directly coupled to the screw or use a 1:1 gear ratio. Most high-speed extruders fall between 60 and 75 mm diam. and none are larger than 90 mm. Kuhne offers only 60- and 72-mm sizes with 33:1 L/D. Battenfeld Extrusionstechnik (BEX) found 75 mm diam. and 34:1 to 40:1 L/D to be the optimum for the biggest output advantage. Below 75 mm, machine builders cannot take full advantage of potential screw flight depth without risking breakage at high rpm. Above 75 mm, they lose the adiabatic effect and have to add heat and cooling. Color changes on small machines are also faster and waste less plastic. The screw holds only 4kg of resin (vs. 40 kg for a 150-mm extruder of the same output) the color changes are quick and save material cost along with processing time.
Huhtamaki Oyj has set up more high-speed extruders than any other processor- installing more than 10 units to reduce energy cost and to enable inline sheet extrusion and thermoforming in plants where space is limited. High-speed extruders are now developing so rapidly that it is hard to define high speed anymore. Traditionally, any rotation of screw circumference over 1 meter/second was high speed, but top rotational speeds today are 3, 4 and 5 m/sec.
In the US, development of high-rpm single-screw extruders took a different route�making color masterbatches instead of sheet for thermoforming.
High speed extrusion equipment is not restricted only to single screw extrusion equipment but also to corotating twin-screw compounding extruders since the early 1990s. The new generation of European high-speed single-screws has been running for 1-4 years in 24/7 operations. Processors� have discovered that besides energy and space savings, high-speed extruders actually improve sheet clarity, particularly improving clarity and crystalline structure of PP sheet. The traditional approach to making thin PP sheet is with slow, controlled cooling. But high-speed extrusion requires faster cooling.
Another surprising discovery about high-speed extrusion is that PS, ABS and other polymers degrade less in a high-speed extruder because of dramatically shorter residence time. Some pioneers are starting to run LDPE at moderately high screw speeds and very high outputs.
Coextruded cast film is another new frontier for high-speed extrusion. Besides Huhtamaki�s barrier film line in France, EDV Packaging in Llinars del Valles, Spain, extrudes cast coex barrier PP and PS film at high speed on a line installed last year. EDV declined to provide details. High-speed extruder screws need a longer melting section and higher L/D (typically 30:1 to 40:1) to get more output at the same screw speed.