The more screws, the better
One new type of extruder has a ring of small screws corotating around a static core. The ring extruder, which can have three to 12 small screws, was developed by 3+ Extruder GmbH in Germany. The fixed core distinguishes the machine from traditional planetary-gear models, in which screws spin around a counter-rotating core. Century Extruders will build 3+ ring extruders in the U.S.
The model shown at K'98 had 20:1 L/D, 12 screws of 30-mm diam., and output of 600 lb/hr. It's said to have excellent low-pressure dispersive and distributive mixing and controllable residence times. Its screws have segmented shear, mixing, and kneading elements that wipe themselves clean twice with each revolution, instead of once like twin-screw elements, because they intermesh on two sides. The screws have an almond-shaped mixing element that lets resin pass from the outer barrel wall to the inner wall, creating a counterflow pattern. Resin on the outside of the ring of screws travels along the barrel in one direction. Resin on the inside travels along the core in the other direction.
A novelty for recycling and compounding is a flat rectangular box with five screws of varying diameters. No walls separate the long screws, so resin rolls in several directions inside the barrel. This "REM" (reactor, exchanger, mixer) device, built by TIM Industrie in France, can devolatilize highly inked or contaminated films or mixed waste. The first line has been operating since July, recycling automotive carpet, bottles, film, bumpers, and car batteries at up to 1000 lb/hr. Material goes first into a short (10:1) twin-screw extruder and is then side fed at low pressure into the REM mixer. A large vacuum pump on top of the unit draws off volatiles. Melt flows out the other side of the REM unit into a single-screw extruder for pelletizing.
Extrude, pelletize, extrude
Sikoplast of Germany showed an unusual use for a strand pelletizing die, situated between two single-screw extruders in a system designed to devolatilize heavily inked films. From the first extruder, melt flows into a strand die with 50-200 holes, depending on output wanted. The strands fall downwards inside a vacuum cylinder, which draws off volatiles. The molten strands are then gathered into a second extruder and pelletized. Sikoplast has installed five commercial lines in the past three years, including one last year in the U.S.
New rolls for PVC
Entex introduced an unusual planetary roller extruder. Model TP/WE 250 has 14 spindles around a central roller. Model TP/WE 150 has six spindles around a central roller. The spindles have shallow spiral grooves to help transport material. The roll extruder can be used alone or teamed with the company's planetary-gear extruder. Entex says a half dozen went into commercial production in the past year. Some are used for compounding, some for in-line extrusion of flexible and rigid PVC film.
Kinder, gentler mixing
Hermann Berstorff in Germany showed the Compex line of low-shear corotating twin-screw compounders, which recently came under its wing. Krauss-Maffei, corporate parent of Berstorff, acquired Compex GmbH in 1995. Compex has a new patented mixing element on the screw that moves material backward, lessening pressure, shear stress, and temperature, Berstorff says.
Berstorff also launched a new mid-sized compounder, preassembled on a skid and ready to install. Mid-size skids come with 110-130 mm extruders of 42-46:1 L/D. The skid format allows a compounding line to be set up and running in several days rather months, Berstorff says. A smaller C.C. Skid (compact compounding) was launched at NPE '97.
Name your price
Eclipse T.S.S. BV in the Netherlands (U.S. offices in Burton, Ohio) showed new software that calculates cost alternatives for plastics compounding recipes. It includes a database of some 1800 raw materials, plus unlimited ability to add more materials. Recipes, mixing instructions, and applications can be stored. A processor can enter up to four price alternatives per ingredient and calculate "what-if" options for a given recipe and product. The new program, called Compeasy, runs in Windows 95 or NT.
Mix up a little batch
Haake GmbH launched a prototype of the Micro Mini Lab, a tiny benchtop conical twin-screw extruder. It can mix small batches of less than 10 grams and simultaneously generate rheological measurements.