Multi-screw extruders | machinery While twin-screw extruders are the
mainstay for the compounding industry there is growing interest in multiple- screw designs for more challenging applications. Peter Mapleston looks at the available options
More the better? The benefi ts of multiple screw extruders
Twin-screw extrusion technology is by far the most widespread when it comes to compounding plastics, but it is certainly not the only option. There are times when single screw extruders are used, while kneaders also have their place. But there is also a not-inconsiderable market for multiple-screw extruders. Compounders today can choose from equipment with three, four, eight, even twelve screws. Then there are the planetary roller extruder designs, popular in the PVC compound- ing sector for some time but which are fi nding a growing number of applications beyond that area. This article aims to count through the options and identify the benefi
ts.Investing in three
Stepping up to three Germany’s Keimei Plastifi zierung Technik fi rst introduced its three-screw extruder development at K2013. Project Manager Levente Szöke says the main target of the technology today is the production of masterbatch containing nano particles for synthetic fi bre applications. “Currently we are developing a medium sized machine with a screw diameter of 38mm, which we will present at K 2016,” he says. The company already offers two larger types with L/D ratios from 20 to 60.
The triple-screw unit is equipped with a “three-start”
screw design, which is also Keimei’s preference for twin-screw models and is said to provide an optimised balance between plasticising effectiveness, output and
www.compoundingworld.com
energy consumption. Szöke says that, compared to conventional two-start screw designs, shearing energy input into the processed polymer material is 50% higher per revolution of the screw. The triple screw unit’s trefoil formation also provides
three cutting points between the three screws, rather than just the one in a conventional twin screw extruder. The combined effect of the three-start screw and the triple-screw design enables the L/D ratio of the system to be much shorter than on a comparable standard twin screw extruder while achieving the same plasticising effect, Szöke says. “Average shear rate in the fl ow channels is signifi -
cantly higher [than in twin-screws] due to a lower channel height,” Szöke points out. “However, the addition of the third screw compensates the loss of channel volume through the lower channel height regarding the maximum output rates.” Because the retention time of the material inside the
barrel is so low, the extruders are claimed to be ideal for processing heat-sensitive polymers. They are also said to be particularly advantageous for polymer blending and grafting work, as well as the compounding and dispersing of powder and polymer additives.
From four to eight The WDR extruder from Japan’s Technovel Corporation is available in two versions; the Quad Screw model is arranged with four screws in a line while the Octa
June 2016 | COMPOUNDING WORLD 15 Main image: Multiple screw
extruders such as Gneuss’s
MRS are fi nding application where
conventional twin-screw
compounding
systems may be near their limit
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86
