processing feature | Top tips for twin-screws

process tasks. Also, this usually causes excessive wear of the first kneader group, because it is being forced to do a disproportionate share of the melting work. Many people ask why screw elements wear when the

material isn’t that abrasive? This can be understood from Newton’s third law, which says that for every force, there is an equal force acting in the opposite direction. So if the kneader lobe is pressing against the cold pellets, the pellets are also pressing against the kneader lobe. Many customers set zones 1, 2, and 3 (Figure 2) at

too low a temperature, which means the energy for melting must come primarily from mechanical work. If these zones are set to higher temperatures, it will lessen the workload on the first kneader group, thus reducing the wear rate. The reason most customers do not set the tempera-

Figure 2: Raising the temperature in zones 1, 2, and 3 cuts the mechanical energy needed to melt the resin

0 2

Set zones 1 and 2 to higher temperatures to decrease wear

of plasticating screw elements In most plastic extrusion processes, the first task after feeding is to melt the material. This must be done relatively quickly, in a short amount of machine length, to leave adequate extruder length for the following process tasks. In order to continuously plasticate cold materials

which are introduced to the machine, a large amount of energy must be imparted to the material[1]

. This energy

Figure 3: Side stuffers help to increase powder loadings, but there are several factors to bear in mind

can come from only two sources – mechanical energy, such as friction, shearing, kneading or squeezing, or from heat. In a twin-screw extruder, both mechanical energy and heat are acting on the material. But many opera- tors rely too much on mechanical “brute force”, and don’t use heat effectively. Running the process this way causes the extruder to use too much torque just for melting, resulting in less torque available for other

tures higher is because they don’t want a high melt temperature. But in fact the melt temperature will not be any higher, as the material is moving through the plasticating zones in a matter of seconds. It is common to have a barrel set-point of 250°C in order to transfer a lot of energy into the material to cause it to melt, but end up with a melt at 180°C.

0 3

Things to consider when side stuffing powders

Side stuffing is widely used with twin-screw extruders for feeding of various fillers (Figure 3). Processors frequently desire very high percentage loadings of fillers, many of which are low bulk density (fluffy) materials. The ultimate loading which is attainable is usually limited by two parameters: the volumetric capacities of the side stuffer and main extruder screws; and the venting capacity to allow air to escape the extruder. The volumetric capacity is based on the free volume

geometry of the side stuffer screws, as well as the main extruder screws, and of course the RPM of both sets of screws. Usually if a test is made running the material through only the side stuffer (with the stuffer unbolted from the extruder, discharging into a drum), it will easily feed a high rate of material. But when the stuffer is attached to the extruder, capacity is often limited by the amount of material the main extruder screws can accept. In terms of extruder screw design, it is best to have

flighted elements with a long pitch (long flight advance) at the stuffer location, extending 2 to 4D downstream of the stuffer. This is to keep the melt material in the extruder moving rapidly forward, to allow the maximum free volume for the filler to enter. If the screw design causes any “dam-up” of material downstream of the stuffer, this will severely limit the amount of filler which can be fed. Important factors when using side stuffers: Back-venting: The object of venting is to allow air to

28 COMPOUNDING WORLD | May 2012 www.compoundingworld.com

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