PROCESSING | SMALL BATCH COMPOUNDING
Above: Buss kneaders allow liquids to be injected at any position and to be mixed efficiently over a very short distance. This image shows a yellow colour mixed in PVC
compounds, with mechanical cleaning a good alternative. In some cases, the processing temperature can be reduced to a level at which the polymer becomes rubbery so, if the material is not sticky, it can be removed relatively easily. When working with liquid colorants, it is generally possible to inject the colour very close to the end of the machine, Niklaus says. Buss kneaders allow liquids to be injected at any position and to be mixed efficiently over a very short distance – often just a couple of L/Ds. This strategy can enable fast product transitions by purging as only a very short section of the machine is “contaminated” with colour.
Feeding considerations It’s not only the core compounding equipment that should be optimised for quick product change. The same applies to up and downstream equipment, from raw material handling to the packaging system, Loviat points out. “Ideally, each raw material should be dosed into the machine with a dedicated
dosing system in order to avoid product change in the gravimetric feeders,” he says. This is because changing product in the gravimetric feeder usually means it must be emptied, dismantled and cleaned, which can be time consuming and result in consid- erable line downtime and lost production. Loviat acknowledges, however, that it is often not possible to dose each single raw material with a dedicated feeder for cost or space reasons. A compromise may be required. “In general, pellet dosing units can be run empty and do not require much cleaning while switching product. It is, therefore, usually a reasonable trade-off to use the same gravimetric units for dosing different pellet raw materials.” The situation is different for powders, though, as
some cleaning is generally required when switching products. Where space is the main issue, Loviat says powder components can be dosed with dedicated gravimetric feeders mounted on a carousel or on rails. This allows unused devices to be placed in a remote “stand-by” position. “In case product changes in the gravimetric feeders cannot be avoided, one should pay attention to the design of the equipment and focus on features making the cleaning of the equipment faster and easier,” he says. “Units assembled with clamps instead of screws are usually easier to dismantle for cleaning. Agitators fixed in the wall of the material hopper instead of the cover are also of advantage. Furthermore, it is in some cases possible to exchange only the raw material hopper of the feeders, keeping the same weighing system. This limits the amount of cleaning work to the minimum.”
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IMAGE: BUSS
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