MACHINERY | SCREENCHANGERS AND MELT FILTRATION
IMAGE: PSI
Above: PSI says its its Super Plate can help to raise the efficiency of screen changers
yield a 30-55% gain in effective filtration area over a conventional design, it claims. The Super Plate is a precision-manufactured grid
of stainless-steel diamond-shaped blades, which are cross-welded to provide high strength and support. The screen pack rests on the edge of the blades. Spacing and support of the blades often allows the cost-saving elimination of a 20-mesh or higher support screen. It also eliminates the risk of screen blow-through and resulting contaminants in the end product that can occur if the delta-P exceeds the tensile strength of the wire cloth sitting directly above a breaker plate hole, says the company.
Low delta P At the same time, its low delta P breaker plate design can improve production run times between screen changes. Every process has a high-pressure limit defined either by the extruder or the yield strength of the screen pack. Starting (clean screen) pressure drop defines the low end of the process pressure window. Because screen micron rating and related wire diameters are generally standard- ised, when working with a defined filtration level, the remaining variable for reducing pressure drop is the diameter of the holes in the breaker plate. Efficient breaker plate design balances open
Right: Recycling of PE and PP are typical applications of Ettlinger’s ERF 1000 continu- ous melt filter
area and yield strength to reduce pressure drop. For most polymers and adhesives processed through screen changers, the industry standard (Class 2) breaker plate hole size works well. But for high viscosity, fractional melt materials, the typical hole diameter generates a high-pressure drop. This reduces the pressure window and corresponding run time available for a given screen pack and results in more frequent screen changes, says PSI. To overcome this, PSI offers Class 3 and Class 4
breaker plates, each having successively larger holes than the standard Class 2 plate. The Class 2 holes are ideally sized to support a 20-mesh screen. The larger diameter of the Class 3 and 4 holes use a Super Plate or a 10-mesh support screen for the finer filter screens. Finally, when space and screen life are considera- tions, an oval screen pocket can offer productivity
16 PIPE & PROFILE EXTRUSION | Spring 2024
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advantages, says PSI. Available for bolt-type screen changers, the elongated pocket increases the filter area by around 168%. This is generally equivalent to upsizing the screen changer to as much as twice the model size but with the space envelope of the smaller version. Pressure drop across the screen pack is lower, yielding higher throughput rates given the lower average back pressure. Shear stress is lower, boosting melt integrity. Screen life is roughly linear to the area increase, giving an increase in time between screen changes. For processes where recyclate can contain ‘bursts’ of contaminants, the longer screen life allows the operator longer intervals between screen mainte- nance. This option is only available for new machines due to the custom housing and bolts.
Cost efficiency Germany-based Maag says that high-performance melt filters – such as the ERF series from its Ettlinger subsidiary – can help to make more high quality recyclate available to converters. The ERF 1000 – with its four rotating, perforated drums – can filter up to 10 tonnes of plastic melt per hour. Benefits include continuous operation in a stable process at consistent pressure and long operating times without filter changes. The very low melt loss – in the range of a just few percent – as well as the possibility of changing each drum individually and without interrupting production also contribute to high cost-efficiency. Lower-throughput versions, such as ERF 350
(around 3,000 kg/h) and ERF 500 (up to 6,000 kg/h) are also available. All three sizes are energy-efficient and suitable for filtering common thermoplastics, including soft PVC. The proportion of contaminants can be up to 16%. In principle, ERF melt filters can be used on any extrusion line – either single- or
IMAGE: MAAG
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