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PROCESSING | EQUIPMENT CLEANING


Today’s stringent product quality requirements demand the highest levels of compounding equipment cleanliness. Schwing Technologies’ Senior Expert Udo Heffungs provides some advice on cleaning options


Cleaning: the key to quality


Main image: Cleaned parts being removed from a thermal cleaning oven at Schwing Technologies’ contract cleaning facility in Germany


The increasing quality requirement for plastic compounds calls for the highest degree of cleanli- ness in the equipment used. Contamination leads to significant losses in quality and to plant shut- downs. Effective cleaning provides the solution but also interrupts production. So when should users clean extruder and melt-conveying components? The expert recommendation is to clean down when changing the polymer being processed, when changing polymer viscosity (particularly when moving from a high to low viscosity resin), when changing color (especially from dark to light) or to remove deposits from flow surfaces and out of dead zones.


The purging option Many compounders would like an “inline” option, allowing equipment to be cleaned without having to disassemble it. A purging or cleaning compound can provide a solution, however, the cleaning mate- rial used must be adjusted individually to ensure it suits the process or quality requirements. There are a variety of cleaning compounds available on the market each with different modes of action: depolymerisation types work by decom- posing low-melting or low-viscosity materials; interlacing types use elastic phases to rub surfaces clean; non-melting types use non-abrasive compo- nents to rub or scrub surfaces; abrasive types contain components that impart a grinding or


60 COMPOUNDING WORLD | October 2018


polishing action. Cleaning compounds can also contain a variety of solvents and reagents selected to achieve a particular result. Unfortunately, there are problems with what is, in principle, an interesting solution. Dead zones may not be reliably purged and purging media, once introduced, can remain in the equipment. Contaminant deposits may remain trapped in the pores of the tool surfaces. Problems can also arise from non-overlapping elements, at pushing/shear edges or undercuts, from partial decomposition, from swelling and extrusion of hot substances, and from the amount of waste generated.


Manual cleaning An alternative is to use a manual cleaning method. Rough blast media, such as duroplast granules, can remove organic contamination from the surfaces of the screws. Cleaning with blast media offers a number of advantages: the surfaces are cleaned without leaving residues; investment and operating costs are comparatively low; the cleaning process is environmentally friendly; and cleaning can be performed economically with low quantities of media. This approach can only be used, however, to remove surface contamination. Internal contaminants in die plates or pelletising dies cannot be tackled. The welding torch (or open flame) is still also frequently used today as a cleaning option. The


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PHOTO: SCHWING TECHNOLOGIES


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