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Purging purge (pûrj)


v. purged, purg·ing, purg·es v.tr. a. To free from impurities; purify.


b. To remove (impurities and other elements) by or as if by cleansing.


Due to the increased demands of customers and the desire for less capital tied up in inventories regular colour and material changes are often required for polymer processors.


Running the same material of the same colour ‘day in day out’ would be ideal but this is the exception rather than the norm for a whole range of plastic processors.


After a material/colour change and before and after start up and shut down, it is essential to remove all traces of the previous material. It is also good practice to use effective purging techniques in order to avoid contamination from residual colour and/or polymer and also avoid the build up and release of carbonised material. The use of specifi cally designed purging agents normally results in higher quality components that are produced more effi ciently and the economics typically justifi es the additional expense of purchasing a specifi c purging compound.


The most common method of purging, particularly with commodity polymers, is to run through virgin material at the end/start of the run to drag out any unwanted colour and polymer. Whilst this method is commonly used it is not very effective or economically viable. This can result in colour streaks and contamination, black specks and polymer contamination resulting in poor quality parts and milkiness when processing clear materials like PS, PC and PMMA.


To this end specifi c compounds designed to improve this process have been formulated.


Contamination sources The most common cause of contamination results from the shutdown procedure, this is anytime that the machine stops; weekend shut down, tool change, breakdown etc. Essentially if the residence time of the polymer in the processing equipment is increased there will be a chance of degradation therefore the equipment must be purged effectively. The reason why either purging with a more inert polymer such as PE or shutting down the machine with the screw forward is not effect is because the screw is not empty it is just less full.


The issue of contamination does not only depend upon the type of polymer, but also the interaction of that polymer with other additives. For example some colourants will leach out of certain polymers whilst remaining fully encapsulated in others. Strong blue being one good example; in POM colour leach out can be so severe that the surface of


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