Purging compounds | processing
Purging compounds are fairly established in general plastics processing but are not so accepted by compounders. Peter Mapleston fi nds out why and looks at what is available for compounding applications
The purge to keep it clean
Processors looking to improve their cost position—which essentially means all proces- sors—could do well to take a closer look at how they can reduce downtime and scrap during colour and material changes. That, at least, is the advice coming from suppliers of commercial purging agents. But while injection moulders and extruders of sheet, fi lm, pipe and profi le are taking heed and adopting the use of specially formulated purging compounds, it seems com- pounders are proving a harder nut to crack. Of course, compounders are well positioned to cook up their own purging recipes but these may not always be the most cost-effective options. Andrew Reeder, sales manager
at Moulds Plus International USA, which produces Ultra Purge purging agents, says plants that are processing different types of dark and light colours often struggle with excessive downtime and scrap when transitioning out of a dark colour and into a light colour and that use of a purging compound when changing for all dark to light colour changes can be a good solution. Purge compounds can also contribute to a “preventative maintenance” strategy as every time the equipment is purged to remove colour, the compound will also remove degraded material that is beginning to build on the screw. Processing companies have traditionally used either
natural resin or regrind to purge their machines when performing dark to light colour changes, and some companies have even come up with their own in-house solutions. “Using resin or homemade purge compounds was not an issue decades ago because business was very good, resin prices were relatively low and the profi t margins were very high,” says Reeder. “Creating extra
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scrap or having excessive downtime was not imperative to a company’s well- being.”
Times have changed, however, and with a more competitive global market processors have more reason to cut costs. Suppliers such as Moulds Plus argue that their products are a good way of helping, but “like anything in life, old habits are hard to break,” Reeder says. “Also, there is a psychological barrier of paying the higher price per pound for a commercial purging compound compared to resin. However, many production managers and purchas- ers have overcome this barrier and
have determined commercial purge com- pounds to be benefi cial,” he says. “For the most part plastic companies have warmed up to the idea of using a commercial purge compound, especially in the injection
moulding sector. Use in the extrusion and blow moulding sectors is less common but more companies in these industries are starting to evaluate commercial purge compounds,” says Reeder. One of the fundamental reasons that colorant cannot
be easily fl ushed out revolves around the general principle that polymer melts, being viscous, tend to have laminar fl ow, and there is little turbulence close to the fl ow channel wall. “If thermoplastics fl owed in a turbulent pattern it would be much easier to clean colour out of plastic processing machines,” Reeder says. Certain types of colorant can also be more diffi cult to purge out than others, Reeder notes. “For example, [some insoluble] organic pigments tend to accumulate in the microscopic crevices on the screw. Using resin to clean out these imperfections tends to take a long time and be ineffi cient.”
January 2016 | COMPOUNDING WORLD 53
Main image: Custom-formu- lated purging compounds are used as cleaning aids across the plastic processing industry but compounders have been slower to adopt them
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