The purge to keep it clean
Processors looking to improve their cost position—which essentially means all processors—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, film, pipe and profile are taking heed and adopting the use of specially formulated purging compounds, it seems compounders 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 profit margins were very high,” says Reeder. “Creating extra scrap or having excessive downtime was not imperative to a company’s wellbeing.”
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 purchasers have overcome this barrier and have determined commercial purge compounds to be beneficial,” 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 flushed out revolves around the general principle that polymer melts, being viscous, tend to have laminar flow, and there is little turbulence close to the flow channel wall. “If thermoplastics flowed 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 difficult 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 inefficient.”
There are two basic types of commercial purging compounds, chemical and mechanical. Chemical purge compounds work at the molecular level to break the bonds of the polymers, which results in a reduction in melt viscosity and an increase in turbulent flow. Mechanical purge compounds work by scrubbing the machine clean. “A good analogy for the two types of purge compound is that a chemical purge is similar to using soap and water to wash dirt from a person’s hands, while conversely a mechanical purge is comparable to cleaning a person’s hands with a brush,” Reeder says. Most manufacturers of purge compounds have multiple grades that are designed to purge specific types of resin and machinery.
Recently introduced Ultra Purge 5150 and the Ultra Purge 5160 grades have been designed for fast and efficiently cleaning of screws and dies in compounding machines. They incorporate Mould Plus’s “Ultra-X” technology, which uses a proprietary mineral filled material that adds a mechanical component to cleaning the machine. “Additionally, Ultra-X is a better alternative to a glass filled carrier because it allows for the Ultra Purge active part to expand more in order to get to the dead spots of the machine,” Reeder claims.
In order to limit the amount of purging grades to have on hand, suppliers recommend having a good multi-purpose purging compound capable of working with many different resins. Running a purge compound through the extruder before pulling the screw will also save the compounder time as commercial purge compounds are typically much easier to clean off the screw compared to standard thermoplastic resins, Reeder claims. After the screw has been pulled, the remnants of the commercial purge compound can be easily unwrapped from the screw.