Processors today generally use three different kinds of purging compounds: chemical; abrasive; and non-abrasive.
Purging compound options are widely used in the plastics industry, with traditional cleaning methods offering reductions in purging time and enhanced quality of the final product.
Each of these options has specific characteristics and offer benefits for processors. Some purging compounds use a combination of mechanical and chemical interaction to most effectively purge processing surfaces.
Mechanical purging compounds primarily utilize shearing force to purge contaminants from processing surfaces. Mechanical purging compounds can also utilize differences in material viscosity or bond with residual resin and other contaminates to purge them from processing surfaces.
For processors, picking the right purging compound increases efficiency and lowers cost by reducing downtime between material and color changeover. Mechanical compounds are effective across a wide variety of applications. They use high viscosity plastics or abrasives to mechanically scrub pollutants from the machine. The process does not require soaking of the product, thereby saving time.
Abrasive-type purge materials typically are glass-filled or mineral-filled materials that scour and scrub the screw and barrel of a machine. Depending on the particle size of the additive, the screen packs may have to be removed to avoid clogging.
Abrasive systems can be expensive, such as glass-filled polycarbonates.
As purging compound manufacturers work towards providing the most cost-effective solutions, many non-abrasive mechanical purging compounds are available. These non-abrasive purging compounds still cause wear of processing surfaces, but this wear is within the range the machinery would be expected to experience from normal operation.
Chemical compounds, used in applications requiring low pressure where mechanical purging can be less effective, operate by utilizing heat-activated chemicals to break down bonds between the various contaminants and processing surfaces in a chemical reaction in the barrel or die.
Chemical purging compounds often require a short period of “soak” time, commonly between 5 and 30 minutes, to most effectively purge contaminates. While almost all mechanical purging compounds are solid, one can expect to find both solid and liquid chemical purging compounds.