Purging compounds are materials used to clean or purging processing equipment such as extruders or injection molding machines of materials lodged in the equipment from previous runs. (Mark Kaptur 8/20/2009)

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Discussion:  If it were possible, manufacturers would dedicate process machines and molding / extrusion tooling to one part and one material.  No transition from one resin to another, or from one color to another, would ever be required. 

Unfortunately, the market place trend is in exactly the opposite direction.  The entire value chain is closely watching inventory levels.  Customers want smaller and smaller quantities with each order.  As a result, manufactures have adopted “Just-In-Time” or “Lean Manufacturing” systems.  Rather than dedicating capital equipment to a single application or material, manufacturers are required to make more and more frequent changes.  

As a result, many plastics processors report that their largest single cause of scrap material is resin lost “purging” out one material or color, prior to producing acceptable parts with the second material or color.  This is also a major category of lost machine time. 

Purging compounds are designed to make this transition take less time, with less lost resin, and with lower cost.  Manufacturers can make more productive transitions if the purge compound quickly and completely removes the previous material(s) from the machine and tooling. 

Purge compounds tend to fall into three categories:

• “Mechanical” or “Scouring” Type Purge Compounds.  Manufacturers often load a base resin with a high level of ground cast acrylic, calcium carbonate, or other abrasive agent.  They then use the highly loaded compound to scrub the process machine.  This type purge can be very effective.  However, those responsible for the cost and maintenance of capital equipment often fear these scouring type purge compounds will damage the machinery.  Since the scouring additive does not fully melt, these purge compounds are not suitable for hot runner injection molds, as the scouring agent can block the gates. 
• “Chemical” Type Purge Compounds.  Many manufacturers react adversely to the term “chemical purge compound”.  This type of purge was originally formulated to produce ammonia when processed.  The ammonia then did an excellent job of cleaning the screw and barrel, but also produced a noxious odor.  This “ammonia” technology, however, is mostly obsolete. 

 Today’s chemical purge compounds, like Ultra® Purge from Moulds Plus International, contain FDA compliant additives (check with the manufacturer of your purge compound for FDA compliance) that break polymers into smaller chemical chains.  An “expandant” or “foaming agent” is included to insure the purge compound reaches “hidden” spots, such as screw flights, injection molding check rings, extrusion breaker plates, or blow molding accumulator heads.   These smaller polymer chains, together with pigment particles embedded in them, are then easily flushed from process machine by the carrier resin.  

• The third category of purge compounds combines the “mechanical” and “chemical” type purge agent into one compound.  

(M Kaptur 8/20/2009)

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