As a component of the Feedscrew and Barrel Maintenance blog series, we are incorporating a three-part series on purging. This blog will focus on effective methods for product changeover.
According to Webster’s Dictionary, the definition of purging is “to make free of something unwanted.” Incorrect purging procedures for product changeover, line shutdown and restart can lead to premature polymer degradation that will significantly impact yield and productivity, and lead to unwanted downtime. Common sense purging methods can increase uptime and minimize waste.
In a polymer processing system, defects consisting of gels, black specs, off-color or degraded polymeric material leads to unwanted, off-spec products. Typically, these defects result from stagnant regions due to poor equipment design; internal damage to flow surfaces because of incorrect cleaning procedures or chemical attack; the wrong operational temperature settings; or improper line shutdown, start-up and product changeover. Not to be overlooked, these defects can also appear in the incoming resin feedstock to the line.
Best Practices for Product Changeover
The first place to begin is in your production planning. We recommend these rules for the best results:
1 Evaluate your production run sequence.
High-viscosity resin will displace lower viscosity materials quickly in an extrusion system. If possible, plan production runs from low-viscosity resins to higher-viscosity resins. This will greatly reduce changeover time.
2 Consider color changes.
Plan your production run sequence to go from light-colored products to dark-colored products. The hiding power of dark colors will quickly absorb lighter colors. If you do this in reverse, you may have to shut down the line and clean it. This leads to unnecessary downtime.
The picture below shows samples taken during a color change of clear PET to dark brown PET.
3 Consider the resin melt point.
If you process materials with significantly different melting points, always run lower melting point materials to higher melting point materials. This will eliminate unmelted gels in the product.
The Disco Purge Method
An effective purge method for product changeovers was introduced by John Vansant of DuPont. It is called the “Disco Purge Method.” According to Vasant, the following procedures can help expedite changeover time after introducing a new polymer:
A general outline of the procedure and issues to watch for are listed below. For a more detailed procedure, please contact Davis Standard.
1. Empty the feed system and make sure it is free of the existing material.
2. Run the existing material down until the top of the screw flight is visible.
3. Avoid the introduction of air into the extruder system when emptying the extruder. (This can lead to gel formation.)
4. Run a range of RPMs to vary the shear rate and change flow patterns. This will help scrub existing resins from slow flow or non-streamlined areas of the system.
5. Be sure to include slower extruder screw speeds to allow resins to bond together and interrupt flow patterns.
6. When changing materials and going to a higher temperature setting, purge the system fully before increasing the extruder temperature setpoints. The lower temperature causes high-shear stresses on the internal flow surfaces when scrubbing the old material out.
7. Never let it “drool;” be active in purging. Running at a low screw speed during changeover will not fully eliminate the existing material. When the screw speed is increased after a drool-type purge, existing material will be released and contaminate the new product.
These common-sense methods are intended to simplify your purging process. Keep in mind that highly dissimilar materials will require additional purging steps. The next blog will discuss the importance of product shutdown and restart procedures.
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The D-S Connect Blog Team