Thursday, September 23, 2010

Purging Tip of the Week: #22

System temperatures are important in purging. Changes in temperature cause movement of metal parts (due to the metal’s coefficient of thermal expansion) and this can dislodge contaminants. Increased temperatures enhance the effectiveness of chemical purging compounds by increasing reaction rates.

But the raising and subsequent lowering of temperatures takes time. The positive and negative impacts of changing temperatures must be carefully evaluated in planning a purging process. Manufacturers’ recommendations are a good guide to optimizing temperatures for the purge.

Tuesday, September 14, 2010

Purging: Observations of an Expert

The September issue of Plastics Technology magazine landed in my "In" basket today with a feature article by noted extrusion expert Chris Rauwendaal, titled "Optimize Product Changeover & Purging." The article is also available on-line. As you might expect, I read it with considerable interest.

It's very nice, from the perspective of someone whose business for twenty years has been centered on purging issues in plastics processing, to see this level of attention being paid. It's also nice to see some theoretical underpinnings offered for some of the recommendations we've been making for a long time.

Dr. Rauwendaal begins with an overview of melt displacement behavior - that is, the way two polymers interact in an extruder when one is the resident polymer and another is the displacing polymer. He uses the power law index as his primary mathematical tool, and assumes that both resins have the same index (as would be the case in, for example, a color change). His analysis makes the point that, all else being equal, materials with low indices will displace more quickly due to greater shear-thinning. So presumably, in a given piece of equipment, you might accomplish a color change in styrene or low-density PE (whose power law indices are in the 0.3 ish range) comparatively quickly, but a transition in Nylon or polycarbonate (indices in the 0.7'ish range) would take much longer.

The author points out that reducing the layer of "resident" polymer in a "typical" extruder to about 0.1 mm (about 0.004", the thickness of a sturdy film) would take about 1/2 hour and getting it down to 0.01 mm (that's really, really thin - almost "gone") could take 4 to 6 hours. Maybe a purging compound could help?

Also, discussion of cases where the resins in the machine have different power law indices is beyond the scope of the article. It would seem to me that higher-index resins would be better able to displace lower-index resins due to the shear-thinning tendencies of the latter, but I'm not a rheologist. (Perhaps John Spevachek or Eric Brown will favor us with a comment.)

Another section of Dr. Rauwendaal's article discusses the effects of viscosity relationships on purging behavior. He suggests that the viscosity ratio (i.e., the viscosity of the resident polymer divided by that of the displacing polymer) is a critical factor. You really need the ratio to be less than 1, meaning that the displacing polymer is more viscous - or, if you like, stiffer than the resident polymer. That's pretty intuitive.

Incidentally, he doesn't come right out and say it but I suspect the author is referring to the absolute viscosities of the fluidized polymers. You shouldn't start making ratios of Melt Flow Indices and expect to get useful results.

The article discusses the effects of color, in a couple of ways. First the author takes note of the well known phenomenon where dark colors cover lighter colors effectively but light colors have difficulty displacing dark colors. He therefore endorses the ancient and honored concept of running a color cycle, from light to dark.

He then discusses research that indicates an inverse correlation between difficulty of purging of color and the size of the pigment particles with carbon black - which has exceedingly small particle sizes - cited as the classic "bad actor."

The research also observed that some colors are prone to "staining" the screw, leading to stubborn and prolonged persistence of the darker shade. (In such cases, frequent purging may be the only way to avoid teardown and manual cleaning.)

The DuPont company developed a purge procedure that they call "Disco Purging." It involves periods of varied screw speed to set up varied conditions of flow and turbulence in the extruder. Dr. Rauwendaal describes this method approvingly but notes that in some cases it works well, and in other cases not so well. It is certainly something in the "worth a try" category.

The article ends by addressing briefly such measures as low-adhesion coatings and the diagnosis and elimination of "dead spots" in the extrusion system. Dr. Rauwendaal also acknowledges that there are many factors effecting purging, some not fully understood. He makes this observation:

"It is unavoidable that at some point a transition needs to be made from a high-viscosity resin to a low-viscosity resin or from a dark to a light color. In these situations a purging compound may be required. If this does not work well, it may be necessary to pull the screw and physically clean the extruder."
As we work with these issues every day, we appreciate the effort that PT magazine and Dr. Rauwendaal have put into this article.

Thursday, September 9, 2010

Purging Tip of the Week: #21

Clear polycarbonate and clear acrylic are prone to post-purge hazing or cloudiness if the purging compound in use is polyolefin-based. To avoid this issue, make sure that the purging products you use do not contain polyethylene or other olefins. Of course, if the polycarbonate or acrylic production resin is pigmented, the hazing issue should not arise.