What You Need to Know About...The Slurry Process

By By Mark D. Barnes
When it comes to improving polishing slurry productivity and profitability, there are three primary points to consider. #1 Investigate slurry performance in your polishing process.

How long before you actually start seeing swirls on lenses? If the goal is 10 days and you begin seeing swirls at day 8 and continue struggling until day 10, then you should consider using different slurry, perhaps one that is more robust with greater longevity.

Does your slurry stay in suspension and maintain Baumé? Many types of slurry perform well when first placed into a system. The real test is whether the slurry will maintain Baumé over time. If your slurry loses Baumé, and you find yourself boosting it, chances are your alumina particles may be settling to the bottom of the reservoir as the alumina has fallen out of suspension. Using slurry that minimizes settling may be needed.

# 2 Focus on a slurry maintenance program. How well do you maintain your slurry? If you answer, “not very well” or worse yet, “not at all” you have overlooked a potential for improved performance and profitability. Depending upon how many lenses you polish; what type of substrate you are polishing; how well the slurry is filtered; and how well the polish temperature is maintained; are all factors that influence slurry life. Just like motor oil in an automobile, it is extremely important to change out slurry with regularity. No matter which slurry you use, it will go bad, unless you change it out periodically.

Do you have good slurry flow to all polishing heads? Make sure you do not have distribution blockage or slurry build-up in the lines. If you do not have adequate flow to all polishing heads, then lenses may be “starved” for slurry, resulting in swirls or unwanted scratches. By creating uninhibited flow, you maximize slurry coverage on each and every lens. Are you keeping your filter clean? Operating with a clean filter will ensure not only good flow, but also minimize the chance for possible contamination scratches on lenses.

#3 Understand overall cost of ownership Are you using slurry that is designed for long-life? Some slurry can appear to cost less initially. But do you know how long the less expensive slurry performs in your system? The key is that you really do not know until you actually install the slurry into your system and observe its performance while measuring results.

To narrow the field of scope, look for slurries that demonstrate robust characteristics such as: Extended polishing life; Excellent suspension properties; Maintains consistent Baumé; Easy to mix; and Easy to clean up.

One area that is typically overlooked is pad-carryout. Some slurry has a tendency to “stick” to polishing pads. Thus, when you remove the pad after use, you actually “carry-out” good usable slurry with the discarded pad. It stands to reason, the more lenses you polish, the more “carry-out” you will experience. In general, to minimize pad “carry-out,” you want to look for slurries that are less viscous and easier to suspend.

Does your slurry corrode polishing equipment and/or fixtures over time? Typically, slurries that contain extra chemicals to increase removal rates tend to be more aggressive toward equipment and personnel. You can spot slurry that is chemically aggressive by looking at exposed metals, copper, and brass fixtures. What you will see is metal discoloration, perhaps some etching, and even erosion. These are signs that your slurry is chemically attacking equipment. To reduce the propensity of chemicals attacking your equipment, look for slurries that are less aggressive chemically, but can still perform to the polishing standards you require.

Written by Mark D. Barnes, global sales and marketing manager for Praxair Electronics. He can be reached at 317-240-2217 or at mark_barnes@praxair.com

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August/September LabTalk 2017