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Temperature
in Process Control:
Surfacing |
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PSI-993-2T, for Fining Water Filtration
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With the rapid growth of technology in
lenses and coatings, controlling temperature variables in
the surfacing area has become much more critical over the
last several years. All thermoset (regular plastic) and
thermoplastic (polycarbonate) resins are subject to
temperature shifts during the blocking, generating, fining
and polishing process.
Studies have concluded that temperature
variations of over 5 degrees between generating, fining and
polishing will affect the stability of the substrate and is
known to cause waves, aberrations and an overall degraded
surface quality. Power problems are also very common.
When blocking with either alloy or
non-alloy mediums, it is highly recommended that the lenses
are allowed to cool down at room temperature for a minimum
of 30 minutes prior to generating. For thin centered High
Index and Polycarbonate lenses, a few more minutes of cool
down is desirable. Never rapid cool blocked lenses by
immersing them in cold water or by placing them in a
freezer. This will cause thermal shock to the lens surfaces
resulting in an incorrect true radius of curvature prior to
the surfacing process. Lenses processed in this condition will typically exhibit
poor surface quality, suspect powers and waves.
The optimum temperature range for wet
generating, fining and polishing should be between 55 to 60
degrees for taped surfaces and 60 to 65 degrees for non –
taped surfaces. Non – alloy blocking mediums have been
known to lose their seal against the front surface of the
lens if it gets too cold resulting in front surface micro
etching as a result of the polishing aluminas “seeping”
into this break. The slightly warmer water and polishing
slurry temperatures will prevent this from occurring. Taping
will obviously eliminate this defect as well. It is
imperative that this range be monitored closely throughout
the course of the production period.
For fining, a recycling system with
filtration and chilling coils not only saves water and
money, but enables more consistent temperature control year
round. The use of a fining water additive is also possible
increasing the lubricity of the water resulting in more
consistent stock removal and improved surface quality. Match
the temperature range in fining to your polishing
temperature range.
Central slurry systems for polishing have
been in wide use for many years in the surfacing department.
The benefits here are quite simple again in controlling
temperature with proper filtration and Baume consistency.
Central slurry systems deliver the same polish chemistry to
all machines and surface quality is improved while extending
the useful life of the polish. Remember to match your polish
temperature with the fining water temperature for process
control that really does help you make great lenses.
One key to system control and trouble
free operation in your laboratory is understanding the
process variables and keeping conditions consistent
throughout your production day.
Are you making lenses under the same conditions by
middle of the afternoon that you were making first thing in
the morning? In doing so, you will create an environment in
which you can gain productivity and profits with less
spoilage at the end of the day!
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