Tips for Surfacing and Finishing Lenses Made from - X

Tips for Surfacing and Finishing Lenses Made from - X
Tips for Surfacing and Finishing Lenses
Made from Trivex™ Material
L
ooking back at days when polycarbonate lenses
were introduced, industry veterans will recall having
to learn a whole new set of skills to surface them
properly. In contrast, optical laboratories will be pleased
to find that surfacing lenses made from Trivex material is
as easy as surfacing other plastic lens materials; and with
a little refining, most of the techniques used for other lens
materials can be used for lenses made from Trivex material.
As with any lens material, there are a few basic lab
techniques one should know to improve their final results.
The following recommendations are provided for both
novice and experienced laboratory personnel.
SURFACING RECOMMENDATIONS
T R I V E X
T M
L E N S
M AT E R I A L
U S E R ’ S
G U I D E
Base Curve Selection
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Many of the lens manufacturers who work with Trivex
material produce their single vision lenses with aspheric
base curve designs. It’s important to choose a base curve
that is within the specified range for the power that is needed.
Manufacturers have base curve selection guides to assist
you in this step. Some of the newer surfacing software will
specify which base curve to select. Going outside the
recommended parameters will cause adverse power
problems for the eyeglass wearer. This is true of any lens
made in any material and is particularly troublesome with
aspheric designs. Eyecare professionals who specifically
request base curves outside of the recommended range for
a specific Rx should be informed that the lenses they receive
may not provide the level of aberration control and vision
correction they expected from the design they ordered.
Surfacing Layout
Single vision aspheric lenses made from Trivex material
have a specific factory-marked base curve center on the
blank. This point is the apex (or steepest part) of the curve
that defines the lens’ asphericity. This center should coincide
with the optical center after the lens has been surfaced.
If it does not, the lens (depending on power) may have
undesirable and unpredictable multi-axis cylinder and may
need to be remade.
Double aspheric lenses have an axis marking line on
them that must be properly aligned with the Rx axis. These
markings can be prevented from coming off when the
surface protector tape is removed if a piece of disappearing
cellophane tape is placed over the markings prior to
applying the surface protector tape. These markings will
be used to verify that the lens’ optical center and axis are
aligned with the aspherical apex during finishing layout.
In order to obtain the full optical and cosmetic benefits
that lenses made from Trivex material have to offer, it is
recommended that layout computing be completed with
software that is specifically designed for these lenses. The
combination of aspheric design, 1.53 index and minimum
thickness variances requires this upgrade to your layout
program.
G U I D E
U S E R ’ S
M AT E R I A L
L E N S
T M
T R I V E X
As with any plastic lens
material, the base curve of the
lenses made from Trivex material
should be covered with surface
protecting tape prior to blocking
and generating. The front of
these lenses has a built-in,
protective, scratch-resistant
layer, but shielding the lens
with surface protector tape offers
enhanced scratch protection and
provides a better bond for the surfacing block. There is also
an increased amount of torque created when surfacing this
material and the surface protector tape will help provide a
good bond with the surfacing block.
Heat is always an enemy of plastic lenses so the less
heat lenses made with Trivex material are exposed to, the
better they will turn out. That is why a low temperature alloy
or wax blocking method is recommended for blocking lenses
made from Trivex material. Be sure to allow adequate
cooling time after blocking so that the blocking bond is
optimal and the lens is not stressed from the heat of the
blocking process.
Photo courtesy of Younger Optics
Surface Taping and Blocking
23
Generating
There are two basic generator types used in today’s
optical laboratory. Different procedures are recommended
depending on the type of generator being used.
Manual Sweep Arm and Diamond
Quill Generators
T R I V E X
T M
L E N S
M AT E R I A L
U S E R ’ S
G U I D E
A combination grit diamond quill meant for both CR-39
monomer and polycarbonate will work well with Trivex
material. It is best to run the coolant throughout the manual
generating process. Generator operators should use slightly
slower sweeps and take less thickness off with each sweep
than they normally do with polycarbonate. There is an
increased amount of torque produced during the grinding
process on lenses made from Trivex material, so slow
things down a bit. A very slow final sweep with a minimal
thickness cut and with coolant running will give the lens
an excellent pre-fining surface.
A fluffy, wool-like swarf that can clog the coolant drain is
produced when generating lenses made from Trivex material
in a diamond generator. Be prepared to clean out the swarf
trap after generating each lens.
24
Single-Point CNC Dry Milling
Generators
A two or three flute cutter will work best with Trivex
material. Cutters must be sharp in order to produce the best
surface quality. If the generator has an optional cribbing
setting, use it. This is because edging lenses made from
Trivex material causes a lot of torque so if the lens diameter
is minimal, there will be less torque. This helps keep lenses
on axis during edging.
Using the pin bevel setting on a dry mill generator will
help reduce wear on your fining and polishing pads and
eliminate sharp edges.
Thickness
While lenses made from Trivex material have excellent
impact resistance with a 1.0mm center thickness, they may
warp over time if placed in frames that exert significant
pressure around the lens’ circumference. Because of this,
some labs add 0.2mm to 0.3mm of thickness to the lenses.
The extra thickness does not alter the final appearance of
the glasses noticeably (for example, a 1.0mm edge vs. a 1.3 mm
edge) and can help avoid the potential warping problem.
G U I D E
U S E R ’ S
M AT E R I A L
L E N S
Some laboratories have found that polishing pads and
polishes designed specifically for Trivex material work very
well. Other laboratories have found that they can use pads
and polishes recommended for polycarbonate with good
results too. Lenses made from Trivex material tend to be very
forgiving to the surfacing process as long as the process is
not rushed.
T M
Polishing
T R I V E X
Some laboratories may choose to
use a two-step fining process to
surface lenses made from Trivex
material. When doing so, use
compensated lap tools. These tools
have been cut with a slightly steeper
curvature to allow for pad stacking.
If a second fining pad is stacked on
top of the first fining pad, it adds
diameter to the lap tool curvature
which can cause a noticeable power
error in these lenses depending on
the lens’ power. Compensated laps correct this problem.
Single-use foam lap tools must also be cut to compensate
as well.
Fining pads made specifically for lenses made from
Trivex material usually have grit ratings of about 300 for the
first fining and 1150 for the second fining depending on the
pad manufacturer. Some labs find that the pads they use
for polycarbonate lenses work well too since there is little
difference in the grit sizes of 280 for the first fining and
1000 for the second fining.
Be sure that cylinder machines are adjusted to their fullyrecommended sweep and stroke settings (on most cylinder
machines this will be about 28mm tall for the stroke by
62mm wide for the sweep). If they are not, there may be
areas on the lenses that do not fine completely.
Use a slightly lower pin head pressure than the usual
20 psi (about 18 psi), a timer setting of 1.5 – 2.0 minutes
and the slow speed setting on your cylinder machines for
lenses made from Trivex material. These lenses fine out very
easily compared to polycarbonate and do not require much
excess pressure or time to bring them to an excellent finish.
Do not over work these lenses; it simply is not necessary.
Photo courtesy of HOYA Vision Care
Fining
25
Backside Scratch-Resistant
Hard Coating
Even though lenses made from Trivex material are
considerably more scratch resistant than uncoated
polycarbonate lenses, you will still want to add a backside
scratch-resistant hard coating. A good quality UV cured
tintable coating will improve scratch resistance as well as
enhance the tintability of these lenses.
Anti-Reflective and Mirror Coating
In general, AR and mirror coating techniques that
provide good results for other plastic lens materials work
well for lenses made from Trivex material also. As with other
lens materials, a good scratch-resistant hard coating is an
essential part of the AR and/or mirror coating’s success.
Coatings adhere well to lenses made from Trivex material.
Therefore, washing lenses with soap and water is preferable
to etching prior to coating. Some coatings can have a
detrimental effect on impact resistance.
T R I V E X
T M
L E N S
M AT E R I A L
U S E R ’ S
G U I D E
Lens Inspection
26
As mentioned earlier, the center markings on single
vision aspheric lenses made from Trivex material must align
with the optical center after surfacing. Measure the lens
thickness using a lens caliper and visually inspect the lens
for any flaws before moving on. Lenses that consistently
come out with waves or warpage suggest that there was too
much heat or pressure applied during surfacing. Check the
chiller and coolant waters in the generator and cylinder
machines to ensure they are keeping the lenses cool at all
times. Double check the pin head pressure on your cylinder
machines to be sure they set properly (around 18 psi).
FINISHING RECOMMENDATIONS
Finish Blocking
A high quality pad should be used to block lenses made
from Trivex material. Pads that stretch too easily may cause
axis problems. Also remember, the smaller the diameter and
the thinner the lens blank edge is, the less torque there will
be in the edger, helping to keep things on axis. Be sure to
use the correctly curved block for the lens’ base curve.
Lenses made from Trivex material have a little flex in them so
using the wrong shaped block may damage the lens when it
is chucked in the edger. Flexible plastic blocks work well to
reduce this problem but avoid using older over-used plastic
blocks that could cause axis problems. Chucking pressure
should not exceed 20 psi or be less than 18 psi or the metric equivalent of 80kg.
G U I D E
U S E R ’ S
M AT E R I A L
L E N S
T M
T R I V E X
A dry, cutter blade edger provides
excellent edging results with lenses
made from Trivex material. Good sharp
cutter blades will not only cut the lenses
easily and with very little additional
torque, they will also produce a semiglossy edge that some technicians feel
needs no additional edge polishing. For
semi-rimless chord mounted jobs, it is
advisable to use the semi-rimless blades
that automatically groove the lenses, if possible. This saves
time and an additional step in the groover. It is also not necessary to clean out the melted swarf that accumulates in a
manual groover with Trivex material.
An airborne particulate and a distinctive smell can be
created when edging lenses made from Trivex material. To
help reduce the particulate and scent, use a dry-cut cycle
that has a vacuum to collect the dry swarf if possible. It may
be advisable for lab techs to wear a dust mask during the
edging and surfacing process.
Photo courtesy of Younger Optics
Edging – For Dry, Blade
Style Edgers
27
T R I V E X
T M
L E N S
M AT E R I A L
U S E R ’ S
G U I D E
Edging – For Wet-Sump
Diamond Wheel Edgers
28
Having a wet edger with the correct diamond wheels is
one of the most important factors in getting lenses made
from Trivex material to finish on axis and with a high quality
bevel. The recommended diamond wheels have diagonal
slots or a series of holes in either of both the roughing wheel
and the beveling/finish wheel. The purpose of these
slots/holes is to allow for cooler roughing and beveling
cycles. The diamond grit on these wheels is very much like
the grit on conventional polycarbonate stones and they work
well on all types of plastic lenses.
If the edger has a “Trivex” setting it should be used.
Most older edgers will not; in which case the, polycarbonate
setting will work best. Lenses made from Trivex material
should be rough cut and rough-finish cut dry (without the
coolant water running on the wheels). Coolant water should
be running on the final finish edging cycle to bring the bevel
to a smooth finish. If the machine has an edge polishing
wheel, use the coolant water on this cycle, too.
There will be a noticeable amount of melted swarf
coming off the lenses during the rough cycle which can
form into hard chunks. These fragments may clog the swarf
drain in the edger if the machine does not automatically
wash them away. They can also get wedged between the
diamond wheels and the grinding chamber walls, causing
a loud bang when they dislodge and propel back into the
grinding chamber. Because of this, it’s important to be very
cautious and wear safety glasses while edging lenses made
from Trivex material.
Sizing and Bevel Placement
Lenses made from Trivex material maintain their size and
shape well over time and do not need to be edged large to
compensate for shrinkage like some materials. Cut these
lenses right on size.
Trivex material hand edges slowly on conventional grit
ceramic-pin-beveling hand stones. Use a dual grit hand
edger stone with a coarser grit on one side and a finer grit
on the other side in order to hand edge it effectively.
Manual Edge Polishing
The automatic edge polishing function (on edgers that
have it) give lenses made from Trivex material a shiny luster
and is a convenient way to edge polish these lenses. For
labs that use manual edge polishing equipment, use a
fresh, new felt polishing pad or muslin buffing wheel in
conjunction with a high-quality dry polish stick designed for
polycarbonate. This dry polish has a lubricating wax in it
that will help keep the lenses from getting pits in them from
too much heat. Always protect the lens surfaces with surface
protecting tape before polishing. Use as little of the polish
stick as possible and use very little pressure when polishing.
Avoid polishing the apex of the lens bevel, if the lenses were
cut exactly to size, or they may turn out too small. Manually
bringing lens edges to a high gloss takes a little time so be
patient and do not overheat the lenses or they will blister.
Manual Grooving
Manually groove lenses made from Trivex material dry
(without using water). After grooving, clean the excess swarf
out of the groove with an optical screwdriver, or similar
device, the same way as with polycarbonate. This removal
will be a little easier with Trivex material than it is with
polycarbonate because the swarf comes off in long strands.
If properly edged and sized, lenses made from Trivex
material will have a high-quality edge enabling easy
glazing. These lenses should not be glazed too tightly or
they may warp, causing distortion and stress aberration.
Be cautious when glazing lenses made from Trivex
material into plastic frames using a heating pan containing
glass beads or salt. Excessive heat can blister the lenses,
so a hot air frame warmer is recommended for this task.
Use standard mounting and insertion techniques to
glaze lenses made from Trivex material into metal and
semi-rimless frames.
U S E R ’ S
M AT E R I A L
Mounting and Inserting
L E N S
Tinting lenses made from Trivex material is as easy
tinting any other plastic lenses. Consult the “Tinting” section
of this manual for tips on getting the best results when
dyeing these lenses.
T M
Tinting
T R I V E X
Lenses made from Trivex material are ideal for rimless
eyewear. For drilling tips, see the section entitled, “Drilling.”
G U I D E
Computerized and Manual Drilling
29
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