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FIELD OF THE INVENTION
This invention relates to optical grinding "laps", particularly to an
optical grinding lap comprised of an easily machinable (and
re-machineable) thermoplastic polymer.
BACKGROUND OF THE INVENTION
In preparing and grinding optical lenses, for example eyeglass lenses, the
primary grinding (i.e., the grinding necessary to put the primary focal or
optical curvature into the lens(es) according to the eye doctor's
prescription) is performed by relatively coarse abrasive materials. The
result is that, after the optical curvature is ground into the lens(es),
the inside (i.e., interior, concave) surface of the lens is almost opaque
from grinding or scratch marks left by the optical grinding cutters.
The prevailing practice in the optometry field is to perform a "secondary"
grinding or polishing operation upon those concave surfaces, to remove the
grinding lines left by the primary optical cutters. This secondary
polishing operation may itself comprise more than one step, but both steps
are performed in substantially the same manner. Basically, the eyeglass
lens is placed "scratched" (i.e., concave) side down upon a polishing
"lap" or, as they are sometimes referred to, polishing master. The
receiving surface of the polishing lap is convex, and is usually ground to
the exact reciprocal of the concave surface of the lens, so there is
virtually complete surface mating between the concave surface of the lens
to be polished, and the convex surface of the lap upon which the polishing
is to be accomplished.
Before the lens is placed upon the lap for polishing, the lap is provided
with an abrasive polishing surface, usually in the form of a flexible
adhesive-backed abrasive pad, whereby the abrasive pad is flexible enough
to conform fully to the surface of the lap. The lens to be polished is
impressed upon the lap (with adhesive pad thereon) in such a way that the
surface of the lens to be ground is in full contact with the polishing pad
upon the lap.
The combined lens-lap-pad is placed onto a polishing machine and the
assembly is caused to oscillated or vibrated in the presence of a
polishing lubricant and in such a way that the polishing pad and lubricant
removes the grinding marks left by the primary optical cutters. Often the
polishing is accomplished in conjunction with providing some sort of a
mildly abrasive slurry, of approximately the same grit as is applied to
the polishing pad, to the lap-pad-lens assembly while the polishing is
taking place.
In two stage polishing processes, such as the polishing process utilized by
this inventor, after the slurry polishing step is completed, the first
polishing pad is removed, and a second, finer (i.e., less gritty, less
abrasive) pad is placed upon the lap. the lens reimpressed thereupon, and
the lap-pad-lens assembly is reoscillated/vibrated, this time usually with
only water as the provided solution (although other solvents, even
including a minor grit solvent, could be used).
It is generally known and appreciated that the laps must be contoured very
precisely, to the inverse curvature of the optometric Prescription, so the
convex surface of the lap will coincide exactly with the concave surface
of the optical lens being polished. This is accomplished on commercial lap
cutters which are generally known in the art.
Because of the relatively high cost of machining and the time it takes to
machine present laps, laps are presently provided in large number of
different starting shapes (as many as 72 lap shapes), shapes which are
provided in an estimate of the final ground contour, so as to minimize the
amount of lap material which must be machined off to exactly match the
prescription contour In addition, often it is necessary to have more than
one set of laps for each pair of glasses, one set for glass lenses, and a
second set for plastic lenses. As you might imagine, the inventory of laps
in many optical labs is very high.
In addition, even though presently available laps are made of relatively
machineable metal (i.e., aluminum; see infra), it still consumes quite a
bit of time to grind a lap.
The techniques and procedures just above described are well known and
understood in the optical grinding art.
The standard lap that is used in the optical lab industry is made of
aluminum, which replaced cast iron as the lap of choice some ten years
ago. However, even though aluminum laps provided advantages over cast iron
laps because of their noncorrosiveness, relative ease of machineablity and
relative (i.e., compared to cast iron, at any rate) low weight, the
aluminum laps in use today have the nagging disadvantage of high machine
time to grind to the reciprocal contour of the prescription (as much as
6-12 minutes per lap), and their still relatively high weight (about 12
ounces) causes wear and tear on the polishing apparatus. In addition, the
time delay caused by multiple passes on the lap cutter tempts the operator
to try to machine off too much aluminum in a single pass, and this often
results in chipping of the aluminum, commonly called "dinks." If a lap
becomes "dinked", it must be thrown away.
For many years, the industry has striven to provide a plastic lap to
replace and avoid the problems associated with the use of aluminum laps,
but without success--until now.
Attempts to provide a viable plastic lap have heretofore failed,
principally for two reasons. First, while it is well known to use
injection molding to produce easily machinable plastic parts, standard
injection molding techniques do not produce an acceptably strong lap
structure to withstand the mechanical stresses and abuses of the polishing
process. This is because, ordinarily, injection molded plastic parts have
only a relatively thin "skin" or "shell" of solid plastic to form the
upper lap surface, and the structural integrity (such as it is) of the lap
is provided by the standard "webbing" practice well known in the injection
molding arts.
The other well known plastic molding process, compression molding, has
proven unacceptable because the plastics which are utilizable in
compression molding (i.e., thermal set plastics, such as phenolics) are
generally brittle, and would crack or chip when attempting to machine them
to required contour.
The present invention overcomes these problems and provides an acceptable
plastic lap through a combination of a unique material selection and a
molding process which departs substantially from the prevailing teachings
ordinarily associated with injection molding. The particular material
utilized is a mineral-filled thermoplastic polymer, which is injection
molded to its final shape into a super-heated mold via an inlet valve or
gate which is much, much larger than ordinarily necessary, and also
utilizing process parameters that exceed the prevailing published process
and design criteria for the material, and can only be accomplished by an
injection molding press that is about three times the size and capacity
that would ordinarily be called for to injection mold a part of the size
and shape of standard optical laps.
SUMMARY AND OBJECTS OF THE PRESENT INVENTION
The primary object of the present invention is to provide a plastic lap
which will provide the same utility as an aluminum lap, but without the
attendant disadvantages of high weight and high machining cost.
It is an object of the present invention to provide an optical polishing
lap which is substantially more easily machinable than an aluminum
polishing lap without dinking, yet is mechanically serviceable to
withstand the mechanical abuses of the polishing apparatus.
It is a further object to provide a polishing lap which is substantially
reduced in cost compared to aluminum laps.
It is a further object to provide a polishing lap that is capable of
multiple remachinings (sometimes referred to as "trueing").
It is a further object to provide a polishing lap which is so easily
machinable that the number of pre-machined sizes can be, and is,
startlingly reduced. At the present time, it is contemplated that, because
the laps of the present invention are so easily and quickly machinable,
the number of stock sizes provided will be four (4), compared to the 72
required be the present art. It is possible that the number of standard
starter sizes provided will ultimately only be two (2).
Other objects and uses will become apparent to those skilled in the art,
after review of the drawings and detailed description below, but such
other objects and uses do not depart from the scope and spirit of the
present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top perspective view of an optical lap made according to the
present invention;
FIG. 2 is a bottom perspective view of an optical lap made according to the
present invention;
FIG. 3 is a front plan view of an optical lap made according to the present
invention, with the device oriented domed surface down;
FIG. 4 is a right side plan view of an optical lap made according to the
present invention, with the device oriented in the same fashion as in FIG.
3; and
FIG. 5 is a bottom plan view of an optical lap made according to the
present invention, with the device oriented in the same fashion as in
FIGS. 3 and 4; FIG. 5 is also provided with "double hatch lines" to reveal
an alternate embodiment of the present invention, as described more fully
below.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT (BEST MODE)
Referring now to FIG. 1, disclosed is a generally round optical lap 11 of
thermoplastic polymer (e.g, 6-6 nylon) comprising a monolithic dome shaped
top portion 13 affixed to and upon a generally rectangular base 15.
Referring to FIG. 3, the lap 11 is provided in circular form of a diameter
D (e.g, 3.165") with cylindrical sides of a height H (e.g, 0.650"), or
other shapes such as oblong (not shown) which are known in the optical
grinding arts. Dome shaped portion 13 is provided with a rise R (e.g, an
arc with a diameter of 3.50") extending from the upper end of sides H to
the apex of the dome, indicated by point A. At present, D, H and R are
provided in numerous varieties of dimensions known in the optical
industry, and one of the principal benefits of the present invention is
that the present invention can eliminate the high number of different
dimensions available to four, or even possibly two.
Unlike most injection molded parts, the base 15 of the lap of the present
invention is substantially integral, without the usual "webbing" pattern
associated with injection molding techniques. Nevertheless, base 15 is
provided with a space 19 in a manner to be adaptable to fit within the
chuck of an optical lap cutter machine, such as one (Model 302) provided
by Coburn Optical Industries of Muskogee Okla. Specifically, for the
Coburn Model 302 lap cutter, there is provided a recess 19A to accommodate
a protrusion on the chuck of the Model 302 Coburn optical lap cutter
machine where the lap is held in place while cutting/grinding it to its
necessary contour. Of course, the specific form of space 19/recess 19A is
not integral to the invention, but is provided merely to accommodate the
lap cutting geometry of the particular lap cutter described.
In the present design, base 15 is provided with parallel rails 17A and 17B
which are provided to fit within the clamps of the chuck of the optical
lap cutter. The end portions of the base 17C and 17D are provided in
conformance with the general contour of the lap. As stated above, one
object of the present invention is provide an integral base, rather than
the usual webbed base associated with injection molding. In FIGS. 1-5, the
base 15 is provided with only two cross webs 21A and 21B in addition to
the four components, side rails 17A 17B and end rails 17C and 17D of the
base 15. For a lap of the present invention, rails 17A and 17B are about
2.0" apart, and end rails 17C and 17D are provided with a curvature of
about a 2.5" inside diameter.
It has been found that to avoid movement of the domed portion 13 imparted
by the clamping pressure of the lap cutter chuck, it is advisable that
side rails 17A and 17B are presented in sloped fashion 23 with the rails
being thicker at the point where they contact the lower surface of dome 13
than they are at their terminal flat surfaces. The amount of slope 23
provided to rails 17A and 17B may vary substantially, such that the amount
of open space 25A 25B will vary accordingly, and in fact spaces 25A and
25B may be completely filled, shown as double "hatch marks" 25C in the
right portion of FIG. 5, thereby providing a virtually monolithic base
except for the recess/space 19/19A necessary to permit the lap to placed
upon a lap cutter.
To make the optical lap of the present invention, an injection molding
process is used, which departs in several material respects from standard
injection molding techniques. At present, the material which is known to
work best for the present invention is a mineral filled thermoplastic
polymer, namely 6/6 nylon with 40% talc filler; the particular brand which
has demonstrated successful results is dupont's MINLON 10B40 nylon,
although no particular reason is known why other thermoplastic polymers
would not also provide adequate results.
The process utilized is beyond the published working ranges in three major
respects. First, the thermoplastic polymer is heated above its published
working range. Second, a substantially larger than ordinary "gate" size is
used, about two to three times the size ordinarily used in "conventional"
injection molding processes, to accommodate the rush of extraheated
thermoplastic material into the mold cavity. Third, the mold itself is
heated to a high degree to avoid the thermoplastic polymer establishing a
"skin" or "shell" when first contacting the mold walls. Fourth, the
molding pressures utilized are above the published working range. Because
of these differences, it has been found that a surprisingly large
injection molding machine is necessary to adequately accommodate this
process and produce acceptable laps. When first confronted with the
desired dimensions and expected volume of the lap (about 6 ounces),
"conventional wisdom" in the injection molding arts predicted that the
laps could be produced on a 100 ton injection machine, which is usually
associated with a 6 ounce "shot" (i.e., the volume of the mold cavity to
be filled). However, because of the substantial departures from
"conventional wisdom", the laps of the present invention require an
injection molding machine with a 300 ton capacity with an 18 ounce shot
capacity. The particular injection molding machine utilized was
manufactured by Reed Manufacturing Company (now no longer producing
machines in its own name), but other injection molding suppliers could
provide adequate substitutes.
The thermoplastic polymer is heated above its recommended temperature
range, in the case of MINLON 10B40 to 610 degrees fahrenheit more or less,
which is above the published working range of 520-580 degrees. Of course,
care must be exercised to not overheat the polymer and destroy its
as-molded properties, but the present upper limit of temperature beyond
the published processing temperatures is unknown. By such raising the
temperature of the thermoplastic polymer, it is possible to obtain higher
working temperatures in the injection molding machine (e.g, a nozzle
temperature of 560 degrees), which renders the thermoplastic polymer
capable of more rapid flow into the mold cavity.
To facilitate the rapid injection of the high amount of polymer into the
mold, it is necessary that a much larger than ordinary gate be used,
depicted in the drawings at numeral 27. In making a three inch diameter
round lap with a 3.5 inch diameter dome with sides H of about 0.650", and
those of similar size, ordinarily a screw runner gate size of about
0.10"-0.15" would be called for, a ratio of gate size diameter to height H
between about 0.154 to about 0.230. To make an acceptable lap of the
present invention, a gate size of 0.250"-0.375" is used, a corresponding
ratio of about 0.33 to 0.75, or some two to five times the ordinary gate
size. This permits a sufficiently rapid flow of hot thermoplastic polymer
into the mold to fill up the cavity before the thermoplastic begins to
set.
The mold itself is made of steel, the usual material for an injection mold,
and perhaps the "female" core portion of the mold is provided with copper
or some other highly conductive metal. The mold is preferably maintained
at a temperature around 200-210 degrees, which is almost twice as hot as
the ordinary temperature (e.g, 120 degrees) called for by "conventional"
injection molding proactices, also a substantial departure from the
"conventional wisdom" of the injection molding art. Again, the elevated
temperature of the heated mold restricts the skin forming tendencies of
the thermoplastic polymer when it first contacts the mold walls.
The final departure from the "conventional wisdom" of the injection molding
art is the use of higher molding pressures than the published working
ranges. In making the laps of the present invention, a two stage pressing
operation is used, the first stage at a pressure of 1200 pounds for about
36 seconds, and the second high pressure range for an additional 36
seconds at 1800 pounds pressure. This converts to a "high" pressure of
about 15-25 percent above the published molding pressure ranges for
mineral filled 6/6 nylon.
When completed, the laps of the present invention provide a dimensionally
stable thermoplastic polymer which is remarkably easier to machine than an
aluminum lap. For example, to grind or cut an aluminum lap, it usually
requires three or more passes to finally grind the lap to its desired
contour, a process which takes about 6-8 minutes. A thermoplastic optical
lap made according to the present invention can machined in one pass on
the same lap cutter, in about 45 seconds. The resulting finished
thermoplastic lap will weigh about 6 ounces, compared to about 11 ounces
for an aluminum lap. This substantial reduction in weight will no doubt
prolong the bearing life of the polishing equipment, as well as the lap
cutters themselves.
The foregoing description should be construed as limiting the scope of the
invention herein disclosed in any fashion, as those skilled in the art
will readily appreciate that the invention may practiced in many obvious
variations, without departing from the scope or spirit of the invention.
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Description |
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