Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATU8 FOR TREATING
AN 0~...~T~C LEN8 MOLD
Prioritv Data
This is a continuation-in-part of application Serial
No. 07/992,884, filed December 21, 1992.
Background of the Invention
This invention relates to a method and apparatus for
the improved removal of molded ophthalmic lenses from the
mold in which they are produced. In particular, this
invention is suited to molded ophthalmic lenses such as
hydrogel contact lenses, although the method is also
suitable for other small, high-precision ophthalmic lenses
such as intraocular lenses.
Soft ophthalmic lenses for placement on the cornea or
within the eye, such as contact lenses or soft intraocular
lenses, can be made by a variety of techniques. Contact
lenses can be made by spin casting a monomer material in
a rotating mold then polymerizing the material so shaped.
Another method used to manufacture both contact lenses and
intraocular lenses is precision lathing of a piece of
material which is then polished and used as a lens.
Recently the molding of soft contact lenses and soft
intraocular lenses has come into favor. This technique
has the advantages of repeatability and speed that
compares favorably with the prior methods of manufacturing
lenses. Techniques for successfully molding such lenses
can be found in U.S. Patents 4,495,313 and 4,640,489 to
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VIA EXPRESS MAIL N0. TB15074564X
MAILED DECEM8ER 28, 1993
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Larsen and 4,889,664; 4,680,336 and 5,039,459 to Larsen
et.al. These patents specifically described the use of a
diluent, a material which substitutes for water during the
molding process, and which is replaced with water after
the molding has been completed. The advantage of this
technique is that the optical properties, size and shape
of the lens thus made does not change as radically as with
methods that do not utilize such diluent.
It is further khown in the art to mold such
ophthalmic lenses by forming a monomer or monomer mixture
in a mold such as one made from polystyrene or
polypropylene.
An example of this art can be found in U.S. patent
4,565,348 to Larsen. Discussed therein is the requirement
for a polystyrene mold that the materials, chemistry and
processes be controlled so that the mold pieces do not
require undue force to separate by sticking to the lens or
to each other.
In contrast to the above polystyrene molds, another
example is the use of polypropylene or polyethylene molds
such as that described in U.S. Patent 4,121,896 to
Shepherd.
A particular problem, however, is that the monomer or
monomer mixture is supplied in excess to the concave mold
piece. Upon mating of the molds, thereby defining the
lens, the excess monomer or monomer mixture is expelled
from the mold cavity and rests on or between the flange of
one or both mold pieces forming an annular ring or
flashing around the formed lens.
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After separating the two-mold pieces, the peripheral
flashing of now polymerized excess material usually
remains with the female mold piece, the same piece that
holds the lens. In order to further process the lens
through hydration, inspection, packaging, sterilization,
etc., it is necessary to remove the flashing of
polymerized material from the female mold piece. When the
flashing remains with the female mold piece with the lens,
it is manually picked off with the finger.
It is the object therefore of the present invention,
to present a means for removing an ophthalmic lens from
the mold in which it is held, along with the surrounding
flashing without human intervention. This invention
greatly simplifies this portion of the lens making process
by reducing cost, increasing throughput and allowing for
automation.
More specifically, it is an object of the present
invention to provide a method and apparatus for separating
an ophthalmic lens from a flashing when the mold pieces
are separated.
SummarY of the Invention
The above objects are accomplished by providing a
method and apparatus that directs accelerated electrons to
at least part of one surface of one of the mold pieces
prior to filling with monomer and lens polymerization.
The mold pieces contain on one piece an edge that makes
line contact with the other piece, such-that when the two
pieces are mated a cavity is formed therebetween to form
the lens. In particular it has been found that generation
of the ionized oxygen by means of a corona treatment
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- 4 -
electrode sufficiently increases the adherence of the
polymer to the mold piece so treated. In the preferred
embodiment, the flange around the convex, male piece of
the lens mold is corona treated so that when the mold
S pieces are separated after lens polymerization, the
flashing of excess polymerized material surrounding the
lens cavity adheres to that male, convex piece flange
while the lens is removed with the female, concave piece.
A process gas flow manifold supplies nitrogen and air
while drawing a vacuum to ensure the appropriate cover gas
on mold portions during the corona treatment.
Brief DescriPtion of the Drawings
Figure 1 shows a characteristic of liquid\solid
surface interaction modified by the present invention and
a means of its measurement.
Figure 2 is an enlarged cross-section of the
electrodes of the present invention along with male mold
work piece.
Figure 3 shows in cross-section the apparatus of the
present invention containing the electrodes of Figure 2
for treating multiple male mold pieces.
Figure 4 shows in cross-section a pair of mated mold
pieces.
Figure 5 is a cross-sectional view of a corona
treatment process gas flow manifold used in particular
applications of the invention.
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DESCRIPTION OF PREFERRED EMBODIMENT
It has been found that the adhesion of the
polymerized monomer material to the mold in which it is
formed and polymerized is related to the surface energy of
the mold material. Surface energy, a material property
analogous to the surface tension of a liquid, determines
the wettability of the material and is measured in dyne
per centimeter.
The surface energy of a material can be determined by
a contact angle measurement. By measuring the contact
angle of a liquid droplet on a solid surface using a
goniometer, the surface energy can be determined. The
smaller the contact angle measured the more wettable the
surface.
Referring to Figure la, there is shown the typical
goniometer scale 10 indicating the contact angle 12 formed
by liquid droplet 14. Figure lb shows liquid droplet 14
on a substrate 16 having poor surface wettability for this
particular liquid forming contact angle 12 which is much
greater than 90. Referring to Figure lc, there is again
shown liquid droplet 14 and substrate 16, in this case
with good surface wettability. In contrast to Figure lb,
here the contact angle is less than 60 indicating a
material that has a surface energy exceeding the wetting
liquids surface tension by at least ten dyne per
centimeter.
Because the wettability of a liquid on a substrate
surface is not strictly a function of the substrate's
surface energy, but rather the result of the difference
between the substrate and the wetting liquid, the surface
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energy alone, although an indication of wettability,
cannot be used alone as the ultimate indicator of the
contact angle for all liquids.
In the preferred embodiment of the present invention
where polystyrene molds are used to form a etafilcon A, a
58% water content hydrogel contact lens, the polystyrene
has a surface energy of 40 dyne per centimeter.
Experimentally, it has been shown that the prepolymer for
the etafilcon A material (in combination with the boric
acid ester diluent to taking the place of water during the
molding process described in the patents identified in the
Background section) in contact with a polystyrene surface
has a contact angle between 28 and 30.
Methods for increasing the surface energy of
polystyrene and other plastics include flame treatment,
plasma and chemical etching and electrical surface
treatment. The method employed in the preferred
embodiment is electrical surface treatment, otherwise
referred to as corona treatment. In effecting this
method, an apparatus includes a set of electrodes which
conform to the area where treatment is desired, a high
voltage transformer and a high frequency generator with
impedance matching electronics. The operating frequency
is adjusted based on impedance up to 25 kHz operating from
14 to 50 kV. With this combination of high frequency and
high voltage, it is possible to maintain a distance of
about 1~ inches and a relatively short treatment time by
making the plasma between the electrodes fairly intense.
After treatment the contact angle between the above
described etafilcon A monomer and polystyrene is between
6 and 12. This corresponds to a surface energy increase
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in the polystyrene to between 65 and 70 dyne per
centimeter.
Referring now to Figure 2 there is shown a specific
embodiment for implementing the invention on polystyrene
mold pieces constructed according to the references in the
background section. There is shown in this Figure, convex
male mold piece 20 intended for treatment. This male mold
piece 20 is held in place by piece support 22. This piece
support is constructed of an electrically non-conductive
material such as poly(ethylene terephthalate) and is
generally cylindrical in shape. Exterior to the piece
support 22 is electrode 24 which is close to, but does not
touch, the mold piece 20.
Located generally on the opposite side of male mold
piece 20 from electrode 24 is counter-electrode 26. This
counter-electrode is also generally cylindrical in shape
but with a hollow interior. This counter-electrode
touches the flange area of the male mold piece 20 and has
a surface which extends to the interior convex surface of
the mold piece proximate, but not touching, the back
surface of the male mold piece generally opposite the
position of electrode 24.
This results in the area of treatment indicated as
28.
The space between the electrode and the area of
treatment ranges between 0.0 and 0.05 inches, while the
area between the counter-electrode and the back surface of
the male mold piece zo ranges from o (in contact) to about
0.07S inches in the area of treatment.
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Referring now to Figure 3, there is shown a plurality
of electrodes and counter-electrodes in an assembly used
to treat a plurality of mold pieces. As shown in Figure
2 there is also shown herein piece support 22 electrode 24
and counter-electrode 26. Not shown is a mold piece for
treatment.
This Figure also shows in the apparatus, electroplate
30 which supplies a common voltage to electrodes 22 as
well as insulating support 32 attached to mounting plate
34. Counter-electrodes 26 are supported by mount 36 and
the assembly rides on guide rods 38. By movement of guide
rods 38, the mount 36 can move the counter-electrodes 26
away from the electrodes 24 and piece support 22 allowing
easy insertion and removal of the mold pieces.
In performing the actual treatment, the electrodes
are placed between 0.25 mm and 0.5 mm from the surface of
the mold piece to be treated.
While the exact mechanism causing the polymerized
material to adhere to the corona treated polystyrene is
not known, electrical surface treatment effectiveness has
been linked by theory to such phenomenon as ablation
(surface degradation), cross linking of the polymer,
oxidation, hydrogen bonding and electret formation. While
the mechanism is unclear, it has been found that one of
the parameters effecting the strength of adhesion between
the polystyrene and the lens polymer is the amount of
oxygen present before and during treatment of the mold
surface. Generally, the lower the oxygen level, the lower
the bound oxygen to the surface, and the less adhesion
between the polystyrene and the lens polymer. For this
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reason it is best that oxygen contact with the polystyrene
molds be minimized prior to treatment.
Other parameters effecting the adhesion strength are
power of the electrodes and time of treatment as well as
treatment frequency and voltage.
For the present invention it was found that a
treatment voltage of 10 kV at a frequency of 20 kHz to
30 kHz with a power between 10 watts and 80 watts, with
30 watts preferred, for a period of at least about 0.2
seconds gave the best results. In the preferred
embodiment with an electrode diameter of 0.79 inches, a
power of 22 watts and a treatment time of 0.3 seconds in
an ambient atmosphere, 100% of the flashing was removed
with the convex male mold piece 20, while only 0.5% of the
lenses being improperly retained by the convex male mold
piece 20.
Referring now to Figure 4, there is shown the mated
mold pieces including the concave, female mold piece 40.
Between the pair of mated mold pieces is lens 42 and
external to the lens, around the periphery and between the
flanges of mold pieces 20 and 40, is flashing 44. The
relative position of the area exposed to corona treatment
28 with respect to the lens and flashing is now apparent.
As can be appreciated by one skilled in the art an
excessive increase in any of these parameters causes
migration of the treatment into the lens surface of the
convex male mold piece that results in adhesion of the
lens to the male mold piece.
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It has been found that if there is no oxygen present
during the electrode discharge to the target surface,
neither extended treatment times or higher power causes
the flashing to stick to the target surface. In short, it
is believe that corona treatment causes oxygen to ionize
and bind to a specific area of the convex mold piece to
chemically alter the surface.
Under certain manufacturing conditions, however, it
is preferred to maintain to the extent possible, an
oxygen-free environment about the mold and monomer. For
this reason, the manufacturing process is done under an
inert gas environment, preferably nitrogen, until the lens
is polymerized. To meet this requirement and still
practice the present invention, a special fixture is
required.
Referring to Figure 5, there is shown a process gas
flow manifold used to practice the invention under
otherwise oxygen-free conditions. This manifold supplies
an oxygen bearing gas, air, to the area to be treated
while preventing the oxygen from contaminating the lens
manufacturing area of the molds or diluting the nitrogen
atmosphere of other lens process areas.
A mold piece 49 sits atop housing 50 constructed of
Lexan and containing three concentric annuluses. Nitrogen
gas is supplied to a supply means such as center annulus
52, travels the length of the housing and washes over the
lens-forming portion of the mold piece by way of nitrogen
cup 54. As a non-reactive gas, nitrogen permits the lens-
forming portion of the mold to remain uneffected by the
treatment. Oxygen containing air is supplied through a
supply means such as housing outer annulus 56 and
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ultimately flows over the flange area desired to be
treated.
A vacuum is drawn from the middle annulus ensuring
that the N2 and 2 gases flow over the desired areas, but
do not intermix. The flow of gases is kept balanced by
appropriate sizing of orifices 60. Corona treatment is
initiated when voltage is applied to electrode 62 as the
gas flow is maintained. After the treatment cycle is
completed (approximately 0.2 seconds), the gas flow is
ceased, the mold piece removed and the next piece put on
place.
Following treatment and filling with monomer, the
monomer is then caused to polymerized by chemical, thermal
or ultraviolet means of polymerization initiation. After
polymerization is completed, the male and female portions
of the mold pair are separated and the lens is removed.
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