Patent Application
20070138666
1. Field of the Invention
The invention relates to applying a coating with a stamp to an insert of an injection molding machine and then injecting resin to form a coated ophthalmic lens.
2. The Prior Art
In-situ coating via direct injection, so called in-mold coating was originally developed to improve the surface appearance of sheet molding compound (SMC) part molded by compression molding. In more recent years it's been applied to injection molded thermoplastic parts.
For the regular injection molding process, the thermoplastic piece is ejected out of the mold once it is rigid enough to resist the deformation caused by ejection. For in-situ coating injection, it integrates with injection molding via injecting thermoset coating liquid on the exterior surface of the thermoplastic piece when the thermoplastic piece is solidified to the degree that it won't be damaged by the coating injection. More coating is injected after the desired surface coverage is obtained to achieve certain coating thickness. The coating thermally cures by the residual heat from the thermoplastic piece and the continuously heated mold.
U.S. Pat. No. 5,943,957 discloses a method for pad printing inked images onto injection-molded pieces while they are still in the mold. The patented method relates to conventional ink that air dries, and does not involve an optical grade coating that will be spread over the lens by re-clamping the mold inserts and allowing the coating to cure via the retained heat in the mold block.
US 2003/0152693 A1 discloses a stamp coating method of applying a coating to an optical surface including the lens surface and the mold surface for lens casting. This method is based on a technology where a balloon made of deformable material is used to pick up a certain amount of coating from a container, a so-called cliché. The deformable balloon with coating on it then stamps the surface of the lens so that the coating is transferred from the balloon surface to the lens surface. The coated lenses are then UV cured. However, lens stamp coating has not been successfully applied to injection molding in order to deliver in-situ coated ophthalmic lenses. Accordingly, it would be desirable to provide a suitable coating composition and method for applying an in-mold coating to injection molded thermoplastic lenses.
In-situ coating integrated with injection molding is the way to in-situ coat injection molded thermoplastic lens, e.g. polycarbonate lens when it is still inside the mold. The coating can be applied to the mold insert via stamp prior to the lens molding. In this case, the coating will be transferred to the lens surface later on.
It is therefore an object of the present invention to provide a method to manufacture in-situ coated thermoplastic lens
Then it is an object of the present invention to disclose an in-mold coating method to manufacture coated thermoplastic lens wherein the coating is applied to the hot insert metal surface prior to the high shear, high temperature thermoplastic melt injection.
It is an object of the invention to provide an in-mold coating system that results in a coating of optical quality.
It is a further object of the present invention to modify an injection molding cycle to incorporate a coating stage therein.
It is yet another object of the present invention to provide a coating composition which is stable and liquid at room temperature, and which will quickly cure at a mold insert temperature.
It is a further object of the present invention to provide a flexible technique to stamp the coating onto the mold insert.
More particularly the optical coating is first applied to the heated mold insert via stamping. Stamping is conducted using a deformable pad of which the material has good wetability to pick up the coating from a cliché and is of certain temperature resistance at the thermoplastic lenses molding temperature. The pad is then pressed against the hot metal insert for 2 minutes to let the coating thermally precure. So when the pad is retracted, a thin layer of coating is formed onto the insert optical surface and no residual coating will be taken away by the pad. The mold is then closed and the thermoplastic melt is injected. The coating layer is further cured by the heat from the hot melt and transferred to the lens surface when the lens is ejected out of the mold.
In a general summary of the various methods covered by the invention, there is provided a method to manufacture an in-mold coated ophthalmic lens by pre-treating a heated mold insert of an open injection molding machine. A cliché is provided with a circular etched recess that contains a heat-activated thermoset acrylic-based coating solution. A silicone membrane is dipped into the recess to contact the coating solution. The liquid coating is transported via the silicone membrane to a heated mold insert and the membrane is then pressed on the insert until the coating is pre-cured. The silicone membrane is removed and the mold is closed. A thermoplastic resin is injected into the closed mold. Once the lens solidifies, the mold is opened and an in-mold coated ophthalmic lens is ejected.
The recess is etched to a depth between 10 μm to 50 μm. The diameter of the circular recess is dimensioned to dispense sufficient coating to completely cover a curved insert surface. The cliché contains between 0.2 ml and 0.5 ml of coating solution. The silicone membrane is an inflatable bladder inflated to a pressure between 5 psi to 20 psi. The pressure of the bladder on the insert is maintained from a time between 1.5 minutes and 5 minutes. The thermoplastic substrate is selected from the group consisting of polymethyl(meth)acrylate, polycarbonate, polycarbonate/polyester blends, polyamide, polyester, cyclic olefin copolymers and polyurethane, polysulfone and combinations thereof. The thermoplastic may be a polycarbonate derivative.
The liquid coating comprises at least a mixture of one or more (meth)acrylate compounds, a catalyst, and a metal salt. The liquid coating comprises at least a mixture of at least one hexafunctional acrylate compound, at least one difunctional acrylate compound, and at least one monofunctional acrylate compound. The catalyst is selected from alkyl aralkyl peracide compounds. The metal salt is cobalt naphthenate. The invention also covers a coated thermoplastic ophthalmic lens manufactured according to the method.
In one embodiment of the invention a cliché is provided on a horizontal surface of a lower mold half at the beginning of the cycle when the movable mold half is opened vertically. The membrane, the cliché and the mold insert all have center points, and wherein the three center points are aligned along a common vertical axis during said dipping step, and wherein said transporting step comprises transporting the membrane so that its center point remains on the vertical axis. The cliché contains between 0.1 ml and 0.8 ml of liquid coating solution.
In an alternate embodiment, the cliché is horizontally-oriented adjacent one mold half at the beginning of the cycle when the movable mold half is opened. The membrane, the cliché and the mold insert all have center points, and wherein the three center points are aligned along a common vertical plane during said dipping step, and wherein said transporting step comprises transporting the membrane so that its center point remains on the vertical plane.
The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings. In the drawings wherein like reference numerals denote similar components throughout the views:
The methods described herein provide an in-mold coated ophthalmic lens. An acrylic based coating solution is provided that is stable and liquid at room temperature. The coating may be applied to the insert as an unpressurized drop and then spread across the insert by an inflatable bladder. Alternatively, the bladder can apply the coating already spread out in a cliché. In either case the bladder pauses while the coating is being pressed against the heated insert to allow the coating to pre-cure. The bladder is next withdrawn. Once the coating is able to resist deformation from the injection flow, the resin is introduced into the cavity. The resin solidifies at the same time as the coating fully cures. An in-mold coated lens is then ejected from the injection molding machine.
The drop coating embodiment can be summarized in the following steps:
A more specific description will be provided with reference to the flowchart of
In step 10, the cycle begins with the top movable mold half opened vertically. As will be discussed in greater detail below, the mold inserts are heated to about 250 degrees F., which is below a resin solidification temperature and represented by T. In step 12, a drop of acrylate based coating solution is deposited onto the lower concave insert. The coating is applied as an unpressurized coating drop from a coating dispenser. For example, the full metered charge of coating, sufficient to cover the entire surface of the optical coating carrier at the desired thickness, is applied as one drop.
Directly thereafter, within 2-10 seconds, an inflatable bladder is placed on to the coating drop, in step 14. The bladder is heat resistant and configured and dimensioned with a smooth exterior to press evenly against the entire insert surface. The outer surface of the bladder must be uniformly smooth at the intended inflation level, thereby spreading the coating into a uniformly thin layer. The bladder may be pre-inflated or its base may be brought into position and then inflated, on top of the coating drop. In step 16, the coating begins a pre-cure phase. Pre-cure continues until the bladder can be withdrawn without disturbing the thin coating layer.
In step 18, the molten resin is injected into the cavity via an edge gate. The coating must be sufficiently per-cured so that it can maintain its shape during injection. If the resin is under-cured it can be flushed away from the gate. The edge gate may be modified to insure the resin in injected in a manner to minimize disturbing the coating. Also it is imperative that the resin should not be injected underneath the coating. The resin is injected at very high pressure, in the order of thousands of pounds per square inch, thus having the effect of compressing downward on to the coating. As a result, the coating becomes intimately bonded to the resin as it sets into a final cured state. If the coating is over-cured, it will be hazy or may fail to adhere to the solidifying lens. The molten resin may enter the cavity at above 500 degrees F.
In step 20, the resin begins to cool to the mold solidification temperature T. During this phase, a packing pressure may be utilized for a span of time. Once the lens is sufficiently rigid to resist deformation, the mold is opened and the in-mold coated lens is ejected.
Before contacting the coating drop with the bladder, it is possible to introduce an ophthalmic wafer, carrier or film onto the coating itself. Photochromic carriers, wafer or films are well suited for use in the inventive process. Polarized wafers, carriers or films are also well suited because they tend to be very delicate, and the invention affords a high degree of scratch protection by sandwiching the polarized film in between a protective coating and a thermoplastic lens. The wafers may be on the order of 0.5 mm to 1.5 mm thick. After the coating cures it remains as a uniformly thin thermoset layer across the entire convex surface of the wafer.
Industrial Manufacturing Installation
Bottom mold half 40a, includes an exemplary three mold cavities, although it should be understood that thermoplastic lenses are usually molded in pairs. An empty cavity 42 is shown on the left, a cavity 44 with a coating drop applied thereon in the middle, and a coated cavity 46 in cross-section on the right. A screw-jack 46c, or similar height adjusting device supports a lens-forming insert 46a flush with a receiver 46b. Circulating channels 48 may be located within receiver 46b, or nearby in the mold block, which have a temperature control fluid circulated therethrough by a thermolator. A screw injector is also heated to melt the resin and deliver it to the cavities via a runner 50, there being a runner provided for each cavity. In the case of polycarbonate, the screw injector may have a heating range of 500 to 600 degrees F. The thermolator may heat receiver 46b to a range of 200 to 300 degrees F. Similar temperature control lines may be located within movable mold half 40b.
A process controller is coupled to clamping unit 40c, resin injector, thermolator and other components to coordinate the molding of lenses as is known in the art. According to an embodiment of the invention we further provide a coating applicator 60 which is coupled to the controller. More specifically, coating applicator 60 is mounted adjacent the mold halves for pivoting, sliding or other reciprocating motion to position an applicator head 60a and a spreader 70 sequentially over the mold cavities.
In
According to the second cliché embodiment, the method may be summarized according to the following steps:
As can be seen in
In step 104 the cliché is withdrawn to reveal the mold cavity directly underneath. The bladder can then repeat its downward advancement directly onto the mold insert. According to this embodiment, we align the center points of the mold insert, the cliché and the bladder. This can be accomplished in a single alignment step.
As an alternative to step 104, the bladder arm is articulated away from the cliché in step 106. The bladder carrying the coating solution can then be pressed onto the mold insert. According to step 106, the mold insert may be oriented at any angle since the coating is able to be carried in a thin layer via the bladder. In this step 106, the bladder may require a first alignment step to properly orient it with respect to the cliché, followed by a second alignment step to properly orient it with respect to the mold insert. Alignment will be discussed in greater detail below.
After either step 104 or 106, the bladder is placed into contact with the heated mold insert, per step 108. A characteristic of the coating solution used herein, is that it will remain stable and liquid at room temperature. It will have a viscosity and flowability adjusted to complement the wetability of the bladder material, for example, silicone. The coating should adhere to the bladder in an even layer without running or dripping.
Once the bladder is pressed against the heated insert, we employ a pause step 110, to allow the coating to pre-cure. During the pre-cure the coating will begin to set and take on some characteristics of a solid. The bladder can be withdrawn, or deflated, or some combination of both to allow the bladder to peel off the coating, without disturbing the coatings setup onto the mold insert surface. Per step 112, the bladder and cliché are removed from the mold. For all embodiments, the bladder may be inflated to a pressure within the range of 5 psi to 20 psi. For example, the bladder may be inflated to a pressure within the range of 8 psi to 10 psi.
Having achieved the necessary clearance, the mold is closed and the resin is injected in step 114 via an edge gate. The edge gate may be modified to insure the resin in injected in a manner to minimize disturbing the coating. For example, it is imperative that the resin should not be injected underneath the coating. The resin is injected at very high pressure, in the order of thousands of pounds per square inch, thus having the effect of compressing downward on to the coating. As a result, the coating becomes intimately bonded to the resin as it sets into a final cured state. The molten resin may enter the cavity at above 500 degrees F.
In step 116, the resin begins to cool to the mold solidification temperature T. During this phase, a packing pressure may be utilized for a span of time. Once the lens is sufficiently rigid to resist deformation, the mold is opened and the in-mold coated lens is ejected.
Industrial Manufacturing Installation
Bladder 70b is then dropped into cliché 80a via vertical controller 70a. As can be seen in
As can be seen in
An alternate alignment system can be most readily seen in
An alternate coating system can be seen in
It should be understood, that one arm and cliché may be provided for each mold cavity. For molding 2-pair, 3-pair or more pairs of lenses simultaneously, the arm/tray assembly may be reproduced to provide one assembly per mold cavity. The assemblies may be supported on a single beam that is extended into the mold for 4, 6 or more coating operations to occur simultaneously. The beam may have a square, rectangular, diamond or other polygonal shape to closely match the layout of the various mold cavities. The beam may have a spider shape with a single source of coating solution at the central head point, with feeder lines extending out each of the legs to replenish the clichés as needed.
The substrate that could be used in this method could be any injection moldable lens material like polymethyl(meth)acrylate (PMMA), polycarbonate, polycarbonate/polyester blends, polyamide, polyester, cyclic olefin copolymers and polyurethane, polysulfone and combinations thereof. In practice, polycarbonate derivatives have worked well with the coating solution and techniques according to the invention.
The injection cycle is as usual and depends from the nature of the thermoplastic. Usually the mold temperature is comprised from 240° F. to 290° F., the melt temperature is comprised from 540° F. to 600° F., the packing pressure is comprised from 5,000 psi to 15,000 psi, the packing time is comprised from 10 sec to 50 sec, and the cooling time is comprised from 60 sec to 265 sec.
A metal plate which has a circular recess about 50 μm deep was positioned horizontally on the parting surface of the mold. An amount of liquid coating, 0.5 ml, was deposited onto the circular recess, so-called cliché.
An air-inflated silicone membrane, driven by a pneumatic cylinder, moved downward to pick up the coating from the cliché and then moved back. After the cliché was removed from the mold parting surface, silicone membrane then moved downward again to press against the heated mold insert and held there to let the coating pre-cure for 2 minutes. After that, silicone membrane was removed and a partially cured thin layer of coating was transferred to the metal insert. No coating residual was taken away by the membrane.
The mold was closed for the PC lens molding. PC lens molding conditions are the same as the regular PC molding conditions and consisted of mold temperature set at 250° F., melt temperature ranging from 535 degrees F. to 565 degrees F., packing pressure set at 6,150 psi for 12 seconds and cooling down for 60 seconds. Coating was further cured by the heat from the hot PC melt. At the end of the molding cycle, the mold opened and the coated lens, which is optically clear, was ejected out of the mold.
Following is the coating composition that was used in the example.
A coating according to the present invention advantageously provides and/or includes at least the following characteristics:
A coating according to the present invention is thermally curable, optically clear, does not show visible interference fringes after coating onto a lens and comprises an optically transparent coating that is compatible with the lens material in order to adhere to it without causing any undesirable effects while imparting the desired features (tint, scratch resistance, etc.) onto the lens material.
A coating composition according to the present invention is preferably solvent less and includes an acrylate compound. The acrylate compound is preferably thermally cured, which means the coating may be cured via, e.g., azo, peroxides, and/or blocked tertiary amine. Chemically speaking, the coating composition preferably includes multi-functional acrylates comprising up to hexa functional groups and with various molecular weights. Preferably, the present invention comprises a multi-functional urethane acrylic coating that is modified to meet various competing requirements. For example, such coating needs to stay in liquid form to flow along a hot mold insert to an even thickness and then polymerize rather quickly, since the lens molding process is being extended by the coating set time. Indeed, a coating used in the present invention advantageously remains in liquid form to flow along a heated mold insert to a uniform thickness and then polymerizes quickly.
More particularly, a coating composition according to the present invention preferably comprises acrylates including monofunctional acrylates and/or monofunctional methacrylates such as isobornyl acrylate and hydroxylpropyl methacrylate, as well as tetrafunctional acrylates and/or tetrafunctional methacrylates and hexafunctional acrylates and/or hexafunctional methacrylates. Exemplary acrylates that may be used in the present invention may include and are not limited to reactive multifunctional acrylates, preferably hexafunctional aliphatic urethane acrylates. For example, exemplary acrylates used in the present invention may include hexafunctional acrylates and at least one difunctional acrylate. As noted herein, the term “(meth)acrylate” refers to either the corresponding acrylate or methacrylate.
Acrylates may be obtained from UCB Chemicals or from Sartomer and Henkel (a German Co.), and may in one embodiment comprise, e.g., Ebecryl™ brand acrylates. A brief general description of various Ebecryl acrylates in EB number formats which may be used according to the present invention is as follows:
Use of hydroxylpropyl methacrylate presents a particular interest to slow down the reaction in the coating composition. Multi-functional acrylates of three functional groups or higher advantageously will provide more cross linking and result in higher abrasion resistance. For example, hexa-functional acrylates will provide a high degree of cross linking due to having six (6) functional groups. The urethane backbone of these high functional acrylates provides flexibility and greater ability to resist heat. Difunctional acrylate species are used to increase the flexibility and toughness and to control the viscosity of the formulation for process-ability to a certain extent.
A monofunctional methacrylate, such as hydroxylpropyl methacrylate, serves as a monofunctional diluent and kinetic modifier. It is used to terminate the reaction or to slow down the propagation of polymerization so that it will have some stability and a window of reactivity for processing. Monofunctional methacrylates used in a composition according to the present invention serve as reactive diluents and kinetic modifiers to improve flow characteristics.
With regards to the term acrylates, it is to be noted that methacrylates and other unsaturated compounds, whether mono- or multifunctional may also be used in addition to, or instead of, acrylates. In some cases methacrylates may experience a slower chemical reaction during polymerization. Acrylate or methacrylate compounds may be selected from the family of aliphatic urethane acrylates which include, e.g., from two to about six functional groups.
In a preferred embodiment of the present invention, high molecular weight acrylates (for example, acrylates having a molecular weight of at least 1000 centipoises (cps) or higher at 25° C.) are preferably used for ophthalmic injection molding according to the present invention. This embodiment presents the advantage of improved control of the viscosity and flow of the coating composition on a heated surface. For example, a high injection pressure requires a high viscosity flow to allow for the higher temperature (i.e., higher than room temperature) during applied extrusion. It is to be noted that the viscosity may further be adjusted as necessary based on the particular injection molding parameters and requirements.
In one embodiment of the present invention, the coating composition preferably comprises an acrylic base cured with an initiator (e.g., t-butyl perbenzoate). In fact, the thermal cure process of the present invention utilizes free radical polymerization. The initiator (t-butyl perbenzoate) obtains energy by absorbing heat to decompose and generate free radicals (that is, the free radical reaction is generated by thermal heating). These free radicals then attach monomers or oligomers (reactive multifunctional acrylates) to generate more free radicals to propagate the reaction to form long molecular chains and eventually a cross-linked network.
An in-mold coating composition according to the present invention preferably may further include at least one catalyst (initiator) and at least one metal salt. The catalyst may be selected from, e.g., alkyl aralkyl peracide, azo derivatives and blocked tertiary amine, is preferably selected from ketone peroxides, diacyl peroxides, dialkylperoxides, diperoxyketals and peroxyesters, and in a very preferred embodiment comprises tert-butylperbenzoate.
The examples disclosed herein preferably use peroxides derived from alkyl aralkyl peracide with a metal salt promoter. Peroxides are used to cure the coating via a free radical reaction. Metal salt promoters help to generate free radicals quickly and minimize oxygen inhibition. The metal salt and peroxide concentration are preferably chosen to fit a curing cycle for the current process. The concentration ratio can be varied as necessary to fit a particular process requirement. Again, although use of peroxides for curing is a preferred method, and more specifically tert-butyl perbenzoate is a preferred candidate, alternative methods for curing may include use of azo and blocked tertiary amine.
The metal salt is preferentially selected from cobalt naphthenate, cobalt octoate, cobalt neodecanoate, copper naphthenate, zinc naphthenate, and potassium octoate, and preferably, the metal salt comprises cobalt naphthenate.
In one embodiment, an exemplary coating composition according to the present invention comprises the following: (a) at least one hexafunctional acrylate and/or hexafunctional methacrylate compound; (b) at least one difunctional acrylate and/or a difunctional methacrylate compound; (c) Hydroxyl propylmethacrylate; (d) Isobornyl acrylate; (e) T-butyl perbenzoate; and (f) Cobalt naphthenate.
An in-mold coating composition according the invention may optionally further include a surfactant which is preferably selected from a fluorinated surfactant or a silicone surfactant. That is, a surfactant such as a fluorinated surfactant (e.g., EFKA 3034) or a silicone surfactant (e.g., Silwet L-7602) may be included in a coating composition according to the present invention. The surfactant in the coating composition may be added to improve wetability of the mold surface.
The coating composition may also optionally include acrylic or epoxy functionalized colloids, for example, OG-101 or OG-103 (available from Clariant), or functionalized colloidal silica with acrylic silanes, or other colloids such as, e.g., cerium colloid, niobium colloid, and antimony colloid.
An in-mold coating composition according to the present invention may further optionally include, e.g., a metal alkoxide which may be selected, for example, from zirconium isopropoxydes, methyl trimethoxysilane and tetraethoxysilane.
A coating composition according to the present invention may further optionally include at least one dichroic dye, a photochromic dye and/or one liquid crystal.
It is to be understood by one of ordinary skill in the art that the coating should preferably retain its qualities at the lens substrate molding temperature, e.g., for a polycarbonate substrate, such temperature is around 250° F.
Upon coating of an optical lens, a coating according to the present invention is optically clear and may have a thickness ranging from about 1 micron to about 100 microns. For example, typical abrasion resistance coating thickness ranges from about 1 micron to about 8 microns, and a photochromic system can be up to about 20 microns or more.
Advantageously, an in-mold coating composition according to the present invention provides very good anti-abrasion properties. To further increase abrasion resistance, it is also possible to include in the coating formulation according to the present invention acrylic or epoxy functionalized colloids, as discussed above. Metal alkoxides and its derivatives may also optionally be added as discussed above to increase refractive index, abrasion resistance and perhaps influence the rate of polymerization.
According to one embodiment, a coating composition according to the present invention comprises the following:
A liquid coating drop, 0.5 ml was deposited in the center of the concave surface of a 6 base metal insert. The insert was enclosed in the bottom mold half and had been constantly heated as the rest of the mold of which the temperature was maintained at 250 degrees F. An auto dispenser or a scaled pipette was used to deposit coating.
An air-inflated silicone membrane moved downward to press the coating drop and spread it out on the insert concave surface. Coating formulations are the same as in Example 1. The silicone membrane was held at that position for 2 minutes to let the coating thermally cured by the heat from the hot insert. When the membrane was retracted, a thin layer of coating was formed on the insert surface. The mold was then closed and plastic melt was injected into cavity. Coating was further cured by the heat from the melt. Lens molding parameters are the same as those in Example 1. At end of the lens molding cycle, the optical clear coated lens was ejected out of the cavity.
In certain tests, a 2 mm thick, 6 base lens was injected into a cavity having one insert stamp coated. A clamp tonnage of 150 tons was used during injection. The coating thickness was 28 μm thick.
Having described preferred embodiments for lens manufacturing, materials used therein for coatings and methods for processing same (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.
