GB2050928A - Method and apparatus for molding ophthalmic lenses without spoilation - Google Patents

Abstract

A method and apparatus particularly for molding ophthalmic lenses in which one of a pair of mold inserts 110a, 122 received in respective mold members 82, 84 to form a mold cavity 90, 92 is resiliently biased toward the other and is positively limited from separating beyond a predetermined extent. After closing the mold members, hot viscous polycarbonate is injected into the mold cavity at a pressure sufficient to overcome the resilient biasing and separate the inserts the predetermined extent. After injection, the resilient biassing compresses the injected resin to preserve the surface curvature as it cools to the glass transition temperature of the resin being employed, while the surfaces of the mold cavity, are maintained at a temperature just below the glass transition temperature of the injected resin to ensure uniform cooling. In a preferred embodiment, resin is injected into a multicavity mold having differently curved insert pairs in each cavity to mold simultaneously lenses of different commonly used foci in the near piano range. A knockout assembly for expelling the molded article from the mold cavity includes ejecting movement of an insert. <IMAGE>

Description

SPECIFICATION Method and apparatus for simultaneously molding ophthalmic lenses of different foci Background of the invention This invention relates to a method and apparatus for molding articles, and in particular, to a method and apparatus for simultaneously molding ophthalmic lenses of different foci.

Various methods and apparatus for molding ophthalmic lenses are known in the prior art. For example, Johnson U.S. Patent 2,443,826 discloses a lens molding method and apparatus in which resin is injected into one or more mold cavities at a pressure sufficient to overcome the force of biasing springs and separate the mold inserts by a predetermined extent determined by stops or the like. After injection, the springs urge the inserts together to compress the injected resin and maintain contact between the insert surfaces and those of the article being molded. Because Johnson does not disclose operating the mold cavities at any particulartemper- ature, however, the performance of his apparatus is uncertain.

Weber U.S. Patent 4,008,031 discloses a molding method and apparatus in which molten thermoplastic resin is first injected through a gate into a heated mold cavity with the hydraulic line used to compress the injected material unactuated so that the mold inserts exert little or no pressure on the injected material. After a delay, the hydraulic line is enabled to compress the resin within the cavity, forcing some of the resin into overflow pockets located generally diametrically opposite the gate. According to the patentee, the final volume of the mold cavity, and hence the thickness of the molded lens, is controlled by varying both the time delay between injection and hydraulic actuation and the duration of hydraulic actuation.While controlling these variables theoretically permits of adjustment of the lens thickness, as a practical matter there is no assurance that the resulting lens will have any particular thickness. Variations in thickness from lens to lens are obviously highly undesirable, since they result in differences in the power of the lenses. Further, to minimize the formation of surface irregularities during cooling, the inserts should be under pressure continuously from injection until final solidification.

Using pressure as a control variable to regulate thickness inevitably results in some sacrifice of surface regularity. While the patentee suggests limit stops as an alternative expedient for controlling final lens thickness, such stops will of course prevent the inserts from exerting pressure on the plastic resin beyond a certain shrinkage point.

In ourcopending application Serial No. 932,442, filed August 1978, we describe a method of molding ophthalmic lenses that is especially suited forthe manufacture of high minus prescription lenses. According to that method, a charge of synthetic resin approximating the mass of the finished lens is injected under relatively low pressure into a space larger than that of the finished lens between relatively movable inserts. After injection the inserts are moved together to press the mass of synthetic resin into the desired shape. The mold and the inserts draw heat away from the molten material, causing it to cool, shrink and ultimately solidify when it reaches the "glass transition temperature".

At the same time, the inserts continuously apply pressure to eliminate any surface irregularities due to uneven cooling.

While this method is capable of producing precision lenses of any power, it does not readily lend itself to the economic production of lenses in the near plane range of powers between about +2.0 diopters and -2.0 diopters. The necessity of introducing a precisely measured charge of resin into the mold cavity dictates a single-cavity mode of operation, as it would be extremely difficult, using a single injection nozzle, to control accurately the amount of resin injected into a particular cavity of a multicavity mold.Although single-cavity operation is commercially practical for manufacturing high minus lenses, owing to the relative expense or unsuitability of alternative methods and materials, the wider availability of substitute materials and processes in the near plano range makes single-cavity operation, with its high unit labor costs, economically unfeasible.

Summary of the invention One of the objects of our invention is to provide a method and apparatus for molding articles which are capable of forming such articles with optically precise surfaces.

Another object of our invention is to provide a method and apparatus for molding articles which are especially suited for the commercially feasible production of near piano lenses.

Still another object of our invention is to provide a method and apparatus for molding articles which are adaptable to multicavity operation.

A further object of our invention is to provide a method and apparatus for molding articles which do not require high injection pressures.

Still another object of our invention is to provide a method and apparatus for molding lenses of precisely determined thickness.

Still another object of our invention is to provide a method and apparatus for molding articles which produce lenses of good ophthalmic and cosmetic quality.

A further object of our invention is to provide a method and apparatus which are capable of simultaneously molding ophthalmic lenses of different foci.

Still another object of our invention is to provide a method and apparatus which are capable of simultaneously molding single vision corrective lenses having spherical power, cylindrical power, or both.

Another object of our invention is to provide a method and apparatus which are capable of molding bifocal magnifying lenses in which the top sections have plano optics while the bottom sections have spherical power additions to +3.00 diopters.

Other and further objects of our invention will be apparent from the following description.

In general, our invention contemplates a method and apparatus for molding ophthalmic lenses or other articles in which respective inserts having facing surfaces complementary to opposed surfaces of the finished article are mounted in first and second mold members relatively movable between an open position and a closed position for relative movement of the inserts toward and away from each other with the mold members closed to form a cavity. The mold inserts are resiliently biased to move toward each other and are positively limited by stops or the like from separating -beyond a predetermined extent. With the mold members closed hot viscous synthetic resin is injected into the mold cavity at an injection pressure sufficient to overcome the resilient biasing and separate the mold members by the predetermined limited extent.

As the injected resin cools from a plasticto a glass state and shrinks in volume, the resiliently biased lens inserts maintain compressive contact with the opposed surfaces of the article being molded to prevent surface irregularities from forming during cooling, while the surfaces of the mold cavity are maintained at a temperature just below the glass transition temperature of the resin to control the rate of cooling of the various parts of the article being molded.

Since the amount of resin injected into the mold cavity, and hence the thickness of the finished lens, is determined by the insert limit stops rather than by injection pressure, our method is readily practiced in a multicavity mold supplied with resin from a single injection nozzle. The constant pressure of the resilient biasing means on the mold inserts further assures that all parts of the mold cavity, including constricted portions, are completely filled during injection. Finally, since the optimum cavity temperature and insert pressure vary only slightly in the near plano range, our method is readily capable of being used to mold simultaneously, in a single multicavity mold, lenses of different foci.

Brief description of the drawings In the accompanying drawings to which reference is made in the instant specification and in which like reference characters are used to indicate like parts in the various views; Figure 1 is a section of one form of molding apparatus illustrating the practice of our invention in a single mold cavity.

Figure 2 is a bottom plan of the upper mold member of multicavity molding apparatus used according to our invention to mold simultaneously ophthalmic lenses of different foci.

Figure 3 is an enlarged section of the apparatus shown in part in Figure 2, taken along line 3-3.

Figure 4 is a flowchart showing the steps performed in practicing our molding method.

Detailed description of the preferred embodiments Referring now to Figure 1, one embodiment of our invention, indicated generally by the reference numeral 10, includes a first mold half 12 and a second mold half 14 relative movable toward each other to meet along a mold parting line a-a. Mold half 12 comprises an outer plate 16 and an inner plate 18, the inner surface of which is flush with the mold parting line a-a in the position illustrated in Figure 1.

A recess in plate 18 carries an annular liner 20 which receives a fixed lens insert 22 having a concave working surface complementary to the convex surface of the lens being formed. A bolt 24 or the like carried by the outer plate 16 secures the lens insert 22 within the liner 20 of the first mold half 12. An annular heating channel 26 formed in inner plate 18 and coaxial with insert 22 receives hot oil or the like from any suitable source, such as shown in our copending application Serial No. 932,442, to maintain the working surface of the insert 22 at a suitable temperature, as further described below.

The second mold half 14 comprises a centre plate 32 sandwiched between an inner plate 34 and a mold base 30. A bore in inner plate 34 carries an annular liner 36 which receives a movable insert 38 having a convex working surface complementary to the concave surface of the lens being molded. We mount the movable lens insert 38 on the end of an ejector 40 rod movable axially with respect to a cylindrical compression sleeve 46. Sleeve 46 is movable axially relative to the center plate 32. A first bushing 42 guides the compression sleeve 46 relative to the center plate 32, while a second bushing 44 guides the ejector rod relative to the compresson sleeve 46.

A plurality of disc springs 52 disposed around ejector rod 40 between the compression sleeve 46 and the mold base 30 urge the compression sleeve 44 and hence the lens insert 38 to the left as viewed in Figure 1 to a limit position determined by an adjustment ring 60 threadably received by compression sleeve 46. We dispose respective washers 48 and 50 between the disc springs 52 and the compression sleeve 46 and between the springs 52 and the mold base 30. A cylindrical bushing 54 locates the ejector rod 40 relative to the mold base 30. A plurality of disc springs 56 stacked in series between the mold base 30 and the head 58 of the ejector rod 40 bias the ejector rod 40 to the right as seen in Figure 1 away from the first mold half 12.

From the foregoing description, it will be seen that mold insert 38 is movable in either axial direction away from the home position shown in solid lines in Figure 1. Whenever insert 38 is moved to the left out of the receiver 36, as occurs when ejecting the molded article, springs 56 urge the insert into the receiver with a compression force of about 26 psi.

Whenever, on the other hand, insert 38 is moved to the right from the solid-line position further into the receiver 36, springs 52 become active to urge the insert outwardly from the receiver with a net compression force of about 760 psi, taking into account springs 56. The linear displacement between the solid-line "home" position and the dotted-line compression position need only be somewhat greater than that necessary to accommodate material shrinkage as the resin cools from its initial injection temperature to its final solidification temperature.

For a typical polycarbonate lens and an injection temperature of 590"F, this shrinkage is about 0.025 inch.

We form inner plate 34 with a heating channel 62 disposed coaxially relative to the mold insert 38 and supplied with hot oil or the like from any suitable source, such as the source previously referred to, to heat the mold half 14 to a desired temperature.

Preferably one or more thermocouples (not shown) are disposed at suitable points in mold halves 12 and 14to monitor the mold temperature and control the temperature of the hot oil supplied to channels 26 and 62.

We form mold halves 12 and 14 with mating recessed portions 64 and 66 for receiving a nozzle 72 through which wffinject hot synthetic resin such as polycarbonate into the mold cavity 28 defined by inserts 22 and 38 and receivers 20 and 36. Recessed surface portions 68 and 70 formed in the mold halves 12 and 14 define a channel or runner communicating between the nozzle 72 and the mold cavity 28.

In operation, mold halves 12 and14 are moved together along mold parting line a-a and the nozzle 72 inserted into the recess defined by portions 64 and 66. Initially, mold insert 38 is urged by springs 52 in the direction of insert 22 to a limit position determined by adjustment ring 60, as shown in full lines in Figure 1. Preferably, springs 52 exert a biasing force of about 785 psi. Next, synthetic resin is injected into the mold cavity from nozzle 72 in a hot and viscous state, preferably at a temperature of about 590 F in the case of polycarbonate resin, at a sufficient pressure of, for example, 1000 psi, to urge lens insert 38 to the position shown in broken line in Figure 1 in which the lens insert 38 abuts the surface of sleeve 46.

Following the injection operation described hereinabove the resin in the mold cavity is allowed to cool to the temperature of the mold surfaces. In the case of polycarbonate resin, the surface of mold halves 12 and 14 are preferably maintained at a temperature of about 280 F. As the resin cools and decreases in volume, springs 52 maintain the resin under a net compressive force of about 760 psi to maintain conformity between the lens surfaces and the surfaces of the inserts 22 and 38. When a sufficient period has elapsed to permit the lens to cool to the glass transmission temperature of about 305 F, the mold halves 12 and 14 are separated to remove the mold lens. In the case of polycarbonate resin in the embodiment shown, a cooling time of about 90 second is sufficient.As the mold halves 12 and 14 are moved away from the mold parting line a-a, the head 58 of ejector rod 40 strikes a stationary knockout plate 74, pushing mold insert 38 out of the liner 36 against the pressure of springs 56 to eject the mold lens from the mold half 14.

It can be seen that if concave insert 28 and convex insert 38 are each provided with lens-forming surfaces that are sections of spheres, the resin lens molded therefrom will exhibit spherical refractive power. If insert 38 is surfaced in the configuration of a torus while insert 28 remains as a spherical section, the lens produced therefrom will have spherocylindrical refractive characteristics and, in this case, would be a negative toric.To further demonstrate the adaptability of this process, if insert 38 is finished with a polished spherical section while insert 28 is made from two spherical sections, the upper having a radius calculated to produce a plano lens and the lower having a shorter radius calculated to produce a spherical or magnifying lens, the lens so molded will be a bifocal type without corrective optics (plano) in the upper, or distance, portion but with magnifying power addition in the lower, or near-viewing, portion. such lenses are useful as magnifying aids.

Referring now to Figures 2 and 3, we show an alternative embodiment illustrating the practice of our invention in a multicavity mold. More particularly, a mold indicated generally by the reference numeral 80 comprises an upper mold half 82 and a lower mold half 84 facing each other along a mold parting line b-b. Mold halves 82 and 84 define six mold cavities 86, 88, 90, 92, 94 and 96 arranged, for example, in a two-by-three array. A nozzle 102 ejects a suitable thermoplastic resin such as polycarbonate resin into the mold cavities simultaneously through a vertical channel 100 communicating between the upper side of mold half 82 and a horizontal runner system 98 formed in the lower surface of mold half 82 communicating with each of the mold cavities 86-96.Since each of the mold cavities is identical except in the respect described below, the structure and operation of the mold 80 will be described in detail with reference to mold cavity 90.

Upper mold half 82 comprises an upper plate 104 and a lower plate 106. Lower plate 106 holds an annular line 108 which receives the fixed lens insert 110. A bolt 112 carried by plate 104 secures the lens insert 110 inside the liner 108. Hot oil from a suitable source (not shown) is circulated through an annular heating channel 114 formed in lower mold plate 106 coaxiallywith lens insert 110 to maintain the mold half 82 at a suitable temperature slightly below the glass transition temperature of the material of which the lens is being molded.

Lower mold half 84 comprises an upper plate 118 backed by a lower plate 116. Upper plate 118 carries a liner 120 which receives a movable lens insert 122.

A bolt 124 releasably secures lens insert 122 to a travelling insert rod 126. A bushing 136 carried by lower plate 116 receives sleeve 126 for axially sliding movement. A plurality of disc springs 134 stacked in parallel fashion in a recess formed in the upper surface of plate 116 bias insert 122 upwardly to a limit position determined by the point at which the head of the travelling rod 126 abuts the lower surface of plate 116. A washer 132 provides a bearing surface for the lowermost of the springs 134, while a washer 130 provides a bearing surface for the uppermost of the springs 134. We dispose one or more spacers 128 between rod 126 and insert 122. A heating channel 138 extending circumferentially around lens insert 122 receives hot oil from a suitable source (not shown) to maintain the surfaces of mold cavity 90 at a suitable temperature just below the glass transition temperature of the lens material.

The adjacent mold cavity 92 need not be identical to cavity 90, but may contain a pair of inserts 110a and 122a shaped to mold a lens of different focal length than that of the lens being molded in cavity 90. Each of the cavities 86, 88, 90, 92, 94, 96 may be used to mold a different power lens in accordance with a desired distribution, since lenses of approximately equal foci have nearly identical optimum molding conditions and resin composition. For each cavity, the volume of the molded lens is determined by the relative separation of inserts 110 and 122 when the insert 122 is in its fully compressed position shown in broken lines in Figure 3.This limit position may be adjusted in individual cavities by any suitable means, such as by using different numbers of spacers 128, spacers of different thicknesses or by providing additional stops (not shown) between spacers 128 and plate 116.

The operation of mold 80 is similar to that of mold 10. With mold halves 82 and 84 moved together, thermoplastic resin is injected in a fluid state into the mold cavities such as cavity 90 from injection nozzle 102 at a sufficient pressure to fill the cavities and to move mold inserts including insert 122 to the broken line position shown in Figure 3. The injected resin is then allowed to cool to its glass transition temperature at a rate determined by the controlled cavity temperature. As the resin shrinks during cooling, the compressive force of the inserts including insert 122 continuously reforms the resin to eliminate any surface irregularities due to uneven cooling. When the entire resin has cooled to the glass transition temperature, its shape is set and the article may be removed from the mold 80.

Experiments conducted with the mold 80 have shown that the quality of the finished lens is critically dependent on cavity temperature. Thus, in the case of polycarbonate resin, at mold temperature between 200 F and 240 F, the cooling of the outer lens portions was so rapid that the inserts were unable to reshape the resin as it shrank. As a result, the lenses produced resembled those produced by fixed-insert molds, exhibiting the usual "picture framing" and "splay" marks emanating from the runner gate.At a mold temperature of 2400F the splay marks dis appeared and circular compression patterns appeared on the lens, indicating that the springs 134 were beginning to function. "Picture framing", which was still evident at 240 F, disappeared when the mold temperature was raised to 2800F. Raising the temperature beyond this point did not improve mold performance, but rather results in wavy distor tion marks.

A cycle time of 90 seconds was found to be adequate to ensure that the lens had completely cooled to the glass transition temperature of 305 F of polycarbonate resin. Preferably, to avoid "blowing" the mold 80 and thereby producing such undesirable effects as flash, the injection pressure should be as low as possible while still being sufficient to over come the spring biasing force. While the thermo plastic resin is preferably injected vertically as shown in Figures 1 and 3, it will be apparent to those skilled in the art that each of the molds 10 and 80 could be operated in any appropriate orientation.

It will be seen that we have accomplished the objects of our invention. Our molding method and apparatus are capable of forming lenses with optic ally precise, cosmetically acceptance surfaces and are especially suited for the production of lenses in the near plano range, including toric and bifocal lenses. Our method and apparatus are adaptable to multicavity operation and do not require high injection pressures. Finally, our method and apparatus are capable being used to mold simultaneously ophthalmic lenses of different foci.

It will be understood that certain features and subcornbinations are of utility and may be employed without reference to other features and subcombina- tions. This is contemplated by and is within the scope of our claims. It is further obvious that various changes may be made in details within the scope of our claims without departing from the spirit or our invention. It is, therefore, to be understood that our invention is not to be limited to the specific details shown and described.

Claims (16)

1. Apparatus for molding an article from synthetic resin including in combination a pair of separable mold members, a pair of relatively movable mold inserts received in said respective mold members to form a mold cavity therewith, means for resiliently biasing said mold inserts toward each other, means for positively limiting the separation of said inserts beyond a predetermined extent, means for injecting hot viscous synthetic resin into said mold cavity at an injection pressure sufficient to overcome said biasing means and separate said inserts said predetermined extent, said biasing means acting on said inserts after injection to compress the injected mass of resin as it cools to the glass transition temperature of said resin, and means for maintaining the surfaces of said mold cavity at a temperature slightly below the glass transition temperature of said synthetic resin to control the cooling thereof.

2. Apparatus as in Claim 1 in which said resin is a polycarbonate resin.

3. Apparatus as in Claim 2 in which said temperature means maintains said surfaces at a tempera ture of about 280 F.

4. Apparatus as in Claim 2 or 3 in which said resin is injected into said cavity at a temperature of about 590 F.

5. Apparatus as in Claim 4 in which said biasing means exert a force on said insert surfaces of about 760 psi.

6. Apparatus as in Claim 2 or 3 in which said biasing means exert a force on said insert surfaces of about 760 psi.

7. Apparatus for simultaneously molding a plu rality of articles from synthetic resin including in combination a pair of separable mold members, a plurality of pairs of relatively movable mold inserts received in said mold members to form respective mold cavities therewith, means for resiliently biasing each of said pairs of mold inserts toward each other, means for positively limiting the separating of each of said pairs of inserts beyond a predetermined extent, means for injecting hot viscous synthetic resin into each of said cavities at an injection pressure sufficient to overcome said biasing means and separate each of said pairs of inserts said predetermined extent, each of said biasing means acting on the associated pair of inserts after injection to compress the injected mass of resin as it cools to the glass transition temperature of said resin, and means for maintaining the surfaces of each of said mold cavities at a temperature slightly below the glass transition temperature of said synthetic resin to control the cooling thereof.

8. A method of forming an article of synthetic resin in a mold having a pair of mold members relatively movable between an open position and a closed position and provided with respective inserts movable relative to each other with said members in said closed position to form a mold cavity, said inserts being resiliently biased toward each other and being positively limited from separating beyond a predetermined extent, said method including the steps of closing said mold members, injecting a mass of hot viscous synthetic resin into said mold cavity at an injection pressure sufficient to overcome said resilient biasing and separate said inserts said predetermined extent, and maintaining the surfaces of said mold cavity at a temperature just below the glass transition temperature of said synthetic resin as the resin cools to said temperature to control the cooling of said mass and to allow said resiliently biased inserts to compress and reform said mass as it cools.

9. A method of simultaneously forming a plurality of articles of synthetic resin in a mold having a pair of mold members relatively movable between an open position and a closed position and provided with a plurality of pairs of respective inserts movable relative to each other with said members in said closed position to form respective mold cavities, each of said pairs of inserts being resliently biased toward each other and being positively limited from separating beyond a predetermined extent, said method including the steps of closing said mold members, injecting a mass of hot viscous synthetic resin into each of said mold cavities at an injection pressure sufficient to overcome the resilient biasing acting on each of said pairs of inserts and separate each of said pairs of inserts said predetermined extent, and maintaining the surfaces of each of said mold cavities at a temperature just below the glass transition temperature of said synthetic resin as the resin cools to said temperature to control the cooling of each of said masses and to allow each of said resiliently biased pairs of inserts to compress and reform the mass in the respective mold cavity as it cools.

10. A method as in Claim 9 in which said articles are ophthalmic lenses, each of said pairs of inserts having facing surfaces complementary to opposed surfaces of one of said lenses.

11. A method as in Claim 10 in which each of said lenses has a spherical power greater than -2.00 diopters.

12. A method as in Claim 10 in which each of said lenses has a spherical power between -2.00 and +2.00 diopters and a cylindrical power between 0 and +2.00 diopters.

13. A method as in Claim 10 in which each of said lenses has a first and a second viewing section, said first viewing section being optically piano and said second viewing section having a spherical power between near zero and +3.00 diopters.

14. A method as in Claim 10 in which said pairs of inserts are complementary to lenses of different foci.

15. Apparatus for molding an article from synthetic resin including in combination first and second mold members relatively movable between an open position and a closed position, respective first and second inserts having facing surfaces complementary to opposed surfaces of said article, means mounting the respective inserts in said mold members for relative movement toward and away from each other with said mold members closed to form a mold cavity, first means for resiliently biasing said second mold insert toward said first mold insert, means for positively limiting the retraction of said second insert beyond a predetermined extent, means for injecting hot viscous synthetic resin into said mold cavity at an injection pressure sufficient to overcome said biasing means and separate said inserts said predetermined extent, said biasing means acting on said inserts after injection to compress the injected mass of resin as it cools to the glass transition temperature of said resin, means for limiting the action of said first biasing means beyond a predetermined forward extent of said second insert, second means for resiliently biasing said second mold insert away from said first insert, and an abutment disposed on the side of said second mold member remote from said first mold member, said abutment and said second mold member being relatively movable toward each other to cause said abutment to push said insert forward of said predetermined forward extent to expel the molded article from said cavity.

16. Apparatus for molding an article from synthetic resin including in combination first and second mold members relatively movable between an open position and a closed position, respective first and second inserts having facing surfaces complementary to opposed surfaces of said article, means mounting the respective inserts in said mold members for relative movement toward and away from each other with said mold members closed to form a mold cavity, said second insert normally being at a home position in said second mold member and being movable from said home position outwardly from said member to an extended position, means for injecting hot viscous synthetic resin into said mold cavity to mold said article, means for resiliently biasing said second mold insert away from said first insert from said extended position to said home position, and an abutment disposed on the side of said second mold member remote from said first mold member, said abutment and said second mold member being relatively movable toward each other to cause said abutment to push said insert to said extended position to expel the molded article from said cavity.