CA1093769A - Method and apparatus for injection molding lenses - Google Patents

Abstract

A method and apparatus for injection molding thermoplastic lenses, such as prescription eyeglass lenses in a single injection molding operation into a finished single vision or multifocal blank which does not require additional molding or polishing. Movable optical inserts (dies) are forced apart by injected polycarbonate, then the inserts are urged together forcing a portion of the injected polycarbonate into a pocket.

Description

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The invention r~lates to the field of molding apparatuses and tech_ niques for lenses.
There have been several attempts) in the prior ~rt~ to mold ophthal-mic lenses. One such method and apparatus is shown in United States Le~ters Paten~ 2,473,588. In accordance wi~h the ~eachings of this patent, blanks are first prodused by cuttîng or grinding. Following the fabrication of the blanksJ the blanks are then compression molded to the desired optical shape.
In other prior art teachings, blanks are fo~med by injection molding tech-niques, and then ~hrough compression molding formed in~o thelr final shapea In one instance a protec~îve coa~ing is placed on ~he blanks, prior to com-pression molding to their inal shape. ~See United States Letters Patent 3,862,261, Columns 9 and 10).
One problem inhe~snt in injection molding a minus or negative lens in a single injec~ion molding opera~ion is that the plastic flo~ within the eavity defining the lens tends to firs~ flow ~o the thicker, ou~er portion of the cavity, and then finally Elow ~hrough the thinner eenter portion. At the seam where the flow from ~he outer portion of the cavity meets, a knit line of~en occurs making the resultan~ lens ~nusable) or requiring substantial polishing or compression molding. This problem is particularly açute for lenses which are thinner in their center, such as a concave lens used to cor-rect myopia (minus lenses). This conventional plastic flow, and the resultant knit line, will be discussed in more detail in con~unction wi~h Figures 3a through 3d, A simila~ kni~ line problem associated with plus lenses occurs.
Ano~he~ proble~ associa~ed wi~h the formation of a lens in a s~ngle in~ection molding operation is that during the curing o ghe plastic~ shrink-age occurs which results in an uneven and wavy ex~erior surface on ~he fi nished lens blank and bubbles and oth~r imperfec~ions in the interior of the le~s causing optical distor~ions and aberrations unacceptable for prescTiption lenses and înstrument lenses~
Ideallr? a lens should be produced in a single injection moldîng 33~76~

operation and as will be seen the presently invented method and apparatus provides for the injection molding o~ an optical lens which results in a finished lens blank. The finished blank may be thin (approximately one milli-meter3 at its center so as ~o result in an optically correct, light weight and aesthetically pleasing eyeglass lens which is shatter proof.
An apparatus and method is disclosed for producing a finished opti-cal blank which may be used for an eyeglass lens, or the like. The invented apparatus and process is particularly adaptable for lenses which are thinner in the center, such as a concave9 or double concave (minus or negative~ lens, including single vision, multifocal and cylindrical lenses.
According to the invention there is provided an apparatus for injec~ion molding a lens comprising:
a housing;
a pair of dies disposed within said housing and a cavity for receiving molten plastic therein;
said housing including means permitting the relative movement of said dies such that the volume Q~ said cavity may be varied;
injection molding means communicating with said cavity for injecting plastic into said cavi~y;
at least one transfer pocket for receiving plastic from said cavity, said ~ransfer pocket communicating with said cavity; and compression means coupled to at least one of said dies, for sequentially reducing the w lume of said cavity and for appl~ing pressure to said plastic in said cavity after said plastic is injected into said cavity to trans~er plastic from said cavity into said transfer pocket;
, ~ whereby molten plastic may be injected into said cavity, and then - the volume of said cavity may be decreased forcing molten plastic ~rom said cavity into said transfer pocket.
Also, according to the lnvention,there is provided a method for in~ection molding lenses, utilizing the inventive apparatus.
This apparatus and method eliminates~the knit line associated with conventional plastic flow, and furthermore compensates for the shrinkage associated with the curing of the blank. The resultant blank is an optically precise configuration. The process and apparatus is suitable for both plus and minus lenses.
It is an object of the present invention to provide an apparatus and ~: :
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method which allows a finished optical blank to be produced in a single injec-tion molding operation.
In the accompanying drawings:
Figure 1 is a perspective view of a finished blank produced in accor-da~ce with the present invention with a cutaway section used to illustra~e the ~hinner center of the blank.
Figure 2 illus~rates ~he finished blank of Figure 1 h~g on a hanger, which hanger is formed concurrently with the moldîng of the finished blank.
Figures 3a, 3b, 3e, and 3d illustrate conventional plastic flow, and ~he forma~ion of a knit line associated with such flow. These figures are used to discuss the prior ar~ problems assoeiated with ~he ~abrication of a finished blank in a single injection molding operation.
Figure 4 is a cross-sectional view of an injection molding apparatus built in accordance wi~h the present invention.
~igure 5 is a plan view illustr~ting the cavity defined by the dies or optîcal inserts of the apparatus oX Figure 4 taken through sec~ion line 5 -5 of Figure 4, Figure 6 is an additional cross-seo~ional ~iew o~ ~he apparatus of Figure 4 ~aken ~hrough ~he angled sec~ion line 6 - 6 of Figu~e 5. This view is utilized to particularly illustrate one of the transfer or overflow pockets.
Figure 7 is a par~ial eross sectional view of ~he apparatus of Figure 1 taken throu~h the angled section line 7 - 7 of Figure 5, and par~icularly illustrates the ejection pins.
The presen~ inven~ion discloses an appara~us and me~hod for fabricat-ing a finished cptical blan~ which blank may be utilized for eyeglasses, or the like. The res~ltant finished blank produced with the presen~ in~en~ion requires little9 or no polishing and the blank may be readily cu~ or edged to ; any desired peripheraI shape such t~at the l~ns ~ay engage an eyeglass fra~e, instrument, or the like.
B~fore describing a preferred embodiment of the present inven~lonJ

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however, the problems associated with injection molding a lens ~ill be des-cribed in order tha~ the present inven~ion be more readily unders~ood. Refer-ring first ~o Figure 3a a eavity 29 o~ an injection molding apparatus is illus-trated which includes an inle~ 28, The injected molten plastic 35 is illustra-ted as the plastic first enters the cavîty 29. Assume, for the sake of dis-cussion, that the lens being fabricated within cavity 29 has a thinner center 33, suçh center being thinner ~han ~he outer edge of the lens, As the plastic 35 enters the cavi~y 29 it tends to flow to the ou~er perimeter of the cavity 29, since the cavity i5 thieker in this region. In Figure 3b the plastic 35 is again illus*ra~ed as it con~inues to fill the cavity 29. As m~y be seen in ~his figure, the plastic 35 does not immediately flow in~o ~he thinner center 33 of the cavity 29 but rather continues to advance about ~he outer rim of the eavity. In Figure 3c the continued flow of the plastic 35 is illustrated;
however, in this figure, as may be seen9 the plastic has flowed through the thinner center of the lens. As the plastic meets within the thinner center of the cavity a knit line 31 develops at the seam alo~g which the flow meets. In Fi~ure 3d the cavity is illus~rated completely filled with the plastic 35, and the knit line 31 which is inher~nt in conven~ional plastic flow (for any cavity having a thin center) is illustrated. This kni~ line prevents the use of the resultant blank as an optical lens wi~hout additional treatment9 such as com-pression molding or polishing.
As will be seen, ~he present in~ention, through the use of transfer or ov~rflow pockçts, and through ~he use of movable op~îcal inserts, pre~ents the fo~mation of ~his knlt lineJ particularly for lenses which are thinner in their center.
Be~ore d~scribing the apparatus a brief description of the finished blank 10 of Figure 1 will be gi~en~ since it will be helpful in unders~anding the apparatus. The finished blank 10 includes a lsns area 11 which is edg d to any appropriate shapeg such as a shape required to fi~ an eyeglass frame.
Disposed about the exterior of the lens area 11 is ~che annular rim 12 which is - 4 ~

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used to eject the blanks. The inlet runner or spruce 23 shows the flow of the molten plastic as it enters the cavi~y defining the blank 10. In the presently preferred embodiment a tip 25 is defined by the cavity, which tip allows the plus curved lens to be placed on a flat surface without scratching the bottom surace of the blank 10. Also defined by the ca~ity is a hanger Z0. This hanger allows the finished product to be hung from a pin 21, shown in Figure

2, or other de~ice during subsequent processing of ~he finished blank lO. A
pair of over10w spruces 16 illus~rate the effects of compressing the molten plastic af~er the plas~ic has entere~ the cavity. Qs will be explained9 once the cavity is filled the dies are co~pressed, forcing or squeezing predeter-mined amoun~s of molten plastic into pockets which define the overflow spru-ces 16, Refe~ring to Figures 4, 6 and 7, the presently preferred embodiment of the apparatus employs a compr~ssioTI molding apparatus disposed within a con-ventional injection molding machine. The conventional i.njection molding ma-chine includes an upper block 41 and a lower block 42 which meet at a mating surface 89, Disposed within these blocks is an injection nozzle 35 for injec-ting l~en plastic. A plurality of heating rods 46 are dispssed within ~hese blocks (and also below the oavi~y 86) for hea~îng the entire injection appara-~us, including the optical inserts 59 and 60. The nozzle 3S communicates with cavity 89 through passageways 38 and 39. The blocks 41 and 42 are secured to-gether through a hydraulic system, exerting, by way of example9 a hundred tons of ~orce.
The disclosed apparatus includes an upper interior block S4 and a lower interior block 55. Within these generally cylindrical blocks is disposed a : hydraulic eylinder 48. A piston 49 which terminates in a flange 62 coopera~
tively engages the cylînder 48 or exerting pressure upon the plas~ic within ca~ity 86. A spacer S2 is disposed between the flange 62 and an upper op~ical : insert o~ die 59. At ~he lower end of cavî*y 86 a spacer 57, supported from plat~orm 45, is disposed directly below, and in con~act with, the lower optical _ 5 ~93~6~3 inser~ or die 60. Thus, the cavity 86 is substantially defined by the adja-cent surfaces of the upper and lower dies 59 and 60, Passageway 39 passes through the blocks 54 and 55 in order that the nozzle 35 communicates with the eavity 86.
A hydraulic inlet lîne 50 is coupled to the eylinder 48 for providing hydraulie fluid ~o th~ eylinder when the plastic contained within cavi~y 86 is to be compressed. The travel of the piston 49 is shown by dimension 88 of Figure 4; this travel is lim;ted by the distance be~ween the flange 62 and ~he annulus 64 defined by the upper bloek 54. However~ in the presently preferred e~bodiment, the travel of piston 49 (and die 59) is determined by the period of time during which pressure is applied ko the plastie within cavity ~6 and by other means as will be tiscussed~
The suraces of the dies 59 and 60 defining the cavity 86 are fabrica-ted from a suitably hard metal or glass, whieh typîeally is highly polished with a precise, compensated, curve configuration in order tha~ the finished blank has an optically clearJ distortion free surface. Such optical inserts or dies are known ;n the prior art. Moreover, ~he curved surfaces of these inserts may be varied, by changing the dies 59 and 60, to obtain desired lens shapes or powers.
A plurlaîty of ejector pins 68 are disposed about the circumferenoe of the cavity 86 and contact the rim 12 o~ the finlshed blank to urge the blank from the cavity once the blank has been properly cured and the dies have been separated. ThPse pins are coupl~d to ~he movable ejection platform 44, Standard known injection devices may ~e used or this application.
Re~errîng to Figures S and 6, in the presently preferred embodi~en~, two transfer or ov~rflow pockets 70 are deined within ~he block 42. Th~se poekets comm~nica~e with the cavity 86 through th~ passageways 72. The pas- -sageways 72 are disposed generally facing the inle~ passageway 23. While in ~he presently preferred embodiment two pockets 70 are illustratedJ a single ~: 30 pocket ~or several pockets) nay nonetheless be used, and in an embodimant : - 6 -3r~ g where a single pocket is used the passageway communicating with this single pocket is disposed directly opposite ~he inle~ passageway. A piston 76 is disposed for movement within the pocke~ 70. The lower end of the piston 76 is urged upward by a spring 74 mounted between the lower end of piston 76 and the ejec~ion platform 44. The upper surface of ~he piston 76 includes a notch 77, the purpose of which will be explained later in this disclosure. An ad-jus~ment 66 is disposed at the lower end of each piston ~o limit the ~ravel of ~he pistons 75, These adjustments 66 in par~ determine the final vol~ne of the cavity 86. When the blank is ejected, the pistons urge the spruces 16 from the pockets. Gates 84 may be disposed within the passageways 72 as a means of limi~ing the flow into ~he pocke~s 70 f~om the cavit~ 86.
The inlet passageway 39 as is best ~llustrated in Figure 5 communicates with the cavity 86 ~hrough a righ~ angle bend 23. This right angle ~which is kno~n in the prior art) is used to dif~use the inlet flow of molten plastic.
Also illustrated in Figure 5 is a hanger cavity or void 82 t~hich defines the blank hanger 20 illustrated and discussed in conju~ction with Figures 1 and 2.
The entire appa~a~us shown in Figures 4, 5~ 6 and 7 ~ay be readily fab~
ricated utilizing known technology.
Assume a lens is to be ~abricated on the apparatus shown in Figures 4, 5r 6 and 7 and that the desired dies 59 and 60 have been placed within the apparatus, In the presently preferred embodiment a polycarbonate or o~her ther~o plastic is metered into ~he nozzle 35 at a te~perat~re of approxi~ately 520F - 560F. Also~ through use of ~he heating rods 46, the cavity is heated to a te~perature of approximately 260~ - 275F. Prior to the injection of ~ the molten plastic into the cavity 86 fr~m the nozzle 35 ~he outer blocks 41 : : ~ and 42 are held secured against one ano~her a~ the ma~ing surface 89 wi~h a force in the order o~ magnitude of one-hundred tons~
When the plas~ic is inject~d into the cavitr 86 via passageways 38 and 39, the force of the ~olten plastic against the die 59 causes the die to rise~
thereby increasing th~ volume of cavity 86 and its thickness. During this ~3~

period of time, little, or no, force is exerted on die 59 through the piston 49. Since the cavity increases in volume, and particularly since the center of the cavity has a greater thickness because of the movement of die 59, the knit llne described in conjunction with Figure 3 is no~ formed. In the pre-sently preferred embodiment the injeetion o the molten plastic into the cavi-ty 86 t~kes approximately ten seconds.
Following the injection of the molten plastic into the cavity 867 hy-draulic fluid is applied to the cylinder 48, through line 50, causing the upper dîe 59 to compress the molten plastic within cavity 86. By way of ex-ampleJ such pressure is exerted for a period of approximately ~hirty seconds with a total force of approximately twenty tons. During this period of time molt0n plastic is forced from the cavi~y 86 ~hrough the passageways 72 into the pockets 70. Referring briefly to Figure 6 the nokch 77 allows the plastic flowing into pocket 70 ~o exert an axial force against the piston 76 sueh that the piston 76 is foreed downward against the force of spring 74. It is during this period of time that the overflow spruces 16 of Figure 1 are formed. The pressure on the die 59 causess the volume and thicknesss of cavity 86 to de-crease, and particularly decreases the eenter thickness of the cavity. In ~his way, a lens of a desired thickness may be fabricated; including bifocal lenses.
In the presently preferred embodiment, the travel of die 59 is control-; led by the length of time elapsing after molten plastic enters cavity 86 and pressure is applied to the die 59 through piston 49O Also the final volume o~
cavity 86 is controlled by the length of time that pressure is applied to the die 59 by piston 49. Thus, if a thinner lens is desired, a shor~er period of time is allowed tolap~ebetween the injection and application of pressure, and the pres~ure is maintained for a longer period of time. If a thicker lens is desired, a longer period of time is allowed to run beore pressure is applied ; ~nd the pressure is maintained for a shorter period of time. The amount of flow rom cavity 86 may also be controlled by the force exerted on the piston 76 by the spring 74, ~hFo~gh the use of ~he gates 84 shown in Figure 5 and .~

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through use of the adjustments 66. While positive stops are not employed on piston 49 in the presently preferred embodiment, such stops may be alterna-tively employed.
Following the period of time during which the overflow or transfer occurs, ~he pressure is relieved from the cylinder 48 allowing a release of the pressure applied ~o the plastic wlthin cavity 86. Typically, the pressure is released for two ~o three seconds. During this period of time the overflow into the pockets 70 ceases, since the molten plastic begins to cure. For some applications this release of pressure may not be necessary before curing be-gins.
Following this ~wo or three second period of time, ~orce ~of approxi-- mately twen~y tons) is again applied to t~e die 59. This ~rce continues for a period of approximately two minutes, durin~ which ~ime the plastic within the savity is cured. The pressure applied to the die during this period of time assures that the surface Qf the blank remains smooth and optically cor-rect, since as the plastic cures i~ tends ~o shrink and pull away from the die surfaces. However, since the die is being urged agaînst the blank, a smooth surface matching the co~pensated optical inserts is maintained during this CUriDg period. The resultan~ blank is distor~lon free.
If the cavity 86 is suf~iciently thick during the period o time that the plastic is injected in~o the cavity, the knit line associated with the prior art injection techniques is, to a lar~e extentJ eliminated. Also, during the period of time that the overflow occurs into the overflow pockets, any knit line that may be present, to some ex~ent, is flushed from the cavity. For this reason in the presently preferred embodiment, the ou~let por~s leading ~o ~he overflow cavities are disposed opposite the inlet port.
While in the disclosed embodiment the upper die 59 moves relative to the fixed lower die 60, it will be apprecia~ed that the lower die may be msved to-wards a fixed upper die, or that both dies may move. More~ver, while in the disclosed embodimen~ electric heating rods 46 are shown, o~her heating means, 1376~

such as 1uid heating means ~ay be utilized to heat the apparatus. Also a vacuum e~haust may be applied ~o the cavity 86 prior to the time that flow begins into nozzle 35. Conventional vacuu~ exhaust ~ay be used for this pur-pose, Cooling fluids may also be used to cool the cavity 86 during curing.
It will be appreciated that the ~imes, tempera~ure and forces set forth in the above example may be varied.
Also, in the presently preferred embodiment, the pressure exerted again~t the upper die 59 by the injected molten plastic is used to raise this die, thus increasing the volume o~ cavity 86. However, the die may be raised lû mechanically prior ~o the time ~hat plastic is injected into the caYity wi~h the .~ame result.
When the finished blank is re~oved rom the cavity 86 by ~he pins 68 its appearance is ~hat of ~he finished blank 10 illustrated in Figures 1 and 2. This blank may then be coated wi~h a scratch resist2nt coa~ing, in a manner known in the prior art. Other known opera*ions such as cutting, color-ing, cl~aning, etc. may be perfor~ed on the finished blank.
A problem simllar to the knit line problem discussed in COnJUnCtiOn with F;gure 3 occurs for lenses which are thinner at their outer edges than at thair centers, such as large plus lenses wi~h approximately one millimeter edge thickness. The disclosed method and apparatus may be used to solve this problem.
Thus, an appara~us and me~hod for injection molding a lens has been disclosed~ The lens is ~abricated in a sîngle injectîon molding operation, and unlike prior art methods, ~ pre-formed blank is not required. With the disclosed apparatus many~high quality ophthalmic an~ instrument lensos may be produced ~rom a single i~jec~ion mol~ing appara*us, a~ substantially low&r costs than is possible with prior art ~echni~ues and ~pparatusas.

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Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for injection molding a lens comprising:
a housing;
a pair of dies disposed within said housing and a cavity for receiving molten plastic therein;
said housing including means permitting the relative movement of said dies such that the volume of said cavity may be varied;
injection molding means communicating with said cavity for injecting plastic into said cavity;
at least one transfer pocket for receiving plastic from said cavity, said transfer pocket communicating with said cavity; and compression means coupled to at least one of said dies, for sequentially reducing the volume of said cavity and for applying pressure to said plastic in said cavity after said plastic is injected into said cavity to transfer plastic from said cavity into said transfer pocket;
whereby molten plastic may be injected into said cavity, and then the volume of said cavity may be decreased forcing molten plastic from said cavity into said transfer pocket.

2. The apparatus defined in claim 1 wherein one of said pair of dies is fixed and said other die moves relative to said fixed die.

3. The apparatus defined in claim 2 including heating means for heating said cavity.

4. The apparatus defined in claim 3 wherein said other die is mounted such that when plastic is injected into said cavity, the volume of said cavity is increased by the pressure of such inflowing molten plastic.

5. The apparatus defined in claim 1 wherein said transfer pocket communicates with said cavity approximately opposite from said inlet means.

6. The apparatus defined by claim 1 wherein said transfer pocket includes an elongated chamber having a piston disposed in said elongated chamber such that molten plastic from said cavity displaces said piston as said molten plastic is forced by said compression means into said chamber.

7. The apparatus defined by claim 6 including adjustment means for limiting the travel of said piston.

8. The apparatus defined by claim 7 wherein said transfer pocket includes a gate for controlling the transfer from said cavity.

9. The apparatus defined by claim 6 wherein said cavity defines a hanger at its periphery such that said lens blank includes a hanger.

10. A method for forming a thermoplastic optical lens blank in an injection molding apparatus where said apparatus includes an optical cavity for defining said optical lens blank comprising the steps of:
injecting molten plastic into said cavity at a first aperture along the edge of said cavity when said cavity has a volume greater than said optical lens said greater volume to assure better flow into the thinner section of said cavity; and compressing said cavity such that the thickness of said cavity is decreased so as to define said optical lens blank, thereby causing a flow of molten plastic from said optical cavity at a second aperture along said edge of said cavity, said second aperture spaced-apart from said first aperture;
whereby said injected molten plastic flows into a cavity having a greater thickness than the thickness of said optical lens blank, thereby avoiding problems associated with conventional plastic flow into a thin cavity.

11. The method defined by claim 10 including the step of curing said lens blanking in said cavity after said flow of molten plastic from said cavity has ceased.

12. The method defined by claim 11 wherein pressure is applied to said blank during said curing so as to correct for shrinkage during curing.

13, The method defined by claim 12 wherein said molten plastic comprises polycarbonate.

14. The method defined by claim 13 wherein said cavity is heated prior to said injecting of molten polycarbonate into said cavity.

15. A method for forming a thermoplastic optical lens in a cavity comprising the steps of:
injecting molten plastic into said cavity at a first aperture along the edge of said cavity when said cavity has a volume greater than said optical lens, said greater volume of said cavity obtained by the force of said injected molten plastic acting upon the surfaces defining said cavity during said injection of said molten plastic;
applying first pressure to said injected molten plastic in said cavity so as to urge molten plastic to flow from said cavity through a second aperture along said edge of said cavity, said second aperture spaced apart from said first aperture;
applying second pressure to said molten plastic as said molten plastic cures;
whereby an optical lens is formed.

16. The method of claim 15 wherein said first pressure decreases the thickness of said cavity thereby urging said molten plastic from said cavity, said first pressure being applied until said cavity substantially defines said optical lens.

17. The method defined by claim 15 wherein said molten plastic comprises a molten polycarbonate.