CA1102973A - Method and means for replicating centrally apertured video disc records - Google Patents

Abstract

ABSTRACT

This invention relates generally to a method and apparatus for molding a centrally apertured part, such as a video disc. Prior art molding techniques are deficient in that extremely tight hole centering tolerances could not be met, and likewise thickness tolerance caused excessive birefringence of a light beam used in reading the disc. The present invention overcomes these deficiencies by providing an improved molding technology In accordance with the invention heated material is injected into an annular cavity, defined by a pair of mold halves, through a sprue passage, defined by a sprue bushing and an end portion of a punch. The annular cavity and the sprue passage form, respectively, the centrally apertured part and a sprue. After partial cooling of the heated material, the punch is locked in place while the annular cavity is displaced relative to the sprue passage so that the sprue is severed from the part along the peripheral surface of the punch end portion. The mold halves are separated to open the annular cavity while the sprue is resting on the punch end portion, and while the part is held by the punch end peripheral surface.
After the mold halves are separated, a sprue ejector member is actuated to eject the sprue from the punch end portion. The finished video disc is then removed, carried by the moving platen of the moving mold assem-bly. An improved injection gate is also provided, as well as a unique manner of cooling the mold assembly.

Description

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MEq~IO~ AND MEANS FOR REPLICATIN~
CENTRALLY APERTURED VIDEO DISC RECORDS
TECHNICAL FIEID
Thls lnvenkion relates generally to a method and apparatus for molding a centrally apertured part;
and more particularly concerns a method and apparatus ~or injection molding a centrally apertured and spirally tracked record, such as a video disc.
BACKGROUND OF T~E PRIOR ART
An apparatus ~or produc-Lng injection molded and centrally aperturecl video dlscs is shown in U.S.
Patent No. 3~989~436, issued November 2, 1976.
BRIEF SUMMARY OF THE INVENTION
The lnvention is not only use~ul for inJec-tion molding centrally apertured vldeo disc recordsg butis also applicable to the ~ormation o~ in~ection molded records requiring an accurately positioned centrally placed aperture.
In certain video disc systems, video ln~orma-tion is recorded in the ~orm of light scattering orlight reflecting members positioned in a spiral track on the sur~ace o~ a centrally apertured record. For ~ playback, the record is mounted on a rotatable turn--~ table havin~ a spindle which engages the record aper- -ture for~centering, and then relative motion ls e~tablished between the record and a reading assembly.
The reading assembly includes a laser optlcal system for generating a laser beam and ~or directing the laser beam to impinge upon a splral track which contains the .~ -'~.

. . - . . . . . .

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o~'~S~'3 video lnformation. The optical system includes an objective lens system which directs the laser beam ko the spiral track and also collects the reflected slgnal from the spiral krack ~or transmisslo~ to associated electron-lc circu~try which translates the detected re~lectlons into s-lg~als suitable ~or a visual display by a standard home type television receiver. A system o~ the above-ment:Loned kype is described in U. S.
Patenk 3,829,622~ issued to James E. Elliott and ass-Lgned to the Assignee ~ the present lnventlon.
In such a video disc system, it has been recogniæed that, not only the average speed of relative motion between khe impinging laser lighk beam and the record must be maintained at a predetermined speed of 1800 r.p.m., but the cyclical variations about the average speed must be limited ~or proper playback.
It is desirable to reduce the cyclical speed variation to assure that the synchronizing pulses in the reco-~ered television signal are fairly stable and within the lock-up range o~ the de~lection circuits ~ the televisionreceiver. The cyclical speed variakions are particu-larly unpleasant when the recorde~ in~ormation is a color television sLgnal.
One of the sources o~ cyclical speed variations is the record eccentricityO For proper playback of the video discg it is important that the replLcated record's center hole is concentrlc wlth the spiral groove center to a high degree of accuracy. As discussed ln the a~orementioned Elliott patent~ the time base cor-3 rection circultry is useable to achie~e correctlon forsuch cycllcal speed variations to within the limlts o~
the time base correcting circuits. More specifically, a time base correcting c^lrcuit has operaking limits itself and can only correct ~or a certain amount of error. ~herefore, it is preferred to form as perfect video disc record as possible during the injection molding operation.
The concentricity bekween the center hole of the record and khe record spiral trac~ is particularly .

important when the spiral tracks are spaced a fraction of a micron from track center to track center~ prefer-ably one-half micron to one micron. The video disc member useable with the Elliott video disc player has light reflective and light scaktering members positioned in track-like fashion on the information bearing sur-face of the video disc member. These light reflective or light scatterlng members are preferably sized to equal one-quarter of a wavelength of the incident light along the beam axis. More speclfically, typical light scattering structure~ are more completely de-_ scribed in Canadian Patent 1,066,411 of Manfred Jarsen, entitled ~'Replication utilizing a casting Process".

~~ In the Elliott system using a laser beam re~lection system for reading the information tracks on the video disc, the video disc must be essentially birefringence free in the ~ense that it does not contain impurities embedded in the plastic material which give a false reflection or light scattering effect to the impinging laser beam. These lmpuritles could include small particles of matter, left over from the previous in~ection cycle, being mixed with the new material needed in the present cycle~ These impuritles also induce stress forces formed withln the plastic material durlng the lnJection cycle.
The concentricity of the ~inished molded video disc member ls required to achieve f'aithful reproduct-lon of the video il~formation contained there-on. The center of graYity must be posltioned withln ten mils from the center of rotat:Lon o~ the video disc to limit the vibration due to static imbalanceO The information tracks should be concentric with the center o~ rotation within one to two mils for good tracking and to fall within the time base correction capability o~ the electronic cirGuitry associated wlth the Elliott player. Concentricity of the information tracks with the center of rotation are achie~edg in , . .

part~ by prov-Lding a stamper member having means ~or allowing expansion of 'che stamper member w-Lth-Ln the video disc cavity in response to the heat assoclated with the injected plastic material. Concentricity can also be da~aged during the sprue e~ection or aperture punch-ln~ steps. The aperture punching step must be performed without set~ing up a lateral shear force which lnherently moves a portion of the v-ldeo disc further towards or further away f'rom the center of rotation during the punchlng operatlon.
lt is an ob~ect of the present invention to provide a video disc record without flow lines and other s~rface de~ects in ~hat portion of the record contalning the spiral shaped -lnformation tracks.
It ls a further obJect of the present inven tion to provide a video disc member having a substan-tially uniform value of birefringence over the entire surface of the v~deo disc used ~or recordlng the video information traclcs.
A still further ob~ect of the present lnven-tion is to provide a platen assembly having means ~or allowing the stamper structures to freely expand within predetermined limits in advance o~ the inJected molten plastic material and the heat created therefromO
Another obJect o~ the present invention is to provide an inJection molding tool having cooling means for the platen subassembly.
The productlon of a video disc record having unifor~ values ~ b-Lrefringence over the entire in-3o formation bearing surface of the video disc member is achleved ln part by employing holdin~ means for allow-lng the v-ideo disc stamper members to ~xpand under the application o~ heat caused by the lnjected molded material; by employing an annular shaped sprue passage lntermediate the principal sprue passage and the video dLsc cavity to cause the molten material to flow into the di~c cavity in a manner avoiding stress gradlents ~ithln the ln~ected material.
A f':Lrst mold~hal~ and a seco~ mold-half are .. : : ,. . : -- - ~ . , reciproca~,ly mounted in the to~l. A sprue bushin~
having an opening in communication with an injection nozzle is secured to the platen. A punch, having an end portion in registr~ with the sprue bushing~ is rec-lprocably mourted relative to the second mold-half.
~hen the second molcl-hal~ is in a closed locationJ
(1) the punch end portion and the opening in the sprue bushing define a sprue passage~ ancl (2) the first and the second mold halves de~ine an annular cavity sur-rounding the sprue passage. The annular cavlty andthe sprue passage rorm3 respectively~ the centrally apertured part and the sprue when heated material is in~ected therein. ~ter partial cooling o~ the heated lnjected materlalg the ~`irst and the second mold halves move from the closed location to a location inter-mediate of the closed locatlon and an open location whlle ~he punch is locked in place to sever the sprue from the part along the peripheral sur~ace o~ the punch end portion. The centering die locator and iixed center stamper clamp portion of the first mold half, where the punch end portion enters9 serves as a dieO
The mold halves are therea~ter separated to open the annular cavity while the sprue is resting on the punch end portion, and while the part is in contact with the punch end peripheral sur~ace. ~ter the mold halves are separated, a sprue ejector member and a part removal member are actuated to remove, respectlvely3 the sprue and the part ~rom the punch end portion.
An improved platen assembly is described 3o having releasable means ~or centering the stamper and ~or holding the stamper in place against the platen sur~ace. The holding means also includes means ~or allowing the stamper to expand under the temperature gradient o~` the heated plastic inàected mater-lalO
The stamper expands uni~ormly in ~ront of the injected material, as the heated injected material ~ills the video disc cavityO The platen assembly ~urther con-tains a plurallty o~ s~parate cooling channels ~or maintain-lng the platen at a uniform temperature across '3 ~

the surface of the platen.
A center punch is described which accurately punches a hole in the center of the injected video disc member.
The center punch is held statlonary while the fixed mold half expanding ~ase plate and the moving mold half move through the punch stroke to an intermediate position set by a punch stroke limiter device.
An annular shaped sprue passage is formed by the closing of the first and second mold half members and the end portion of the punch. The sprue passage comprises a first section de~ined by the sprue bushing itself and a second section ~
defined by a combination of members including a portion of the sprue bushing, the end portion of the punch and portions of the inner centeriny locators and clamps.
In accordance with one broad aspect, the invention relates to an injection molding apparatus for molding centrally apertured parts, said apparatus comprising: a port for receiving molten material, an annular cavity formed by first and second mold halves into which the molten material is injected, and an injection gate positioned intermediate said port and sai~
cavity, said injection gate including a disc shaped passage whose height decreases with increasing radius such that the molten material flows thereacross at a substantially constant rate.
BRIEF DESCRIPTION OE' THE DR~INGS
FIGURE 1 is a cross-seckional view, partly schematic, taken along the line 1-1 of Figure 7 of an injectlon molding apparatus ror replicating centrally apertured and spirally tracked records pursuant to the principles of the present invention;
FIG~-RE 2 is a cross-sectional view, similar to Figure 1, 7~

~t.s.~ 3 taken along the line 2-2 of Figure 7, of an injection molding apparatus for replicating centrally apertured and spirally tracked records pursuant to the principles of the present invention;
- FIGURES 3 through 6 show in schematic form the sequence of operation executed by the injection molding apparatus shown in Figures 1 and 2;
FIGURE 7 is a plan view oE the moving half shown in Figure l;
FIGURE 8 is an inverted plan view of the fixed half of the mold assembly shown in Figure l;
FIGURE 9 is an enlarged sectional view taken within the circle identified as 9 and as shown in Figure l;
FIGURE 10 is an enlarged sectional view of the area contained within the circle identified as 10 shown in Figure l;

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FI'~U~E 11 is an enlarged sectlonal view of the area con~alned within the circle ldentif~led as 11 shown in Figure 19 FIGURE 12 ~s an enlarged sectional view taken along the llne 12-]2 o~ Ftgure 1.
FIGURE 13 is a graph showlng a changing rela-tionshlp in value of birefrlngence accompanying a changing relationship in the t;hickness of an accept-able video discg and FI~URE lLI -ls a graph showing a changing rela-tionship in value of b-Lrefringence accompanying a changing relationship in the thickness of an un-acceptable v-ldeo disc~
DET~ILED DESCRIPTION OF THE _~E TION
Ref'erring to Figures land 27 there is shown a tool 10 for use in combination with a standard in-jection molding machine, such as the 375 ton model manufactured by the Stokes Division o~ Penwalt M~g.
Co. The tool 10 is used for replicating a centrally apertured video disc recordg and comprises a fixed mold half (first mold half) 12 and a movi.ng mold half (second mold half) 14~ The fixed mold half 12 com-prises a fixed mold f-lxed base plate 16 which is attached to the fixed ~rame member of a standard mold-ing machine (not shown) and a fixed mold half expandingbase plate 18. The base plate 16 carrles in integral attachment therewith a plurality of maJor guide and support pins9 one of which is shown at 20. A fixed mold half ma~or guide and support pin bushing is shown at 22 for reclprocally mounting the base plate 18 with the base ~ate 16 during the punching step. The punching step is defined in part by the reciprocal movement of the expanding base plate 18 with re~erence to the fixed base plate 16.
The moving mold half 14 comprises a moving mold half carrler plate 309 a moving mold half spacer plate 32 and a moving mold half fixed base plate 34.
The moving mold half fixed base plate 34 is attached to t~e ~rame of a standard in~ection molding machine . .

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, (not shown). The spacer plate 32 is attached to the fixed base plate 34 by a plurality of moving mold half spacer plate ~olts~ one o~ which is shown at 360 The bolts 36 are countersunk wlthin the spacer plate 32 and are uniformly spaced around the periphery of the spacer plate 32 for f-lrmly attaching it to the fixed base plate 34.
~ he base plate 34 is additionally attached to the carrier plate 30 by a plural-.Lty of moving mold hal~ clamp bolts, one of WhiC~l iS shown at 38. Each of the clamp bolts 38 passes through the spacer plate 32 as indicated by the dotted lines at 40 and are threadably attached to the carrier plate 30 as indi-cated by the dotted lines shown at 42. The clamp bolts 15 38 are uniformly spaced about the periphery of the base plate to securely fasten together the carrier plate 30, the spacer plate 32 and the base plate 34.
Each cr the bolts ~8 is countersunk within the base plate 34 to provide a smooth contact surface 43 to the 20 plate 34.
A moving mold half major support pin bushing is shown at 44 carr-led by the carrier plate 30. The support pin 20 is positioned within the bushing 44 and provides reciprocal movement bet~een the fixed mold 25 half fixed base plate 16 and the carrier plate 30 during the punching operation. The support pin 20 also provides conjunctive movement between the fixed mold half expanding base plate 18 and the moving mold half carrier plate 30 during the Punching operation.
With the completion of the punching operation, the support pins 20 are fully withdral~n from the bushings 44 during the remaining portion of the opening step.
At the full~ opened position, the support pins are spaced from the moving stamper 144 to the same extent 35 as the prlmary pnnch plate assembly stroke limiter 90 is shown spaced from the eY~panding base plate 18 in Figure 5.
A punch plate assembly 50 comprises a punch plate assembly clamp plate 52 and a punch plate assembly ~q _9_ support plate 54. The punch plate assembly 50 is carried within the moving mold half fixed 'base pla.te 3 and is reciprocally mounted therewith by a plurality of punch plate assembly guide pins integrally a~'cached to the base plate 3~. One o~ the punch plate assembly guide pins .Ls shown at 55. A punch plate assembly clamp plate 'bushirlg is shown at 56 and a punch plate assembly support plate bushing is shown at 58. The ~uide p-lns 55 e~Ytend through the plates 52 and 54.
10 The plates 52 and 5ll are reciprocally mounted upon the pins 55 by the 'bushi.ngs 56 and 5~, respectively.
The guide pins 55 extend into the moving mold half carrier plate 303 as shown by -the dotted lines 60.
A plurality o~ moving mold half carr-ler support bars 6~ are attached to the base plate 34 by lndiv-ldual bolts~ one o~ which is shown at 66. The support bars 64 extend through openings in the plates 52 and 54 as ind-Lcated by the dotted lines at 72 and 74g respectively.
The support bars 6~ provide added support to the back surface 76 of the carrier plate 30 during the inJection of the molten material into the video disc cavity.
Re~'erring to Figure 2; a punch plate stop bar is shown at 77 positioned -lntermedlate the punch plate assembl~ support plate 54 and the fixed base plate 34.
The stop bar is attached to the base ~ate 34 by a plurality of punch plate stop bar bOltsg one of which ls shown at 78. The s-top bar 77 is c-~rcular in cross section. A portion of the bar is shown at both the left and right hand portion of Figure 2. The stop bar 77 adds rigidity to the tool allowing it to withstand the full force of' the closing force of the main ram assoclated with the inJection molding machine. In this capacity~ it cooperates with the side members 34a of the fixed base plate 34 ln withstanding the ram pressure during the closing and closed ~ortion of the molding operatlon. While the stop bar 77 is described as clrcular in cross section~ it can also be a single .

plate~ If 3. single plate con~iguratlon ~, used, a number of such plates are disposed around the per-1pher~J
of the base plate 34 so that the combined effect of the single member 77 is to uniformly separate the sup-port, plate 54 from the base plate 34.
The punch plate assembly support plate 54 isintegrally attached to the pllnch plate assembly clamp plate 52 by a plurality of support plate to clamp plate bolts~ one of whLch is shown at 80. Disassembly of the clamp plate 52 from the support plate 54 allows assem-bly of a plurality of primary punch plate assembly stroke llmiters~ one of which :Ls shown at 90, to be positioned within an openlng 92 carried by the clamp plate 52. Each primary stroke limlter rests at an inter~ace 94 with the support plate 54. The stroke limlter 90 extends through openings 96 and 98 carrled by the carrler plate 30 and the base plate 18. The stroke limiter is in engagement with the base plate 16 at an interface ldent-lfied as 100.
A plurality of~ secondary punch plate assembly stroke llmlters are carried by ~he punch plate assembly clamp plate 52. One of the stroke limiters is shown at 102 attached to the plate 52 by a bolt 104~ In Figure lg an end surface 106 of the secondary punch plate assembly 102 Ls shown spaced ~rom the under-surface 107 of the carrier plate 30 b~ the distance indlcated by a llne 108. This distance 108 represents the distance to be traveled by the carrler plate 30 from the open posit:Lon to the intermed-late posit-Lon, as described in greater detail with ref'erence to Figures 4 and 5.
The f'ixed mold half fl~ed base plate ~
carries a sprue bushing at 110 held in place by a sprue bushing lock ring 112. ~he sprue bushing 110 has an open-lng 114 which communicates at one end 115 wlth an inJection nozzle 116 of the injection moldlng machine and at another end 117 with the video disc c~vit~.
The fixed mold half expanding base plate I8 . ~ ' , : , ' , , , .

~ J ~J~'~3 carrles a fixed pla~en 1~0 attached thereto by a nu.rnber o~ bolts, one of whi.ch is s'nown at 122. The ixed platen 120 carries a fixed stamper ~24 held against the fixed platen 1~0 at its inner radius by a centering die locator and fixed center stamper clamp 126, and held at lts outer radius by a flxed outer stamper ring clamp 128. The clamp 126 -ls held ln place by a center clamp retaining bolt 130 which passes through the fixed mold hal~ expanding base plate 18 and the fixed platen 120. The flxed outer stamper ring clamp is shown in greater detail in Figure 10. The relationship between the ~ixed center stamper clamp 126 and the fixed stamper 124 is shown ln greater detail with reference to ~igure 9O
The moving mold half carrier plate 30 carries a moving platen 140 attached thereto by a number o~
bolts, one of which is shown at 142. The moving platen 140 carries a moving stamper 144 held against the platen 140 at lts inner radius by a centering punch locator and moving center stamper clamp 146 and at lts outer radius by a moving outer stamper ring clamp 148.
Both outer ring clamps 128 and 148 can be effeckively held ln place by bolts countersunk in their respectlve platens 120 and 140 and extending through the platens into the rings 128 and 148. The bolts would provide a ~ixed connection while the con~igura-tion schematically shown to the left edge of the platens 120 and 140 provide a releasable connection.
The moving platen 140 is releasably attached to the moving mold half carrier plate 30 by a center clamp locking assembly 150. The assembly 150 can also comprise a number of bolts which would bolt the clamp 146 to the platen 140 as does the assembly 150. The bolts replacing the assembly 150 are countersunk with-in the carrier plate 30 in a manner similar to the positioning of the bolt 142.
A sprue ejec'cor pin 156 has a sprue e~ector pin base nut 158 resting against a piston 159 of an air cyl~nùer 160. ~ center hole punch 162 compr~ses a :
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vertical center hole forming punch member 164 and a horizontal center hole forming punch member 1~6 havlng a punch end peripheral surface 167. A center hole punch adjusting nut 168 is attached to the bottom end 170 of the ver~ical punch member 164 and rests aga:inst the surface 9~ of the punch plate assembly support plate 5L~. The horizontal puncll mem~er 166 of the center hole punch 16~ carr~es an undercut 173; as best seen in ~`lgure 5~ on an lnterior surface 174 whlch f`orms a portlon of the sprue region. During inJection of the molten plas~ic material, some sprue material fills the undercut region. During separation of the ~ixed mold half expandlng base plate 18 from the movlng mold half carrier plate 30, the plastic in the undercut reglon holds the sprue 175 to the punch 162. The centrall~ apertured part ls shown at 175a.
The alr cylinder 160 is bolted to a surface 180 of the assembly support plate 51~ by bolts 182. As a design conslderation, the cylinder 160 fits within an openlng deflned by a surface 184 carrled b~ the base plate 3~. An alr intake passage lnto the cyllnder 160 is schematlcally shown at 1~6 whlle an exhaust port - is s~hemat-lcally shown at 188.
Referring generally to Figures 3 through 6~
the operation of the injection mold apparatus is de-scrlbed. Schematic representations are shown illus-trating basic movements provided by the standard injection molding machineg previously identified~
whlch cooperates with the remaining elements of the tool 10 to provide the molding apparatus of the present invention. These basic steps form a poxtion of the method of the present lnvention and the basic machine elements which perform these movements also ~orm a portion of the injection molding apparatus of the pre-Isent invention. These basic movements and apparatusprovided b~J and contained ~thin a standard injection molding machine is schematically shown with re~erence to Flgure 3.
The in~ection moldlng apparatus is shown in ' ' : , ~
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the closed position -Ln Figure 3. This closed posi~lon is de~lned ln part with the end surface 100 of the primary punch plate assembly stroke limiter 90 in contact with khe fixed molcl half fi~ed base plate 16.
The secondary punch plate assembly stroke limiter 102 ls spaced a distance 108 ~rom the surface 76 of the moving mold half carrier plate 30. The ejector pin 156 is in its retracted position. The horizontal port~on 166 of the punch assembly 162 is ~n its re-tracted poslt-lon. An end surface 189 of the punch assembly 162 and the opening 114 of the sprue bushlng 110 derines the sprue passage~ The first and second mold halves define an annular cavity surrounding the sprue passage. The annular cavity and the sprue passage form, respectlvely, a centrally apertured part 175a and a sprue 175 when heated material is inJected therein.
A flrst selectively actuated means 190 causes motlon of ~he second mold half 14 between a closed location (Figs. 1 and 3) and the open locations (Figs.
5 and 6). The first selectively actuated means com-prises a piston 191 (Fig. 3) rldlng ln a cylinder 192.
A connectlng rod 193 connects the plston ~1 to the base plate 3L~. Pressurlzed fluid is admitted into the cylinder 192 through a fluid valve 194 to move the second mold half 14 ~rom the closed location (Figs. 1 and 3) to the open location (F-lgs. 5 and 6). Pressur-ized fluid ls admitted -lnto the cylinder 192 through a fluid valve 195 to move the second mold hal~ 14 from the open locat~on (~igs. 5 and 6) to the closed loca-t-lon (Flgs. 1 and 3).
When the first and the second mold halves 12 and 14 are ln the closed locatlon (Flgs~ 1 and 3), ~a) an end portion 189 o~ the punch 162 and the sprue bush-ing openlng 114 de~lne a sprue passage and (b) the flrstand second mold halves de~lne an annular cavity sur-rounding the sprue passage. The annular cavity and the sprue passage form, respectlvely3 a centrally apertured part 175a and a sprue 175 when heated J'~ ~'3 makerial is injected therein.
A second selectively actuated means causes motion of the :~irst mo].d half 12 with the second mold hal~ in response to the motion of the second mold half from the closed location (Flgs. 1 ~ 3) to a loca-tion wllich ls interrnediate (Fig. 4) of khe closed location and an open locatlon (~-Lgs. 5 ~ 6) whereby the an~ular cavi.l,y remains closed while the firsk and second mold halves move f'rom the closed location to the intermedlate location. The second selectively actuated means comprlses a latching means 197 com-prislng a fixed base pla'ce 216 attached to the fixed mold half expanding base plate 18 by a plurality of bolts 218. ~ firsk latch member 220 is bolted to the fixed mold half flxed base plate 16 by a plurallty of bolts 222. A second latch member 22~ is bolted to the moving mold hal~ carr-Ler plate 30 by a pluralikcy of bolts 226. While this latch is a standard lateh, lts mode of opera'cion has been specifically ineorpor-ated into the present invention and as such it repre-sents an integral operating mechanism within the inven-tive combination. This latch mechanlsm is a standard eommeroially available lateh identified as the "~iffy lateh" Model LL-201 manufactured by the De~roit Mold and Engineering Company.
Briefly stated, the method of operakion of the lateh e~nters around a horizontal d~sposed pivoting lateh member 230 having a pivot pin 232 having a sur-faee 234 whieh engages latehing s~rfaces earried by eaeh lakceh member 220 and 224. In the views9 it shows the surfaee 234 and the la'cching surfaees of the member~
220 and 224 are all represented by 'che line 23~ sinee all these members are shown lrom the 'cop view. The len~th of the pivot pin indicated by the length of the llne 23l~ represents the extent to whlch the first lateh member 220 ean move away from and wlkh relatlon to the seeond la'cch member 22~, whlle the lateh re-mains in the la'cched eondltion. Thls dlstanee ls represented by the len~th of the line 235 shown ~n , , , ' , ' ',' ', ' ' ' ', ' :

Figure 4. The latching means 197 functions to hold the ~ixed mold halr expandin~ base plate ~ against the movir~ mold half carrier plate 30 while both last -ldentified members move a distance lndlcated by the length o~ the line 235 shown in Figure 4 ~ igure 4 shows the pivot pin 232 in its fully extended ~sition at the instant the first latch member 220 has moved the full d~stance with relation to the second latch member 224. The latch mechanism 197 keeps the annular cavity closed while the first and second mold halves move from the closed location to the intermediate location.
Referring back to Figure 3~ an expandlng base stroke limiter comprises a plurality of bolts~ one of which is shown at 240, The bolt 240 has a shank por-tion 21~2 fitting within an opening 244 carried by the fixed mold half fixed base plate 16. A head member 245, carrying a shoulder 246, is ~ntegrally connected to the shank member 242. The stroke limiter 240 is threadingly enga~ed with the expanding base plate 18 as shown at 24~. A plurality of such stroke limiters 240 are provided around the periphery of the base plate 16 and function to limit the movement to khe fixed base plate 16 with relation to the expanding base plate 18 during the rotation of the pivot latch member 230 during the movement of the tool from its closed ~sition shown in Figure 3 to its intermediate position shown withln Figure 4.
Third selectively actuated means 250 locks 3o the punch assembly 50 in place while the first and second mold halves 12 and 14 move from the closed location (Figs. 1 ~ 3~ to the intermediate location (Fig. 4), at which position the sprue 175 is fully severed from the part 175a by the peripheral surface 167 of the punch end portion 166. A portion of the sprue bushing ~here the punch end portion 166 enters ; serves as Q die. Third selectively actuated means 250 comprises a piston 252 riding in a cylinder 254.
connecting rod 256 connects the piston 252 to the -~6-punch plate assernbly support plate 54 thrGugh an open-ing ln the base plate 34 deflned by a surface 257.
Pressurlzed fluid or a-lr is admitted to the cylinder 254 through valve 258 to lock the punch plate assembly support plate 54 in contact with the base plate ~ at interface 100 while the flrst and second mold halves 12 and 14 move from the closed location (Figs. 1 ~ 3) to the lntermediate locatlon (Fig. 4). After the second mold half 14 reaches the intermed:late location (Flg.
4)~ the punch plate assembly support plate 54 moves in conjunction with the second mold half 14 from the intermediate location (Fig. 4) to the open locatlon (Figs. 5 & 6). This is accomplished by opening the valve at 258 and thus reduclng the pressure holding piskon 252 against the punch plate assembly support plake 54 alone or in conjunction with the admission of pressurized fluid or air through valve 260 to drive p:lston 252 back to lts retracted position.
Referring to Flgure 49 the tool 10 is in its intermedlate ~sition. The intermediate positi.on is defined, in part~ as that position wherein the shoulder 246 of the expanding base stroke limiter 240 is in contact with the f-lxed mold half fixed base plate 16 at the interface identified as 246. The first latch -, 25 member 220 has retracted from the second latch member 224 by a dlstance indicated by the line 235 and the pivot pin 232 has rotated in the direction lndlcated by an arrowhead 270 to its maximum latched position JUSt prlor to opening. The connectlng rod 256 of khe third selectively actuated means 250 bears against the sur-face 180 of the punch plate assembly support plate 54 malntaining contact of the primary punch plate assembly stroke llmiter 90 with the fixed mold half fixed base plate 16 at the interface 100. With the third selec-tively actuated means 250 maintaining contact ofthelim~ter 90 with the base p ate 16 and holding the punch assembly 162 stationary with relation to the sprue bushing 1109 the fixed mold half expanding base plate 18 and the moving mold half carr-ler plate 30 move in the direction indicated by an arrow 272 under tile admission o~ fluid by the valve 194 intothe cylincler 192 o~ the first actuated means 190. A punchlng action occurs between tne horizontal portion 166 of the punch 162 and the centering die locator and fixed center stamper clamp 126 for severing the sprue 175 from the part 175a. An annular disposed portion 274 of the part 175a remains attached to the sprue 175. The portion 274 ls that part positioned between and in contact with the end surface 189 of the punch 162 and the end sur~ace 299 o~ the sprue bushing 110. The sprue 175 is held by its u~dercut 173 to the punch end surface 189 and the part 175a is resting on the punch end peri-pheral surface 167. It should be noted a-~ this point that the main par~ing line indicated at 27~ between the expanding base plate 18 and the carrier plate 30 illustrates that the two plates are still firmly held together by the second selectively actuated means 197.
~len the combined movement of the expanding base plate 18 and the carrier plate 30 is such that surface 76 of the carrler plate 30 contacts surface 106 of the secondary punch plate stroke limiter 102~
and when surface 245 of the stroke limiter 240 contacts fixed base plate 159 the latching means 197 opens.
The expanding base stroke limiter 240 stops the move-ment of the base plate 16. The actuating means con-tinues to open the cavity by continuing movement of the second mold half 14 including the punch assembly 50 to the open position shown-in Figure 5.
3 Figure 5 shows the mold injection tool 10 ln its open position. In the open position the shoulder 246 of the expanding base stroke limiter 240 is in contact with the fixed mo~d half flxed base plate 16 at the interface identified by numeral 246. The pri-mary punch plate stroke limiter 90 is fully withdrawn from the base plate 18. The latching member 230 is fully rotated in the direction indicated by the arrow 270 such that the pivo~ pin 232 is di~engaged ~rom both the upper latch member 220 ancl the lower latch ~'s'', member 224~ A latching surface 280 of the first latch member 220 is shown in its disengaged position from the pivot pin 232. A latching surface 282 of the second latching member 224 is shown in its disengaged positi~
from the latching surface 234 ~ the latching means 197. The sprue 175 is attached to the end of the e~ector pin 156 and carries an annular protuberance at 284 which corresponds to the undercut 173. Durlng the injection of the molten material into the sprue passage and the annular cavity~ theprotuberance 284 was formed ln the undercut 173.
The plston 159 of the air cylinder 160 ls shown in its extended position whereby the eJector pin is moved forward carrying the sprue 175 and its annular part 274 free of the end 189 of the punch 162.
The piston 159 is caused to move its ~orward posltion by admission of pressurized fluid through the valve 186.
With the sprue 175 separated from the part 175a, the next function to be performed is the removal of the sprue from the end of the eJector pin 156.
Referring to F-Lgure 6~ the tool 10 is shown in the fully open position and the ejector pin 155 is shown in its retracted position. The retracted posi-tion is caused by 'che admission of fluid into the valve188 for moving the piston to its second retracted position as shown in Figure 6. In the movement of the e~ector pin 156 back to its retracted posltion in the direction as shown b~ the arrow 286~ the procuberance at 284 engages the surface 189 o~ the punch ~or separ-ating the sprue from the ejector pin 156 as shown in Figure 6.
Referring to Figure 9, there is shown an expanded vlew of that portion of Figure 1 shown within circle 9. The fixed pla'cen is shown at 120 having the fixed s'camper 124 held thereto by a finger 126a of the centering die and flxed center stamper clamp 126.
The moving platen is shown at 140 havlng the moving stamper 144 held thereto b~ a flnger 146a of the cenkerin~, punch locakor and moving center stamper clamp 1l~6~ The sprue bushing 110 is s'nown having its lower end 117 posit-ioned in prOx:~mlt~J to the ejector pln 156. The horizontal punch member 166 is shown 5 having the undercut 173 positloned in the inner surface 17L~. Assuming that the cavity de~inecl by the members Just above describecl is ~illed wlth hardened plastic mater:lal injected during the in~ection cycle o~ the lnJectlon molding machine, the sprue is shown at 175 19 the annular port-Lon 274 is shown attached integrally ~ormed with the sprue 175. The protuberance 284 is also shown in integral formation wlth the sprue 175.
It has been ~ound that the design of the sprue passage, including an annular shaped gate passage 298 15 positioned intermediate the sprue opening 114 and a video disc cavity 306 formed between stampers 12~ and 14~, should possess a conflguratlon such that the inJected material advances across the stamper sur~ace at a uniform rate. In order to achieve th-ls desired 20 ef~ect, the annular shaped gate passage 298 assumes a unique form comprising a plurality o~ annularly shaped passage sections and each section has an entry region and an exit region. The exit region o~ one section corresponds to the entry passage of t~le ~ollow-25 ing section.
The ~lrst annular sect-lon ls formed between an end surface 299 o~ the sprue bushing 110 and an end sur~ace 189 o~ the punch 166 as indi¢ated by the ; bracket 300. Each o~ the surfaces 299 and 189 lie at an angle ~ three percent with the horizontal. Sur-~ace 299 lies three percent above the horizontal and line 189 lies three percent below the horizontal. The entry region o~ this ~irst section 300 is at llOaO
The e~it section is at llOb. The distance between ~; 3~ sur~aces 299 and 189 at the exit region equals the thickness o~ the cauity 305. ~y observation~ the entrance section between llOa and 166a is thicker than the e~it region hetween points llOh and 166b. This means that a pressure dif~erential e~ists between regions llOa and 110~.
A second annular section i3 formed by a por-tion of the surface 126b and a portion of a surface 146c ~ the fixed center stamper clamp 12~ and the moving center stamper clamp 146. This second region is indicated by a bracket 302. The surfaces 126b and 146b are spaced apart a distance equal to the thlc~ness of the video disc cavity 306 and are coextensive over their entire length.
A thlrd annular section of the annularly shaped gate passage 298 comprises relatively short portions 12~c and 1~6b of the fixed center stamper clamp 126 and the moving center stamper clamp 146, respectively, as indicated by the bracket 304. The 15 entrance region of the third annular section equals the thickness cf the video disc cavity 306 and the exit region is considerably smaller than the entrance region.
A fourth annular section of the annularly shaped gate passage 29$ is formed by a portion 125d and 146d of the fixed center stamper clamp 126 and the moving center stamper clamp 146 as indicated by bracket 305. The exit section of the fourth annular section ~s an entry nozzle into the video disc cavity 2g 306.
In operatlon, the hot material to be in~ected into the video disc cavit-; for forming the video disc record enters the sprue passage 114 as a hot melt and then spreads out circumferentially around the sprue 30 passage 114 into the annularly shaped gate passage 298 and finally into the video disc cavity 30~ until it reaches the outside dimensions of the cavity as more fully discussed with reference to Figure 10. The machine is held in a quiescence posltion until the molten plastic solidifies to a certain temperature as ls more completely discussed with reference to Figures 7 and 8. During the previously descrlbed inJection cycle, the hot melt enters the entrance region ~
passage 2~v at a gr:eater rate than it can exit the , ,~..~, same passage 298 through the section identified b~J
the bracket 305 because the exit aperture from the fourth annular section 305 is necked down relative to the entrance section at the first annular section 300.
The third annular section 304 acts as a partial re-strictor to the flow of molten material. The flrst and second sections 300 and 302 act as a pressure reservoir and distribution header for the flow of material to -lnsure that an even flow of molten plastic flows into the video disc 306 with minimum disturbance to material flow. This control of the molten material through the restrictor section 304 provides the added advantage of makillg very round video disc records with information tracks on the video disc records which closely approximated perfect circles. This control in formation of both the round video disc records and the tracks closely approximating perfect circles would not be available if the annular gate passage were not used.
Referring to Figure 13g there is shown a graph showing the relationship between the thickness of an acceptable video disc and the birefringence at a radius from the center of the sprue passage. Curve - A of Figure 13 shows a variation in thickness of plus or minus two thousandths of an inch from a nominal value of forty-four thousandths of an inch over the in~ormation carrying portion of the video disc sur-face. Curve B shows the change in birefringence over the same region of the disc surface. The change in birefringence is from two nanometers to seven nano-meters.
Referring to Figure 14, curve A shows the relationship between birefringence of an unacceptable video disc member having a thickness of plus two to minus five thousandths of an inch from a nominal value of forty-four thousandths of an inch over the informa-tion carrying portion of the video disc surface.
Curve B shows the change in birefringence over the same region a~ the disc surface. This change in --22_ birefringence varies from a maximum of twenty-two to a minimum of two nanometers. It has been found that a video disc l,~ill only operate when the birefringence of the video disc is substantially uniform over the 5 playing surface. Referring back to Figure 13, the information carrying surface cf the video disc lies between f-lfty-flve millimeters and one hundred and flfty millimeters wherein the value of birefringence varies ~rom a maximum of seven to a minimum of two.
10 It has been found by experimental study that the video disc exhibiting I;he characteristics shown with refer-ence to Figure 13 is suikable for play on a vldeo disc player as described in Elliott patent9 U.S. Patent No.
3,829~622 where the disc having the characteristics shown with reference to Figure 14 does not perform satisfactorily on the same player.
Referring to Figure 10; there is shown the exploded vie~ cf that portion of Figure 1 shown within the circle 10. The fixed outer stamper ring clamp is shown at 128 and the moving olter stamper ring clamp is shown at 148. The fixed pla~en is shown at 120 of Figure 1 for holding ~he fixed stamper 124. The fixed stamper 124 terminates a distance from the moving fixed outer stamper ring clamp 128 as indicated by the length of the bracket 307. This allows the stamper 124 to expand outwardly until contact is made with the ring clamp 128. Thls expansion àrea for the :Eixed stamper 124 allo~s the stamper to expand as the heated mater-ial t5 inJected into the video d-lsc cavity.
The mov-lng stamper 144 is held to the moving platen 140 by the moving outer stamper ring clamp 148.
The fixed stamper 144 terminates at a point removed from the rlng clamp 148 a distance as indicated by the bracket 308. A foot 310 of the flxed outer stamper ring clamp 128 helps to hold the moving stamper 144 in place yet allow the stamper 144 to expand through the distance as indicated by the brac-ket 308 in response to the heat from the in~ected molten material.

,i'~ .

I,~en the molten plastic is in~ec~ed lnto 'che video disc cavlt~J, it -Ls important that the v~deo dlsc have a substantially un-lform thickness across khe pla^~
area o~ the video disc as illustrated in Figure 13.
~hen the thlckness of the video d~sc varies six thousandths o~ an inc~lg a non-playable disc results as lllu,strated ln Figure 1~ lhen the thickness of the video dlsc ls maintained with:Ln two thousandths of an inch as shown in Flgure 13, a playable disc results.
It has been foun~ that allow-lng the upper stamper and the lower stamper to expand in a horizontal direction as indicated by the brackets 307 and 308~ a playable disc results because the stamper expands ko fill these areas and does not buckle when subjected to the heat of the inJected molten plastic material causing a varlatlon in thickness ~ the vldeo disc member. A
passage 312 allows venting ~ the video dlsc cavity durlng the in~ection cycle.
The cooling system provided in the instant -lnvention ls calculated to remove heat from the tool 10 caused b~J the injection o~ molten plastic into the sprue passage and video disc cavity. Coollng pre-vents stress imperfectlons in the finished video disc record. The absence of stress imperfections improves the birefringence characteristic of the finlshed video disc record.
The cooling channels will be descrlbed with reference to Figures 1~ 2~ 7 and 8. Referring collectively to Figures 1 and 8g a sprue bushing cool-ing channel is shown at 350 having an entry valve at352 and an exit valve at 35L~. As seen best in Figure 1 the cooling channel 350 is spirall~ shaped and has four threads per -lnch extending from a position closer to the end 115 of the sprue passage 114 and extending down towards the end 117 of the sprue passage 114.
The spiral then reverses itsel~ back up the sprue bushing and exits f'rom the exit valve 354O An 0-ring is shown at 356 to provide a ~luid tight connection between the sprue bushing cooling channel 350 and the , ~ ' .. .
. . . .
.

f~32~

sprue bushing lock ring 112.
Referring to Figure 8) a fixed platen sprue region cooling channel is shown at 357 having an entrance valve at 358 and an exit valve at 359. The plurality of spiral shaped turns in the cooling channel 357 is represented by a single tu m 357a shown in Figure 8 at the center of the Figure. A flxed platen inner region cooling channel 3~0 is shown having an entrance valve at 362 and an exit valve at 364. The entrance point is at the inner radius and spirals out a plurality of turns before exiting the exit valve 364 f'or providing an inner cooling zone f'or the fixed platen. A fixed platen intermediate cooling channel is at 3~0 shown having its input valve at 372 and its exit valve at 374. The intermediate region cooling channel provides a second cooling zone to the fixed platen.
A fixed platen outer reglon cooling channel 380 has its input valve at 382 and its exit valve at 384. The flxed platen outer region cooling channel provides a further cooling zone to the ~ixed platen.
Referring to Figures 2 and 7, there is shown a plurality of cooling zones associated with the moving mold hal~ 14. A moving platen, punch region cooling channel is shown at 391 having an input valve at 393 and an exlt valve at 395. This punch region cooling channel makes one turn around the punch region as shown at 390a with reference to Figure 2 and then exits the exit valve at 395. The hottest portion of the tool 10 is at the sprue and punch region where the molten material enters from the injection molding machine.
A moving platen inner region cooling channel ls shown at 400~ having an input valve at 402 and an exit valve at 404. The moving platen inner region cooling channel makes a number ~ turns around the platen before exiting through the exit valve 404. The moving platen inner region cooling zone provides a further cooling zone f'or the moving platen. A moving platen intermediate region cooling channel 410 is shown having an input valve at 412 and an exit valve shown at 414. The moving platen intermediate region cooling channel provides a further cooling zone for the moving platen. A moving platen outer region cool-ing zone 420 is shown having its ~put valve at 422 and the output valve at 424. The moving platen outer reglon cooling channel provides for the coollng zone for the moving platen. Any suitable cooling fluidg including water, can 'be utilized in any one or all cooling zones.
Both the moving platen 140 and the fixed platen 120 are cored in the manner shown with refer-ence to Figures 1 and 2 to provide for the plurality of cooling channels previously described. A pair of 0-rlngs are provided at 366 and 368 to provide a fluid tight connection for the ~ixed platen sprue region cooling channel 357 o~ the fixed mold half 12.
A second pair of 0-rings are provided at 390 and 3~2 20 to provide a fluid tight connection for the plurality of cooling zones 3609 370 and 380 formed in the fixed platen 120. A further set of 0-rings are provided at 394 and 396 to provide a fluid tight connection for the plurality of cooling zones 400g 410 and 420 f`ormed 25 in the moving platen 140. Another set of 0-rings are provided at 430 and 432 to provide a fluid tight connection for the moving platen punch region cooling channel 391.
Referring to Fi~ure 7g the moving platen 140 3o includes a plurality of transvérse cooling channels.
A f'irst moving platen transverse cooling channel is sho7~n at 440 having its input valve at 442 and -lts eæit valve at 444. A second moving platen transverse cool-ing channel ls shown at 446 having its ~put valve at 44~ and its exit valve at 450.
Ref'err~ng to Figure ~g the fi~ed platen 120 includes a plurality of transverse cooling channels.
A first ~ixed platen transverse cooling channel is sho7~n at 452 having its input valve at 454 and its exit , ., ~

-2~-valve ak 455. r~ S econd ~-lxecl platen transverse cool-lng channel is shown at 45~ havlng its input valve at L~60 and lts exit valve at L~62.
It is important to note that a high degree Or concentricity between the record spiral krack and khe record center hole is obtainecl because both operations, the formakion of the splrally tracked record and punch-lng o~ the sprue to de~ine the record center hole, are perfo~med in a slngle set-up. In the set-up, the -Lnner sur:~aces on the stamper cooperate wlth the correspond-lng mating surfaces on the mold to obtain highly accur-ate poslkioning of the stamper on the mold. The outer surface of khe skamper is allowed to float as previously dlscussed with reference to Figure 10. Similarlyg the punch peripheral surface which defines the record center hole is very precisely located relative to the making surfaces on the flxed center stamper clamp 12~.
Slnce both the skamper~ which forms the record spiral ! krack~ and the punch per~pheral surfaceg which defines the record center holeJ are very carefully aligned with respect ko the mold, a high degree of concenkricity between the record spiral track and the record center hole is obkained~

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:' '

Claims (4)

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

1. An injection molding apparatus for molding centrally apertured parts, said apparatus comprising: a port for receiving molten material, an annular cavity formed by first and second mold halves into which the molten material is injected, and an injection gate positioned intermediate said port and said cavity, said injection gate including a disc shaped passage whose height decreases with increasing radius such that the molten material flows thereacross at a substantially constant rate.

2. The apparatus as claimed in Claim 1, wherein said disc shaped passage includes a plurality of annular shaped sections for regulating the pressure of the molten mixture as the mixture is entering the cavity for providing a stress free molten mixture within said cavity.

3. The apparatus as claimed in Claim 2, wherein the annular shaped gate passage further comprises: a first annular section having a first end and a second end and having a uniform radial length and having a gradually diminishing thickness across its radial length beginning at said first end and continuing to said second end, and said first end being positioned to receive the injected molten mixture from said port for providing a partial reservoir for said molten mixture as it abruptly leaves said port and for removing a portion of the stress carried within the molten mixture due to the abrupt change; a second annular section having a first end and a second end and having a uniform radial length and having a uniform thickness across its radial length, and said first and being positioned to receive the injected molten mixture from said second end of said first annular passage for providing a further reservoir for said molten mixture and for establishing a charge of said molten mixture of dimensional stability with the dimension of the cavity; a third annular section having a first end and a second end, and having a uniform radial length and having a gradually diminishing thickness across its radial length beginning at said first end and continuing to said second end, and said first and being positioned to receive the injected molten mixture from said second end of said second annular section for partially restricting the free flow of molten mixture from said reservoir Into the cavity for reducing stress forces in the molten mixture injected into said cavity; and a fourth annular section having a first end and a second and, and having a uniform radial length and having a uniform thickness across its radial length, and said first end being positioned to receive the injected molten mixture from said second and of said third annular section and said second end being positioned to inject said molten mixture into the cavity with minimun disturbance,

4. The apparatus as claimed in Claim 1, wherein the height of said disc shaped passage decreases with increasing radius to maintain an essentially constant radial cross sectional area in said disc-shaped passage.