Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Changing mold size or shape during molding or with shrinkage...
Reexamination Certificate
1999-06-07
2001-09-18
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Changing mold size or shape during molding or with shrinkage...
C264S040600, C264S328700, C264S328800, C264S328160, C425S144000, C425S162000, C425S548000, C425S552000, C425S555000, C425S573000, C425S808000, C425S810000
Reexamination Certificate
active
06290882
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method and apparatus for thermoplastic injection molding using multiple substantially-opposing edge gates, to mold hard-to-fill product configurations without objectionable knitlines or unsatisfactory molded-surface replication, by starting each molding cycle with mold surfaces heated with circulating heat transfer fluid to retard melt solidification before starting injection, then cooling with circulating heat transfer fluid of a lower temperature source after the mold is filled to solidify the thermoplastic before opening the mold.
BACKGROUND OF THE INVENTION
Optical moldings are particularly limited by the prior art. The present invention may be used in optical-grade thermoplastic molded products such as edge-gated Rx spectacle lens, plano eyewear, instrument optical lenses & prisms, flat-panel display lenses, information bearing optical data cards, halographic displays, reflective and/or transmissive optics, precision molded plastic mirrors, refractive optical elements with a light bending function using multiple concentrically arrayed facets (such as fresnels) or multiple molded lenslet arrays. Even very large shapes such as fresnel- or mirror-type solar collector panels, or front/rear projection screens, or automotive windows could be uses.
The limitations learned from the below-mentioned patents drove us to look at ways to overcome filling problems. Typical computer simulation software would show filling problems, if the meltflow pathlength was too long, or the aspect ratio is too high. Such product configurations have a large aspect ratio for filling, defined as the length of the meltflow pathlength divided by the cross-sectional thickness. Conventional alternatives all have drawbacks if the mold cavity has just 1 gate, even if centrally located. Substituting lower melt viscosity resin with less molecular weight or less reinforcement gives poorer properties to the molded part. Substituting thicker cross-sectioned part gives slower cycle time & higher material costs to the molded part. lower melt viscosity resin or thicker molecular weight. Thus, in search of a way to get shorter meltflow pathlength and lower aspect ratios, multiple gates can be spaced out along the perimeter of the mold cavity. However, when the resulting multiple meltfronts converge and intersect, cosmetically unacceptable knitlines (visible surface flaws) and weldlines (internal weak-spots of poor mechanical strength are not created.
One approach to this problem is multi-gated sequential filling of injection molds for large-surface-area parts and/or thinwalled parts. Flow lengths are now shortened, but the knitlines can be avoided by opening one valve gate at a time. In this sequential fill, the meltfront from the first-to-be-opened gate must pass by the location of the second-to-be-opened gate before this second gate is opened, and so on. As each subsequent gate opens, its melt blends into melt from the previously-opened gates to ideally provide a single smoothly-flowing meltfront driven by multiple short-flowpath gates. No gate is opened to injection before the single smoothly-flowing meltfront has swept by, thus avoiding multiple meltfronts converging and intersecting in knitlines. A recent example of multi-gated sequential filling in Betters et al (U.S. Pat. No. 5,762,855 issued Jun. 9, 1998). It uses mechanically closed valve gates to keep the melt within the next shot at desirably high pressurizations, to avoid splay and other surface defects. It mentions briefly . . . “improved knitline appearance” . . . on column 2, line 18 without further elaboration or support. Another example of multi-gated sequential filling is Hunerberg et al (U.S. Pat. No. 5,135,703 issued Aug. 4, 1992), which also comprises gas injection behind the moving meltfront. Multi-gated sequential filling is reportedly successful in thinwalling opaque electronic housings (i.e. cellphones and laptops) and hard-to-fill large opaque automotive moldings (i.e. bumpers and body panels), but no known optical lenses use it.
A different prior art approach teaches to allow knitlines and weldlines to form, but then to use a plurality of substantially opposing gates to alternately pressurize and depressurize the melt, in coordination with ohe another. So, when one gate is acting to pressurize against the melt, its opposing gate is depressurizing. Then in accordance with a programmed control, they switch roles. And so on, such that at the original intersection of the 2 opposing meltfronts, shearing forces may cause molecular entanglements while the melt is still mobile. Such reciprocating “push-pull” forces are believed to strengthen the weldlines (internal weak-spots of poor mechanical strength) and improved fiber reinforcement orientation. One of the better-known such “multi live feed” approaches is offered by Cinpres, (Allan et al, U.S. Pat. No. 4,925,161 issued May 15, 1990, U.S. Pat. No. 5,156,858 issued Oct. 20, 1992 and U.S. Pat. No. 5,160,466 issued Nov. 3, 1992), in its “Scorim” process available for licensing. Similar in effect but requiring 2 separate injection barrels to implement in Klockner's approach, per Gutjahr et al (U.S. Pat. No. 4,994,220 issued Feb. 19, 1991), dealing with improved orientation of liquid chrystal polymers. Also similar in effect but perhaps with simpler hardware (requires only injection barrel to implement, and capable of running multiple mold cavities) is Husky's approach, per Arnott (U.S. Pat. No. 5,069,840 method patent issued Dec. 3, 1991; U.S. Pat. No. 5,192,555 apparatus patent issued Mar. 9, 1993). Thermold employs an accumulator between plastication and mold, in Ibar (U.S. Pat. No. 5,605,707 issued Feb. 25, 1997). Other newer ones attempting to be similar in effect by trying to act locally upon just the weldline include Groleau (U.S. Pat. No. 5,766,654 issued Jun. 16, 1998), and Gardner et al (U.S. Pat. No. 5,538,413 issued Jul. 23, 1996) both employing reciprocating “packing pins” located beneath the weldline to pulse, and Furugohri et al (U.S. Pat. No. 5,225,136 issued Jul. 6, 1993), employing a “well” as a controllable reservoir for molten resin, located between the gate and the weldline, to . . . “cause migration of the resin at the weld”. However, it is believed that none of these “multi live feed” approaches are successfully employed to the optical lens molder's problems of how to eliminate cosmetically unacceptable knitlines (surface flaws) on the usable portion of transparent amorphous thermoplastic molded lenses. Inasmuch as each of these “multi live feed” approaches still predicates that the multiple meltfronts converge and intersect, they can only remedy the weldlines (internal weak-spots of poor mechanical strength) after they are now created.
All of the above-mentioned multi-gated sequential filling and “multi live feed” approaches are still running substantially isothermally, with respect to the measured temperature of the injection mold cavity blocks and circulating coolant therein. That is to say, those metal mold temperatures and coolant temperatures are always set well below the glass transition temperature Tg of the amorphous thermoplastic throughout the whole injection molding cycle.
Such is also true of the only prior art reference known to Applicants, wherein at least 2 opposing gates are employed to feed a transparent amorphous thermoplastic melt into a single mold cavity, to improve filling and packing of an optically-functioning molding. Kanewske III et al (U.S. Pat. No. 5,376,313 issued Dec. 27, 1994) is molding a lowcost disposible testtube-shaped plastic assay cuvette from . . . “acrylic, polystyrene, styrene-acrylontrile, polycarbonate . . . ” (col. 9, ln 51-2) at . . . “temperature of the mold cavity 508 is preferably from between about 100 F. and about 140 F.; and the temperature of the mold core 504 is preferably from between about 60 F. and about 100 F.” . . . (col. 9, ln 63-67), all of which are very far below the Tg of any of their resins mentioned. In order to get the desired lo
Filipe John R.
Galic George J.
Maus Steven M.
Galic Maus Ventures LLP
Vargot Mathieu D.
LandOfFree
Reduced-knitline thermoplastic injection molding using... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reduced-knitline thermoplastic injection molding using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reduced-knitline thermoplastic injection molding using... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2453074