Reduced-knitline thermoplastic injection molding using multi-gated non-sequential-fill method and apparatus, with a heating phase and a cooling phase in each molding cycle

Hard to fill large-surface-area parts and/or thinwalled configurations of optical lenses and reflective optical elements are among the difficult-to-mold thermoplastic products which require precision replication of the molding surfaces, in micro detail. We combine multiple opposing gates (to reduce the meltflow pathlength) with non-isothermal steps of firstly, heating these mold surfaces (with circulating heat transfer fluids supplied by a hot side supply system, to a temperature setpoint sufficiently high to retard solidification), then secondly, injecting the melt through these opposing gates, then thirdly, rapidly cooling to solidification (by circulating heat transfer fluids of much lower temperature, supplied from a cold side supply system). To run fastest injection molding cycle time, the fast heating phase comes from combining high-thermal-conductivity copper alloy mold cavity materials with very large thermal driving force (high "delta T") from high-heat-transfer-rate fluids (preferably, steam). Fluid control units and electronic process sequence control units are interconnected to govern the flow of these fluids into and out of the injection mold and the supply systems in accordance with the predetermined logic of the process flowsheet. To reduce geometric constrictiveness during filling of the mold cavity, an injection compression process sequence with pre-enlarged variable-volume mold cavity can be created before injection starts, then compressed toward original dimensions during injection.

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 low levels of molded-in stresses in
 the "optical read region" (through which absorbance of a known light beam
 is measured), this patent says that it was necessary to locate the gate at
 least some minimum distance away from "optical read region", and since
 this reduces ease of filling, the preferred embodiment employs at least 2
 such gates at substantially opposing locations with substantially
 simultaneous injection into both (presumably, to maintain symmetry). This
 patent is silent on the presence of a knitline, but those skilled in the
 art would expect a very long knitline.
 Let us now move away from these prior art approaches which are still
 running substantially isothermally. Various "non-isothermal" prior art
 approaches have already been previously cited by Applicants in our U.S.
 Pat. No. 5,376,317 (Maus et al) issued Dec. 27, 1994, incorporated herein
 by reference. In the USPTO examination of U.S. Pat. No. 5,376,317, the
 closest prior art was determined to be Muller (U.S. Pat. No. 4,963,312
 (method) issued Oct. 16, 1990 and U.S. Pat. No. 5,055,025 issued Oct. 8,
 1991 (apparatus)). Muller is drawn to thinwall packaging molding, which
 has somewhat different problems than Applicants' focus on optical lens and
 quality of microstructured replicated surfaces. Like Applicants, Muller
 heats the mold surface before injection, so as to overcome the melt's
 resistance to flow and to improve filling of its difficult product
 configurations. Unlike Applicants, Muller apparently constructed his mold
 cavity members without regard to their heat transfer rates (he is silent
 on his materials of construction; we assume tool steels), so to achieve
 fast cycle times, he chose to make his mold cavity members very thin,
 which then required him to stop flowing his circulating heat carrier and
 hold it at maximum static pressure . . . "during injection of plastic . .
 . for supporting the thinwalled members" (see his Abstract) against
 unwanted mechanical deflection. Applicants solved this problem by our
 choice of materials of construction and sufficient thicknesses of same for
 proper loadbearing, so our heat transfer fluid can continue flowing
 through the mold during filling and packing phases of the molding cycle.
 In addition to those cited "non-isothermal" prior art approaches, there are
 a couple newer ones which need to be commented on.
 Yamaguchi et al (U.S. Pat. No. 5,399,303 issued Mar. 21, 1995) is drawn to
 a similar field--optical lenses--as Applicants, but with a very different
 sequence of method steps . . . "filling resin in a metal mold held at a
 temperature lower than a glass transition temperature of the resin, then
 pressurizing the resin under comparatively high pressure so as to expedite
 hardening of the resin by raising its glass transition temperature, then
 reducing the pressurization of the resin to comparatively low pressure,
 and increasing, generally in association, the temperature of the mold
 cavity surface higher than the glass transition temperature of the resin
 so as to form a molten layer on a front face of the resin surface, and
 finally raising the pressurization of the resin to a medium pressure and
 lowering, generally in association, the cavity surface temperature of the
 metal mold so a to reduce the temperature for the withdrawal of the
 product" . . . (see Yamaguchi's Abstract). In this sequence of steps,
 Yamaguchi resembles the Uehara (U.S. Pat. No. 5,093,049 issued March 1992)
 cited in our U.S. Pat. No. 5,376,317. By this sequence of steps, Yamaguchi
 does nothing to prevent the formation of knitlines or weldlines, were he
 to have been running myopic-prescription polycarbonate Rx spectacle lenses
 having thin centers and thick edges. Once the knitline is formed on the
 molded lens surface, merely re-heating per Yamaguchi . . . "so as to form
 a molten layer on a front face of the resin surface" . . . will not be
 adequate to remove the objectionable knitline. Nor does Yamaguchi teach
 the benefits of reducing geometric resistance to filling (like Applicants'
 use of variable volume mold cavity with injection compression sequence),
 nor reducing meltflow pathlength (like Applicants' use of opposing gates).
 Byon (U.S. Pat. No. 5,762,972 issued Jun. 9, 1998) is not drawn to a
 similar field--optical lenses--as Applicants, but rather general problems
 of filling. Byon heats the mold before injection (by induction) to improve
 the fluidity of the resin melt, so there is no need to increase injection
 pressure. He does so by induction because it is faster than by the heat
 pipes employed in his cited prior art. Only very briefly is there any
 mention of weldlines, and no mention of knitlines . . . the fluidity of
 the resin fluid filling up the cavity is increased. In addition to these,
 the luster of the product is enhanced while reducing flow marks, weld
 lines, etc." (col. 4, ln 22-24). Byon is silent on the criticality of
 heating the mold to at least the Tg. Apparently, he is satisfied by . . .
 "The rise in mold temperature inhibits the cooling of the flowing resin
 fluid to maintain the fluidity of the resin fluid, so that there is no
 need to increase the injection pressure" . . . (col. 2, ln 1-3). Also,
 Byon does not teach the benefits of reducing geometric resistance to
 filling (like Applicants' use of variable volume mold cavity with
 injection compression sequence), nor reducing meltflow pathlength (like
 Applicants' use of opposing gates).
 SUMMARY OF INVENTION
 The present invention is 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 microstructured 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 thermoplasic before
 opening the mold. Applicants' present invention is believed to be an
 improvement on their previous invention, U.S. Pat. No. 5,376,317 (Maus et
 al) issued Dec. 27, 1994, incorporated herein by reference, from which
 several elements are drawn. This 1994 patent employs the concept of
 initially injecting molten thermoplastic into a mold cavity having surface
 temperature greater than the glass transition temperature (Tg)--for an
 amorphous thermoplastic polymer--or above the melting temperatures (Tm)
 for a--crystalline thermoplastic polymer. This 1994 patent has already
 been put to successful commercial use for optically transparent amorphous
 thermoplastics injection molded into certain optical lens and reflective
 optical elements. It has specifically shown enhanced quality of
 microstructured replicated surfaces. Compared to the conventional state of
 art, improved filling and packing (to gain best possible replication of
 mold surface microstructure) have been achieved. However, Applicants' 1994
 U.S. Pat. No. 5,376,317 still has limitations in molding hard-to-fill
 product configurations without objectionable knitlines or unsatisfactory
 lack of fidelity in molded-surface replication. Large-surface-area parts
 and/or thinwalled parts which cannot be center-gated are hard to fill.
 Such configurations have a large aspect ratio for filling, defined as the
 length of the meltflow pathlength divided by the cross-sectional
 thickness.
 The problem of hard to fill large-surface-area parts and/or thinwalled
 configurations was earlier addressed in part by Applicants' previous
 invention, U.S. Pat. No. 4,828,769 (Maus et al) issued May 9, 1989, also
 incorporated herein by reference. See FIG. 20, and columns 4-6. This
 method patent and its corresponding apparatus patent (U.S. Pat. No.
 4,900,242 to Maus et al issued Feb. 13, 1990, also incorporated herein by
 reference) have also already been put to successful commercial use for
 optically transparent thermoplastics injection molded into certain optical
 lens and discs. These 2 patents use an injection compression process
 sequence wherein a pre-enlarged variable volume mold cavity is first
 created before injection starts, then this pre-enlarged variable volume
 mold cavity is secondly compressed toward original dimensions during
 injection. Increasing the mold cavity's height dimension by means of
 increasing distance between the opposing mold cavity surfaces will
 decrease aspect ratio during filling of said mold cavity and thereby
 reduce geometric constrictiveness during filling of the mold cavity. In
 one example, Applicants' 2 patents have been commercially successful in a
 center-gated example configuration of the newest DVD disc, mold to only
 half the normal CD thickness (0.6 mm vs. 1.2 mm), with a proportionately
 worse aspect ratio (120:1 for DVD vs. 60:1 for CD discs). In a different
 example, Applicants' 2 patents have also been commercially successful in
 an edge-gated example configuration, for myopic-prescription polycarbonate
 Rx spectacle lenses having thin centers (1.0 to 1.5 mm) and thick edges
 (7.0 to &gt;10 mm), with minimal knitlines. However, even when Applicants'
 use of pre-enlarged variable volume mold cavity combined with
 injection-compression process, to reduce aspect ratio (as perceived by the
 melf during filling) in this application, a significant knitline (3 mm to
 8 mm) can be seen opposite the gate at the lens' O.D. with the naked eye,
 and this knitline must then be cut away when the round molded lens blanks
 are cut down to fit into the spectacle frame. So, these 2 Applicants'
 injection-compression patents also have limitations in large-surface-area
 parts and/or thinwalled parts. So, the limitations learned from these 3
 above-mentioned Applicants' patents drove us to look at other ways to
 overcome filling problems.
 To summarize, Applicants now combine multiple opposing gates (to reduce the
 meltflow pathlength and thereby reduce aspect ratio) with non-isothermal
 steps of firstly, heating these mold surfaces (with circulating heat
 transfer fluids supplied by a hot side supply system, to a temperature
 setpoint sufficiently high to retard solidification), then secondly,
 injecting the melt through these opposing gates, then thirdly, rapidly
 cooling to solidification (by circulating heat transfer fluids of much
 lower temperature, supplied from a cold side supply system). Each
 injection molding cycle thus starts with a heating phase, wherein the fast
 rise in mold surface temperature comes from a combination of
 high-thermal-conductivity metal (preferably, copper alloy) mold cavity
 materials, plus a very large thermal driving force ("delta T") being
 supplied by the hot side supply system fluid (preferably, steam). Ideally,
 this fluid's temperatures are well above the melt-solidifying temperatures
 (Tg or Tm) characteristic to the thermoplastic. The heating phase and
 injection is then followed by a fast cooling phase, wherein molding
 surface temperature decrease is thermally driven (preferably also a large
 "delta T") by cold side supply system fluid (preferably cold water)
 temperatures well below the melt-solidifying temperatures (Tg or Tm)
 characteristic to the thermoplastic. The greater these temperature
 differences ("delta T") are, the faster this "non-isothermal" molding
 cycle will be.
 To overcome the problems of poor mold surface replication by the molded
 thermoplastic article and, more specifically, to be able to maximize
 microreplication of the finest surface detail and contour (i.e. the
 fidelity of the molded part to the molding surface) we heat the mold
 cavity part forming surfaces at least above a characteristic solid-liquid
 phase-change temperature which is characteristic of the thermoplastic
 polymer. Since the most desirable of optically-clear thermoplastic
 polymers are amorphous in nature (especially, polycarbonate and acrylic),
 the preferred setpoint could be the glass transition temperature (Tg).
 (For crystalline thermoplastic polymers, another melt-state temperature
 would be its melting point (Tm). For either type of polymer, the preferred
 setpoint temperature would be sufficiently high so that the thermoplastic
 being molded is not form stable at any higher temperatures, then after the
 mold cavity has at least been completely filled by the molten
 thermoplastic and before the mold is opened at the parting line, mold
 surface temperature is dropped to below the Tg or Tm of that crystalline
 thermoplastic material. Although most optical uses do not use crystalline
 thermoplastics, new catalysts for polyolefins are changing their prior
 limitations. One possible use may be to mold the microstructured surface
 of large front/rear projection screens.)
 Reduction of "knitlines" (and to a lesser extent, "weld lines") is another
 benefit of the present invention, as is the ability to mold large
 surface-projected-area parts in smaller injection molding machines having
 less clamp tonnage (due to less "brute force" high injection pressures
 being used to fill), less "wedge" (non-uniform part thickness) problems
 and more uniform packing and shrinkage (due to more balanced fill pattern
 and shorter flowpaths). Preferably, the mold surface temperature is
 maintained above Tg or Tm until the mold cavity is not only filled with
 melt but is sufficiently pressurized to reach a peak value for melt
 pressure as measured within the mold cavity. This preferred embodiment
 retards solidification of the plastic onto the mold surfaces at least
 until the maximum packing pressures withing the cavity has been attained,
 thereby forcing a still-mobile polymer molecule against the micro detail
 of the partforming surfaces of the mold cavity construction. Retarded
 solidification during mold cavity filling also minimizes the well-known
 problem of high melt pressure near the gate and much lower melt pressure
 near the end-of-fill cavity wall; this "hydraulic melt pressure drop"
 incurred during conventional injection filling causes a corresponding
 difference in volumetric % shrinkage and resulting mechanical inaccuracies
 and warpages.
 From the plastic's perspective, it sees only the mold surface temperature;
 however, in operating the molding hardware to perform the present
 invention, one must take into consideration the existence of time delay
 between changes made to the apparatus and/or heat transfer media, before
 actual mold surface temperature changes occur. Therefore, a process
 sequence chart of mold surface temperature changes will inevitably lag at
 least a bit from the actual changes made which provide the driving forces
 for changes in the mold surface temperature. Larger thermodynamic driving
 forces reduce this delay time and minimize total molding cycle time when
 maximum temperature differentials exist between the heat transfer media
 and the mold surface temperature. Therefore, the preferred embodiments of
 the present invention deliberately maintain a high temperature
 differential ("delta T") as a means of speeding response time and
 converting changes made by the molding apparatus into changes perceived by
 the plastic (by means of changes induced in the molding surface
 temperatures, accordingly).
 In order to start the molding cycle at an optimally hot mold surface
 temperature and subsequently to drop rapidly that mold surface temperature
 to quicken solidification after a predetermined point in the molding cycle
 is reached, and thereby to minimize a total molding cycle time while also
 maximizing molding productivity and output quality, the present invention
 necessarily must consider mold materials construction which have
 relatively good thermal conductivity and heat transfer coefficients, as
 well as thermal diffusivity (i.e., to minimize point-to-point temperature
 non-uniformities within the molding surface). Certain tool steels, for
 example, are substantially better in heat transfer than others; high alloy
 content steels such as stainless steels would NOT be preferred. For
 example, substituting a low alloy steel such as H13 or P20 in place of a
 high alloy stainless steel such as 420 grade provides some improvement;
 polishability and tarnish resistance of such non stainless steels can be
 enhanced by means of electroplated nickel or chromium coatings.
 However, even higher productivity is achieved in a preferred embodiment
 using copper based alloys such as copper bronze (Ampcoloy 940 (tm) from
 Ampco) and beryllium copper alloys (Moldmax (tm) or Protherm (tm) alloys
 from Brush Wellman), or hardened aluminum alloys. One preferred optical
 mold construction would be of the type disclosed and claimed in applicants
 U.S. Pat. No. 4,793,953 issued Dec. 27, 1988, incorporated herein by
 reference. These preferred molds for optical thermoplastic high pressure
 molding combine the high surface polishability and mechanical damage
 resistance of electroplated nickel or chromium at the outward face of the
 mold cavity mold cavity support block with a high conductivity substrate
 metal such as low alloy beryllium copper (typically 2% or less of
 beryllium, and 98% or more copper) or similar copper alloys, although
 conceivably other well known high conductivity metals such as aluminum
 alloys could be used. (Similarly, precious metals such as silver and gold
 could conceivably be functional equivalents but too soft and too
 expensive). By joining the desired surface qualities of the plating with
 the desired mechanical (typically, a compressive yield strength of at
 least 50,000 psi is required for satisfactory injection mold cavity mold
 cavity support blocks, and 70,000-100,000 psi is preferred) and
 thermodynamic properties of the substrate, a resulting monolithic mold
 mold cavity support block well suited for operating the present invention
 process sequence is obtained. Another specially preferred combination uses
 Protherm (brandname from Brush Wellman, for genetic C17510 BeCu alloy)
 with a hard, wear-resistance TiN (titanium nitride) surface coating.
 (We earlier discussed prior art Muller U.S. Pat. Nos. 4,963,312 &
 5,055,025. He constructed his mold cavity members without regard to their
 heat transfer rates, so to achieve fast cycle times for non-optical
 thinwall packaging moldings, he must make his mold cavity members very
 thin, which then required him to stop flowing his circulating heat carrier
 and hold it at maximum static pressure to support his thinwalled members
 against unwanted mechanical deflection. Applicants solved that problem by
 our choice of materials of construction and sufficient thicknesses of same
 for proper loadbearing, so our heat transfer fluid can continue flowing
 through the mold during filling and packing phases of the molding cycle.
 It depends on the specific alloy, but the critical thickness dimension can
 be as little as 0.250 inch of metal, measured between the actual molding
 surface and the deepest cut of machined-in cooling channel.)

DETAILED DESCRIPTION OF THE INVENTION
 Refer to FIG. 1. An injection mold of the present invention is shown in
 cross-section view. Its movable half (2) is mounted onto the movable
 platen of the injection molding machine (not shown), which operates a
 conventional ejector assembly, consisting of ejector pins (5) located
 under each gate (for clean push-out without contacting optically-usable
 areas of the molded part) and held by ejector cover plate (25) and ejector
 plate (24). Its fixed half (1) is mounted onto the stationary platen of
 the injection molding machine (not shown), whose nozzle is in fluid
 communication with the mold's melt channel (15) within the hot runner
 manifold (13), and melt enters the mold cavity (6) through opposing gates
 (7) & (8). At the plane of the parting line (Section 1-1), the mold cavity
 (6) is formed by opposing mold cavity support blocks, (18) on the fixed
 moldhalf & (31) on the movable moldhalf.
 On each injection molding cycle, the mold is opened and closed along the
 parting line by an injection molding machine (not shown), guided by leader
 pins (3). The mold cavity support blocks (18) & (31) are preferably made
 of the copper alloys, earlier described in more detail. The mold cavity
 support blocks are mechanically supported and housed within a stationary
 moldhalf (1) and a moveable moldhalf (2). Note that for optimum results it
 is desired to create dead air space (19), to the extent possible without
 giving poor mechanical rigidity/support to the mold assembly, between the
 mold cavity support block (18) and its supporting mold half (1) by means
 of thermal isolation. Importantly, note that whereas the mold cavity
 support block's (31) heat transfer fluid circulation channels (27) are
 alternately filled with relatively hotter and relatively colder heat
 transfer fluids within each individual molding cycle, the supporting
 moldhalf (2) is fitted with flow channels (17) through which heat transfer
 fluid may also flow, but that these moldhalf flow channels are preferably
 supplied by an independent source (not shown) of heat transfer fluid
 maintained, to the extent possible, at a constant temperature
 ("isothermal") throughout the whole molding cycle. This constant
 temperature is characteristically less than the heat transfer fluid
 temperature flowing in mold cavity support block channel (31) at the
 "heating phase" (at the start of each individual molding cycle), but that
 the fluid temperature in moldhalf flow channel (17) will be greater than
 the heat transfer fluid temperature within molding cavity mold cavity
 support block flow channel (27) after the process control unit has
 transferred from the "heating phase" to a "cooling phase". An alternate
 but less preferred embodiment not shown in FIG. 1 has NOT located the heat
 transfer fluid circulation channels within the opposing mold cavity mold
 cavity support blocks (18) & (31), but rather within one or more support
 plates having a mating-contoured-surface onto which opposing mold cavity
 mold cavity support blocks can be removably mounted (for easier
 changeover), whereby intimate thermal contact is maintained therebetween.
 Such alternate but less preferred embodiment will generally have slower
 heat transfer and poorer thermal diffusivity, of course.
 In the case of optical lenses, their molding surface is typically highly
 lapped and smoothly polished surfaces which may be electroplated with
 chrome or nickel, or vacuum hardcoated with TiN toolcoatings for wear
 resistance. Alternatively, where a microstructure consisting of a
 repeating pattern of prismatic elements or spherical lenslets is desired
 to be replicated onto the molded part, at least one of the molding
 surfaces (or both) may be firstly plated with several thousandths of an
 inch thickness of an electroless nickel (preferred composition: greater
 than 12% P), then secondly, using diamond turning to machine into the
 electroless nickel plated layer the desired mirror-image microstructure
 (adding in a % shrinkage factor to such calculations). Yet another
 especially preferred embodiment of the present invention has at least one
 (and possibly both) inward-to-the-mold cavity faces of the support blocks
 fitted with a thin nickel electroform (removably mounted thereto), which
 then acts as a partforming surface onto which the molded plastic will
 replicate itself. Such nickel electroform "stampers" (not shown) can vary
 widely in geometry and thickness. Typically for the essentially planar
 microstructure with micron-sized depths, such nickel electroform
 "stampers" are only 0.012" thick (0.3 mm)). But extremely deep
 microstructure such as certain fresnel concentrically-faceted lenses with
 large facet depths may need to have much thicker nickel electroform
 "stampers".
 Consider an example of how the present invention solves the problems that
 the prior art cannot. "Underfill" (lack of fidelity) at the very tips of
 the dead-sharp "knife edge" facets on any fresnel lens configuration will
 produce poor optical transmission, a functional (not cosmetic) problem.
 The problem is seen when the lens is cut into cross-section, then these
 tips are examined under the microscope. Any measurable "radiusing" or
 rounding of what is supposed to be a sharp "knife edge" facet tip is
 unacceptable, its light-bending performance will be inadequate to meet the
 design intent. Under the best of conditions, injected melt will have a
 hard time to fully fill out or replicate such a deeply grooved sharp
 corner configuration. For this reason, prior art ways for injection
 molding always failed this test, and so such parts needed to be
 compression molded, which takes very long cycle times and is more labor
 intensive. Then came Applicant's U.S. Pat. No. 5,376,317. With it,
 less-deeply-faceted fresnel concentrically-faceted lenses of smaller size
 could be molded on short automatic cycles in a conventional injection
 molding machine, not a compression press manually loaded & unloaded by an
 operator. However, even with it, more-deeply-faceted fresnel
 concentrically-faceted lenses of a large size (perhaps 1 foot square or
 larger) could not be completely successful in avoiding such "underfill" at
 the very tips of the "knife edge" facets. This was found to be especially
 true for the 1/2 of the meltflow pathlength which is away from the single
 edge-gate (center gating isn't possible for any fresnel
 concentrically-faceted lens configuration, as the very center must be both
 cosmetically and optically perfect). Further examination in such
 underfilled tips of more-deeply-faceted fresnel concentrically-faceted
 lenses of a large size showed proper filling was easier for the 1/2 of the
 meltflow pathlength which is nearest to the single edge-gate. Thus, the
 cross-sectional asymmetry of the space between the facets of such fresnel
 concentrically-faceted lens is found to be easier to fill when the
 meltfront comes from the vertical side of the facet, and conversely harder
 to fill when the meltfront comes from the angled non-vertical side of the
 facet. So, for any single edge-gated prior art approach, even Applicant's
 U.S. Pat. No. 5,376,317 this underfill problem of imprecise surface detail
 was found for the 1/2 of the meltflow pathlength which is farthest away
 from the single edge-gate. But if one were to try to simply solve this
 problem by adding a second edge-gate at a location across the mold cavity
 from the first edge-gate, then a very visible surface flaw which is
 cosmetically rejected is always formed where these 2 meltfronts meet and
 form their knitline. This was found to be true of any "isothermal"
 process, even when higher mold & coolant temperatures (still below Tg),
 higher melt temperatures, faster injection rates, and higher packing
 pressures were used. Also, even when Applicant's U.S. Pat. No. 4,828,769
 pre-enlarged variable-volume mold cavity type of injection-compression
 molding process was run, still the same problem. However, when Applicants
 experimentally combined this addition of a second edge-gate at a location
 across the mold cavity from the first edge-gate, then ran the
 "non-isothermal" process steps of Applicant's U.S. Pat. No. 5,376,317 with
 substantially simultaneous injection through both gates (a preferred
 embodiment), the previous underfill problem of imprecision surface detail
 was no longer found for the 1/2 of the meltflow pathlength which is
 farthest away from the original single edge-gate, and no visible surface
 flaw or knitline (when inspected under normal lighting with the unaided
 eye, without using magnifying aids) was found, and so these lenses were
 not cosmetically rejectable. Only under polarized light was an internal
 weldline seen, where these 2 meltfronts meet.
 FIG. 2 shows in a plan view (into an open parting line) this single-cavity
 injection mold of the present invention, with 2 opposing edge gates and
 machined-in mold cooling channels for circulating heat transfer fluid. For
 our example of the more-deeply-faceted fresnel concentrically-faceted
 (pictured as 2 concentric dotted-line rings) lenses of a large size, we
 show the original single edge-gate (7), in a fan-gate design (located over
 ejector pin (5)), and addition of a second edge-gate (8) at a location
 across the mold cavity from the first edge-gate. For best results, both
 gates should inject a balanced (substantially equal) flowrate of melt and
 be started substantially simultaneously through both gates. As shown in
 this example, at the ideal balanced fill from 2 gates, the first
 intersection of the 2 meltfronts will be located at the center of the
 fresnel concentrically-faceted lens, then this juncture of merged
 meltfronts moved outward toward the perimeter, as shown, at a point more
 than halfway through the filling step. This is a preferred way, because it
 eliminates the problem of trapped gases as the lens' very center, which
 must be both cosmetically and optically perfect. So, this preferred
 pattern of balanced filling can be seen also as a way to improve venting
 by "inside-out" filling, so as to driving gases out toward the mold cavity
 perimeter, where abundant venting is possible (vs. in its center, where
 venting passageways cannot be located, without making an objectionable
 surface flaw within the usable optical area of the molded part).
 See now FIG. 3, which shows in cross-sectional split view a preferred but
 optional embodiment of our single-cavity injection mold of the present
 invention, with its 2 opposing edge gates and machined-in mold cooling
 channels for circulating heat transfer fluid. To reduce geometric
 constrictiveness during filling of the mold cavity, an injection
 compression process sequence with pre-enlarged variable-volume mold cavity
 is shown. In the lefthand split view, the mold is seen before injection
 starts, with a greatly increased mold cavity height. In the righthand
 split view, the mold is seen after injection starts, with the mold cavity
 height having been compressed toward original dimensions during or after
 injection. Means for forming such pre-enlarged variable-volume mold cavity
 and various alternative driving forces and timing sequences for the
 compression stroke are disclosed in Applicant's U.S. Pat. Nos. 4,828,769
 (method) & 4,900,242 (apparatus), also incorporated herein by reference.
 FIGS. 2-8 show such molds in their various steps within a single molding
 cycle, and its operation described in columns 25-29. Unlike "isothermal"
 optical disc or lens molding processes described, wherein the timing
 between injection and compression steps can be critical to best optical
 and cosmetic quality of the molded part, in the present invention's
 "non-isothermal" process, it has been found that timing between injection
 and compression steps is not so critical to best optical and cosmetic
 quality of the molded part. Specifically, Applicants' U.S. Pat. No.
 4,828,769 (method) specifys that compression should be started before
 injection is ended. When a pre-enlarged variable-volume mold cavity is run
 in an injection compression process sequence under "isothermal" mold
 temperature conditions, an objectionable "parison line" is seen when
 optical disc or lens molding processes when compression was started AFTER
 injection is ended. However, in the present invention's "non-iosthermal"
 process, both types of timing between injection and compression steps can
 make satisfactory optical and cosmetic quality of the molded part. So, in
 an optical but less preferred embodiment, the compression step starts
 after injection is ended & no more melt is entering the mold cavity.
 FIG. 4 shows in a plan view (into an open parting line) a single-cavity
 injection mold of the prior art, with only 1 conventional design edge
 gate. Note the single half-moon-shaped meltfront (10) is partway through
 the filling step, as melt is being injected from the single edge gate (8).
 For our example of the more-deeply-faceted fresnel concentrically-faceted
 (pictured as 2 concentric dotted-line rings) lenses of a large size, we
 show the single edge-gate (8) in a fan-gate design. Any "isothermal"
 process will still have "underfill" at the facet tips, both facing into
 and away from the approaching meltfront, even when higher mold & coolant
 temperatures (still below Tg), higher melt temperatures, faster injection
 rates, and higher packing pressures were used. Also, even when Applicant's
 U.S. Pat. No. 4,828,769 pre-enlarged variable-volume mold cavity type of
 injection-compression molding process was run, still the same problem.
 With the "non-isothermal" process steps of Applicant's U.S. Pat. No.
 5,376,317, the underfill problem of imprecision surface detail was no
 longer found for the 1/2 of the meltflow pathlength which is closest to
 the single edge-gate, but not solved for the 1/2 of the meltflow
 pathlength which is farthest away from the single edge-gate.
 As previously mentioned, the materials of construction of the mold cavity
 support block need to be of sufficient mechanical loadbearing strength and
 thickness to exceed deflection forces exerted by the peak packing melt
 pressure without being internally pressurized within the coolant channels,
 and desirably of very high thermal conductivity to be suitable for rapid
 thermodynamic change, which is necessary if a minimal molding total cycle
 time is to be attained. Copper based high strength mold alloy materials
 are preferred, and one optional embodiment would be use of Applicant's
 U.S. Pat. No. 4,793,953 herein incorporated by reference, or functional
 equivalence thereof. The mold cavity support block is preferably fitted
 directly with heat transfer field circulating channels, but within a
 single monolithic piece of high conductivity metal. However, a less
 preferred embodiment would place a high conductivity mold element into an
 assembly joined mechanically or adhesively to a backing plate wherein the
 channels for circulating heat transfer fluid could be housed.
 FIG. 5 shows in cross-sectional view an injection mold of the present
 invention, with heat transfer fluid plumbing and electronic control
 circuitry shown in schematic form, connected to a process controller
 device. Look now at the fluid control unit (2), showing a schematic
 diagram of the heat transfer fluid supply system which feeds into supply
 line (7) to the injection mold (1) and out of injection mold (1) by fluid
 return line (8). Supply line (7) is shown feeding into both mold halves
 and both cavity mold cavity support blocks (6), but it would be obvious to
 provide at least 2 such fluid control unit (2) so that each moldhalf and
 mold cavity support block could be maintained independently--conventional
 demolding practices assist part removal of running one moldhalf colder
 than the other throughout the whole cycle just so one side of the molded
 part shrinks more than the other by the time the mold is opened to eject
 the part. Similarly, return lines (8) allow the cyclic heat transfer fluid
 to exit both mold cavity mold cavity support blocks and mold halves.
 Looking at the fluid control unit (2), on the left hand side of the center
 line we see the "hot side" (which is activated during the heating phase of
 each injection molding cycle), including a heat transfer fluid supply
 system (22) being maintained at a higher fluid temperature than a maximum
 surface temperature of the mold mold cavity support blocks, plumbed with
 and outlet line which passes through a pump (20) in turn plumbed with 3
 valves:
 1. A check valve (17) to prevent backflow from the supply line (7) which
 feeds into the injection mold (1).
 2. A control valve (18) operating under electronic control of the process
 control unit (3) through control wire (50). When control valve (18) is in
 its closed position (as shown here, during the heating phase of the
 molding cycle), then the pumped fluid is forced to feed the mold and is
 prevented from returning to the supply system, but when the control valve
 is opened (not shown here, during the cooling phase of the molding cycle),
 the "hot side" heat transfer fluid seeking the path of least resistence
 will dump into the "hot side" supply system (22), and the supply line (7)
 feeding into the mold will become pressurized by pump (30) by "cold side"
 heat transfer fluid supplied from "hot side" supply system (32).
 3. Relief valve (19) will only open when high pressure in the supply line
 is reached which exceeds the preset value for the relief valve, thereby
 dumping heat transfer fluid back into the heated supply system.
 The supply system is heated by means of heating element (21), which
 operates under the electronic control of the process control unit through
 wire (3) through wire (50) (note temperature sensor (25) connected by
 control wire (50) to process control unit (3).)
 Turning now to the right hand side of the fluid control unit (2), we view
 the "cold side" of the system (which is activated during the cooling phase
 of each injection molding cycle), including supply system (32) storing
 heat transfer fluid maintained at a lower fluid temperature than a minimum
 surface temperature of the mold mold cavity support blocks (note
 temperature sensor (35) connected by control wire (50) to process control
 unit (3). In addition to heating element (31), a heat exchanger (36) is
 operated under the control valve (37), to either increase, reduce, or
 eliminate entirely cooling water flow which acts to bring down the fluid
 temperature within supply system (32), under the control of process
 control unit (3) through cooling water supply line (49).
 In operation, during the cooling phase of the molding cycle, pump (30)
 feeds heat transfer fluid through check valve (27) into supply lines (7).
 As shown in FIG. 1, the mold is in the heating phase and not in the
 cooling phase, therefore control valve (28) is shown in the open position,
 wherein the heat transfer fluid is diverted away from supply line (7) and
 is dumped back into the supply system. Similarly, as similar to the case
 with the hot side, control valve (28) is wired by control wire (50) into
 the process control unit, as is also the fluid return line with
 temperature sensor (33) (23 for the hot side). When control valve (34) is
 shown in the closed position, return line (8) feeds heat transfer fluid
 coming from the mold into the hot side through its control valve (24)
 (shown in the open position) and not into the cold side, since control
 valve (34) is shown in its closed position. Control valves (24) and (34)
 are wired by control wire (50) to the process control unit (3), and work
 in opposition to each other.
 Look now at process control (3), shown with typical value settings
 characteristic of the heating phase of the molding cycle. Indicator light
 (41) is shown lit up, which means that heating is going on at this time
 (similarly, indicator light (45) is shown not lit up, which means that
 cooling is not being done at this time). Temperature sensor (25) provides
 the heat transfer fluid temperature within the hot side supply system, and
 similarly temperature sensor (35) provides the heat transfer fluid
 temperature within the cold side supply system (32). Settable temperature
 (38) for the fluid in the hot side supply system (22) is shown, in this
 example, at 430 degrees F. Actual mold surface temperature (displayed as
 (47)) is read by cavity mold cavity support block temperature sensor (10),
 and compared against a settable "minimum temperature" value (39) (at
 least=Tg or Tm for the thermoplastic), and the control logic requires that
 at least this minimal value be read by the temperature sensor (10) before
 injection is allowed to start. In this example of a polycarbonate optical
 lens with Tg=296 F., the settable mold surface temperature reading from
 sensor (10) must be at least 380 degrees before start of injection is
 allowed.
 Cavity mold cavity support block temperature sensor (10) could be any
 conventional thermocouple (Type J, Type K, etc) but is preferably a
 faster-responding (0.001 second or less) thermister. Sensor (10) should be
 mounted within the mold cavity mold cavity support block in a position
 very close to that partforming surfaces to be wetted by the molten polymer
 (about 0.100" or 2.5 mm setback distance is recommended). Sensor mounting
 can be any of the following; surface mounting, bayonet lock, magnetic
 probe; removable or permanent.
 Settable transfer temperature (40) is shown here at 330 degres F. When the
 heat transfer fluid temperatures sensed by sensor (23) in the return line
 feeding hot side supply system (22) falls below this set point (in this
 example, 330 degrees F.), then the process control unit (3) sends a signal
 through control wire (50) to flow return control valve (24) to close, and
 valve (34) is opened to divert the return flow of heat transfer fluid into
 cold side supply system (32).
 Holding temperature settable time value (48) shows the time from the
 "mold-closed" signal from sensor (12) ("time=0") before the heating phase
 is ended (by opening control valve (18)). In this example, the setting
 value shown is eight seconds. That means that for eight second after start
 of injection, control valve (18) is closed and thus, after the timer
 exceeds this settable time delay value, then control valve (18) opens and
 bypasses heat transfer fluid away from supply line (7) back into hot side
 supply system (22), thus bypassing the mold. Actual mold cavity
 temperature sensed by sensor (10) is displayed in temperature reading (47)
 (in this example, shown at 380 degrees F.).
 Depending upon actual choice of mold materials and temperature
 differentials, this starting point ("time=0") can be retarded (i.e. a
 settable delay time AFTER "mold-closed" signal from sensor (12)) or
 advanced (i.e. start timer at start of "clamp decompression" or "start of
 mold opening" or off a robotic part-verification "mold-clear" signal, all
 of which start BEFORE "mold-closed" signal from sensor (12)). It would be
 an obvious functional equivalent to use one of these alternative means for
 triggering start of the "heat-on" and or "timer-start-for-transfer"
 instead of this preferred embodiment.
 Looking now at the right hand or cold side of the process control unit,
 supply system temperature settable value (42) shows the desired supply
 system temperature (in this example 150 degrees F.). When the sensor (35)
 is reading higher temperatures than temperature setting (42), then the
 process control unit increases the flow rate through heat exchanger valve
 (37) to increase the rate of heat removal out of supply system (32).
 Cooling mold temperature value (43) can be set for determining when to open
 the mold and eject the part, at a point where (47) is sufficiently below
 Tg for the plastic's temperature (not easily directly measured) to be
 shape stable; shown here in this polycarbonate optical disk example at 200
 degrees F. Importantly, this is substantially lower than the "control
 band" range of mold & coolant temperatures desired in conventional optical
 disk molding, which chooses a mold & coolant temperature between 240-265
 F., and once set, the conventional optical disk molding process attempts
 to maintain this set value + or -5 degrees F. throughout the whole molding
 cycle, wherein typically best optical disk properties are obtained by
 these conventional processes and apparatus employing only one fluid supply
 system operating at only one temperature setpoint throughout each molding
 cycle.
 Settable temperature value for onset of transfer from return line (8) fluid
 back to supply system for the temperature sensor (33) shows a value of 280
 degrees F. When this value is exceeded, control valve (34) is closed and
 will not permit returning fluid to enter.
 In actual operation being shown, the molded thermoplastic part has been
 removed from the open mold and the injection molding machine has just
 closed the two mold halves together to seal the parting line (16), which
 is confined by limit switch (12), which then signals process control unit
 (3) that a new injection molding cycle may now start. In practice, if and
 only if actual mold surface temperature (47) is at least equal to the
 settable value (39) for start of injection (in this case, both (47) and
 (39) equal at least 380 degrees F.), then molten thermoplastic is injected
 through sprue (14) into mold cavity (13).
 FIG. 6 shows the changing mold cavity surface temperature as a function of
 changing molding process sequence of the present invention. It shows that
 as the mold is opened and the molded article is ejected, the limit switch
 (12) signalling mold opening can be used to trigger start of heating
 phase, while the molded part is being ejected and the mold is again
 closing. (Even before actual mold opening motion trips the limit switch,
 it may be desirable to start heating phase BEFORE clamp decompression is
 started). As mold surface temperature rises up to the required minimum
 setpoint, heating continues, then once the desired setpoint is reached,
 injection can start. Sometime after the injection is ended and packing has
 commenced, the mold surface temperature may be allowed to fall without
 reducing the surface replication and quality of the molded plastic part.
 In actual practice, because of the delay time, it is possible that hot side
 fluid pumping may be stopped at or before the start of injection even, due
 to this induction time or delay time effect. What counts, of course, is
 the mold cavity surface temperature which is seen by the plastic; as long
 as it remains sufficiently high so that the polymer molecules are not
 prematurely set in place near the mold surfaces, then high fidelity
 replication can occur. As shown in FIG. 6, the settable value for start of
 injection is substantially above Tg and is preferably maintained there
 sufficiently long to assure peak cavity melt pressures have at least been
 attained before mold surface temperature is allowed to drop quickly by
 means of onset of pumping cold side heat transfer fluids. Once active
 cooling is started, the mold surface temperature drops quite quickly, due
 to high conductivity materials and thermal isolation away from the
 thermal-cycling mold cavity support blocks, and good proximity of the
 flowing coolant to the molding surfaces wetted by the molten plastic
 polymer.
 FIG. 7 shows a "prior art" embodiment, of a "conventional cooling" process,
 wherein by intentions the inlet fluid temperature into the mold cavity is
 maintained within a tight range of temperature, with minimum change within
 each molding cycle being desired. In contrast to FIG. 6, FIG. 7 shows
 actual cavity surface temperature never reaches Tg or Tm and fluctuates
 between a lower temperature during cooling phase and a warmer temperature
 when the mold is being heated by the hot plastic. The important thing to
 note is that the transfer fluid temperature is deliberately maintained
 substantially constant throughout each injection molding cycle
 individually. This "isothermal" process is in contrast to the present
 invention, which exceeds the Tg temperature at the time when injection is
 to start, then switches over to a much lower temperature heat transfer
 fluid during the actively cooling stages.
 FIG. 8 shows the process flow sheet with the decision tree logic necessary
 for the process control device to operate. First, when in "standby"
 (non-production), the injection molding machine is in "manual mode", with
 valves (18), (24) and (28) open and (34) closed. To transfer into
 production, the injection molding machine is switched over to "automatic
 cycle", as shown at the top of the page. The heating cycle is started at
 least as soon as the mold is opened (as sensed by limit switch sensor
 (12)), and heating is started by closing control valve (18). Once
 temperatures (25) and temperatures (35) are within the set limits, and the
 actual mold surface temperature (T10) at least equals if not exceeds the
 settable value (39) inject, then injection is permitted to start. If not,
 then heating continues until the setpoints are reached (it is possible to
 add a timer which would sound an audible alarm if an abnormally long time
 interval has passed due to some malfunction). The next step is to close
 the heating circuit opening control valve (18) to stop flow from the
 "hotside" supply system into the mold through supply lines (7). This is
 done as soon as injection has started, or could alternatively be delayed
 by some predetermined way. (In this preferred embodiment shown in FIG. 3
 the heating circuit is essentially closed simultaneously to the start of
 injection, in order to minimize total cycle time, but a longer heating
 phase could be run without harm to quality of the molded plastic, if one
 is willing to reduce output quantity with this longer total molding cycle
 time.) At typically the same time, the hold timer (48) is started and
 after that settable value for time is attained, then start of the cooling
 phase is initiated by closing valve (28). Note that FIG. 3 shows these
 steps in a sequential, serial order, but it would be obvious to those
 skilled in the art to minimize cycle time by performing concurrently (i.e.
 in parallel) those steps which are not specifically contingent upon an
 outcome or measurement preceeding it.
 Transfer of heat transfer fluid from the return line (8) depends on whether
 sensed temperature from sensor (23) reads less than the heating transfer
 temperature value (40). It not, then cooling is continued as before. If
 so, then the return heat transfer fluid is diverted from the hot to the
 cold side supply system by opening control (34) and closing control valve
 (24). Next, if the actual mold surface temperature T10 is less than the
 prescribed value for cooling setpoint (43), then move ahead to close the
 cooling circuit by opening control valve (28). If T10 is less than cooling
 setpoint (43), then continue through previous step. When T10 is greater
 than cooling setpoint (43), then go back to start the cooling circuit by
 closing valve (28) again. When that happens, in parallel to it, the mold
 is opened as sensed by limit switch (12), then the next step starts.
 Recycle loop checks to see that the injection molding machine is still in
 automatic mode, then closes valve (18) to start heating again. Only when
 sensed temperature from sensor (33) is greater than the settable value for
 cooling transfer (44), then the heat transfer fluid from return line (8)
 is diverted away from the cold side supply system (32) and over to hot
 side supply system (22) by opening control valve (24) and closing control
 valve (34).
 In case there is imbalance in the total flows out of and into each supply
 system, there is an interconnect line (26) between supply systems (22) and
 (32), such that if the liquid level of either supply system rises to that
 point, then gravity transfer will spill over the excess heat transfer
 fluid from into the other. Note that both supply systems are maintained at
 atmospheric pressure, in the case of oil or glycol being used as the heat
 transfer fluid. (Not applicable in the case of the optional but preferred
 use of steam; excess condensate or cooling water can be dumped if need be,
 if imbalanced flow happens. Those skilled in the art of steam generation
 and plumbing would make suitable accomodation). This simple
 self-equilibrating, self-adjusting heat-transfer-fluid-level-balancing
 apparatus is one embodiment, but is could be replaced by standalone
 electronic level controls in fluid communication with optional auxillary
 3rd ("mixed hot+cold" supply system) and/or 3rd+4th fluid supply systems
 (i.e. separate backup hotside and coldside supply system). Also,
 substitution of a 3-way valve for the combination of control valves 24+34,
 governing the return of fluid to the supply systems. Return lines exiting
 the mold could optionally deliver the returning heat transfer fluid into a
 third supply system which in turn is maintained in fluid communication
 with both hot side heating supply system and cold side cooling supply
 system. These examples show how the present invention can be implemented
 by several alternative means, & should not be limited to just this
 hardware as illustrated in FIG. 1.