RADIANT PRE-HEATING OF MOLD SURFACES

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims Paris Convention priority of U.S. Provisional Patent Application SN 60/739,103, filed November 21 , 2005.

BACKGROUND

[0002] The invention relates to the field of molding thermoplastic materials and specifically concerns techniques for precisely establishing predetermined temperature conditions over the surfaces of a mold for introducing a hot plastic melt into the mold. The invention is particularly useful for molding articles that are small in thickness compared to area, such as compact data disks and DVDs, and enables repetitive molding of quality products at a high rate of mold cycling.

[0003] When molding by injecting hot plastic melt into a mold cavity, it is necessary to introduce the melt at a sufficient temperature to obtain a low viscosity that permits the melt to flow, so as to fill and conform to the mold cavity. A melt may be introduced at a temperature that is higher than the temperature at which the melt will flow, into a mold cavity that is at or lower than the temperature at which the melt will set. In that case, the melt may initially heat the mold cavity surface, to a temperature higher than the solidifying temperature, permitting the melt to flow into conformity with the mold surfaces. Thermal conduction from the melt into the mold cools the melt, which sets in the required shape defined by the surfaces of the mold cavity.

[0004] , In a cyclic operation, it is desirable to obtain a molding cycle that is as short as possible. A short mold cycle maximizes the number of molded units that can be produced with a given number of molds in a given time. In order to achieve a short molding cycle, it is desirable that the melt be brought to below the solidifying temperature rapidly. The melt temperature

can be reduced to near the solidifying temperature when the melt is introduced into the mold. Thus there is less cooling time needed to reach solidifying temperature. The initial mold cavity surface temperature also can be kept low, to accelerate the rate at which heat is transferred from the melt to the mold.

[0005] If the mold cavity surfaces are too cool, the melt may solidify unevenly. For example, portions of the melt may solidify on cool cavity walls while hotter portions of the melt are still flowing as needed to fill the mold. In the case of mold cavities that are thin or have constrictions, a solidified portion of material may set or freeze on the surface of the cavity and obstruct the melt flow path so that the cavity does not fill completely. Thermal expansion and phase change considerations can also result in stresses and structural defects if the melt sets and/or cools unevenly. [0006] An accumulation of solidified melt material on the mold cavity surface reduces the thickness of the flow channel for the melt, which is a problem of itself if the cavity is thin. Assuming a molding processes for production of an 0.6 mm thick DVD optical disc, for example, frozen layers of 0.09 mm thickness decrease the width of the melt flow path approximately 30 percent to 0.42 millimeters. This affects the mold cycle time adversely, because other things being equal, a relatively constricted flow path results in a relatively slower rate of flow.

[0007] This is especially true in very thin wall articles, as the ratio of cavity surface area to cavity volume is markedly increased and hence results in a proportionally more rapid rate of cooling. In the case of a 0.25 mm thick disc, the width of the flow path may be reduced by as much as approximately 72 percent to 0.07 mm. The required injection pressure to fill the mold under these conditions would be unattainable using existing methods and equipment.

[0008] Some developers have concluded that an 0.1 mm thickness optical surface (cover) layer disc for digital information capacities over 15 GB cannot be molded. For example, U. S. Patent No. 6,440,516 describes

using a pressure sensitive adhesive film or a dry photopolymer film for the surface layer. However, this film is not cost effective due to material costs and trimming the film after attachment, which adds a step and a work station to the process.

[0009] U.S. Patent No. 6,512,735 describes a spin coated ultraviolet cured layer. Spin coating a layer with satisfactory properties and geometric tolerance has proven difficult Neither the film nor the spin coating have digital information on the substrate side to support dual data layer discs. [0010] In order to mold high quality parts on a short molding cycle, including thin data discs or disc layers, provisions have been attempted including careful presetting of the melt temperature versus the mold cavity temperature, temperature controllers, mold heating or cooling provisions such as resistive heating elements or coolant circulation paths, layers of thermal insulation, supplemental heaters for edge zones and other problem areas, etc. Some examples of such provisions are disclosed in commonly owned U.S. Patents 6,276,656; 6,019,930; and 5,324,473. [0011] It would be desirable to overcome limitations of prior art methods by providing new and improved ways to create and employ molds and molding processes, especially for molding very thin wall articles such as data discs or data disc layers, in economically short cycle times. In particular, an optimized way is needed to pre-heat molding surfaces to near or above manufacturer recommended melt temperatures at the time hot plastic melt is injected into the mold. The technique needs to be robust and preferably uncomplicated, capable of control to achieve and maintain a steady cyclic operation, and capable of adapting to all aspects of this and similar products. [0012] The molds and molding processes described herein overcome the problems of setting and/or increased melt viscosity, which tends to obstruct and potentially block necessary melt flow before the cavity of a very thin walled article is filled. The molds and processes provide for controllable temperature conditions, in particular to provide melt conditions during molding that permit uniform compression across the thickness of the melt,

i.e., by providing for low viscosity of the melt across the molding surfaces to be pressed toward one another so as to compress the melt. This is achieved in part by providing a radiant heater that is moved over the cavity surface prior to introducing the melt.

[0013] The subject molds and processes are apt for thin molded structures such as data discs or layers thereof, as well as other structures. By way of example, the disclosed mold and molding techniques enable the molding of optical disc substrates, cover layers, and other articles having a wall thickness of approximately 0.3 mm to less than 0.075 mm in economical cycle times. In one application, an 0.10 mm thick cover layer or a 0.25 mm thick center substrate for an optical disc is molded with pits or grooves in one or both surfaces. Such a thin configuration of molding material has relatively little thermal inertia. A mold lining layer is provided that can be insulated from the thermal mass of the mold and is surface heated by the radiant heater prior to introducing the melt. Neither the thin melt nor the surface of the cavity (e.g., an insulated liner) has a great deal of thermal inertia. The application of radiant energy to the cavity surface quickly brings the melt- contact cavity surface up to temperature where the melt is permitted to flow without substantial obstructive surface freezing, until the mold is full. The mold surfaces and the melt then quickly drop to the solidifying temperature and the part is complete.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Fig. 1 is a simplified schematic side elevation view of a molding apparatus according to the subject invention.

[0015] Fig. 2 is a simplified cross section through certain mold elements as discussed herein.

[0016] Fig. 3 is a flow diagram showing steps in an inventive molding method.

[0017] Fig. 4 is a plot of temperature versus time, demonstrating a preheating temperature-time history of molding surfaces at three locations using a profiled visible light or infrared (IR) heat source.

[0018] Fig. 5 is a temperature profile versus distance, through a cross section of the mold at time of melt injection, illustrating temperature conditions as a function of distance from a flow inlet for the melt along a flow path approaching a remote destination for the flow.

DETAILED DESCRIPTION

[0019] The subject invention is advantageously applied to molding thin articles and is particularly useful when molding very thin walled articles, namely articles that have at least one substantial ratio of wall surface dimension (length, width or area) to wall thickness. In connection with data discs and the like, a "very thin wall article" can be regarded as an article with a wall thickness of approximately 0.3 mm or less, versus a diameter of about 120 mm.

[0020] According to an aspect of the invention, the thin walled nature of the article to be molded is complemented by a quick radiant heating technique to raise the temperature of the mold cavity surface prior to introducing the melt. The heated melt-contact surfaces prevent or delay surface solidification of the melt on the cavity surfaces as the mold is filled. With continuing heat conduction from the melt into the respective mold body, the melt solidifies.

[0021] In connection with such dimensions, the "mold heating position" or "mold heater position" as used herein refer to a point between spaced edges of the mold cavity, for example a point across a diameter or other chord. The term "mold heating position" notably is used to define a spatial position of a radiant heater element relative to a line extending across a mold cavity between cavity walls. The mold heating position is a point between a near edge and a far edge along the imaginary reference line. Thus, a "zero" mold heater position is the end of the line at a near mold edge, the sixty mm

position of the heater (assuming a 120 mm diameter) is on the imaginary line at the center of the molding surface layer and the one hundred twenty mm position of the heater is aligned with the imaginary line, at the far mold edge. [0022] The disclosed technique comprises advancing the mold heating position from the zero mold heating position toward the far mold edge along the imaginary line. The "far mold edge" is the edge of the mold cavity that is closely exposed to energy from a radiant heater last, as the radiant heater is moved according to the invention along this imaginary line. The far mold edge may also be referred to as the one hundred twenty mm position in a standard size data disc wherein the imaginary line corresponds to a diameter. The "near mold edge" refers to the edge of the cavity that is exposed to energy from the radiant heater first. The near mold edge may also be referred to as a zero position.

[0023] As shown in Fig. 1 , a surface heated mold 10 is provided in an injection molding machine 12, which generally compresses the melt material to conform the material to the mold cavity surfaces, while the melt material cools and solidifies, preferably in a minimal amount of time. [0024] Conventional mold temperature control equipment 14 preferably is provided to control the temperature of the mold together with temperature variations that result from the introduction of the hot melt and the flow of heat transfer fluid circulated through passages in the mold body. This establishes a temperature cycle over time: the mold is prepared, then melt is introduced tending to heat the mold surfaces, a molded part is formed by solidifying the melt material due to conduction of heat energy away from the melt and into the mold body, and finally the molded part is removed, freeing the mold for another molding cycle. An article removal robot 16 extracts the article from the mold at the end of the cooling period.

[0025] According to one aspect, a visible light or infrared heater 18 is attached to a heater positioning robot 20, for providing an elevated starting temperature at the surfaces of the mold cavities.

[0026] Fig. 2 further details the surface heated mold 10. Each side of the surface heated mold 10 is comprised of a molding surface layer 10A covering at least a major portion of the mold cavity, a thermal barrier layer 10B, a metal die 10C, a joining means 10D, a joining material 10E, and cooling passages 10F.

[0027] The molding surface layer 10A preferably has emissivity equal to or greater than approximately 0.50. Emissivity is a multiplicative product of thermal conductivity times specific heat times density, and represents a thermal energy capacity of the material. The emissivity preferably is equal to or less than that of zirconia. The surface layer should not degrade or react chemically with the melt when exposed to a rapidly cycled molding surface temperature, which is cycled, for example, between 100 degrees C and 525 degrees C. As additional properties, the surface layer 10A should be resistant to chipping, should release the molded article at least as easily as 420 stainless steel, and should have a coefficient of thermal expansion compatible with that of the thermal barrier layer 10B throughout the temperature cycling range, so as to minimize stress from differences in thermal expansion characteristics.

[0028] The thickness of molding surface layer 10A may optionally be nonuniform, so as to tailor the pre-heat temperature of the molding surface to compensate for uneven cooling. Specifically, cooling may be faster near margins of the cavity than at areas spaced from the margins. [0029] Still further, the molding surface layer 10A may also optionally comprise a stamper that carries digital information in pits or grooves for transfer to an optical disc substrate or cover layer that is molded with the stamper defining part of the mold cavity surface. The digital information can be applied to the cavity surface by ion beam milling, etching or other methods known in the art. Alternatively, the digital information may be applied to the molding surface layer 10A by applying a deposited material that is one micron or less thick and defines the necessary digital bits. If layer

10A is a stamper, it may be held against thermal barrier layer 10B using a vacuum as joining means 10D.

[0030] The thermal barrier layer 10B must withstand exposure to cycled temperatures, e.g., between 80 degrees C and 300 degrees C, and is preferably made of a material with a product of thermal conductivity times specific heat times density that is much less than that of zirconia (5.63E-06 J ^λ 2/(sec-mm ^λ 4-F ^λ 2).

[0031] A metal die 10C that forms a remaining part of the mold body provides structural support for the thermal barrier layer 10B and molding surface layer 10A. The die 10C has passages for fluid circulated by the mold temperature control equipment 14, and defines a high thermal conductivity path for heat flow from the cavity to the fluid. Metal die 10C, in cooperation with platens of molding machine 12, provides the rigidity required for cover layer wall thickness variations that meet stringent optical disk production requirements, such as those of the Blu-ray™ variety. [0032] Joining means 10D can be a vacuum, or joining materials can be used such as adhesive, solder, braze, and others known in the art. Joining materials must adhere to molding surface layer 10A and thermal barrier layer 10B, and maintain structural integrity at the operating temperatures and pressures. When joining means 10D is a joining material, the minimum thickness of the molding surface layer 10A is selected so that voids in 10D do not result in an unacceptable article due to molding surface deformations when exposed to melt pressure. The maximum thickness is selected in cooperation with the thermal barrier layer 10B to ensure acceptable molding cycle time.

[0033] Joining material 10E can be an adhesive, solder, braze, or the like. It must adhere to thermal barrier layer 10B and metal die 10C, and maintain structural integrity at the operating temperatures and pressures. [0034] Cooling passages 10F can be formed by grooves in the top surface of die 10C and layer 10B. The cooling passages 10F can be smaller, more numerous, and closer to molding surface layer 10A than

represented in Figure 2 so as to shorten cooling time, or may be less numerous or even absent depending upon operating conditions. [0035] The injection and compression aspects of molding machine 12 are conventional, with the exception that for creating very thin walled articles the screw is significantly smaller relative to the clamp size. The platen also advantageously can have a parallelism control and should be substantially rigid to preclude deflections under clamping force that would cause unacceptable variations in the thickness of the molded article, such as those imposed by the Blu-ray™ specifications.

[0036] The mold temperature control equipment 14 comprises temperature controllers, lines for flow of fluid, and optionally other components known in the art.

[0037] A part removal robot 16 preferably is provided and tailored for efficient handling of the molded part when removed. For optical disc substrates, the robot 16 commonly has suction cups to grasp the molded disc in its clamping area, and two fingers to grip the waste sprue. For very thin molded optical discs, the disc may be grasped by a metal or ceramic vacuum chuck, which may optionally be heated as well, to prevent problems such as premature cooling or warping of the molded disc. [0038] The radiant heater 18 is an arrangement of radiation heat sources, such as quartz or sodium vapor lamps designed to heat both mold halves simultaneously. The heat source of heater 18 can reach 2500 degrees C or more, and raises the temperature of the cavity surface to a lesser temperature determined by the exposure time of the heater 18 to the cavity surface. To promote uniform temperatures across the molding surface layer 10A, the heater temperature is profiled by varying lamp spacing or current; however a non-linear profile may be used as necessary for a given article or molding surface temperature exposure profile.

[0039] Preferably, to mold very thin walled articles, the molding surface layer 10A is pre-heated to a point such that temperatures at first contact by the hot plastic melt are, for example, at least approximately 195% of the

manufacturer's maximum recommended melt (consider degrees C). The radiant heating technique is such that the extreme surface of the molding surface layer can be heated quickly, but by quick application of the radiant heater, the elevation in temperature is concentrated at the surface. [0040] Preferably, at the start of compression, the melt closest to the just heated cavity surface flows more readily than the rest of melt. As the molding surfaces cool, the flow at the surface decreases relative to the rest of the melt, which is the converse of the typical situation without radiant preheating, i.e., wherein a solidified melt layer typically occurs on a cavity surface that is at or below the solidifying temperature. As the cavity surfaces cool, solidification ensues but the result of radiant preheating is more uniform flow through the cavity cross section by the time that final thickness is achieved by compression and the part solidifies.

[0041] Prior to introducing the melt, the heater positioning robot 20 moves the radiant heater 18 from a home or rest position to a mold heating position between the two sides of the mold 10 where it commences heating of the molding surfaces. It is preferred that the heater positioning robot 20 move quickly to the mold heating position to minimize uneven heating of the molding surface. The robot 20 can be designed to insert and extract the heater for a second mold while the melt is compressed and cooled in the first mold. When outside the molds and when in the home position, the radiant heater 18 is preferably covered for safety but remains powered. [0042] The molding apparatus initially is brought to a beginning temperature with the mold 10 closed by operation of molding machine 12. Specifically, the mold temperature control equipment 14 is activated and brings the mold to a predetermined control temperature. This temperature is typically near or slightly below the temperature of the melt to be injected. Meanwhile, the radiant heater 18 is also powered up to bring it to the desired temperature profile. The radiant heater 18 is then in the home position. During this preparatory stage, the melt is brought to the desired temperature

using molding machine 12, which temperature is above the solidifying temperature of the molding material.

[0043] Cyclic stages of mold preheating, charging, cooling and part extraction are then undertaken. At the beginning of each cycle, the molding surfaces 10A of mold 10 are preheated by opening the platen of molding machine 12 to separate the two sides of the preheated mold and then rapidly inserting and applying radiant heat energy from heater 18 to the cavity defining surfaces of the mold halves. Heater 18 is positioned by a positioning robot 20.

[0044] The heater 18 remains in a stationary position between the mold halves long enough to heat the molding surfaces 10A of the surface heating mold 10 sufficiently for filling. The heater quickly brings the molding surfaces 10A to a temperature higher than the melt solidifying temperature. Inasmuch as the heating is radiant, the heat energy is incident on the surface. The surface is elevated in temperature. The heat of the surface dissipates by conduction from the surface into the mold body, but a profile of temperature versus depth is achieved wherein the surface for a time is at a higher temperature than the mold body and a higher temperature than the melt solidifying temperature.

[0045] After the molding surfaces 10A have reached a sufficient temperature, the radiant heater is retracted and the mold 10 is partially closed to the melt fill position. This step is accomplished rapidly, both to minimize cooling of the molding surfaces 10A and to maintain a difference in temperature versus depth from surfaces 10A into the mold body. The hot melt is injected into the mold cavity either while the mold 10 is still closing or shortly thereafter. It is preferred that the molding surfaces not cool to less than 95% of the manufacturer's minimum recommended melt temperature if a short cycle time is desired.

[0046] Near completion of the melt injection, a clamp applies compression between the opposed portions of the mold 10 rapidly and for a

length of time sufficient to cause the melt to flow and to completely fill the cavity.

[0047] The mold is held in the closed position as the melt solidifies and cools to an article removal temperature that is below the solidifying temperature. Clamp pressure can be permitted to decrease as the melt cools to solidifying and the molded part holds its shape. The mold 10 is then opened and the article ejected with the assistance of the removal robot 16. [0048] The invention is now described with reference to certain specific Examples. These Examples are provided for illustration of operational aspects, and the invention is not limited to the Examples, but rather encompasses variations that are evident as a result of the Examples and other teachings provided herein and set forth in the claims.

Example 1

[0049] A Blu-ray™ compatible 0.075 mm thick high definition optical disc is molded using the molding apparatus as described above. The mold is prepared by first closing the mold 10 using an injection/compression molding machine 12 having a platen parallelism control.

[0050] The mold 10 is preferably comprised of a molding surface layer

10A covering at least a major portion of the mold cavity, a thermal barrier layer 10B, a metal die 10C, a joining means 10D, a joining material 10E and cooling passages 10F.

[0051] The molding surface layer 1OA preferably comprises zirconia preferably 0.3 to 1.5 mm thick, more preferably 0.8 to 1.3 mm thick, and still more preferably 1.3 mm thick, with a preferred emissivity of approximately

0.56. The molding surface layer may comprise two sub-layers, one joined to the thermal barrier layer 10B and the other a replaceable stamper that can bear data to be impressed in the molded part.

[0052] The thermal barrier layer 10B preferably comprises Cotronics'

Rescor™ 914 Glass Ceramic, preferably 0.2 to 1.5 mm thick and more preferably 0.4 to 1 mm thick.

[0053] Metal die 10C provides structural support for the thermal barrier layer 10B and molding surface layer 10A, passages for fluid circulated by the mold temperature control equipment 14, and a high thermal conductivity path for heat flow to the fluid. Die 10C can comprise a durable steel. [0054] Joining means 10D and joining materiaHOE are preferably active metal solders able to maintain structural integrity at operating temperatures of 45 to 300 degrees C and pressures of approximately up to 5000 psi. [0055] Preferably, the cooling passages 10F are approximately 7.8 mm from molding surface layer 10A.

[0056] The molding machine 12 preferably has about forty metric tons of clamping capacity and rigidized platens, and more preferably has less than forty metric tons to lessen distortion of the platens and mold. As opposed to the common twenty-five mm diameter for optical disc molding in twenty to sixty metric ton machines, the screw is preferably twelve to fourteen mm diameter, more preferably twelve mm diameter, and injects a full shot of melt material in approximately 40 milliseconds.

[0057] Once the mold temperature control equipment 14 is turned on, the fluid temperature should be set to control for 25 degrees C, which carries away heat energy that is conducted from the melt into the mold body. However before injecting the shot of melt material, the facing surfaces of the mold are heated quickly and thus shallowly, to above the solidifying temperature of the melt.

[0058] The radiant heater 18 preferably is kept at a desired operational temperature profile while in the home position. Preferably, the heater element is arranged to define a temperature profile, in this embodiment 1864 degrees K at the zero position, 2013 degrees K at the sixty mm position, and 2200 degrees K at the one hundred twenty mm position, with a substantially linear temperature profile between these positions. [0059] The melt is heated to the desired injection temperature using molding machine 12. In an exemplary embodiment, the melt temperature is approximately 380 degrees C. The specific melt temperature depends on

the characteristics of the melt material but is sufficiently above the solidifying temperature of the material that the melt is free at the injection temperature to flow under the pressure of injection and compression. [0060] Immediately before melt injection, the molding surface layer 10A of mold 10 is preheated by opening the platen of molding machine 12 to separate the two sides of the mold and then rapidly inserting the heater 18 between the mold halves using positioning robot 20, which preferably moves at a rate faster than 240 mm/second. The heater 18 preferably has an emissivity of approximately 0.9.

[0061] The heater 18 remains in a stationary position between the mold halves long enough to sufficiently heat the molding surface layer 10A of the mold 10 prior to filling. A temperature at least 55% above the manufacturers recommended minimum melt temperature is preferable. In this example of a melt injection temperature of about 380 degrees C, a temperature profile is obtained on the mold surface of 472 degrees C at the near mold edge, 481 degrees C at the center of the molding surface layer 10A and 514 degrees C at the far mold edge. A linear temperature distribution between positions is most preferable. By such radiant heating, the heating at the molding surface layer 10A is substantial but shallow. The heater 18 is then removed and although the molding surface layer begins to cool, the melt is injected before the molding surface layer 10A falls substantially. [0062] After the molding surface layer 10A has reached the desired temperature profile, the mold 10 is partially closed to the fill position. This should be accomplished rapidly to minimize cooling of the molding surface layer 10A. It is preferred that the molding surface layer 10A not cool at any point between the near and far edge to less than 95% of the manufacturer's minimum recommended melt temperature if a short cycle time is desired. Preferably, closing is accomplished in approximately less than 0.6 seconds, resulting in the molding surface layer 10A retaining a temperature of approximately 98% of the manufacturer's minimum recommended melt

temperature. Preferably the manufacturer's minimum recommended melt temperature is approximately 300 degrees C. [0063] Fig. 4 is a graphic representation of the temperature of the molding surface layer 10A from the start of heating to the start of injection under preferred conditions. Fig. 5 is a graphic representation of the temperature through a cross-section of the mold 10 at the start of injection of the hot melt, namely along the line from the near to far edge. [0064] Once in the fill position, hot melt is injected into the mold cavity. Hot melt may be injected while the mold 10 is still closing up until any such time thereafter that the molding surface layer 10A becomes cooled to the extent that further flow of the hot melt is obstructed. It is preferred that the mold cavity is approximately 0.5 mm thick in the fill position and that the molding surface layer 10A be less than 10 degrees C below the manufacturer's minimum recommended melt temperature when the cavity is completely full.

[0065] Near the time that melt injection is complete, a clamp applies compressive pressure to the mold 10 so as to compress the cavity. The compression is as rapid as possible and can be maintained for a length of time sufficient to cause the melt to flow and to fill the cavity completely. A ramp rate of 1000 metric tons per second up to the full clamp force is preferred, and a total time of approximately 0.83 seconds is generally sufficient.

[0066] Mold 10 is held in the closed position until the melt cools and solidifies. The mold is cooled to an article removal temperature, which is preferably 130 degrees C at molding surfaces 10A. A time of approximately 3.6 seconds is sufficient. Clamp pressure decreases as the melt cools. [0067] The mold 10 is opened and the now solid article is ejected with the assistance of the removal robot 16. Preferably, this step takes approximately 1 second with the assistance of a heated ceramic vacuum chuck as part of the removal robot 16 assembly.

[0068] The total cycle time is preferably less than approximately 8.68 seconds. Two machines producing 0.075 mm cover layers can be teamed to produce a 1.10 mm substrate layer in approximately 4.34 seconds in BIu- ray™ molding equipment configuration. These times can be further reduced by faster mold opening and closing speeds, faster robots, smaller cooling passages that are closer to the molding surface layer, and the disclosures herein or U.S. Patent No. 6,019,930 to shorten the cooling time for a 1.10 mm substrate.

Example 2

[0069] In an alternative exemplary embodiment, the heater 18 is inserted between the open sides of the mold 10 while the heater 18 is enclosed by shutters. The shutters are retracted, exposing the radiation heat sources of the heater 18 to molding surfaces 10A for a predetermined time interval sufficient to achieve the desired surface temperature. The heater 18 in this example can be held in a stationary position with the shutters being opened and closed by movable louvers or the heater 18 and the shutters can be movable relative to one another such that the heater 18 is advanced or retracted relative to the shutters or vice versa.

Example 3

[0070] In another alternative embodiment, a near black body material is used in place of radiant heater 18, otherwise according to Examples 1 or 2. The near black body material may be profiled to apply more heat at desired locations of the cavity surfaces. For example, the cavity of an optical disc may be heated more at the outer diameter to compensate for the longer exposure to air cooling in this area while the melt flows outward from the inner diameter toward the outer diameter to fill the cavity.

Example 4

[0071] In another alternative embodiment, otherwise in accordance with Examples 1 to 3, outer fluid passages in metal die 10C are part of one or more separate circuits that circulate hotter fluid to heat the outer edge area of molding surface layer 10A to compensate for the longer exposure to air cooling as the mold 10 fills.

Example 5

[0072] Another alternative embodiment is according to Examples 1-4 except that two molding machines 12 and two molds 10 are employed together with one mold surface heater 18 that alternates heating each mold as the melt is compressed, cooled and removed from the other mold.

[0073] The invention has been disclosed with reference to specific embodiments. Other embodiments and variations of the invention can be devised without departing from the spirit and scope of the invention and are encompassed within the scope of the appended claims together with their reasonable equivalents.