Abstract:
An injection molding machine for forming a multi-layer plastic article by over molding where the second layer of the article includes a portion having a different geometrical profile than the first. The injection mold machine comprises an array of one or more cores which engage arrays of one or more first cavity and arrays of one or more composite cavities. Each composite cavity is formed from the combination of a second cavity and a cavity extension which carries at least a portion of the different geometrical profile. In one embodiment, the cavity extension comprises a pair of cavity portions which are mounted adjacent the core to laterally moveable slides on a movable platen. In another embodiment, the cavity extension is a single element which is moved between a disengaged position wherein the core can be inserted into the first cavity and an engaged position wherein the core is inserted into the composite cavity.

Description:
This application is a divisional of App. No. 09/018,655 filed Feb. 4, 1998 (incorporated herein by reference; now U.S. Pat. No. 6,322,738), which is a continuation of App. No. 08/899,752 filed Jul. 24, 1997, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a novel injection mold, injection molding machine and an injection molding method for producing over-molded articles that can be made of one or more moldable materials. The invention is suitable for use with plastic resins, but can also be applied to glass, ceramics, powders or to combinations of them. 
     BACKGROUND OF THE INVENTION 
     Machines and molds for producing injection molded plastic articles of two or more layers of one or more different resins are well known. Such composite articles can be used in a wide variety of applications. Multi-materials can include materials with different molding properties, such as PET and PEN, the same material with various additives such as dyes, or combinations of these. For example, multi-layer multi-material articles which arc injection molded include the keys used in personal computer keyboards, wherein the indicia of the function assigned to the key is formed from a different colored material than the remainder of the key and components such as multi-colored lenses used for the stop and turn signal indicator lights in automobiles. 
     Another common use is in the manufacture of multi-layer, multi-material articles for the packaging of food wherein, for example, U.S. FDA regulations require that only virgin plastic materials be employed in locations which contact the food. Generally, it is desired to reduce the amount of virgin material which is employed in such packages for both environmental reasons (wherein it is preferred to use recycled materials) and for cost reasons (as virgin material is more expensive than recycled plastic materials). Accordingly, multi-layer multi-material packages have been produced which include a first, thin, layer of virgin plastic material which contacts the food and a second, thicker, layer of recycled or otherwise less expensive material which is laminated to the first layer during injection molding to provide strength to the package. 
     One area where such multi-layer packaging is employed is in bottles and other vessels manufactured from PET or other materials. PET bottles and vessels are commonly blow molded from “preforms” in a well known manner, the preforms having been manufactured by injection molding to form a thread on the neck portion of the bottle to receive a bottle closure. It is known to form a multi-layer preform of PET, the inner most layer of which is virgin plastic and at least some portion of the remainder of the preform being recycled plastic material. In such cases, when the bottle or vessel is blow molded from the preform, the virgin material forms a continuous inner layer within the vessel and the recycled or other material surrounds the outside of the inner layer to increase the overall strength of the vessel to an acceptable level. 
     In other circumstances, the layers employed in multi-layer articles can have properties other than, or in addition to, being different colors and virgin and recycled materials, for example layers can have different chemical properties, etc. Also, more than two layers can be employed, if desired. It is known, for example, to produce a multi-layer preform for blow molding PET bottles and vessels wherein a layer of barrier material is located between the inner layer of virgin material and the recycled material, the barrier layer inhibiting take-up of CO 2  gas from carbonated beverages stored in the blown bottle by the PET materials behind the barrier. 
     Various systems and techniques for molding multi-layer, multi-material plastic articles are known. Generally, such systems are based on either co-injection, over-molding and/or insert-molding systems. In all co-injection methods, the mold remains closed until the cavity is filled by the injection of two or more plastic materials into the cavity, either simultaneously or sequentially. 
     In sequential co-injection, a measured amount of a first material is injected into the cavity and an amount of a second material is then injected into the first material within the cavity. Due to a “skin” effect, the first material maintains its contact with the cavity walls and the second material pushes the first through the cavity, such that the materials fill the cavity with the second material sandwiched between inner and outer layers of the first. 
     In simultaneous co-injection, both materials are injected into the cavity at the same time, for at least part of the injection operation, and the differing viscosity, skin effects and other characteristics of the materials and the injection process result in the desired formation of layers of the materials within the cavity. 
     In the majority of co-injection methods, the article is made of maximum three different materials displaying different characteristics or/and functions. For example, one material can be a virgin resin, the second one can be a recycled version of same or different resin and the third can be a chemical barrier layer (such as EVOH, Nylon, MXD6) formed between them, or as a first layer. In common applications using two materials, an article can be formed having three or five layers (2M3L or 2M5L). If three materials are used, the article can have either three (3M3L) or five layers (3M5L). 
     Sequential co-injection systems for preforms are discussed in U.S. Pat. No. 4,781,954 to Krishnakumar et al. and U.S. Pat. No. 4,717,234 to Schad et al., the contents of each of which are incorporated herein by reference. A more recent co-injection system, shown in U.S. Pat. No. 5,582,788 to Collette et al., shows the use of a turret injection molding machine for co-injection which allows for improved cooling of molded articles. 
     Simultaneous co-injection systems for preforms are discussed in several U.S. Patents, such as those assigned to American National Can. Of interest in this regard is U.S. Pat. No. 5,523,045 to Kudert et al. which shows a multi-material co-injection nozzle design suitable for multi-layer preforms. 
     An innovative mold design capable of performing either simultaneous or sequential molding is described in U.S. patent application, Ser. No. 712,481 to Bertschi et al. and assigned to the assignee of the present invention (now U.S. Pat. No. 5,651,998). This application shows the first mold design wherein hot runner injection nozzles are located on the opposite sides of a cavity to inject two or more different resins. This approach simplifies the mold and allows for injecting into cavities which are arranged in a more compact, denser manner, as the nozzles for a single cavity are not on the same side of the mold. 
     While conventional co-injection methods offer some advantages as they use a single cavity and all the injection units are on one side of the injection molding machine, they also have several significant drawbacks. One of them is that it is difficult to obtain continuous and uniform layers of the different materials as they interact in a complete molten state and proper metering of the materials is often difficult. This is especially true when three materials are to injected. Further, the mold design and the hot runner design become very complicated as a single manifold or a single nozzle must be able to work with different materials having different processing parameters. These problems are further exacerbated for high cavitation molds, such as 48 or 96 cavity molds. Another difficulty is cooling, wherein thick articles require longer residence time in the mold close position, which affects the cycle time. 
     Some of the disadvantages of the co-injection systems are overcome by over-molding systems, where each injection operation is performed in a different mold cavity. Generally, the first injection operation is performed in a mold cavity to create the first layer of an article and the cavity is then changed to increase the volume and, commonly, to alter the geometry of the cavity space. Usually this is accomplished by changing the cavity and using the same core that holds the molded article. A second molding operation is then performed with the first layer of the article, which is retained by the core, being placed in the changed cavity. During the second injection the new molded material bonds to the previously molded layer in the mold to form the multi-layer article. As will be apparent, while the second cavity has a larger volume than the first, it will be understood by those of skill in the art that the actual cavity volume which must be filled in the second injection operation can be less than the volume filled in the first injection operation, with the balance of the volume being occupied by the first layer. As will also be apparent, over-molding can include more than one over molding operation to form articles using more than two resins and/or with more than two layers, if desired. 
     While good results can be obtained by over-molding, the necessity to open the mold to move a previously molded layer of an article to a second mold cavity for molding of the next layer has been difficult to achieve in a cost effective and reliable manner, especially if there are geometrical profile differences between the over-molded layers. U.S. Pat. No. 3,914,081 to Aoki shows an early attempt to perform over-molding employing a rotary stripper plate which is used to extract, hold and transfer a molded first layer of an article to a second mold cavity, wherein a second layer of resin of a different color is injected. U.S. Pat. No. 3,947,176 to Rainville shows a split mold design that allows ejection of the article after the molding of a threaded neck portion of the article by splitting the mold laterally. Rainville-type molds have proven to be difficult to manufacture, need more “real estate” to allow opening of the mold walls, present sealing problems over a greater area and tend to leave injection marks on the molded article. 
     Attempts to produce a more suitable over-molding system include U.S. Pat. Nos. 4,744,742 and 4,830,811 to Aoki which shows a two cavity mold design for preforms-which is used with a rotary injection blow-molding machine. In these systems, the core enters a first cavity in which the first, inner, layer of a preform to molded. The core is then removed from the first cavity with the molded layer still in place and is inserted into a two portion second cavity, the lower portion of which is a single piece cavity of a larger diameter than the first and the upper portion of which is a two-part, split, cavity which defines threads for the neck portion of the preform. The second layer is then injected into the two portion cavity and the core is removed from within the cavity. The upper, threaded, portion of the cavity extracts the molded preform from the lower portion of the cavity and moves it to a blow molding station. After blow molding, the upper portion of the cavity is split to allow removal of the finished bottle. 
     The system taught by Aoki suffers from a number of disadvantages. First, the design is not readily applicable to forming more than two preforms per cycle, due to the complexity of the transfer platen used to move articles and the upper portions of the molds. Also, after the second injection operation is-performed, the. core is removed from the molded article prior to its transfer to the blow-molding station, preventing cooling of the interior of the preform by the core during the transfer. Thus, the bulk of the cooling must be performed before removal of the core, resulting in a relatively long cycle time. 
     A more recent attempt to produce over-molded preforms having a thread on the neck portion is shown in published European Patent Application 715,937 A1 to Massano. This reference teaches an injection mold to perform two-layer over-molding of a two-material PET preform wherein the mold comprises a stationary cavity plate, a moveable stripper/cavity and core plates. The cavity plate comprises adjacent pairs of single piece cavities of two different diameters and the stripper/cavity plate includes adjacent pairs of two-part cavity portion elements which can be split laterally. One cavity portion of each pair, which is aligned with the smaller diameter cavity in the cavity plate, has a smooth bore of the same diameter as the smaller diameter cavity and the other cavity portion of each pair, which is aligned with the larger diameter cavity, includes a thread to define the threaded neck portion of the preform. 
     The core plate has. rotatable pairs of adjacent cores and molding is performed by inserting the pairs of cores into the pairs of cavities with the stripper/cavity plate contacting the cavity plate so that the cavity portions on the stripper/cavity plate form part of the cavity for the injection operation. 
     A complete injection operation is performed by injecting a first layer of material into the smaller diameter cavity and cavity portion, then the core and cavity/stripper plates are each moved away from the cavity plate until the end of the molded preform has been completely removed from the cavity, after which the core plate continues to move away from the cavity plate while the stripper/cavity plate remains in place. The core plate moves away from the now stationary stripper/cavity plate to remove the molded first layer, which remains on the core, from the smooth-bored cavity portion on the stripper/cavity plate. Just prior to the core being completely removed from the cavity portion, the two parts defining the pair of cavity portions arc separated to allow the completed preform (commenced in the previous injection cycle) to fall from the threaded cavity portion, having been removed from the core by the engagement of the molded threads with the threaded cavity portion. 
     The molded first layer remains on the other core, being pulled through the smooth-bored cavity portion. Once the core and the molded first is completely removed from the smooth-bored cavity portion, the pair of cores are rotated one hundred and eighty degrees on the cavity plate so that the core with the molded first layer can now be inserted into the larger diameter cavity, through the threaded cavity portion on the stripper/cavity plate, and the now empty other core can be inserted into the smaller diameter cavity through the smooth-bored cavity portion to commence another injection molding cycle. 
     The core plate and the stripper/cavity plate are closed to the cavity plate and the second layer is injected into the larger diameter cavity and the threaded cavity portion to complete the molding of the preform on this core (a first layer is injected into the other cavity with the smooth-bored cavity portion to commence the molding of the perform on that core). The cavity plate and stripper/cavity plate are then moved away from the cavity plate, as described above, to eject the completed preform and to rotate the cores for the next portion of the cycle. 
     The Massano system described above suffers from several disadvantages. In particular, the core plate must be moved away from the cavity plate for a distance exceeding at least twice the length of the molded articles while the stripper/cavity plate must be moved away from the cavity plate for a distance exceeding the length of the molded articles to allow ejection of the molded articles. These opening requirements result in a slower cycle time, while the plates move the required distances, and in a machine which requires a relatively large amount of floor space in which to operate. Also, the molded first layer is pulled through the smooth-bored cavity portion at the end of the first injection operation and this can result in damage to the molded first layer. Further, the requirement to rotate each pair of cores increases the expense of manufacturing the machine and can lead to leaking of cooling fluid from the cores, etc. 
     Published PCT patent application WO 97/02939 to Collette et al. shows two other injection molding machines for over-molding. The first machine shown is a turret machine with a number of cores mounted on each of a pair of opposed sides of the turret and a pair of cavity plates, each with a set of a corresponding number of cavities, facing each turret face. The first set-of cavities is used to form the first layer of the molded article and the second set of cavities each including cavity extension portions to define the threads of a preform neck. One cavity plate and the turret move relative to the other cavity plate, and the turret rotates to move cores with a first molded layer from the first set of cavities to the second set of cavities where the second layer is molded with the cavity extension portions closed. The turret mold shown in Colette is used in conjunction with a conventional three platen injection molding machine. As shown in FIG. 2 a  of Colette, the second injection station unit (more exactly the second cavity plate) is located opposite the first one and in front of the clamping unit (not shown). The clamping unit thus prevents the injection unit from being located perpendicular to the mold plate, and instead it must be located at 90° to the stroke of the clamping unit. This results in Collette&#39;s machine having a large total foot-print. Further, Collette system requires an additional ejection system on the core plate to eject the molded articles from the cores which have been retracted from the second set of cavities. Such ejections systems are expensive and/or difficult to provide and can introduce other problems in the molding operation, such as core shift. 
     The second machine taught in Collette is a shuttle-type system wherein the cavity plate has two sets of first cavities surrounding a set of second cavities and two sets of cores are mounted to a core plate which shuttles the cores between a first position, wherein the first set of cores is aligned with one set of first cavities and the second set of cores is aligned with the second set of cavities, and a second position, wherein the first set of cores is aligned with the second set of cavities and the second set of cores is aligned with the other set of first cavities. The core plate is laterally “shuttled” between the first and second positions each time the mold is opened to sequentially insert a core in one of the first sets of cavities, where a first layer is molded, and then in one of the second set of cavities where the second layer is molded. This machine suffers from disadvantages in that it requires an extra set of cavities, i.e.—three sets of cavities produce two sets of articles, which increases the expense of the mold . 
     Multi-layer articles can also be formed by insert-molding wherein an insert, formed by extrusion, injection molding, thermoforming, etc., is placed into a mold cavity and a layer of another material is then injected to fill the cavity. In fact, insert-molding can be combined with over-molding or co-injection to encase the insert between multiple layers of different materials, if desired. 
     It is desired to have an efficient, reliable and cost-effective injection molding machine and mold therefore to form multi-layer molded articles. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel injection molding method, machine and mold therefore to produce multi-layer molded articles. 
     According to a first aspect of the present invention, there is provided an injection mold for producing over-molded articles, comprising: 
     a cavity plate having first and second cavities mounted thereon; 
     a core plate having a core mounted thereon; 
     a cavity extension comprising a pair of cavity elements located about said core; 
     cavity extension operating means on said core plate to move said pair of cavity elements between an open position wherein said core can be inserted into said first cavity between said pair of cavity elements and a closed position wherein said pair of cavity elements are combined with said second cavity to form a composite cavity of greater volume than said first cavity; and 
     a mold clamping unit operable with said cavity operating means to close said mold by inserting said core into said first cavity when said cavity extension elements are in said open position and to close said mold by inserting said core into said composite cavity when said cavity elements are in said closed position. 
     According to another aspect of the present invention, there is provided an injection molding machine for producing over-molded articles, comprising: 
     a cavity plate having first and second cavities mounted thereon, said second cavity having a cavity depth less than the cavity depth of said first cavity; 
     a core plate having a core mounted thereon, said core plate being movable relative to said cavity plate; 
     a cavity extension comprising a pair of cavity elements located about said core, said cavity extension having a depth substantially equal to the difference between the cavity depths of said first cavity and said second cavity and defining a geometric configuration different from that of said first cavity; and 
     cavity extension operating means on said core plate to move, said pair of cavity elements between an open position wherein said core can be inserted into said first cavity between said pair of cavity elements and a closed position wherein said pair of cavity elements are combined with said second cavity to form a composite cavity which receives said core. 
     According to another aspect of the present invention, there is provided an injection mold for producing over-molded articles comprising: 
     a cavity plate having first and second cavities mounted thereon, said second cavity having a cavity depth less than the cavity depth of said first cavity; 
     a core plate having a core mounted thereon, said core plate being movable relative to said cavity plate; 
     a cavity extension comprising a pair of cavity elements located about said core, said cavity extension having a depth substantially equal to the difference between the cavity depths of said first cavity and said second cavity and defining a geometric configuration different from that of said first cavity; and 
     cavity extension operating means on said core plate to move said pair of cavity elements between an open position wherein said core can be inserted into said first cavity between said pair of cavity elements and a closed position wherein said pair of cavity elements are combined with said second cavity to form a composite cavity which receives said core. 
     According to yet another aspect of the present invention, there is provided a method of injection molding an over-molded article, comprising the steps of: 
     (i) moving a cavity extension associated with a core to a disengaged position; 
     (ii) inserting said core into a first cavity having a defined volume and a first geometrical configuration; 
     (iii) performing a first injection operation into said first cavity to form a first layer of said article; 
     (iv) removing said core from said first cavity with said first layer of said article on said core; 
     (v) moving said cavity extension to an engagement position; 
     (vi) inserting said core and said first layer into a second cavity, said second cavity and said cavity extension engaging to form a composite cavity having a larger volume than said defined volume and defining a second geometrical configuration;. 
     (vii) performing a second injection operation into said composite cavity to overmold said first layer to form an article; 
     (viii) separating said core and cavity extension from said second cavity to remove said article therefrom; and 
     (ix) moving said cavity extension relative to said core to remove said article from said core. 
     According to yet another aspect of the present invention, there is provided an injection molding machine for producing over-molded articles, comprising: 
     a cavity plate having first and second cavities mounted thereon, said second cavity having a cavity volume greater than the cavity volume of said first cavity, each of said first and second cavities having means to receive an injection nozzle; 
     at least one injection unit to perform an injection operation into said first and second cavities; 
     a core plate having a core mounted thereon; 
     a cavity extension adjacent said core; 
     cavity extension operating means on said core plate to move said cavity extension between a disengaged position, wherein said cavity extension is distal said first cavity, and an engaged position, wherein said cavity extension combines with said second cavity to form a composite cavity; and 
     a mold clamping unit operable with said cavity operating means to close said mold by inserting said core into said first cavity when said cavity extension is in said disengaged position and to close said mold by inserting said core into said composite cavity when said cavity extension is in said engaged position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
     FIG. 1 shows a schematic representation of section through the turret of an injection molding machine and a mold in accordance with a first embodiment of the present invention; 
     FIG. 2 shows a schematic representation of a section through a portion of the core plate, stripper plate, cavity plate and a pair of slide operators of an injection molding machine and a mold in accordance with another embodiment of the present invention; 
     FIG. 2 a  shows the center portion of the machine of FIG. 2 with two cores and two cavities; 
     FIG. 3 shows the injection molding machine of FIG. 2 a  with the core and stripper plate separated from the cavity plate with the slide operators in a first position; 
     FIG. 4 shows the injection molding machine of FIG. 3 with the stripper plate separated from the core plate; 
     FIG. 5 shows the injection molding machine of FIG. 4 wherein the stripper plate has been moved back to the core plate with slide operators in a second position; 
     FIG. 6 shows the injection molding machine of FIG. 5 after the core plate has been rotated with respect to the cavity plate; 
     FIG. 7 shows the injection molding machine of FIG. 6 after the cavities have been closed; 
     FIG. 8 shows a cross section through a core, cavity extension and first cavity in accordance with another embodiment of the present invention; 
     FIG. 9 shows a cross section through the core and cavity extension of FIG. 8 and a second cavity; 
     FIG. 10 shows a cross section through an article produced in the mold of FIGS. 8 and 9; and 
     FIG. 11 shows a top view of the article of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, an injection molding machine in accordance with an embodiment of the present invention is indicated generally at  20 . As shown, machine  20  is a turret mold that can be advantageously operated on an innovative two-platen injection molding machine, similar to that described in co-pending U.S. patent application Ser. No. 08/772,474, filed Dec. 23, 1996 to Koch et al. and assigned to the assignee of the present invention (now U.S. Pat. No. 5,817,3452, and the contents of which are included herein by reference. This two platen injection molding machine is not only faster then than three platen machines, but also allows placing of various molding stations at any location around the turret mold, which saves space and provides manufacturing flexibility. 
     Machine  20  includes, for example, a four-sided turret  24  which can be rotated about axis  28  (that can be vertical, horizontal, etc.), in the direction indicated by arrow  32 . As will be apparent to those of skill in the art, turret  24  is rotated as desired to move four mold core assemblies  36   a ,  36   b ,  36   c  and  36   d  between four different molding operation stations  40 , 44 ,  48  and  52 . In the FIG. 1, and in the following discussion, similar components on each side of turret  24  are identified with like reference numerals to which an “a”, “b”, “c” or “d” is appended to identify the particular station the component illustrated is located at. 
     Further, FIG. 1 shows a partial cross-section through turret  24  so that only one core assembly  36  per side is visible. However, as will be apparent to those of skill in the art, turret  24  can include multiple core assemblies  36  on each side with corresponding numbers of components being located at each station, 40 , 44 , 48  and  52  as needed. In a presently preferred embodiment of the invention, each side of turret  24  includes forty eight core assemblies  36  on each side of turret  24 . 
     In FIG. 1, station  40  is an ejection station, station  44  is a first molding station, station  48  is a second molding station and station  52  is a cooling station. As shown in FIG. 1, each mold core assembly  36  includes a mold core  56 , a stripper plate  60 , a pair of cavity extension elements  64 , each of which is attached to a slide means  68 ,  68 ′. While not shown in the Figure, core  56  is provided with suitable means for circulating cooling fluid within core  56  and turret  24  is provided with suitable means for moving stripper plate  60 , which movement is described below in more detail. 
     Each slide means  68 ,  68 ′ is connected to a respective one of a pair of slide operators  72  via a tierod  76  and slides  68 ,  68 ′ can move toward or away from core  56  under the control of a respective slide operator  72 . Slide operators  72  operate as cavity extension operating means, as further described below. It should be noted that, for clarity, only one of the two slide operators  72  and one of the two tierods  76  of each core assembly  36  is shown in Figure  1  but in practice two slide operators  72  are provided on each side of turret  24 . 
     Each slide means  68  extends longitudinally along the side of turret  24  with each cavity extension element  64  on element  64  on a first side of each core  56  being mounted to slide means  68  adjacent that first side and each cavity extension element  64  on a second side of each core  56  being mounted to slide means  68  adjacent that second side such that, movement of a slide means  68  by its respective slide operator  72  results in all the cavity extension elements on a side of core  56  moving in unison toward or away from core  56 . 
     In the embodiment of FIG. 1; each slide operator  72  comprises a cam support  80  mounted to turret  24 , each cam support having a cam track  84  therein with two legs  86  and  86 ′ in which a cam follower  88  can move. Each cam follower  88  is connected to a respective slide means  68  by a respective tierod  76  and cam follower  88  moves with slide means  68  and with stripper plate  60 . At the extremity of cam track  84  distal turret  24 , there is a cam director  92  which operates to switch cam follower  88  between a leg  86  and a leg  86 ′ via a gate  96 . As shown in the Figure, each leg  86  and  86 ′ includes an inclined portion adjacent cam director  92  and a straight portion adjacent the side of turret  24 . 
     Cam director  92  is rotatable to move gate  96  into communication with the inclined portion of either leg  86  or  86 ′, as described below in more detail. Cam director  92  can be rotated by any suitable means as will occur to those of skill in the art, and in a presently preferred embodiment is rotated by pneumatic means. 
     The process of creating a multi-layer injection molded article with machine  20  will now be described, by discussing the operations performed at each station in turn. It will be apparent to those of skill in the art that, while the following discussion relates to the molding of a single article on a single side of turret  24 , in operation of machine  20  multiple article are being molded and/or operated on each side of turret  24 , at each station  40 , 44 ,  48  and  52 . 
     The injection molding operation for a multi-layer article commences with a core assembly  36   a  at station  40 . As shown in FIG. 1, core  56   a  is empty, a previously formed multi-layer article  100  (if any) having been stripped form core  56   a  by stripper plate  60   a  moving away from turret  24  as will be described further below. Cam followers  88   a  in each cam operator  72  are located in gate  96   a  so that tierods  76   a  have slides  68  in a half-open position, allowing previously molded article  100  (if any) to be ejected. Cam directors  92   a  are then rotated to bring gate  96   a  into alignment with leg  86 ′ and turret  24  is rotated ninety degrees in the direction indicated by arrow  32   
     At station  44 , the second step of the process is shown wherein stripper plate  60   b  is moved adjacent turret  24 . Stripper plate  60   b  is moved toward or away from turret  24  in any suitable manner as will occur to those of skill in the art and, in a presently preferred embodiment of the invention, is performed via hydraulic cylinders. As stripper plate  60   b  moved toward turret  24 , cam followers  88 b move along the inclined portions of legs  86 ′ to the straight portions adjacent turret  24 , moving tierods  76   b  away from core  56   b  and thus moving slides  68   b  and cavity extension elements  64 b mounted thereon, to a fully opened position. As will be apparent to those of skill in the art, the movement of stripper plate  60  can be performed simultaneously with the rotation of a side of turret  24  to station  44  from station  40 , to reduce total cycle time, or can be performed once that rotation is complete. 
     A first cavity  104  is then brought into engagement with core  56   b , extending between cavity extension elements  64   b , and a first injection operation is performed. As shown in FIG. 1, the base of core  56   b  directly engages cavity  104  via corresponding inclined surfaces  108  and  112  which aid in sealing cavity  104 . First injection operation can be performed with a single material or can be a co-injection operation, either simultaneously or sequentially, as will be apparent to those of skill in the art. 
     When the first injection operation is complete, cavity  104  is retracted from core  56   b  and stripper plate  60   b  is moved away from turret  24  to move cam followers  88  into gates  96   b . Cam directors  92  are then rotated to align gates  96   b  with the inclined portions of legs  86  while turret  24  is rotated to move core  56  with the molded article thereon to station  48 . 
     At station  48 , or while rotating to station  48 , stripper plate  60   c  is moved to a position adjacent turret  24 , thus moving cam followers  88  along the inclined portions of legs  86  to the straight portions of leg  86  proximal turret  24 . As cam follower  88  is moved along the inclined portions of legs  86 , tierods  76  and slides  68  are moved towards core  56   c , bringing the two halves of cavity extension element  64   c  into engagement about core  56   c . As stripper plate  60   c  continues to move toward turret  24 , cam followers  88  moves along the straight portion of leg  86  and the inclined surface  116   c  of the engaged cavity extension elements  64   c  engages the inclined surface  108   c  at the base of core  56   c.    
     A second cavity  120  is then moved into engagement with cavity extension elements  64   c , second cavity  120  having an inclined surface  124  complementary to an inclined surface  128  on engaged cavity extension elements  64   c.    
     As will be apparent, cavity  120  has a shorter length and a greater diameter than cavity  104 . As will also be apparent, cavity  120  is combined with the cavity formed by cavity extension elements  64   c  to obtain the required total length of the cavity. As will also be apparent, the portion of the combined cavity formed by cavity extension elements  64   c  defines different geometric features for a portion of the article to be molded in the combined cavity. In the illustrated embodiment, these different geometric features comprise threads for the neck portion of a preform, although any other features of differing geometries can be provided as will occur to those of skill in the art. 
     A second injection molding operation is then performed at station  48  to fill the combined cavity comprising cavity  120  and cavity extension elements  64   c . The second injection operation can be performed with a single material or can be a co-injection operation, either simultaneous or sequential, as will be apparent to those of skill in the art. 
     When the second injection molding operation is completed, cavity  120  is removed, leaving molded article  100  on core  56   c  and turret  24  is rotated to move core  56   c , with article  100  still thereon, to station  52 . At station  52 , article  100  is cooled, both by cooling fluid circulated within core  56 d and by cooling air blown over article  100 . 
     Next, turret  24  is rotated to bring core  56   d , and article  100  thereon, to station  40  to complete the molding operation. At station  40 , stripper plate  60   a  is moved away from turret  24 , moving cam followers  88 a along legs  86 . Cavity extension elements  64   c  are still engaged with each other and with article  100  and force article  100  along core  56   a  as stripper plate  60   a  moves away from turret  24 . As stripper plate  60   a  approaches the limit of its movement away from turret  24 , each cam follower  88  engages the inclined portion of legs  86 , moving tierods  76  to disengage cavity extension elements  64   a  from each other and from article  100 . As article  100  is substantially free of core  56   a  at this point, article  100  is ejected from machine  20  and can be removed from the vicinity of machine  20  by any suitable means such as a conveyor. Each cam follower  88  enters a respective gate  96   a , movement of stripper plate  60   a  ceases and the molding cycle is complete and machine  20  is ready to commence another cycle. 
     While the description above discusses a single molding cycle, it will be apparent to those of skill in the art that, in fact, four molding cycles are performed simultaneously, with each station  40 ,  44 ,  48  and  52  performing its respective operations on a different one of four different cycles. 
     While in the embodiment of FIG. 1 machine  20  includes the abovementioned four stations, it will be apparent to those of skill in the art that the number of stations and the corresponding number of sides of turret  24  can be selected as required by the molding operation to be performed. Further, while machine  20  of FIG. 1 includes the above-mentioned four different molding stations, it will be apparent to those of skill in the art that all the stations need not be different. For example, if eight stations are provided, they can comprise two repeated sets of the four stations described above to allow to complete articles to be produced on each half of a complete rotation of turret  24 . In either case, the number of simultaneous machine cycles which can be performed can be selected as desired. Also, it is contemplated that in some circumstances it may be desired to have a second cavity extension, formed from a second pair of extension elements, which can be used to form a composite cavity with a third cavity for a third injection operation. In such a case, the second pair of extension elements can move between their open and closed positions in a direction perpendicular to the pair of cavity elements for the first cavity extension, such that either both sets can be open at the same time or either set can be closed, as desired. 
     One of the significant problems which must be faced when injection molding articles is that the material, or materials, which are injected can be damaged by a slow transition from liquid to solid states as the article is cooled. The present inventors have determined that this damage, commonly referred to as the crystallinity problem, is mitigated or eliminated if adequate cooling and short cavity residence times can be obtained. As will be apparent, over-molding can aggravate the crystallinity problem in two aspects, the first being that the first layer acts as an insulator between the core and the second layer, inhibiting the transfer of the heat from the second layer to the core and the second being that the first layer is reheated, to some extent, by the injection of the second layer, thus enabling the formation of crystalline areas in the first layer during the second injection. In an over-molded article, this crystallinity problem can lead to failure of the inner layer, for example allowing food to contact the second layer of recycled material, or even total failure of the article. 
     Accordingly, as determined by the present inventors, the provision of cooling station  52  in the embodiment of FIG. 1, which allows both internal cooling of the article from the core and external cooling from the blown air or other cooling fluid at station  52 , is believed to provide significant advantages in allowing the reduction of crystallinity in over-molded articles. It is further contemplated that another cooling station can be provided in some circumstances, between station  44  and station  48 , to provide external cooling to the first layer between injection operations. In such a circumstance, turret  24  can have more than four sides or one or more stations, such as station  40  and station  52 , can be combined. 
     If it is desired to produce an article which is over-molded over an insert, it is contemplated that, in the situation wherein the insert is pre-formed by a separate process, an insert loading operation can be combined with the ejection operation at station  40 , and the insert placed on core  56   a  after ejection of a completed article  100 , or can combined with the machine operation at any other appropriate station. In the situation wherein the insert is to be molded in place by machine  20 , an appropriate additional station can be added at an appropriate location, as will also occur to those of skill in the art. In the situation wherein it is desired to mold over an insert between the injection operations, an appropriate additional station to load the insert onto a first, or subsequent, layer of the molded article can be provided between injection stations. 
     A second embodiment of the present invention will now be described with reference to FIGS. 2 and 2 a  through  8  wherein another molding machine in accordance with the present invention is indicated generally at  200  and similar components to those of the embodiment of FIG. 1 are identified with like reference numerals, although in these Figures the letters “a” and “b” are appended to distinguish between two sets of components. As described below, machine  200  is a rotary machine. 
     Machine  200  comprises a core plate  204  which includes a series of identical core assemblies  36  including cores  56 , a stripper plate  60  and a set of slides  68 , each of which has one or more cavity extension elements  64  mounted thereon. Machine  200  includes a pair of slide operators  72  which are mounted to core plate  204  and cam followers  88  in each slide operator  72  move with stripper plate  60 , as described above with respect to machine  20 . Each cam follower  88  is directly connected to the slide means  68  closest to it via a tie bar  76  and the remaining slides  68  are connected to alternating remaining slides via additional tierods  76  extending between slides  68  such that every second slide means  68  is operated by one slide operator  72  and the remainder of slides  68  are operated by the other slide operator  72 . For example, in FIG. 2 a  slides  68   b  and  68   a  are operated by slide operator  72   b  while slide means  68   c  is operated by slide operator  72   a.    
     In a preferred aspect of the present invention, cooling fluid is circulated to slides  68 , and thus to cavity extension elements  64 , via tierods  76  which are hollow, providing closed conduits between slides  68  through which cooling fluid is circulated. This use of tierods  76  to circulate cooling fluid to slides  68  is believed to be particularly advantageous and eliminates the need for cooling fluid hoses to be provided each slide  68 . 
     Machine  200  also includes a manifold plate  208  and a mold cavity plate  212  to which a plurality of pairs of cavities  216  and  220  are mounted. As shown, cavity  216  has a smaller diameter than cavity  220  and has a greater depth than cavity  220 . As is indicated in FIG. 2 a , only a portion of core plate  204 , stripper plate  60  and cavity plate  208  are shown for clarity and, in use, machine  200  can include forty-eight or more core assemblies  36  on core plate  204  and a corresponding number of cavities, arranged as adjacent pairs of cavities  216  and  212 , on cavity plate  208 . Accordingly, core plate  204  and mold cavity plate  212  have like numbers of cores and cavities, respectively, which can be arranged in a square, rectangular or other shaped array, as desired. 
     Core plate  204  is rotatable about central axis  224  and cavities  216  and  220  are arranged in the array on mold cavity plate  208  such that rotation of core plate  204  through one hundred and eighty degrees will result in each core  56  which was axially aligned with one of cavities  216  and  220  before the rotation, being axially aligned with the other of cavities  216  and  220  after the rotation. In a presently preferred embodiment, rotation of core plate  204  is reciprocating, i.e.—turning one hundred and eighty degrees in a first direction and then turning one hundred and eighty degrees in the opposite direction. While reciprocal rotation simplifies the various connections which must be effected to core plate  204  and the components mounted thereon, reciprocal rotation is not required and continuous rotation in a single direction is also possible. 
     The operation of machine  200  will now be described. For clarity, the molding of a single article on a single core  56   a  will be described, although it will be apparent to those of skill in the art that each core  56  is identical to each other core  56  and that an article is generally always being molded on each core  56 , albeit at one of two different stages, except at start up or shut down of machine  200 . 
     In FIG. 2 a , a molding cycle is commenced with core  56   a  inserted into cavity  216 . A shown, cam followers  88   a  and  88   b  are located in the straight portions of legs  86  of cam tracks  84  and slides  68   a  and  68   c  are thus moved away from each other, allowing cavity  216  to be inserted therebetween to engage the base of core  56   a . As shown, the base of core  56   a  includes an inclined surface  108   a  which engages a complementary inclined surface  112   a  on cavity  216  to assist in sealing cavity  216 . A first injection operation is then performed into cavity  216  to form a first layer of a molded article. 
     Next, core plate  204  is moved away from mold cavity plate  208 , as shown in FIG.  3  and the first layer molded onto core  56   a  in cavity  216  remains on core  56   a . Next, as shown in FIG. 4, stripper plate  60  is moved away from core plate  204  and, as can be seen, cam followers  88   a  and  88   b  move with stripper plate  60  and enter gates  96   a  and  96   b  respectively, moving slides  68  to the mid-points of their range of movement with respect to each other. 
     As will be apparent to those of skill in the art, the movement of stripper plate  60  will remove a finished article, if present, from core  56   b  until cam followers  88   a  and  88   b  enter the inclined portions of leg  86 , moving the pair of cavity extension elements  64   b  away from each other, allowing the completed article to fall, or be otherwise removed, from machine  200 . 
     Next, as shown in FIG. 5, cam directors  92   a  and  92   b  are rotated to bring gates  96   a  and  96   b  into alignment with legs  86 ′ and stripper plate  60  is moved toward core plate  204 . As cam followers  88   a  and  88   b  ride on the inclined portion of legs  86 ′, tierods  76   a  and  76   b  move slides  68   b  and  68   c  away from each other and slides  68   c  and  68   a  toward each other, thus closing the pair of cavity extension elements  64   a  about the article formed on core  56   a  in the first injection operation and opening the pair of cavity extension elements  64   b  about core  56   b  as shown. Closed cavity extension elements  64   a  define an inclined surface  116   a  which is complementary to and engages inclined surface  108   a.    
     Next, as shown in FIG. 6, core plate  204  is rotated about center axis  224  to align core  56   a  with cavity  220  and core  56   b  with cavity  216 . As will be apparent to those of skill in the art, cavities  216  and  220  can be arranged in a variety of manners on cavity plate  212 . For example, all of cavities  220  can be on one side of central axis  224  and all of cavities  216  can be on the other. Alternatively, cavities  216  and  220  can be arranged in repeating sets of pairs on either side of center axis  224 , with the ordering of the pairs being reversed on either side of center axis  22 . Other arrangements of cavities  216  and  220 , including mixtures and combinations of those mentioned above, will occur to those of skill in the art. 
     Next, core plate  204  is moved toward cavity plate  208  to close the mold, as shown in FIG. 7, and a second injection operation is performed in cavity  220 , over-molding the first layer previously formed on core  56   a  with a second layer. As shown in FIG. 7, closed cavity extension elements  64   a  define a second inclined surface  128   a  which is complementary to and engages inclined surface  124  of cavity  220 . 
     When the second injection operation of FIG. 7 is complete, machine  20  is in the same state as that shown in FIG. 2 a , albeit with the two cores in a reversed configuration, and the another molding cycle commences with the machine repeating the steps discussed above with respect to FIGS. 3 through 7. 
     As was the case for machine  20 , either or both of the injection operations of machine  200  can be injections of single materials or can be co-injection operations, either simultaneous or sequential, as desired. 
     While each of machines  20  and  200  have been described as having cavity extension elements  64  on the cavity used for the second injection operation, it will be apparent to those of skill in the art that this can be reversed, if desired, to allow creation of features of different geometries on the first layer which are then covered by the second layer. For example, the jeweled diffraction areas of an automotive indicator light lens can be molded in a first cavity having cavity extension elements to define the jeweled area and then inserted into a larger, second cavity in which a second layer of material is over-molded on the lens to form a smooth outer layer. The first layer can be molded in red translucent material, for example, and the second layer in transparent material. 
     It is also contemplated that the present invention can by employed in circumstances wherein a single, common, cavity is employed with different cores. In such an embodiment, the molded article is formed by a first injection operation into the cavity with a large core in place. The large core is then removed and replaced with a smaller core, while the article remains in the cavity, and a second injection operation is then performed to complete the article. The article is then ejected from the cavity and the cycle is repeated. In over-molding processes wherein the article remains on the core for each molding operation, there can be difficulty in providing adequate cooling through the core after the injection molding operation. This is because the first layer formed on the core acts to some extent as an insulator, inhibiting heat transfer between subsequent layers and the core. In the common cavity-multiple core embodiment of the present invention, this difficulty can be avoided by changing the core between injection operations. 
     FIG. 8 shows another embodiment of the present invention comprising a core  300  and a first cavity  304  and a cavity extension  308 . In FIG. 8, core  300  and first cavity  304  form a mold cavity  312  and cavity extension  308  is in a disengaged position, as shown. When an injection operation is performed, through inlet  316 , a first layer of an article is formed in cavity  312 . Core  300  is then removed from cavity  304 , with the first layer of the article on it. 
     As shown in FIG. 9, core  300  is then inserted into a second cavity  320  with the first layer of the article  324  on core  300  and a composite cavity  328  is formed by moving cavity extension  308  into an engaged position with cavity  320 . In this example, composite cavity  328  overlaps only a portion of article  324  and it will be apparent to hose of skill in the art that the present invention is not limited to the complete over-molding of a first layer of an article and can instead be employed to over-mold only portions of a first layer. 
     An injection operation is performed through inlet  332  to fill cavity  328  and core  300  and cavity extension  308  are separated from cavity  320  with the over-molded article on core  300 . Cavity extension  308  can then be moved along core  300 , toward over-molded article  324 , to eject article from core  300 . As will be apparent to those of skill in the art, cavity extension  308  engages only a portion of cavity  320  in this embodiment to form composite cavity  328  and cavity extension  308  is a single part. 
     FIGS. 10 and 11 show an example of the irregular geometry of article  324  which can be obtained with the present invention. Further, while in this example both the first and second injection operations employed the same materials, resulting in article  324  having a homogenous structure, it will be apparent to those of skill in the art that the first and second injection operations can employ different materials and can in fact be co-injection operations, if desired. 
     While the description above only specifically refers to turret and rotary machines, it will be apparent to those of skill that the present invention is not so limited and can be employed with shuttle-type or other machine types. 
     The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.