Abstract:
Post-mold cooling of injection molded plastic articles such as preforms is achieved by transferring the articles directly from the mold cavities onto cooling cores carried by a take-out plate. The molded articles are supported on the cooling cores until they become sufficiently frozen that they can be stripped from the cores.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefits from U.S. provisional application Ser. No. 60/330,541 filed Oct. 24, 2001 which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the plastic molding art. More particularly (though not exclusively) the invention is concerned with the manufacture of relatively large blow molded hollow articles such as bottles for carbonated drinks. 
     BACKGROUND OF THE INVENTION 
     Bottles for carbonated drinks typically are made in a wide variety of sizes, including quite large sizes (e.g. 2 litre capacity). These bottles are required to meet rigorous testing standards to guard against the possibility that a bottle might leak or burst in use. The bottles are required to withstand severe, extraordinary mechanical impacts without leaking. 
     In order to ensure that the finished bottles meet the required standards, close attention is paid to the bottle manufacturing process. One expedient that is adopted is to make the finished bottle via an intermediate product known as a “preform”. Preforms are essentially elongate plastic tubes that are closed at one end and formed with a thread and collar at the opposite end that will become the neck of the bottle. The body of the preform is much shorter and of much less diameter than the eventual bottle and is usually reheated and then blow-molded to the final bottle shape and size. 
     Preforms typically are made by injection molding and are designed to have a molecular structure that results in the final bottle having the required strength characteristics. Attention must also be paid to the clarity of the plastic material of the preform, to ensure that the eventual bottle has the required visual characteristics. For example, the plastic material must not be allowed to crystallize, otherwise the preform will be “cloudy” and the quality of the eventual bottle will be impaired. Rapid post-mold cooling of the preforms is essential if the objective is to be met. Rapid cooling is also important in terms of cycle time and therefore productivity of the overall molding process. 
     DESCRIPTION OF THE PRIOR ART 
     Injection and injection blow-molding of the preforms is well known technology and numerous proposals to speed up the cycle time are disclosed in the technical and patent literature. As noted previously, a key issue is the handling and temperature conditioning of the molded preforms. Reduced injection molding cycle times may result in soft preforms in which the plastic material may also crystallize and become cloudy. 
     A typical injection molding machine includes a mold comprising a mold cavity part having an array of cavities for defining the exterior of the articles to be molded, and a mold core part having corresponding cores that are received in the mold cavities when the mold parts are closed, for defining the interior of the molded articles. A number of prior art solutions to the problem of reducing cycle time involve retaining the molded preforms on the molding cores after the cores have been removed from the mold cavities and internally cooling the cores. A disadvantage of this approach is that the molding apparatus must have several sets of mold cores so that a set can be used for molding while one or more other sets are used for cooling. Each core must also have associated “neck rings” that form the threaded neck portion of the preform. In other words, specialized molds and specialized injection molding machines are required. 
     The best known examples of this type of injection molding machine are known as the “shuttle mold” or the “turret” or “index type” machine. 
     The shuttle mold approach requires a mold core part that has twice as many mold cores as mold cavities. This approach is disclosed in several patents including U.S. Pat. No. 4,473,515 (Ryder), U.S. Pat. No. 5,501,593 (Marcus) and U.S. Pat. No. 6,095,788 (Manen). 
     Turret or index type injection molding machines use a rotary block that in most cases has four faces provided with respective mold core plates in different planes. The block is indexed about an axis perpendicular to the axis of injection to bring different core plates into the molding position, while other core plates are exposed to the air for cooling of preforms carried by those cores. 
     Another approach is to transfer the molded preforms directly from the mold cores onto a take-out plate having cooling tubes for post mold cooling. Examples of this approach are disclosed in U.S. Pat. No. 4,592,719 (Bellenhache) and in WO 00/29193 and EP 937566 (both to Biraghi). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an injection molding method and apparatus that provides for improved post-mold cooling of preforms or other injection molded articles. 
     In one aspect, the method of the invention comprises the steps of: 
     a) providing a molding system comprising a mold cavity plate having a plurality of injection cavities, and a mold core plate having a corresponding plurality of injection cores receivable in the cavities for forming a plurality of molded articles; 
     b) with the mold plates in a closed position, injecting plastic material into the mold cavities to form the molded articles; 
     (c) separating the mold plates while retaining the molded articles in the injection cavities; 
     (d) providing a take-out plate having retaining cores; 
     (e) positioning the take-out plate between the cavity plate and the core plate; 
     (f) transferring the molded articles directly from the cavities onto the retaining cores; and, 
     (g) cooling the molded articles on said retaining cores. 
     While the molded articles may be allowed to cool passively on the retaining cores, the articles preferably are cooled actively, for example, by means of cooling fluid jets that are directed internally and/or externally onto the molded articles. Typically, the retaining cores will incorporate passageways for internal cooling of the molded articles. 
     The mold plates themselves may be designed to permit partial cooling of the molded articles while they are still in the mold, i.e. before the articles are transferred to the cooling cores. 
     In prior art approaches in which the freshly molded articles are cooled on a take-out plate having tubes, it can be difficult to provide proper support for the molded articles and ensure that they do not sag against the walls of the tube while the articles are in only a partially frozen condition. According to the present invention, the molded articles are retained in the mold cavities and then transferred directly to a take-out plate having cooling cores. Supporting the molded articles on cooling cores avoids sagging of preforms. The cooling cores can be configured identically or very similarly to the mold cores. 
     In another aspect, the invention provides an apparatus that includes a mold comprising a mold cavity plate having a plurality of injection cavities and a mold core plate having a corresponding plurality of injection cores receivable in the cavities for forming a plurality of articles. The mold plates are movable between open and closed positions and the apparatus further includes a take-out plate including a plurality of retaining cores corresponding to said plurality of cavities, and means for transferring molded articles from said cavities directly to said cores. 
     Preferably, the apparatus includes cooling means located on the take-out plate, for example, means for delivering cooling air internally and/or externally to said molded articles. 
     The retaining cores may correspond in number and position to the number and position of the cavities in the mold cavity plate so that the retaining cores can be inserted directly into the mold cavities for transfer of molded articles onto the cores. Alternatively, the number and configuration of retaining cores can be different from the number and/or configuration of mold cavities, in which case it will be necessary to completely remove the molded articles from the mold cavities and transfer them onto the retaining cores. 
     The word “plate” as used herein in referring to a mold cavity plate, a mold core plate and a take-out plate represents commonly accepted terminology in the art and, where appropriate, is to be interpreted broadly as including structure that may be more complex than a simple flat plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate schematically a number of preferred embodiments of the invention by way of example, and in which: 
         FIG. 1  is a perspective view of a typical bottle preform that may be manufactured in accordance with the present invention; 
         FIG. 2  is a perspective view of the cavity mold plate of an injection molding apparatus in accordance with the invention; 
         FIG. 3  is a detail perspective view of part of  FIG. 2  showing components of the cavity mold plate in an open position for release of a preform; 
         FIG. 3   a  is a view similar to  FIG. 3  illustrating an alternative embodiment; 
         FIG. 4  is a perspective view showing the mold plates in an open position with a take-out plate interposed between the mold plates for removing molded articles from the mold cavities; 
         FIGS. 5  to  9  are side elevational views of the apparatus shown in  FIGS. 2 ,  3  and  4 , illustrating successive steps in the overall molding cycle; 
         FIGS. 10  to  14  illustrate successive steps involved in molding an individual preform; 
         FIGS. 15 ,  16  and  17  are sequential views illustrating an alternative embodiments of cooling core; and, 
         FIG. 18  is a view illustrating an embodiment of the invention in which the molded articles are blow molded immediately following injection molding. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1 , an injection molded bottle preform is denoted by reference numeral  20  and has a generally cylindrical overall shape with a closed outer end  22  and an open inner end  24 . The portion of the preform adjacent end  24  will form the neck of the bottle to be made from the preform and includes a cylindrical threaded section  26  and an annular flange  28 . 
       FIGS. 2  to  6  schematically illustrate the principal components of a molding system for making multiple preforms  20  in batches. For purposes of illustration, the drawings show a system for making batches of 12 preforms; in practice, the apparatus will be designed to make much larger batches, as is conventional in the art.  FIG. 2  shows the cavity plate of the mold, generally at  30 . Open inner ends of the mold cavities (arranged in vertical rows) are visible at  32 . A conventional hot runner system for delivering molten plastic material to the mold cavities is indicated at  34  in FIG.  6 . 
     A mold core plate is generally indicated by reference numeral  36  in  FIGS. 4 ,  5  and  6  and carries mold cores  40  in an array that corresponds to the array of cavities seen in FIG.  2 .  FIG. 5  shows the mold core plates in an open position prior to injection, in which the mold cores  40  are spaced from but aligned with the mold cavities.  FIG. 6  shows the mold core plates in a closed position in which the mold cores  40  are received within the mold cavities (denoted  42 ) ready for injection of molten plastic material. 
     In a conventional injection molding machine used for making preforms, the threads  26  and annular flange  28  of the preform ( FIG. 1 ) are formed by so-called “neck rings” that are carried by the core plate  36 . Each ring is made in two halves so that the ring can open to release the molded part at the end of the injection cycle. 
     In contrast, the corresponding mold elements are carried by the cavity plate  30  in the present invention. This differentiates the mold overall from the prior art. Thus, neck rings for forming the threads  26  and flanges  28  on the preform are incorporated in plates that are carried by the cavity mold plate  30 . As seen in  FIG. 2 , the mold cavities are aligned in vertical rows represented by the openings that are denoted  32 . Neck rings for forming the threads and flanges on the preforms made by each vertical row of cavities are incorporated into a pair of neck ring plates  44  that meet on a vertical line  46  that bisects the open ends  32  of the cavities. The two plates are formed with respective half-rings  48  that are aligned with the mold cavities and the plates are movable between the closed positions in which they are seen in  FIG. 2  during molding, and open positions in which the plates are spaced apart and moved outwardly away from the cavity mold plate  30  as shown in FIG.  3 . 
     This movement of the neck ring plates  44  is accomplished by providing appropriately shaped cam tracks shown at  50  in  FIG. 3  that co operate with corresponding followers  52  carried by rearward extensions  44   a  of the neck ring plates  44 . The cam tracks  50  are formed in a stationary plate  54  above and at right angles to the neck ring plates  44 . Actuators (not shown) are provided for displacing the neck ring plates  44  outwardly in the directions indicated by arrow  56  in  FIG. 3  (and back) at appropriate times, to cause the required outward displacement and separation of the neck ring plates  44 . It will be seen from  FIG. 2  that a corresponding set of cams and cam tracks are provided on a plate  58  below the cavity mold plate  30 . A corresponding set of actuators (also not shown) are provided in association with a rearward extension  44   b  at the bottom of each neck ring plate  44 . 
       FIG. 3   a  illustrates an alternative embodiment in which the cam tracks  50  (and the corresponding cam tracks at the bottom of the cavity mold plate) are incorporated into top and bottom faces of the cavity mold plate and co-operate with followers  52  on the underside of the rear extensions  44   a  and  44   b  of the neck ring plates. 
     Reverting to  FIG. 6 , the neck ring plates  44  are of course in the closed position shown in  FIG. 2  during injection of molten plastic material into the mold cavities  42 . Once the cavities have been filled with molten plastic material, the mold is opened. Core plate  36  moves back to the position shown in  FIG. 5 , withdrawing the mold cores  40  from the mold cavities and leaving the molded articles within the cavity mold plate  30 . In accordance with the method of the invention, a series of cooling cores carried by a take-out plate are then interposed between the two mold plates and the molded articles are transferred directly to the cooling cores. 
       FIG. 7  shows a take-out plate  60  carrying cooling cores  62  positioned between the open mold plates, ready to receive the molded articles. In the illustrated embodiment, there are in fact two sets of cooling cores and the take-out plate  60  is carried by a robot arm  64  so that the take out plate can be manipulated to receive two sequential batches of molded articles, one of which can be cooling while the second set of cooling cores receives a second batch of freshly molded articles.  FIG. 4  shows schematically the robot arm  64  and the two sets of cooling cores  62  that are carried by plate  60 .  FIG. 4  also makes it clear that each set of cooling cores includes one core for each of the mold cavities. In practice, the two sets of cores normally will be interlaced on plate  60  rather than in two separate sets. 
       FIGS. 8 and 9  illustrate the step of transferring the freshly molded preforms to the cooling cores  62 . As seen in  FIG. 8 , the two neck ring plates  44  have been moved under the control of the cam tracks  50  and  52  ( FIG. 3 ) to advanced positions for transferring the preforms  20  onto the cooling cores  62 . As described in connection with  FIG. 3 , the neck plates  44  not only move outwardly away from the cavity mold plate  30 , but also move apart so that the preforms are released after having been positioned on the cooling cores  62 . The neck ring plates  44  are then retracted to the position shown in FIG.  2 .  FIG. 9  shows the components of the molding apparatus with the freshly molded preforms  20  positioned on the cooling cores  62  and the neck ring plates  44  retracted. 
       FIGS. 10  to  14  illustrate the steps of transferring the molded preforms onto the cooling cores with reference to a single preform.  FIG. 10  shows one of the molding cores  40  entering a corresponding mold cavity  42 .  FIG. 11  shows the mold plates closed with the core plate  36  in contact with the neck ring plates  44  and plastic material having been injected into the mold cavity as indicated at  66 . 
       FIG. 12  shows the mold plates open with core plate  36  retracted to the left and the take-out plate  60  positioned between the mold plates as shown in FIG.  9 . In  FIG. 13 , the neck ring plates  44  have been advanced to transfer the molded preform onto the cooling core  62 , and have moved apart to release the preform. As seen in  FIG. 14 , the neck ring plates  44  have been retracted to the position shown in FIG.  9 . 
     The take-out plate  60  is provided with means for retaining the molded preforms on the cooling cores  62 .  FIGS. 12  to  14  illustrate one possible form that these means may take. As shown in those views, the take-out plate  60  carries, adjacent each cooling core, a pair of gripper arms  68  that are pivotally coupled to the take-out plate  60  at their inner ends, as indicated at  70 . The arms  68  are spring-biassed to the retracted positions in which they are shown in FIG.  12  and the take-out plate  60  is provided with an actuator plate  72  that can be moved outwardly with respect to the take-out plate to move the arms  68  inwardly against the spring biassing effect. Appropriate actuators (not shown) are provided on the take-out plate  60  for displacing plate  72 . 
     As seen in  FIG. 13 , the gripper arms  68  are in their outwardly biassed positions and the preform  20  has just been released by the neck ring plates  44  onto the core  62 . Plate  72  is then moved outwardly as shown in  FIG. 14  to pivot the arms  68  inwardly and engage the annular ring  28  of the preform, drawing the preform onto the cooling core  62 . When the preform has been cooled and is to be released, plate  72  is simply returned to its rest position, allowing the arms  68  to open under the effect of their spring-biassing, and release the preform. Robot  64  may invert the cooling cores so that the preforms will be stripped from the cores by gravity. Alternatively, an air jet may be used to blow the preform off the cooling core. Typically, air or other fluid will be used to internally cool the preform so the core  62  will be provided with an air outlet and a return, as indicated by way of example at  74  and  76  respectively in FIG.  14 . 
     External jets of air or other fluid for cooling the performs are shown schematically at  77  and may be used in combination with or separately from internal cooling means. 
     Alternative means for retaining the preforms on the cooling cores may comprise neck ring plates similar to the plates  44  but carried by the take-out plate  60 , for engaging the preforms after they have been placed on the cooling cores and then releasing the preforms after cooling. 
     Another possibility is to use suction to draw the preform onto the cooling core, and hold the cooling core in place, either by suction alone or in combination with a mechanism such as the gripper arms  68  referred to previously.  FIG. 15  shows one form of cooling core,  62 ′, that is provided with both a suction port  78  for drawing the preform onto the core and a pressure port  80  for directing cooling air into the preform. 
       FIG. 16  shows a further alternative form of cooling core, denoted  62 ″ having air outlet and return ports  82  and  84  respectively for cooling the preform.  FIG. 17  shows a still further form of cooling core, denoted  62 ′″ having air outlet and return ports  86 ,  88  respectively through which air can be directed to blow the cooled preform off the core. 
     Finally,  FIG. 18  shows an embodiment in which molded preforms  20  are removed from the take-out plate  60  by a further robot arm  90  and then placed directly in a blow molding machine  92  for forming the preforms into bottles. In this case, the preforms will not normally be cooled. In other words, the apparatus performs a take-out function without active cooling. 
     Broadly speaking (not only in the embodiment of FIG.  18 ), the cores may function generally as “retaining” or “transfer” cores having no active cooling function. 
     The retaining cores may be made of a thermally conductive material (e.g. aluminum) for passively cooling the molded articles. 
     The invention may be carried on using any known injection molding machine having an injection unit to provide moldable material under pressure to a mold cavity space formed between a mold core and a mold cavity. Any known machine clamp unit is used to maintain a mold cavity plate and a mold core plate in contact and in a closed position during the injection process. 
     In one embodiment, the take-out plate comprises retaining cores to hold the molded articles transferred directly from the injection mold cavities. The retaining cores are made of any suitable material. In one instance the retaining cores have an inner diameter that allows the core to make contact with the molded article. In this case, the core may be made of a thermally conductive material that allows the heat transfer from the article to the core. Therefore the cores may be made of steel, steel alloys, aluminum or aluminum alloys, copper and copper alloys and other materials. 
     It will of course be understood that the preceding description relates to particular preferred embodiments of the invention only and that many modifications are possible within the broad scope of the invention, some of which have been indicted and others of which will be apparent to a person skilled in the art. It should be noted in particular that the precise expedients that are used for transferring the freshly molded preforms from the mold cavities to the cooling cores are given by way of example only and may vary. Also, while reference is made specifically to bottle preforms (i.e. an intermediate product), it is to be understood that the method and apparatus of the invention may be applied to the cooling of finished molded articles.