Abstract:
A press arrangement having ball-screw drive units for providing rapid opening and closing movement of the press, and having a pneumatic closure arrangement for providing a final small-stroke closing movement and holding force for the mold. The pneumatic closure arrangement acts through the mold to cause the ball-screw drive units to be axially displaced a small amount sufficient to cause automatic engagement of a friction brake which nonrotatably holds the screw shaft to prevent back driving thereof due to the mold closure force.

Description:
FIELD OF THE INVENTION 
     This invention relates to a press system for forming plastic parts, such as urethane parts, and more particularly relates to an improved press which utilizes a ball-screw driving arrangement for high-speed closure of the press and an air pressure arrangement for holding the press closed. The press also incorporates a brake which is automatically applied when molds associated with the opposed press platens are loaded so as to prevent back driving of the ball-screw drive arrangement. 
     BACKGROUND OF THE INVENTION 
     Copending application Ser. No. 09/041 068, owned by the Assignee hereof, illustrates therein a press arrangement which is particularly desirable for permitting molding of large plastic parts, such as urethane parts. The press arrangement of the aforesaid application incorporates ball-screw drive shaft arrangements for effecting relative opening and closing movement of the opposed mold bearing press platens. The drive arrangements of this press have proven highly desirable with respect to the performance they provide, and are particularly desirable where the press is mounted on a mobile carrier, such as an overhead support track, since the press does not have to rely on the use of hydraulic actuators and the like for the major performance functions carried out thereby. In the aforementioned press, the ball-screw drive actuators are controlled by both high speed and low speed drives which react with at least one of the press platens to provide rapid opening and closing movement of the press platen, and provide for slow movement throughout a small stroke which exists at the time the opposed mold parts are being moved into a closed relationship. This arrangement, in conjunction with the brakes associated with the drive motors for the screw shafts, provides the desired control over the press platens. 
     The press arrangement of the aforesaid application has proven highly desirable, but nevertheless in some situations the press is of a more sophisticated and complex construction than is necessary or desired. 
     In other known rim presses, the final closure of the mold has often been accomplished using fluid pressure devices such as air bags or the like. In these known presses, however, the final closure stroke has generally been large (i.e., three-fourths inch or greater), and this has imposed limitations with respect to adapting the press to different tool sizes, and limitations due to the required structure of the air bag and the required large quantity of pressurized air. This press construction also encourages users to insert spacers into the press to minimize the stroke and increase the closure force, but this can often cause damage to the tool (i.e., mold). 
     Accordingly it is an object of this invention to provide an improved press arrangement which, while it utilizes ball-screw drive units for providing rapid but desirable control over the primary opening and closing movement of the press, it in addition is provided with a pneumatic closure arrangement which provides the final small-stroke closing movement and holding force for the mold, whereby the overall press arrangement is mechanically simplified, and such press arrangement is equally usable either when mounted on a mobile overhead carrier or when provided in association with a stationary floor-supported frame. 
     In the improved press arrangement of this invention, the ball-screw drive arrangements are activated to permit rapid movement of one of the press platens (the upper press platen in the illustrated embodiment) during the basic opening and closing movements of the press. When substantially in a closed position, that is in a position which is closely adjacent but spaced from a fully closed position by only a very small distance, then full closure between the mold parts is effected by a mold displacing arrangement such as an air pressure lifting device associated with one of the press platens (the lower press platen in the illustrated embodiment) to effect full closure of the mold and holding of the mold in the closed position during the molding operation. The activation of the mold displacing arrangement also acts through the mold to cause the ball-screw drive arrangement to be axially displaced a small amount sufficient to cause automatic engagement of a friction brake which effectively nonrotatably holds the screw shaft to prevent back driving thereof due to the mold closure force, whereby separate latching of the upper press platen is not required. The friction brake automatically disengages when the mold closing force is released. 
     In the improved press arrangement of this invention, as aforesaid, the upper and lower press platens are preferably respectively provided with booking plates which mount the mold halves thereon, and which are each preferably supported in such fashion as to permit the booking plates to be pivotally moved so as to face sidewardly, preferably toward opposite sides of the press, to facilitate access to the mold halves mounted thereon. In the preferred and illustrated embodiment, the lower booking plate and the lower press platen in its entirety is vertically swingable through a limited extent about a horizontal pivot axis to permit sideward tilting of the lower press platen to facilitate access to the lower mold half. 
     The improved press arrangement of this invention, as aforesaid, also includes, as an option thereon, preferably on the lower press platen, a release mechanism which acts in opposition to the mold displacing arrangement so as to facilitate cracking (i.e., initial opening) of the mold in the event that deactivation of the mold displacing arrangement is inadequate to effect mold separation. 
     The improved press, as aforesaid, also permits a short-stroke, high-force closure of the mold by a pneumatic device which can be maintained of simple construction and function. 
    
    
     Other objects and purposes of the invention will be apparent to persons familiar with constructions of this general type upon reading the following specification and inspecting the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a press arrangement according to the present invention and illustrating the upper press platen in an open position, and with the booking plate of the upper press platen tilted sidewardly into its access position. 
     FIG. 2 is an end elevational view of the press as appearing in FIG.  1 . 
     FIG. 3 is an end elevational view similar to FIG. 2 but also illustrating the lower press platen tilted into its side access position. 
     FIG. 4 is an end elevational view of the press substantially in its closed position. 
     FIG. 5 is an enlarged fragmentary sectional view taken generally along line  5 — 5  in FIG.  4 . 
     FIG. 6 is a fragmentary sectional view taken generally along line  6 — 6  in FIG.  5 . 
    
    
     Certain terminology will be used in the following description for convenience in reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the apparatus and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import. 
     DETAILED DESCRIPTION 
     Referring to the drawings there is illustrated a carrier  10  provided with a press arrangement  11  according to the present invention. The carrier includes a frame structure  12  having, as a part thereof, a top frame  13  which is movably supported under a pair of overhead support rails  14 . The support rails  14  are typically positioned in upwardly spaced relation from a floor, such as adjacent a ceiling, and typically define a closed loop so that a plurality of like carriers can be moved along a closed path into a plurality of workstations defined along the path. 
     In the illustrated embodiment, the top frame  13  is provided with a plurality of support rollers  15  mounted on upwardly projecting yokes  16 , which rollers engage the overhead rails  14  to permit the carrier to be rollingly suspended therefrom. The top frame also mounts thereon one or more drive motors  17 , typically electric drive motors, which rotate one or more drive rollers  18  which are disposed in driving engagement with the overhead rails. 
     The frame structure  12  also includes a pair of end frames  19  which, in the illustrated embodiment, are fixed to the top frame  13  adjacent opposite longitudinal ends thereof, and project vertically downwardly in generally parallel but spaced relation. These end frames  19  resemble vertically elongate posts or columns and, in the illustrated embodiment, are defined by structural elements such as I-beams. 
     The press arrangement  11  as provided on the carrier  10  is preferably of the type which is often referred to as a rim press, and as such is used for forming parts which are least partially molded of a plastics material such a polyurethane, which parts are typically rather large in size and may comprise parts such as interior door panels for vehicles and the like. The press arrangement  11  includes an upper or top press structure or platen  21  which in the preferred embodiment is vertically movably supported on and between the end frames  19 , and a lower or bottom press structure or platen  22  which is supported on and between the lower ends of the end frames  19 . The top press structure  21  is vertically slidably guided on guides or gibs  20  which are fixed to and project vertically along the frame posts  19 . 
     The press  11  is adapted to support thereon a mold which typically includes a top mold part  23  which is fixedly supported on and projects downwardly from the upper press structure  21 , and a bottom mold part  24  which is fixedly supported on and projects upwardly from the bottom press structure  22 . The top and bottom mold parts  23  and  24  are disposed in vertically aligned relation when the mold is in a closed relationship as illustrated by FIG. 4, and they respectively define thereon opposed mold cavity walls which cooperate to define a mold cavity for forming the desired part when the top and bottom mold parts have been relatively moved into a closed engaged relation as diagrammatically illustrated by FIG.  4 . The construction of the mold parts is conventional, and is designed in accordance with the specific part being molded, whereby further description thereof is believed unnecessary. 
     The top press structure  21  includes a top press support  25  which extends transversely between the frame posts  19  and is vertically slidably supported thereon. The top press structure  21  also preferably includes a top press plate  26  which is positioned under the top press support  25  and defines thereon, in the normal operating position of the press, a generally horizontally enlarged and downwardly facing surface to which the upper mold art  23  is fixedly secured. This top press plate  26 , often referred to as a booking plate, is secured along one longitudinally extending side edge thereof to the top press support  25  by a hinge structure  27  which defines a generally horizontally extending hinge axis, the latter extending generally in the longitudinal direction of the carrier and being disposed adjacent one side thereof. The hinge structure  27  permits the top booking plate  26  and the top mold part  23  mounted thereon to be vertically swung downwardly through an angle approaching 90° so that the top booking plate is suspended generally vertically downwardly between the end frames  19  adjacent one side of the carrier, substantially as illustrated by FIGS. 1 and 2. When in this downwardly suspended position, the upper mold part  23  is positioned so that the mold cavity thereof can be readily accessed by a worker standing adjacent one side of the press to permit performance of the necessary manual operations required relative to the upper mold part. 
     To effect movement of the top booking plate  26  between the upper and generally horizontal molding position shown in FIG. 4, and the vertically suspended access position shown in FIG. 3, an activating mechanism  31  is provided for effecting movement of the top booking plate  26  between the positions shown in FIGS. 3 and 4. This control mechanism  31  includes a fluid pressure cylinder (i.e. an air cylinder)  32  whose housing  33  is supported at its upper end from the top frame by a hinge  39 . A conventional cylinder rod  34  projects downwardly from the cylinder housing  33  and at its lower end is connected by a hinge  35  to an elongate control link  36 , the latter at its opposite end being connected by a hinge  37  to an arm  38  which is fixed to the booking plate  26 . The lowermost chamber of the pressure cylinder  32  is constantly supplied with pressurized air so as to always tend to urge the piston rod  34  upwardly into a retracted position, substantially as shown in FIG. 3, in which position the booking plate is maintained in its open access position whenever the upper press structure  21  is in its raised open position. Movement of the upper press structure  21  downwardly toward the closed position, however, causes the upper booking plate  26  to automatically swing upwardly about the hinge  27  into a horizontal position wherein it seats on the underside of the top press support  25 . Appropriate automatically activated latches  40  are typically provided on the edge of the top press support  25  for locking the booking plate  26  in the horizontal position. 
     The activating mechanism  31  for causing automatic swinging of the booking plate  26  to and from its sideward access position in response to vertical displacement of the supporting press structure  25  is described in copending Ser. No. 09/039 920, also owned by the Assignee hereof. The disclosure of this latter copending application is, in its entirety, incorporated herein by reference. 
     The bottom press structure  22  as shown in FIGS. 5 and 6 also includes a horizontally enlarged bottom press plate  42  (often referred to as the bottom booking plate), the latter defining thereon a horizontally enlarged upper surface on which the bottom mold part  24  is fixedly positioned. This bottom booking plate  42  is supported above and on a bottom press support  41 , the latter being supported on and extending horizontally between the lower ends of the frame posts  19 . 
     The bottom press support  41  includes a main structural support  43  defined by generally parallel and horizontally enlarged top and bottom support plates  44  and  45 , respectively, the latter being rigidly joined together by a plurality of intermediate support channels  46 . Opposite ends of the top and bottom plates  44  and  45  are in turn rigidly joined to vertical end plates  47 , the latter being positioned inwardly of but in close proximity to the respective end posts  19 . 
     The bottom booking plate  42  is supported on the main support  43  for limited vertical displacement, and for this purpose a lifting device  48  cooperates between the main support  43  and the bottom booking plate  42  for controlling vertical raising and lowering of the latter. This lifting device  48 , which in use functions for effecting final mold closure, includes a plurality of inflating devices  49 , such as flexible air hoses, positioned in interposed relation between the bottom booking plate  42  and the top support plate  44 . These hoses  49  extend transversely of the plates  42 ,  44  in the region therebetween, and are closed at one end, and are connected to an air supply conduit  51  at the other end, the latter being connectable to a suitable source of pressurized air. 
     The bottom booking plate  42  has a plurality of pins  52  secured thereto and projecting downwardly therefrom so as to slidably project through the top plate  44 . These pins  42  at the lower ends thereof are provided with enlarged heads, and springs  53  surround the pins  52  and coact between the enlarged heads and the underside of the top plate  44  to normally urge the bottom booking plate  42  downwardly into a seated position which is determined by spacers  54  positioned between the plates  42  and  44 . These spacers maintain a minimal clearance between these latter plates to accommodate the air hoses  49  therebetween when the air hoses are deflated. 
     The bottom press structure  22  is supported on and between the lower ends of the frame posts  19  by means of a swivel arrangement  55  which cooperates between the frame posts  19  and the main support  43 . This swivel arrangement  55  includes a pair of hubs  56  which are fixed to and project outwardly in generally aligned relation from the opposite end plates  47  so as to define a generally horizontal pivot or swivel axis  57 . The hubs  56  project into and are rotatably supported within bearing sleeves  58  which in turn are secured to support blocks  59 . Each support block  59  in turn is fixedly secured between a pair of side plates  61 , the latter in turn being fixedly secured to and projecting generally transversely relative to the respective end posts  19  adjacent the lower free end thereof. This swivel arrangement  55  enables the lower press structure  22 , in its entirety, to be pivotally displaced relative to the frame about the horizontal swivel axis  57  so that the lower press structure can be moved into a sidewardly inclined position substantially as illustrated in FIG. 3 so as to facilitate sideward access to the lower mold part  24 . This sideward access position, in the illustrated embodiment, is preferably disposed at an angle of about 20 to about 45 degrees relative to the horizontal. 
     The sideward tilting of the lower press structure  22  is controlled by a tilt actuator  63  which, in the illustrated embodiment, comprises a fluid (i.e. air) pressure cylinder  64  whose housing  65  is connected at its upper end by a hinge  66  to a bracket  67  which is fixed to and projects transversely from the respective frame post  19 . A piston rod  68  projects outwardly of and is extendible from the lower end of cylinder housing  65 , and the outer (i.e. lower) free end of the piston rod  68  has a connecting pin  69  thereon which couples to the main support  43  so as to permit tilting of the lower press structure  22  from the horizontal molding position of FIG. 2 into the sideward access position of FIG. 3 when the cylinder  64  is energized so as to extend the piston rod  68  downwardly. 
     The lower press structure  22  also has, as an option, a mold opening assist mechanism  71  (FIGS. 5 and 6) associated therewith. This latter mechanism includes a generally horizontally enlarged actuator plate  72  which is disposed adjacent and is vertically movable through a limited extent relative to the bottom plate  45 . For this purpose, the actuator plate  72  has a plurality of pins  73  secured thereto and projecting upwardly through the bottom plate  45 , which pins have enlarged upper ends. Springs  74  surround the pins and cooperate between the enlarged heads thereof and the upper surface of the bottom plate  45  so as to normally urge the actuator plate  72  upwardly into a raised position wherein it abuts spacers  75  which in turn abut the underside of the bottom plate  45 . In the clearance space defined between the plates  45  and  72 , due to the presence of the spacer  75  therebetween, a plurality of actuators formed as elongate inflatable air hoses  76  are disposed so as to extend transversely across the actuator plate  72 . These air hoses  76  are closed at one end, and at the other end are connected to a suitable supply conduit  77 , the latter in turn being connected to a source of pressurized air. 
     The mold opening or actuator plate  72  also has a plurality of elongate connector pins  78  fixed thereto and projecting upwardly therefrom. These latter pins  78  freely pass upwardly through all of the other plates and in fact pass upwardly through openings formed in the bottom booking plate  42  so as to terminate in enlarged heads  79  which are normally spaced upwardly a selected distance above the bottom booking plate  42  when the latter is in its lowered position, and also when the bottom booking plate  42  is in its normally raised position as illustrated by dotted lines in FIG.  5 . 
     With the arrangement as described above, and assuming that the upper air hoses  49  are inflated so that the bottom booking plate  42  is raised to provide a closing force between the upper and lower mold parts, then inflation of the lower air hoses  76  (either simultaneous with or subsequent to deflation of the upper air hoses  49 ) causes the enlarged heads  79  of the pins  78  to engage the bottom booking plate  42  and exert a downward mold opening force thereon in response to downward displacement of the actuator plate  72  due to inflation of the lower air hoses  76 . 
     As an alternative, the pins  73  and springs  74  (FIG. 5) as associated with the actuator plate  72  can be deleted, and in place thereof the elongate connector rods  78  can have the upper and lower ends thereof respectively fixedly connected to the booking plate  42  and actuator plate  72 . With this fixed connection between the plates  42  and  72 , the actuator plate  72  will normally be maintained downwardly a greater vertical spacing below the bottom plate  45  when the booking plate  42  is in the nonactivated position shown in FIG. 5 so that when the air bags  49  are activated, both plates  42  and  72  will be vertically lifted simultaneously. In such situation, if an assist is necessary to crack open the mold following the molding operation, then the lower air bags  76  are energized simultaneous with or following de-energization of the top air bags  49  so that the actuator plate  72  acting through rods  78  exerts a downward force on the booking plate  42 . 
     To effect closing and opening of the press, which respectively requires downward and upward displacement of the top press structure  21 , the press arrangement  11  includes a pair of screw-type drive units  81  which are disposed adjacent opposite sides of the frame and provide a driving connection between the frame and the upper press structure  21  to effect desired vertical movement of the latter. The pair of screw-type drive units  81  in the preferred embodiment are identical, one being positioned in close proximity to each of the frame posts  19  so as to extend generally parallel therewith, and thus only one of the screw-type drive units will hereinafter be described. 
     The screw-type drive unit  81  is preferably a ball-screw drive and includes a vertically elongate drive shaft  82  which is disposed adjacent and generally parallel with the respective frame posts  19  so that the axis  83  of the shaft extends generally vertically. The shaft  82  is drivingly connected at its upper end to a drive motor  84  which is carried on the top frame  13 . The drive motor  84  is typically a high speed electric motor which is drivingly connected in a conventional manner through a speed reducer or other drive transmitting device, such as a belt drive, to the upper end of the drive shaft  82 . The drive motor  84  is of conventional construction and possesses its own internal brake which activates to prevent rotation of the drive motor shaft whenever the drive motor is de-energized. While each drive unit  81  may be provided with its own drive motor, it is also feasible to provide a single drive motor for both drive units. Preferably each drive unit has its own drive motor, but the two drive shafts are drivingly connected through a timing belt or the like to ensure synchronization of driving thereof. These latter variations are disclosed in copending U.S. application Ser. No. 09/041 068, and further description thereof is believed unnecessary. 
     The drive shaft  82  has the lower end portion thereof disposed so as to extend through an opening  87  which projects vertically through the support block  59 , with the lower end of the drive shaft being rotatably supported thereon by a conventional anti-friction bearing  85 . The upper end of the drive shaft  82  similarly projects through a frame member associated with the top frame  13  and is also appropriately rotatably supported by means of an anti-friction bearing  85 A, such being conventional and well understood. The drive shaft  82 , adjacent the lower end thereof, has a stop collar  88  fixedly mounted thereon which under normal operation is adapted to abut against the upper end of the inner race of the lower bearing  85  to maintain a normal vertical position of the drive shaft  82 , although the upper and lower bearings which support the drive shaft  82  do permit limited upward axial displacement of the drive shaft  82  for a purpose as described hereinafter. 
     The elongate intermediate portion of the drive shaft  82  is externally threaded and cooperates with a rotatable ball-nut unit  86  which surrounds the screw portion of the drive shaft. This ball-nut unit  86  incorporates therein a plurality of balls (not shown) which cooperate in a rolling and recirculating manner with the screw portion of the drive shaft  82  in response to relative rotation between the nut  86  and the drive shaft  82 , such ball-nut-screw drives and their construction being conventional and well understood. 
     The housing of the ball-nut unit  86  is fixed to the top press support  25  associated with the top press structure  21  so that the latter is vertically displaced upwardly or downwardly along with the ball-nut units  86  in response to rotation of the drive shafts  82 , depending upon the direction of rotation of the latter. 
     To provide a positive braking of the drive shafts  82  independent of the brakes associated with the driving motors therefor, the drive shaft  82  of each drive unit  81  has a friction brake  91  (FIG. 5) which cooperates between the lower end of the drive shaft  82  and the frame of the press. This friction brake  91  includes an annular brake plate  93  which is fixed to the respective drive shaft  82  adjacent the lower free end thereof, whereby this brake plate  93  is disposed closely adjacent and substantially directly under the support block  59 . The brake plate defines thereon a substantially flat upper brake surface  94  which is normally disposed in opposed but slightly axially spaced relation (typically between about 0.020 and about 0.040 inch) from a substantially planar lower brake surface  95  as defined on the lower end of the support block  59 . These opposed brake surfaces  94  and  95  are conventional friction brake surfaces, and during normal rotation of the drive shaft  82  are spaced from one another in non-engaged relation. However, when the drive shaft  82  is displaced axially upwardly a small extent, as explained hereinafter, the brake plate  93  is moved upwardly so that the upper brake surface  94  thereon engages the lower brake surface  95  provided on the stationary support block  59 , thereby creating a frictional braking engagement between the opposed surfaces  94 - 95  to provide a positive restraint against rotation of the drive shaft  82 . 
     The operation of the press  11  will now be briefly described. 
     In the following description of a typical operational cycle of the press arrangement  11 , it will be assumed that the press is initially in its open position wherein the upper press structure  21  is in its raised or uppermost position, and that the upper and lower booking plates  26  and  42  are angled sidewardly into their access positions, substantially as illustrated in FIG.  3 . In this latter position the booking plates are angled so as to face sidewardly toward opposite sides of the press, whereby workers positioned adjacent opposite sides can respectively access the upper and lower mold parts  23 - 24  to carry out whatever operations are required with respect to preparing the mold. The press cycle is then activated by supplying air to the bottom of the tilt cylinder  64  so as to cause the piston rod  68  to be pulled upwardly, thereby causing the bottom press structure  22  to be pivotally displaced so as to return to its substantially horizontally oriented position as illustrated by FIG.  2 . This latter position can be determined by appropriate stops (not shown) associated either with the frame or the pressure cylinder, and in addition appropriate latches (not shown) can optionally be provided and activated so as to latch the lower press structure in this horizontal orientation if desired. 
     With the lower press structure  22  in its horizontal orientation so that the lower mold part  24  is oriented with its cavity projecting generally upwardly, then if the press involves an open pour, a predetermined quantity of plastics material, particularly polyurethane, is poured or injected into the mold cavity of the lower mold part. While an open pour technique is conventionally utilized, it will be appreciated that in some situations the polyurethane is injected into the mold cavity when the press is closed. 
     Subsequent to or simultaneous with the movement of the lower press structure  22  into its horizontal orientation, the upper press structure  21  is activated so as to move downwardly toward the closing position of the mold. For this purpose the motors  84  of the drive units  81  are energized, causing the internal motor brakes to release, and causing synchronous rotation of the drive shafts  82 . These rotating shafts  82  react with the ball-nut units  86  and hence cause the top main support  25  to be slidably displaced downwardly away from its uppermost position. During this downward displacement of the top press support  25 , pressurized air acts upwardly against the piston of the pressure cylinder  32  so that as the top press support  25  is moved downwardly carrying with it the hinge  27 , the control link  36  and cylinder rod  34  are urged upwardly due to the internal pressure in the cylinder  32 , thereby causing the top booking plate  26  to swing upwardly (clockwise in FIGS. 2 and 3) about the hinge  27 . The top booking plate  26  continues to swing upwardly synchronously with the downward displacement of the top press support  25  until the top booking plate  26  is disposed substantially horizontally and seats against the top press support  25 , at which time the latches  40  will be automatically activated. This now results in the upper mold part  23  being generally vertically aligned with but spaced upwardly from the lower mold part  24 . 
     The drive units  81  remain energized so that the continued rotation of drive shafts  82  causes the entire upper press structure  21  to move downwardly toward the lower mold part, and during this continued downward movement the cylinder rod  34  due to its connection to the top booking plate is extended downwardly in opposition to the internal cylinder pressure. 
     When the upper mold part  23  has been moved downwardly so as to be positioned close to a position of closure with the lower mold part  24 , but normally spaced a small distance from such closure, which small distance will typically be in the order of about 0.100 inch or less, the drive units  81  are de-energized and the internal brakes associated with the drive motors  84  are automatically applied. The stoppage of the top press structure  21  can be readily controlled by appropriate programming of the overall drive system or by appropriate position sensors, such being conventional. 
     After stoppage of the upper mold part  23  in very close positional relationship to the lower mold part  24 , pressurized air is supplied to the air hoses  49  which cause the bottom press plate  42  to be lifted upwardly away from the bottom press support  43  whereby the lower mold part  23  is lifted upwardly to effect a full closing engagement with the upper mold part  24 . The inflation of the air hoses  49  is also of sufficient magnitude to cause the lower mold part  23  to exert a significant closure force against the upper mold part  24  so as to permit the mold parts to be maintained fully closed during the molding process which takes place within the mold. This imposition of a high closure force from the lower mold part to the upper mold part, however, results in this force being applied to the upper press structure  21  and hence to the ball units  86  which react against the screw portions of the drive shafts  82 . This upward force as transmitted from the ball-nut units  86  to the screw shafts  82  causes the latter to lift axially upwardly a small extent, as permitted by the top and bottom bearings  84  and  84 A, thereby causing the friction brake plates  93  at the lower ends of the drive shafts to also be lifted upwardly so that the upper brake surfaces  94  move into frictional engagement with the stationary lower brake surfaces  95  defined on the support blocks  59 . This thus effectively prevents the shafts  82  from rotating, and hence the continued imposition of the upward force on the upper mold part, and the transmission of this force to the drive shafts, is thus ineffective in causing reverse driving (i.e. reverse rotation) of the drive shafts due to their coaction with the ball-nut units. 
     Once the molding cycle has been completed, then the air hoses  49  are de-energized and, in some situations, this release of the upwardly directed force on the lower mold part is sufficient to effect release between the mold parts, thereby enabling the lower mold part to move downwardly out of engagement with the upper mold part. At the same time, the release of the upward force from the upper mold part enables the upper press structure  21  and the drive shafts  82  to move downwardly a small extent due to the urging of gravity, thus causing a separation between the brake surfaces  94  and  95 , with the limited downward movement being controlled by the stop collars  88  re-engaging the bearings. 
     In many situations, however, the magnitude of the force applied to effecting closure of the mold parts is such as to cause the mold parts to become tightly adhered together, and thus the mere release of the upward mold closure force may be incapable of effecting cracking or separating of the upper and lower mold parts. In such situation, when the press is equipped with the optional mold release assist mechanism  71 , the lower air hoses  76  can be inflated substantially simultaneous with the deflation of the upper air hoses  49 . The inflation of the lower air hoses  76  forcibly drives the actuator plate  72  downwardly which, acting through the pins  78 , cause the enlarged heads  79  of the pins  78  to engage the bottom booking plate  42 . A downwardly directed opening or cracking force is thus imposed on the bottom booking plate  42 , and on the lower mold part  24  attached thereto, so as to effect opening or cracking of the lower mold part  24  from the upper mold part  23 . Once this has been accomplished, then the lower air hoses  76  are also de-energized. 
     With the upper and lower mold parts cracked or opened, then the drive motors  84  are energized to cause reverse rotational driving of the drive shafts  82  so that the upper press structure  21  is quickly moved upwardly away from the mold closure position. During the initial upward movement, the pressure in the cylinder  32  causes the cylinder rod  34  to be retracted through its maximum extent. Thereafter continued upward movement of the upper press structure  25  causes the upper booking plate  26  to swing sidewardly (counterclockwise) about the hinge  27  so as to cause the top booking plate to swing into its full sideward access position wherein it is substantially vertically suspended simultaneous with the continued upward displacement of the top press support  25  into its uppermost open position, substantially as illustrated in FIG.  3 . 
     Simultaneous with or subsequent to the upward opening movement of the top press structure, the lower press structure is sidewardly tilted by applying fluid pressure to the upper end of the pressure cylinder  64  so that the piston rod  68  is extended downwardly to cause the lower press structure to be sidewardly tilted into its side access position as illustrated by FIG.  3 . This also enables the molded workpiece to be readily removed from the mold cavity associated with the lower mold part. 
     The molding cycle has now been completed, and the press is again in its open and access position so as to permit initiation of a new molding cycle. 
     With the press  11  of the present invention, when it is mounted on and associated with a mobile overhead carrier  10 , the carrier and press can be moved sequentially along a closed path into and through a plurality of workstations, whereby various operations can be carried out at sequential workstations disposed along the closed work path. This thus enables the mold operation when the mold is closed to take place as the carrier is moving between two adjacent workstations. At the same time, electricity can be supplied to the carrier and to the press from an overhead arrangement such as is conventional, and the press can be automatically coupled to a supply of compressed air when it is disposed in a desired working station. 
     As an alternative, it will be appreciated that the press and its frame can be disposed in a stationary arrangement on a support floor, and all of the operational relationships are still substantially the same with respect to the basic molding cycle. 
     For further detail with respect to a rim press employing screw-type drive arrangements, and the general type of molding operation carried out in such press, attention is directed to aforementioned copending U.S. Ser. No. 09/041 068, the entire disclosure of which is incorporated herein by reference. 
     Although a particular preferred embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.