Abstract:
An electrically powered, molding clamp assembly enables rapid closure of platens, while exerting a force sufficient to maintain the platens in the closed position during molding without the necessity of large horsepower motors of high energy. Force accumulators join at least one of the platens to rotatable threaded rods connecting the platens. A first electric drive means rotates the threaded rods at a first rotational speed to move one or both of the platens between open and closed positions, and a second electric drive means rotates the threaded rods at a second rotational speed less than said first rotational speed to move the accumulators between said uncompressed and compressed positions when the movable platen is in its closed position, creating a clamping force on the platens.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The present invention relates to an electric molding clamp assembly, and in particular to a clamp assembly that can be rapidly closed, followed by exertion of a high clamping force during molding.  
           [0003]    (2) Description of the Prior Art  
           [0004]    A plastic molding clamp assembly is normally comprised of a pair of supports, known as platens, having inwardly facing, parallel mounting surfaces to carry mold sections. Each mold section includes a part of a mold cavity, so that the mold sections, when moved into facing contact, define a mold cavity corresponding to the exterior surface of the desired part. In some molding assemblies, e.g., assemblies of the type typically used for injection molding, one platen remains stationary while the other platen is moveable between open and closed positions along a pathway that is perpendicular to the mounting surfaces of the platens. In other molding assemblies, e.g., assemblies of the type used for blow molding, both platens are moveable between open and closed positions.  
           [0005]    A molding clamp must meet two requirements. First, the clamp must be adapted to open and close rapidly, so that the time required to mold a given part is as short as possible and economically viable. Second, the clamp must exert a considerable force on the mold sections during molding to prevent molten plastic from escaping between the mold sections during molding.  
           [0006]    Conventional molding clamps have relied on mechanical toggle arrangements or hydraulic cylinders to meet these requirements. More recently, efforts have been made to develop acceptable injection molding clamps using electrical drive means, such as an electric motor coupled to rotatable threaded rods, to carry the moveable platen between open and closed positions. The use of a single motor for this purpose has been unworkable, either because the size of the motor was cost prohibitive, because the speed of opening and closing was too slow, or because the clamping force was insufficient to prevent escape of plastic during molding.  
           [0007]    Some prior art disclosures have suggested using two electric motors to achieve the desired objectives, with one motor being used to rapidly rotate threaded rods to effect rapid opening and closing, and a second motor to rotate the rods at a considerably slower speed, but at a considerably higher torque, to exert a clamping force on the mold sections once the molds are brought to their clamping position by the first motor. Representative of such disclosures are U.S. Pat. No. 4,929,165 to Inaba et al, and U.S. Pat. No. 5,190,714. Drive mechanisms other than a second motor have also be suggested, as exemplified by U.S. Pat. No. 6,186,770 to Ziv-Av.  
           [0008]    However, prior art assemblies require large and expensive motors, with corresponding high-energy requirements, to maintain a sufficient clamping force. These motors and the energy required render the assemblies uneconomical, and thus commercially unacceptable. Thus, there still remains a need for a clamping assembly that will address these needs in an effective and economical manner.  
         SUMMARY OF THE INVENTION  
         [0009]    Generally, the molding clamp assembly of the invention is comprised of first and second platens, at least one of the platens being moveable, to support mating mold sections; resilient force accumulators carried on at least one of the platens; rotatable threaded rods connecting the platens; and a drive means to first move the platens between open and closed positions at a relatively high speed, and then to compress the force accumulators after the platens are in the closed position, thereby tightly clamping the mold sections together.  
           [0010]    In one embodiment, the clamping assembly of the present invention is comprised of a base, which may be oriented horizontally, vertically or at an intermediate angle, that supports a fixed stationary platen, and a moveable platen slideable along the base between open and closed positions. The platens have facing, parallel, mold mounting surfaces to support mold sections that join to form a mold cavity when the moveable platen is in the closed position. In another embodiment, both platens are moveable inwardly during closing.  
           [0011]    The platens, whether one or both are moveable, are connected by rotatable, threaded rods, also known as lead screws. The threaded rods are parallel to each other and to the pathway of the moveable platen or platens and perpendicular to the platen mold support surfaces. Preferably, the ends of the rods are rotatably attached to a support frame attached to the base and/or to a stationary platen. The rods may also extend through bores in one or both of the platens.  
           [0012]    The threaded rods are joined to at least one of the platens by compressible force accumulators, preferably mounted on an exterior surface of a platen. For purposes of the present disclosure, a “force accumulator” is intended to mean a device that is compressible, and which stores energy when compressed, urging the device toward its uncompressed state. The force accumulators of the present invention generally include one or more springs, which may be in the form of washers having cupped and flattened states. A preferred force accumulator is commonly known as a disc spring or disc spring pack. A disc spring pack generally includes a plurality of springs arranged in a desired configuration. As used herein, it will be understood that the term “disc spring” encompasses disc springs with a single spring, as well as disc spring packs that incorporate a plurality of springs. A representative disc spring is known in the industry as a Belleville washer. Other force accumulators include compression springs, compressible urethanes, and pre-charged gas cylinders.  
           [0013]    In the present invention, the force accumulators are adapted to be moved between uncompressed and compressed states after the moveable platen is in the closed position. In the assembly of force accumulators, e.g., disc springs, the compressible member may be held within the assembly under a small compressive force, primarily to maintain desired positioning of the components. For purposes of the present invention, assemblies of this type will still be considered to be “uncompressed,” the terms “uncompressed” and “compressed” as used herein referring to the deformation, or lack of deformation, of the compressible part of the force accumulator due to the force of the drive means.  
           [0014]    The electric drive means used to reciprocate the platens between open and closed positions and to move the force accumulators between their uncompressed and compressed states is comprised of a plurality of threaded rods operatively connecting to a common drive shaft that is rotated by the electric drive means. During the molding cycle, initial rotation of the threaded rods moves the platens to their closed position. Compression of the force accumulators is effected by further rotation of the threaded rods. For example, the force accumulators may include threaded components, such as planetary rollers or ball screws, meshing with the threads of the threaded rods, so that rotation of the threaded rods causes linear movement of the force accumulators along the threaded rods.  
           [0015]    Various transmission arrangements, such as belts and pulleys, may be used to connect the drive shaft and threaded rods. Other transmission means, e.g., gears or chain drives, will be apparent to one skilled in the art. Generally, the drive means includes a first drive configuration for reciprocating the platens between open and closed positions at a relatively high speed, and a second drive configuration for compressing the force accumulators at a considerably slower speed, but at the considerably higher force that is required to compress the force accumulators.  
           [0016]    The drive means may be a single electric motor with gearing to provide alternate high speed and low speed drive connections to the drive shaft. However, the drive means is preferably comprised of two electric motors. A first motor is adapted to rotate the drive shaft at relatively high speed, and may be directly connected to the drive shaft by a belt and pulleys, or other transmission means. A second motor is adapted to rotate the drive shaft at relatively low speed, and may be connected to the drive shaft through a reduction gearbox. The drive shaft of the gearbox may serve as the common drive shaft. The drive and transmission means is configured to rotate all threaded rods at the same rate, thereby maintaining the platen mounting surfaces parallel to each other during opening and closing of the clamping assembly.  
           [0017]    In operation, first and second mold sections are mounted on the platen mold section mounting surfaces, with each mold section including a portion of a desired mold cavity. The first motor is then engaged to rotate the drive shaft at a high speed to rapidly move one or both platens to their closed position, whereby the mold sections are substantially in contact with each other, i.e., in contact or only a small distance apart. The second motor is then energized to rotate the drive shaft at a slow speed, but at a high torque. Since the platens are in the closed position, rotation of the drive shaft by the second motor does not result in any significant linear movement of the moveable platen or platens.  
           [0018]    Instead, rotation of the threaded rods results in linear movement of the force accumulators, compressing the springs or other resilient elements within the force accumulators. Preferably, the resilient elements are on the exterior side of a platen, creating a considerable force against the exterior surface of the platen, urging the platen toward the other platen. As a result, the mold sections held between the platens are clamped tightly together during molding.  
           [0019]    Compression on the mold sections is maintained until the plastic has solidified. To prevent the compressed force accumulators from returning to their uncompressed states during the molding cycle, the second motor may be energized during all or a part of the molding cycle. Unlike prior art proposals, however, high-energy usage is not required during molding due to the presence of the force accumulators. Alternatively, the motor can be de-energized and a brake on the motor shaft can be used to prevent rotation of the lead screws during the molding cycle.  
           [0020]    After the plastic has solidified, the high-speed motor is engaged in reverse to rapidly withdraw the moveable platen to the open position. Once the moveable platen is in the open position, the cycle can be repeated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a side view of the preferred clamping assembly including a stationary platen and a moveable platen with the disc springs being mounted on the moveable platen.  
         [0022]    [0022]FIG. 2 is a top view of the preferred clamping assembly.  
         [0023]    [0023]FIG. 3 is a proximal end view of the preferred clamping assembly.  
         [0024]    [0024]FIG. 4 is a proximal end view of the moveable platen of the preferred clamping assembly showing the four disc springs.  
         [0025]    [0025]FIG. 5 is a detailed sectional side view of a disc spring carried on the moveable platen of the preferred embodiment.  
         [0026]    [0026]FIG. 6 is a side view of a first alternative embodiment including two moveable platens carrying disc springs.  
         [0027]    [0027]FIG. 7 is a side view of a second alternative embodiment including a stationary platen and a moveable platen, with the disc springs being mounted on the stationary platen. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    In the following description, terms such as horizontal, upright, vertical, above, below, beneath, and the like, are used solely for the purpose of clarity in illustrating the invention, and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale.  
         [0029]    [0029]FIGS. 1-5 and the associated description are of an electric molding assembly with one stationary platen and one movable platen, especially adapted for use in injection molding. After reading the description, however, it will be apparent to one skilled in the art that the principles of the invention may also be applied to other plastic molding assemblies, such as blow molding clamp assemblies. Also, it will be apparent that assemblies can be designed with two moveable platens, and with force accumulators positioned on a stationary platen, or on both of two moveable platens. FIGS. 6 and 7 illustrate two of these alternative configurations.  
         [0030]    As best illustrated in FIGS. 1-4, the preferred clamping assembly, generally  10 , is comprised of a base  12 , which supports stationary platen  14  and moveable platen  16 , which is slideable along platen carriage  18 . Platens  14  and  16  include facing, parallel, mold support faces  20  and  22 , respectively.  
         [0031]    Platens  14  and  16  are connected by threaded rods  24 , each rod having a proximal end rotatably attached to support frame  26 , and a distal end rotatably attached to stationary platen  14 . Threaded rods  24  are parallel to each other and perpendicular to mold support faces  20  and  22 , and extend through bores  28  in moveable platen  16 .  
         [0032]    Threaded rods  24  are joined to the proximal side of moveable platen  16  by disc springs  30 . FIG. 5 provides a detailed sectional side view of disc spring  30 . Disc spring  30  is comprised of slideable cylindrical housing  32  having a cylindrical internal bore  34  for receiving rod  24 , and flange  36  having a circumference greater than housing  32  adjacent the proximal end of housing  32 .  
         [0033]    A plurality of planetary rollers  38  having threads meshing with the threads of rod  24  are carried within the wall of bore  34 . Disc spring  30  is carried on a plurality of bolts  40  bolted into the proximal side of platen  16 . Bolts  40  extend through bores  42  in flange  34 , with bores  42  having a greater diameter than bolts  40 , so that disc spring  30  is slideable on bolts  40  between uncompressed and compressed positions.  
         [0034]    Disc spring  30  also includes a plurality of annular washers or discs  46 . As illustrated, sixteen discs  46  are used in alternating sets of four each. It will be understood, however, that a greater or lesser number of discs can be used, and that the disc springs may be arranged in a different manner. Disc springs  46  are slidably positioned around rod  24 , and between the distal side of flange  36  and the proximal side of platen  16 .  
         [0035]    Threaded rods  24  are simultaneously rotated by a common drive means, generally  50 , comprised of a first electric motor  52 , and a second electric motor  54  joined to a reduction gear box  56 . A drive shaft  60  extends along a pathway parallel to, and equidistant from, the pathways of threaded rods  24 . Belt  62  extends around the shaft of motor  52  and clutch  64  on the shaft of gear box  56 . The shaft of gear box  56  is joined to drive shaft  60 , with coupling  66 .  
         [0036]    Drive shaft  60  is connected to threaded rods  24  with a transmission means comprised of belts  70  extending around pulleys on drive shaft  60  and rods  24 . Tension on belts  70  is adjusted with belt tensioners  72 . All belts and pulleys are as identical, as are the diameters and thread configurations of threaded rods  24 , in order to maintain surfaces  20  and  22  of platens  14  and  16  parallel during closing of clamping assembly  10 .  
         [0037]    Motor  52  preferably has a horsepower rating of from about 1 to about 30, and is adapted to rapidly rotate drive shaft  60  at a relatively high speed to move platen  16  between an open position, shown in FIGS. 1 and 2, and a closed position (not shown) adjacent stationary platen  14 , with spacing being provided for first and second mold sections (not shown) carried on faces  18  and  20  of platens  14  and  16 . Upon closing of mold assembly  10 , the mold sections abut to form a complete mold cavity, which is subsequently injected with the desired plastic through inlets (not shown) in stationary platen  14 .  
         [0038]    Movement of platen  16  to its closed position serves to bring mold sections on platens  14  and  16  substantially together, i.e., either touching or almost touching. Platen  16  is then moved to its clamping position, in which a high pressure is exerted on platen  16 . Movement of platen  16  and creation of the high clamping pressure is achieved by engaging low-speed motor  54 .  
         [0039]    Motor  54  preferably has a horsepower in the range of from about 5 to about 50, i.e., from about 2 to about 4 times greater than motor  52 . In addition, motor  54  is connected to drive shaft  60  through gearbox  56 , which may include reduction gearing of, for example, from about 150:1 to about 450:1. As a result, motor  54  rotates drive shaft  60 , and thereby rods  24 , at a much slower rate than the rotation by motor  52 , but with considerably greater torque. Motor  52  may be de-energized during molding. Motor  52  may also be configured with a shaft encoder (not shown) to monitor the position of the moveable platen and determine the activation and direction of the two motors. If so, motor  52  may remain energized during the clamping stage, effectively resulting in two motors contributing to the compression of disc springs  30 .  
         [0040]    Since platen  16  is in the closed position upon actuation of motor  54 , rotation of drive shaft  60  by motor  54  does not result in any significant linear movement of platen  16 . Instead, rotation of the rods  24 , causes rotation of planetary rollers  38  within each of disc springs  30 , causing disc springs  30  to move in a linearly in a distal direction within bore  28  in platen  16 , compressing springs  46 . Compression of springs  46  in disc spring  30  results in a considerable force against the proximal face of platen  16 , thereby clamping mold sections held between platens  14  and  16  tightly together during injection molding. Depending on the motor size and gearing, and the construction of the disc springs, a force on the order of 100-1000 tons of force may be exerted against the moveable platen.  
         [0041]    To maintain the desired force during molding, motor  54  can be run during all or part of the molding process, e.g., during the injection stage, and/or motor  54  can include brake  74 , which is engaged if motor  54  is disengaged during molding. After the molding cycle is complete, motor  54  is disengaged, and motor  52  is run in reverse to rapidly move platen  16  to its open position in preparation for the next molding cycle.  
         [0042]    [0042]FIG. 6 illustrates an alternative clamping assembly using two platens  114  and  116 , both platens being moveable between open and closed positions on threaded rods  118 . Disc springs  30  are mounted on the exterior of surfaces of both platens. In operation, rotation of rods  118  causes platens  114  and  116  to move inwardly toward each other to their closed position. Further rotation of rods  118  causes disc springs  30  to move to their compressed positions, clamping platens  114  and  116  tightly together.  
         [0043]    [0043]FIG. 7 illustrates another clamping assembly using a stationary platen  124  and a moveable platen  126 , with disc springs  30  being mounted on stationary platen  124 . In preparation for molding, rods  24  are rotated to move platen  126  inwardly to its closed position, causing mold sections (not shown) on the inner mounting surfaces of platens  124  and  126  to come together. Further rotation of rods  24  causes disc springs  30  to move inwardly creating the desired clamping force.  
         [0044]    Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.