Abstract:
A platen of an injection molding machine includes a plate-shaped center area having corners, each corner formed with a recess. A plurality of sleeves is received in the recesses of the center area and defines bores for accommodation of column-like holding and/or guide elements. The sleeves have an outer surface in confronting relationship to the center area and are configured for only partial connection to the center area, thereby establishing an elastic connection between the center area and the sleeves.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a continuation of prior filed copending PCT International application no. PCT/EP02/04913, filed May 4, 2002, which designated the United States and on which priority is claimed under 35 U.S.C. §120, the disclosure of which is hereby incorporated by reference. 
   This application claims the priority of German Patent Application, Serial No. 101 23 104.0, filed May 12, 2001, pursuant to 35 U.S.C. 119(a)–(d), the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a platen of an injection molding machine. The present invention further relates to a two-platen injection machine equipped with such a platen. 
   U.S. Pat. No. 5,593,711 discloses a platen for use in a plastics injection molding machine, with the platen including a base plate, a frustoconical center part, and a front plate in parallel relationship to the base plate for carrying a molding tool. The base plate has corners formed with bores for receiving tie bars. The center part is hollow and has a base disposed at a central location of the base plate, with the front plate situated upon the truncated top of the center portion. The front plate is thus supported solely in midsection by the center part. In this way, the front plate is presumed to remain planar, when a clamping force is applied so as to prevent an opening of the half-molds of the molding tool. 
   European Pat. No. EP 0 789 648 B1 discloses an injection molding machine with a generally C-shaped machine frame with two frame limbs in the absence of interconnecting tie bars. Supported by one limb is a fixed platen, while the other limb has mounted thereto a flange plate of a clamping unit. The clamping unit includes a hydraulic piston and cylinder unit to displace the moving platen, supported by the machine frame. The flange plate and the fixed platen are secured via so-called holding elements to the limbs of the machine frame. During closing operation, the holding elements of the flange plate are exposed to a tensile load while the fixed platen is loaded by a compressive force. The holding elements resemble the operation of springs whose elasticity is smaller than that of the machine frame. The holding elements are distributed over the vertical dimension of the fixed platen and the flange plate and exhibit different elasticity, whereby the lower holding elements have a greater elasticity than the upper holding portions. The holding portions are made of spring steel and are formed with slots for effecting material weakenings. The lower portion includes hereby several or deep slots to enhance the elasticity. When pushing the frame limbs of the machine frame apart at the top during the injection operation, the fixed platen and the flange plate can undergo a tilting movement relative to the machine frame to thereby maintain parallelism of the platens. The holding elements thus form an axis-less hinge which besides the rotary movement also permit a translatory movement. 
   European Pat. No. EP 0 381 107 B1 discloses a clamping unit for a horizontal two-platen injection molding machine for plastics. The injection molding machine includes a fixed platen and a moving platen for carrying confronting half-molds. The fixed platen has a rectangular shape and is connected with the moving platen by spindles arranged at the four corners or an imaginary tetragon. The spindles are hereby arranged non-rotatably and cantilevered on the fixed platen. The moving platen is displaceable in the direction of the fixed platen along the spindles at the corners for opening and closing the half-molds. Mounted on each spindle on a side of the moving platen distal to the half-mold is a spindle nut for operation of the moving platen. The spindle nuts are rotated together by an electric motor and configured as a ball screw nut. After closing the half-molds, a clamping force is applied upon the half-molds by a hydraulic piston and cylinder unit which act on the spindles and spindle nuts and include an annular piston through which the respective spindle is guided. To prevent opening of the half-molds after injection operation, the piston and cylinder unit is double-acting. A tilting or deformation of the platens during application of the clamping force is accompanied by a bending of the spindles which are thus subject to increased wear, ultimately resulting in their destruction. 
   It would therefore be desirable and advantageous to provide an improved platen and improved two-platen injection molding machine to obviate prior art shortcomings and to be of simple construction and yet enabling an elastic securement for a connection of the platen to a holding and/or guiding element. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a platen of an injection molding machine includes a plate-shaped center area having corners, each corner formed with a recess; and a plurality of sleeves received in the recesses of the center area and defining bores for accommodation of column-like holding and/or guide elements, whereby the sleeves and the recesses are placed into one-to-one correspondence, wherein the sleeves have an outer surface in confronting relationship to the center area and configured for only partial connection to the center area. 
   The present invention resolves prior art problems by providing an elastic connection between the center area and the sleeve through separation of the stiff center area from the elastic, quasi resilient support of the sleeves in the center area. The elastic connection can be implemented by providing between the center area and the sleeves a transition zone which is comprised of an annular gap to define a clearance between the center area and the sleeves and an annular bridge to connect the sleeves to the center area. Suitably, the ring-shaped gap has a depth which is greater than half a thickness of the center area, thereby enabling an adjustment of the desired elasticity. 
   According to another feature of the present invention the center area and the sleeves may be made in one piece, suitably of cast steel or gray cast iron. In this way, the platen can be made in a simple manner. 
   According to another feature of the present invention, the center area may be of generally rectangular shape, and may have a substantially star-shaped or cross-shaped configuration, whereby the sleeves project beyond the bores and have an outer perimeter which is connected to an adjacent rim zone of the center area by a stay. As a clamping force is applied, the stays are subjected to torsion and bending. The stays may have a wedge-shaped configuration and can be connected with the adjacent rim zone across an entire thickness of the center area and length of the sleeve. 
   A platen according to the present invention is especially useful for plastics injection molding machines without tie bars, whereby the platens are connected via the holding and/or guiding elements, placed in the openings of the sleeves, with the normally U-shaped machine frame. This type of suspension of the platen in the form of an axis-less hinge effectively prevents the limb of the U-shaped machine frame to give way, when subjected to the clamping force, and thus maintains a parallelism of the platens. 
   According to another aspect of the present invention, a two-platen injection molding machine includes a machine frame, a fixed plated mounted to the machine frame and carrying a half-mold, a moving platen carrying another half-mold, a clamping unit including a plurality of spindle mechanisms, each having a spindle extending between the fixed and moving platens, for displacing the moving platen in a direction of the fixed platen, wherein each of the fixed and moving platens includes a plate-shaped center area having corners, each corner formed with a bore, and a plurality of sleeves received in the bores of the center area, whereby the sleeves and the bores are placed into one-to-one correspondence, wherein the sleeves have an outer surface in confronting relationship to the center area and configured for only partial connection to the center area. 
   According to another feature of the present invention, the spindles may be positioned in the corners of the fixed platen, so that a total of four spindles may be provided to extend in the direction of the moving platen through the corners of the moving platen for connection to associated spindle nuts of the spindle mechanisms. These spindle nuts are rotatable by a drive unit in order to open or close the half-molds, mounted to the platens. 
   According to another feature of the present invention, the moving platen may be supported by two bearings upon the machine frame. Suitably the bearings are constructed as support rollers which are arranged on the underside of the moving platen in the lateral peripheral zones, respectively. 
   Suitably, the spindle mechanisms are constructed as ball screw mechanisms. This is desired because they run substantially free of friction and thus generate little noise and require little maintenance. 
   According to another feature of the present invention, the clamping unit may include a piston and cylinder unit for applying a clamping force onto the half-molds. In this way, the spindle mechanisms are used exclusively for opening and closing the half-molds and not required to apply the required clamping force. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
       FIG. 1  is a schematic perspective view of a clamping unit of a horizontal two-platen injection molding machine, having incorporated therein the subject matter according to the present invention; 
       FIG. 2  is a frontal view of a platen according to the invention; and 
       FIG. 3  is a side view of the platen of  FIG. 2  in an area of a sleeve. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
   Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic perspective view of a clamping unit  1  of a horizontal two-platen injection molding machine for plastics. The injection molding machine includes a machine bed  3 , a fixed platen  2 , which is secured upright onto the machine bed  3  and carries a half-mold of a molding tool  14 , and a moving platen  6  displaceable in relation to the fixed platen  2  and carrying another half-mold of the molding tool  14 . The fixed platen  2  has a generally block-shaped configuration, approximately square in shape, and has corners  2   a  which are each formed with a bore  4  for receiving a spindle  5 . As shown on the right-hand side of  FIG. 1 , each spindle  5  is non-rotatably connected on the backside of the fixed platen  2  via a detachable fastening element  15  with the fixed platen  2 . 
   The four spindles  5  extend cantilevered and substantially a right angle to the fixed platen  2 . As the fixed platen  2  is essentially aligned in vertical direction, the spindles  5  thus extend horizontal. At its front side confronting the moving platen  6 , the fixed platen  2  carries the half-mold of the molding tool  14  in a space between the spindles  5 . The spindles  5  extend through bores in the corners  6   a  of the moving platen  6  and jut out with their spindle ends  5   a  for connection to a spindle mechanism, generally designated by reference numeral  7 . For sake of illustration, the spindle mechanism  7  at the upper right-hand side on the moving platen  6  has been partially broken away to show internal components which will be described furtherbelow. 
   Each spindle mechanism  7  includes a spindle nut  8  which is rotatably supported on the backside of the moving platen  6  and is in meshed engagement with the spindle  5 . The spindle nut  8  is suitably constructed as a ball screw nut. Opening and closing of the half-molds and thus displacement of the moving platen  6  along the spindles  5  in the direction of the fixed platen  2  is implemented by a drive unit, generally designated by reference numeral  9  and operatively connected to the spindle nuts  8 . The drive unit  9  is constructed as a belt drive including a belt  9   b  which is trained about rotatable pulleys  9   a  disposed in concentric relationship to the spindles  5  in the area of the drive mechanisms  7  and operatively connected with the spindle nuts  8 . As all four pulleys  9   a  are driven by the common belt  9   b , the spindle mechanisms  7  and thus the moving platen  6  run in synchronism. 
   As further shown in  FIG. 1 , the moving platen  6  is rollingly supported via two support rollers  13  on the machine bed  3 . The support rollers  13  are supported on the underside of the moving platen  6  via horizontal axles which extend transversely to a movement direction S of the platen  6 . Suitably, the support rollers  13  are disposed at the lateral peripheral zone of the moving platen  6 . 
   Turning now to  FIG. 2 , there is shown a frontal view of one of the platens,  2 ,  6 , here, by way of example, the fixed platen  2 . As the platens  2  and  6  are essentially of identical construction, except for their support on the machine bed  3 , it will be understood by persons skilled in the art that a description of one of the platens is equally applicable to the other platen. The platen  2  has a block-shaped, substantially rectangular, configuration with a substantially cross-shaped center area  10  with four circular recesses  10   d . The center area  10  is of light-weight construction and has a base board  10   c  which is reinforced by a rib structure  10   a . Disposed in the recess  10   d  of each of the four corners is a sleeve  11  to form the bores  4  for receiving the column-like holding and/or guide elements. In the non-limiting example of  FIG. 2 , the column-like holding and/or guide elements are represented by the spindles  5  which extend through the bores  4  of the sleeves  11  of the platen  2  and extend through the bores  4  of the sleeves  11  of the moving platen  6  for connection with the respective spindle nut  8  of the spindle mechanism  7  adjacent the sleeve  11 . 
   As shown in  FIG. 2 , the sleeves  11  are received within the recesses  10   d  at a clearance about a portion thereof to define an annular, sickle-shaped gap  12  and to separate the sleeve  11  from the center area  10 , except for a ring-shaped bridge  17  that connects the sleeve  11  to the central area  10 , as shown in particular in  FIG. 3 . The angular length of the circular ring-shaped gap  12  in relation to the center point of the sleeve is about 110°. The gap  12  terminates in the peripheral zone of the center area  10  of the platen  2 .  FIG. 3  in concert with  FIG. 2  show the disposition of the annular gaps  12  on the platens  2 ,  6  of the two-platen injection molding machine on the sides facing away from one another. In  FIG. 1 , the area of the sleeves  11  of the platen  2  is broken away here to show the respective gap  12 . Suitably, the annular gap  12  has a depth T which is greater than half a thickness of the center area  10 , preferably about ⅘ of the thickness d of the center area  10 . 
   The sleeves  11  are only partially received in the recesses  10   d  of the center area  10  and project beyond the center area  10  by about a third of their diameter. As shown in  FIG. 2 , the sleeves  11  are further connected to the lateral edge  10   b  of the center area  10  by stays  16 . Each sleeve  11  is hereby provided with two stays  16  of generally wedge-shaped configuration to connect the outer surface  11   a  of the sleeve  11  with the edge  10   b  of the center area. The outer side of the stay  16  terminates hereby tangentially into the outer surface  11   a  of the sleeves. The stay  16  extends substantially across the entire thickness d of the platen  2  and the opposite area of the outer surface  11   a . Each sleeve  11  is thus connected with the center area  10  by a ring-shaped bridge  17  that remains at the root of the ring-shaped gap  12  and the two stays  16  disposed laterally to the bridge  17 . 
   The center area  10  is further provided with an aperture  18  for insertion of an injection unit (not shown) to connect to the molding tool  14 . 
   The configuration of the platens  2 ,  6  with the rib structure  10  and the sleeves  11 , connected to the center area  10  via the stays  16  and the ring-shaped bridge  17 , is especially suitable for realizing a single-piece construction of casting material, e.g. cast steel or gray cast iron. In this way, the platen  2 ,  6  is lightweight and easy to manufacture. In addition, the center area  10  is formed with holes  10   c  to further contribute to the lightweight character of the platen. 
   The stays  16  and the ring-shaped bridge  17  define an elastically yielding connection between the sleeves  11  and the center area  10  so that a transmission of deflections of the platens  2 ,  6  as a consequence of the applied clamping force to the sleeves is greatly reduced. The stays  16  and the bridge  17  are hereby subjected to torsion and bending. As a result, the spindle mechanisms  7  and the spindles  5  are subjected to little bending stress so that parallelism between the platens  2 ,  6  is maintained and overly stressing of the spindle mechanisms  7  as well as the spindles  5  is avoided. 
   While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: