Abstract:
A self-compensating support skate ( 46 ) for an annular hydraulic ram ( 15 ) mounted in the front stationary platen ( 10 ) of a “two-platen” injection molding machine ( 1 ) is disclosed. The hydraulic ram ( 15 ) fits into a central bore ( 22 ) in the stationary platen ( 10 ) and connects directly to a relatively thin die platen ( 11 ) that provides a mold mounting surface. The purpose of the skate ( 46 ) is to prevent movement (tilting) of the die platen ( 11 ) and the resulting misalignment with the movable platen ( 20 ) when the mold ( 30 ) is attached. The skate ( 46 ) is adjusted during assembly to compensate for the tolerance stack-up in the ram assembly. The “zero” clearance is maintained by disc springs ( 56 ) within the skate ( 46 ) that compensate for the expansions and contractions of the large diameter parts due to temperature variations. The die platen ( 11 ) is held in the aligned position by contact with the lower tie rods ( 7 ).

Description:
This application claims priority to Provisional Application Serial No. 60/046,627, filed May 16, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mold clamping system for an injection molding machine and, more particularly to apparatus to support a short-stroke hydraulic ram in a two-platen injection molding machine. 
     2. Description of the Related Art 
     The clamping system of a typical injection molding machine generally comprises two rectangular stationary platens; a front platen adjacent the machine&#39;s injection unit, and a rear platen, with tie rods connecting the four corners of the two platens. A movable platen is located between the stationary platens and is supported by the tie rods in a way that allows for translational movement between the two stationary platens. A mold constructed in two “halves” is mounted so that one half is attached to the front stationary platen and the other half attached to the movable platen. The force for the clamping system is usually provided by a hydraulic cylinder that acts directly or via a toggle mechanism to open and close the mold halves by applying the appropriate force to the movable platen. With the mold held closed by the clamping system, plastic melt is injected into the cavity formed by the mold halves, forming a molded product. 
     One of the drawbacks of the conventional, three platen injection molding machines is that the overall length of the machine is relatively long, meaning that the machine occupies a significant amount of valuable space on the manufacturing floor. 
     Accordingly, the prior art has proposed various “two-platen” constructions for injection molding machines that have only one stationary platen. More specifically, prior art two platen clamping systems generally include a front (stationary) platen, a movable platen, four tie rods fixedly connected to one of the platens, releasable nuts attached to the other platen to engage the tie rods, means for traversing the movable platen, and one or more clamping cylinders to hold the mold closed. In operation, the movable platen is traversed into position to close the mold; the nuts are then closed on the tie rods to provide a rigid link between the two platens; and clamp tonnage is applied by the clamp cylinder in preparation for injection of the plastic melt. 
     While two-platen machines have the advantage of significantly shorter overall length, their configuration also presents potential problems. For example, since the two-platen design is primarily used for larger machines, the associated molds tend to also be very large and heavy. The weight of the mold must be supported by the platens in such a way that the platen surfaces remain parallel when the mold is open, so that it will close smoothly and properly form the mold cavity. However, the eccentric weight makes this difficult, particularly since the tie rods are fixedly supported at only one end when the movable platen is in motion. 
     In addition, most prior art two-platen machines have employed clamping cylinders associated with all four tie rods, thereby applying force to the mold via the tie rods. Clamping the mold in this manner tends to be rather complex since it requires that four clamping mechanisms be carefully synchronized to assure that uniform pressure is applied to the mold. 
     A few prior art attempts have been made to apply the clamping force centrally through a mold mounting surface. However, these clamping systems have not been satisfactory because of their failure to maintain alignment of the mold surfaces. In particular, due to the variation in weights of different molds, combined with the manufacturing tolerances of the components and associated running clearances connected with assembly, the mold mounting surface tilts from vertical after the mold is attached. In other words, the prior art has failed to provide a means of compensating for the manufacturing tolerance stack-up. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a two-platen clamping system that applies uniform clamping force to the mold and includes means to ensure vertical alignment of the associated mold mounting surface. 
     The present invention is generally described as a self-compensating support skate for an annular hydraulic ram mounted in the front stationary platen of a “two-platen” injection molding machine. More specifically, the hydraulic ram fits into a central bore in the stationary platen and connects directly to a relatively thin mold mounting (die) platen. The purpose of the skate is to prevent movement (tilting) of the mold mounting platen and the resulting misalignment of the platens when the mold is attached. Without the skate, movement of the die platen would occur due to the variation in weights of different molds combined with the manufacturing tolerances of the components and associated running clearances in the normal ram assembly. 
     The skate is positioned at the bottom of the ram on the injection side of the stationary platen, where it is adjusted during assembly to compensate for the tolerance stack-up. In particular, after the tie rods are leveled, the skate is adjusted so that the upper surface of the ram is in contact with a mating bronze sleeve in the stationary platen; a lock nut holds the skate in this position. The die platen is then raised until its mold mounting face is parallel with the movable platen. Appropriately sized shims are installed to establish the desired spacing between the die platen and lower tie rods thereby maintaining parallelism. 
     Adjustment and support of the hydraulic ram in this manner effectively reduces the tolerance stack-up to zero and virtually eliminates movement (tilting) of the die platen due to the weight of the attached mold half. This “zero” clearance is maintained by disc springs within the skate that compensate for the expansions and contractions of the large diameter parts due to temperature variations. The disc springs also prevent massive overloading of the support skate when an “out-of-parallel mold” is used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of the clamping system for a two-platen injection molding machine incorporating the ram support skate of the present invention. 
     FIG. 2 is a top view of the clamping system for a two-platen injection molding machine incorporating the ram support skate of the present invention. 
     FIG. 3 is an enlarged view of the stationary platen assembly of the clamping system shown in FIG.  1 . 
     FIG. 4 is a view showing the back side of the stationary platen assembly, taken along the line  4 — 4  in FIG.  3 . 
     FIG. 5 is a view showing the mold mounting side of the stationary platen assembly, taken along the line  5 — 5  in FIG.  3 . 
     FIG. 6 is a sectional view of the stationary platen assembly, taken along the line  6 — 6  in FIG.  5 . 
     FIG. 7 is an enlarged, fragmentary, sectional view of the ram skate assembly, as shown in the sectional view of the stationary platen assembly in FIG.  6 . 
     FIG. 8 is an enlarged, fragmentary view showing the manner in which the die platen of the stationary platen assembly shown in FIG. 5 is supported by the tie rods. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As illustrated in FIGS. 1 and 2, the clamping system  1  associated with the present invention generally includes a stationary platen assembly  10  (including a die platen  11  and a cylinder platen  12 ), a movable platen  20 , tie rods  7 , traverse cylinders  2 , a stationary mold half  30   a , a movable mold half  30   b , a rear support plate  35  and tie rod locking mechanisms  26 . 
     Specifically, the stationary mold half  30   a  is mounted to the die platen  11  of the stationary platen assembly  10  which is, in turn, fixed to a suitable machine base (not shown). The movable mold half  30   b  is mounted to the movable platen  20  which is, in turn, located at the other end of the machine base. The traverse cylinders  2  are fixedly mounted to the stationary platen  10  at the diagonally opposed positions shown. The piston rod  6  of each traverse cylinder  2  is connected to the movable platen  20 , so that simultaneous operation of the cylinders  2  causes the movable platen  20  to move linearly on the machine base with respect to the stationary platen  10 . FIGS. 1 and 2 show the movable platen  20  in the extreme open position; the extreme closed position is indicated by the phantom line  20   a  shown in FIG.  2 . 
     Four tie rods  7  are fixedly connected by conventional means to the cylinder platen  12 , extend through die platen  11  and the movable platen  20 , and are stabilized at the opposite end by rear support plate  35 . All of the tie rods  7  have a series of circumferential grooves  8  along an intermediate section of each rod  7 . When the movable mold half  30   b  is brought into contact with the stationary mold half  30   a , the grooves  8  come into engagement with locking mechanisms  26  mounted on the movable platen  20 , locking the movable platen  20  on the tie rods  7 . The locking mechanisms  26  are constructed as generally known in the art, including a split collar having a series of rings configured to mate with the grooves  8 , and an actuator to move the collar portions into and out of engagement with the tie rods  7 . With the locking mechanisms  26  engaged, the distance between the movable platen  20  and cylinder platen  12  is fixed. 
     The primary force for the clamping system  1  is provided by an annular hydraulic cylinder in the stationary platen assembly  10 . Referring to FIGS. 6 and 7 in particular, the cylinder platen  12  is provided with a bore  22  configured to receive a ram  15 . The ram  15  is of an annular configuration to allow the barrel of an injection unit (not shown) to pass therethrough and communicate with the stationary mold half  30   a . The ram  15  also has a flat front face  24  that serves as a means for attachment of the die platen  11 , and an elongated section  28  that extends into the central bore  22  of the cylinder platen  12 . With this construction, the stroke of the ram  15  is short. The maximum distance of travel for the ram  15  and die platen  11  is approximately equal to the spacing of the grooves  8  on the tie rods  7 . The drawings all show the stationary platen assembly  10  with the ram  15  and die platen  11  fully retracted. The phantom line  11   a  in FIGS. 2 and 3 shows the extent of travel of the die platen  11 . 
     A number of components are fitted between the bore  22  of cylinder platen  12  and the ram  15  to support and guide the ram, as well as providing a sealing means to contain the hydraulic fluid that is used to actuate the ram  15 . As best seen in FIG. 7, these components include cylinder ring  40  that attaches to the end of the elongated section  28  of ram  15 . Surrounding the cylinder ring  40  and the adjacent portion of the elongated section  28  of ram  15  is a bronze sleeve  42  fitted into the bore  22  of cylinder platen  12 . Additional sealing elements, as known in the art, are fitted around the cylinder ring  40  and bronze sleeve  42 , as required to contain the hydraulic fluid. 
     Although the components associated with the ram  15  and cylinder platen  12  as described above are effective to construct a functional hydraulic cylinder, the fact that this assembly includes several close-fitting, manufactured components results in a “stack-up” of manufacturing tolerances assigned to each of the individual components. In addition, further variation is introduced by the associated running clearances necessary for normal operation of the ram assembly. Accordingly, a ram skate assembly  46  is positioned at the bottom of the ram  15  on the injection side of the stationary platen assembly, as shown in FIGS. 4 and 6. In this position, the ram skate  46  enables adjustment of the ram  15  relative to the cylinder platen  12 , during the assembly process to compensate for the tolerance stack-up. 
     As best seen in FIG. 7, the ram skate assembly  46  preferably includes a lower bearing support  48  attached by a bolt that passes through the flange of the bronze sleeve  42  and into the cylinder platen  12 . An upper bearing support  50  is similarly attached to the end on the ram  15  by bolts that pass through the cylinder ring  40 . The remaining elements of the ram skate assembly  46  include an adjustment screw  52 , a load pin  54 , spring washers  56 , skate washer  58 , wear pad  60  and skate plate  62 . 
     The assembly process requires that the tie rods  7  be positioned in the stationary platen assembly  10 , fixedly connected to the cylinder platen  12  and leveled. The ram skate  46  is adjusted by rotating the adjustment screw  52  so that it acts through the load pin  54 , spring washers  56 , skate washer  58 , wear pad  60  and skate plate  62  on lower bearing support  48 , moving the upper surface of the ram  15  into contact with the mating bronze sleeve  42  in the cylinder platen  12 . After the ram  15  is adjusted into the desired position, reducing the tolerance stack-up to “zero”, a lock nut  64  is used to hold the adjustment screw  52  in place and maintain the zero tolerance. 
     With one portion of the ram  15  thus properly positioned, the assembly process continues by raising die platen  11  (and the forward end of ram  15 ), using floor jacks, for example, until the mold mounting face of the die platen  11  is parallel with the four tie rod collar mounting surfaces of the cylinder platen  12 . (By construction the faces of the rod collars are perpendicular to the tie rods and thus parallel to the mold mounting surface of the movable platen  20 ). Raising the die platen  11  results in a gap between the bronze shoe  66  that rides on the lower tie rod and the bottom surface of the die platen  11 , see FIG.  8 . This gap is measured so that an appropriately sized shim  68  can be made and installed to fill the gap, thus ensuring that the mold mounting face of the die platen  11  remains aligned with the corresponding surface of the movable platen. 
     Adjustment and support of the ram  15  in this manner effectively reduces the tolerance stack-up to zero and virtually eliminates movement (tilting) of the die platen  11  due to the weight of the attached mold half  30   b . The desired “zero” clearance is maintained by the spring washers  56  within the skate assembly  46  that compensate for the expansions and contractions of the large diameter parts that occur due to temperature variations. It should also be appreciated that the spring washers  56  will also prevent overloading of the ram skate  46  when an “out-of-parallel” mold is run in the clamping system  1 . As described above, the machine is assembled so that the mold mounting surfaces of die platen  11  and moving platen  20  are parallel. If the mounting surfaces of the mold are not parallel when the mold is closed, the clamping tonnage will tend to force the platens to align with the mold, the spring washers  56  will absorb some of the platen displacement in this situation. 
     Operation of the clamping system  1  will now be described. Prior to production operation of the machine it is necessary to establish (set-up) certain operating parameters associated with the size of the mold. More specifically, after the mold halves  30   a ,  30   b  are attached to the mounting surfaces of the die platen  11  and movable platen  20  respectively, the operator uses the machine control to initiate an automatic die height set-up process. As a first step, the ram  15 (along with the stationary mold half  30   a  on die platen  11 ) is fully retracted (i.e., zero stroke forward) with respect to the cylinder platen  12 . The traversing cylinders  2  will then operate to position the movable platen  30  so that the mold is closed. The machine uses feedback of this movable platen position to determine whether the locking mechanisms  26  are aligned with the grooves on the tie rods. If they are not aligned, the control will calculate the distance to a new position for the movable platen where the locking mechanisms will properly align with the grooves on the tie rods. This distance will determine the required stroke of the ram  15  to pre-position the movable mold half properly with respect to production operation. 
     To begin a cycle of machine operation, the traversing cylinders  2  are operated to bring the mold halves together, so that the movable platen  20  together with the locking mechanisms  26  move along the tie rods  7  toward the stationary platen assembly  10 . The grooves  8  formed on the tie rods  7  pass through holes  34  of the movable platen  20  as it traverses along the machine base. The movable platen  20  continues toward the stationary platen  10  so that the movable mold half  30   b  is slowly brought into close proximity with the stationary mold half  30   a . When the movable platen  20  reaches a predetermined position (as determined by the mold set-up procedure), the locking mechanisms  26  are actuated to engage with the grooves  8  of the tie rods  7 . 
     After the locking mechanisms  26  have fully engaged the grooves  8 , hydraulic oil is supplied to a chamber  5  adjacent the ram  15  and cylinder platen  12  to close the mold fully and initiate a clamping force by the ram  15  on the die platen  11  and the two mold halves  30   a ,  30   b . With the desired force thus applied to hold the mold closed, plastic melt is injected into the mold cavity. 
     The retraction of the clamp system is essentially the reverse of the procedure described above. Hydraulic pressure is applied to another chamber  4  adjacent the ram  15  so that clamp pressure is removed from the die platen  11  and stationary mold half  30   a , with the ram  15  retracting slightly. After the clamping pressure on the mold is released, the pressure on the locking mechanisms  26  is also released and they are actuated to disengage from the grooves  8 . The traversing cylinders  2  are then operated to move the movable platen  20  and movable mold half  30   b  away from the stationary mold half  30   a  a sufficient distance to allow removal of the molded part. 
     While the invention has been illustrated in some detail according to the preferred embodiment shown in the accompanying drawings, and while the preferred embodiment has been described in some detail, there is no intention to thus limit the invention to such detail. On the contrary, it is intended to cover all modifications, alterations, and equivalents falling within the spirit and scope of the appended claims. For example, although the ram, die platen and tie rods are shown and described as part of the stationary platen assembly, it is conceivable to connect some or all of these elements to the movable platen.