Abstract:
The present invention relates to a pre-stressed tie rod and method of manufacture. The pre-stressed tie rod can be used on injection molding machines to increase the length of the tie rod and movement of the platen, wherein the tie rods are loaded to restrict mold separation. The pre-stressed tie rods could also be used with other types of presses.

Description:
This application claims priority of U.S. provisional patent application Ser. No. 60/548,412, entitled “Pre-Stressed Tie Rod and Method of Manufacture,” by Bryan R. Kirchmer and Derek Campbell, and having a filing date of Feb. 28, 2004, the description of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a new and improved pre-stressed tie rod and a method of manufacturing the same. More particularly, this invention is directed toward use of an improved pre-stressed tie rod to increase the capacity of plastic molding machines to mold larger parts. Still more particularly, the present invention can also be applied to other types of devices such as presses. 
   2. Prior Art 
   Injection molding machines are categorized by the amount of force they are able to exert on a work piece or molding dies. Generally speaking Injection molding machines fall into two types. The first type, the direct force injection molding machine, has been around for many years and uses a large hydraulic cylinder to directly exert a large force on the mold. The second type, the restrained force injection molding machine, is a relatively new concept which uses the strength of tie rods to restrain the molds against the forces created by the molten plastic or other material forced into the die. 
   The direct force injection molding machine has a static platen, a dynamic platen, a base, a mold or die with two halves and a source of force such as a large hydraulic cylinder. The hydraulic cylinder and static platen are anchored on the base. The hydraulic cylinder moves the dynamic platen in relationship to the static platen. When in use the two halves of the die are contained between the static platen and the dynamic platen. The hydraulic cylinder exerts a force on the dynamic platen which forces the two halves of the die together. The force exerted by the hydraulic cylinder must be greater than the force exerted by the hot molten plastic injected into the die. If it is not the plastic will seep between the two halves of the mold and cause the parts being formed to flash. If the amount of flashing is too great, the parts must be discarded. 
   The greatest short coming of the direct force injection molding machine is the high energy costs. When the piece is being formed the hydraulic cylinder must maintain the force on the platens and mold the entire time. This requires a large hydraulic pump powering a large hydraulic cylinder to achieve high clamping loads. This translates into higher energy costs to run the molding operation. 
   The second type of injection molding machine, the restrained force injection molding machine, relies on the strength of the tie rods to hold the two platens and two mold halves together during the molding process. It does not require large hydraulic cylinder to apply force throughout the molding cycle. It uses much smaller hydraulic cylinders to move and lock into place the dynamic platen and the attached half of the mold, the energy consumed by those hydraulic cylinders is much less than the energy consumed using a similar sized direct force injection molding machine. 
   U.S. Pat. No. 6,241,508, entitled “Multiple Mold Workstation with single Injection Feeder and Hydraulic Pumping Station”, issued to John Michael et al., which is incorporated herein by reference, discloses a restrained force injection molding machine. The primary limiting factors in the ability of a restrained force injection molding machine is the capacity of the tie rods. The capacity of the tie rods is determined by the number of tie rods, the cross-sectional area of each tie rod, and the Young&#39;s modulus, or otherwise known as the modulus elasticity of the material from which the tie rods are constructed. 
   One way to increase the capacity of tie rods in other types of devices is to have a pre-stressed tie rod. However, heretofore use of pre-stressed tie rods on injection molding machines is not known. U.S. Pat. No. 4,240,342, entitled Frame Structure for a Press Assembly, issued to Philip T. Delmer on Dec. 23, 1980. The Delmer patent discloses a press assembly having an improved frame, including a crown, bed, cylinder and ram assembly, tied together by tie rods and compression members in which a platen assembly secured to the ram assembly guides directly on the inward facing surface portion of the tie rods. 
   U.S. Pat. No. 6,250,216, entitled Press Deflection Controller and Method of Controlling Press Deflection, issued to John B. Bornhorst on Jun. 26, 2001. The Bornhorst patent discloses a mechanical press having a press deflection controller. The press includes press members which have work surfaces, such as a slide in a bed. The press deflection controller includes a tie rod which is encased in a tube. The tie rod is connected to the press member and is maintained in tension while the tube is maintained in compression. Adjusting the tension in the tie rod during press operation works to adjust the deflection in the press member. 
   The Bornhorst patent differs from the Delmer patent in that the tie rods in the Delmer patent run parallel with the direction of the force being applied whereas the pre-stressed tie rod in Bornhorst runs perpendicular to the direction of the force being applied. 
   When the press is in use, the tie rods are in tension. The tension force in the tie rods is equal to the compression force being exerted on the work piece. The capacity of the press for exerting force on the work piece is limited by the tensile strength of the tie rods. The tensile capacity of the tie rods can be increased by pre-loading the tie rods with compression. When the tie rods have been pre-loaded with a compression force, the total tensile capacity of the tie rod is then equal to the original tensile strength of the tie rod plus the pre-loaded compressive force in the tie rod. 
   The pre-stressed tie rods disclosed in Delmer, Bornhorst and other prior art typically include a center portion which is threaded on both ends. The center portion is then extended through the center of a tubular member. A nut or other threaded fixture is then engaged on the threads on either end of the center piece. As the nuts travel along the threads of the center piece, they travel toward one another, capturing the tubular member between them. Once the nuts engage the tubular member, they exert a force on the tubular member. This creates a tension in the center piece which is equal to the compression exerted on the tubular members. One of the shortcomings of the prior art is that they rely upon the movement of the nut on the threaded centerpiece to tension the center portion and compress the outer tubular member. The force exerted by this arrangement can be difficult to gauge and balance as well as adjust. This oftentimes leads to less than optimal use of the device. 
   Because of the innate inaccuracy and difficulty of use of prior art pre-stressed tie rods, heretofore restrained force injection molding machines use single piece tie rods which are not pre-stressed.  FIG. 1  shows a restrained force injection molding machine  20 .  FIG. 2  shows a partial top view of the restrained force injection molding machine shown in  FIG. 1 . The shortcomings of the prior art tie rods did not allow for the use of pre-stressed tie rods on an injection molding machine. 
   As can best be seen in  FIG. 2 , there is a stationary platen  26 . The stationary platen  26  serves as a work surface for the injection molding machine  20 . The base of the mold  28  is attached to the stationary platen  26 . Four tie rods  30  extend perpendicular from the stationary platen  26 . The dynamic platen  32  moves in relationship to the stationary platen  26  along the tie rods  30 . The second half of the mold  34  is attached to the interior surface of the dynamic platen  32 . 
   In operation, the dynamic platen  32  is moved toward the stationary platen  26  by a hydraulic cylinder  36  until the base and top of the mold  28  and  34  are in contact with one another. Each tie rod  30  has a collar  38  which is fixedly attached to the tie rod  30 . The base of the mold  28  and the second half of the mold  34  are forced together by a locking mechanism  40 . The locking mechanism  40  is comprised of a hydraulic cylinder  42 , with a wedge  44  located on either end of the hydraulic cylinder  42 . The locking mechanism  40  creates a force holding the base of the mold  28  and second half of the mold  34  together by the hydraulic cylinder  42  forcing the wedge  44  between the outside surface of the dynamic platen  32  and the collar  38 . This places the base and second half of the mold  28  and  34  into compression. At the same time, it puts the tie rods  30  into tension. The tension in the tie rods  30  is equal to the compression force exerted on the base of the mold  28  and second half of the mold  34 . 
   A hot molten plastic is then pumped into the mold at very high pressure. The tie rods  30  deflect in proportion to the force exerted by the hot molten plastic. The force created by the pressure of the molten plastic will cause the tie rod  30  to deflect, causing a gap between the base of the mold  28  and the second half of the mold  34 . If the gap is large enough the molten plastic will flash between the base of the mold  28  and the second half of the mold  34 . If the flashing is large enough it can take the part being formed out of acceptable tolerances, in which case the part must be discarded. 
   The deflection of the tie rods  30  can be calculated by the following equation: 
           δ   =     PL   AE           
wherein δ is equal to the length of the deflection, P is equal to the force applied, L is equal to the length of the tie rod, A is equal to the cross-sectional area of the tie rod, and E is equal to the modulus of elasticity of the material from which the tie rods are made.
 
   As can be seen by the equation, one of the limiting factors in determining the deflection is the length of the tie rod. For this reason, restrained force injection molding machine as shown in  FIG. 1  typically use a rather short length tie rod  30 . This in turn limits the height of the piece which can be formed. For this reason, restrained force injection molding machine such as the one shown in  FIG. 1  have been limited to applications such as molding pallets or other short or flat objects. This limitation has prevented them from being used to mold taller objects such as traffic barrels. 
   SUMMARY OF THE INVENTION 
   Due to the shortcomings of the prior art, it is an objective of the present invention to provide a pre-stressed tie rod which is easy to use. 
   Another objective of the present invention is to provide a pre-stressed tie rod which can be used on an injection molding machine to provide a longer usable tie rod than is possible with the prior art. 
   It is a further objective of the present invention to provide a method for manufacturing an improved pre-stressed tie rod. 
   It is yet another objective of the present invention to provide a pre-stressed tie rod to enable an injection molding machine to mold tall pieces such as traffic barrels. 
   It is still another objective of the present invention to provide a pre-stressed tie rod which can be used on a traditional press, thus providing the same capacity while reducing the amount of metal or other material used to construct the frame of the press. 
   Other objects, features, and advantages will be apparent to persons of ordinary skill in the art in view of the following detailed description of preferred embodiments and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, the needs satisfied thereby, and the features and advantages thereof, reference now is made to the following descriptions taken in connection with the accompanying drawings in which: 
       FIG. 1  is a perspective view of a prior art restrained force plastic injection molding machine. 
       FIG. 2  is a partial top view of the prior art plastic injection molding machine shown in  FIG. 1 . 
       FIG. 3  is a top view of a tie rod incorporating one embodiment of the present invention. 
       FIG. 4  is a cross-section view of the tie rod taken along line  4 — 4  shown in  FIG. 3 . 
       FIG. 5  is an exploded view of the tie rod shown in  FIG. 3 . 
       FIG. 6  is a partial top view of a restrained force plastic injection molding machine using one embodiment of the present invention. 
       FIG. 7  is an exploded view of the preferred embodiment of the collar and improved pre-stressed tie rod. 
       FIG. 8  is a schematic diagram of a press using an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows a top view of the preferred embodiment of the improved tie rod  70 .  FIG. 4  shows a cross-sectional view of the same tie rod  70  taken along the line  4 — 4 , shown in  FIG. 3 . The same tie rod is shown in an exploded view in  FIG. 5 . The improved tie rod has a center  72 , which is surrounded by an compression sleeve  74 . In the preferred embodiment, the center  72  generally has a dumbbell shape, comprising an elongated tensioned core  76  connecting two bases  78 . Each base  78  has a shoulder  80  located on the side of the base facing the opposing base  78 . 
   The compression sleeve  74  in its preferred embodiment shown in  FIGS. 3 ,  4  and  5 , is constructed from two elongated semi-circular pieces, as best seen in  FIG. 5 . It should be noted that while the preferred embodiment of the pre-stressed tie rod  70  is shown in  FIGS. 3 ,  4  and  5 , other geometric configurations could also be used to achieve the same result and thus should be considered to fall within the scope of the present invention. 
   In constructing the preferred embodiment of the improved tie rod  70 , the center  72  can be formed by casting, forging, machining or any other manufacturing process, out of any material with a suitable modulus of elasticity such as low carbon steel. The compression sleeve  74  can be constructed in a similar manner. 
   In order to assemble the improved tie rod  70 , the center  72  is heated to a desired temperature. This causes the material in the center  72  to expand, thus increasing the distance between the two shoulders  80 . Once the center  72  is heated to the desired temperature, the two portions of the compression sleeve  74 , being at an ambient temperature or a temperature below that of the center  72 , are positioned around the tensioned core  76  of the center  72 . The entire assembly of the improved tie rod  70  is then allowed to cool. 
   During this cooling process, the material in the center  72  contracts. This reduces the distance between the two opposing shoulders  80 , thus capturing the sections of the compression sleeve  74  between the two shoulders  80 . This in turn creates a tension in the tensioned core  76  of the center  72  and a compression force in the portions of the compression sleeve  74 . If necessary, the seams between the sections of compression sleeve  74  can be welded or otherwise joined. Likewise, the seam between the ends of the compression sleeve  74  can be welded or otherwise joined to the shoulder  80  of the center  72 . 
   The pre-loaded compression and tension of the improved tie rod  70  are determined by the amount the overall length of the outer sleeve  70  exceeds the distance between the shoulders  80  of the center  72 . It is also a function of the properties of the material used in making the center  72  and the compression sleeve  74 . In assembling the improved tie rod  70 , the center  72  must be heated to a temperature sufficient to allow the tensioned core  76  of the center  72  to expand to a length such that the distance between the two shoulders  80  of the center  72  is equal to or greater than the overall length of the compression sleeve  74 . This is a function of the thermal expansion coefficient of the material used to make the center  72 . 
     FIG. 6  shows a partial top view of restrained force injection molding machine  20  using the improved tie rods  70 . As can be seen in comparing  FIG. 6  to  FIG. 1 , the injection molding machine  20  operates in the same manner. However, with the improved tie rod  70  the amount of force the molding machine is capable of exerting on the base and second half of the molds  28  and  34  is increased by the amount of pre-loaded compression in the compression sleeve  74 . This increase in the amount of force can be translated into an increase in the overall length of the improved tie rod  70  over what is available with the prior art while using the same material, applying the same amount of force, same cross-sectional area of the tie rod and achieving the same deflection. This means that there can be greater movement of the dynamic platen  32 , which in turn means that the injection molding machine  20  using the improved tie rod  70  would now be able to mold larger pieces. 
     FIG. 7  shows an exploded view of the preferred embodiment of the collar  38 . The collar  38  has a wedge shaped sleeve  90  and a split collar  92 . The interior of the wedge shaped sleeve  90  is sized to fit the outer diameter of the improved tie rod&#39;s  70  compression sleeve  74 , thus allowing it to be slid along the improved tie rod  70  during assembly or for maintenance purposed. There is a groove  94  which is cut into the outer surface of the compression sleeve  74  of the improved tie rod  70 . It should be noted that the depth of the groove  94  is less than the thickness of the compression sleeve  74 . The interior diameter of the split collar  92  is sized to fit the inside diameter of the groove  94 . 
   In assembling the collar  38 , the wedge shaped sleeve  90  is slid so that it is adjacent the groove  94 . The first piece of the split collar  92  is then inserted into the groove  94 . Bolts are inserted into the bolt holes  96  and tightened to fasten the split collar  92  to the wedge shaped sleeve  90 . The second half of the split collar  92  is then inserted into the groove  94  and bolted to the wedge shaped sleeve  90  in a similar fashion. It should be noted that other types of fasteners could also be used. Further, it is possible to use other configurations of collar  38  while still being within the spirit of the present invention. 
     FIG. 8  shows a schematic of a press  100  using one embodiment of the improved tie rod  70  of the present invention. The press  100  has a crown  102 , a work surface  104 , two or more tie rods  70  and a hydraulic cylinder  106  or other source of force. The crown  102  and work surface  104  are attached to one another by two or more pre-tensioned tie rods  70 . A collar  108  is fastened to the compression sleeve  74  just above and below the tie rods&#39;  70  intersection with the crown  102  and work surface  104 . The design of the collar  108  can be similar to the collar  38  shown in  FIG. 7 , with the wedge shaped sleeve  90  replaced with a cylindrical shaped sleeve. It should also be noted the tie rods  70  can be secured to the crown  102  and work surface  104  by any number of means commonly known while still falling within the spirit of the present invention. 
   When the press  100  is in use the force exerted by the hydraulic cylinder  106  is equal to the tension in the compression sleeve  74  of the tie rods  70 . The capacity of the tie rods is equal to the compression force stored in the compression sleeve plus the tensile strength of the compression sleeve  74 . This means the press  100  can be build using less material in the tie rods  70  than would be needed with a single piece unstressed tie rod. This means the cost of the press  100  is less. Further the press  100  is lighter and easier to move thus reducing installation costs. 
   The foregoing specifications and drawings are only illustrative of the preferred embodiments of the present invention. They should not be interpreted as limiting the scope of the attached claims. Those skilled in the arts will be able to come up with equivalent embodiments of the present invention without departing from the spirit and scope thereof.