Abstract:
A bracket for linking guide rods to a wheel of a vertical blow molding machine and a system for blow molding containers and a mechanism for opening and closing a pair of mold halves linked to the wheel of the vertical blow molding machine using the bracket, comprising a first portion and a second portion made of iron, a plurality of pivot apertures, and a plurality of guide rod apertures for receiving the guide rods.

Description:
TECHNICAL FIELD 
     The invention is directed to extrusion blow molding. More particularly, the invention is directed to an apparatus for extrusion blow molding polyethylene terephthalate. 
     BACKGROUND OF THE INVENTION 
     Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The myriad of advantages of PET include toughness, clarity, good barrier properties, light weight, design flexibility, chemical resistance, and good shelf-life performance. Furthermore, PET is environmentally friendly because it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles. 
     There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube (also called a “parison”). The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold. 
     To make PET suitable for EBM, PET manufacturers have developed special grades of PET also referred to as extrusion grade PET or “EPET.” Typically, EPET is high molecular weight PET having an inherent viscosity (“I.V.”) of 1.0 dl/g or greater as measured by solution viscosity. 
     One EBM process used to manufacture EPET containers includes a vertical rotary blow molding machine, which can have an output of, for example, over 100 bottles per minute depending on the number of cavities and molds. Vertical rotary blow molding machines index circumferentially spaced mold halves on a wheel in steps around a horizontal axis. The mold halves each capture a vertical, continuously growing parison at an extrusion station. In one type of machine, for example, the flow head extruding the parison moves up and away from the mold halves after the mold halves close to capture the parison. The parison is severed adjacent the top of the mold halves, the mold halves are moved away from the extrusion station, and a top blow pin is moved into the end of the captured parison at the top of the mold halves to seal the mold cavity and blow the parison. Subsequently, the flow head and dependent parison are lowered back to the initial position so that the new parison is in position to be captured by the next pair of mold halves. The blown parison cools as the mold halves are rotated around the machine, following which the mold halves open at an ejection station and the finished article, commonly a container, is ejected from between the mold halves. In operation, the rotary wheel can produce at least 110 bottles per minute for a 22 cavity mold machine (22 molds) rotating at a rate of 5 RPM. 
     In EBM processes, high clamp forces are often required and deflection of the mold halves may occur. As a result, there may also be increased part wear and subsequent mold misalignment. Accordingly, there is a need in the art for a system for extrusion blow molding EPET containers that reduces deflection and operates at high clamp forces. 
     BRIEF SUMMARY OF THE INVENTION 
     To meet these and other needs, and in view of its purposes, the invention provides an improved angle link pivot bracket for the wheel of a vertical blow molding machine. One aspect of this invention provides a bracket for linking guide rods to a wheel of a vertical blow molding machine comprising a first portion and a second portion made of iron having a tensile strength of at least 65,000 psi and a yield strength of at least 45,000 psi, a plurality of pivot apertures, each for receiving a pivot bushing having a width of 1.65 to 1.85 inches; and a plurality of guide rod apertures for receiving the guide rods. 
     Another aspect of the invention provides a system for blow molding containers comprising a rotatable wheel and a plurality of molds with mold halves movable between an open position and a closed position, where each mold half is linked to the wheel by a linking system having a plurality of guide rods and a pivot bracket. The pivot bracket is comprised of a first portion and a second portion made of iron having a tensile strength of at least 65,000 psi and a yield strength of at least 45,000 psi, a plurality of pivot apertures, each for receiving a pivot bushing having a width of 1.65 to 1.85 inches, and a plurality of guide rod apertures for receiving the guide rods. 
     Yet another embodiment of the invention provides a mechanism for opening and closing a pair of mold halves linked to a wheel of a vertical blow molding machine comprising a plurality of guide rods mounted on the wheel by a pivot bracket, first and second mold halves mounted on the guide rods, where the mold halves are movable along the guide rods toward and away from one another between an open position and a closed position, and a plurality of actuator rods for moving the mold halves between the open position and the closed position. The pivot bracket is comprised of a first portion and a second portion made of iron having a tensile strength of at least 65,000 psi and a yield strength of at least 45,000 psi, a plurality of pivot apertures, each for receiving a pivot bushing having a width of 1.65 to 1.85 inches, and a plurality of guide rod apertures for receiving the guide rods. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures: 
         FIG. 1  is an illustration of a vertical rotary blow molding machine; 
         FIG. 2  is a front view of the wheel linkage system according to the invention; 
         FIG. 3  is a top view of the wheel linkage system of  FIG. 2 ; 
         FIG. 4  is a front view of a pivot bracket according to the invention; and 
         FIG. 5  is a side view of the pivot bracket shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is directed to an apparatus for extrusion blow molding PET, and preferably extrusion grade PET (EPET), at high output rates that are characteristic of vertical wheel-type continuous extrusion blow molding machines. Referring now to the drawings, in which like reference numbers refer to like elements throughout the various figures that comprise the drawings,  FIG. 1  shows a vertical rotating blow molding machine  100 . The machine  100  includes a rotatable wheel  114 , supported on a base  112 , and an extruder  115 . 
     The wheel  114  has a plurality of in-line molds  116 , each mold  116  having an identical inner cavity  117  defining a container. Rotary blow molding machines  100  typically have from 6 to 30 molds, but may have any number of molds. The in-line molds  116  are mounted on the wheel  114 , preferably about the perimeter of the wheel  114 , for rotation about a horizontally disposed rotational axis  113 . The in-line molds  116  each preferably have a pair of mold halves that split the respective inner cavities  117  and that open and close at various stations during rotation about the rotational axis  113  consistent with operation of a conventional wheel-type extrusion blow molding apparatus. 
     One mold half  116   a , which is shown in  FIG. 2  and  FIG. 3  in an open position, is mounted on a platen  210  that slides on the guide rods  200 . The mold half  116   a  is linked to a cam follower  199  of the wheel  114  via a link system, including a pivot bracket  201 . With the addition of the pivot bracket  201 , the portion of the guide rods  200  that is exposed is reduced in length to about 1.5 to 2 inches, i.e., a portion of the guide rod  200  is supported by and contained within the pivot bracket  201 . The shortening of the exposed guide rod  200  reduces guide rod deflection and twisting and increases the life of the guide rod  200 . Reduction of guide rod deflection also results in reduction of part wear and subsequent mold misalignment. The link system may also include a spring assembly  211 , which may include a spring shaft and spring discs, engaging a yoke  202  connected to the guide rod  200 , a connector link  203  connected to the yoke  202 , an angle link  204  connected to the connector link  203 , such as a bull gear, and a cam link  205  that connects the angle link  204  to the cam follower  199 . The pivot bracket  201  is connected to the angle link  204  by a pin (not shown) that passes through pivot apertures  302  (shown in  FIGS. 4 and 5 ) of the pivot bracket  201  and the angle link  205  to create a pivot. Bushings  206  are also used as sleeves in the pivot apertures  302  to create a bearing surface, where the bushings  206  distribute bearing forces over a surface area. An actuator rod (not shown) is also connected to the cam follower  199  to move the mold halves between an open and a closed position. 
     The pivot bracket  201  is shown in more detail in  FIGS. 4 and 5 . The pivot bracket  201  has a first portion  300  and a second portion  301  and a front  310  and back  311 . The first portion  300  has a plurality of pivot apertures  302  adapted to receive a pivot bushing (not shown). The pivot apertures  302  have a diameter of 1.65 to 1.85 inches. Preferably, there are two pivot apertures  302  and each has a diameter of 1.75 inches. The second portion  301  has a plurality of guide rod holes  304  and  305  that receive the guide rods  200 . Preferably, there are two guide rod holes  304  and  305 , and the diameter of the guide rod holes  304  and  305  are slightly larger than guide rod to provide a snug fit. Preferably, the diameter of the guide rod holes ranges from 0.03 to 0.3 inches greater than the guide rod. The guide rod holes  304  and  305  are, preferably, larger at the back  311  than at the front  301 , i.e., the guide rod holes  304  and  305  increase in diameter from front  310  to back  311  and have a conical shape. Many of the dimensions of the pivot bracket  201  depend upon the parts of the extrusion blow molding machine  100  and can vary from one machine to another. Regardless of the magnitude of the dimensions for the pivot bracket  201 , the dimensions are predetermined before the pivot bracket  201  is cast. By “predetermined” is meant determined beforehand, so that the predetermined dimensions must be determined, i.e., chosen or at least known, in advance of casting. 
     The first and second portions  300  and  301  are made of iron having a tensile strength of at least 65,000 psi and a yield strength of at least 45,000 psi. Exemplary materials include 80-55-06 ductile iron and 65-45-12 ductile iron. Other suitable materials may also be used provided that they are stronger than low grade cast iron. Preferably, the first portion and the second portion comprise one contiguous piece, i.e., the pivot bracket  201  is comprised of one integral piece. By “integral” is meant a single piece or a single unitary part that is complete by itself without additional pieces, i.e., the part is of one monolithic piece formed as a unit with another part. 
     Referring back to  FIG. 1 , the extruder  115  includes a die  130  located adjacent to the wheel  114  and defining a filling station  182  at which a series of hot plastic parisons are extruded from the die  130 . The die  130  is positioned such that when the in-line molds  116  are rotated, seriatim, to the filling station  182  by the wheel  114 , the hot plastic parisons extruded from the die  130  may be accepted into the respective inner cavities  117  according to conventional wheel-type blow moldings techniques. 
     To construct a proper bottle, a parison supplied by the die  130  will typically have a varying thickness profile along a length of the parison to assure that upon the stretching experienced during blowing, the bottle will have the desired wall thickness at the various portions of the bottle. For example, it is often desirable to increase the thickness of a neck or bottom portion of a bottle relative to the remainder of the bottle to provide increased strength at those portions. Accordingly, the thickness of the parison for such a bottle may be, for example, relatively thicker at a top and bottom of the parison corresponding to a neck and bottom portion of a bottle blown from the parison. 
     To facilitate the production of such a profile of a parison, a pin  128  of the die  130  controls the thickness of a wall of each parison extruded. The pin  128  is coupled to a hydraulic cylinder  131  by a drawbar (not shown) such that the hydraulic cylinder  131  may vary the position of the pin  128  during extrusion of a parison. As is known to one of ordinary skill in the art, the hydraulic cylinder  131  thus raises and lowers the pin  128  to vary the thickness of the parison as it is extruded. The controller  120  controls the hydraulic cylinder  131  and thus the positioning of the pin  128 . The controller  120  has programmed in it, in a manner well-known to those skilled in the art, a preset pin control program or routine to control the vertical movement of the pin  128  during extrusion of each parison and manifests control of the pin  128  via the hydraulic cylinder  131  as is also known in the art. In this manner, the pin control program determines the thickness profile of the parison. The controller  120  also induces extrusion of the parison from the die  130  upon indication from the trigger mechanism  188  that a mold is in place, that a parison is required, or both. 
     In operation, a parison is extruded from the die  30  and the mold halves of one of the in-line molds  116  close around the parison such that the parison is received by the inner cavity  117 . Having received a parison within one of the inner cavities  117 , the wheel  114 , in a conventional manner, continues rotation to a blowing station  184  where air is introduced to the inner cavity  117  through a blow needle (not shown). The air from the blow needle expands the parison to conform to the shape and size of the inner cavity  117  and form a container with conventional blow molding techniques. Once blown and at least partially cooled, the container is thereafter released by opening the mold halves  116   a  and  116   b  at a release station  186 . Continued operation of the wheel  114  can produce at least 75 bottles per minute. 
     Suitable EPET for use in accordance with the invention is, for example, POLYCLEAR® EBM PET 5505 available from Invista (Spartanburg, S.C.) and Eastman Copolymer PM32177 available from Eastman Chemical Company (Kingsport, Tenn.). However, any PET or EPET or any other material may be used. 
     Although the invention has been described in detail and with reference to specific embodiments, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. For example, the pivot bracket  201  may be made of any material and may be used with different blow molding machines  100 . Thus, it is intended that the invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.