Patent Publication Number: US-2016236388-A1

Title: Injection molded elongated objects

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to injection molding and more particularly to apparatus and methods for injection molding elongated objects having a high aspect ratio. 
     injection molding is a well-known process in which plastic is melted, then forced into the cavity of a mold and then allowed to solidify, resulting in finished parts. The temperatures and pressures used in the molding process are significant. For example, the fluid plastic may be injected a pressure of about 207 mPa (30,000 lbf/in 2 ) and a temperature between 260 to 316° C. (500 to 600° F.). 
     Some part geometries are challenging to produce through injection molding. For example,  FIGS. 1 and 2  illustrate an exemplary part  1  which has been injection molded and therefore has a structure which may be described as unitary or monolithic. The part  1  has a generally cylindrical sidewall  2  closed off at one end by an endwall  3 . The ratio of the overall length “L” of the part  1  to the thickness “t” of the sidewall  2  is quite high, for example the ratio may be on the order of  100 . 
     A mold cavity for making such elongated parts typically includes a cylindrical cavity with a long cylindrical core disposed coaxially therein. When plastic (for example polyethylene terephthalate or “PET”) is injected, it tends to flow into the space between the two mold components asymmetrically. This causes unavoidable lateral shift of the core. Core shift results in a lower-quality part, increases the chance of total rejects (e.g. because of voids in the part), and can result in damaging contact between the core and the cavity. 
     Accordingly, there is a need for reliably produced injection molded elongated parts. 
     BRIEF SUMMARY OF THE INVENTION 
     This need is addressed by the present invention, which according to one aspect provides a an injection molded part having a monolithic structure including a sidewall with a closed perimeter, an endwall closing off one end of the sidewall, and a boss extending axially inward from an interior surface of the endwall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a side view of a part made using the apparatus and method of the present invention; 
         FIG. 2  is an end view of the part of  FIG. 1 ; 
         FIG. 3  is a schematic, partially cross-sectioned diagram of a molding apparatus constructed according to an aspect of the present invention; 
         FIG. 4  is an enlarged view of a portion of  FIG. 3 ; 
         FIG. 5  is an enlarged cross-sectional view of a portion of a mold assembly shown in  FIG. 3 , in a first position; 
         FIG. 6  is an enlarged cross-sectional view of a portion of a mold assembly shown in  FIG. 3 , in a second position; and 
         FIG. 7  is an enlarged, cross-sectional view of a portion of the part of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 3 and 4  illustrate an injection molding apparatus  10  constructed according to an aspect of the present invention which is useful for molding plastic parts in general, and especially elongated parts as shown in  FIG. 1 . The basic components of the injection molding apparatus  10  include a plastic supply  12 , a molder/extruder  14 , a mold assembly  16 , and a controller  18 . 
     The plastic supply  12  comprises apparatus of a known type such as a hopper, tank, etc. suitable for storing and dispensing plastic in the form of small solid pellets. 
     The molder/extruder  14  is a known type of device operable to receive the plastic from the plastic supply  12 , melt it to an appropriate temperature so that it forms a viscous fluid, and inject the fluid into the mold assembly  16  at suitable temperatures and pressures. For example, the fluid plastic may be injected at about 207 mPa (30,000 lbf/in 2 ) and between 260 to 316° C. (500 to 600° F.). 
     The mold assembly  16  includes a cavity which receives the fluid plastic. After injection, the plastic cools and solidifies in the cavity, which is then opened to eject the finished part. One or more actuators  20  are coupled to the mold assembly  16  and are used to open and close various portions of the mold assembly  16  during the molding procedure. The specific type of actuator is not critical and may be electrical, pneumatic, or hydraulic, for example. 
     The controller  18  comprises apparatus effective to control one or more aspects of the operation of the injection molding apparatus  10 . It may be implemented as a general-purpose electronic computer, as one or more application-specific electronic processors, or as one or more discrete mechanical or electrical components or modules. The controller  18  is depicted with single-line connections indicating control and/or sensing paths to the plastic supply  12 , molder/extruder  14 , and actuators  20 ,  44 ,  76 . 
     Referring to  FIGS. 3, 4, and 5 , the mold assembly  16  will now be described in more detail. In the illustrated example, the mold assembly  16  is modular in nature, being built up from several plates  22 A- 22 F. Each plate  22 A- 22 F has a pair of parallel, spaced-apart mating faces. The mating faces of adjacent plates  22 A- 22 F abut each other when the assembly is closed. Each plate  22 A- 22 F incorporates one or more functional components of the mold assembly  16 . The individual plates  22 A- 22 F can be coupled to each other or to static structure in any combination. Furthermore, single plates  22 A- 22 F or groups of plates  22 A- 22 F may be mounted to the actuators  20 , so as to be selectively moveable along a main axis “A”. Stated another way, groups of the plates  22 A- 22 F may be pushed together or pulled apart along the main axis A to effect opening and closing of the mold assembly  16 . It is noted that the principles of the present invention are equally applicable to other mold configurations which are not modular in nature. 
     The mold assembly  16  has a front end  24  and an rear end  26 . Movement of a component towards the front end  24  may be described as “forward” motion and movement towards the rear end  26  may be described as “aft” motion. These directional terms are used herein solely for the purpose of convenience in description and do not imply that any particular orientation of the components described is required. 
     Considering  FIG. 3  from left to right along the main axis A, the first two plates  22 A,  22 B carry a valve assembly  28  and a valve block  30 . The valve assembly  28  includes a valve nozzle  32  with a frustoconical nozzle end  34  and a cylindrical valve gate pin  36 . The valve block  30  includes a chamber  38  that receives the valve nozzle  32 . One end of the chamber  38  is closed off by a conical endwall  40  that matches the shape of the nozzle end  34 . The nozzle end  34  is spaced a short distance from the endwall  40 . An injection orifice  42  sized to receive the valve gate pin  36  extends through the center of the endwall  40 . The valve assembly  28  also includes an actuator  44  operable to selectively move the valve gate pin  36  so that it is in contact with the injection orifice  42  or retracted away from the orifice  42 . in the retracted position, a fluid flowpath is open between an interior channel  39  of the valve nozzle  32  and the injection orifice  42 . In operation the molder/extruder  14  described above supplies the pressurized fluid plastic to the interior channel  39  of the nozzle  32 . 
     The next plate  22 C carries a mold block  46  which has an internal block cavity  48 . The block cavity  48  is shaped and sized to define a portion of the outer surface contours of the part  1 . In the specific example illustrated, the left end of the block cavity  48  mates with a domed end cavity  50  in the valve block  30 . Collectively the block cavity  48  and the end cavity  50  constitute a mold structure and cooperatively define a complete mold cavity. This end cavity  50  communicates with the injection orifice  42 . 
     The next plate  22 D carries an ejector  52  which has a generally frustoconical shape with an end face  54  that mates against an aft face  56  of the mold block  46 . The end face  54  has a hole  58  formed therein which is slightly smaller in diameter than the block cavity  48 , and a conical bore  60  that communicates with the hole  58 . 
     The next plate  22 E carries a core element  62 . The core element  62  includes a cylindrical main body  64 , a conical transition section  66 , and a cylindrical core  68 . The transition section  66  is sized and shaped to mate with the conical bore  60  of the ejector  52  when the mold assembly  16  is closed. The transition section  66  is also shaped and sized to close off the aft end of the block cavity  48  when the mold assembly  16  is closed. The core  68  is shaped and sized to define the interior surface contours of the part  1 . When the mold assembly  16  is closed, the core  68  is positioned coaxially within the block cavity  48  but not touching it, the space between the two components defining a mold envelope  70  that is filled with fluid plastic during molding process. 
     Generally stated, the mold envelope  70  is bounded by an inner sidewall, an inner endwall disposed at one end of the inner sidewall, an outer sidewall spaced apart from the inner sidewall, and an outer endwall spaced apart from the inner endwall. The sidewalls each have a closed perimeter, and can take on any shape such as circular, elliptical, or polygonal. In the specific example illustrated, the outer sidewall is defined by the block cavity  48 , the inner sidewall and the inner endwall are defined by the core  68 , and the outer endwall is defined by the end cavity  50  in the valve block  30 ; however different arrangements are possible. For example, the end cavity  50  could be eliminated and the core  68  simply spaced away from the valve block  30  to define the outer and inner endwalls. 
     A central bore  72  extends through the core element  62 , and a locking pin  74  is received in the central bore  72 , mounted so that it can move back and forth parallel to the main axis A. The aft end of the locking pin  74  is coupled to an actuator  76  carried in the last plate  22 F. The actuator  76  is operable to selectively extend or retract the locking pin  74 . 
       FIG. 5  illustrates the spatial relationship of the valve block  30 , valve nozzle  32 , core  68 , and locking pin  74  in more detail. The flat end face  77  of the core  68  includes a generally frustoconical void  78  that receives the tip  80  of the locking pin  74 . The tip  80  of the locking pin  74  is chamfered and includes one or more shallow slots  82  formed around its periphery.  FIG. 5  shows the locking pin  74  in its extended position. In this position, the tip  80  is registered in the injection orifice  42  of the valve block  30 , and functions to prevent any relative lateral movement of the core  68  inside the block cavity  48 . The shallow slots  82  communicate with the interior channel  39  and the block cavity  48 , and provide a flow path for fluid plastic. 
     The molding operation will now be described in sequence. Initially, the mold assembly  16  is closed with the valve block  30  abutting the mold block  46  and defining the mold envelope  70  as shown in  FIG. 5 . The locking pin  74  is extended into the injection orifice  42  of the valve block  30 , and the valve gate pin  36  is in an open position. 
     During the actual mold “shot”, plastic in a fluid state is forced past the valve gate pin  36 , through the slots  82  in the locking pin tip  80 , and begins to flow into the mold envelope  70 . At the very beginning of this process the plastic enters the frustoconical void  78  in the core  68 , where it surrounds the locking pin  74  and begins to solidify. This has the effect of rigidly locking the relative lateral position of the locking pin  74  to the core  68 , and the combination of those two elements to the valve block  30 . This prevents lateral shifting of the core  68  relative to the block cavity  48 . 
     Subsequently, fluid plastic flows around the end of the core  68  and into the mold envelope  70 . During this process the plastic may flow asymmetrically down the elongated sides, resulting in significant lateral pressure on the core  68 . However, the stabilization provided by the locking pin  74  prevents deflection of the core  68 . Stated another way, the part wall thickness “t 1 ” on one side of the core  68  remains substantially equal to the part wall thickness “t 2 ” on the opposite side. 
     As the mold envelope  70  nears complete filling, the locking pin  74  is retracted as shown in  FIG. 6 , creating a flow path into the frustoconical void  78 . Simultaneously, the valve gate pin  36  is extended rearward, extruding a small volume of plastic into the opening left by the locking pin  74 . Once this volume of plastic solidifies, it closes off the endwall  3  of the part  1 . This entire process is rapid, for example it may take less than one second. Subsequently, the mold assembly  16  is opened, exposing the part  1 , and the part  1  is ejected from the block cavity  48  by moving the ejector  52  forward. The mold assembly  16  is then re-closed, ready to form another product. 
     The steps of the injection process may be controlled by various means. For example, a discrete electronic or mechanical timer (shown schematically at  84  in  FIG. 3 ) may be started when the mold shot is initiated by the controller  18 . The timer  84  counts a predetermined time and then signals the locking pin  74  and valve gate pin  36  to begin their movement. Alternatively, a timer could be incorporated into the software or hardware of the controller  18  itself. 
     An example of a resulting part  1  is shown in  FIGS. 1, 2, and 7 . As noted above, the part  1  is unitary or monolithic molded plastic and has a closed perimeter sidewall  2 , which can take on any shape such as circular, elliptical, or polygonal. In the specific example illustrated, the sidewall  2  is generally cylindrical. The sidewall  2  is closed off at one end by an endwall  3 . The ratio of the overall length “L” of the part  1  to the thickness “t” of the sidewall  2  is quite high, for example the ratio may be on the order of  100 . A boss  90  extends axially from an interior surface  92  surface of the endwall  3 . The boss  90  is annular, with a closed perimeter wall  94  and one open end. The outer surface  96  of the perimeter wall  94  matches the shape of the frustoconical void  78  described above. The inner surface  98  of the perimeter wall matches the chamfered shape of the tip  80  of the locking pin  74 . The shape of the boss  90  is defined by fluid plastic flowing into the space between the void  78  and the locking pin  74  during the molding process, as described above. The presence of the boss facilitates the use of the molding method described above, which results in the sidewall  2  having a highly uniform thickness around its periphery. 
     The method and apparatus described above provides a means for molding plastic parts. It is especially useful for producing elongated parts with good quality and good yields while protecting the molding equipment from damages. These principles may be applied to molding all kinds of plastic parts as well as the molding of other materials where support of a core within a mold is necessary or desirable, such as elastomers and low-melting-point metals. 
     The foregoing has described a method and apparatus for injection molding. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying potential points of novelty, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.