Abstract:
A method and apparatus for extruding plastic articles, the method comprising the steps of injecting at least one stream of plastic material into a mold, the mold including a first portion and a second portion, the first portion of said mold being used for forming at least one article, and the second portion of said article forming a sprue attached to the at least one article, and, terminating the at least one stream of plastic material in the second portion.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to an injection molding process, and in particular, a method and apparatus for reducing the size of a core layer hole in an injection molding process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Presently, many plastic articles are formed by injection molding processes. These articles include common items such as test tubes and pre-forms for forming items such as beer and ketchup bottles. Many of these articles are produced from injection molding machines having the ability to inject multiple plastic layers at the same time (i.e., co-injection). Thus, the injection-molded articles may have two or more layers of plastic in their final form (i.e., multi-layer plastic articles).  
           [0003]    A common configuration of multi-layer plastic articles includes an interior or “core” plastic layer which is surrounded on all sides by an outer plastic layer. For example, see U.S. Pat. Nos. 5,914,138 and 6,187,241, both assigned to Kortec, Inc. The disclosures of both of these patents are incorporated herein by reference. Typically, the interior (core) layer is formed of a material such as Ethyl Vinyl Alcohol (EVOH), and the outer layer is formed from a material such as Polyethylene Terephtholate (PET). This construction produces a sandwich structure wherein the outer layer (e.g., PET) forms both the exterior and the interior of the article, and the interior (core) layer (e.g., EVOH) is sandwiched therebetween.  
           [0004]    However, a common problem experienced when injection molding such articles is that a hole or gap is formed in the interior (core) layer at the base of the molded article where the interior (core) layer enters the mold. The hole is formed because the interior (core) is formed by an annular stream with a diameter that decreases towards the base of the molded article. The diameter of the annular core stream at the base of the article corresponds directly to the diameter of the hole or gap. In particular, the interior (core) layer enters the mold as an annular stream which is surrounded on both sides by inner and outer covering layers. When the flow of the interior (core) layer is stopped, a tail of the interior (core) layer continues up the sidewall of the molded part, thereby creating a hole at the base of the molded part which is typically much larger than a gate of the nozzle which injects the interior (core) and outer layers.  
           [0005]    [0005]FIG. 14 shows a conventional injection molding system  500  which includes injection molding apparatus  510  and a mold  550 . The injection molding apparatus  510  includes a nozzle  515  which has various passageways for transferring plastic materials to the mold  550 . A first series of passageways  520  are used for delivering plastic material  521  to the mold  550 . Plastic material  521  forms both an inner covering layer (IL) and an outer covering layer (OL). A second series of passageways  525  are used for delivering an interior annular layer (IA) of plastic material  526  to the mold  550 . The interior annular layer IA may be, for example, a barrier layer that prevents passage of gases into or out of the molded article. The first and second series of passageways  520 ,  525  come together at a gate portion  530  of the nozzle  515 . The nozzle gate portion  530  comprises a relatively narrow portion of the nozzle  515  which feeds directly into the mold  550 . The injection molding apparatus  510  also includes a throttle pin  535  for controlling the flow of plastic material ( 521 ,  526 ) through the nozzle gate portion  530 .  
           [0006]    Particularly, if the interior (core) layer IA is stopped too soon, the interior (core) layer IA will travel up the sidewall of the molded part, thereby creating a large hole or gap  595  at the base of the part. If the interior (core) layer IA is stopped too late, some interior (core) material IA will be left in the nozzle  530  of the injection molding apparatus  510 . This remaining material will contaminate the next molding by flowing into the next molded part and possibly ending up on an outside wall of the part. The result of this type of contamination is often referred to as ‘scale.’ Scale can occur inside and/or outside of the molded part.  
           [0007]    Controlling the size of the hole or gap created by the annular interior (core) streams is fundamental in present day injection molding systems. If this gap is too large, the barrier properties of the molded part will be significantly reduced. In other words, a vacuum created within a substance-containing portion of the part cannot be maintained for a long period of time because exterior gases will enter the part through the hole, or conversely pressure can not be maintained in the part because gases within the molded part will seep out through the hole.  
           [0008]    Thus, there is presently a need for a method and apparatus for injection molding articles where the size of a gap or hole in the interior plastic layer is efficiently controlled.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is a method and apparatus for extruding plastic articles, the method comprising the steps of injecting at least one stream of plastic material into a mold, the mold including a first portion and a second portion, the first portion of said mold being used for forming at least one article, and the second portion of said mold forming a sprue attached to the at least one article, and, terminating the at least one stream of plastic material in the second portion.  
           [0010]    The above and other advantages and features of the present invention will be better understood from the following detailed description of the exemplary embodiments of the invention which is provided in connection with the accompanying drawings. 
       
    
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a cross sectional view of an injection molding system according to a first exemplary embodiment of the present invention during a first stage of a fill sequence.  
         [0012]    [0012]FIG. 2 is a cross sectional view of the injection molding system of FIG. 1 during a second stage of a fill sequence.  
         [0013]    [0013]FIG. 3 is a cross sectional view of the injection molding system of FIG. 1 during a third stage of a fill sequence.  
         [0014]    [0014]FIG. 4 is an isometric view of a molded article formed using the injection molding system according to the first exemplary embodiment of the present invention.  
         [0015]    [0015]FIG. 5 is an isometric view of the molded article of FIG. 4 with the sprue portion removed.  
         [0016]    [0016]FIG. 6 is a cross sectional view of the injection molding system according to a first exemplary embodiment of the present invention during a first stage of an ejection sequence.  
         [0017]    [0017]FIG. 7 is a cross sectional view of the injection molding system of FIG. 6 during a second stage of an ejection sequence.  
         [0018]    [0018]FIG. 8 is a cross sectional view of the injection molding system of FIG. 6 during a third stage of an ejection sequence.  
         [0019]    [0019]FIG. 9 is a cross sectional view of the injection molding system of FIG. 6 during a fourth stage of an ejection sequence.  
         [0020]    [0020]FIG. 10 is a cross sectional view of an injection molding system according to a second exemplary embodiment of the present invention during a first stage of a fill sequence.  
         [0021]    [0021]FIG. 11 is a cross sectional view of the injection molding system of FIG. 10 during a second stage of a fill sequence.  
         [0022]    [0022]FIG. 12 is a cross sectional view of the injection molding system of FIG. 10 during a third stage of a fill sequence.  
         [0023]    [0023]FIG. 13 is an isometric view of a molded article formed using the injection molding system of the second exemplary embodiment.  
         [0024]    [0024]FIG. 14 is a cross sectional view of a conventional injection molding system. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1 shows a cross section of an injection molding system  100  according to a first exemplary embodiment of the present invention, including an injection molding apparatus  110  and a mold  150 . The injection molding apparatus  110  includes a nozzle  115  which has various passageways for transferring plastic materials to the mold  150 . A first series of passageways  120  are used for delivering plastic material  121  to the mold  150 . Plastic material  121  forms both an inner covering layer (IL) and an outer covering layer (OL) (See FIG. 2). A second series of passageways  125  are used for delivering an interior annular layer (IA) of plastic material  126  to the mold  150 . The interior annular layer IA may be, for example, a barrier layer that prevents passage of gases into or out of the molded article. The first and second series of passageways  120 ,  125  come together at a gate portion  130  of the nozzle  115 . The nozzle gate portion  130  comprises a relatively narrow portion of the nozzle  115  which feeds directly into the mold  150 . The injection molding apparatus  110  also includes a throttle pin  135  for controlling the flow of plastic material ( 121 ,  126 ) through the nozzle gate portion  130 .  
         [0026]    The mold  150  includes a mold cavity  155  with a first portion  160  and a second portion  165 . As will be understood from the foregoing description, the first portion  160  of the mold cavity  155  comprises a sprue portion, and the second portion  165  comprises a molded article portion. Disposed between the injection mold apparatus  110  and the mold  150 , there is an ejector member  140  used for separating a sprue  220  (See FIG. 5) formed by the first portion  160  of the mold  150  from the injection molding apparatus  110  (See FIGS. 8 and 9). The sprue portion  160  includes a sprue gate  161  at an end thereof which serves as an injection point for plastic material ( 121 ,  126 ) into the mold cavity  155 .  
         [0027]    The sprue portion  160  of the mold cavity  155  contains the sprue  220 , which forms no part of a final molded article  210  formed by the mold  150  (See FIG. 5). The sprue portion  220  is merely an additional portion which is used to assist in controlling the flow of plastic materials  121 ,  126  into the mold  150 , but which may be discarded after the molded article  210  has been produced (as explained below).  
         [0028]    [0028]FIG. 1 shows a first exemplary embodiment of an injection molding system  100  according to the present invention during a first stage of a fill process for filling the mold  150  with plastic material ( 121 ,  126 ). As shown in FIG. 1, a first plastic material  121 , such as PET or Polypropylene (PP), which forms the inner and outer covering layers IL, OL of the molded article travels through passageways  120  of the nozzle  115 , passes through the gate portion  130  of the nozzle, and is passed to the mold  150 . It will be noted that the passageways  120  of the injection molding system  100  are annular, thereby creating annular streams of the first material  121 . The advantages of using annular flow are explained in U.S. Pat. No. 6,187,241 referenced above. These annular streams come together at the nozzle gate portion  130  to form a circular stream, until they are injected into the molded article portion  165  of the mold  150  (through sprue gate  161 ) where the streams again flow in an annular fashion. In the exemplary embodiment shown in FIG. 1, the flow of the material  121  is initiated before the flow of the material  126 . The material  121  forming the layers OL and IL flows through the sprue portion  160  of the mold cavity  155 , and into the molded article portion  165 . A flow front  180  of the material  121  is shown in FIG. 1.  
         [0029]    [0029]FIG. 2 shows the injection molding system  100  of FIG. 1 during a second stage of the fill process. In the second stage, the flow of IA material  126 , such as EVOH has been initiated. The IA material  126  flows from the passageways  125  of the nozzle  115 , through the nozzle gate portion  130 , through the sprue portion  160  of the mold cavity  155 , and into the molded article portion  165  of the mold cavity. It will be noted that passageways  125  of the injection molding system  100  are annular, thereby creating an annular stream of the IA material  126 . This annular stream remains annular through the nozzle gate  130  and into the mold  150 . A leading edge  185  of the IA material  126  is shown in FIG. 2.  
         [0030]    The IA material  126  flows through the approximate center of the material  121  already flowing in the mold  150 , thereby creating an IL stream and an OL stream from the single stream of plastic material  121 . In the exemplary embodiment of the present invention, the ratio of the IL stream to the OL stream is 50:50, however, it will be understood by those skilled in the art that this ratio can be varied (e.g., 25:75, 75:25, etc.).  
         [0031]    [0031]FIG. 3 shows the injection molding system  100  of the present invention during a third (and final) stage of the fill process. In the third stage, the flow of material  126  is terminated, such that a trailing edge  190  of the core material is at least partially disposed within the sprue portion  160  of the mold cavity  155 . It will be noted that, due to the annular nature of the IA stream, a gap  195  is created in the interior layer at the base of the molded article. The IL, OL stream is initiated before the initiation of the IA stream, and is terminated after the termination of the IA stream. The IL, OL layer must continue flowing until the IA layer is pushed out of the nozzle, thereby cleaning the nozzle of the IA stream.  
         [0032]    This feature of terminating the interior (core) material  126  within the sprue portion  160  overcomes a problem of the prior art of trying to precisely control termination of the core material. Because the termination occurs before the beginning of the molded article  210  in the present invention, there is little or no risk of the hole  195  in the core layer being too large. Further, because the core material termination is clearly beyond the nozzle gate  130  of the nozzle  115 , there is little or no risk of contamination and scaling in the next molded article produced. The sprue portion  160  of the mold  150  provides a long zone of tolerance within which the core material can be terminated without impacting the quality of the finished part, or contamination of the nozzle.  
         [0033]    As seen in FIG. 3, a gap or hole  195  in the interior annular layer IA exists near the base of the molded article (i.e., the upper portion of the mold  150 ) which is approximately equal to the inner diameter of the annular stream of IA material  126 . Due to the configuration of the sprue portion  160  of the mold cavity  155 , and due in part to the ratio of IL to OL, this gap  195  has a diameter which is approximately 50% of the diameter of the sprue gate  161  (as opposed to conventional injection molding apparatus where this gap  595  is much larger than the diameter of the gate  530 ; See FIG. 14) in the first exemplary embodiment. Moreover, the configuration of the sprue portion  160  of the mold cavity  155  allows the size of the gap  195  to be controlled reliably. Preferably, the sprue portion  160  of the mold cavity  155  is shaped in such a way that the gap  195  is in a range approximately 40-60% of the diameter of the sprue gate  161  in the exemplary embodiment. By decreasing the size of this gap  195 , the barrier properties of the molded article are comparatively increased. In other words, the molded article will be able to retain gases stored therein for longer periods, and will be able to prevent the entry of exterior gases for longer periods. In the foregoing description, reference will be made to the “protected” and “unprotected” portions of the molded part. The “unprotected” portion comprises that portion of the molded part which fails to include an interior annular layer IA (i.e., the portion of the part where the gap  195  exists). The “protected” portion comprises that portion of the molded part which includes an interior annular layer IA (i.e., the remainder of the part).  
         [0034]    For a typical tube shaped part with a length L of approximately 75 mm and a diameter D of approximately 12 mm, the total surface area SA of the tube shaped part may be expressed as follows: 
           SA (part)= SA (tube)+ SA (spherical end) 
           SA (tube)=( L−D/ 2)*(π)* D= 2601 mm 2   
           SA (spherical end)=( D   2 /4)*(π)=113 mm 2   
         Thus,  SA (part )=2714 mm 2   
         [0035]    Without running the interior annular layer IA through the sprue portion  160  and through sprue gate  161 , the unprotected gap diameter might vary between 2.0-5.0 mm. Additionally, the surface area of the gap may be expressed as: 
           SA (gap)=(π)* D   2 /4 
         [0036]    Accordingly, a gap with a diameter in the above range will have a surface area between 3.14 mm 2  and 19.6 mm 2 . Then, the ratio of unprotected area to protected area is in the range from 1:864 to 1:138. At this ratio, the gap could play a significant role in determining the total barrier performance of the part.  
         [0037]    Running the interior annular layer IA through the sprue portion  160  and through the sprue gate  161  will create a significantly smaller gap  195 . For a typical sprue gate  161  diameter of 0.8 mm, a ratio of IL to OL of 50:50, and a typical polymer material, the gap diameter will be about  0 . 4 mm. The area of the hole in the protective interior annular layer IA will be: 
           SA (gap)= D   2 /4*(π) 0.126 mm 2   
         [0038]    When using the sprue portion  160 , the ratio of unprotected area to protected area is about 1:20,000. This ratio shows that the unprotected gap  195  will have a negligible effect on the total barrier properties of the part. The gap surface area has also been significantly reduced by a factor of between 23.1 and 145.  
         [0039]    The ratio of the thickness of the IL, OL layers also has an effect on the size of the gap  195  in the interior annular layer IA. In the exemplary embodiment described above, the ratio is 50:50 (i.e., the IL and OL layers are divided evenly on each side of the IA layer). However, if the ratio of IL to OL were about 25:75, then a gap  195  which is in a range of approximately 25-50% of the diameter of the sprue gate  161  is achievable. Moreover, if the ratio of IL to OL were about 75:25, then a gap  195  which is in a range of approximately 50-75% of the diameter of the sprue gate  161  is achievable. The actual gap diameter can be calculated by those skilled in the art based on the flow properties of the particular materials and the ratio of flow rates of IL to OL.  
         [0040]    The above-described injection molding apparatus  100  forms a molded article  200 , as shown in FIG. 4. The molded article  200  comprises an article portion  210 , and a sprue portion  220 . Although the molded article  210  may be of any desired shape (depending upon the shape of the mold  150 ), the molded article shown in FIG. 4 comprises a test tube for retaining blood with a first end  211  (which is typically open to receive blood), and a second end  212  (which is typically closed).  
         [0041]    A thin gate member  221  attaches the sprue portion  220  of the molded article  200  to the article portion  210 . The gate member  221  is coupled to a first shaft  222  of the sprue portion  220 , which is in turn connected to a disk  223  of the sprue portion. The disk  223  of the sprue portion  220  is connected to a second shaft  224  of the sprue portion.  
         [0042]    The gate member  221  is coupled to the article  210  and the sprue  220  in such a way that, if sufficient force is exerted on the article in a direction away from the sprue, the article will separate from the sprue, as shown in FIG. 5. This separation of the article  210  and the sprue  220  provides an article which is ready for use, and a sprue part which may be discarded.  
         [0043]    FIGS.  6 - 9  show an exemplary ejection process for removing the molded article  200  from the mold  150  once the article has been molded. FIG. 6 shows a first step in the ejection process where the throttle pin  135  is moved towards the gate portion  130  of the injection molding apparatus  110 . In FIG. 7, once the throttle pin  135  completely occupies the gate portion  130 , thus cutting off any residual plastic in the nozzle  115  from the mold  150 . A mandrel portion  166  of the mold  150  is moved way from the injection molding apparatus  110  in the direction indicated by the arrows. The movement of the mandrel portion  166  of the mold  150  away from the injection molding apparatus  110  creates sufficient force to break the thin member  221  which connects the article  210  (carried on the molded article portion of the mold) to the sprue  220 .  
         [0044]    [0044]FIG. 8 shows a third step in the ejection process where the sprue portion  160  of the mold  150  is moved away from the injection molding apparatus  110 . This action leaves the sprue  220  attached to the injection molding apparatus  110 . In FIG. 9, to remove the sprue  220  from the injection molding apparatus  110 , an ejector member  140  is moved away from the injection molding apparatus, thereby creating sufficient force to remove the sprue.  
         [0045]    Referring to FIG. 10, there is shown a cross section of an injection molding system  300  according to a second exemplary embodiment of the present invention. The injection molding system includes an injection molding apparatus  310  and a mold  350 . The injection molding apparatus  310  includes a nozzle  315  which has various passageways for transferring plastic materials to the mold  350 . A first series of passageways  320  are used for delivering an inner covering layer (IL) and an outer covering layer (OL) of plastic material  321  to the mold  350  (See FIG. 11). A second series of passageways  325  are used for delivering an interior annular layer (IA) of plastic material  326  to the mold  350 . The first and second series of passageways  320 ,  325  come together at a gate portion  330  of the nozzle  315 . The gate portion  330  comprises a relatively narrow portion of the nozzle  315  which feeds directly into the mold  350 . The injection molding apparatus  310  also includes a throttle pin  335  for controlling the flow of plastic material ( 321 ,  326 ) through the gate portion  330  of the nozzle  315 .  
         [0046]    In the second exemplary embodiment, the gate portion  330  forms both a ‘nozzle’ gate and a ‘sprue’ gate (i.e., there is no separate sprue portion of the mold as in the first exemplary embodiment).  
         [0047]    [0047]FIG. 10 shows the injection molding system  300  during a first stage of a fill process for filling the mold  350  with plastic material ( 321 ,  326 ). As shown in FIG. 10, a first plastic material  321 , such as PET or PP, which forms the inner and outer covering layers IL, OL) of the molded article travels through passageways  320  of the nozzle  315 , passes through the gate portion  330  of the nozzle, and is passed to the mold  350 . It will be noted that the passageways  320  of the injection molding system  100  are annular, thereby creating annular streams of the first material  321 . These annular streams come together at the gate portion  330  to form a single non-annular stream, until they are injected into the molded article portion  365  of the mold  350  where the streams again flow in an annular fashion. A flow front  380  of the material  321  is shown in FIG. 10.  
         [0048]    [0048]FIG. 11 shows the injection molding system  300  during a second stage of the fill process. In the second stage, the flow of IA material  326 , such as EVOH has been initiated. The IA material  326  flows from the passageways  325  of the nozzle  315 , through the gate portion  330 , and into the mold  350 . It will be noted that passageways  325  of the injection molding system  300  are annular, thereby creating an annular stream of the IA material  326 . This annular stream remains annular through the gate  330  and into the mold  350 . A leading edge  385  of the IA material  326  is shown in FIG. 11.  
         [0049]    [0049]FIG. 12 shows the injection molding system  300  of the present invention during a third (and final) stage of the fill process. In the third stage, the flow of IA material  326  is terminated. A trailing edge  390  of the IA material  326  is shown in FIG. 12.  
         [0050]    As will be seen in FIG. 12, a gap or hole  395  in the interior annular layer IA exists near the base of the molded article (i.e., the upper portion of the mold  350 ) which is approximately equal to the inner diameter of the annular stream of IA material  326 .  
         [0051]    Due to the long and narrow structure of the gate portion  330 , this gap  395  has a diameter which is approximately 50% of the diameter of the gate  330  (as opposed to conventional injection molding apparatus  500  where this gap  595  is much larger than the diameter of the gate  530 ; See FIG. 14) in the exemplary embodiment. Preferably, the gate portion  330  is formed in such a way that the gap  395  is approximately 40-60% of the diameter of the gate  330  in the exemplary embodiment. By decreasing the size of this gap  395 , the barrier properties of the molded article are comparatively increased. In other words, the molded article will be able to retain gases stored therein for longer periods, and will be able to prevent the entry of exterior gases for longer periods.  
         [0052]    As discussed above with reference to the first exemplary embodiment, the ratio of IL, OL layers also has an effect on the size of the gap  395  in the interior annular layer IA. In the exemplary embodiment described above, the ratio is 50:50 (i.e., the IL and OL layers are divided evenly on each side of the IA layer). However, if the ratio of IL to OL were about 25:75, then a gap  395  which is in a range of approximately 25-50% of the diameter of the gate is achievable. Moreover, if the ratio of IL to OL were about 75:25, then a gap  395  which is in a range of approximately 50-75% of the diameter of the gate is achievable. The above equations dictate the relationship between the IL:OL ratio and the gap size.  
         [0053]    [0053]FIG. 13 shows a molded article  400  formed using the injection molding system  300  according to the second exemplary embodiment of the present invention. The molded article  400  comprises an article portion  410 , and an unwanted portion  420 . When the unwanted portion  420  is removed from the article portion  410  (by cleaving or some equivalent process), the molded article appears much as the article  210  in FIG. 4.  
         [0054]    Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.