Abstract:
A method of extrusion blow molding is disclosed; the method allows the finished part to have consistent and even wall thickness within a broad range of local mold radii. This result is achieved without resorting to injection blow molding or other common techniques of deforming the parison.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/311,368 filed Aug. 13, 2001. 

   FIELD OF THE INVENTION 
   This invention relates to blow molding and more particularly to blow molding parts whose radius varies substantially. Additionally, this invention relates specifically to blow molded parts with wide open ends. 
   BACKGROUND OF THE INVENTION 
   Blow molding is a commonly used technique to produce a variety of thin walled containers. Unlike injection molding, blow molding does not rely on very high pressure to form the finished part. Instead, a parison is provided in a mold and inflated or blown. Air between the parison and the mold escapes the mold through venting channels. Since the pressure used is low in comparison to injection molding, the mold is relatively inexpensive. This procedure is used to make a wide variety of plastic containers such as plastic bottles and pails. 
   A variety of prior art exists that describes different ways of producing different blow molded shapes. For example, in U.S. Pat. No. 4,724,116 to Aoki et al., a parison having a varying thickness is used to produce a blow-molded bottle. The bottle has a substantially elliptical cross section. Thus, the parison used in this process features thin walls in the region where more expansion is desired and thicker walls where less expansion is desired. 
   A person of skill in the art of blow molding, after having reviewed Aoki et al. might conclude that a substantially conical shape would be produced by varying the thickness of the parison as a function of parison length. Although this will work to an extent, it is flawed in that the thin section of the parison is most easily expanded, and consequently, if the thin section is too thin relative to the thick section the parison will fail by rupturing prior to taking the shape of the blow mold. This failure represents a waste of time and material. 
   Alternatively, a parison with consistent thick walls is used. Although this technique will work to an extent, it results in a container having walls with different thickness. Ideally, a less wasteful container will have uniformly thin walls regardless of the shape of the container. This saves material and consequently reduces operating costs. 
   Additionally, many shapes with a substantial variation in radius have at least one open end as well. Typically, producing such a shape involves blowing a sealed container shape and cutting away the openings. As the area of the opening is quite large, material that is cut away represents a substantial portion of the original parison. 
   It would be advantageous to provide a new technique of extrusion blow molding that produces even wall thickness over areas of differing radii while keeping the costs of the parts and the tools to a minimum. Additionally, it would be beneficial if such a technique supported shapes with a large opening without the operations associated with cutting away material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the drawings in which: 
       FIG. 1  is a simplified section view of an open mold with an extruded parison therein according to the prior art; 
       FIG. 1   a  is a simplified section view of a closed mold with a partially inflated parison according to the prior art; 
       FIG. 1   b  is a simplified section view of a closed mold with a finished molded parison according to the prior art; 
       FIG. 2  is a simplified section view of a closed conical mold in which a parison with a thin wall has failed to inflate properly according to the prior art; 
       FIG. 3  is a simplified section view of a prior art blow mold with a parison whose interior cavity shape is not circular; 
       FIG. 4  is a simplified section view of a prior art injection mold for producing a hollow conical parison; 
       FIG. 4   a  is a simplified section view of a prior art blow mold with a conical parison whose walls are not equally thick for producing a substantially conical shape; 
       FIG. 5  is a simplified section view of a prior art conical blow mold with a retractable extrusion head and a parison with varying wall thickness; 
       FIG. 6  is a simplified section view of a prior art conical blow mold in which a parison of varying wall thickness is being inflated wherein the thin wall of the parison is the first to be stretched; 
       FIG. 7  is simplified section view of a blow mold and parison whose shape is based upon an embodiment of the invention; 
       FIG. 8  is a simplified section view of a blow mold with a parison shown at varying stages of inflation, which are consistent with an embodiment of the invention; 
       FIG. 9  is a simplified section view of a blow mold in which the outside wall of the parison has formed a seal with the inside surface of the mold whose shaped is based upon an embodiment of the invention, and; 
       FIG. 10  is a section view of a blow mold with post extending upwards from the bottom surface of the mold. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a parison  11  is extruded into an open mold  12 . Referring to  FIG. 1   a , the mold  12  is then closed, pinching off one end of the parison  11 . The pinching process also results in the fixing of the pinched portion  15  at the pinching location. A nozzle  14  injects a fluid; usually air, into the cavity  10  of the parison. The parison  11  expands as the cavity  10  within it expands, creating a large hollow shape. Referring to  FIG. 1   b , the shape is eventually constrained when the parison reaches the interior surface of a mold  12 . Typically, when the parison  11  makes contact with the mold  12 , it is cooled and thus hardened. When the interior of the mold  12  has been covered by the parison  11  and the parison  11  is cool enough, the mold may be opened and the finished part removed. 
   This process is well suited to creating cylindrical shapes with thin walls, such as bottles. When this process is used to create a shape with an open end, such as a pail, it is often necessary to cut away a substantial portion of the material in order to provide the pail with a wide mouth. Unfortunately, cutting away this portion results in a large amount of wasted material. Clearly, the amount of material wasted depends upon the shape of the product and the material properties of the parison however it is not unknown have a finished product that is less than half the mass of the extruded parison. Alternatively, it is known to produce two pails from a single mold such that the two pails are joined at their wide openings. Once removed from the mold cutting away material that connects to the two openings separates the pails. Unfortunately, it is very difficult to achieve a consistent wall thickness between the two pails created this way. Since the parison is not solid when it is extruded it tends to flow downwards causing the bottom pail to receive more material than the top pail. Consequently, the pails are not identical. Clearly, this problem is dependent upon a number of factors; however, generally as the parts become larger producing them with consistent wall thickness becomes more difficult. 
   It is well known that creating shapes whose radius varies, such as a cone, causes the thickness of the walls of the finished part to vary. As the radius increases the wall thickness of the finished part decreases. Referring to  FIG. 2 , in an extreme case, the cavity  10  within the parison  11  breaks through the wall of the parison and causes the parison to stop expanding. As the wall of the part being molded becomes thinner it is more prone to rupturing resulting in a failure of the molding process. Optionally, a parison with more material and hence generally thicker walls is provided, however this is wasteful and more costly. 
   In U.S. Pat. No. 4,724,116, filed Apr. 1, 1983, Aoki demonstrates a method of creating a blow molded elliptical cross section bottle from an extruded parison. Referring to  FIG. 3 , the parison  1   b  used in this technique has a cross section which is round on the outside but whose interior shape is not round. This allows the parison to expand substantially more in one direction than another allowing the elliptical bottle to be formed. While this technique is effective, it can only be used properly to produce a part of mainly constant cross section. Therefore, this technique cannot be used to effectively and efficiently produce a shape whose cross section varies along the extrusion axis, such as a cone. 
   Alternatively, injection blow molding is used to produce shapes that are not cylindrical. Referring to  FIG. 4 , an injection blow molding primary mold  40  is shown. The primary mold  40  has a parison  41  injected into it. Since the parison  41  is injected and not extruded the parison  41  is not limited to extruded shapes. Thus, this technique involves molding a parison  41  in a specific shape with predetermined wall thickness. Referring to  FIG. 4   a , the parison  41  is then blown in a second mold  42 . This process permits a wide variety of blow-molded shapes to be produced, including shapes whose radius varies substantially. Unfortunately, this technique is significantly more costly than conventional blow molding. It uses two molds, instead of one and injecting the parison  41  involves high-pressure equipment. 
   Additionally, it is known to use a nozzle to vary the thickness of the parison. This technique works to a limited extent. In this case, the nozzle controls the thickness of the parison walls as the parison is extruded. Until the parison reaches the inner surface of the mold it tends to expand most in the thinnest section. Consequently, the areas of the parison with the most material are best suited to resisting the deformation caused by blowing. Ideally, the behaviour of the parison would be opposite. Referring to  FIG. 5 , a nozzle  55  is used to control the thickness of the parison  51  as it is extruded. If the parison must fill a large radius in a first region and a small radius in a second region then the thin section  50  of the extruded parison  51  is matched to the small radius  50  and the thick section  52  of the parison is matched to the larger interior radius  53  of the mold. 
   Referring to  FIG. 6 , once the process of inflating the parison  51  begins the thin section  50  of the parison  51  deforms while the thicker section  52  resists deformation. It is advisable that care be taken to ensure that the parison  51  does not rupture as it is being blown. Clearly, this technique is preferably used with relatively small changes in radius and is other than preferably useable if relatively large changes in radius along the finished part are required. 
   Referring to  FIG. 7 , in a first embodiment of the invention, the parison  71  enters the mold  72  according to a conventional extrusion blow molding process. Unlike the prior art molds described previously, this mold has a bottom surface  77  positioned relative to the extrusion head (not shown) that provides the parison  71 . 
   Referring to  FIG. 8 , when the mold closes, it pinches the parison  71  creating a fixed closed end  85 . At the opposite end of the mold a nozzle  84  is used to inject air into the parison  71 . The parison  71  contacts the mold  72  sealing against it in a ring contact. Unlike the prior art, the sealing of the parison at the nozzle end is achieved by sealing the outside surface of the parison  71  against the inside surface of the mold  72 . As the parison  71  is blown, the free moving end  83  slides within the mold supporting an overall increased wall thickness of the parison  71 . To further assist the blowing process, vents  86  in the mold ensure that any gasses in the mold escape as the parison  71  is inflated. The bottom surface  77  of the mold  72  is maintained at a higher temperature than the sidewalls  78  of the mold  72  to reduce the likelihood of the parison from bonding to the bottom surface  77 . 
   Of course, it is essential that at least a partial seal be formed between the parison and sidewall of the mold to ensure that the gas entering through nozzle  84  does not escape though the vents  86  without substantially expanding the parison. 
   As shown in  FIG. 8 , an end of the parison  71  moves relative to the bottom of the mold. Initially, the parison  71  forms a seal proximate a bottom of the mold. As gas is introduced into the parison  71 , it expands in circumference and in an intermediary state of expansion. As is evident in the figure, the bottom  83  of the parison  71  has moved but still forms a seal with the bottom surface  77  of the mold. As more gas is introduced, the parison moves to a more molded position where the end of the bottom  83  of the parison  71  is now sliding along an edge of the mold as its circumference expands. Finally, with a little more expansion the part is fully molded. This technique is highly advantageous for making parts with a large opening at one end because it reduces the need for cutting away unwanted material. 
   Optionally, another embodiment of the invention is created by combining an apparatus consistent with the first embodiment of the invention and extruding head with a nozzle that permits varying the thickness of the extruded parison. Varying the thickness of the parison and using the technique of sliding along a sealing surface permit the production of a finished shape with large variation in radius while maintaining the advantage of providing an open end and therefore conserving parison material. 
   Referring again to  FIG. 1   a , producing such a part involves adding additional material in the parison to ensure that it fills the mold without breaching. Further, much of this additional material is removed when the bottom end is opened to form the finished product—a pail. In contrast to the prior art, the finished pail is molded with an open end thereby eliminating the need to either cut out an open end or separate two pails that are made in a same mold and separated after the molding process. Thus, fewer operations are needed to make the pail thereby reducing costs. 
   When the part has cooled it is removed from the mold. Finishing of the part is optionally performed at this stage. The present invention allows for molding of an open-ended part with only a small excess of material to be removed. Further, since one end of the parison is free to slide within the mold, the resulting molded part has more uniformity in sidewall thickness. This allows the mold operator to use a minimum of material without compromising the strength of the finished product. 
   In another embodiment, shown in  FIG. 9 , the tooling for the blow molding process is simplified by tapering the surface that the parison  91  seals against. This is achieved by decreasing the radius of the mold to an amount that equal to the outside radius of the parison  91  prior to inflation. Thus, the parison  91  slides along this surface  98  as it is extruded. In this embodiment, the opposite end of the parison  91  is pinched, forming a pinched end  95 . When blowing occurs the parison  91  expands against this surface, improving a quality of the seal already formed. As the parison  91  passes the minimum interior radius of the part it expands radially and therefore it maintains the seal between it and the mold. In prior art systems, the parison is pinched—sealed and fixed—at opposing ends of the mold, however, using the present inventive technique, it is not required that both ends of the parison  91  be pinched. Consequently, it is now possible to vary the length of the extruded parison to adjust the thickness of the walls of the finished part. Clearly, it is advantageous that the parison  91  does not bond rigidly to the surface  98 , but instead, be sealed against it and slide along it. To achieve this result, the bottom portion of the mold, henceforth referred to as the “bucket”, is kept at a temperature sufficient for reducing the likelihood of bonding between it and the parison  91 . 
   Preferably, the mold is designed to prevent the parison from losing its seal with the mold in order to improve a reliability of the mold for use with the present process. 
   Referring to  FIG. 10 , in yet another embodiment of the invention, a mold  105  is illustrated featuring an extrusion head  101 , pinching plates  102 , an extruded parison  103 , a sealing surface  104 , a post  106 , and a sealing block  107 . The mold  105  is shown in the closed position. The extrusion head  101  provides the molten parison  103  that slides over the post  106 . The parison  103  forms a seal with the mold  105  at the sealing surface  104 . When the extrusion head  101  has provided a sufficient amount of material, the pinching plates  102  secure the parison  103  and form a seal between the parison  103  and the sealing block  107 . The parison  103  is blown forming a shape according to the interior surface of the mold  105 . Once again, since the parison  103  is permitted to slide along the bottom surface of the mold  105  this technique supports a variety of shapes having varying radii. This embodiment is highly beneficial because it allows the mold operator to produce parts with two open ends, such as a funnel. Allowing both ends to be open further reduces the amount of wasted material and effort associated with trimming and disposing of the wasted material. Although the sealing block  107  is shown attached to a post  106  that extends from the bottom surface of the mold this need not be the case. In an alternative embodiment, the sealing block  107  is attached to the extrusion head  101 . 
   In an alternative embodiment of the invention when the parison  103  has formed a seal with the sealing surface  104 , the operation of blowing the parison  103  is initiated. Thus, the parison  103  is inflated sooner and the mold is capable of producing parts at a faster rate than would be anticipated by a similar machine according to the prior art in which the mold provides the sealing surface when the mold closes. 
   In yet another embodiment of the invention, the blowing of the parison begins prior to pinching off the end with the pinching plates  102 . Thus, the parison  103  is being blown before the step of extruding the parison  103  is finished. This is also highly advantageous as it further reduces cycle time for the mold  105 . As will be appreciated by a person of skill in the art of blow molding, the inflation of the parison prior to closing the mold is limited. 
   Numerous other embodiments of the invention may be envisioned without departing from the spirit or scope of the invention.