Patent Publication Number: US-2015061179-A1

Title: Ceramic die pin for molten plastic extrusion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/872,332, filed Aug. 30, 2013, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The disclosed subject matter is related to apparatus and methods for the extrusion of molten plastic material, and particularly to die pins for the same. 
     BACKGROUND OF THE INVENTION 
     Molten thermoplastic extrusion is widely employed for the manufacture of plastic articles, including plastic containers and the like. Generally, plastic polymer pellets are melted within a heated extrusion apparatus under pressure and extruded through an outlet. In certain circumstances, the molten plastic or extrudate is extruded in the form of a tube or other hollow member, such as for blow molding techniques. For example, molten plastic is extruded from the extrusion apparatus in an annular space defined between an outlet and a tooling or die pin disposed within the outlet to shape the molten plastic into a hollow parison. 
     Die pins for extrusion of molten plastic for container blow molding are conventionally made of tool steel. However, steel die pins are susceptible to the accumulation of degraded molten plastic during repeated cycles of plastic extrusion. In rare instances, on the order of one in ten million, the accumulated degraded molten thermoplastic material may be ejected into a parison, resulting in a solid plastic contaminant. A representative contaminant associated with prior art die pins is shown in  FIG. 1A , and a comparative infrared spectroscopy analysis of the contaminant and of a finished container is shown in  FIG. 1B . The contaminant has an irregular appearance with a brown and black color, and spectroscopic and melting point analyses confirm the contaminant is composed of degraded thermoplastic material of the same polymer composition as the finished container. 
     To prevent container contamination associated with prior art die pins, it may be necessary to undertake laborious and expensive measures, including periodic phase resets to dislodge the contaminants and line shutdown to permit cleaning of the tooling. However, such efforts are time-consuming and costly, and further require line shutdown, resulting in reduced output. Line shutdown is also associated with increased energy consumption, as the molten plastic must be reheated upon restart. 
     Accordingly, there is a need for a method and system capable of minimizing the undesirable accumulation of thermoplastic material in extrusion equipment. 
     SUMMARY OF INVENTION 
     The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings. 
     In one aspect of the disclosed subject matter, a die pin for extrusion of molten plastic is provided. The die pin comprises a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core pin. The ceramic head member has a mounting cavity therein, and the core member has a dimension less than the corresponding dimension of the mounting cavity to allow thermal expansion of the core member therein. The ceramic head member can be made of a ceramic selected from alumina ceramic or zirconia ceramic, and particularly high purity aluminum oxide or ytrria stabilized zirconium oxide. 
     In accordance with another aspect, an apparatus for extrusion of molten plastic is provided comprising a die ring having an inner surface defining an extrusion opening with a central axis, and a die pin disposed within the extrusion opening and aligned with the central axis, the die pin comprising a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member, wherein an annular space is defined between an outer surface of the ceramic head member and the inner surface of the die ring. Additionally, a method for extrusion of molten plastic is provided comprising providing an extrusion apparatus comprising a die ring having an inner surface defining an extrusion opening with a central axis, and a die pin disposed within the extrusion opening and aligned with the central axis, the die pin comprising a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member, wherein an annular space is defined between an outer surface of the ceramic head member and the inner surface of the die ring. The method includes directing molten plastic through the annular space between the die ring and the die pin without accumulation of the molten plastic on the die pin. 
     It is to be understood that both the foregoing description and the following detailed description are exemplary and are intended to provide further explanation of the disclosed subject matter claimed. 
     The accompanying drawings, which are incorporated and constitute part of this specification, are included to illustrate and provide a further understanding of the systems of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter. The exemplified embodiments of the disclosed subject matter are not intended to limit the scope of the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosed subject matter will now be described in conjunction with the accompanying drawings in which: 
         FIG. 1A  is a photograph of a degraded plastic contaminant associated with prior art die pins, and  FIG. 1B  is an infrared spectroscopy analysis of the degraded plastic contaminant shown in  FIG. 1A  and a reference infrared spectroscopy analysis of the plastic material of a corresponding container. 
         FIG. 2A  is a side view of an exemplary embodiment of a pin base member for a die pin according to the present disclosure, and  FIG. 2B  is a side view of a representative embodiment of a ceramic head member for use with the pin base member of  FIG. 2A . 
         FIG. 3  is a photograph of one embodiment of a ceramic die pin made from the pin base member depicted in  FIG. 2A  and the ceramic head member depicted in  FIG. 2B  according to the present disclosure. 
         FIG. 4  is a schematic illustration of an extrusion blow molding apparatus suitable for use with the die pins of the disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the various aspects of the disclosed subject matter. The method of the disclosed subject matter will be described in conjunction with the detailed description of the system, the figures and examples provided herein. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which the disclosed subject matter belongs. Although methods and materials similar or equivalent to those described herein can be used in its practice, suitable methods and materials are described below. 
     It is to be noted that the term “a” entity or “an” entity refers to one or more of that entity. As such, the terms “a”, “an”, “one or more”, and “at least one” can be used interchangeably herein. The terms “comprising,” “including,” and “having” can also be used interchangeably. In addition, the terms “amount” and “level” are also interchangeable and can be used to describe a concentration or a specific quantity. Furthermore, the term “selected from the group consisting of” refers to one or more members of the group in the list that follows, including mixtures (i.e. combinations) of two or more members. 
     The term “about” or “approximately” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, based upon the technique used to measure the value, i.e., the limitations of the measurement system. For example, “about” can mean within 3 or more than 3 standard deviations, per the practice in the art. Alternatively, “about” can mean a range, for example, of up to +/−20%, or up to +/−10%, or up to +/−5%, or up to +/−1% of a given value. 
     In accordance with one aspect of the present disclosure, a die pin is provided for extrusion of molten plastic. The die pin disclosed herein comprises a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member is provided. The base portion and the core member together define a pin base member. For purpose of illustration and not limitation, an exemplary embodiment of the disclosed subject matter is depicted in  FIGS. 2A ,  2 B, and  3 , comprising a ceramic head member  210  and a pin base member  220 . Particularly,  FIG. 2A  is a side view of a representative embodiment of a pin base member  220  for the die pin,  FIG. 2B  is a side view of a representative embodiment of a ceramic head member  210  to be mated to the base member, and  FIG. 3  is a photograph of a representative embodiment of an assembled die pin  200 . As embodied herein, the pin base member  220  includes a base portion  226  and a metallic core member  221  extending from the base portion  226 . Particularly, as depicted, the base portion has a generally cylindrical body  223  with a fastener region  225  extending therefrom, such as a threaded portion or the like to be coupled with a mounting portion of an extrusion apparatus. The base portion  226  depicted herein further includes a polygonal region  224  to facilitate a conventional tool to rotatably engage the fastener region  225  with the mounting portion. Although cylindrical body  223  is depicted for the base portion  226 , other suitable structure or shapes can be provided if needed. 
     The fastener region  225  is configured to be compatible with the die stem of the flow head tooling of an existing extrusion apparatus. For example, and as depicted in the exemplary embodiment of  FIG. 2A , fastener region  225  has a thread for fixedly engaging a flow head tooling. As embodied herein, a series of curved recesses (i.e. fillets) and flat and beveled surfaces are disposed between the cylindrical member  223  and the fastener region  225 . In the exemplary embodiment illustrated, the fastener region  225  has a length of about 1.125 inches and a diameter of about 1 inch along the threaded portion, although any suitable dimensions can be used, as determined by the flow head tooling to which the die pin is to be mounted. 
     The pin base member, and particularly the base portion, can have any suitable size and shape as required for the intended purpose. As noted above, in some embodiments, including the embodiment illustrated in  FIG. 2A , the base portion  226  includes a cylindrical body  223 . In the exemplary embodiment depicted in  FIG. 2A , the base portion generally has a cylindrical body  223  with an overall diameter of about 1.5 inches, and a total length of about 3.25 inches. Furthermore, and as embodied herein as depicted in  FIG. 2A , the base portion  226  can include the polygonal region  224 , which is adapted to permit fixing or mounting of the die pin onto the flow head tooling such as by a wrench. The base portion  226  can have any suitable shape and size, which will be determined primarily by the extrusion apparatus and the dimensions of the parison to be extruded 
     Additionally, and as depicted herein, the pin base member  220  includes a core member  221  extending from the base portion  226 . The core member has a free end  222  opposite the base portion  226 . As embodied herein, the free end  222  of the core member  221  has a tapered shape, such as a frustroconical shape with a rounded tip as shown. Alternative shapes for the core member likewise can be used as described further below. 
     In the exemplary embodiment depicted in  FIG. 2A , the base portion  220  includes core member  221  formed as a monolithic single-piece structure. In alternative embodiments, the metallic core member can be separately formed and incorporated onto the base portion by any suitable means, including, without limitation, threading, welding, or the like. For example, and in certain embodiments, the base portion can further comprise a mounting cavity (not shown) for the core member, which can be bonded or threadingly engaged to the base portion. The base portion and the core member can be composed of any suitable material, including metal, ceramic, or plastic. In certain embodiments, including the embodiment depicted in  FIG. 2A , the base portion and the core member are each composed of tool steel and are formed as a single-piece structure. 
     As previously noted, the die pin of the disclosed subject matter further includes a ceramic head member to be mounted on the core member of the pin base member. In the exemplary embodiment depicted in  FIG. 2B , ceramic head member  210  has cylindrical base portion  212  with a generally uniform diameter over a first length and frustroconical tapered end portion  213  over a second length proximate to the tip  211 . The ceramic head member  210  can have any suitable dimension and shape as needed for its intended use. For example, in alternative embodiments, the ceramic head member can have a substantially cylindrical shape with a rounded tip or a hemispherical tip, or a substantially frustroconical shape along the entire length of the head member. Additionally, the relative dimensions of the ceramic head member can be selected as desired. For example, the ceramic head member embodied herein for use with the pin base member of  FIG. 2A  has an overall length of about 2.5 inches and an overall diameter of about 1.25 inches. Generally, the shape and dimensions of the various embodiments of the die pin disclosed herein are primarily determined by the dimensions of the parison to be extruded and the extrusion apparatus to be used. Furthermore, the overall shape and dimensions of the die pin once assembled can be substantially the same as prior art tool steel die pins, familiar to those of ordinary skill in the art. 
     In some embodiments, cylindrical body  223  of the base portion  226  will have the same outer diameter as the cylindrical base portion  212  of ceramic head member  210 , which is mounted flush against the cylindrical member  223  such that the die pin  200  has a substantially continuous exterior surface. For purpose of illustration and not limitation, an exemplary die pin according to one aspect of the present disclosure is shown in  FIG. 3 . In accordance with the disclosed subject matter, the die pin comprises a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the metallic core member. As shown in the exemplary embodiment depicted in  FIG. 3 , at least a portion of the ceramic head member is cylindrical and flush with the base portion to form a substantially continuous exterior surface of the die pin. 
     Additionally, in accordance with the present disclosure, the ceramic head member has a mounting cavity formed therein. For example, and as embodied in  FIG. 2B , the internal mounting cavity  214  can have a substantially cylindrical first length and a tapered second length. In alternative embodiments, the internal mounting cavity is substantially cylindrical along the entire length. In some embodiments, the dimensions and shape of the internal mounting cavity are substantially complementary to the dimensions and shape of the metallic core member  221 . For example, and with reference to the exemplary embodiment illustrated in  FIG. 2A , as noted above, the core member  221  can have a free end  222  opposite the base portion  220 , the free end  222  having a frustroconical shape with a rounded tip. In accordance with another aspect of the disclosed subject matter, however, the core member can have at least one dimension less than a corresponding dimension of the mounting cavity to allow thermal expansion of the core member within the mounting cavity. For example, and as embodied herein, and depicted in the exemplary embodiment of  FIGS. 2A and 2B , the metallic core member  221  has a dimension or dimensions less than a corresponding dimension or dimensions of the mounting cavity  214  to compensate for thermal expansion of the metallic core when the die pin  200  is heated to a working temperature of up to or even in excess of about 400° F. 
     In the various embodiments of the present disclosure, the die pin comprises a metallic core member and a ceramic head member mounted on the core member. The ceramic head member embodied herein includes a mounting cavity defined within the proximal surface of the head member which abuts the base portion (i.e., opposite the tip  211 ). The metallic core member comprises a shape and dimensions for mounting via complementary male-female engagement within the mounting cavity. In certain embodiments, the mounting cavity of the ceramic head member has an internal volume greater than the volume of the metallic core member to allow for thermal expansion of the metallic core member within the cavity of the ceramic head member. Depending on the coefficient of thermal expansion at ambient temperature, the difference between the volume and/or dimension of the mounting cavity and that of the core member can be about 2%, or about 3%, or about 5%, or about 10%, or about 15%, or about 20%, or any value therebetween to ensure that the metallic core member can thermally expand within the mounting cavity without cracking or dislocating the head member. For purpose of illustration and not limitation, a representative linear thermal expansion coefficient for tool steel is 13.0×10 −6  mm/mm/° C., with the corresponding volumetric coefficient being 39.0×10 −6  mm 3 /mm 3 /° C. By way of illustration, a tool steel core member mounting projection 10 mm in length and a working temperature of about 400° F. (˜200° C.), will expand by a length of approximately 0.3 mm, or ˜3% of initial length, as it is heated from ambient temperature to working temperature. 
     In various embodiments, the internal mounting cavity of the ceramic head member can have a diameter of between about 10% and about 60% of the diameter of cylindrical portion  212 , and can have a length between about 10% to about 75% of the length of the ceramic head member  210 . In one exemplary embodiment, the length of the ceramic head member is 1.5 inches, but can be smaller or greater depending on the flow head tooling and the desired size of the parison. While tip  211  is depicted in  FIG. 2B  as being rounded, in alternative embodiments the tip is, for example, blunt or flat. In some embodiments, the tip of the ceramic head member is hollow. In alternative embodiments, the tip of the ceramic head member is not hollow. 
     In certain embodiments, the ceramic head member is mounted onto to the base portion by matingly engaging the mounting cavity of the ceramic head member and the metallic core member of the base. Additionally or alternatively, the head member can be bonded to the base portion with adhesive or the like. Suitable bonding agents include, by way of example and not limitation, epoxide and acrylate based bonding agents. The ceramic head member can be affixed to the base portion in any other suitable manner. For example, in alternative embodiments, the ceramic head member can comprise an engagement feature within the mounting cavity to engage a corresponding feature on the metallic core member. 
     As disclosed herein, it was determined that die pins comprising a head member formed of a ceramic are not susceptible to the accumulation of molten thermoplastic material and therefore do not result in container contamination. Without limitation to theory, it is believed that the higher specific heat capacity of the ceramic prevents localized cooling of the pin tip as it is subject to continuous flow of pressurized air during parison formation. The disclosed die pins are therefore believed to be less susceptible to condensation of the molten thermoplastic material on the die pin. Additionally, the ceramic metal itself is believed to be less susceptible to non-covalent (e.g., electrostatic, hydrogen-bonding, and Van der Waals) interactions with the molten thermoplastic material than conventional tool steel pins. The ceramic die pins disclosed herein can be used with existing molten thermoplastic extrusion apparatus. 
     In accordance with one aspect of the present disclosure, at least the head member of a die pin for molten thermoplastic extrusion is composed of at least one ceramic. As used herein, ceramics refer to inorganic, nonmetallic materials made by the action of heat at temperatures sufficient to cause sintering, solid-state reactions, bonding, or conversion partially or wholly to the glassy state, and include without limitation crystalline oxides, nitrides and carbide materials. Suitable ceramics for use with the present disclosure include, without limitation, alumina-based ceramics, boron-based ceramics, zirconia-based ceramics, and silicon-based ceramics. Alumina based ceramics suitable for use with the present disclosure include, without limitation, alumina oxides having a composition of 90% or greater alumina oxide, including 96% alumina oxide, 99.5% alumina oxide, and 99.8% alumina oxide. Suitable boron-based ceramics include boron nitrides and boron carbides. Suitable silicon-based ceramics include silicon carbides and silicon nitrides. Suitable zirconia based ceramics include zirconia oxides such as magnesia stabilized zirconia, yttria stabilized zirconia, and zirconia toughened alumina. 
     In accordance with another aspect of the disclosed subject matter, the ceramic material is provided with a relatively smooth surface finish. Depending on the extrusion application, surface finishes of less than about 25 μm (1000 μinch) average roughness (R a ) will suitable for the at least one ceramic material component of the die pin. For example, the at least one ceramic material can have a surface finish of between about 0.1 μm R a  (i.e. approximately 4 RMS) to about 12.5 μm R a  (i.e. approximately 500 RMS). More particularly, the at least one ceramic material can have a surface finish of between about 0.1 μm R a  and about 1.6 μm R a  (i.e. approximately 64 RMS). Furthermore, the at least one ceramic material can have a surface finish of about 0.1 μm to 0.8 μm R a  (i.e. approximately 32 RMS). 
     The ceramic die pin disclosed herein is suitable for use for extrusion of molten thermoplastic polymers for plastic containers. The die pin is suitable for extrusion of, without limitation, polyethylene, low density polyethylene, high density polyethylene, polyethylene terephthalate, polypropylene, polystyrene, and polyvinylchloride. 
     In a further aspect of the disclosed subject matter, an apparatus for extrusion of molten thermoplastic material is provided. The apparatus includes a die ring having an inner surface defining an extrusion opening with a central axis and a die pin as described above positioned within the central axis of the extrusion opening. An annular space is thus defined between the outer surface of the ceramic head member and the inner surface of the die ring. Particularly, the die pin comprises a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member. 
     An exemplary extrusion blow molding apparatus suitable for use with the disclosed die pin is shown schematically in  FIG. 4  for purpose of illustration. The blow molding apparatus  400  depicted herein, for example, is an upward extruding blow molding apparatus as disclosed in U.S. Pat. No. 8,388,333, the contents of which are incorporated herein in their entirety by reference herein. Generally, the apparatus includes an inflow  410  and melt pipe  415  connected to an extruder having a flow head  420 . The molten plastic is extruded outward to form a parison  430  which is captured by a mold  440  with mold halves  440   a  and  440   b . The mold  440  can travel on a wheel  445 . In the interior of the flow head a mandrel  470  is present having a die stem which can be actuated up and down, for example, by a servo actuator. A spacer  480  is additionally provided. A die pin  200  in accordance with the description above is operably connected to the die stem. The die pin  200  is positioned within the central axis of extrusion opening  460  of the flow head  420 . The molten thermoplastic material flows over the die pin  200  to create the inner diameter of the parison  430  and past the extrusion opening  460  to create the outer diameter of the parison  430 . The parison  430  is thus formed from the molten thermoplastic material in the flow head  420 , and the parison  430  continuously flows upward from the flow head  420  into the mold  440 , where the molten thermoplastic is blown into a desired shape. 
     Although reference is made, for purpose of illustration and not limitation, to an extrusion apparatus having an upward extruding blow molding configuration, any suitable extrusion configuration using a die pin can be used in accordance with the disclosed subject matter. 
     In an additional aspect, a method of extruding molten thermoplastic material using a ceramic die pin according to any aspect of the foregoing disclosure is provided. The method disclosed herein includes providing an extrusion apparatus comprising a die ring having an inner surface defining an extrusion opening with a central axis and a die pin disposed within the extrusion opening and aligned with the central axis, such that an annular space is defined between an outer surface of the ceramic head member and the inner surface of the die ring. The method further includes directing molten plastic through the annular space between the die ring and the die pin without accumulation of the molten plastic on the die pin. The die pin provided comprises a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member. For example, and as embodied herein, with reference to  FIG. 4 , the method comprises providing an extrusion apparatus comprising a die ring having an inner surface defining an extrusion opening  460  with a central axis and a die pin  200  disposed within the extrusion opening and aligned with the central axis, the die pin  200  comprising a base portion, a metallic core member extending from the base portion, and a ceramic head member mounted on the core member, wherein an annular space is defined between an outer surface of the ceramic head member of die pin  200  and the inner surface of the extrusion opening  460  of the die ring. The method further comprises directing molten plastic through the annular space between the die ring and the die pin without significant accumulation (e.g., accumulation sufficient to result in contamination) of the molten plastic on the die pin. In additional embodiments, the method comprises contacting a ceramic die pin with molten thermoplastic material in an extrusion apparatus. Additional aspects of the method of extrusion can be determined from U.S. Pat. No. 8,388,333, which is incorporated by reference herein. 
     Due to the higher shear modulus of ceramics relative to tool steel, and in accordance with an additional aspect, the tooling incorporating a ceramic die pin can be subject to an extended heat soak upon line restart to ensure no unmelts are present in the thermoplastic material for extrusion. Unmelts can exert high shear stress on the ceramic head member and can thereby fracture the ceramic head member of the die pin. By thoroughly remelting the thermoplastic material present from the previous run which had cooled and by heating the tooling to the temperature of the molten thermoplastic material, e.g., 260° C. for PET, prior to extrusion, unmelts and associated pin fracture can be prevented. The duration of the heat soak will depend primarily on the specific tooling apparatus being employed. In certain embodiments, a heat soak of at least twenty minutes prior to extrusion is employed, although a heat soak of up to about 80 and even up to about 210 minutes can be employed. 
     In addition to the specific embodiments claimed below, the disclosed subject matter is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features disclosed herein can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter should be recognized as also specifically directed to other embodiments having other possible combinations. Thus, the foregoing description of specific embodiments of the disclosed subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed. 
     The disclosed subject matter can be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents. All references recited herein are incorporated herein in their entirety by specific reference.