Patent Application
20150061179
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.
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
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.
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.
The disclosed subject matter will now be described in conjunction with the accompanying drawings in which:
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
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
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
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
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
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
Additionally, in accordance with the present disclosure, the ceramic head member has a mounting cavity formed therein. For example, and as embodied in
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 mm3/mm3/° 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
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 (Ra) 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 Ra (i.e. approximately 4 RMS) to about 12.5 μm Ra (i.e. approximately 500 RMS). More particularly, the at least one ceramic material can have a surface finish of between about 0.1 μm Ra and about 1.6 μm Ra (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 Ra (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
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
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| 61872332 | Aug 2013 | US |
