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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of cooling a foam extrusion mixture to an extrudable temperature within a thermoplastic extrusion assembly, the method resulting in the production of a substantially homogeneous thermoplastic foam
product, and which achieves the desired extrusion temperature of the extrusion mixture and at an increased throughput rate over what is normally available, without requiring that a complex, costly and/or oversized extrusion assembly be implemented and
further, without requiring substantial modification to existing manufacturing devices and procedures that may be integrated therein.
2. Description of the Related Art
The field of art associated with thermoplastic extrusion, and thermoplastic foam extrusion in particular, is quite specialized, and indeed, is quite different from that typically associated with metal, rubber, non-foamed plastic extrusion.
Specifically, foam extrusion generally requires an initial step of melting pellets, usually made of a thermoplastic material, and a subsequent step of mixing the melted thermoplastics with a foaming agent, such as a fluorocarbon (whether CFC, HCFC and/or
HFC) or hydrocarbons (such as propane, butane, pentane, etc.), and possibly other agents such as nucleating agents, fire retardants and/or coloring agents, in an isolated extrusion environment. Moreover, the most effective foam extrusion techniques
completely contain the extrusion material during the melting and mixing stages, maintaining the material in a non-foamed, viscous form until passed through an extrusion die and exposed to external forces. Indeed, it is when the extruded material exists
the die of the
foam extrusion assembly that it will foam (i.e. inflate and stiffen) into its ultimately usable form, such as films, planks, and large sheets from which meat trays, egg containers, small containers for butter and jelly, and the like, are formed. Accordingly, precision is imperative in order to ensure that an effective and complete mixing of the ingredients is achieved, thereby providing for a properly configured and homogeneous product, and further, to ensure that the entire extrusion system is
well contained until the material passes through the die, thereby avoiding premature foaming of the extrusion material.
Of course, in addition to the above concerns associated with the formation of a foam product is the need to maintain the extrusion mixture at a rather precise extrusion temperature, corresponding the polymer or substance being used as the basis
for the extrusion mixture, so as to achieve a proper viscosity of the extrusion mixture and permit proper forming of the extrusion mixture through a die, such as a profile die, tube die, sheet die, annular die, flat die or several other common types of
dies. The rather precise extrusion temperature at which a desired range of viscosity is achieved is unfortunately, less than the initial "melt temperature", i.e. temperatures at which the pellets of thermoplastic extrusion material are melted, but
cannot be too much less than that initial "melt temperature" for reasons about to be explained. As such, a substantial balance must be maintained. For example, if the melted extrusion mixture is permitted to cool too much, it will become too viscous
and will fail to achieve the desired product density, becoming unusable, as in general, it will not move effectively through the extrusion assembly, let alone, out through the die. Conversely, if the temperature of the melted extrusion material is too
high, its viscosity decreases significantly and the material is not dimensionally stable or shapeable as it flows through and importantly, from the die.
As can be appreciated, any one of the previously mentioned factors can have a significant effect on the productivity rate associated with the various phases of thermoplastic foam extrusion. Accordingly, typical thermoplastic foam extrusion
processes are often designed to maximize the regulation and control over each factor. For example, a typical foam extrusion process is broken down into two separate phases, and indeed, often requires two separate extrusion devices linked with one
another. The first phase of the procedure typically involves the effective melting of extrusion material pellets, such as a thermoplastic material particularly suited for the foam desired, and the subsequent mixing of those material pellets with the
foaming agent and other various agents, as needed. Usually, this first phase of the procedure is performed within a very large and elongated mechanical extrusion device wherein a thermoplastic extrusion screw, and more specifically a melt screw, urges
the material pellets through an elongate barrel causing the generation of frictional heat, and also from which melting heat is being applied. Indeed, the surface of the thermoplastic extrusion screw is rather slippery be design (i.e. often chrome plated
and polished) and the material moves through the barrel as a result of a frictional engagement or shear when the extrusion material frictionally contacts the barrel surface. Moreover, it is the shear effect combined with the heated barrel which provides
for effective melting of the extrusion material.
In addition to ensuring that proper homogeneous mixing is achieved, the first phase is also limited by the need to establish and maintain an isolated system. In particular, the isolated system prevents premature foaming of the thermoplastic
material, and ensures that sufficient heating energy is being applied in order to effectively melt the material and permit complete mixing thereof. Many advances have been achieved in the industry so as to maximize the flow-through rates attainable by
or within this first phase of the thermoplastic extrusion process. Unfortunately, however, actual output of the extruded product is still limited to levels well below those achievable in this initial phase as a result of the requirements and limitations
of the second phase of the foam thermoplastic extrusion process.
In particular, unlike conventional thermoplastic extrusion which primarily requires a homogeneous mix, form extrusion also includes a second phase which requires the evenly distributed and uniform cooling of substantially all of the melted and
mixed extrusion material to a point where it is at a necessary extrusion temperature and consistency throughout. Usually, this second phase of the procedure is also performed within a very large and elongated mechanical extrusion device wherein a
central thermoplastic extrusion screw, such as a foam cooling screw with a helix or paddle type configuration, urges the melted extrusion material through an elongated barrel to effect a cooling of the material. Given the need for a rather precise
temperature, however, significant limitations relating to the turn rate of the central cooling screw and to the heat extraction rate achieved apply to this second phase of the process. Specifically, the turn rate of the central or cooling screw is
limited because of the need to minimize the heat which results from shearing of the extrusion material with the wall surface of the barrel and internally by the material itself. As such, an increased flow-through rate of high quality cannot be achieved
merely by speeding up the rotation of the screw. Furthermore, one cannot indefinitely merely counter the excess shear heat that is generated by providing for faster cooling merely by decreasing the temperature of the barrel, because if the mixture cools
too much, an optimal viscous flow of the extrusion mixture is not maintained and productive passage through the die is dramatically impeded. Also, merely increasing the size of the extrusion barrel in the cooling phase is not an effective solution as
such an assembly would become excessively large, cumbersome and financially impractical die to using such a large scale factor. Large scale cooling stage processes also involve potentially undesirable operation cost implications due to the typically
longer product change-over time factor and the related materials costs of such large machines. Moreover, if one merely increases the size of the extrusion barrel in the cooling phase and the passage through the die is made long enough so as to allow
temperature tempering of the extrusion mixture, the resistance to flow generated by such passage would detrimentally promote heat generation in the extrusion mixture within the barrel. Further, this restricted long passage would negate the normal
tendency of the thermoplastic extrusion mixture to swell after passage from the die is retarded.
Accordingly, it is seen that one must balance the needs of a productive flow through rate with the requirements of practicality and an effective and evenly distributed cooling.
Many in the industry have nevertheless failed to recognize the above-described limitations. For example, some in the industry have sought to increase the productivity of the second or cooling phase of the extrusion process by increasing the
amount of heat which is extracted at the extrusion barrel's surface. Unfortunately, these procedures have proven ineffective because when an amount of heat is extracted so as to effectively cool substantially all of the extrusion material throughout,
the perimeter layers of the extrusion material, which are in more direct contact with the barrel surface, cool excessively and no longer provide a satisfactory extrudate. Thus, a primary difficulty associated with this cooling phase is the fact that the
quantities of the extrusion mixture which are closest to the shaft of the central screw do not get effectively cooled, as a majority of the heat that is extracted comes first from the extruded material located about the perimeter area within the barrel
of the second mechanical extrusion device. For example, as heat is extracted, the perimeter quantities of the extrusion material continue to get cooler and cooler while the interior quantities gradually cool to the desired, rather precise extrusion
temperature. This yields extrusion material which is not of uniform consistency for proper extrusion. Indeed, in this second phase, as well as in the first phase, it is also noted that complete homogenization of the extrusion mixture is sometimes
lacking, and as a result the finished product can be of diminished quality.
Accordingly, it is seen that the overall productivity of the present industry is still limited by the cooling phase of the extrusion process. To date, the only effective means of ensuring an effectively cooled extrudable material is to provide a
slow, lengthy, and gradual cool down process so as to thicken the extrusion material without overly cooling only portions thereof, and so as to achieve a longer mixing time for increased homogenization. The method of the present invention, however,
addresses the problems and needs which remain in the art and is able to significantly increase the flow-through rate of extrusion material without compromising the quality of the finished product.
SUMMARY OF THE INVENTION
The present invention relates to a method of cooling a foam extrusion mixture to an extrudable temperature, preferably as the extrusion mixture passes through the elongate barrel of a thermoplastic foam extrusion assembly of the type structured
to produce an extruded foam product in any of a variety of extruded shapes.
The preferred method of the present invention includes an initial step of drawing heat from a perimeter surface of the barrel, preferably utilizing a heat extraction structure cooperatively disposed with the barrel and structured to draw heat
therefrom. Preferably simultaneously with the drawing of the heat from the barrel, an elongate extrusion screw is preferably rotated within the barrel. By rotating the elongate extrusion screw, the extrusion mixture is thereby urged towards an outlet
of the barrel where it may pass through a die.
While the extrusion screw rotates, at least some of the extrusion mixture is passed through a circulation channel defined in at least one screw flight of the extrusion screw. The circulation channel preferably includes an inlet area larger than
its outlet area, and thereby maximizes a flow velocity and swell of the extrusion mixture exiting the circulation channel. Furthermore, upon passage of the extrusion mixture through the circulation channel, at least some quantities of the extrusion
mixture which are disposed a spaced apart distance from the perimeter surface of the barrel are circulated towards the perimeter surface of the barrel. As the heat extraction preferably takes place preferably at the barrel, substantially uniform cooling
of the extrusion mixture to the extrudable temperature is achieved.
Preferably, but not necessarily, the method of cooling the extrusion mixture of the present invention is achieved utilizing a foam extrusion assembly having a melt region, which receives and melts a plurality of pellets, preferably thermoplastic
material pellets, and an integral, in-line or separate, heat extraction region.
The melt region of the foam extrusion assembly includes a mixing assembly structured to thoroughly mix the melted material pellets and preferably a foaming agent, thereby achieving a homogeneous foam extrusion mixture.
The heat extraction region preferably associated with the method of the present invention is preferably where the cooling takes place, as it is structured to extract excess heat from the extrusion mixture so that the mixture can achieve an
extrudable temperature and consistency. Preferably, the heat extraction region includes an elongate barrel through which the mixture flows. The barrel, which is preferably substantially enclosed, includes at least one inlet and at least one outlet
defined therein. The inlet is structured to receive the extrusion mixture therethrough, preferably from the melt region, for passage into the barrel.
Cooperatively disposed with the barrel of the heat extraction region is the heat extraction structure. Specifically, the heat extraction structure draws heat from the barrel, such as by liquid cooling, therefore removing heat from the extrusion
mixture in order to ensure that the extrusion mixture arrives at the extrudable temperature range.
Disposed within the barrel of the heat extraction region is the thermoplastic extrusion screw. This extrusion screw is preferably a cooling screw axially disposed within the barrel and as indicated, is structured to rotate therein when the
barrel contains a quantity of the extrusion mixture. In order to urge the extrusion mixture towards the outlet of the barrel, the extrusion screw further includes at least one screw flight. The screw flight is preferably structured to wrap around the
extrusion screw and thereby rotate upon rotation of the screw in order to urge the mixture towards the outlet of the barrel. To achieve effective, complete and evenly distributed cooling of the extrusion mixture, however, the screw flight further
includes at least one, but preferably a plurality of the circulation channels defined therein. In particular, the circulation channels are generally precisely positioned in the screw flight so as to receive quantities of the extrusion mixture
therethrough upon rotation of the extrusion screw. Accordingly, upon receipt of the quantities of the extrusion mixture through the circulation channel, an effective circulation of the mixture relative to the barrel is achieved and substantially all of
the extrusion mixture will substantially uniformly come into close proximity with the barrel for effective and even heat extraction and cooling thereof. Moreover, at least a portion of the flow through area defined by the circulation channel is smaller
than the inlet area of the channel. As a result, a funneling type flow is achieved and the flow velocity and swell of the extrusion mixture from the circulation channel is maximized.
Lastly, the foam extrusion assembly includes a die. In particular, the die is cooperatively disposed in fluid flow communication with the outlet of the barrel, and can take on any of a number of desired forms so as to receive the extrusion
mixture, preferably at the extrudable temperature, therethrough in order to form a desired shape. Indeed, it is upon exit from the die that the extrusion mixture begins to "foam" and thereby produce the finished, extruded product.
BRIEF
DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawing in which:
FIG. 1 is a perspective, partial cut away view of the foam extrusion assembly preferably utilized with the method of the present invention;
FIG. 2 is a sectional, partial cut-away view of the barrel and extrusion screw of the heat extraction region of the foam extrusion assembly preferably utilized with the method of the present invention;
FIG. 3 is an isolated sectional view of the screw flight of the extrusion screw preferably utilized with the method of the present invention;
FIG. 4 is a cross sectional view of the screw flight of the extrusion screw preferably utilized with the method of the present invention;
FIG. 5 is a cross sectional, isolated view illustrating a first embodiment of the circulation channel defined in the screw flight of the extrusion screw preferably utilized with the method of the present invention;
FIG. 6 is a cross sectional, isolated view illustrating an alternative embodiment of the circulation channel defined in the screw flight of the extrusion screw preferably utilized with the method of the present invention; and
FIG. 7 is a sectional, perspective view of an alternative embodiment of the extrusion screw preferably utilized with the method of the present invention.
Like reference numerals refer to like parts throughout the several views of the
drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed towards a method of cooling a foam extrusion mixture to an extrudable temperature as the foam extrusion mixture passes through an elongate barrel 50.
Once a foam extrusion mixture is formed, the present method includes the initial step of drawing heat from a perimeter surface of the elongate barrel 50. As such, heat is also drawn from the extrusion mixture itself, resulting in an ultimate
cooling thereof to the extrudable temperature. As will be described, a heat extraction structure is preferably cooperatively disposed with the barrel 50 so as to draw the heat.
Preferably simultaneously with the drawing of heat from the barrel 50, and
accordingly the extrusion mixture, the extrusion mixture is urged through the barrel 50, towards an outlet 44 thereof. In doing so, an elongate extrusion screw 60, preferably contained within the barrel 50 is rotated. By rotating the extrusion
screw 60, a leading surface 70 of at least one screw flight 64 of the extrusion screw 60 engages the extrusion mixture, urging it towards the outlet 44 of the barrel. Moreover, in order to urge the extrusion mixture towards the outlet 44, the screw
flight 64 also preferably, at least partially frictionally engages the extrusion mixture with a perimeter surface 51 of the barrel 50. In the preferred embodiment, the screw flight 64 of the extrusion screw 60 that is rotated includes perimeter edge 80
that is beveled with a trailing portion thereof angled away from an interior surface of the barrel 60. As a result, the extrusion mixture is urged towards the outlet 44 of the barrel 50 without substantial generation of shear heat which could counter
the cooling process.
Also preferably simultaneously with the drawing of heat and rotation of the extrusion screw 60, at least some quantities of the extrusion mixture which are disposed a spaced apart distance from the perimeter surface 51 of the barrel 50 is
circulated towards the perimeter surface 51 of the barrel 50 so as to achieve substantially uniform cooling of the extrusion mixture. In particular, and as will be described, because the heat is generally drawn at the perimeter surface 51 of the barrel
50, portions of the extrusion mixture remote from the perimeter surface 51 are not cooled to the same extent as those portions near the perimeter surface. Accordingly, the circulation ensures that portions of the extrusion mixture at the perimeter
surface 51 are not over cooled, while portions remote from the perimeter surface 51 are under cooled. Preferably the quantities of extrusion mixture are circulated away from the outlet 44 of the barrel 50 so as to urge trailing quantities of the
extrusion mixture which are disposed at the spaced apart distance from the perimeter surface 51 of the barrel 50 towards the perimeter surface 51 of the barrel 50. Moreover, the circulation is preferably achieved by passing at least some of the
extrusion mixture through one or more circulation channels 90, preferably defined in the screw flight 64 a spaced apart distance from the perimeter surface 51 of the barrel 50. As a result, the quantities of the extrusion mixture which are disposed the
spaced apart distance from the perimeter surface of the barrel and at the trailing surface of the screw flight are urged towards the perimeter surface 51 of the barrel 50. Also in the preferred embodiment, the portions of the extrusion mixture are
passed through a circulation channel 90 which may include an inlet area larger than its outlet area such that the flow velocity and swell of the extrusion mixture exiting the circulation channel 90 is maximized. In addition to, or instead of the reduced
outlet area, the extrusion mixture may be passed through a circulation channel 90 which includes at least one inwardly tapered edge, so as to maximize the quantity and flow velocity of said extrusion mixture which enters said circulation channel, and/or
an asymmetrical inlet area, so as to disrupt a flow pattern of said extrusion mixture through said circulation channel, thereby achieving substantially uniform cooling of the extrusion mixture.
Although not necessarily, the thermoplastic extrusion assembly which can be utilized with the present method is preferably a thermoplastic foam extrusion assembly, as illustrated in FIG. 1, and generally indicated as 10. Specifically, the
illustrated extrusion assembly 10 is structured to produce an expandable polymer plastic which foams upon exposure to an external environment in order to produce a finished or semi-finished product. To this end, the foam extrusion assembly 10 preferably
of the method of the present invention may be utilized with a number of thermoplastic polymers, including but not limited to polystyrene, polyethylene (PE), polypropylene, PET or other similar thermoplastics, including foaming or expandable
thermoplastics or other materials which may be utilized or developed in the future.
As illustrated in FIG. 1, the preferred embodiment of the foam extrusion assembly 10 that may be used with the method of the present invention defines a tandem type assembly, although a single, in-line and/or intermeshing twin screw assembly may
also be utilized, and which if utilized should also be considered as being within the scope and spirit of this invention. With regard to the tandem assembly illustrated in FIG. 1, however, the foam extrusion assembly 10 includes a first "melt region",
generally indicated as 20, and a second "heat extraction" region generally indicated as 40. The first melt region 20 is structured to receive and melt a plurality of thermoplastic material pellets, which preferably form the basis for the product, such
as the foam product, to be produced. As is conventional in the industry, the preferred embodiment includes a larger funnel type solids inlet 24 wherethrough large quantities of the raw material, such as material pellets and the like, may be introduced
into the melt region 20. Of course, it is noted that the material pellets may include small bead or cylindrical type pellets, larger cubes, blocks, chunks, flakes, powder defining particulate, or any other configuration of the material which can be
conveniently introduced into the melt region 20 for subsequent melting thereof. Furthermore, although the melt region 20 may include a large vat or other heating container for direct and immediate melting of the material pellets, in the preferred
embodiment, the melt region 20 preferably includes an elongated extrusion barrel 50' through which the material pellets are urged. Specifically, a thermoplastic extrusion screw, such as an interior melt screw 60' is contained within the barrel of the
melt region 20 and is preferably driven by a large gear assembly 22 so as to rotate within the melt region 20. As the material pellets are urged through the melt region 20, preferably by the interior melt screw 60' so as to have essentially a
"meat-grinder" effect, heat energy is produced to melt the material pellets disposed within the barrel 50'. In the illustrated embodiment, at least one heating source, preferably surrounding the wall of the barrel 50' of the melt region 20, applies heat
to the barrel 50'. Moreover, the interior melt screw 60' contained in the barrel 50' preferably urges, if not forcing, the material pellets against the interior surface of the barrel 50' and against one another, thereby causing a shear effect which in
practice adds the largest quantity of heat to the material pellets and improves the overall melting thereof until a smooth, yet viscous melted material is provided.
As the material pellets alone will generally not provide the necessary foaming reaction when hardening, unless a material with previously incorporated or micro-encapsulated agents is used, and will not become the desired finished foam product,
the foam extrusion assembly 10 may further include an agent addition assembly 26. In the preferred embodiment, the agent addition assembly 26 adds a foaming agent to the melted or melting material pellets, preferably as they pass through the melt region
20. Moreover, in the preferred embodiment, the foaming agent may include fluorocarbon, hydrocarbon, and/or other equivalent foaming agents or mixtures thereof which will add volume to the finished product and will promote the foaming action when the
extruded melt emerges from the foam extrusion assembly 10. Of course, it is understood that other foaming agents may also be developed or provided in the future depending upon the desired finished product, and may indeed be combined with the
thermoplastic material prior to its introduction into the melt region 20. Further, if desired, and preferably along with the agent addition assembly 26, a coloring agent and/or a nucleating agent may further be added to the material pellets being
melted.
Also, preferably included at the melt region 20, is a mixing assembly 27. The mixing assembly 27 is preferably defined by either all or at least a part of the melt screw 60' which preferably urges the melted material pellets through the melt
region 20, and provides for more effective melting as a result of the shear effect produced internally and with the barrel 50'. The mixing assembly 27 is structured to substantially mix the melted material pellets and the blowing agent with one another
so as to provide a substantially homogeneous extrusion mixture of melted material pellets and forming agent. In this regard, it is thought to be very important that a thorough and uniform foam extrusion mixture of the melted material pellets and the
foaming agent result in order to provide a desired finished product that contains few, if any, deformations, imperfections or irregular structures such as air pockets, irregular cell structures, etc.
Once the extrusion mixture has been effectively homogenized, it is preferably passed directly into the second, "heat extraction" region 40, wherein the steps of the method of the present invention preferably take place. As illustrated in FIG. 1
with regard to the preferred tandem assembly, this heat extraction region 40 may comprise a separate structure, but alternatively, could merely be a continuation of the melt region 20 as is the case especially in non-foaming thermoplastic extrusion.
More specifically, in the embodiment illustrated in FIG. 1, a connector type conduit 30 may be provided so as to transfer the extrusion mixture from the melt region 20 to the heat extraction region 40. The heat extraction region 40 is structured to
receive and process the extrusion mixture therethrough and to uniformly extract excess heat from that extrusion mixture such that the mixture will achieve an appropriate extrudable temperature, often between 200 to 300 degrees fahrenheit, depending upon
the thermoplastic being used and the amount of gas present therein. In particular, in the foam extrusion process it is necessary for the extrusion mixture to be provided at an appropriate extrudable viscosity. If the temperature of the extrusion
material is too hot, the material will have too low of a viscosity, meaning it will be runny and difficult to foam into a finished or useable product. Thus, the extrusion material must be sufficiently cooled or viscous, i.e., thickened and gel-like in
order to be moldable and formable through a die 44, without being so viscous as to prematurely harden. Accordingly, the heat extraction region 40, preferably utilizing the method of the present invention, is structured to extract excess heat from the
extrusion mixture in a controlled, uniform and evenly distributed manner such that substantially all of the extrusion mixture will achieve a uniform and effective extrudable temperature, and such that the extrudable temperature will be achieved
throughout the extrusion mixture at an increased pace than is conventionally available.
Referring now to the heat extraction region 40 and the remainder of the Figures, it is seen that the | | |
