Method of Cooling a Foam Extrusion Mixture to an Extrudable Temperature

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.

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.

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.

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 illustrated embodiment of the heat extraction region 40 includes the elongated barrel 50. The elongate barrel 50 includes at least one inlet 31, such as at the connector conduit 30, seen in FIG. 1, or a transition point between the melt region and the heat extraction region of an in-line structure, and at least one outlet, such as at the die 44, defined therein. As such, in the preferred tandem configuration shown in FIG. 1, the extrusion mixture is preferably received from the melt region 20, in a fully or substantially blended form, through the inlet 31 for passage and processing through the barrel 50 prior to formation by the die 44. In this illustrated embodiment, the processing to be performed in the heat extraction region 40 is preferably the cooling of the extrusion mixture pursuant to the method of the present invention, although added phases such as further mixing may also be achieved therein.

Cooperatively disposed with the barrel 50 of the heat extraction region 40 is the heat extraction structure. Specifically, the heat extraction structure is structured to draw heat, preferably from the barrel 50, and accordingly, to draw heat from the extrusion mixture contained within and passing through the barrel 50. In the preferred embodiment, the heat extraction structure includes a cooling structure such as a plurality of cooling coils or channels 52 preferably disposed generally about a perimeter of the barrel 50. As such, the heat extraction structure 52 can draw a substantial amount of heat from the perimeter of the barrel 50, and indeed will preferably draw substantial quantities of heat from the quantities of the extrusion mixture disposed in a vicinity of the perimeter 51 of the barrel 50, as will be described in further detail subsequently.

Contained within the barrel 50 and preferably sized to achieve a precise yet rotatable fit, is the thermoplastic extrusion screw 60, such as a foam cooling screw. In particular, the extrusion screw 60 is preferably made of stainless steel and is also made to be generally elongate. More preferably, the extrusion screw 60 includes an integrally formed screw that extends the length of the barrel 50. The extrusion screw 60 is also preferably connected to a gear assembly, as at 42, seen in FIG. 1, which rotatably drives the extrusion screw 60 within the barrel 50, although it is also possible for the extrusion screw to be stationary but with an input force applied to urge the extrusion mixture through the barrel. Regardless, the extrusion screw 60 preferably includes at least one screw flight 64, which in a more preferred embodiment is structured to wrap completely about a central elongate shaft 62 so as to define a continuous elongate screw flight 64, although one or more angled scraper blades could also define the flight. Furthermore, while the preferred embodiment includes a single screw flight 64, in an alternative embodiment, as illustrated in FIG. 7, two or more screw flights 64 and 64' may be disposed in line with one another or side by side as in the illustrated alternative as so to define at least one flow path for the extrusion mixture. Of course, it should be noted that the screw flight 64 may but need not extend entirely along the shaft 62 or the barrel 50 depending upon the mixing, heating and/or cooling requirements of the particular system and the particular polymer or material be used as the basis for the extrusion mixture. Moreover, the specific angle of incidence of the screw flight 64 can be varied so as to regulate the speed through which the extrusion mixture flows through the barrel 50.

The screw flight 64, which may be separately or integrally formed with the shaft 62 in one or more continuous or segmented pieces is preferably structured to rotate upon rotation of the entire extrusion screw 60. Moreover, the screw flight 64 is structured to extend from the elongate shaft 62 into substantially close spaced relation with an interior surface 51 of the barrel 50. Indeed, that closely spaced relation between the screw flight 64 and the interior surface 51 of the barrel 50 is generally to very precise tolerances (usually to within generally about 0.001 times the inside barrel diameter which is often about 6 inches ) such that minimal seepage of the extrusion mixture passes therebetween upon rotation of the extrusion screw 60 within the barrel 50. Moreover, although the surface of the screw flight 64 is preferably smooth and generally of a lower friction than the interior surface 51 of the barrel 50, the nature of the interior surface 51 of the barrel 50 is such that movement through the barrel 50 is achieved. Accordingly, as the extrusion screw 60 rotates, the relative movement between the screw flight 64 and the interior barrel surface 51 tends to urge the extrusion mixture forward, as illustrated by arrow B in FIGS. 1 and 2, in accordance with the rotation of the extrusion screw 60, as illustrated by arrow A. This movement of the extrusion mixture towards the end that preferably includes the outlet 44 therefore functions to drag or push the extrusion mixture forward, forcing the melt through the die 44 so as to form the foam product 45 which is ultimately utilized in the formation of the finished products.

As previously indicated, the heat extraction structure 52 is preferably disposed about a perimeter of the barrel 50 as shown in FIG. 2. As indicated, more heat is generally extracted from the quantities of the extrusion mixture which are nearest to the interior perimeter surface 51 of the barrel 50. As has been explained, however, substantial considerations relate to the rate at which the extrusion mixture can pass through the heat extraction region 40 in order to attain an appropriate extrudable temperature. For example, while merely increasing the heat extraction rate in conventional systems would appear to increase a throughput rate, if excessive heat is extracted too quickly in an attempt to speed up the process, the quantities of extrusion mixture which are closet to the perimeter of the barrel 51 become over cooled relative to the quantities of the mixture which are closest to the shaft 62 of the extrusion screw. As a result, the heat extraction process of conventional systems must be substantially slow in order to ensure that all of the extrusion mixture arrives as the extrudable temperature uniformly.

Conversely, the method of the present invention is such that a faster throughput rate of the extrudable mixture through the heat extraction region 40 can be achieved by permitting a greater degree of heat to be withdrawn by the heat extraction structure 52. Specifically, and as illustrated in FIGS. 2 through 7, the screw flight 64 of the thermoplastic extrusion screw, and especially the foam cooling screw includes at least one, but preferably a plurality of the circulation channels 90 defined therein. The circulation channels 90 are preferably disposed at a spaced apart distance from one another along the screw flight 64, and may extend along the entire screw flight 64 if desired. Preferably the circulation channels 90 are disposed in a generally offset alignment from one another along the preferred continuous screw flight 64, which may be viewed from either a top or bottom view of the extrusion screw 60, so as to interrupt a cross-channel flow and velocity and so as to provide a more turbulent and mixed flow. The circulation channels 90 are also preferably disposed in a lower region of the screw flight 64 so as to be in closer proximity to the screw shaft 62 than to the interior surface 51 of the barrel 50, and most preferably, are generally disposed directly adjacent to the root of the extrusion screw 60. Moreover, a perimeter surface of the circulation channels 90 is preferably, but not necessarily, completely enclosed and defined by the screw flight 64.

As best shown in FIG. 2, each screw flight 64 preferably includes a leading surface 70, a trailing surface 72 and a perimeter edge 80. Accordingly, as the screw flight 64 rotates, the leading surface 70 essentially scoops up the extrusion mixture and urges it towards the lower pressure trailing surface 72, displacing the extrusion mixture near the screw shaft 62 up towards the interior surface 51 of the barrel 50, and interrupting the normal laminar melt flow, while simultaneously dragging it forward towards the die 44. In the illustrated embodiment, the circulation channel 90 extends from the leading surface 70 to the trailing surface 72 such that the extrusion mixture contained within the barrel 50 passes through the circulation channel 90, as in the direction illustrated by arrow C, shown in FIG. 4, upon rotation of the screw flight 64. Moreover, as the quantities of extrusion mixture move from the leading surface side of the screw flight 64 to the trailing surface side of the screw flight, the quantities of extrusion mixture disposed generally at the trailing surface 72 of the screw flight 64 are displaced and pushed upward towards the interior surface 51 of the barrel 50. This circulation of the extrusion mixture through the circulation channels 90 substantially ensures that an even, uniform distribution of cooling is achieved throughout the entire quantity of the extrusion mixture, and that accordingly, a greater quantity of heat can be uniformly extracted so as to permit a more rapid flow through rate. Specifically, more heat can be extracted and the rate of movement can be increased utilizing the method of the present invention since the greater heat extraction is spread out through all of the extrusion mixture and no portion of the mixture is cooled more than another in an attempt to extract heat from neglected regions. Indeed, by way of experimentation it has been noted that with the method of the present invention, foam production output in some systems has increased by over 400 pounds per hour, representing a substantial production increase over previously exhibited maximums of about 950 pounds per hour, representing a 40% increase in productivity.

Turning to FIGS. 3, 5 and 6, so as to more effectively provide for the circulation of the extrusion mixture through the circulation channels 90, in an illustrated embodiment of FIG. 5, the circulation channels 90 are preferably inwardly tapered from the leading surface 70 to the trailing surface 72 of the screw flight 64. Such a configuration essentially scoops in maximum quantities of the extrusion mixture for circulation therethrough and tends to provide a "jet" effect that maximizes a flow velocity through the circulation channel 90 and an increased disruption of the state of the extrusion mixture behind the trailing surface 72 is achieved. Furthermore, the circulation channel 90 is preferably also structured to be substantially short in length. Preferably, but not necessarily, in addition to the tapered configuration of the circulation channel 90, at least a portion of the circulation channel 90 which trails the leading surface 70 includes a flow through area smaller than the inlet area generally at the leading surface 70. The reduction may be abrupt, varied, or gradual, with the reduced flow through area being at any point along the length of the circulation channel, in the illustrate embodiment, the circulation channel includes a greater inlet area than an exit area, which of course, may or may not be directly at the trailing surface. Accordingly, as the flow converges into and passes quickly through the circulation channel 90 the extrusion mixture tends to exhibit a maximum swell upon exiting the circulation channel 90, a characteristic that also tends to disrupt the flow patterns and increases a mixing and circulation of the extrusion mixture. As a separate characteristic, or additionally, as in the illustrated embodiment best seen in FIG. 3, at least one and preferably two of the edges of the circulation channel 90, such as the left and right edges 101 and 103 are inwardly tapered to achieve the above-described jet effect and flow convergence, while at least one, but preferably two of the edges, such as the top and bottom edges 100 and 102, are generally flat. The effect of the flat edges is to generally result in a slippage and erratic shooting through the circulation channel 90, creating flow vortices and disrupting the flow on the trailing surface side of the screw flight. Indeed, as a result of the above recited, preferred asymmetrical inlet area structure of the circulation channel, increased homogenization of the extrusion mixture is achieved, and in the case of the foam cooling screw, a greater circulation for more even cooling of the extrusion mixture is achieved. In an alternative, less preferred embodiment of the circulation channel 90', as illustrated in FIG. 6, an inward taper may be provided from both the leading surface 70 and the trailing surface 72 so as to funnel the extrusion mixture through the circulation channels 90'.

Lastly, and as previously indicated, the perimeter edge 80 of the screw flight 64 is disposed in substantially close spaced relation with the interior surface 51 the foam extrusion barrel 50 in order to minimize seepage therebetween as the screw flight 64 rotates and the foam extrusion mixture is urged forward. In the illustrated preferred embodiment, and so as to achieve an improved and more effective rotation, the perimeter edge 80 of the screw flight 64 is preferably beveled with a trailing portion 84 angled away from the interior surface 51 of the barrel 50 and a leading portion 82 generally confronting and corresponding the contour of the interior surface 51 of the barrel 50. Such a configuration achieves the necessary scraping effect, while also minimizing the surface area that comes into close proximal contact with the interior surface 51 of the barrel 50, to result in the reduction of potential heat shearing.

Since many modifications, variations and changes in detail can be made to the described preferred embodiment of the invention, it in intended that all matters in the foregoing description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents.

Now that the invention has been described,