Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2000-10-16
2003-08-26
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S090000
Reexamination Certificate
active
06609818
ABSTRACT:
1. Field of the Invention
The present invention relates to a thermoplastic extrusion assembly which provides a substantially homogeneous product, preferably a foam thermoplastic product, and which produces said product at a desired extrusion temperature 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, or 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 exits 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 to 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, and 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 by 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, foam 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 due 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 extrus
Cooley Charles E.
Malloy & Malloy P.A.
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