The present invention relates generally to polymeric foam processing, and more particularly to systems, methods, and articles for manufacturing polymer foams, including microcellular foams.
Microcellular foam is typically defined as having cell sizes of less than 100 microns and a cell density of greater than 106 cells/cm3 of the original solid material. Generally, the requirements for forming microcellular foams include creating a single-phase solution of polymeric material and physical blowing agent, and subjecting the solution to a thermodynamic instability to create sites of nucleation of very high density which grow into cells.
Methods for molding microcellular material have been described. U.S. Pat. No. 4,473,665 (Martini-Vvedensky) describes a molding system and method for producing microcellular parts. Polymeric pellets are pre-pressurized with a gaseous blowing agent and melted in a conventional extruder to form a solution of blowing agent and molten polymer, which then is extruded into a pressurized mold cavity. The pressure in the mold is maintained above the solubility pressure of the gaseous blowing agent at melt temperatures for the given initial saturation. When the molded part temperature drops to the appropriate critical nucleation temperature, the pressure on the mold is dropped, typically to ambient, and the part is allowed to foam.
U.S. Pat. No. 5,158,986 (Cha et al.) describes an alternative molding system and method for producing microcellular parts. Polymeric pellets are introduced into a conventional extruder and melted. A blowing agent of carbon dioxide in its supercritical state is established in the extrusion barrel and mixed to form a homogenous solution of blowing agent and polymeric material. A portion of the extrusion barrel is heated so that as the mixture flows through the barrel, a thermodynamic instability is created, thereby creating sites of nucleation in the molten polymeric material. The nucleated material is extruded into a pressurized mold cavity. Pressure within the mold is maintained by counter pressure of air. Cell growth occurs inside the mold cavity when the mold cavity is expanded and the pressure therein is reduced rapidly; expansion of the mold provides a molded and foamed article having small cell sizes and high cell densities. Nucleation and cell growth occur separately according to the technique; thermally-induced nucleation takes place in the barrel of the extruder, and cell growth takes place in the mold.
The use of check valves, including ring check valves, is known in injection molding to prevent the molten plastic accumulated at the distal end of a reciprocating screw from flowing backwards during an injection of the plastic into a mold.
The following U.S. Patent Applications describe typical check valve configurations used in plastic processing systems. U.S. Pat. No. 4,512,733 (Eichlseder et al.) describes a check valve on the end of a plastifying screw for an injection molding apparatus. The check valve comprises a valve housing and an axial displacable valve member that is received in this housing.
U.S. Pat. No. 5,164,207 (Durina) describes a plastic extruder having a rotating screw within a cylindrical shell which is used to feed molten plastic to a high pressure injection molding apparatus. An automatic shut off valve is mounted at the forward end of the screw. During the extrusion step, the valve is forced open to allow molten plastic to flow from the extruder to the injection molder. The valve automatically closes under the action of a spring during the high pressure injection molding operation to prevent backflow of plastic through the extruder.
U.S. Pat. No. 5,258,158 (Dray) describes a positive type non-return valve that is used to positively stop the reverse flow of material in injection molding machines. The valve can be connected at a downstream end of the screw with a thread, or can also be an integral part of the screw. The valve allows material to pass when the screw is rotating, but closes when the screw translates forward, as in an injection molding cycle, with no screw rotation.
While the above and other reports represent several techniques and systems associated with the manufacture of foam material and microcellular material, a need exists in the art for improved systems for foam processing, and in particular for microcellular foam processing.
It is, therefore, an object of the invention to provide systems, methods, and articles useful in the production of microcellular foams, and also useful in the production of conventional foams.
The present invention is directed to systems, methods, and articles useful in the production of foams, and in particular, microcellular foams. The systems include a restriction element that reduces the backflow of polymer melt in an extruder while injecting polymeric material into a mold or ejecting polymeric material through a die. The restriction element is positioned upstream of a blowing agent injection port to maintain the solution of polymer and blowing agent in the extruder above a minimum pressure, and, preferably, above the pressure required for the maintenance of a single-phase solution of polymer and blowing agent. The systems can be used in injection molding, blow molding, or in any other processing techniques that accumulate and inject polymeric material into a mold or eject polymeric material from a die. In some embodiments, the systems utilize reciprocating screws for injection or ejection or, in other embodiments, the systems include an accumulator connected to an outlet of the extruder, in which a plunger moves to inject or eject polymeric material.
In one aspect of the invention, a system is provided for processing polymeric material operable to cyclically inject polymeric material into a mold or eject polymeric material from a die. The system includes a barrel having an upstream direction and a downstream direction and a polymer processing screw constructed and arranged to rotate within the barrel to convey polymeric material in a downstream direction within a polymer processing space defined by the barrel and the screw. The system also includes a blowing agent port connecting to the polymer processing space and positioned to introduce a blowing agent into polymeric material in the polymer processing space to allow formation therein of a solution of polymer and blowing agent. The system also includes a restriction element positioned within the polymer processing space upstream of the blowing agent port that restricts the upstream flow of polymeric material therethrough during at least a portion of an injection or an ejection cycle.
In another aspect of the invention, a system is provided for processing polymeric material. The system includes an extruder including a screw constructed and arranged to rotate within a barrel to convey polymeric material in a downstream direction within a polymer processing space. The extruder has a first inlet for receiving a precursor of foamed polymeric material, an outlet to deliver a solution of polymeric material and blowing agent from the extruder, and a blowing agent inlet positioned downstream of the first inlet and upstream of the outlet to introduce a blowing agent into the polymeric material in the polymer processing space to allow formation therein a solution of polymer and blowing agent. The extruder is constructed and arranged to maintain the solution of polymer and blowing agent in the polymer processing space between the blowing agent inlet and the extruder outlet at a pressure of at least 1000 psi throughout an injection or an ejection cycle.
In another aspect of the invention, a polymer processing screw is provided. The polymer processing screw is constructed and arranged to rotate within a barrel of a polymer processing system to convey polymeric material in a downstream direction within a polymer processing space defined by the barrel and the screw, and to reciprocate within the barrel between an accumulation position and an injection position. The polymer processing screw includes a blowing agent receiving section, and a restriction element constructed and arranged upstream of the blowing agent receiving section to restrict upstream flow of polymeric material therethrough during at least a portion of an injection or an ejection cycle.
In another aspect of the invention, a method of processing polymeric material is provided. The method includes the steps of conveying polymeric material in a downstream direction within a polymer processing space between a polymer processing screw and a barrel, introducing a blowing agent into the polymeric material in the polymer processing space through a blowing agent port and forming therein a solution of polymer and blowing agent, and restricting the upstream flow of polymeric material through at a location upstream of the blowing port during at least a portion of an injection or an ejection cycle.
In another aspect of the invention, a method of processing polymeric material is provided. The method includes the steps of conveying polymeric material in a downstream direction within a polymer processing space defined between a screw and a barrel of an extruder, and introducing a blowing agent into the polymeric material within the barrel through a blowing agent inlet to form a solution of polymer and blowing agent therein. The method further includes maintaining the solution of polymer and blowing agent at a pressure of at least 1000 psi within the polymer processing space between the blowing agent inlet and an outlet of the extruder throughout an injection or an ejection cycle.
In preferred embodiments in each of the above aspects, the restriction element is constructed and arranged to restrict the upstream flow of polymeric material therethrough to maintain the polymeric material downstream of the restriction element at a pressure greater than the critical pressure required for a single-phase solution of polymeric material and blowing agent. In certain preferred embodiments, the restriction element is a ring-check valve. In some preferred cases, the ring-check valve is spring-loaded.
Among other advantages, the restriction element restricts backflow (upstream flow) of polymeric material and maintains the downstream pressure of the polymer and blowing agent solution throughout an injection or an ejection cycle. This enables the single-phase solution of polymer and blowing agent formed during microcellular processes to be continuously maintained in the extruder. Because microcellular processing requires the maintenance of the single-phase solution, the restriction element is particularly useful in forming microcellular foam.
The restriction element is intended to be used to maintain pressure in systems that include reciprocating screws for injection or ejection, and also in systems that have an external accumulator that utilize a plunger for injection or ejection. In typical processing systems that do not include the restriction element and involve the injection or ejection of polymeric material, it is difficult, if not impossible, to maintain pressure throughout an injection or ejection cycle. For example, in systems using reciprocating screws for injection, polymeric material will typically flow backwards when the screw reciprocates in a downstream direction to inject material which results in a pressure drop in the polymeric material in the extruder oftentimes below that required for the maintenance of the single-phase solution. In other systems that use an accumulator external of an extruder, the pressure typically drops when the screw idles during injection.
The restriction element is advantageously located upstream of the blowing agent injection port so that the entire solution of polymer and blowing agent is maintained at high pressures. This location distinguishes from other valves located at a distal end the screw that only prevent the backflow and pressure drop in accumulated polymeric material downstream of the screw and, thus, would not be effective in maintaining the entire solution of polymer and blowing agent at high pressures.
In certain embodiments of the invention, the restriction element permits limited upstream flow of polymeric material therethrough. This limited upstream flow can prevent unsafe, high-pressures from arising during injection, but is not significant enough to reduce the pressure downstream of the restriction to that below that required for the maintenance of the single-phase solution.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although methods and systems similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and systems are described below. All publications, patent applications patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the systems, methods, and examples are illustrative only and not intended to be limiting.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention, and from the claims.
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