Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/34—Auxiliary operations
  • B29C44/3442—Mixing, kneading or conveying the foamable material
  • B29C44/3446—Feeding the blowing agent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/34—Auxiliary operations
  • B29C44/3442—Mixing, kneading or conveying the foamable material
  • B29C44/3446—Feeding the blowing agent
  • B29C44/3449—Feeding the blowing agent through the screw
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/34—Auxiliary operations
  • B29C44/3442—Mixing, kneading or conveying the foamable material
  • B29C44/3446—Feeding the blowing agent
  • B29C44/3453—Feeding the blowing agent to solid plastic material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/92—Measuring, controlling or regulating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92009—Measured parameter
  • B29C2948/92019—Pressure
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92009—Measured parameter
  • B29C2948/92209—Temperature
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92323—Location or phase of measurement
  • B29C2948/92361—Extrusion unit
  • B29C2948/9238—Feeding, melting, plasticising or pumping zones, e.g. the melt itself
  • B29C2948/924—Barrel or housing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92504—Controlled parameter
  • B29C2948/92514—Pressure
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92504—Controlled parameter
  • B29C2948/92571—Position, e.g. linear or angular
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92504—Controlled parameter
  • B29C2948/92609—Dimensions
  • B29C2948/92657—Volume or quantity
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92504—Controlled parameter
  • B29C2948/92704—Temperature
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92819—Location or phase of control
  • B29C2948/92828—Raw material handling or dosing, e.g. active hopper or feeding device
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92819—Location or phase of control
  • B29C2948/92857—Extrusion unit
  • B29C2948/92876—Feeding, melting, plasticising or pumping zones, e.g. the melt itself
  • B29C2948/92885—Screw or gear
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C2948/00—Indexing scheme relating to extrusion moulding
  • B29C2948/92—Measuring, controlling or regulating
  • B29C2948/92819—Location or phase of control
  • B29C2948/92857—Extrusion unit
  • B29C2948/92876—Feeding, melting, plasticising or pumping zones, e.g. the melt itself
  • B29C2948/92895—Barrel or housing
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T137/00—Fluid handling
  • Y10T137/7722—Line condition change responsive valves
  • Y10T137/7758—Pilot or servo controlled
  • Y10T137/7762—Fluid pressure type

Definitions

• the present invention relates generally to polymer foam processing and, more particularly, to systems and methods for introducing blowing agent into polymeric material in a polymeric foam process.
• Polymeric materials are processed using a variety of techniques. Many techniques employ an extruder which includes a polymer processing screw that rotates within a barrel to plasticate polymeric material.
• Some processing techniques such as injection molding and blow molding, may be discontinuous. That is, during operation, the screw does not plasticate polymeric material continuously. Discontinuous processes may have repetitive cycles which include a plastication period, in which the screw rotates and polymeric material is accumulated, followed by an injection (or ejection) period, in which the screw does not rotate and the accumulated polymeric material is injected into a mold (or ejected' through a die).
• Polymeric foam materials including microcellular materials, may be processed by introducing a physical blowing agent into the polymeric material within the extruder through a blowing agent port in the barrel.
• Many conventional blowing agent introduction systems introduce blowing agents continuously into the polymeric material within the barrel. In discontinuous processes, including certain injection molding and blow molding processes, such continuous introduction systems may cause a lack of control over the percentage of blowing agent injected into the polymeric material and may lead to an uneven distribution of the blowing agent in the polymeric material.
• the polymeric material in the vicinity of the blowing agent port when the screw ceases to plasticate polymeric material, may contain higher amounts of blowing agent because of its increased residence time in proximity to the blowing agent injection port.
• blowing agent may result in viscosity variations within the polymeric material in the extruder which can cause output inconsistencies and other problems. Such effects may generally reduce control over the process and may narrow the processing window.
• conventional blowing agent introduction systems may be adequate.
• the conventional introduction systems may impair the process for one or more of the reasons described above.
• certain processes for producing small molded articles and/or microcellular foam articles may be adversely affected if the blowing agent is not precisely controlled.
• the invention provides blowing agent introduction systems, methods for introducing blowing agent into a polymeric foam processing system, and polymer processing systems that include blowing agent introduction systems.
• the invention includes a blowing agent introduction system.
• the blowing agent introduction system includes an accumulator having an inlet connectable to a source of blowing agent and an outlet connectable to a blowing agent port of an extruder.
• the system further includes a pressure regulating device positioned between the source of blowing agent and the inlet of the accumulator.
• the pressure regulating device is designed to control the pressure of blowing agent delivered to the accumulator.
• the system further includes a pressure measuring device constructed and arranged to measure pressure of polymeric material in the extruder.
• the system further includes a control system capable of receiving a first input signal from the pressure measuring device representative of the pressure of polymeric material in the extruder and sending a first output signal to the pressure regulating device to control the pressure of blowing agent delivered to the accumulator to a value greater than the pressure of polymeric material in the extruder.
• the invention in another set of embodiments, includes a blowing agent introduction system.
• the system includes an accumulator having an inlet connectable to a source of blowing agent and an outlet connectable to a blowing agent port of an extruder.
• the system further includes a pressure regulating device positioned between the source of blowing agent and the inlet of the accumulator.
• the pressure regulating device is designed to control the pressure of blowing agent delivered to the accumulator.
• the system further includes an outlet valve associated with the outlet of the accumulator, and a pressure measuring device constructed and arranged to measure pressure of polymeric material in the extruder.
• the system further includes a control system capable of receiving a first input signal from the pressure measuring device representative of the pressure of polymeric material in the extruder and sending a first output signal to open the outlet valve when the pressure of blowing agent delivered to the accumul ator is greater than the pressure of polymeric material in the extruder.
• the invention includes a blowing agent introduction system in another set of embodiments.
• the system includes an accumulator having a substantially fixed volume, an inlet connectable to a source of blowing agent and an outlet connectable to a blowing agent port of an extruder.
• the system further includes a control system capable of adjusting pressure of blowing agent in the accumulator in response to pressure of polymeric material in the extruder.
• the invention provides a system.
• the system includes an extruder including a screw designed to rotate within a barrel to plasticate polymeric material during a plastication period of a molding cycle.
• the extruder has a blowing agent port.
• the system further includes an accumulator having an inlet connectable to a source of blowing agent and an outlet connectable to the blowing agent port.
• the system further includes an outlet valveassociated with the outlet of the accumulator, and a control system capable of receiving an input signal representative of the start of the plastication period and sending an output signal to open the outlet valve.
• the invention includes a method.
• the method includes the steps of plasticating polymeric material in an extruder, introducing a first dose of blowing agent into the polymeric material while plasticating the polymeric material in a first plastication period of a molding cycle, introducing a second dose of blowing agent into the polymeric material while plasticating the polymeric material in the first plastication period of the molding cycle, and injecting a mixture of blowing agent and polymeric material into a mold.
• the invention includes a method of introducing blowing agent into polymeric material.
• the method includes the steps of plasticating polymeric material in an extruder, and sequentially introducing blowing agent into the , polymeric material during a plastication period of a first molding cycle at a first rate, a second rate, and a third rate.
• the second rate has a minimum value that is less than 50% of the smaller of the maximum values of the first and third rates.
• the invention includes a method of introducing blowing agent into polymeric material.
• the method includes the steps of measuring pressure of polymeric material in an extruder and controlling pressure of blowing agent in an accumulator to a value between about 50 psi and about 1000 psi greater than the pressure of polymeric material in the extruder.
• the method further includes introducing blowing agent from the accumulator into the polymeric material in the extruder.
• Fig. 1 schematically illustrates a blowing agent introduction system used to introduce blowing agent into an extruder of an injection molding system
• FIG. 2 schematically illustrates another blowing agent introduction system used to introduce blowing agent into an extruder of an injection molding system
• Fig. 3 schematically illustrates another blowing agent introduction system used to introduce blowing agent into an extruder of an injection molding system
• Fig. 4 schematically illustrates a section of an extruder barrel including multiple blowing agent ports connected to a conduit of a blowing agent introduction system
• Fig. 5 schematically illustrates a valve assembly of the blowing agent introduction system positioned in an extruder barrel
• Figs. 6A and 6B are copies of SEM photos that show representative cross-sections of parts produced in Example 3;
• Figs. 7A and 7B are copies of SEM photos that show representative cross-sections of parts produced in Example 3;
• Figs. 8A and 8B are copies of SEM photos that show representative cross-sections of parts produced in Example 3.
• the present invention provides blowing agent introduction systems and methods for introducing blowing agent into polymeric foam processing systems.
• the methods may involve introducing one or more doses of blowing agent into polymeric material in an extruder during a plastication period of a molding cycle.
• a dose Prior to introduction, a dose is confined in an accumulator which, for example, has a volume defined between an inlet valve and an outlet valve.
• a control system may receive inputs related to system parameters (e.g., pressure of polymeric material in extruder, axial position of screw in barrel, whether screw is rotating, and the like) and based on the inputs coordinates operation of the valves to confine and introduce the blowing agent dose(s).
• the introduction systems may be used in discontinuous plasticating processes, such as injection molding and blow molding, and can be particularly useful in processes that use precise quantities of blowing agent.
• blowing agent introduction system 10 is shown connected to an extruder 12 of an injection molding system 14.
• Introduction system 10 introduces blowing agent from a source 22 through a blowing agent port 24 into polymeric material in extruder 12.
• Conduit 26 connects the blowing agent source to various components of the introduction system and to the blowing agent port.
• the introduction system includes a pump 85 to increase the pressure of blowing agent, a high pressure tank 83 to store blowing agent, and a pressure regulating device (e.g., a pressure reducing regulator 81) to regulate blowing agent downstream of the device to a fixed value.
• a pressure regulating device e.g., a pressure reducing regulator 81
• the introduction system also includes an accumulator 16 having a volume defined between an inlet valve 28 and an outlet valve 30.
• the accumulator includes a chamber 20 which defines the majority of the volume of the accumulator. In ot her embodiments, the chamber may not define the majority of the accumulator volume.
• Inlet valve 28 is positioned between the blowing agent source and the accumulator to permit/prevent the flow of blowing agent into the accumulator and outlet valve 30 is positioned between the accumulator and the blowing agent port to control the flow of blowing agent to the blowing agent port. In some cases and as shown, it may be preferable to position the outlet valve proximate to port 24.
• a control system 29 which, as described further below, may receive input signals from an injection molding controller 31 and/or introduction system 10 and send output signals to open and close valves 28, 30, thus, controlling the operation of the introduction system.
• the control system 29 may receive inputs from components of the injection molding system.
• the introduction system may have a variety of different configurations in other embodiments of the present invention.
• the accumulator may not include a chamber.
• the introduction system may not include certain components such as a pump or a high pressure tank.
• the pressure regulating device may be a back pressure regulator as shown in Fig. 2 and described further below.
• Injection molding system 14 may be any suitable type known in the art. Examples of suitable injection molding systems have been described, for example, in International Publication No. WO 98/31521, entitled “Injection Molding of Microcellular Material,” by Pierick et. al. which is incorporated herein by reference.
• the injection molding system includes extruder 12 having an outlet 33 fluidly connected to an injection mold 32.
• the extruder includes a polymer processing screw 34 that is mounted within barrel 36. Polymer processing screw may be rotated and moved axially in a downstream direction, for example, by a motor 37. Polymeric material, typically in pelletized form, is fed into barrel 36 from a hopper 38 through an orifice 40.
• Screw 34 and barrel 36 define therebetween a polymer processing space 42 in which polymeric material is conveyed in a downstream direction 44 during the plastication period of the cycle by the rotation of the screw.
• Blowing agent port 24 is formed in the barrel to permit introducing of blowing agent into the polymeric material in the polymer processing space.
• Barrel 36 may be equipped with temperature control units 46 at selective positions along its axial length. Temperature control units 46 may be used to heat the barrel, for example to facilitate melting of pelletized polymeric material, or cool the barrel, for example to increase the viscosity of the polymeric melt.
• Extruder 12 may also include measurement instruments (not shown) such as thermocouples and pressure transducers to monitor, respectively, the temperature and pressure of the polymeric material at various locations along the length of the barrel.
• blowing agent introduction system may be cou pled to the plasticating period of the molding cycle.
• screw 34 is positioned at a downstream end of barrel 36 as shown in Fig. 1 and, typically, a quantity (or "dose") of blowing agent is confined at a relatively high pressure in accumulator 16.
• Screw 34 begins to rotate within barrel 36 to plasticate polymeric material and convey the polymeric material in a downstream direction in the polymer processing space.
• outlet valve 30 may be opened by control system 29.
• the control system for example, may open the outlet valve in response to a specified condition as described further below.
• control system may open the outlet valve when blowing agent pressure in the accumulator is greater than the pressure of polymeric material in the extruder.
• the outlet valve When the outlet valve is opened, the blowing agent in the accumulator expands, thus, passing through the blowing agent port and entering the polymeric material to form a mixture of polymeric material and blowing agent in the polymer processing space.
• the outlet valve After the dose is introduced, the outlet valve is closed.
• the inlet valve is then opened, for example by a signal from the control system, which allows blowing agent to flow into the accumulator.
• the inlet valve may then be closed to confine a dose of blowing agent in the accumulator between the closed inlet and outlet valves.
• multiple blowing agent doses may be introduced during a single plastication period.
• the polymeric material and blowing agent mixture is conveyed downstream by the rotating screw and accumulated in a region 52 within the barrel downstream of the screw.
• the accumulated mixture creates a pressure that causes the screw to retract axially in an upstream direction in the barrel.
• plastication continues until a desired quantity of mixture (or shot) is accumulated.
• the amount of blowing agent introduced during a plastication period may be controlled to create a shot having the desired weight percentage of blowing agent.
• the amount of blowing agent may be controlled by introducing multiple blowing agent doses in a single plastication period or controlling the dose size (e.g., by accumulator volume and/or pressure of blowing agent in accumulator and/or temperature of blowing agent in the accumulator).
• control system 29 may receive input signals from components of the molding system and/or send output signals to open and/or close valves 28, 30 and to control the operation of other components (e.g., pressure regulating device 81) of the introduction system.
• control system 29 receives input signals from a temperature measuring device 94 positioned within accumulator 20.
• the controller may determine the mass of blowing agent confined in the accumulator (i.e., dose size) from the blowing agent temperature and other inputs (e.g., blowing agent pressure).
• the temperature measuring device as shown is positioned within chamber 20, although it should be understood that it may be positioned elsewhere in the accumulator.
• the control system receives an input signal from a pressure measuring device 86 related to the pressure of polymeric material in the polymer processing space in the vicinity of the blowing agent port. In the embodiment of Fig. 1, pressure measuring device 86 is inserted through an instrument port 88 formed in the barrel proximate the blowing agent port.
• instrument port 88 may be positioned within about 12 inches, within about 5 inches, or within about 1 inch from blowing agent port 24.
• the pressure measuring device may indirectly measure the pressure of polymeric material in the vicinity of the blowing agent port, for example, by measuring or determining a pressure elsewhere in the system (e.g., hydraulic pressure at back of the screw) and inferring or calculating the pressure of the polymeric material.
• the control system may send an output signal to a component of the introduction system to control the pressure of blowing agent in accumulator 16.
• the control system may send an output signal that controls the pressure of blowing agent in the accumulator in response to the input signal from pressure measuring device 86.
• the pressure of blowing agent in the accumulator may be adjusted to a desired value by sending an output signal, for example, to a pressure regulating device (e.g., pressure reducing regulator 81, Fig. 1; back pressure regulator 19, Fig. 2).
• the pressure regulating device fixes the pressure of blowing agent delivered to the accumulator when inlet valve 28 is opened.
• the pressure of blowing agent in the accumulator may be adjusted by sending an output signal to other components capable of fixing the pressure delivered to the accumulator.
• the control system can set the pressure of the blowing agent supplied to the accumulator at a value greater than the pressure of polymeric material in the extruder in the vicinity of the blowing agent port.
• the control system sets the blowing agent pressure greater than the polymeric material pressure by at least a critical amount . The amount is selected so as to ensure there is sufficient driving force to introduce the dose of blowing agent and to maintain blowing agent pressure greater than the polymeric material pressure throughout introduction even when the polymeric material pressure fluctuates in the extruder.
• the control system may send an output signal that sets the blowing agent pressure in the accumulator at least 50 psi greater than the polymeric material pressure in the vicinity of the blowing agent port.
• control system may send an output signal that sets the blowing agent pressure in the accumulator at least 200 psi, or even at least 400 psi, greater than the polymeric material pressure in vicinity of the blowing agent port.
• Higher pressure differences may be desirable in certain processes such as those that are characterized by having relatively high pressure fluctuations in the extruder. Higher pressure differences may also increase the rate of blowing agent introduction.
• the control system sets the pressure of the blowing agent supplied to the accumulator greater than the pressure of polymeric material in the extruder but less than a critical amount. If the pressure difference between the pressure of blowing agent in the accumulator and the pressure of polymeric material in the extruder is too great than the molding process may be negatively effected. For example, if the pressure difference is too high than the blowing agent may not be properly mixed in the extruder which can effect the quality of molded parts. Furthermore, if the pressure difference is too high than blowing agent may cause backflow in the extruder. These effects may impair process stability.
• control system may send an output signal that sets the blowing agent pressure in the accumulator greater than the polymeric material pressure in the vicinity of the blowing agent port by a value less than 1000 psi greater than the polymeric material pressure.
• control system sets the blowing agent pressure in the accumulator greater than the polymeric material pressure in the vicinity of the blowing agent port by a value less than 500 psi greater than the polymeric material pressure.
• the specific pressure difference may depend on the process. For example, some processes may be more sensitive to higher pressure differences.
• the controller sets the blowing agent pressure in the accumulator greater than the polymeric material pressure within a critical range.
• the lower limit of the range may be any of the values disclosed above (e.g., 50 psi, 200 psi, and 400 psi) and the upper limit of the range may be any of the values disclosed above (e.g., 500 psi or 1000 psi).
• the controller may set the blowing agent pressure in the accumulator between 50 psi and 1000 psi greater than the pressure in the polymeric material in the extruder in the vicinity of the blowing agent port.
• the controller sets the blowing agent pressure in the accumulator between 50 psi and 500 psi greater than the pressure of the polymeric material in the extruder in the vicinity of the blowing agent port.
• the control system does not need to send an output signal to a component of the introduction system to control the pressure of blowing agent in the accumulator.
• a user may manually set the pressure delivered to the accumulator, for example, by manually setting the pressure regulator.
• the blowing agent may be provided to the accumulator (e.g., by source 22) at a sufficient pressure without the need for adjustment.
• the control system may send an output signal that opens outlet valve 30 to introduce blowing agent into the polymeric material.
• the introduction system includes a pressure measuring device 90 to confirm when the blowing agent pressure in the accumulator has reached its desired value. In these cases, pressure measuring device 90 may send input signals to the control system. In response to these input signals, the control system sends an output signal to open the outlet valve when the appropriate pressure condition has been achieved.
• the introduction system may not include a device that measures pressure in the accumulator.
• the pressure in the accumulator is assumed to be a certain value, for example, the value set by the pressure regulating device. Elimination of pressure measuring device 90 may advantageously simplify the design of the introduction system in certain cases.
• the device When pressure measuring device 90 is present, the device may be inserted via an instrument port in chamber 20 of the accumulator. It should also be understood that the pressure measuring device 90 may be inserted into other regions (e.g., conduit 26) of the accumulator.
• pressure measuring device 90 may indirectly measure the pressure of the blowing agent port, for example, by measuring or determining a pressure elsewhere in the system and inferring or calculating the pressure of the blowing agent.
• control system 29 may send an output signal to open the outlet valve in response to an input signal from injection molding controller 31 (or from components of the injection molding system) related to the onset of the plastication period of the molding cycle (e.g., based on axial position of the screw or time after onset of screw rotation) instead of in response to an input signal related to the pressure of polymeric material as described above.
• injection molding controller 31 or from components of the injection molding system
• control system 29 may receive input signals from the injection molding controller 31 related to the onset of the plastication period and receive input signals related to the pressure of polymeric material in the extruder.
• the control system when the control system receives an input signal related to the onset of the plastication period, receives an input signal related to the axial position of the screw within the barrel, for example, as the screw retracts during plastication.
• the control system may, in turn, send an output signal that opens the outlet valve when the screw retracts to a given axial position in the barrel.
• the retraction distance prior to the outlet valve opening is generally long enough to ensure that the pressure in the extruder has relatively stabilized.
• the control system sends an output signal that opens the valve when the screw has retracted at least 0.5 inches from its original position. In other cases, the control system sends an output signal that opens the valve when the screw has retracted at least 0.1 inches.
• the screw retraction distance at which the output signal is sent depends on a variety of factors (e.g., total retraction distance throughout cycle) and other retraction distances may be suitable.
• the control system receives an input signal related to the onset of screw rotation.
• the control system may, in turn, send an output signal that opens the outlet valve when the screw begins to rotate or, more typically, a specified time period thereafter.
• the specified time period is generally long enough to ensure that pressure in the extruder has relatively stabilized.
• the control system sends an output signal that opens the valve at least 0.5 seconds, or at least 1.0 seconds, after the onset of screw rotation. Other times are also suitable.
• the control system may also send an output signal to close outlet valve 30.
• the outlet valve is typically closed after the dose of blowing agent has been introduced into the polymeric material. That is, the outlet valve is typically closed after blowing agent pressure in the accumulator has decreased to approximately equal the pressure of polymeric material in the extruder. However, it should be understood that in some cases, the outlet valve may be closed prior to the blowing agent pressure becoming approximately equal to the pressure of polymeric material in the extruder.
• the control system sends an output signal to close the outlet valve a specified time period after the outlet valve has been open. That is, the control system opens the valve for a specified time period.
• the specified time period may, for example, be 0.5 seconds or more.
• the specified time period may be 1.0 seconds or more; in other cases, the specified time period may be 5.0 seconds or more. Other specified time periods are also suitable. The specified time period depends on a variety of factors including dose size, number of doses, and plastication period time, amongst others.
• control system may send an output signal to close the outlet valve based on other system parameters.
• control system may send an output signal to close the outlet valve because of a pressure condition in the extruder or in the accumulator.
• the control system also typically controls the operation of inlet valve 28.
• the inlet valve is closed when outlet valve 30 is open.
• the control system may send a signal that opens the inlet valve.
• the control system also sends a signal to regulate or control blowing agent pressure supplied to the accumulator (e.g., by use of a pressure regulating device).
• the inlet valve may be opened after the desired supply pressure is reached. Once the inlet valve is open, blowing agent may flow into the accumulator. Once the desired dose has been accumulated, the inlet valve may be closed by the control system to confine the blowing agent. In some cases, the inlet valve may be left open even after the dose has been accumulated and then closed immediately prior to opening the outlet valve.
• control system 29 sends multiple open/close signals to valves 28 and 30 during each plastication period, allowing several doses of blowing agent to be delivered.
• the control system may cause at least 2, 3, 5, 7, 10, 15, or 20 doses of blowing agent to be delivered during a given plastication period.
• the number of doses introduced during a single plastication period depends on a number of factors including the desired weight percentage of blowing agent in a shot, the shot size, and the dose size. It should be understood that certain processes may utilize a single dose or any other suitable number of doses.
• multiple doses allows for increased control over the total amount of blowing agent introduced into the polymeric material in a given shot.
• the increased control arises, in part, from using higher accumulator pressures which reduces inaccuracies resulting from polymeric material pressure variations.
• higher accumulator pressures result in improved blowing agent introduction into the polymeric material.
• the increased control enables precise quantities of blowing agent to be introduced in a given shot.
• This precision may be important in certain processes that produce shots having relatively small amounts of blowing agent (e.g., less than about 2% of CO2 or less than 0.1% of 2), and/or produce relatively small molded articles (e.g., less than 15 g) and/or produce microcellular foam articles. Introducing multiple doses throughout a plastication period may also increase the uniformity of blowing agent distribution within a shot.
• blowing agent e.g., less than about 2% of CO2 or less than 0.1% of 2
• relatively small molded articles e.g., less than 15 g
• the desired blowing agent percentage in a shot depends upon the particular process and the blowing agent type.
• the desired blowing agent percentage may, for example, be less than about 5% by weight of polymeric material and blowing agent mixture.
• the level may be less than about 4% and, in others, less than about 2% by weight of polymeric material and blowing agent mixture.
• N2 is used as the blowing agent, for example, the level may be less than about 0.5%, less than about 0.3%, or less than about 0.1% by weight of polymeric material and blowing agent mixture.
• the introduction system of the invention may be particularly well suited to forming blowing agent mixtures at low weight percentages of blowing agent.
• the shot size also depends upon the particular process and, for example, may be between about 1 g and about 200 g.
• the introduction system may be particularly well suited to introduce blowing agent into relatively small shot sizes, for example, shot sizes of less than 15 g. In some cases, the shot sizes may be between about 1 g and 15 g.
• the dose size is typically measured by mass.
• the dose size is the mass of blowing agent confined in the accumulator that is introduced into the polymeric material. As described above, multiple doses may be introduced in a single plastication cycle. In cases where multiple doses are introduced, the total amount of blowing agent in the shot is the number of doses multiplied by the dose size.
• control system 29 may control the total amount of blowing agent introduced into a shot and/or the number of blowing agent doses introduced into a shot and/or the size of the dose.
• the control system may also receive manual inputs of the desired blowing agent percentage in the mixture of polymer and blowing agent and/or the desired dose size and/or the desired shot size.
• the control system may process the inputs and sends an output signal to control the introduction of blowing agent into the mixture so as to form a mixture having the desired blowing agent percentage.
• introduction system 10 may utilize control system 29 to receive inputs from pressure measuring device 90 and a temperature measuring device 94 related, respectively, to the pressure and temperature of the blowing agent in the accumulator.
• the control system may also receive manual inputs related to the selected fixed mass of the blowing agent to be confined in the accumulator; or, alternatively, may calculate the selected mass of the blowing agent to be confined in the accumulator from other inputs.
• the control system may be programmed to deliver one or more shot sizes, or a desired blowing agent percentage of the blowing agent and polymeric material mixture.
• Control system 29 may also be configured to calculate the volume and/or mass of blowing agent to be confined in the accumulator using the Ideal Gas law or other phase equilibria relation, from the inputs of blowing agent temperature, blowing agent pressure, and selected mass of the blowing agent.
• the volume of accumulator 16 is defined between the inlet valve and the outlet valve. In some cases, the volume of the accumulator may be partially defined within the valve assemblies.
• the accumulator has a substantially fixed or fixed volume in some embodiments. As used herein, a fixed volume refers to a volume that is constant when blowing agent from the accumulator is introduced into the polymeric material and/or constant during the plastication period of a molding cycle.
• the accumulator does not include a wall or plunger assembly that moves when blowing agent is introduced and/or during plastication. Elimination of the movable wall or plunger assembly may be advantageous in certain cases because such designs can complicate construction and operation.
• the accumulator has a volume that is fixed during blowing agent introduction and plastication, but can be adjusted between molding cycles. These types of accumulators also have fixed volumes as defined herein. In one set of embodiments, the accumulator may be adjusted, for example, by adjusting a piston or a screw between cycles that controls the volume of the accumulator, or by replacing the accumulator with another accumulator having a different size.
• the accumulator may not have a fixed volume and may utilize movable walls and/or plungers.
• the volume of the accumulator depends on the specific process and, in particular, on the desired dose size. In some cases, accumulator 16 may have a volume of less than about
• the accumulator volume may be less than about 10 cm ⁇ , less than about 1 cv ⁇ , less than about 0.1 cm*, or less than about 0.01 cm- .
• accumulator 16 includes a chamber 20 having a cross- sectional area greater than that of the conduit.
• the chamber may be in series with the fluidic pathway between valves 28 and 30, as shown in Fig. 1 ; or, the chamber may have any configuration such that it remains in fluidic communication with valves 28 and 30, as illustrated in Fig. 3.
• the chamber may have any size or shape necessary for producing a desired volume and containing a pressurized gas or fluid, for example, rectangular, spherical, or cylindrical.
• the chamber may be any of the kind known in the art able to contain a blowing agent.
• the chamber may be made of a suitable material for containing a pressurized gas, liquid, and/or supercritical fluid, such as a metal tube, preferably made of stainless steel.
• a suitable material for containing a pressurized gas, liquid, and/or supercritical fluid such as a metal tube, preferably made of stainless steel.
• accumulator 16 may be defined by conduit existing between valves 28 and valve 30, as illustrated in Fig. 2.
• Valves 28, 30 may be any device in the art that selectively permits flow of pressurized gas, liquefied gas, or supercritical fluid therethrough in one configuration, and prevents flow of these fluids therethrough in another configuration. Suitable types of valves include solenoid, spool or other equivalent valves, and they may be shut-off valves.
• the valve may include a valve body or assembly, which may define at least a portion of the volume of accumulator 16.
• valves 28 and 30 may be the same type, while in other cases the valves may be of different types.
• outlet valve 30 is positioned near (e.g., less than 12 inches, less than 3 inches or less than 1 inch) or essentially adjacent to blowing agent port 24. In some cases, this arrangement may be preferred to minimize the volume of blowing agent that may be confined in the conduit between the outlet valve and port 24. However, it is to be understood that the position of the outlet valve may be elsewhere relative to blowing agent port 24 so as to control the flow of blowing agent to the blowing agent port.
• outlet valve 30 When positioned near or adjacent blowing agent port 24, outlet valve 30 may be of the type shown in Fig. 5 and described in commonly owned, co-pending, U.S. patent application serial no. 09/710,756, filed November 10, 2000, and entitled “Naive for Injection Molding", and commonly owned, co-pending, U.S. patent application serial no. 09/782,673, filed February 13, 2001, and entitled “Blowing Agent Delivery System", the disclosures of which are incorporated herein by reference.
• Fig. 5 shows outlet valve 30 as part of an injector assembly that is inserted within the blowing agent port, thus, is essentially adjacent to the port. As shown, the outlet valve is formed, in part, from a valve body 168 that is inserted within an injector sleeve 170.
• the illustrated valve includes a valve stem 171 which is actuatable, for example by compressed air, relative to a valve seat 172 to open or close the valve.
• a valve stem 171 which is actuatable, for example by compressed air, relative to a valve seat 172 to open or close the valve.
• the valve stem In the open position (as shown in Fig. 5), the valve stem is separated from the valve seat to provide a pathway that permits blowing agent to flow through an internal passageway 173 of the valve that is connected to conduit 26 (Fig. 1).
• Valve body 168 also may optionally include a back-flow valve.
• the back-flow valve includes a ball check 174 which is upwardly biased and held in position by a spring 175, though other valve constructions may also be used.
• the spring is not required and the ball may move freely in response to the blowing agent and polymeric material.
• the pressure of the blowing agent forces the ball check 174 away from a sealing surface 176 to provide a pathway for blowing agent flow to port.
• Blowing agent port 24 is formed in barrel 36 of the extruder at a location where the polymeric material is generally in a fluid state.
• the blowing agent port connects the blowing agent introduction system to the polymeric material in polymer processing space 42.
• Blowing agent port 24 may be a single port or a plurality of ports arranged in the barrel. When multiple ports are utilized, ports can be arranged radially about the barrel or in a linear fashion along the length of the barrel (Fig. 4). As shown in Fig. 4, an arrangement of ports 24a, 24b, 24c along the length of the barrel can facilitate injection of blowing agent at a relatively constant location relative to the screw when the screw moves axially (in an upstream direction) within the barrel as the mixture of polymeric material and blowing agent is accumulated.
• a plurality of ports 24 may be placed at the 12:00 o'clock, 3:00 o'clock, 6:00 o'clock and 9:00 o'clock positions about the extruder barrel, or in any other configuration as desired.
• shut-off valves 62a, 62b, 62c may be provided at each blowing agent port 24a, 24b, 24c.
• Shut-off valves 62a, 62b, 62c can be individually opened and closed, during the plastication time period, so as to control injection of blowing agent at desired location relative to the position of the screw, for example, to ensure that blowing agent is introduced at blowing agent injection section of the screw.
• One or more valves may be opened at the same time.
• each blowing agent port is connected to the same blowing agent introduction system. However, it should be understood that in some embodiments, each blowing agent port may be connected to a single blowing agent introduction system.
• Blowing agent port 24 may include a single orifice or a plurality of orifices (178, Fig. 5).
• the port may include at least about 2, and some cases at least about 4, and others at least about 10, and others at least about 40, and others at least about 100, and others at least about 300, and others at least about 500, and in still others at least about 700 blowing agent orifices.
• port 24 includes an orifice containing a porous material that permits blowing agent to flow therethrough and into the barrel, without the need to machine a plurality of individual orifices.
• blowing agent port 24 may be located at a blowing agent injection section 48 of the screw during the plastication period of the cycle.
• the blowing agent injection section of the screw may include full, unbroken flight paths. In this manner, each flight, passes or "wipes" the blowing agent port including orifices periodically, when the screw is rotating. This wiping increases rapid mixing of blowing agent and polymeric material in the extruder the result is a distribution of relatively finely divided, isolated regions of blowing agent in the polymeric material immediately upon injection into the barrel and prior to any mixing. This promotes formation of a uniform polymer and blowing agent mixture which may be desired in certain types of polymeric foam processing including microcellular processing.
• the screw Downstream of the blowing agent injection section, the screw may include a mixing section 50 which has highly broken flights to further mix the polymer and blowing agent mixture to promote formation of a uniform mixture within the extruder.
• a preferred uniform mixture in some cases, is a homogeneous, single-phase solution.
• a homogeneous, single-phase solution of polymeric material and blowing agent is accumulated in region 52 and is nucleated as the charge is injected into the mold.
• Source 22 has a larger volume than accumulator 16.
• the source may supply any type of physical blowing agent known to those of ordinary skill in the art including nitrogen, carbon dioxide, hydrocarbons, chlorofluorocarbons, noble gases and the like, or mixtures thereof.
• the blowing agent may be supplied to the chamber in any flowable physical state, for example, a gas, liquid, or supercritical fluid.
• source 22 provides carbon dioxide as a blowing agent.
• source 22 provides nitrogen as a blowing agent.
• solely carbon dioxide or nitrogen may be supplied by the source. Blowing agents that are in the supercritical fluid state after injection into the extruder (and, optionally, before injection as well), in particular supercritical carbon dioxide and supercritical nitrogen, are especially preferred in certain embodiments.
• Conduit 26 may be any of the kind known in the art suitable for transporting a blowing agent.
• the conduit may be a tube made of a suitable material for transporting pressurized gas, liquid, and/or supercritical fluid, such as a metal tube, preferably made of stainless steel.
• the conduits also may be defined by passageways within a block of material, such as drilled passageways within a block of metal such as stainless steel.
• the blowing agent conduits typically have a cross-sectional diameter between about 0.1 mm and about 1.0 cm, though other dimensions are possible.
• the length and configuration of conduits 26 are not constrained and generally depend upon factors such as available manufacturing space, and the layout of the blowing agent introduction system and injection molding system.
• conduit assembly in some embodiments and as illustrated, may have one or more branches, for example, to facilitate connection to accumulator 16 (Fig. 3) or to provide connection to multiple blowing agent ports (Fig. 4).
• Control system 29 and injection molding controller 31 may be any suitable type known in the art. Suitable control systems and controllers have been described, for example, in co-pending, commonly-owned U.S. patent application serial number 09/826,603, entitled “Method and Apparatus for Controlling Foam Molding Processing,” which was filed on 4/05/01 and is incorporated herein by reference. Some systems and processes of the invention do not utilize an injection molding controller.
• Pump 85 may be any suitable type known in the art for increasing the pressure of blowing agent supplied from the source.
• High pressure tank 83 may also be any suitable type known in the art for storing blowing agent.
• Tank 83 has a volume that is less than source 22 but greater than the volume of accumulator 16.
• the high pressure tank is provided to limit the effects of pressure surges into the accumulator when the pump cycles.
• Typical pressures in the tank may be between about 5,000 psi and 10,000 psi (e.g., about 7,000 psi) .
• the pressure regulating device may also be any suitable type known in the art for regulating downstream pressure to a fixed value. In some cases and as shown in Fig. 1, the pressure regulating device is a pressure reducing regulator.
• the pressure at the downstream end of the pressure reducing regulator may be fixed lower than the pressure at the upstream end of the device.
• the pressure at the upstream end of the device may be about 7,000 psi and the pressure at the downstream end of the device may be reduced to about 4,000 psi.
• the fixed lower pressure is the pressure of blowing agent delivered to the accumulator.
• the pressure regulating device may be a back pressure regulator 19 that is positioned within a branch of the conduit as shown in Fig. 2.
• the back pressure regulator provides a fixed upstream pressure and, for example, may have an outlet vented to the environment.
• the fixed upstream pressure is the pressure of blowing agent delivered to the accumulator.
• the pressure regulating device receives input signals from the control system and, in response, fixes the pressure delivered to the accumulator.
• the pressure regulating device must have a suitable design capable of fixing the pressure delivered to the accumulator in response to input signals from the control system.
• blowing agent introduction system 10 is illustrated in conjunction with an injection molding system, it is to be understood that the blowing agent introduction system according to the invention may be used in conjunction with other polymer processing systems including other discontinuous systems and, in particular, blow molding systems. Examples of suitable blow molding systems have been described, for example, in
• the blowing agent introduction systems may be used to form a wide variety of molded polymeric material articles and, in particular, molded foam articles.
• a molded microcellular article may be formed. Suitable microcellular articles have been described in International Publication No. WO 98/31521 (Pierick et al.) which is incorporated herein by reference above.
• the microcellular articles have an average cell size of less than 100 microns, in other embodiments less than 50 microns, in other embodiments less than 25 microns, in other embodiments less than 5 microns, and, in other embodiments, even smaller cell sizes are achievable.
• the example illustrates the use of one embodiment of the invention.
• the injection molding machine used was a 55-ton machine having a 20 mm screw.
• a blowing agent introduction system was connected to the injection molding machine.
• the blowing agent introduction system included an inlet valve, an outlet valve which was incorporated into a blowing agent injector assembly, and a specific length of conduit to establish the accumulator volume in which the charge or dose of the blowing agent was accumulated prior to introduction into the polymer melt, as described below.
• the pressure supplied to the accumulator volume was monitored and controlled using a back-pressure regulator on a T-shaped portion of the conduit with an outlet vented to air.
• the temperature of the tubing was measured using a handheld surface thermocouple device.
• the accumulator volume was calculated as the sum of the injector assembly volume, the inlet valve volume downstream of the shut-off valve seat, and the volume of the conduit connecting the valve to the injector.
• the injector assembly had a volume of 0.18 cm ⁇
• the valve had a volume of 0.41 cm ⁇
• the tubing had a volume of 0.069 c ⁇ v , for a total volume of 0.659 cnr*.
• the mold in the injection-molding machine was used to produce a test polypropylene part having a solid part weight of 1.6 g.
• the nitrogen mass in the control volume introduced into the polymer melt was calculated based on the initial pressure in the accumulator volume, and the ending pressure within the accumulator volume.
• the initial pressure was a pre-set value controlled by the back pressure regulator to be greater than the ending pressure.
• the apparatus did not include a pressure transducer within the control volume, so it was assumed that the ending pressure within the control volume was substantially the same as that of the pressure of the molten polymer.
• the polymer melt pressure was monitored using a pressure transducer located in the molten polymer, adjacent to the injector valve outlet. To introduce the dose of nitrogen, the shut-off valve downstream of the accumulator was opened for one second.
• a single dose of nitrogen was introduced during the plastication period.
• the shut-off valve upstream of the accumulator was opened with a time delay of 0.5 seconds after the shut-off valve downstream of the accumulator was closed to confine an additional dose of nitrogen.
• the process parameters during plastication are shown below.
• the polymer melt pressure at the location for nitrogen injection was measured to be 2310 psi to 2360 psi during screw recovery.
• the nitrogen pressure in the control volume was set to 2550 psi.
• the calculated dose of nitrogen was 6 mg, using a dosage of 0.375% per 1.6 g of plastic weight.
• Example 2 The process was repeated for one hour with no changes to any process setting. Approximately 120 foam parts were produced. The parts throughout the process had similar characteristics. The consistency of the test parts illustrate the stability of the process, and the ability to dose small amounts of nitrogen consistently using one embodiment of the invention.
• Example 2 The consistency of the test parts illustrate the stability of the process, and the ability to dose small amounts of nitrogen consistently using one embodiment of the invention.
• Process parameters in the apparatus in this example were the same as that of Example 1, but the nitrogen pressure in the accumulator volume was set to 2475 psi with a polymer melt pressure of 2350 psi.
• the calculated amount of nitrogen delivered was approximately 3.8 mg, and the temperature was about 250 °F.
• the system parameters were not changed during the run, and the parts were consistently produced for approximately two hours.
• the example illustrates the use of another embodiment of the invention.
• this example illustrates the use of the blowing agent introduction system in connection with a process where the total part weight required a small percentage of the stroke of the machine.
• the stroke required was about 7% of the total machine capacity.
• Typical blowing agent introduction systems cannot be easily operated with consistency at less than 15% of the machine stroke capacity.
• the injection molding machine used was a 88-ton machine having a 30 mm screw.
• the injection molding machine was connected to the blowing agent injection assembly described in Example 1.
• the mold in the injection-molding machine was used to produce a U-shaped part having a solid part weight of 6.5 g.
• the mold had two cavities which were connected by a centered, cold sprue.
• the material used was a glass-filled Nylon 6.
• the nitrogen mass in the control volume introduced into the polymer melt was calculated based on the initial pressure in the accumulator volume, and the ending pressure within the accumulator volume.
• the initial pressure was a pre-set value greater than the ending pressure.
• the apparatus did not include a pressure transducer within the control volume, so it was assumed that the ending pressure within the control volume was substantially the same as that of the pressure of the molten polymer.
• the polymer melt pressure was monitored using a pressure transducer located in the molten polymer, adjacent to the injector valve outlet.
• the inlet valve was closed and the outlet valve downstream of the accumulator was opened for one second. A single dose of nitrogen was introduced during the plastication period.
• the inlet valve of the accumulator was opened with a time delay of 0.3 seconds after the outlet valve of the accumulator was closed to confine an additional dose of nitrogen.
• the first setting was calculated to deliver 9.3 mg (0.14% concentration by wgt) of Nitrogen at a accumulator pressure of 3300 psi.
• the system was adjusted and approximately 20 cycles were run prior to collecting parts.
• the system configuration remained unchanged for approximately one hour.
• High quality foam parts were produced.
• the parts had a density reduction of about 3.1%> relative to the solid plastic.
• the parts had a closed cell structure with an average cell size of about 100 microns.
• Figs. 6A and 6B are copies of SEM photos that show representative cross-sections of the parts.
• the second setting was calculated to deliver 13 mg (.20% concentration by wgt) of Nitrogen at an accumulator pressure of 3400 psi. Again parts were collected for one hour and the process and parts were monitored. High quality parts were produced and the system configuration was stable. The parts had a density reduction of about 4.1%) relative to the solid plastic. The parts had a closed cell structure with an average cell size of about 75 microns. Figs. 7A and 7B are copies of SEM photos that show representative cross-sections of the parts. Two other settings were tested. One was set to deliver 16.5 mg (0.25% concentration by weight) and the other 18.5 mg (0.28% concentration by weight) of Nitrogen with accumulator pressure settings of 3500 and 3600 psi, respectively.
• Figs. 8 A and 8B are copies of SEM photos that show representative cross-sections of the parts.

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