US20070033900A1

US20070033900A1 - Apparatus and method to control the temperature of a melt stream

Info

Publication number: US20070033900A1
Application number: US11/486,787
Authority: US
Inventors: Antoine Rios; Bruce Davis; Paul Gramann
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-07-19
Filing date: 2006-07-14
Publication date: 2007-02-15

Abstract

Disclosed is an apparatus for controlling the temperature of a melt stream, a method of using the apparatus to make variable-density polymeric articles, and the polymeric article formed thereby. The apparatus for making the article includes a row of baffles wherein the baffles include a temperature-control conduit defined or disposed within at least one of the baffles. A polymer melt is passed through the apparatus while the temperature of the baffles is varied (which has the effect of altering the density of certain polymer melts). The process can be used to yield a continuous, monolithic polymeric article of variable density. The apparatus can also be used to control the temperature of the material passing through the apparatus.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is hereby claimed to provisional application Ser. No. 60/700,294, filed Jul. 19, 2005, incorporated herein.

FIELD OF THE INVENTION

The invention is directed to an apparatus that can be used as a static mixer head and/or a temperature regulator in processes requiring temperature control, such as when heating or cooling a melt stream, or other flowing material. The invention is also directed to a continuous method for making composite articles of manufacture, such as composite cores for structural and insulating applications, and the articles of manufacture produced using the method.

BACKGROUND

The transportation, construction, and manufacturing industries are in constant need for a cost-effective composite core material with good mechanical properties, low weight, and efficient insulating capabilities. For instance, composite structural panels to be used in vehicles and in constructing commercial and residential structures should have good thermal-insulating, sound-dampening, and/or shock-absorbing characteristics. For these applications it is common to use sandwich composites having a foamed polymeric core.
Composite sandwiches are commonly used as structural panels in applications where a stiff and lightweight construction is required. FIG. 1 shows the cross-section of a conventional, prior art composite sandwich composed of one middle core and two outer skins. The cores and the skins are fabricated separately and then joined in face-to-face orientation. The resulting sandwich construction is stiffer, less expensive and lighter than an equivalent solid panel. The sandwich construction takes advantage of the fact that the bending stiffness of the resulting panel is proportional to the square of the distance between skins. The relatively low-weight core separates the skins to the greatest extent practicable, which dramatically increases the bending stiffness of the panel, while adding very little additional weight to the panel. Further, when a low-stiffness core is used, such as low-density foam, the core does not directly contribute to the stiffness of the panel. Although a stiffer core can contribute to the bending stiffness of the panel, the main advantage of using a stiffer core is to increase the compression and shear stiffness of the core. Depending on the application, the core also improves the thermal insulating, impact-cushioning, and sound-dampening qualities of the panel. The nature of the core can also help to prevent buckling and wrinkling of the panel. While less expensive than a solid panel, a sandwich-type construction requires a relatively complex series of manufacturing steps to join the skins with the core.
Currently, extruded polymeric composite cores are produced from foamed polymers or from non-foamed polymers with internal reinforcing profiles. Foamed sheets are very commonly used as core materials [1-7].
Extruded high-density structural foams possess desirable mechanical properties; however, they are very heavy. Conversely, softer, low-density foams with higher foaming ratios have lower mechanical properties, yet they are relatively lightweight. Both foam cores are good thermal insulators, but softer foams are better sound insulators and have better load absorption capabilities. Further, extruded non-foamed polymeric cores offer good mechanical properties while remaining lightweight. These cores include internal reinforcement profiles in order to increase stiffness, while maintaining the remainder of the core hollow [1, 7-8]. However, the hollow cavities offer poor thermal insulating capabilities and provide no structural stiffening to the core. Therefore, a secondary process is often required to fill the hollow cores with low-density polymeric foam. The foam acts as a thermal insulator while simultaneously increasing the compressive and impact mechanical properties of the core. Unfortunately, these reinforced foams are very expensive because of the complicated multi-step manufacturing process required to make them.
Other lightweight composite panels are known. For example, composite panels made of plastic, paper and metal have good mechanical and insulating properties [9-12]. However, these types of panels typically require more than one secondary process during their fabrication, such as injecting, cutting, adhering, or other secondary assembling steps. The more secondary processes necessary for manufacture, the greater the cost of the final structural insulating core. In contrast, conventional cores manufactured using continuous processes are extruded foams. These foams possess good insulating properties, but generally poor structural properties. Therefore, there remains a long-felt and unmet need to develop a cost-effective manufacturing technique to produce structural insulating cores in a one-step, continuous process. The continuous process would thus replace the existing, cumbersome, and expensive multi-step processes.
Additionally, in conventional foamed products, as well as many other extrusion processes (foaming and non-foaming), controlling the temperature of the melt is a result-effective variable. For example, olefin-based polymers can be foamed only within limited temperature ranges. If the polymer melt enters and/or exits the die outside of the optimum temperature range, the resulting product will be of lesser quality. In short, in many melt-flow processes (and especially those involving foaming of a polymer melt) the temperature must be controllable within a given range to yield products having uniform physical characteristics. The apparatus described herein allows for precise temperature control of a flowing melt, in both foaming and non-foaming processes.

SUMMARY OF THE INVENTION

The present invention provides an innovative manufacturing process for the continuous fabrication of articles of manufacture, such as lightweight foamed panels of varying density, structural insulating composite cores, and the like. The invention also encompasses the products produced using the process. The proposed manufacturing process utilizes an extruder to produce a panel or other cross-sectional design in a continuous manner, with the panel having varying foam densities and/or varying foam densities and solid sections throughout its thickness. The variability of the foam densities (and/or the positioning of the solid sections) within the panel can be controlled using the method. The controlled density variations within the panel can range between highly foamed to solid (non-foamed) polymers. The controlled density variations in the panel result in a product with excellent structural and insulating properties while remaining extremely lightweight. Exemplary cross-sectional configurations of the products according to the present invention are shown in FIGS. 2A, 2B, 2C, 2D.
The present invention allows the manufacture of articles, such as composite panels, without the need for any secondary processing. Stiffening sections within the article are preferably non-foamed and have a density at or near the density of the unprocessed polymer resin used. The stiffening sections are preferably separated with low-density foamed sections. (Because of their foamed nature, these sections naturally act as thermal and sound insulators.) The low-density foamed sections also add still further stiffness to the article, especially if the article is a large panel or structural core. The density of the foamed sections can be further customized to optimize for desired end uses, such as for thermal insulation, sound-dampening, and/or shock absorption. The invention affords significant cost benefits to conventional fabrication techniques and yields articles having comparable or improved structural and insulating properties as compared to conventional products.
The process can be used to extrude single-ply articles, or to co-extrude multi ply articles, such as laminates, co-axial co-extrusions, co-extrusions encompassing a reinforcing matrix, etc. The resin can be extruded in any shape or profile, without limitation, including (but not limited to) sheet form, circular, hollow, square, or any other geometric cross-section.
A first version of the invention is directed to an apparatus for controlling the temperature of a polymer melt or other flowing material. The apparatus comprises a row of baffles. The row itself comprises a plurality of baffles. Each baffle defines a longitudinal axis that is preferably parallel to the longitudinal axis of another baffle in the row (although this is not required; see FIG. 20). Each baffle includes an upstream portion and a downstream portion. In one version of the invention, the downstream portion of each baffle has a width that is wider than the upstream portion. In another version of the invention, each baffle has a diamond-shaped cross-section. In all versions of the invention, the downstream portion of each baffle is convex or pointed and each baffle has a non-cylindrical cross-section perpendicular to its longitudinal axis. A closed-circuit temperature-control conduit is defined or disposed within at least one of the baffles. As used herein, the term “closed-circuit” means that the temperature-control medium disposed within the conduit does not come into contact with the melt flowing through the apparatus.
The temperature-control conduit can take a number of different forms. For example, the conduit can be a void defined within the baffle. The void is configured to allow a temperature-control medium to flow within the conduit. In this fashion, a thermostatically-controlled liquid medium (such as process water or mineral oil) can be circulated through the voids within each baffle. Alternatively, the temperature-control conduit can be a solid, thermal-control device disposed within the baffle, such as a thermostatically-controlled metallic or ceramic heating element. The apparatus may be configured so that there is a conduit defined or disposed within each baffle, or only in selected baffles. The temperature of each baffle can be controlled independently from any of the other baffles.
The apparatus according to the present invention may optionally comprise a die lip or body dimensioned and configured to yield an extrudate having a predetermined profile, such as a planar profile. Alternatively, the apparatus may be situated as an intermediate device in a modular arrangement of devices. When the present apparatus is placed at an intermediate position within the flow path, and a predetermined profile is desired, a final die that yields the desired profile is placed downstream from the apparatus according to the present invention.
Another version of the invention is directed to a corresponding method for manufacturing variable-density polymeric articles. Thus, the method comprises passing a polymer melt through an apparatus comprising a row of baffles as described in the immediately preceding paragraphs. Again there is a temperature-control conduit defined or disposed within at least one of the baffles to control the baffle's temperature. The temperature of the baffles is varied via the temperature-control conduit as the polymer melt passes through the apparatus. The temperature variations cause the density of the polymer melt passing proximate to the baffle to be altered as compared to density of the polymer melt passing distal to the baffle, thereby yielding a polymeric article having variable density. In the preferred embodiment, the temperature of the baffles is regulated to be colder than the bulk temperature of the polymer melt. This causes the density of the polymer melt that touches the baffles or passes proximate to the baffles to be of greater density than those portions of the melt that pass more distant from the temperature-controlled baffles.
The resulting polymeric articles are also within the scope of the invention. Thus the invention encompasses a variable-density polymeric article comprising a continuous, monolithic, polymeric body, without joints or seams, and having defined therein areas of higher density disposed adjacent to areas of lower density. The polymeric body can take any desired cross-section shape. For example, the polymeric body can be planar, in which case the areas of higher density may be disposed substantially perpendicular to the planar profile, substantially parallel to the planar profile, or at non-perpendicular, non-parallel angles to the planar profile (or any combination thereof).
It is therefore an object of the present invention to provide a continuous method and device for manufacturing lightweight composite articles at a significantly reduced cost.
It is a further object of the present invention to provide a continuous method and device for manufacturing a lightweight composite panel that provides the ability to control the density of the panel at any point throughout the cross-section of a profile during the fabrication process. These varying densities are essential to obtain the desired structural and insulating properties within a suitably lightweight panel. It is a further object of the present invention to provide a continuous manufacturing process that yields articles of manufacture having varying density, but which does not require secondary manufacturing steps.
It is a further object of the present invention to provide a low-cost, continuous method (and a corresponding device) for manufacturing lightweight composite articles, such as panels, wherein the method requires no added secondary manufacturing processes. This eliminates the need for adhesives or other chemicals currently required during secondary processing. Further, a method for manufacturing lightweight composite panels, structural insulating cores, and the like, in a one-step continuous process offers enormous cost advantages.
It is a further object of the invention to provide a versatile method and device for manufacturing lightweight composite articles. The versatility of this manufacturing technique allows easy adaptability to more sophisticated products, as well as for more complex applications where a variety of properties are required from the same product at the lowest cost possible.
It is a further object of the present invention to provide a continuous method and device for manufacturing a lightweight composite article having a wide applicability in sandwich composites for many industrial sectors and in particular for the transportation and construction sectors. The transportation sector devotes significant efforts toward developing lighter, more cost-effective products. Further, in the construction sector, polymers and composites are experiencing greater acceptance and use. These trends illustrate a driving market force for lightweight structural and insulating panels that can be manufactured at a reduced cost.
It is a further object of the present invention to provide a method and device for manufacturing lightweight composite cores for wide soft foams, structural foams, foam-filled structural cores, and multi-processed cores.
It is a further object of the present invention to provide a lightweight composite panel made from the same polymeric material, thereby providing a 100% recyclable product. Panels constructed using such cores is also encompassed within the present invention.
It is yet a further object of the present invention to provide a continuous method and corresponding device for manufacturing lightweight composite articles wherein both the process and the resulting articles have advantages over conventional manufacturing methods and cores. The advantages of the present invention include: better quantification of the key process physics that impact the density of foamed products as it applies to continuous article manufacturing and the ability to gain insight into process physics that induce varying foaming densities on the extrudate's cross-section. The process of the present invention is continuous, thus yielding considerable cost savings as compared to batch-type manufacturing methods.
The invention has many utilities. Primarily, the temperature-controlled apparatus can be used to make polymeric panels that can be used as structural members, as thermal insulation panels, as acoustic insulation panels, and the like. The temperature-controlled apparatus can be used as an intercooler to control the temperature of a polymer melt stream during processing. The apparatus can also be used to control the temperature of any other materials passed through the apparatus When the apparatus includes several off-set rows of baffles, the apparatus can also be used as both a mixer head and an intercooler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a conventional, prior art composite sandwich panel showing opposing skins 40 and core 42.
FIGS. 2A, 2B, 2C, and 2D show exemplary cross-sections of variable-density cores manufactured according to the present invention having foamed areas of lower density 44 and foamed or non-foamed areas having increased density 46. FIG. 2A depicts alternating layers of foamed and non-foamed areas perpendicular to the surface of the core. FIG. 2B depicts diagonal layers of non-foamed areas. FIG. 2C depicts a honeycomb-like arrangement of foamed and non-foamed areas. FIG. 2D depicts alternating layers of foamed and non-foamed areas parallel to the surface of the core.
FIG. 3 is a schematic of an apparatus according to the present invention showing the location of the apparatus 10 disposed between the extruder (not shown) and a die 50. The arrow depicts the direction of material flow.
FIG. 4 is a cross-sectional view of the inventive apparatus 10 showing cooling lines or temperature-control conduits 60 embedded within teardrop-shaped baffles 12. The arrow depicts the direction of material flow.
FIG. 5 illustrates the flow streamlines around the baffles 12.
FIG. 6 is a schematic of a sheeting die apparatus 50 according to the present invention having internal cooling coils 60 disposed within teardrop-shaped baffles 12.
FIG. 7 illustrates temperature contours of flow melt through the apparatus (with gap distance 13) with cylindrical baffles 12 and gap space 13 (cooling conduits removed for clarity). The resulting foamed areas 44 and non-foamed areas 46 of the melt as it exits the apparatus are also depicted.
FIG. 8 is a graph illustrating the temperature profile of the melt passed over the apparatus at different gap distances between cooling lines.
FIG. 9 is a graph illustrating the pressure drop rate of the melt as it flows through the apparatus at different gap distances.
FIG. 10 is a front perspective view of an apparatus according to the present invention.
FIG. 11 is a rear perspective view of the apparatus of FIG. 10.
FIG. 12 is a side view of the apparatus of FIG. 10.
FIG. 13 is a front view of the apparatus of FIG. 10.
FIG. 14 is a side view of two rows of baffles from the apparatus illustrated in FIG. 10 with cooling conduits 60 shown only in the bottom row of baffles for clarity.
FIG. 15 is a side view of two rows of baffles from an alternative version of the present invention with cooling conduits 60 shown only in the bottom row of baffles for clarity.
FIG. 16 illustrates alternative baffle shapes with cooling conduits 60 shown only in the bottom row of baffles for clarity.
FIG. 17 is a longitudinal cross-sectional view of another version of the invention having baffles with a roughly diamond-shaped cross-section. Here, each row of baffles is parallel with every other row of baffles.
FIG. 18 is a perspective view of another version of the invention wherein the baffles 12 have a roughly diamond-shaped cross-section, but the two right-hand rows of baffles are rotated 90 degrees with respect to the two left-hand rows of baffles.
FIG. 19 is another perspective view of FIG. 18 more clearly depicting the conduits 60 and the modular construction of the apparatus.
FIG. 20 is a perspective view of another version of the invention wherein the baffles within a single row are not parallel to one another.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and an apparatus for the continuous, one-step manufacture of lightweight, low-cost, articles of manufacture having a controlled density. The articles so produced are also included within the present invention. The method and apparatus of the present invention better quantifies the process physics that impact the density of foamed products as it applies to continuous manufacturing, as well as the quality of both foamed and non-foamed products. The invention thus providing optimum processing conditions to produce foamed resins with varying densities on an extruder and to ensure the quality of non-foamed products.
The apparatus according to the present invention will function using any polymer resin that can be extruded, as well as any other product that can be passed through the apparatus. The principal utility of the invention, though, is in the extrusion of polymeric articles. Thus, a non-limiting list of polymers that can be used in the present invention include styrenic resins, olefinic resins, acrylates, methacrylates, acrylimides, methacrylimides, carbonates, poly(arylene) oxides, polyvinyl alcohols, co-polymers of any of these (e.g., ABS), and the like. Elastomeric polymers and rubbers (natural and synthetic) may also be used in the present invention. Explicitly included within the list of polymers that can be used in the invention are polystyrenes (PS) (preferred), polyethylenes (PE), polypropylenes (PP), polyvinylchlorides (PVC), polyvinylidene chlorides (PVdC), polyurethanes (PU), polyphenylene oxides (PPO), polycarbonates (PC), polyvinylalcohols (PVOH) and polymethacrylimides (PMI).
As noted above, the method of the present invention brings together the proper combination of material, foaming agent ratios, and equipment for the continuous manufacture of lightweight composite articles, such as panels and cores.
Numerous researchers have studied the effect of processing parameters on foaming density [13-21]. The simplest studies vary the foaming agent concentration, the melt temperature, or the pressure differential during foaming. However, varying these parameters requires multiple steps in the manufacturing process. Thus, in the present invention, varying the foaming agent concentration is not a critical, or even a useful, parameter. Therefore, in one version, the present invention comprises premixing foaming agent with polymer resin pellets [25] by adding a suitable amount of foaming agent per unit mass of the resin. The resin and the foaming agent are then thoroughly mixed, for example, by means of a mixing screw. Then, approximately 0.2% to 5.0% by weight of a chemical or physical foaming agent is added. In this manner, an extruded product with a controlled foam density across its cross-section is produced as depicted in FIGS. 2A, 2B, 2C, and 2D. In these figures, the extruded product is depicted as a planar sheet. Thus, the areas of increased density may be disposed perpendicular to the surface of the sheet (FIG. 2A), parallel to the surface of the sheet (FIG. 2D), at any other angle with respect to the surface of the sheet (FIG. 2B), or in any other desired pattern, such as a honeycomb-like pattern (FIG. 2C).
The invention will function using either chemical or physical foaming or blowing agents, which are equally preferred. Chemical foaming, can be accomplished using endothermic or exothermic foaming agents. Physical foaming using any type of physical foaming or blowing agent (e.g., carbon dioxide, nitrogen, alkanes, halogenated alkanes, other hydrocarbon based blowing agents, etc.) may also be used in the present invention. The apparatus can also be used in non-foaming applications for mixing, for temperature control, or both.
By isolating the effects of die temperature and pressure, Park et al. [19] show that the most important factor on nucleation rate, and thus foam density, is the pressure drop rate. This is because the pressure difference that induces foaming is the real source of thermodynamic instability. If the nucleation time is kept constant, this thermodynamic instability is larger as the pressure drop rate increases. Therefore, even though it is well settled that the pressure drop rate is the most influential variable affecting foam densities, temperature and pressure combine to have a greater effect on foam quality [13-14, 16-21]. Temperature and pressure also have a large influence on secondary variables that affect nucleation and foam density, such as melt viscosity, solidification, coalescence and the diffusion of the foaming agent out of the extrudate. All of these variables must be considered when developing a manufacturing technique that is able to vary foam densities across the extrudate.
The present invention uses an internal apparatus to control and vary temperature (and thus pressure drop) on the melt to induce varying foaming densities on the extruded polymer or to maintain the melt within a desired and predetermined temperature range. The apparatus comprises baffles with a converging section followed by a diverging section through which the polymer melt can flow. One embodiment is a teardrop cross-section as seen in FIGS. 3, 4, and 5 (other cross-sections, described hereinbelow, are also within the scope of the invention). Referring specifically to FIGS. 3, 4, and 5, the inventive apparatus 10, comprises a row of baffles 12 (see FIG. 5). At least one of the baffles 12 includes a temperature control conduit 60 disposed therein or passing therethrough, as shown in FIG. 4. The temperature control conduit functions to regulate the temperature of the material that comes into contact with the baffles (or passes in close proximity to the baffles. The temperature control conduit is a closed circuit, meaning that the material disposed within the conduit 60 (either a circulating fluid or a heating/cooling element) is not in direct contact with the material passed through the apparatus. In the embodiment depicted in FIGS. 3 and 4, the apparatus 10 is placed in the flow path between an extruder (not shown) and a second die 50 as shown in FIG. 3. The die 50 can be dimensioned and configured to yield any desired cross-sectional profile to the extruded product (e.g., circular, regular or irregular polygons, planar, etc.) As the melt flows between the teardrop baffles, the melt undergoes elongational flow mixing. The melt that contacts the baffles 12 is either heated or cooled (depending upon the temperature of the baffles relative to the temperature of the bulk melt as the melt impacts and passes over the baffles).
To use the apparatus 10 as a temperature control device (i.e., an intercooler), a temperature control conduit 60 is defined or disposed in each baffle 12. This can be done, for example, by defining temperature-control conduits 60 through the length of the baffles, as seen in FIG. 4. (See also FIGS. 17-20, described below.) These conduits are connected at each end to a source of temperature-controlled liquid or gas (not shown). As the liquid/gas flows inside the conduits, it changes and controls the temperature of the baffles 12, which are in contact with the polymer melt, thereby resulting in a user-variable and user-controllable temperature field downstream from the baffles. Alternatively, the temperature-control conduits may be solid heating or cooling elements (e.g., metallic or ceramic elements) embedded within or otherwise incorporated onto or into the baffles 12.
The converging sections of the baffles also act as restrictors, defining a converging flow path. The converging sections of the baffles act to influence the temperature of the material as it exits the gap between the baffles toward the diverging section of die 50 (see FIG. 3). Varying the temperature of the fluid, gas, or heating/cooling element inside the temperature-control conduit 60, as well as varying the gap between baffles, has a direct influence on the temperature and pressure variations of the melt stream. Because foam density is affected by pressure and temperature, the apparatus described herein here is able to vary the density within defined regions of the extrudate. Thus, by judiciously selecting the temperature of the baffles and by selecting suitable gap sizes between the baffles, cores of variable density can be created at will.
The temperature of the circulating fluid passing through the conduits 60 is controlled by an external circuit that preferably includes a heater/refrigerator unit as well as suitable thermostat elements. If desired, the temperature within each baffle can be selectively adjustable independently of the other baffles. If this is desired, each baffle includes its own external conduit and associated temperature control elements to maintain each baffle at a desired temperature.
The preferred version of the invention utilizes the teardrop shape of the baffles to maximize heat conduction and convection from the heating element or liquid circulating inside the baffles toward the polymer melt flowing around the baffles. Thus, it is important to ascertain the appropriate dimensioning of the equipment before large-scale manufacturing commences. In short, the apparatus 10 must be “dialed in” to establish the appropriate values for melt pressure drop, temperature, the dimensions of the baffles 12, the spacing between the baffles, the number of baffles, the orientation of the baffles, the temperature of the temperature control conduit 60, the liquid circulation rate (if a liquid temperature control mechanism is used in the conduit 60), and the polymer melt flow rate to achieve the desired variability in the density of the core. These parameters are established empirically.
The apparatus of the present invention can be inserted within the flow path of any extruder capable of extruding polymeric resins, without limitation. The device is preferably located at the end of the flow path (e.g. see FIG. 6) or near the end of the flow path (e.g., see FIG. 3) and may comprise multiple sections (e.g. see FIGS. 12 and 19) to permit customized cores or other desired profiles to be produced.
If the temperature control device is placed immediately before the flow path exit, the sudden pressure drop will induce foaming downstream from the baffles. FIG. 5 shows the computed streamlines as the material flows around the teardrop baffles 12. Here the expected foaming area inside the melt, downstream from the baffles, is also shown. In the present invention, heating or cooling is taking place at the same time the melt is experiencing a large pressure drop as it exits the die.
The present invention differs significantly from earlier processes (such as the Celuka process [24]) by utilizing internal cooling (or heating) to induce controlled varying densities within the cross-section of the extrudate. The present invention therefore includes an apparatus 10 as shown in FIGS. 3 and 4. In these two figures, the apparatus is depicted as having a circular cross-section, which is generally preferred. That being said, other, non-circular cross-sections for the apparatus 10 as a whole are within the scope of the invention. The present invention also includes a die 50 as shown in FIG. 6 to extrude a foamed sheet, as an extruded sheet can be directly used as the core material in a composite sandwich panel.
Specifically referring to FIG. 6, this figure depicts another embodiment of the present invention in which the baffles 12, with their associated temperature-control conduits 60, are integrated within the final die 50 that gives the extrudate the desired final profile. Here, the die 50 is depicted as a sheet die, and the baffles 12 are placed just before the die exit.
To improve heat transfer, the temperature control conduit 60 inside each baffle is preferably as large as possible. However, a careful balance between heat transfer and structural integrity must be maintained to avoid failure during processing. Finite element structural analysis (FEA) combined with non-isothermal flow analysis can be employed to balance these properties (i.e., to balance the structural integrity of each baffle 12 versus the void volume of the temperature-control conduit 60 within each baffle).
The apparatus of the present invention is preferably of a modular design to allow the addition of various internal cooling devices. See FIG. 19. The apparatus is preferably dimensioned and configured to allow changing the axial location of the apparatus with respect to the die exit.
An exploratory non-isothermal flow simulation was done to estimate the temperature and pressure differentials as the material flows by the temperature-regulated baffles. The exemplary flow simulation, performed in two dimensions, assumes the baffles have a circular cross-section as shown in FIG. 7 (a cylindrical cross-section simplifies the calculation). For simulation purposes, the walls of the baffles were assumed to be at 343 K, and the melt at 513 K. Material properties for a typical PS resin were selected. The cross-section was set at 46 mm in height, the diameter of each baffle 12 at 10 mm, and the gap 13 between baffles at 3 mm. (This is a simplified analysis for sake of illustration. The baffle shape and the actual processing parameters employed during any given manufacturing run will vary considerably.) The results, depicted on the right-hand portion of FIG. 7 is an extrudate having lower-density areas 44 and higher-density areas 46.
FIG. 8 shows the temperature contours obtained from this flow simulation. The contours show a layered temperature that varies through the melt. The temperature variations have an effect on the foaming density of the extrudate. The temperature variations throughout the melt, as shown in the graph of FIG. 8, can be purposefully altered by adjusting the distance of the gap between the baffles. Thus, for example, FIG. 8 depicts the temperature of the melt as a function of the distance of the melt from the baffles when the gap between the baffles is 3 mm vs. 15 mm. (FIG. 8 assumes a flow speed of 10 mm/s.) As shown in FIG. 8, temperature variations of up to 60 degrees are observed at flow speeds of 10 mm/s and a 3 mm gap. The temperature variations thus give rise to corresponding density variations once the extrudate hardens into its final form.
The other variable controlled by the geometry of the apparatus is the melt pressure. The influence of the gap distance between baffles on the pressure drop rate of the melt is shown in FIG. 9, which is a graph of melt pressure with respect to the axial distance for two gap geometries (3 and 15 mm). Here, the effect of the gap size is observed to vary the pressure drop rate more than 60-fold. The results of the exploratory flow simulation shown in FIGS. 7-9 demonstrate that the present invention yields a polymeric article of manufacture with varying densities.
Conversely the influence of the gap distance between baffles, baffle geometry, and conduit design can be such to control the melt stream temperatures. These temperatures can be so fashioned as to induce temperature variations or to thoroughly homogenize the temperature variation of the melt stream.
Fractional factorial information is preferably used to facilitate the correlation of the many processing parameters and design variables with the desired measurements. The results obtained can be further correlated back to the simulated temperature and pressure results to establish a connection between simulated temperature and pressure variations with experimental results to facilitate design, setup and scale-up of the present invention. Non-Newtonian non-isothermal flow simulations are preferably performed to guide the design of the processing equipment and the temperature control conduits.
The apparatus 10 according to the present invention is illustrated in greater detail in FIGS. 10-20. (For clarity, the temperature control conduits 60 are omitted in FIGS. 10-13.
The apparatus 10 includes a series of baffles 12 arranged in rows. In the embodiment illustrated in FIGS. 10-13, each baffle 12 is teardrop-shaped and includes a large rounded head portion 14 on one end and a small tail portion 16 on the opposing end. The head portion 14 and tail portion 16 are interconnected by a gradually diverging portion 18. In the versions depicted in FIGS. 10-19, each baffle 12 defines a longitudinal axis 15 that is parallel to the longitudinal axis(es) of the other baffle(s) in the same row. In FIG. 20, the longitudinal axes of the various baffles 12 within the row are not parallel.
The head portion 14 of each of the baffles 12 shown in FIGS. 10-16 is semi-cylindrical in shape. The diameter of the head portion 14 is dimensioned to be the same as the maximum width of the baffle 12 so that a smooth transition occurs between the diverging portion 18 and the head portion 14 of each baffle. The large diameter head portion facilitates smooth flow of the melt stream through the apparatus 10 with reduced dead zones (i.e., reduced stagnation).
Preferably, the baffles 12 are positioned within the apparatus so that they are parallel to and slightly spaced from the other baffles within the same row. See FIGS. 14 and 15. Although this is not required, as illustrated in FIG. 20. The baffles are oriented relative to the material flow (illustrated by arrows in FIGS. 10 and 12) such that the material passes between the baffles from the tail portion 16 to the head portion 14. In this orientation, the tail portion 16 and diverging portion 18 of each baffle form an upstream portion 17, while the head portion 14 forms a downstream portion 19, as shown in FIG. 12. With the above-described orientation of the baffles 12, it can be seen that the diverging portions 18 of adjacent baffles 12 form a converging pathway at a converging angle (FIG. 12, top) through which the melt stream will flow. This converging pathway provides compressive forces on the melt stream, resulting in elongation and dispersion of the melt stream. It should be appreciated that there are other orientations of the baffles that could also form the desired converging pathway. For example, instead of the illustrated straight walls of the diverging portion 18, the converging pathway could instead be formed by curved walls that achieve the same elongational results.
The precise dimensions of the apparatus 10 will vary depending on the materials to be processed and the cross-sectional area of the flow path. For example, the length, width, and number of baffles can be chosen to meet specific needs. Furthermore, the converging angle and the gap (i.e., at the narrowest point) between adjacent baffles can further be varied to achieve different compressive and elongation forces as well as control temperatures.
In the embodiment shown in FIG. 12, the converging angle is between about 14 degrees and about 100 degrees. Preferably, the converging angle is between about 20 degrees and about 80 degrees, and more preferably the converging angle is between about 40 degrees and about 70 degrees. In addition, the ratio of the baffle gap to the baffle width (i.e., the gap:width ratio) is preferably between about 1:7 and about 2:5.
Adjacent rows of the baffles 12 may be transversely oriented (e.g., rotated) relative to each other in order to facilitate distributive mixing and temperature homogenization. See FIGS. 13 and 19. More specifically, the illustrated longitudinal axes 15 of the baffles 12 of one row are angled about 90 degrees relative to the longitudinal axes 15 of the adjacent rows, as shown in FIG. 11. With this design, it can be seen that the baffles 12 of alternating rows will be parallel to each other. In addition to being parallel, the baffles 12 of alternating rows may also be staggered slightly to further promote distributive mixing and temperature homogenization. It should be appreciated that the angular change of the baffles 12 could be less than 90 degrees, such as 45 degrees, thereby promoting a more gradual twisting of the melt stream as it passes through the apparatus.
In addition to promoting compression and elongation of the melt stream, the teardrop-shaped baffles 12 also reduce the amount of dead zones within the apparatus 10. Static mixers and intercoolers typically include dead zones within sharp corners, and particularly in transition regions with concave portions that face downstream. The baffles 12 alleviate this problem by providing downstream portions 19 that are generally convex or pointed in shape (e.g., the rounded head portions 14). The rounded head portions 14 promote flow around the downstream end of the baffles 12 to reduce the amount of dead zones within the apparatus 10.
The apparatus 20 illustrated in FIG. 14 further reduces the amount of dead zones by overlapping the baffles 22 of one row with the baffles 22 of the adjacent row. By doing this, the amount of downstream dead zones are further reduced. In the illustrated embodiment, the tail portion 24 of one row of baffles 22 is positioned approximately at the point of maximum compression 26 of the previous row of baffles 22. Such positioning of the baffles forces the melt stream to pass immediately from the zone of maximum compression of one row of baffles into the compression zone of the next row of baffles. This further enhances the temperature homogenization and mixing of the materials. FIG. 15 depicts the analogous embodiment wherein the baffles 12 of one row do not overlap with the baffles of the adjacent row.
FIG. 16 illustrates alternative baffle shapes, and specifically illustrates alternative head portion shapes. For example, the head portion 30 could have a semi-elliptical or semi-oval shape, which provides a more gradual downstream transition zone and is believed to further reduce material stagnation. Alternatively, the head portion 32 could be pointed, which provides a constant downstream expanding angle.
FIGS. 17-19 depict another version of the apparatus 10 wherein the baffles are not tear drop-shaped but roughly diamond-shaped or oval-shaped. Here, the baffles have a pointed tail region and a pointed head region. As depicted in FIG. 17, the pointed head and tail regions of each baffle are symmetrical in the plane perpendicular to the flow path (the direction of material flow)—that is, each baffle has planar symmetry through a vertical plane passed through the center of each baffle as shown in FIG. 17. Because the head and tail regions of the baffles as shown in FIGS. 17-20 are symmetrical, this version of the apparatus does not have a directionality. A melt passed from left-to-right in the version depicted in FIG. 17 experiences the same forces as a melt passed from right-to-left. (The same is not the case for the version of the invention depicted in FIG. 4, for example, wherein the baffles lack such planar symmetry. Note, however, that this fact does not preclude running material through the apparatus as shown in FIGS. 4, 5, 7, and 12 in the direction opposite to the arrow if desired.) As shown in FIGS. 17-19, in this version a temperature-control conduit 60 is present in every baffle. In FIG. 17 each row of baffles 12 is parallel to the row before it; the rows are not rotated. In FIGS. 18 and 19, however, some of the rows are rotated with respect to one another.
FIG. 19 shows how the apparatus of the present invention can be fabricated in a modular fashion. Here, each row of baffles is disposed within its own separate module. The order and orientation of the modules can be controlled by the user to vary the overall geometry of the collection of modules.
FIG. 20 depicts yet another version of the invention wherein the longitudinal axes of the baffles 12 are not parallel. As shown in FIG. 20, baffle 12′ is disposed in a non-parallel relationship to baffle 12.
The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain best modes known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.

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Claims

1. An apparatus comprising:

a row of baffles, the row comprising a plurality of baffles wherein each baffle defines a longitudinal axis, wherein each baffle includes an upstream portion and a downstream portion, wherein the downstream portion of each baffle is convex or pointed, wherein each baffle has a non-cylindrical cross-section perpendicular to its longitudinal axis, and further wherein the baffles of each row define a converging flow path for materials passing through the apparatus; and

a closed-circuit, temperature-control conduit defined or disposed within at least one of the baffles.

2. The apparatus of claim 1, wherein the longitudinal axis of each baffle in the row is parallel to the longitudinal axis of another baffle in the row, and wherein the downstream portion of each baffle has a width wider than the upstream portion, and wherein the downstream portion of each baffle has a convex surface.

3. The apparatus of claim 1, wherein the temperature-control conduit is a void defined within the baffle, wherein the void is configured to allow a temperature-control medium to flow within the conduit.

4. The apparatus of claim 1, wherein the temperature-control conduit is a solid thermal-control device disposed within the baffle.

5. The apparatus of claim 1, comprising a conduit defined or disposed within each baffle.

6. The apparatus of claim 1, wherein the downstream portion of each baffle has a convex shape.

7. The apparatus of claim 1, comprising a plurality of rows of baffles, wherein the longitudinal axes of the baffles in one row are transverse to the longitudinal axes of the baffles in an adjacent row.

8. The apparatus of claim 7, wherein the axes of one row of baffles are rotated about 90 degrees relative to the axes of the baffles in an adjacent row.

9. The apparatus of claim 7, wherein the axes of the baffles in one row are parallel to the axes of the baffles in a non-adjacent row.

10. The apparatus of claim 7, wherein the baffles in one row overlap with the baffles of an adjacent row.

11. The apparatus of claim 1, wherein the baffles are substantially teardrop in shape.

12. The apparatus of claim 1, wherein the downstream portion of at least one baffle is semi-cylindrical in shape.

13. The apparatus of claim 1, wherein the downstream portion of at least one baffle is semi-elliptical in shape.

14. The apparatus of claim 1, wherein the upstream portion and the downstream portion of each baffle are pointed.

15. The apparatus of claim 14, wherein the upstream portion and the downstream portion of each baffle are symmetrical about a plane perpendicular to direction of flow of material through the apparatus.

16. The apparatus of claims 1, further comprising a die lip dimensioned and configured to yield an extrudate having a predetermined profile.

17. The extrusion die of claim 16, wherein the die lip is dimensioned and configured to yield an extrudate having planar profile.

18. A method for manufacturing variable-density polymeric articles, the method comprising:

(a) passing a polymer melt through an apparatus comprising a row of baffles, the row comprising a plurality of baffles wherein each baffle defines a longitudinal axis, wherein each baffle includes an upstream portion and a downstream portion, wherein the downstream portion of each baffle is convex or pointed, wherein each baffle has a non-cylindrical cross-section perpendicular to its longitudinal axis, and further wherein the baffles of each row define a converging flow path for materials passing through the apparatus; and a closed-circuit, temperature-control conduit defined or disposed within at least one of the baffles to control the baffle's temperature; and

(b) varying the temperature of the baffle via the temperature-control conduit as the polymer melt passes through the apparatus, wherein the temperature is varied to cause density of the polymer melt passing proximate to the baffle to be altered as compared to density of the polymer melt passing distal to the baffle, thereby yielding a polymeric article having variable density.

19. The method of claim 18, wherein in step (a) the longitudinal axis of each baffle in the row is parallel to the longitudinal axis of another baffle in the row, and wherein the downstream portion of each baffle has a width wider than the upstream portion.

20. The method of claim 18, wherein step (b) comprises varying the temperature of the baffle such that the density of the polymer melt passing proximate to the baffle is increased as compared to the density of the polymer melt passing distal to the baffle.

21. The method of claim 18, wherein in step (a), the apparatus is dimensioned and configured to yield a polymeric article having a predetermined profile.

22. The method of claim 21, wherein in step (a), the apparatus is dimensioned and configured to yield a polymeric article having a planar profile.

23. The method of claim 18, further comprising, after step (b),

(c) and then passing the polymer melt through a die dimensioned and configured to yield a polymeric article having a predetermined profile.

24. The method of claim 18, further comprising, after step (b),

(c) and then passing the polymer melt through a die dimensioned and configured to yield a polymeric article having a planar profile.

25. The method of claim 18, wherein step (b) comprises varying the temperature of the baffle to yield areas of increased density within the polymer article, and further wherein the areas of increased density are disposed substantially perpendicular to the planar profile, substantially parallel to the planar profile, or at non-perpendicular, non-parallel angles to the planar profile.

26. The method of claim 18, further comprising: (c) co-extruding at least one additional polymer melt to yield a composite, variable-density polymeric article.

27. A variable-density polymeric article comprising:

a continuous, monolithic, polymeric body, without joints or seams, and having defined therein areas of higher density disposed adjacent to areas of lower density.

28. A variable-density polymeric article of claim 27, produced by:

(a) passing a polymer melt through a die comprising a row of baffles, the row comprising a plurality of baffles, each baffle defining a longitudinal axis that is parallel to the longitudinal axis of another baffle in the row, wherein each baffle includes an upstream portion, and a downstream portion having a width wider than the upstream portion; and a closed-circuit temperature-control conduit defined or disposed within at least one of the baffles to control the baffle's temperature; and

(b) varying the temperature of the baffle via the temperature-control conduit as the polymer melt passes through the die, wherein the temperature is varied to cause density of the polymer melt passing proximate to the baffle to be altered as compared to density of the polymer melt passing distal to the baffle, thereby yielding a polymeric article having variable density.