WO2009117270A2 - Method and system for making a thin wall high density polyethylene catheter tubing - Google Patents

Abstract

The invention relates to systems and methods of using the systems to fabricate thin wall tubing for use in medical devices. Some embodiments provide high density polyethylene tubing with an inner diameter of about 7 French to about 13 French (0.015 inch to 0.168 inch), and with a wall thickness between about 0.0015 inch to about 0.0025 inch. Embodiments of the tubing may be used as a liner for the fabrication of catheters that have an outside diameter in the range of about 9 to about 15 French. Embodiments of the tubing are fabricated with a zoned gas cooling tunnel that provides a precisely controlled and circumferentially homogeneous gas flow around the cooling extrudate. Gas flow into the tunnel is controlled with high precision by a flow controller; gas flow within the tunnel is independently controlled in each zone, gas flow within the interior of the runnel zones is controlled by a pattern and alignment of gas flow inlets and a common exhaust.

Description

METHOD AND SYSTEM FOR MAKmG A THIN WALL HIGH DENSITY POLYETHYLENE

CATHETER TUBING

CROSS REFERENCE TO RELATED APPLICATIONS [001] This application claims priority to U.S. Provisional Patent Application No. 61/037,151 Rettke, entitled "Method and system for making a thin wall high density polyethylene catheter tubing", as filed on March 17, 2008, to U.S. Provisional Patent Application No. 61/042,660 Rettke, entitled "Method and system for making thin wall high density polyethylene catheter tubing with a very small inner diameter", as filed on April 4, 2008, and to U.S. Provisional Patent Application No. 61/043,646 Rettke, entitled "Method and system for making extrusion products with a high level of conformational control", as filed on April 9, 2008, all of which are incorporated by this reference in their entirety.

FIELD OF THE INVENTION

[002] The invention relates to the fabrication of guide catheters or delivery catheters made of thermoplastic liner material extruded into thin wall tubing, HDPE tubing in particular, such tubing being appropriate for fabricating multi-layered catheters, typically including a liner, a braid or coil, and an outer jacket layer.

INCORPORATION BY REFERENCE

[003] All publications, patents and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference.

BACKGROUND OF THE INVENTION

[004] Conventional multi-layered catheters typically include a polymeric lubricious liner material such as polytetrafluoroethylene (PTFE). PTFE is amenable to the manufacture of thin walled tubes by way of extrusion processes. Thin walls are desirable for the manufacture of small diameter catheters because minimization of liner wall thickness allows optimization of the inner diameter with respect to the outer diameter. Nevertheless, PTFE has features that make it less than ideal for catheters. Importantly, for example, PTFE is not stable under irradiation which is a desirable form of industrial scale sterilization because of its fast throughput. An alternative form of sterilization is provided by exposure to ethylene oxide gas which does not compromise the PTFE material. This method of sterilization, however, is less desirable on an industrial scale for a number of reasons, including long exposure times and the necessity of handling large volumes of sterilizing gas.

[005] Materials that provide an alternative to PTFE and which are stable under irradiation include polymers such as Nylon and Pebax®; these materials, however, are not fully satisfactory as catheter materials because of their relatively high coefficient of friction. Thus, the advantage of Nylon and Pebax® provided by their irradiation stability is offset by a lesser degree of Iubriciousness in comparison to PTFE.

[006] Polyethylene material, especially in high density grade (HDPE), has a coefficient of friction that is not as low as that of PTFE, but it is still substantially lower than that of other commonly extruded polymers such as polypropylene. Like Nylon and Pebax, high density polyethylene (HDPE) is irradiation- stable, so a catheter made from the HDPE may be sterilized using irradiation.

[007] The "French catheter scale" or "French gauge" is commonly applied to catheter diameter sizing. The "French" unit (Fr) is equal to the diameter in millimeters multiplied by three; with reference to inches, one French unit is approximately equal to 0.013 inch. A conventional multi-layer configuration of catheters has an inside diameter of about 7 - 13 French, and includes a PTFE Teflon liner with a wall thickness of about 0.0015 inch. The next layer of the catheter is typically a form of braided fabric or metallic material which aids in torque transmission and kink resistance. A final top protective braid layer or jacket layer completes the catheter construction. The inner PTFE layers are in intimate interfacial contact with their respective overlaying layer, and may further have some degree of contact with a higher layer.

[008] In general, optimal luminal diameters of catheters are determined by the volume or diameter required for effective performance with regard to the particular application of the device, but beyond that dimensional constraint, there is an advantage associated with thin walls, which maximize the inner diameter of a device for a given outside diameter. To some extent, the current state of the art of fabricating small-diameter catheters with a HDPE liner is limited by the thickness of the inner-most liner, and more particularly, the consistency of the thickness. There is a need in the catheter market for improved materials or improved processes that can deliver catheter component materials that can be reliably and uniformly produced at high volume, which have small diameters by virtue of thin but robust walls, and which are compatible with irradiation as a sterilizing method. SUMMARY OF THE INVENTION

[009] Manufacturing high density polyethylene thin-wall tubing by conventional methods and arrangements of conventional equipment has not been successful. Conventional approaches include direct or nearly direct transfer of hot extrudate into a water trough, for example. Air cooling, when conventionally applied with a simple blower prior to immersion into a water trough has also not succeeded. Similarly, conventional methods of pulling hot extrudate through a water trough have not been adequate for making thin-walled tubing. Equipment and methods to address inadequacies in conventional approaches, as provided by this invention, have centered on several aspects of the manufacturing process, as summarized below.

[0010] In one aspect, embodiments of the method and the arrangement of equipment and tools are directed to minimally perturbing the hot extrudate and preserving its integrity as it emerges from an extruder head. In another aspect, the method and equipment are focused on immediately establishing a highly controlled gas-cooling environment of the emerging extrudate, and zone-wise, continuing to step its temperature down in a highly controlled manner. Such control includes establishment of conditions that create high-resolution homogeneity around the extrudate with regard to temperature, pressure, and cooling gas flow patterns during the cooling process. Gas cooling is performed by the innovative positioning of a j acketed cooling tunnel between the extruding head and a conventional water trough. Homogeneity of conditions within the tunnel is provided by several features, including the use of clean dry gas, controlling gas flow to the cooling tunnel with highly precise metering, the creation of segregated cooling zones within the tunnel, and controlling the flow of cooling gas within the zoned spaces of the tunnel through the use of a precise arrangement of gas inlets and outlets. [0011] The invention generally relates to a system and a method for fabricating a thin wall high density polyethylene tubing, with a focus on the portion of the system and method that relates to cooling the hot tubular extrudate as it emerges from the head of the extruder. The invention further relates to a thin wall tubing itself, made by the system and method, the invention further relates to larger products such as catheters that include the thin wall tubing as a component. [0012] A method of cooling tubular extrudate as it emerges from an extrusion line includes feeding hot extrudate from an extruder head into a cooling tunnel that includes a zoned space between a jacket and an inner insert through which the extrudate passes, providing a flow of clean dry air or other suitable cooling gas, controlling the clean dry air flow to the jacket with high precision, controlling the air flow through one or more inlets in the jacket such that air flow into the jacket-insert space is substantially uniform among the inlets within a zone, and controlling the air flow into an air tunnel insert from the jacket-insert space such that an air flow pattern, as it encounters the hot extrudate within the insert, is substantially homogenous with regard to flow rate and temperature, with high resolution, across the extrudate surface.

[0013] A system for cooling tubular extrudate as it emerges from an extruder head includes a source of clean dry air or other suitable cooling gas, a cooling tunnel including a jacket and an insert with an interior cooling space; the jacket configured to flow the air homogeneously from the source into a zoned jacket-insert air space, the insert configured to flow air from the jacket-insert space to the interior of the insert such that air flow around the extrudate in the interior of the insert is homogeneous, and an air flow controller for conveying the clean dry air to the tunnel jacket.

[0014] The invention further relates to a thin wall high density polyethylene tube that includes the features of having a wall of a thickness of less than about 0.0025 inch, and an inside diameter that ranges between about 0.015 inch and about 0.168 inch, wherein the thickness and diameter dimensions are statistically valid at a high degree of confidence. In some embodiments, an inside diameter may range from about 0.015 inch to about 0.091 inch.

[0015] The invention further relates to products such as medical catheters that include the tubing of the invention. Three-layer catheters, for example, commonly include a liner for which the tubing of the present invention is suitable. An exemplary medical catheter has an outside diameter that ranges between about 9 French and about 15 French and includes a high density polyethylene liner having a wall of a thickness between about 0.0010 inch and about 0.0025 inch and an inside diameter between about 0.015 inch and about 0.168 inch, a braided envelope enclosing the high density polyethylene liner, and a jacket enclosing the braided envelope.

[0016] The tubing of the invention is a product of a manufacturing process that includes feeding hot tubular extrudate from an extruder into a cooling tunnel that includes a zoned insert within a jacket, and an air space between the jacket and the insert, providing a flow of clean dry air, controlling the clean dry air flow to the jacket with high precision, controlling the air flow through one or more inlets in the jacket such that air flow into the jacket-insert space is substantially uniform among the inlets within an insert zone, and controlling the air flow into an air tunnel insert from the jacket-insert space such that an air flow pattern, as it encounters the hot extrudate within the insert, is substantially homogenous with regard to flow rate and temperature, with high resolution, across the extrudate surface.

[0017] In another aspect, the invention provides a method of cooling tubular extrudate as it emerges from an extruder that includes feeding hot extrudate from an extruder into a zoned gas cooling tunnel, the tunnel including an insert within a jacket and an air space between the jacket and the insert. [0018] In still another aspect, the invention provides a method of cooling tubular extrudate as it emerges from an extruder comprising that includes feeding hot extrudate from an extruder into a zoned gas cooling tunnel, the tunnel including an insert within a jacket and an air space between the jacket and the insert; and feeding partially cooled extrudate from the zoned cooling tunnel into a water trough.

[0019] And in yet another aspect, the invention provides a method of cooling tubular extrudate as it emerges from an extruder that includes conveying hot extrudate from an extruder through a zoned gas cooling tunnel prior to feeding the extrudate downstream into a water trough, and supporting the hot extrudate within the zoned gas cooling tunnel by providing a substantially inward and downstream directed air flow through the tunnel, the air flow radially homogeneous around the extrudate.

[0020] Embodiments of the method of cooling hot tubular extrudate may also be applied to the manufacture of thick wall tubing, such as to tubing with walls in the range of 0.003 inch to about 0.035 inch, such methods being advantageous when a high degree of wall thickness uniformity is desired. Such thick wall tubing may be formed from resins such as high density polyethylene, polyamides, polyimides, and fluoropolymers. Such products as formed from these methods are included as embodiments of the invention. [0021] Further still, embodiments of the method cooling hot polymeric extrudate may be applied to extrusion products other than tubes, such as non-tubular hollow articles and solid forms, such methods being advantageous when a high degree of dimensional control and shape stability is desirable, and/or when extrusion product disfigurations common to conventional processes are highly undesirable. Such products as formed from these methods are included as embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 provides a schematic view of the system for making thin-walled HDPE tubing. [0023] Figure 2 A and 2B are flow diagrams of aspects of the inventive method. Figure 2 A provides an overview of a comprehensive embodiment of the process that includes such aspects as selection of resin, assembling components of the fabrication line, and finally, forming a hollow thin-walled tubing. Figure 2B is a flow diagram of aspects of Figure 3, focusing on the tube-forming process. The flow diagram focuses on cooling airflow within the system, particularly on airflow through a manifold into the cooling tunnel.

[0024] Figure 3 is a schematic diagram of air flow within the system that focuses on airflow through a manifold into the cooling tunnel.

[0025] Figures 4A and 4B are linear cut away sectional view of the air tunnel; Figure 4A shows the air tunnel jacket and Figure 4B shows the air tunnel insert.

[0026] Figure 5 is perspective view of an assembled air tunnel rendered schematically as a half-pipe, with a focus on the air flow within the tunnel.

[0027] Figure 6 is a schematic representation of the extruder, cooling tunnel, and water trough, which focuses on the presence of upstream directed cooling gas which immediately engages hot extrudate as it emerges from the extruder head.

[0028] Figure 7 is a linear cross sectional view of the air tunnel insert wall that focuses on the air holes traversing the wall at various angles.

[0029] Figure 8 is a perspective view of the air tunnel jacket. [0030] Figure 9 is a perspective view of the air tunnel insert. [0031] Figure 10 is a perspective view of the air tunnel insert within the air tunnel jacket, with the jacket rendered as a phantom around the solidly rendered insert.

[0032] Figure 11 is a cross section detail of the air space between the jacket and the insert, focused on the abutment of the barrier wall of the insert and an O-ring around the insert that fits into a groove in the jacket wall. Figure 11a shows an enlarged detail of the abutment. [0033] Figure 12 is a perspective view of the air tunnel insert within the air tunnel jacket, with the jacket rendered as cut-away half-pipe surrounding the solidly rendered insert.

[0034] Figure 13 shows a schematic profile of the temperature of extrudate within separate zones of a cooling tunnel.

[0035] Figures 14A and 14B show schematic views that relate the temperature of various points along the length of an extrudate product 200 and the overall time course of temperature as those points move through a cooling tunnel. Figure 14A shows the temperature of separate surface points, and relates those point temperatures to a chart showing the temperature of those points through the total course of passage through a cooling tunnel. [0036] Figure 14B shows the temperature of separate points distributed within circumferential bands around the periphery of the product, again, in relation to the temperature of these sets of points as they transit through a cooling tunnel.

[0037] Figure 15 provides a schematic view of a catheter fabricated around a thin-walled HDPE tube, as provided by this invention, with approximate dimensions included.

DETAILED DESCRIPTION OF THE INVENTION Overview of Cooling Tunnel and Method for Cooling Hot Extrudate

[0038] The invention described herein provides a system and methods for fabricating a polymeric tubing or catheter liner by extrusion to form tubing with a wall thickness of 0.0025 inch or less, thus enabling construction of an optimally-sized multi-layered catheter. Particular embodiments are described using high density polyethylene (HDPE) as an exemplary polymer, because HDPE has various advantageous features as detailed in the background. However systems and methods described herein may be applied to other polymers, and tubing products produced by these inventive systems and methods may be used for purposes other than inclusion in medical catheters. Currently available processes do not allow for the extrusion of a HDPE liner with wall thickness of .0015 inch or less for tubing with an inside diameter dimension in the range of 7Fr to 13Fr (0.091 - 0.168 inch).

[0039] Figures 1, and 2A and 2B provide overviews of embodiments of the inventive system and method respectively. Figure 1 shows the tubing fabrication system 10 which includes an extruder 15 that feeds early stage product (extruded melt) 201 into a cooling tunnel 100. Also feeding into the cooling tunnel 100 is clean dry air 22 produced by a clean dry air source 20, the flow of which is controlled by a mass flow controller 25. Product 201, air-cooled, emerges from the air cooling tunnel 100 and is directed into a water trough 40; the product 201, now water-cooled is pulled by a puller. Product 201 emerging from the puller 50 is HDPE tubing with a wall thickness of 0.0025 inch or less. Figures 2A and 2B are flow diagrams of the overall process of making thin-walled HDPE tubing, and the air-cooling aspect of the method, as described below. Figures 3 - 12 provide detailed views of embodiments of the cooling tunnel portion of the system which are described further below.

[0040] Figure 2A depicts a comprehensive embodiment of the process that includes such aspects as selection of resin, assembling components of the fabrication line, and finally, forming a hollow thin- walled tubing. Figure 2B provides a schematic flow diagram of an embodiment of the thin-wall tubing fabrication method, with a focus on the aspect of the process whereby the hot extrudate from the extrusion line is air-cooled. Figure 2B, thus depicts the following basic steps:

1. Extruding a thin-wall HDPE tube from an extruder into a gas-cooled tunnel.

2. Cooling with well-controlled conditions in a first gas-cooling zone within the tunnel.

3. Cooling with well-controlled conditions in a second gas-cooling zone within the tunnel. 4. Cooling further in a water trough. [0041] In some embodiments, further cooling steps in further cooling zones (i.e., more than two zones) are provided before entry of the extrudate into a cooling water trough. As described in further detail below, well-controlled cooling is provided by the inventive application of appropriate cooling gases (typically dry, and typically clean), a high precision controlling of the rate of cooling gas into a cooling tunnel, and an inventive physical arrangement of the cooling gas flow path (including air inlet dimensions, air inlet number, spacing between the air inlets, and air inlet angles with respect to the linear axis of the air cooling tunnel). As indicated by the cooling steps, the cooling gas flow path includes separate cooling zones, typically two or more zones that are each independently controlled. Typically, the first cooling zone captures emerging extrudate immediately as it emerges from a head, as shown in Figures 5 - 7, and as described in further detail below.

[0042] In another aspect, the method may be described by the following steps:

1. Providing a source of clean dry cooling gas such as air.

2. Controlling with high precision the flow rate of clean dry air from the source to the jacket of an air cooling tunnel. 3. Controlling the rate of airflow through multiple inlets into the zones within the tunnel jacket such that flow rates into zones are substantially uniform among the inlets of each zone (inlet-to- inlet), although total air flow rate may vary among the zones (zone-to-zone). 4. Controlling the air flow into a tunnel insert from a zoned air space between the tunnel jacket and the tunnel insert from entry inlets into the insert, the inlets equally spaced circumferentially and through radial holes, such that the air flow pattern is substantially homogenous with high resolution with regard to flow rate, pressure, and temperature circumferentially around the extrudate, as extrudate advances downstream through the tunnel insert.

[0043] The method may include other particular steps that address ways in which to handle pressure that could build up in the system (up to 6,000 p.s.i., for example) were it not for inventive features and methods, and which would pose problems of product turbulence and disruption of the integrity of nascent thin-walled tubing. Such method steps may include:

1. Extruding the tubing from an extrusion head with a tool set annular gap that minimizes back pressure that may otherwise create turbulent product, such turbulence compromising the integrity of the tubing. 2. Providing a bore through the center of a zoned cooling tunnel assembly sufficiently large so as to minimize pressure differences between the zones that could otherwise create turbulent motion of the product within the bore, such turbulence compromising the integrity of the tubing.

[0044] The claims of the invention are not bound by theory or rationale provided, but theory is provided to aid in understanding the invention as described below. Those familiar with the art of tubing extrusion generally believe that a skin forms at the surface of the hot extruded product as an initial step as cooling begins, and the product takes on a final form. This skin is a transient physical feature, difficult to observe or measure, nevertheless it is thought to be a likely intermediate stage between the viscous fluid state of melted polymer and the solid state of cooled polymer, especially as cooling begins at the tubing surface, and is followed by progressive heat loss on an inwardly directed gradient. The tubing skin, once formed, provides a stabilizing structure that tends to support uniformity of size and shape along the length of the cooling tube. It is believed that the cumulative features of the inventive air cooling system and the method of using the system, which are designed to create homogeneity with regard to surface temperature and air pressure along the whole of the extrudate surface, support a uniformity and conformational consistency in the development of the tubing skin. Since the wall of the tubing of the invention is thin, there is relatively little room for error or inconsistency in the thickness of the wall. Thus, the ability to fabricate thin wall tubing relies to at least some degree on the consistency of the thickness, or the statistical confidence of the thickness falling within desired specification. The ability of embodiments of the system and method to operate the zones of the cooling tunnel independently of each other further contributes to the development of a uniform skin, as well as the thinness of the wall as a whole. The skin is believed also to serve the function of stabilizing the structure as it passes through the gas cooling tunnel, and enters the cooling water trough.

[0045] Figure 3 depicts an exemplary embodiment of the invention, and focuses on an aspect of the overall system as seen in Figure 1 that involves the flow of clean dry air (or any suitable cooling gas) from a source ultimately to the internal space within the air tunnel insert. A clean dry air source 20 delivers air into a gas flow- or mass flow controller 25, which in turn, directs air flow 22 at a highly controlled rate into a manifold 30 which directs air into the cooling tunnel 100. More specifically, air flow 22 enters the air tunnel jacket 120 through air inlets 130. Typical embodiments of the air tunnel jacket include two or more inlets (the depicted embodiment depicts three equally-spaced inlets) equally spaced circumferentially around the jacket 120. Incoming air, upon passing through air inlets 130, disperses into the air space 145 between the jacket 120 and the air tunnel insert 150. From there, air 22 flows through the cooling tunnel's air holes 160, into the interior of the air tunnel insert 150. Details of the air holes of the cooling tunnel insert are described further below and show in some detail in Figure 7.

[0046] The invention includes a number of embodiments that may be understood as variations of the embodiment shown in Figure 3. For example, the air source 20 can provide any appropriate and effective cooling gas. For example, the composition of the gas may vary from that of ambient air. In some embodiments, the gas may be nitrogen, or helium, or oxygen, or any suitable gas or mixtures of gases that can be used for cooling. The gas flow controller 25 may be understood as any device that provides a highly precise control of cooling gas flow. As described further below, embodiments of the invention may include more than one gas flow controller 25, typically multiple gas flow controllers operate in parallel, each supplying a separate zone within the cooling tunnel 100. Each gas flow controller 25 may be set independently of the others, but each is typically set to control flow at a rate within a range between about 15 standard liters per minute (SLPM) and about 50 SLPM. The settings may vary between production runs and also may vary according to product size, for example flow rates are generally higher for larger tubing, and lower for smaller tubing. In a typical manufacturing run, flow rates are set to specified value, and then tuned until the product is on size, and the settings are then maintained for the duration of the run.

[0047] The manifold 30 conveying gas from a mass flow controller to the air tunnel is depicted schematically; any arrangement of gas conveying tubes or pipes may be used that provide a substantially uniform flow of gas through multiple pathways may be included in embodiments of the invention. The gas inlets 130 into the tunnel jacket 120 are shown as being three in number, but any number less than or more than three may be appropriate in various embodiments of the invention. Typical embodiments have the gas inlets arranged around the cooling tunnel jacket 120 equidistantly, as this arrangement is generally supportive of uniform and homogenous gas flow within the tunnel. [0048] Further, although Figure 3 depicts a single cooling gas source 20, in some embodiments, more than one gas source may be included. Further, a single precision gas flow controller 25 is depicted, but more than one gas flow controller may be included in some embodiments. For example, a single gas source 20 may provide gas to three separate gas flow controllers, each gas flow controller having a dedicated gas source. In other embodiments, there may be multiple gas flow controllers, each having its own dedicated cooling gas source. The invention typically is directed toward providing a flow of gas into the cooling tunnel such that flow into and within the tunnel is homogeneous within the tunnel. Any arrangement of cooling gas source(s), gas flow controller(s), manifold(s), and air inlets into the tunnel that serve this end are included as embodiments of the invention. As will be described further below, the air tunnel 100, more particularly the air space 145 between the jacket 120 and insert 150 is separated into a variable number of independently operable zones. Thus, the schematic depiction of an air tunnel assembly in Figure 3 may also represent, in particular, a cross-sectional slice through a single zone, as determined by a particular portion of airspace 145 between jacket 120 and insert 150. In complex arrangements of the gas source, gas flow controller, and manifolds, a computer or controller (not shown) may be included to supervise the operation of the various components. Detailed views of aspects of zoned air spaces 145 are provided in Figures 5 - 6.

[0049] Figures 4 A and 4B provide linear cutaway or half-pipe sectional views of the air tunnel jacket 120 (Figure 4A) and the air tunnel insert 150 (Figure 4B), respectively. The air space between tunnel jacket 120 and tunnel insert 150 is divided into zones 145 (see Figure 5) which have O-ring grooves 142 to accommodate O-rings (not shown), which mark the site of contact with barrier walls 170 within the jacket-insert space and segment it into zones. This particular exemplary embodiment is shown with three zones, but other embodiments may have fewer than three zones, or more than three zones.

[0050] Continuing with reference to Figures 4A and 4B, air inlets 130 are seen at the uppermost aspect of the air tunnel jacket in each zone, as well as on the inner aspect of the wall. A third air inlet set is not seen, for having been cut away in this view. These inlet holes into the jacket insert may be configured into rings around the circumference of the insert, as seen in Figures 8, 10, and 12. Boundary walls 170 (seen in Figure 4B) form internal zones within the jacket-insert space when the insert 120 and the jacket 150 are assembled. The assembled tunnel 100 and the zoned jacket-insert spaces may be seen in Figures 5 and 6. The air tunnel insert (Figure 4B) is separated into internal zones 145, as created by boundary walls 170. The walls 170 extend beyond the circumference of the wall itself, to form ribs that meet with O-rings that fit into grooves 142 within the inner aspect of the wall of the jacket 120 (as mentioned above). The abutment of the circumferential edge of the barrier wall 170 and the O-rings in grooves 142 creates separate zones within the jacket-insert space 145 as well. The flow of air within each of these zones of the jacket-insert space 145 is thus separate and independent of the other zones.

[0051] Figure 5 is a perspective view of an assembled air tunnel 100 rendered schematically as a half- pipe (insert 120) within a half-pipe (jacket 150), with a focus on the air flow within the tunnel. In general, with an exception as noted below, the predominate and net direction of air flow 22 within the tunnel is in the same direction as that of the product flow 200, a direction which may be understood as downstream. Details of the holes 160 within the wall of insert 120 (as seen in Figures 9, 10, and 12) are not shown. Figure 5 does show the entry of air or other cooling gas (arrows 22) into zoned spaces 145 between the jacket 150 and the insert 120. It can be seen that zoned spaces 145 are bounded on either side by barrier walls 170. Air flows from separate zoned spaces 145 through the wall of insert 120 into the interior space 165 within insert 120, again as indicated by arrows 22. Air flow within the interior 165 of the insert 120 is not physically segmented into zones as in spaces 145, but the general parallel directionality of air flow as it enters into space 165 preserves to some extent the separateness of the air flow streams as they originate in separate spaces 145. Thus, although a physical separation defines the zone only in spaces 145, the tunnel 100 as a whole may be understood to be segmented into air flow zones, as initially determined by spaces 145, and as further differentiated by the independently controlled rates of air flow entering into space 165 from zoned spaces 145.

[0052] The direction of air flow in the proximal-most or most upstream portion of the first zoned space is upstream, in the direction of the extruder, and directed at the extrudate 201 as it emerges from the extruder head, as shown in Figure 6. This is an exception to the general direction of air flow which is otherwise generally downstream. The direction of the flow path 201 of extrudate 201 through the interior space 165, also downstream, is indicated by arrow (201). The flow path 200 of product 201 through interior space 165 is supported by the balanced and centrally-directed airflow 22 which centers the product in space 165, and by the distally- or downstream-directed gas exhaust through the central bore 175 at the distal end of the tunnel. A typical flow rate of extrusion product through the tunnel is in the range of about 25 feet/min to about 100 feet/min, a value that understandably corresponds to the rate at which a puller operates (see operational details of the puller, below). In particular embodiments, the product throughput rate may range above or below the typical range of about 25 - 100 feet/min.

[0053] Figures 6 and 7 focus on air flow within tunnel 100 and on the interface between an extruder head 16 and the cooling tunnel. Figure 6 is a schematic representation of the extruder 15, cooling tunnel 100 and water trough 40. Figure 7 is a linear cross sectional view of the wall of air tunnel insert 150 that focuses on the air holes traversing the wall at various angles. Figure 6 shows the most downstream portion of an extruder 15, where hot extruded product 201 emerges from the head 16 and enters the cooling tunnel 100. Since it is schematic, Figure 6 does not include details of the jacket and insert portions of the tunnel, such as inlet holes 160 that are seen elsewhere. The arrows represent the general directionality of the cooling gas flow within the insert portion, in this exemplary and non-limiting embodiment. It can be seen that the first or most-upstream portion of the cooling gas flow is directed upstream, toward the head 16. The head 16 is configured in a way that provides a small space or gap 17 intervening between the downstream exit portion of the extruder 15 and the upstream entry end of the tunnel 100; and the most proximal gas streams are directed toward that gap 17. The next portion of the cooling gas flow within the zone is directed perpendicularly, directly at the advancing extrudate 201, and the next portion and then substantially throughout the remaining portion of the interior 165 cooling tunnel insert 120, gas flow is directed distally or downstream, in the same direction as the extrudate flow {i.e., left to right as shown).

[0054] Specific angles of the gas flow may vary according to the specific size and relative proportions of the air tunnel; the proximally-directed flow, perpendicular, and distally-directed flow as indicated by arrows are representational approximations. It can further be seen that the extrudate product 201 advances through three zones in this exemplary embodiment. One aspect of the function of the proximally directed air flow at the proximal portion of the air tunnel 100 is that hot extrudate emerging from the head 16 is immediately met with cooling flow of gas, precisely-directed, with a flow rate that is highly controllable, and with a circumferential homogeneity around the extrudate. It is theorized that this precise, circumferentially homogeneous and immediate engagement of the hot extrudate is important in the formation of a solid skin around a viscous mass, and the skin is further important in maintaining the integrity and dimensions of the advancing extruded tube.

[0055] Significantly, the hot extrudate is not exposed to room air, which would have a random or uncontrolled flow pattern, and have uncontrolled temperature. In this way, hot extrudate is immediately captured by a controlled and homogeneous cooling environment. Per the theoretical considerations elaborated above, this immediate capture of the extrudate in a controlled cooling environment may be significant in the formation of a stabilizing skin around the extrudate. The overall pattern of cooling gas flow within the tunnel supports and stabilizes the hot extrudate in its transit through the tunnel; no other physical support or physical pulling is required within the tunnel (although a puller engages the extrudate for its transit through the downstream water trough). [0056] One of the functions of the first portion of the cooling air flow within interior space 165, as directed by the direction of holes 160 as they enter space 165 from space 145, thus is to cause the immediate capture of the hot extrudate and quickly and homogeneously step it down to an appropriate temperature (see Figure 13). A function of the subsequent linear portion within space zone 165 is to further step down the temperature of the extrudate such that when it moves into the water trough for a final cooling step, that that transition into water is a mild step that does not physically shock the extrudate. A typical three-zone cooling tunnel embodiment may have a length of about 3 inches to about 6 inches, although particular embodiments may be of a shorter or longer total length. Tunnel zones are not necessarily of the same length; two zones is a minimal number as provided by the invention; some embodiments may have three or more zones. Another feature of the cooling tunnel relates to the throughbore within the end walls of the cooling tunnel 10, through which the extruded product passes. As provided by this invention, the bore 175 is of a diameter sufficiently large relative to the diameter of air supply holes 160 so as to minimize pressure differences that could create turbulent motion of the product within the bore.

[0057] Figure 7 shows a cross sectional view of inlet holes 160 that traverse from the air space 145 between the air cooling jacket 120 and through the wall of air cooling tunnel insert 150. The holes pass through the wall at varying angles in order to provide high resolution homogeneity of flow pattern within the insert volume of the throughbore. The entry angles include forward or downstream directed entry (i.e., in the direction of the extrudate flow, perpendicular entry, and upstream-directed entry (toward the extruder and toward oncoming extrudate). Typical forward and rearward entry angles are about 45 degrees from perpendicular entry (perpendicular to the plane tangential to the surface of the linear axis of the tubular wall), but particular embodiments include angles both smaller and larger than 45 degrees. [0058] Figure 8 is a perspective view of an embodiment of the air tunnel jacket 120, and shows air inlets 130 equally spaced circumferentially. In this embodiment, which is typical, each section of the jacket 120 has a set of air inlets that are connectable with an arm of a manifold 30 (see Figure 3). Visible inside the jacket are O-ring grooves 142, into which an O-ring fits, per details as provided in Figure 11. The embodiment shown has three air inlets per section corresponding to an insert zone, but other embodiments may have fewer (one or two) air inlets, or more (four or more), in all cases, typically spaced equally apart.

[0059] Figure 9 is a perspective view of an embodiment of the air tunnel insert 150. Air inlet holes 160 are typically arrayed in annular patterns as schematically represented here, but may be distributed into any appropriate arrangement. The angles of the air holes through the walls typically are at varied angles, as seen in figure 5. The outer portions of barrier walls 170 appear as ribs projecting out from the base circumference of the insert. This particular embodiment is shown with three zones 165, as represented by the space between the ribs, however the number of zones may vary from fewer than three to more than three.

[0060] Figure 10 is a perspective view of an embodiment of the air tunnel insert 150 within the air tunnel jacket 120, with the jacket rendered as a phantom around the solidly rendered insert. Visible in the phantom rendering of the jacket are air inlets 130, spaced equidistantly around the circumference of the jacket. Visible also are the air holes 160 arranged in rings around the insert. Barrier walls 170 that separate the insert into zones project beyond the circumference of the body of the insert to appear as ribs.

[0061] Figure 11 is a cross section detail of the air space 145 between the jacket and the insert, focused on the abutment of the barrier wall of the insert and an O-ring 140 around the insert that fits into a groove 142 in the wall of jacket 120. When the tunnel is assembled, the O-rings and outer rim of the barrier walls separate the air space 145 into zones that complement the zones within the insert. Figure 11a, within a circle, is a detail expanded from the encircled portion of Figure 11 that shows the arrangement and relationship at the point of contact between the outer portion of the barrier wall 170 of the insert (projecting beyond the wall of insert 150) and the tunnel jacket 120, with an O-ring 140, more specifically, occupying an O-ring groove 142 within the wall of tunnel jacket 120. The O-ring, when in place, conforms to the shape of the available space, thereby creating a seal between the wall of jacket 120 and the wall of insert 150, as well as between lateral zones within the jacket-insert air space 145.

{0062] Figure 12 offers another view of the cooling tunnel; a perspective view of the air tunnel insert 150 rendered as a solid within the air tunnel jacket 120 also rendered as a solid but cut-away half-pipe surrounding the insert. Seen within the wall of the jacket 120 are cut-away views of air holes 130, and within the wall of the insert 150 are air inlet holes 160.

[0063] Figures 3 - 12, as described above, show exemplary embodiments of the invention. A section below, entitled "Detailed Aspects of the Cooling Tunnel and Tubing Fabrication Method", provides further detail on these and other embodiments. As mentioned throughout the application, and as may be understood by those familiar with the art, other variations in the particular embodiments presented here are included within the scope of the invention.

[0064] Figure 13 is a schematic representation of a profile of the temperature of hot extrudate in separate zones within a tunnel insert, through which the hot extrudate flows. In this particular example, a source 20 of cooling gas, clean dry air for example, feeds cooling gas into three gas flow controllers 25, and each gas controller provides a highly controlled gas flow independently to three tunnel zones 165. In this example schematic illustration, the tunnel jacket and the tunnel insert are each represented as a single integrated unit. Product flow path 200 extends from the extruder 15, into the series of three tunnel zones, and ultimately into a water cooling trough 40. In the lower portion of the Figure 13 a representational temperature profile 300 shows that upon exit from the extruder, the extrudate has a temperature typically in the range of about 500° F. As the extrudate advances through the zones, it undergoes a step-wise drop upon entry into each zone, and then a gradual decrease in temperature through the zone. Upon exiting the gas cooling tunnel and about to enter the water trough 40, the extrudate temperature has dropped to a level such that entry into the water does not physically shock or distort the now-cooled extrudate.

[0065] Figures 14A and 14B show schematic views that relate the temperature of various points along the length of an extrudate product 200 and the overall time course of temperature as those points move through a cooling tunnel. Figure 14A shows the temperature of separate surface points along the length of the product at a single point in time, and relates those point temperatures to a chart showing the temperature of those points as would be through the total course of passage through a gas cooling tunnel. From the view provided by Figure 14A it can be seen that all of the surface individual points lie on the same temperature profile line, thus demonstrating the homogeneity of the cooling profile of points along the length of the extrudate as it transits through an embodiment of the cooling tunnel provided by the invention. Figure 14B shows the temperature of separate points distributed within particular circumferential bands around the periphery of the product 200, and shows that all points within that band are at the same temperature, such temperature being displayed in the context of an overall chronological profile of temperature through the cooling tunnel. This view illustrates another aspect of the homogeneity of cooling as provided by embodiments of the invention: not only is there point-to-point homogeneity with regard to cooling rates, but the cooling also occurs at the same time at points on the extrudate that are linearly or longitudinally near each other around the periphery of the product. The parallel nature of cooling from these two perspectives contributes to the formation of a uniform skin around the extrudate, and that uniform skin, in turn supports stabilization of the overall shape and dimension of the extruded product during the cooling period, when shape and size are highly vulnerable to distortion by any type of perturbation. [0066] Figure 15 provides a schematic view of a catheter fabricated around a thin-walled HDPE tube, as provided by this invention, with approximate dimensions included. In some embodiments, a three-layer catheter (liner, braided layer, jacket layer) is provided with an outside diameter of 9 French to about 15 French, and an inside diameter (corresponding to the inside diameter of the liner) that ranges between about 7 French to about 13 French. In these embodiments, the liner includes HDPE, and has a wall thickness of between about 0.0015 inch and about 0.0025 inch. In other particular embodiments, the outside diameter of the tubing may be as low as 1 French. In other embodiments of the invention (not illustrated, but similar in form to the depiction of Figure 12), a three-layer microcatheter is provided that has an inside diameter of less than 6 French (i.e., the inside diameter of the liner), thus allowing the fabrication of microcatheters with an outside diameter of less than 9 French. In these embodiments, the thickness of the wall of the HDPE tubing comprising the liner ranges between about 0.00075 inch and about 0.0010 inch. Various processing for assembling a three-layered catheter are well known in the art; the improvement provided by this invention includes the use of a thin-wall tube as a liner, the thin-wall tube being of dimensions and character as described herein, and as fabricated by embodiments of a system and embodiments of methods, as described herein. [O067] At the dimensions and the configuration of tubing in the small diameters and with the thin wall sizes, as provided by this invention, unexpected material properties may emerge. It is generally accepted, for example, that Teflon (PTFE) is more resistant to abrasion than HDPE, as measured for example with a Tabor testing method. It has been observed by the inventor, however, that with the type of abrasion that thin-wall tubing may encounter under conditions in which they are normally used, in fact, HDPE tubes appear to be more resistant to abrasion than PTFE tubes of the same size. A property of higher-than- expected abrasion resistance would be an important advantage of the use of HDPE in thin wall tubing, particularly as components of medical devices such as catheters.

Aspects of the Overall Process of Fabricating Thin-Wall Catheters

[0068] In embodiments of the inventive method, liner material is placed over a suitably-sized lubricious mandrel, typically coated with PTFE to facilitate easy removal of the mandrel from the completed catheter structure. The liner, still on the mandrel, is passed through a braiding or coiling machine capable of producing the desired braid or coiled pattern with the desired braid or coiling material. In another aspect of the method, a continuous length of HDPE liner with a wall thickness of

0.0015 inch ± 0.0005 inch is extruded over a mandrel for further manufacturing operations such as braiding or coiling, and then jacketing continuously (overlaying another layer of polymer) to encapsulate the braid or the coil. [0069] Following the braiding operation, a jacket layer is constructed over the braided or coiled structure. The jacket layer may be formed of a single layer of an irradiation-stable polymer such as HDPE thin wall material by installing the extruded HDPE thin wall tubing of 0.0015 inch ± 0.0005 inch wall thickness over the braided or coiled structure described above. In other embodiments, Pebax or any irradiation stable polymer may be substituted for HDPE. A heat-shrinkable tubing comprising PTFE for example, may be placed over the HDPE layer and then heated, thus driving the construction together. These particular aspects of the process, including the removal of the PTFE layer, are familiar to those skilled in the art of this methodology. Another approach is jacketing a continuous length of braided composite structure by passing the length through an extruder of proper capability and design, and jacketing the structure continuously with an irradiation-stable thermoplastic resin to attain a suitable resin depth that yields tubing with an inside diameter in the range of 7 to 13 Fr.

[0070] Depending on the particulars of varying aspects of jacketing, continuous length jacketing or constructing one catheter at a time, subsequent manufacturing steps may variously include the methodological approaches A and B, below.

[0071] (A) If heat-shrink tubing has been used during the operation, ultimately it must be removed. After removal of the heat shrink material, any number of further operations such as removal of the mandrel, the addition of markers, balloons, side holes and the like may be accomplished. (B) If the jacket has been applied on a continuous basis, then any number of operations may be performed at this time, such as cutting to length, removal of the mandrel, adding markers, balloons, side holes and the like.

[0072] Adhesion of the thin walls of catheters to other structures by way of heat seals or glued joints is a desirable aspect of catheters, as it can be exploited to add device features and functionality. A native PTFE surface is not generally amenable to such interaction, and requires etching or other surface alterations in order to accommodate such bonding with other materials. A native HDPE surface, on the other hand, has a molecular structure that is much more amenable to heat sealing or gluing directly, and as such, this represents another advantage of HDPE over PTFE. [0073] Following final construction, and with an appropriate choice of add-on polymer components, the HDPE-lined catheter may be irradiated without being degraded. Compatibility with irradiation, as addressed in the background, is another advantage for any type of catheter shaft constructed with the thin wall HDPE material, per the inventive method described herein.

[0074] Further description below provides detail on aspects of embodiments of a method for extruding thin-wall HDPE tubing with an inside diameter in the range of about 7 French to about 13 French. Such thin wall tubing may be applied toward the fabrication of multi-layered catheters, which have a desirably low coefficient of friction, and which may also be irradiated without compromising their structural integrity.

Thin Wall HDPE - Extrusion System and Method

[0075] The invention provides a system and methods for extruding thin wall HDPE tubing for fabricating tubing embodiments with wall thicknesses in the range (in descending order) of about 0.001500 inch, about 0.00100 inch, about 0.00075 inch, about 0.00050 inch, and about 0.00025 inch. Tubing of these thicknesses may be used in the fabrication of catheters with inner diameters that range between about 7 French to about 13 French (0.091 - 0.168 inch). This extrusion technology applies to liner materials, additional layer materials, and final jacket layers of thermoplastic resins such as HDPE, formed into extruded tubing or as a form extruded over a substrate.

[0076] Through the practice of embodiments of the method, by the technological approaches described below, a highly robust process can be operated that results an inside diameter quality control reject rate that approaches zero. Technological aspects of the method, detailed below, are grouped into the following areas: (1) extruder capability and methodology, (2) cooling capability, (3) size control system, (4) puller capability, and (5) storage of extruded product.

(1) Extruder Capability and Method

[0077] a. Various brands and sizes of extruders may be used with embodiments of the method using smaller size extruders such as a ³A inch diameter, 24: 1 L/D single screw machine, although method embodiments may be implemented with other sizes and types of extruder. It is preferable that the machine be "super clean", i.e., having had all ports and orifices opened, brushed out of all contaminants, and then reassembled with clean parts.

{0078] b. Per embodiments of the method, it is preferable that the extrusion machine have tight screw RPM-control to facilitate high quality pumping for uniform mass flow, as accomplished by the motor and motor control types, screw design, or other methods of uniform mass flow control such as feed grooves, gear pumps or twin screw extruders. Preferable screw RPMs for the machine may range from about four (4 RPM) to thirty (30 RPM), although other particular embodiments may make use of a broader RPM range.

[0079] c. Per embodiments of the method, the extrusion machine preferably has a quality temperature control system(s) for all heating and cooling zones common to any type or size of extruder. For purposes of this invention, the heating zones for a ³A inch diameter, 24:1 L/D size extruder would include the feed throat, zone one (feed zone), zone two (transition zone), zone three (metering zone), zone four (clamp zone), zone five (adapter zone), zone six (head zone), and zone seven (die heater).

[0080] d. As part of the pumping system, per embodiments of the invention, an extruder uses a screw or screws commonly known in the extrusion art for any size extruder; screws may include double wave barrier screws, Maddock mixing screws, Saxton mixing screws, and CRD Mixer screws. [0081] e. Following the insertion of the extruder's screw to the extruder barrel, the next component typically installed is a breaker plate with screen pack. Preferably for a ³A inch diameter extruder, the breaker plate has a set number of through-holes of a diameter and length along with a design to allow uniform resin flow through the breaker plate, with minimum resin holdup caused by dead zones with an L/D ratio of about 3.6. Where applicable, a screen pack may be used to improve the condition of the melt, and would include screens of the following mesh sizes: 40, 60, 80, 100, and 150.

[0082] f. Per embodiments of the invention, the extruder adapter and cross-head are typically of a modern design of stainless steel construction that limits knit lines, such designs and materials being known in the extrusion arts. Where used, these components are preferably pre-cleaned, assembled, and installed using proper care to eliminate damage, and may further be pre-adjusted for general concentricity with the system cold.

[0083] g. Per embodiments of the invention, the process of tubing extrusion requires a tool set of suitable dimensions to obtain the desired tubing dimensions. A process may be further specified with regard to the ratios that relate such dimensions to each other. A group of draw-down ratios can be extracted from the dimensions (in absolute terms) of the tool set, die and tip. Draw-down refers to the process of pulling or drawing the molten plastic exiting from the annular gap of the tool set down to the specified dimension of the tubing. The success of an extrusion process may be associated with a preferred range for each of these ratios, as defined by the expressions below:

Wall DRAW (WD): (Die ID - Tip OD) / (Tube OD - Tube ID) Diameter Draw (DD): (Die ID + Tip OD) / (Tube OD + Tube ID)

Area Draw: (WD) x (DD)

Draw Ratio Balance: (Die ID / Tube OD) / (Tip OD / Tube ID)

[0084] In addition to the draw-down ratios that relate the tool set and the tubing product, the ratio of the cross sectional areas of the tool set itself and the annular gap between the die and tip is also of relevance to the success of an extrusion process. If the cross sectional area is too low, the head pressures and shear rates of the resin passing through the small annular gap will be such that the high pressures will limit the extruder RPM and line speed and the physical properties of the tubing may be adversely affected by high shear. In general, the difficulty of the run may be increased because of the low cross section areas and resultant high pressures and high shear. The tool sets chosen for the extrusion of HDPE thin wall tubing, per embodiments of this invention, have specific preferred draw-down ratios as well as specific cross sectional areas as related in Table 1 below.

[0085] The "tooling" may be of a solid type design typical of dies and tips known in the extrusion art with suitable land lengths from 0.025 inch to 0.200 inch. Also the tooling, with similar land lengths as mentioned above, may be assembled as components or inserts to the die or tip, such as hypo-tube inserts which are all known in the art.

(2) Detailed Aspects of the Cooling Tunnel and Tubing Fabrication Method

[0086] a. A typical extrusion method is a horizontal "open trough" method, with a cooling system of water and/or air-cooling; it may also include extrusion using vacuum sizing, as well as other methods. The use of air pressure, per embodiments of the invention, preferably and typically ranges from about 1 PSI, or less, to about 50 PSI, and is performed with air flow controls that support maintenance of uniform pressure and volume output. Embodiments of the inventive method further include the use of an air cooling tunnel as part of the cooling system. The tunnel may touch the front of the head, or it may be spaced away from the head; the same is true for the tunnel position with respect to a water trough if used, although the position is not limited to any of these positions or locations. The tunnel may also have complete mobility in that it can be moved forward, backward, up, down, left, right, or in a circular fashion to maintain alignment to the extruder head, the extrudate, and the water trough.

[0087] b. Embodiments of the cooling tunnel may include an outer jacket that seals and limits air flow to and from the desired air flow locations. The jacket covers and seals an inner insert that includes cooling zone sealing barrier walls, cooling zones, and air flow holes going from the cooling zone through the insert wall to the bore of the cooling tunnel. The volume of the cooling zone is regulated so as to not exceed the volume of the air flow input.

[0088] Clean dry air (or other suitable cooling gas) enters the zoned air spaces 145 between the jacket 120 and insert 150 of the cooling tunnel assembly 100, and exits therefrom through holes 160 that penetrate through the wall of the insert 120 and enter the longitudinal central bore of the insert 165. As air flowing from the zoned spaces 145 by way of holes 160, air is traveling in the direction of travel of the product 201, and a low vacuum is created at the proximal end of the air tunnel, closest to the head of the extruder. The direction of flow of the air from the holes 160 causes the tunnel air to exit at the end of the tunnel furthest from the head of the extruder. This positive, controlled, uniform and directional exhaust air flow from the tunnel assembly longitudinal bore enhances the uniformity of the cooling of the molten extrudate as it passes through the air tunnel assembly in the same direction as the exhaust air.

[0089] c. Embodiments of the cooling tunnel typically may have a length that ranges between about 3 inches to about 12 inches, but is not limited to these dimensions. The cooling tunnel may have an insert with a through-bore of preferably about 1 inch, but can range from about three eighths of an inch (3/8") to about one and one half inches (1 ¹A.").

[0090] It is also important to maintain a suitable relationship between the diameter of the air flow holes to the diameter of the through bore of the tunnel insert. For example, if the bore of a jacket insert is about 1 inch, a suitable diameter of the air-flow holes would be about 3/32 inch, in accordance with a ratio of 10.6 (i.e., 1/ 3/32 = 10.6). As the throughbore of the insert varies up or down in size, the diameter of the air flow holes needs to vary in order to maintain a suitable ratio between the two diameters, which is preferably between about 5 and 30. Maintaining the proper ratio between the diameters of the air-flow holes to the bore of the insert creates a significant improvement in cooling capability as well as dimensional control over the product and run efficiency. [0091] d. A typical embodiment of the invention includes two or three cooling zones, each with its own air flow control, independently controllable. Embodiments of the tunnel are not limited to this number of cooling zones with flow controllers, and may include from one to six or more cooling zones, each zone having its own air-flow control. Embodiments of typical cooling zones have two to six equally spaced air inputs, and preferably at least three inputs for each zone to insure air flow uniformity. Uniformity of air flow is preferred for this method, other arrangements or numbers of cooling zones that support uniformity of air flow and extruded product control are included as embodiments.

[0092] e. Embodiments of the cooling tunnel zone air-flow holes that extend from the cooling zone through the insert wall to the bore of the cooling tunnel may typically have the following features: they are located radially 6° to 30° apart, of a sufficient quantity, generally about 12 to 60 holes per ring of holes, between 60 to 400 holes per cooling zone, of a suitable diameter of about 0.010 inch to about 0.187 inch, of a suitable length of bore, of a suitable angle of about ±10° to ±90° to the extrudate allowing forward as well as downstream or upstream-directed flow to allow control of cooling of the extrudate melt-exiting the die and tip gap of the tool set at the head of the extruder. Control of the cooling tunnel, per embodiments of the method, generally includes maintaining a uniform cooling of the extrudate melt. Such control provides for a product with the proper dimensions as required for any given inside diameter tube with any given wall thickness, and yielding a high quality control efficiency for any given extrusion run or production rate.

[0093] f. The temperature of the cooling air, per embodiments of the method, may be monitored and controlled within a range from about 7° C to 37° C. Various methods of air temperature control may be utilized such as chilled water, chillers, and vortex-style units. [0094] g. Per embodiments of the method, the tubing exiting the cooling system is typically well- controlled along its length by a system of guides. These may be, by way of example, fixed guides, rotary guides, rod guides, roller guides, wire guides, or any combination thereof.

(3) Inner Diameter Size Control System [0095] A subset of various components of the system may be grouped together for their common role in controlling the inner diameter (ED) of extruded tubing. These components include Tube ID air input control unit, screw RPM, cooling method, line speed and the extruder tool set (die and tip). The tube ID air input controller receives clear dry air (CDA) from a CDA source 20 and distributes it back to the extruder head 16, where it passes through the head and into the inner diameter of the tubing. A tube ID air input controller is known in the art, however per embodiments of the invention, it has been modified so as to have a two-decimal point digital display and to be able to exert control at that level of resolution. Typically, the tube ID air-input controller is operated in a range of about 0.09 inches of water to about 3.0 inches of water.

(4) Puller Capability [0096] a. Per embodiments of the invention, the extrusion method may be preferably accomplished with a puller of suitable quality to allow a uniform rate of pull, ensuring high accuracy of the unit's set line speed, as measured, for example, in feet per minute. Typical puller rates, per embodiments of the invention, are in the range of about 25 feet/min to about 100 feet/min, although in particular circumstances, puller rates could be lower or higher than this typical range. This puller rate control can be established with a variety of known methods that include, for example, motor type and motor controller package, high accuracy high ratio gear boxes or inputs to the gear box. Various types of pullers, including multiple roller units of two or more rollers, double belt units, capstans and other types of pullers may be utilized in embodiments of the method.

[0097] b. Embodiments of the puller are preferably fitted with proper quality and type of roller size and material, puller belt size, and type of material and surface coating of the rolls and belts, as well as other components and materials. A puller, thus-fitted, will have uniform grip of the tube surface without damaging the tube as it is extruded, and it will also release the tube at the proper moment. An air release system may also be utilized at the exit of the puller to assist with tubing removal from the puller.

[0098] c. Various guides may be utilized in embodiments of the method to control the passage of the tubing from the cooling system into and out of the puller system. These guides can be fixed guides, rotary guides, rod guides, roller guides, wire guides or combinations of but not limited to these units. They can be in contact with the product but should not deform it or otherwise alter the physical properties of the product.

(5) Take-up and Storage Method [0099] Any suitable method of storing the extruded bulk product such as reels, coils, cut lengths or other mass storage devices may be applied to properly handle this type of critical product, many such methods are known in the art. Any undue stress added to the extruded product can reduce the quality of the product and therefore care must be taken in handling, storing and packaging this precision product.

[00100] While the methods and devices have been described in some detail here by way of illustration and example, such illustration and example is for purposes of clarity of understanding only. It will be readily apparent to those of ordinary skill in the art in light of the teachings herein that certain changes and modifications may be made thereto without departing from the spirit and scope of the invention.

Examples of implementing the method to provide tubing with a small inside diameter

[00101] Two thin wall extrusion procedures were run to determine the feasibility of extruding tubing with smaller diameters than those originally disclosed in U.S. Provisional Patent Application No. 61/037,151, as filed on March 17, 2008, and as per the preceding disclosure. The methods described in that application were used for all the tooling calculations, setup, and processing conditions, with the processing conditions requiring only slight modification during the Runs 1 and 2 as reported below. These procedures yielded tubing products with inside diameters and wall thicknesses as follows:

Run l Inside Diameter: 0.054" (54 thousandths of an inch) ± 0.0015"

Wall Thickness: 0.0012" (1.2 thousandths of an inch) to 0.0013"

Run 2

Inside Diameter: 0.015"(15 thousandths of an inch) ± 0.00075" Wall Thickness: 0.0015" (1.5 thousandths of an inch) ± 0.0005" [00102] Both extrusion runs (1 and 2) required new tools sets which had not been used previously, as the dimensions had not previously been attempted. Both runs were completed with no complications due to the dimensions of the products nor were significant changes required to the general processing technology. Details of the experimental runs and the processing conditions were as follows:

Run l 1. Temperatures of the clamp, adapter, and head zones were slightly reduced with respect to previous operating conditions: the clamp from 525° F. to 500° F., the adapter from 525° F. to 500° F., and the head from 525° F to 480° F. 2. The cooling tunnel air volumes were run within normal ranges: zone 1 (front) 48.10, and zone 2 (output) 50.0. 3. The tool dimensions were as follows: Die ID = 0.2635" x 0.124", Land Length, and Tip OD

= 0.253" x 0.188" Land Length Run 2

1. Temperatures of the clamp, adapter, and head zones were slightly reduced with respect to previous operating conditions: the clamp from 525° F. to 500° F., the adapter from 525° F to 500° F, and the head from 525° F. to 480° F.

2. The cooling tunnel air volumes for zones 1 (front) and 2 (output) were reduced: front from 15 to 5, output from 28 to 10. 3. The tool dimensions were as follows: Die ED = 0.068" x 0.119" Land Length and Tip OD

= 0.058" x 0.145" Land Length

[00103] Experiments 1 and 2 were run to determine if tubing with significantly lower inside diameters could be produced with the methods as disclosed in Provisional Patent Application No. 61/037,151. Thus, whereas tubing with inside diameters in the range of 91 - 168 thousandths of an inch were disclosed in that application, applicant now discloses examples of HDPE tubing with inside diameters of 54 thousandths of an inch and 15 thousandths of an inch. The fact that only minimal modifications of processing conditions were required to achieve these new dimensions demonstrates the robustness of the overall method. The two products produced from the experiments are apparent "firsts" in the industry, as the dimensions of the high density polyethylene tubes cannot be made via standard extrusion processing techniques. Based on this present disclosure, a claim to this range of inner diameter dimension is now being made; see claim 3a in the listing of claims.

Examples of implementing the method to provide products other than thin wall tubing

[00104] Embodiments of the method which have been directed toward the manufacture of thin wall tubing and small inner diameter tubing, may also be advantageously directed toward the manufacture of other types of extrusion products. For example, embodiments of the method may be directed to the manufacture of thick wall tubing, with walls of a thickness in the range of about 0.003" to 0.035" (about 3.0 to about 35 thousandths of an inch) in thickness and higher. The thick walled tubing can be made from a large variety of resin types including high density polyethylene (HDPE) and a large class of engineering grade resins, which include, for example, polyamides, polyimides, and fluoropolymers.

[00105] In addition to the manufacture of tubing products the technology is also applicable to hollow profile extrusions as well as to solid extrusion profiles of any shape. In all of these types of products other than thin wall tubing (thick wall tubing, as well as other hollow and solid forms), embodiments and aspects of the described method provide a uniform cooling to the surfaces of the hot extrudate, thereby supporting the formation of a skin at a uniform rate across the whole of the extrudate surface. This surface skin, once formed, provides dimensional control and stability to the shape of the product such that the product undergoes minimal shape distortion or disfiguration during further cooling, as provided either by exposure to cooling gas or water. Extrusion product disfigurations common in the industry include product ovality, product warpage, and surface blemishes such as divots, dimples, and pits. [00106] Theoretical considerations connect the application of embodiments of the system and method to both tubing and non-tubular products. Figures 14A and 14B provide schematic representations of aspects of uniform cooling as they can apply to extruded tubing as it transits through an embodiment of a cooling tunnel. The invention, as directed toward the manufacture of thin wall tubing, reliably yields thin walls and small inner diameters, at least in part, because of a cooling of hot extrudate that is uniform with respect to the rate of cooling across the surface of the product (comparing point-to-point, as in Figure 14A), and uniform with respect to that cooling rate occurring contemporaneously at points on circumferential bands around the tube (as in Figure 14B). The same basic principles of uniform cooling, which create a skin on tubing with high resolution homogeneity and integrity, and which, in turn lend integrity to shape and dimensions of the tubing, are applicable to improving the cooling aspect of extrusion processes that produce non-tubular products as well.

[00107] A beneficial application of the method to the production of sheet products can be described as an example with reference to the theoretical aspects of cooling that have been provided. Extruded sheet products are typically extruded with standoffs that are extruded into or onto the surface of the sheet. The standoffs are locking fingers that mutually engage each other when the sheet is folded into a three dimensional shape such as a tube. Alternatively, one tube can be extruded inside another tube along with thin webs that attach to the surfaces of both tubes, keeping the tubes mutually centered and at a set distance. The space between the webs and bounded by the tube surfaces form flow paths for fluids or gasses and then the space is filled by another composition later in the process. Returning to the point being made, it is the heat remaining in these webs or locking finger standoffs that the causes the parts to change shape.

[001081 ^m the case of engineering resins, their high processing temperatures (700° F. or higher) are the source of the distortion problems. Distortion occurs, at least in part, because a thin layer of water forms at the surface of the extrudate and is trapped there as the extrudate enters the cooling water. That water layer quickly vaporizes into steam bubbles that travel with the product down the water trough. A pit of divot forms at the point where the bubble contacts the surface of the extrudate, and this consequent distortion is a flaw that is very hard to prevent or remove. These are common problems known in the industry. In all of these various examples, uniformly pulling the heat out of the skin of the product before it contacts the water portion of the cooling process stiffens the outside of the product thereby protecting the surface against distortion by process complications such as these. Absent the uniformity of cooling per conventional processes, quality control reject rates can be high, and highly variable. Inclusion of the system and methods described herein are anticipated to be able to stabilize processes and significantly lower quality control rejection rates.

[00109] The value of the inventive method to the type of products described here is that it provides new levels of conformational control of products, including control of shape and control of dimensions in absolute terms, as well as decreased incidence of surface blemishes created during cooling of the products. Thus, applications of embodiments of the method are anticipated to yield products that meet high quality standards or specifications regarding, for example, tolerances in terms of dimensions, homogeneity of surfaces, trueness of lines and planes, smoothness of curves, and accuracy in angles. This type and level of product conformation control has not been available in the industry prior to the development of the methods described herein.

[00110] As examples of embodiments of the method being directed toward the manufacture of thick wall tubing, applicant ran experiments that are summarized as follows.

[00111] Thick wall example 1 : embodiments of the method were directed to the production of thick wall tubing. Dimensions of the tubing were as follows: OD = 0.105" (105 thousandths of an inch), ID = 0.086" (86 thousandths of an inch), wall thickness = 0.0095" (9.5 thousandths of an inch). The product was very round, without oval distortion, or other deformation that could have otherwise been caused by the water bath step in the cooling process.

[00112] Thick wall example 2: embodiments of the method were directed to the production of thick wall polypropylene tubing. The run was first attempted as an open trough water cooling only (without gas cooling) type extrusion with the result being the product was highly distorted and it was impossible to extrude the product using only a water trough. Dimensions of the tubing manufactured with gas cooling followed by water cooling, as provided by the invention, were as follows: OD = 0.128" (128 thousandths of an inch), ID = 0.119" (119 thousandths of an inch), wall thickness = 0.0045" (45 thousandths of an inch). As soon as the air-cooling system was installed before the water trough a very round product was produced without any distortion of the extrudate.

[00113] Unless defined otherwise, all technical terms used herein have the same meanings as commonly understood by one of ordinary skill in the art of manufacturing thin wall polymeric tubing. Specific methods, devices, and materials are described in this application, but any methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. While embodiments of the invention have been described in some detail and by way of exemplary illustrations, such illustration is for purposes of clarity of understanding only, and is not intended to be limiting. Further, while some theoretical considerations have been advanced in order to convey an understanding of the invention, such elaboration of theory has no bearing on claims to the invention. Various terms have been used in the description to convey an understanding of the invention; it will be understood that the meaning of these various terms extends to common linguistic or grammatical variations or forms thereof. Moreover, any one or more features of any embodiment of the invention can be combined with any one or more a features of any other embodiment of the invention, without departing from the scope of the invention. Still further, it should be understood that the invention is not limited to the embodiments that have been set forth for purposes of exemplification, but is to be defined only by a fair reading of claims that are appended to the patent application, including the full range of equivalency to which each element thereof is entitled.

Claims

CLAIMSWhat is claimed is:

1. A method of cooling tubular extrudate as it emerges from an extruder, comprising: feeding hot extrudate from an extruder into a zoned cooling tunnel, the tunnel including an insert within a jacket and an air space between the jacket and the insert; providing a flow of clean gas; controlling the flow of clean gas to the jacket with high precision; controlling gas flow through the jacket such that within the jacket-insert space gas flow is substantially uniform within a zone; and controlling gas flow from the jacket-insert space into the tunnel insert such that air flow around the extrudate is radially homogenous with regard to flow rate and temperature.

2. A system for cooling tubular extrudate as it emerges from an extrusion line, comprising: a source of cooling gas; a gas flow controller configured to control gas flow from the source with high precision; and a multiple-zone cooling tunnel configured to receive gas flow from the gas flow controller, the zones configured for independent gas flow paths, the tunnel including a jacket and an insert within the jacket, a zone-specific gas flow path extending through the jacket and into the interior of the insert, the gas flow path within the interior of the insert configured for radially homogeneous gas flow around the extrudate.

3. A thin wall high density polyethylene tube comprising: a wall of a thickness between about 0.0010 inch and about 0.0025 inch and an inside diameter between about 0.015 inch and about 0.168 inch, the tube being fabricated by a process that includes: feeding hot tubular extrudate from an extruder into a zoned cooling tunnel that includes an insert within a jacket and an air space between the jacket and the insert; providing a flow of cooling gas; controlling the cooling gas flow to the jacket with high precision; controlling the gas flow through one or more inlets in the jacket such that air flow into zones of the jacket-insert space is substantially uniform within each zone; and controlling the air flow into a cooling tunnel insert zone from the jacket-insert space such that an air flow pattern within the insert is radially homogenous around the extrudate with regard to flow rate and temperature.

4. A medical catheter with an outside diameter that ranges between about 9 French and about 15 French, the catheter comprising: a high density polyethylene liner having a wall of a thickness between about 0.0010 inch and about

0.0025 inch and an inside diameter between about 0.015 inch and about 0.168 inch,; a braided envelope enclosing the high density polyethylene liner; and a j acket enclosing the braided envelope.

5. A method of cooling tubular extrudate as it emerges from an extruder comprising feeding hot extrudate from an extruder into a zoned gas cooling tunnel, the tunnel including an insert within a jacket and an air space between the jacket and the insert.

6. The method of claim 5 further comprising: feeding partially cooled extrudate from the zoned cooling tunnel into a water trough.

7. A method of cooling tubular extrudate as it emerges from an extruder comprising: conveying hot extrudate from the extruder through a zoned gas cooling tunnel prior to feeding the extrudate downstream into a water trough; and supporting the hot extrudate within the zoned gas cooling tunnel by providing a substantially inward and downstream directed air flow through the tunnel, the air flow radially homogeneous around the extrudate.