WO1992016350A1

WO1992016350A1 - Apparatus for transporting an open tube of material

Info

Publication number: WO1992016350A1
Application number: PCT/US1992/002035
Authority: WO
Inventors: Ronald P. Swanson; Eugen Will
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1991-03-21
Filing date: 1992-03-09
Publication date: 1992-10-01
Also published as: AU1675192A; CN1066021A; MX9201187A

Abstract

A transport mechanism (28) for pulling and transporting a rotating open tube (16) in a downline direction, without distorting or closing the tube, and while allowing access to a central longitudinal cavity (60) of the tube from the downline side of the tube, is disclosed. The transport mechanism includes a plurality of drive rollers (30) positioned to engage the tube on only the outside of the tube. The drive rollers pull the tube downline without contacting the inside of the tube. The drive rollers are uniformly distributed around the tube and are mounted on preferably all of the sides of a regular polygon. The transport mechanism also includes a motor (78) which rotates the drive rollers around their respective axes and a motor (74) which spins the drive rollers around a central axis substantially colinear with the central axis of the open tube to rotate the open tube.

Description

APPARATUS FOR TRANSPORTING AN OPEN TUBE OF MATERIAL

TECHNICAL FIELD

The present invention relates to an apparatus for transporting a tube of material. More particularly, the present invention relates to an apparatus for transporting an open tube from the outside.

BACKGROUND OF THE INVENTION

Existing blown film processes involve blowing a film tube or bubble into a cylinder and maintaining the cylindrical form until the tube is converted to a web. The tube is blown and drawn out from a die and rotated at a substantially constant speed. Prior to conversion, the tube is collapsed into a two ply sheet to permit the blown film to be further treated and cut into a web. Recent representative examples of collapsed tube blown film processing are found in U.S. Patent No. 4,189,288 to Halter and U.S. Patent No.

4,699,580 to Co. Collapsing boards, rollers, or other tube restricting devices are used to seal and drive the film tube.

However, these closed nip processes have a propensity for wrinkling the film during collapsing. The folds formed during collapse of the tube are permanently creased and the portions around the folds are usually cut off and discarded. Also, the laminating force provided by the nip often causes the two film layers to stick together, thereby eliminating the possibility of coating or coextruding any material that this laminating would damage. Moreover, as the tube is closed, the nip blocks access to the inside of the tube, severely restricting processes such as internal tube cooling or coating. This causes blown film dies to be very complicated and expensive when the interior of the tube is to be treated; air channels must be machined through the extrusion die. Even with these air channels, air passing through the channels is warmed by the die as the die is cooled by the air, both of which are disadvantageous to the web forming process.

In U.S. Patent No. 3,342,657 to Dyer, an apparatus for forming a thermoplastic film is disclosed which forms the film in a tube and cuts the tube into webs without closing the tube. However, although this system uses an annular rotating extrusion die, the tube is not blown. Rather, the tubular film is pulled over mandrels within the tube which serve as cooling and heating devices. Additionally, the tube is stretched and oriented between the mandrels. These mandrels block access to the interior of the tube during tube formation and no devices control the external diameter of the tube. Moreover, rollers must be placed inside the tube to transport the tube downline.

Japanese Kokai Publication No. JP63-151429 to Goto is directed to a method of producing flat films using an open tube process. However, the apparatus appears to be inoperative as shown. Additionally, there is no way to prevent the tube from contacting the internal stage barriers and stopping operation.

SUMMARY OF THE INVENTION

An open tube or open bubble type web forming process overcomes these disadvantages of closed tube processes and offers many additional advantages such as internal tube cooling and coating. This invention provides a transport mechanism for an apparatus which continuously produces an elongate strip of material, such as a strip of tape having an adhesive-coated surface, without intermediate manual handling steps and without wasteful trimming of the edges. The open tube process allows a much wider range of internal tube operating pressures and the transport mechanism allows relatively easy access to the interior of the tube from downline of the extrusion die by not contacting the inside of the tube.

The transport mechanism can be used as part of an open tube film forming apparatus as described in detail in U.S. Patent Serial No. 07/673,285 filed March 21, 1991 concurrently with this application, the specification of which is incorporated by reference. The open tube film forming apparatus includes a frame, an extrusion die mounted on the frame, a transport mechanism mounted on the frame downline of the extrusion die, and a tube cutter mounted on the frame downline of the transport mechanism. The extrusion die continuously extrudes thermoplastic material from a stationary extruder in the downline direction to form a tube with a central generally cylindrical longitudinal cavity. The transport mechanism pulls the tube in the downline direction. A driver rotates the extrusion die and transport mechanism together relative to the tube cutter to provide relative rotation between the tube and the tube cutter. The tube cutter cuts the tube into a continuous elongate web of the desired width as the tube moves downline. An air seal is mounted on the frame for substantially sealing the tube cavity to prevent air from escaping through the open end of the cavity. A tube pressurizer controls air volume in the central longitudinal cavity of the tube to expand the tube laterally outwardly to within the desired range. A novel constraining structure locates the tube concentric with the extrusion die and physically prevents the tube from expanding beyond the inner diameter of the constraining structure.

The transport mechanism pulls and transports an open tube in a downline direction without distorting or closing the tube, and while allowing access to a central longitudinal cavity of the tube from the downline side of the tube. The transport mechanism includes a plurality of drive rollers which engage the tube on only the outside of the tube and pulls the tube downline without contacting the inside of the tube. The drive rollers are uniformly distributed around the tube and are mounted on preferably all of the sides of a regular polygon. The transport mechanism includes two motors. One motor rotates the drive rollers around their respective axes. The other motor spins the drive rollers around a central axis substantially collinear with the central axis of the open tube to rotate the open tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of the primary components of the apparatus of the invention. Figure 2 is a front elevation of the apparatus of Figure 1, illustrating the arrangement of an extrusion die, constraining structure, air seal, transport mechanism, and tube cutter. Figures 3a and 3b are schematic illustrations showing the diameter ranges of the transport mechanism. Figure 4 is an enlarged side elevation of the transport mechanism and air seal of Figure 2.

Figure 5 is a cross-sectional view along line 5-5 of Figure 4.

Figure 6 is an enlarged front elevation of the extrusion die and constraining structure of Figure 2, with portions cut away to illustrate details.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to the schematic illustration of Figure 1 and the side view of Figure 2, apparatus 10 is designed to continuously produce an elongate web or strip 12 of material, including single and multiple layer strips 12, such as adhesive-coated tape. The apparatus 10 generally includes a rotatable extrusion die 14 for extruding a flowable material, such as a thermoplastic polymer material to form a bubble or tube 16. The flowable material travels to the extrusion die 14 via conduits or passageways 18 from one or more extruders 20. A swivel joint 22 permits the passageways 18 to transport material to the extrusion die 14 while it is rotating. Two extruders 20 are used, for example, to produce an adhesive-coated backing. One extruder 20 produces the backing material and the other extruder 20 produces the adhesive material. Upon leaving the extrusion die 14 the tube 16 passes through a diameter constraining device 24 which maintains a constant tube diameter. A lip seal 26 is disposed within the tube 16 downstream of the diameter constraining device 24 and a transport mechanism 28, including a plurality of rollers 30, is located downstream of the lip seal 26. The tube 16 then passes over a mandrel 32 and a slitter or tube cutter 34 cuts the tube 16 into at least one elongate strip 12. The tube cutter 34 continuously cuts the tube 16 into the strip 12 as the extrusion die 14 and tube 16 rotate relative to the tube cutter 34. The strip 12 then passes around an idler roller 36 and is wound on a winder, such as level winder 38.

As used herein, "downline" refers to the general direction that material, e.g., the tube 16, moves when being processed by the apparatus 10. In the figures, the downline direction is upward from the extrusion die 14, as illustrated by the arrow designated by the reference numeral 40 in Figure 1. Downline does not imply any relationship to vertical or horizontal directions.

The extrusion die 14 may be of a conventional heated design used in blown film processes, and preferably is of the type for co-extruding a multilayer tube 16 having, for example, a polymeric backing layer and a pressure-sensitive adhesive ("PSA") or other adhesive layer along the outside of the tube 16. See U.S. Pat. Nos. 3,342,657; 4,643,657; and 4,753,767; Japanese Kokai No. 63-151429; and British Patent No. 1,553,881 (which are incorporated herein by reference) for descriptions of various blown film extrusion dies. Referring to Figures 1, 2, and 6, the extrusion die 14 is rotatable, and may be mounted on the center of a rotating table 42, which is mounted on a frame 44. The table 42 rotates at speeds of up to 115 rpm. A delivery line 46 is located directly below the die at the center of the axis of rotation and is connected to a rotary union such as the swivel joint 22. The swivel joint 22 can be any commercially available part used for handling high viscosity fluids at temperatures up to 288°C (550°F) and pressures up to 41 x 10^β N/m² (6000 psi). The stationary end of the swivel joint 22 is connected by another delivery line 48 to the stationary extruders 20 so that the flowable material can be delivered to the rotating die 14 from each extruder 20. The die 14 is electrically heated, with the wiring connections being made through the slip ring assembly mounted concentrically to the delivery line 46 and centered on the axis of rotation. This arrangement allows for complete rotation of the die 14 during operation. This rotational motion is transmitted to the extruded tube 16.

The extrusion die 14 preferably includes a mandrel 50 and an annular opening 52 surrounding the mandrel 50. The flowable material is substantially continuously extruded through the annular opening 52 to form a generally cylindrical tube 16, which moves along its central longitudinal axis in the downline direction as the tube 16 is formed. An air passageway 54 is provided through the mandrel 50 and extrusion die 14. One end of the air passageway 54 is in fluid communication with a source 66 of pressurized air via an air conduit 58 and the other end of the air passageway 54 is in fluid communication with the central cavity 60 of the tube 16. Pressurized air is provided through the air passageway 54 to the central cavity 60 of the tube 16 during the initial start-up of the apparatus 10 to inflate and expand the tube 16. The air passageway 54 is sealed with a suitable valve mechanism (not shown) during regular operation of the apparatus 10. The inflation of the tube results in a blown tube as distinguished from a non-inflated tube. Two stationary extruders 20 may be provided for melting, mixing and delivering the flowable material and any adhesive materials to the extrusion die 14 via the swivel joint 22. These extruders 20 may be of any suitable design, such as a screw-operated extruder having a driven screw that melts and pushes material through a heated barrel for delivery to the extrusion die 14. The extruders 20 do not rotate with the extrusion die 14.

A driver 64 is mounted on the frame 44 for rotating the extrusion die 14 and transport mechanism 28 relative to the tube cutter 34. The tube cutter 34 can rotate while the extrusion die 14 and transport mechanism 28 remain stationary, although it is preferred that the cutter 34 remain stationary while the extrusion die 14 and transport mechanism 28 rotate. This provides relative rotation between the tube 16 and the tube cutter 34 to enable the tube cutter 34 to cut the tube 16 into a substantially continuous web of a desired width as the tube 16 moves downline. The driver 64 can be a DC servomotor with a tachometer which controls the rotation speed of the die 14 by an input voltage signal.

The transport mechanism 28 is mounted on the frame 44 downline of the extrusion die 14 and pulls the tube 16 in a downline direction. The transport mechanism 28 pulls the open tube 16 without distorting or closing the tube 16 thereby allowing access to the tube 16 from the downline side of the tube 16. Additionally, the transport mechanism 28 drives from the outside of an open, flexible tube 16 rather than from the top of a closed portion of the tube as in known methods. Thus, the tube 16 is driven from only one side by engaging the tube 16 on only the outside of the tube 16 without contacting the inside of the tube 16. The transport mechanism 28 includes a plurality of, and preferably four, drive rollers 30 positioned to engage and pull the tube 16 downline. The drive rollers 30 are preferably uniformly distributed around the tube 16. Thus, the drive rollers 30 are mounted on preferably all of the sides of a regular polygon, such as a square when four drive rollers 30 are used. When four drive rollers 30 are used, the drive rollers 30 are positioned on the sides of a square having a perimeter preferably no smaller than the desired circumference of the tube 16 and no larger than eight times the desired radius of the tube 16 as illustrated in Figures 3a and 3b. The drive rollers 30 are mounted on a rotary table 68 so that the downline velocity of the tube 16 and the rotation of the tube 16 can be independently controlled by similar motion of the drive rollers 30 through a slip ring assembly 70. The rotary table 68 is mounted on a support track 72, which serves as a bearing for the transport mechanism 28, and the slip ring assembly 70 is disposed upstream of the support track 72. Thus, the transport mechanism 28 controls the motion of the tube 16 in both the vertical and rotational directions. In the embodiment illustrated in Figures 4 and 5, separate motors are used to control downline velocity and rotation. A rotational motor 74 rotates the rotary table 68 through a drive chain 76 to provide rotational velocity. A drive motor 78 drives or rotates the drive rollers 30 through a drive belt 80 to provide downweb velocity. The motors 74, 78 can be conventional servomotors which operate through standard bevel gears or flexible cable. A tachometer 82 measures the rotational speed of the drive rollers 30. The surface of the drive rollers 30 is preferably coated with a high friction, low tack silicon rubber which combines high friction characteristics with high release characteristics. The internal tube pressure forces the tube 16 against the rollers 30 and combines with the high friction to draw the tube 16 through the transport mechanism 28. The high releasability permits the tube 16 to pass between the rollers 30 without adhering to the rollers 30 and being damaged. The internal tube pressure which governs the operation of the transport mechanism 28 is controlled by the operation of an air mandrel 32 described below. The air pressure below the lip seal 26 is controlled by one or more support tubes 84 which are inserted into the interior of tube 16 through the lip seal 26 at one end and are connected to a source of pressurized air (not shown) at the other end. These tubes 84 inject or remove air to control the diameter of the tube 16. Additionally, it has been found that the transport mechanism 28 with four rollers 30 can generate over 89 N (20 lbs) of lifting force depending on the material, far greater than is needed to draw the tube 16 through the apparatus 10 and far greater than expected to be provided by any one-sided transport mechanism 28.

The diameter constraining structure 24 is mounted on the frame 44 between the extrusion die 14 and the transport mechanism 28 and serves two important functions. It locates and aligns the tube 16 with the other components of the apparatus 10 and it controls expansion of the tube 16 to an outside diameter within a desired range after the tube 16 is formed by the extrusion die 14. The constraining structure 24 of the illustrated embodiments is cylindrical with both ends open to allow passage of the tube 16 through its interior. The constraining structure can also be elliptical or polygonal having a low aspect ratio as long as the circumference is nearly constant along the axis of the tube 16 in the region of the frost line 86. The constraining structure 24 is located to physically prevent excessive radial or lateral expansion of the blown film tube 16 in the region at and below the frost line 86, the line at which the flowable material of the tube solidifies. "Solidifies" is defined herein as the state change from a liquid to a solid phase. More precisely, the frost line 86 is the location where the tube film has cooled to a temperature at which the resistance of the film to further axial stretching is greater than the force exerted by the pressure difference between the interior and the exterior of the tube 16. Downline of the frost line 86, the tube 16 does not expand further.

The constraining structure 24 is a cylindrical tube positioned downline of and concentric with the extrusion die 14 and air ring 62 and includes an annular sleeve 92. The sleeve 92 has an inside diameter approximately equal to or slightly larger than the desired final tube diameter. The tube 16 is pressurized to reach a diameter which is close to the internal diameter of the sleeve 92.

A tube sensing and controlling system senses a portion of the tube 16 before the tube 16 enters the constraining device 24 and monitors the shape of the tube 16. The tube sensing system then provides a signal to a tube pressure regulator which adjusts the air volume inside the tube 16 to move the frost line 86 and adjust the tube diameter when the sensed portion of the tube 16 varies from the desired range. The tube sensing and controlling system can include two infrared (IR) beam sensors mounted upline of the constraining structure 24 which monitor the shape of the tube 16 and send signals to the pressure regulator to adjust the air volume in the central longitudinal cavity 60 of the tube 16. The sensors sense when a portion of the tube 16 entering the passageway 94 has an outer diameter larger than a first predetermined size or smaller than a second predetermined size smaller than the first predetermined size. The sensors then provide a feedback signal to solenoid valves to decrease or increase air volume in the tube cavity 60.

A pulse generator 122 is connected to one roller 30 of the transport mechanism 28 and supplies a feedback signal to the digital controller of the drive motor 78 of the transport mechanism 28. This signal also is sent to servomotors which control the rotation of the die 14, transport mechanism 28, and tube cutter 34.

The lip seal 26 is mounted on the frame 44 for sealing the cavity 60 of the tube 16 to prevent air from escaping through the open end of the cavity 60 and help control the pressure within the tube 16. The seal 26 is a disk-shaped object with an elastomeric outer lip.

The tube cutter 34 is mounted on the frame 44 downline of the transport mechanism 28 for cutting the tube 16 into a continuous elongate strip 12. The cutter 34 also defines the open end of the central cavity 60 of the tube 16, and the tube cutter 34 is offset at an angle with respect to the downline direction. The tube cutter 34 can use score, shear, or razor slitting systems, depending on the web material.

The apparatus 10 operates to produce an elongate strip of material 12 in the following manner. First, flowable material such as a polyolefin like polyethylene is extruded through the annular die orifice of the extruder 20 in a downline direction to form the open-ended tube 16 with a generally cylindrical central longitudinal cavity 60. As the tube 16 is formed, the central longitudinal cavity 60 of the tube 16 is pressurized with a fluid, preferably air, to expand the tube 16 laterally outwardly to a desired outer diameter within a predetermined range. The tube 16 is transported downline by the transport mechanism 28 which engages and pulls the tube 16, and rotates in synchronism with the extrusion die 14.

The position of the tube 16 and its outer diameter are controlled by moving the tube 16 through the diameter constraining structure 24 which does not rotate. The outer diameter of the portion of the tube 16 prior to entering the constraining structure is sensed to adjust the air pressure inside the tube 16 to expand or contract the tube 16 when the sensed portion of the tube 16 varies from the desired range. A feedback signal is provided to change air pressure within the tube 16 when the outer diameter is not at the desired size. After passing through the constraining structure 24, the tube 16 is pulled over the lip seal 26 which maintains the pressure in the upline section of the tube 16, while allowing controlled access through passageways in the seal 26. Finally, the tube 16 floats over the air bearing mandrel 32 and is then cut with a cutter 34 into a continuous elongate strip 12 at an angle offset from the downline direction. The rotational velocity of the the extrusion die 14, the transport mechanism 28, and the cutter 34, and the downweb speed of the tube 16 are controlled by the sensing and controlling system to vary the width of the elongate strip 12.

Claims

CLAIMS :

1. A transport mechanism (28) for pulling and transporting an open tube (16) in a downline direction, without distorting or closing the tube (16), while allowing access to a central longitudinal cavity (60) of the tube (16) from the downline side of the tube (16), the transport mechanism (28) comprising: a plurality of drive rollers (30) positioned to engage the tube (16) on only the outside of the tube (16) and pull to the tube (16) downline without contacting the inside of the tube (16), wherein the drive rollers (30) are uniformly distributed around the tube (16) and are mounted on preferably all of the sides of a regular polygon; and means (78) for rotating the plurality of drive rollers (30) around their respective axes.

2. The transport mechanism (28) of claim 1 further comprising means (74) for continuously spinning the plurality of drive rollers (30) around a central axis substantially collinear with the central axis of the open tube (16) to rotate the open tube (16).

3. The transport mechanism (28) of claim 1 comprising four drive rollers (30) which form a square.

4. The transport mechanism (28) of claim 2 wherein the drive rollers (30) are positioned to generate over 89 N of lifting force depending on the material used for the open tube (16).

5. The transport mechanism (28) of claim 1 wherein the perimeter of the polygon formed by the drive rollers (30) is no smaller than the circumference of the tube (16) and no larger than eight times the radius of the tube (16).

6. The transport mechanism (28) of claim 1 wherein the surface of the drive rollers (30) combines high friction characteristics with high release characteristics such that during use the internal tube pressure of the open tube (16) forces the tube (16) against the rollers (30) and combines with the high friction to draw the tube (16) through the transport mechanism (28) while the high releasability permits the tube (16) to pass between the rollers (30) without adhering to the rollers (30) and being damaged.

7. The transport mechanism (28) of claim 1 wherein the downline direction is vertically upward.