Mandrel structure for use in manufacture of cellulose food casing

Abstract: A mandrel 26 over which a tube 16 of non derivatized cellulose is extruded depends from the extrusion orifice 15. The mandrel 26 has a shaft portion 28 and a sizing portion 30 which is of greater diameter than the orifice 15 and serves to "size" the extruded cellulose tube. The sizing portion 30 has a plurality of orifices 60 which form a diamond pattern on the outer surface of the sizing portion 30. The orifices allow gas bubbles which are evolved during extrusion to pass to the interior of the mandrel 26 to be vented through a conduit (54 in figure 2). An upper band (59 in figure 2) of the sizing portion 30 has no orifices and forms a seal between the tube 16 and the sizing portion 30 to prevent passage of gas therebetween.

Description: TECHNICAL FIELD The present invention relates to a method and apparatus of forming a seamless cellulose tube, suitable for use as a food casing, using a solution of nonderivatized cellulose, tertiary amine N-oxide and water. BACKGROUND OF THE INVENTION Cellulose food casings are well known in the art and are widely used in the production of stuffed food products such as sausages and the like. Cellulose food casings used in the manufacture of small diameter sausages such frankfurters and the like generally are seamless tubes formed of a regenerated cellulose and contain a plasticizer such as water and/or a polyol such as glycerine. Plasticization is necessary because otherwise the cellulose tube is too brittle for handling and commercial use. Cellulose food casings of pure regenerated and non-reinforced cellulose are hereinafter referred to simply as "cellulose casing". When intended for the manufacture of frankfurters, cellulose casing generally has a wall thickness ranging from about 0.025 mm to about 0.076 mm and is made in tube diameters of about 14.5 mm to 203.2 mm. Cellulose casing is most commonly produced by the well known and so called "viscose process" wherein viscose, a soluble cellulose derivative, is extruded as a tubular film. The annular extrusion die extends through the bottom of a coagulating and regenerating bath so that the extrusion proceeds in an upward direction through the bath. By extruding upwardly directly into the regenerating bath, the bath liquid supports the extrusion which is fragile and not self supporting during the initial phase of regeneration. After sufficient regeneration to be self supporting, the tube undergoes additional processing steps and is then washed, plasticized with glycerine or other polyol, and dried. Drying usually is accomplished while the tube is inflated with air at a pressure sufficient both to maintain a constant tube diameter and to orient the film. The present invention involves an alternate cellulose production method in which a cellulose solution is formed by means of a simple dissolution rather than requiring the formation of a cellulose derivative prior to forming a soluble substance (as in the viscose process). A cellulose dissolution process is described, for example, in US-A-2179181 wherein a natural cellulose is dissolved by a tertiary amine N-oxide to produce solutions of relatively low solids content. The cellulose in the resulting solution is "nonderivatized" in that the natural cellulose was not chemically reacted prior to dissolution to produce a soluble cellulose derivative as would occur for example, in the viscose process. US-A-3447939 discloses use of N-methyl-morpholine-N-oxide (NMMO) as the tertiary amine N-oxide solvent wherein the resulting solutions, while having a low solids content, nevertheless can be used in chemical reactions involving the dissolved compound, or to precipitate the cellulose to form a film or filament. More recent patents such as US-A-4145532 and US-A-4426288 improve upon the teachings of US-A-3447939. US-A-5277857 discloses a method and apparatus for manufacturing cellulose food casing from a solution comprising nonderivatized cellulose, NMMO and water. In US-A-5277857, nonderivatized cellulose in a molten state, and contrary to the viscose process, is extruded as a tubular film downwardly through an air space and into a non solvent liquid such as a water bath. In the water bath, the NMMO solvent is removed to regenerate or precipitate the nonderivatized cellulose which is then washed of residual solvent, plasticized and dried. In US-A-5451364 the manufacturing method as disclosed in US-A-5277857 is improved by the discovery that extruding the thermoplastic cellulose solution through a long air gap improves the properties of the resulting tubular cellulose film. In particular, US-A-5451364 discloses that the air gap should be over 152.4 mm and preferably from 152.4 mm to 304.88 mm long and perhaps longer. Both US-A-5451364 and US-A-5277857 further disclose the use of a mandrel which depends from the extrusion die and about which the thermoplastic cellulose solution is extruded. This mandrel extends through the air gap and has its lower end face disposed below the level of the non solvent liquid bath. The mandrel for most of its length is a slender shaft. The lower portion, however, is larger in diameter and is as large as, or larger than, the extruded tube diameter so it contacts around the entire inner circumference of the extruded tube. The mandrel shaft, being smaller in diameter, is radially spaced from the inner surface of the extruded tube. The large diameter lower portion of the mandrel serves to size the tube as it enters the bath. Also, since it confacts the extruded tube, the enlarged lower portion of the mandrel together with the extrusion die provide spaced bearing points for stabilizing the extruded tube and preventing it from wandering. The mandrel also is used to introduce a non solvent liquid into the interior of the extruded tube. One function of this introduced non solvent liquid, among others, is to lubricate around the circumference of the lower portion of the mandrel to prevent the extruded tube from binding as it passes over the surface of the lower portion or blocking when it later is collapsed to a flat width. In this regard, a non solvent liquid or "inner bath" is introduced inside the extruded tube through ports in the mandrel shaft. This inner bath flows down the mandrel and pools where the extruded tube meets the enlarged lower end of the mandrel. This pooling distributes the non solvent around the mandrel so the entire outer circumference of the mandrel lower portion is wetted. Non solvent liquid then flows off the mandrel and to the bath within the extruded tube. US-A-5766540 discloses a mandrel structure according to the preamble of claim 1 allowing extrusion through even longer air gaps of up to 500 mm or more. US-A-2987765 relates to a process and apparatus for continuously forming tubular thennoplastic stock to a predetermined cross section by the use of gas pressure. US-A-5759478 discloses that certain properties of the cellulose film formed by the tubular extrusion as described in US-A-5277857 are enhanced by increasing the length of the enlarged lower or "sizing portion" of the mandrel. It is speculated that maintaining the extruded tube in a stretched condition for a longer time by keeping it in contact with the sizing portion of the mandrel allows desirable orientation characteristics of the gel tube to "set" during the solvent extraction process. Whatever the reason, a longer contact time with the sizing portion of the mandrel was desirable and US-A-5759478 indicates that a preferred length of the sizing portion is about 50 mm. However, as disclosed in US-A-5759478, increasing the length of the sizing portion of the mandrel gives rise to another problem. This involves the removal of gas bubbles from the interface between the surface of the sizing portion and the inside surface of the extruded tube that likely are formed by out gassing of air from the extruded tube. In US-A-5759478, these bubbles are removed through a series of interconnected circumferential grooves formed in the surface of the sizing portion. It now has been found that both the stability of the extrusion process and properties of the casing are further improved when the length of the sizing portion is increased to lengths greater than the 50 mm disclosed in US-A-5759478. Increasing the length of the sizing portion necessitated additional circumferential grooves to provide for the removal of gas bubbles. However, adding more grooves made difficult the drawing of the leading end of the extrusion over the sizing portion on the start-up of extrusion. This is because the increase in the number of grooves increased the likelihood of the leading end of the extrusion snagging on the edge of a groove. Also, contrary to the disclosure of US-A-5759478, the additional grooves did not act as air bearings to facilitate the passage of the extrusion over the sizing portion. Instead, each additional groove added an amount of frictional force so that the total drag caused by the extrusion passing over the many circumferential grooves was not compatible with the smooth passage of the extrusion over the mandrel sizing portion. If there are too many grooves, the frictional force may become so great that the extrusion breaks rather than being pulled over the mandrel. Accordingly, the addition of grooves to provide for gas removal as taught by US-A-5759478 limited the mandrel length and interfered with the benefits that were seen from the use of a mandrel longer than 50 mm. Thus, while it was found that mandrels of up to 152 mm or more were desirable from the standpoint of the stability of the extrusion process and casing properties, the additional circumferential grooves needed to accommodate gas removal over this length prevented use of the more desirable longer length mandrel. Accordingly, it is an object of the present invention to provide an improved apparatus and method for extruding a seamless tube of a thermoplastic nonderivatized cellulose solution to permit the use of a mandrel having a longer sizing portion to diametrically expand the extruded tube including means to accommodate and remove gas bubbles that are generated at the interface between the surface of the sizing portion and the inner surface of the extruded tube. SUMMARY OF THE INVENTION The present invention provides the method of independent claim 1. The dependent claims specify preferred but optional features. In the present invention, a tube of thermoplastic nonderivatized cellulose is downwardly extruded through a long air gap and into a bath of non solvent liquid as is generally disclosed in US-A-5277857 and US-A-5451364. For purposes of this specification "nonderivatized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces such as hydrogen bonding. "Non solvent" means a liquid which is not a cellulose solvent. As shown in US-A-5277857 and US-A-5451364, extrusion is about a mandrel which depends from the extrusion die. The mandrel has a shaft portion and a lower end portion which is larger in diameter than the shaft. The lower end or sizing portion of the mandrel has a diameter larger than the extruded diameter of the thermoplastic tube so the tube is stretched as it passes over the sizing portion. As noted above, gas bubbles tend to generate at the interface between the outer surface of the sizing portion and the inside surface of the extruded tube. In the present invention, the sizing portion is hollow for most of its length and openings in the outer surface of the sizing portion provide passages to conduct these bubbles directly from the surface to the hollow interior of the sizing portion. A conduit extending up from the hollow interior and through the shaft portion provides for the removal of gas from the hollow interior of the sizing portion. The present invention provides a method of diametrically sizing a tubular extrusion composed of a thermoplastic cellulose solution comprising: a) forming a seamless cellulose tube by downwardly extruding a tube of thermoplastic solution composed of non derivatised cellulose, a tertiary amine oxide and water through an air gap and into a non solvent bath; b) during the course of extruding, causing the extrusion to pass over and about a vertically oriented mandrel having a sizing portion generally in the shape of a hollow cylinder that has a diameter larger than the extruded tube, the mandrel having a lower end disposed below the level of the bath; wherein the mandrel has a smooth outer surface provided with a plurality of openings to the interior of the hollow cylinder, the openings being arranged in a plurality of circumferentially-spaced rows extending longitudinally of said sizing portion which has a length of at least 60 mm, the openings of each row being offset longitudinally from those of its adjacent rows; and the method further comprising: c) passing gas bubbles evolving from the extruded tube at the interface between the extruded tube and sizing portion through the openings and into the hollow interior; and d) removing the gas from the hollow interior upwardly through the mandrel. A seamless tubular film may be prepared from a thermoplastic solution composed of non derivatized cellulose, a tertiary amine oxide cellulose solvent and water by extruding a tube of the solution downwardly from an annular die, through an air space and into an outer bath of a non solvent liquid for regenerating the cellulose from the solution, the extrusion being about a mandrel comprising a shaft which depends from the die and extends through the air space and a sizing portion at a lower end of the shaft which is disposed for the majority of its length below the level of the liquid in the bath. The sizing portion is formed by an annular wall that defines a hollow cylinder having a length of from 60 to 152.4 mm or more. Its outside diameter is greater than the diameter of the tubular extrusion. The outer surface of the sizing portion is smooth and is provided with a plurality of openings that extend through the annular wall to the hollow interior of the cylinder. These openings allow gas bubbles generated at the interface between the outer surface of the sizing portion and the inner surface of the extrusion to pass through the annular wall directly into the hollow interior of the sizing portion. Preferably, the openings arc arranged such that any drag created as the extrusion passes over the openings does not create a moment of force tending to turn or twist the extrusion with respect of the sizing portion. From the interior of the sizing portion, the gas is removed upwardly through a conduit in the stem that opens into the hollow cylinder. Other objects and advantages of this invention will be apparent from the ensuing detailed disclosure and appended claims. DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example with reference to the drawings of this specification, in which: Figure 1 is a schematic view showing a first embodiment of the present invention during the course of steady-state extrusion; Figure 2 is a view on an enlarged scale, partly broken away and in section showing the sizing portion of the mandrel of Figure 1 in greater detail; and Figure 3 is a view similar to Figure 2 showing another embodiment. DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, Figure 1 shows the apparatus generally indicated at 10 during the course of steady-state openation. The apparatus includes a die 12 arranged to downwardly extrude a thermoplastic cellulose solution. The die inlet 14 receives the molten solution under pressure from any appropriate source (not shown). The solution is a solution of a natural cellulose (such as wood pulp) dissolved in a cellulose solvent comprising N-methyl-morpholine-N-oxide (NMMO) and water. Methods of making an appropriate solution (hereafter called "dope") for extrusion as a tubular film are weD known in the art and comprise no part of the present invention. The die has an annular extrusion orifice 15 so the molten dope is extruded as a seamless tube indicated at 16. For purposes of extruding a small diameter food casing, the extrusion orifice is about 12-16 mm in diameter. As shown in Figure 1, the die is arranged above a vessel 18 which contains a bath 20 (sometimes referred to as an "outer bath") comprising non solvent liquid. Water or an aqueous solution containing a low concentration of NMMO are preferred non solvents for purposes of the present invention. The die is spaced well above the level 22 of the bath so the tube is extruded downwardly through a long air gap 24. This air gap may be 304mm to 500mm or more in length. Depending from the die is a mandrel, generally indicated at 26, which extends from the die to below the bath level 22. A preferred mandrel structure is more particularly disclosed in US-A-5766540. In general, the mandrel has a shaft portion 28 and a lower portion 30. The lower portion preferably is larger in diameter than the diameter of the annular extrusion orifice 15. Thus, the lower portion 30 comprises a means for diametrically stretching or "sizing" the extruded tube and is referred to hereafter as the mandrel sizing portion. The mandrel sizing portion 30, including a conical transition section 32 preferably are made of a hydrophobic material such as Teflon® or a filled Teflon®. As the extruded tube 16 enters bath 20, the NMMO solvent is drawn from the tube causing regeneration of the dissolved cellulose to form a so called gel tube 36 of pure cellulose. This gel tube is collapsed to a flat width by a series of fingers or baffles 38 and then is drawn around a driven roll 40 and directed up and out of the vessel 18 for further processing. The tubular extrusion process, as disclosed in US-A-5277857 and US-A-5451364, further includes the introduction of air and a non solvent liquid into the interior of the extruded tube 16. Both air and the non solvent, hereafter called "the inner bath", are introduced through the mandrel. In this respect, an air line 42 connected to the upper end of mandrel shaft 28 provides an air flow through the mandrel to openings (not shown) in the shaft that vent the air into the interior of the extruded tube to hold it open at the start of the extrusion process. Non solvent liquid for the inner bath likewise is introduced into the mandrel through a line 44 that is connected to one or more ports 46 in the mandrel stem 28. After the non solvent exits the mandrel, it flows down the surface of the mandrel shaft 28 and flows down the mandrel shaft to form a pool 48. This pool provides lubrication to permit the passage of the extruded tube over the mandrel sizing portion 30 and starts the regeneration of the cellulose at the inner surface of the extruded tube. The inner bath from the pool passes down between the mandrel sizing portion and the extruded tube and flows into the volume 50 of bath liquid within the tube. As best seen in Figure 2, the mandrel sizing portion 30 is formed by an annular wall 51 that defines a hollow cylinder between the conical transition section 32 and the lowermost end face 34. The lower face 34 of the sizing portion is open and the upper end 52 is slightly dome-shaped. An exhaust conduit 54 extending through the mandrel stem 28 opens into the hollow interior 56 adjacent the upper end 52 of the sizing portion. The outer surface 58 of the cylinder is smooth and unbroken except for a plurality of openings 60 that extend through the annular wall 51 and communicate with the hollow interior 56. In one embodiment, the openings 60 are arranged in four rows (A, B, C and D) extending substantially the full length of the sizing portion wherein each of the two diametrically opposite rows of openings 60A and 60C are spaced at 90° around the circumference of the sizing portion from the two diametrically opposite rows of openings 60B and 60D. Moreover, as shown in Figure 2, the openings of diametrically opposite rows A and C are longitudinally offset from the openings of the diametrically opposite rows B and D so that each of the holes 60A and 60C are about mid way between the openings 60B and 60D. This arranges the openings in a diamond shape pattern 62, represented by dotted line 62, about the surface of the mandrel. It is believed that the placement of the openings at 90° intervals around the circumference of the sizing portion and the longitudinal offset of the openings prevent the drag created by the holes from imparting any rotary motion to the extruded tube around the sizing portion. This ensures that the extrusion is solely in a linear direction with no angular motion that could adversely affect the properties of the casing that is produced. When extrusion is through a long air gap of 300mm and up to 500 mm or more, it is preferred that the mandrel be an extendable mandrel as disclosed in US-A-5766540. With an extendable mandrel, extrusion is started with the mandrel in a raised position such that the lower end face 34 is above the liquid level 22. Extrusion is started and the leading end of the extruded tube 16 is drawn down and over the sizing portion 30. Once the leading end of the extruded tube is beyond the mandrel lower end face 34, the mandrel is extended to the position shown in Figure 1 wherein most, if not all, of the sizing portion is disposed below the liquid level 22. The inner bath introduced through ports 46 collects in a pool 48 which forms around the lower end of the mandrel stem. Non solvent liquid from this pool is carried out from between the inner surface of the extruded tube and the cylinder surface 58 to lubricate this area and permit the passage of the extruded tube over the sizing portion. During the course of extrusion, gas bubbles may appear at the interface between the extruded tube and the mandrel sizing portion. It is not known with certainty why these gas bubbles form. It is speculated that the gas is air which becomes dissolved in the molten thermoplastic solution during the formation and extrusion of the solution. This so called "soluble air" remains in solution so long as the molten thermoplastic solution is under pressure. However, once the solution is extruded, this soluble air is released as an out gas from the extruded tube. In any event, a portion of gas released from the inner surface of the tube becomes trapped between the inner surface of the extruded tube and the surface of the mandrel sizing portion. The openings 60 shown in Figure 2 provide the means for allowing the gas bubbles to pass through the annular wall 51 and into the hollow cylinder. In this respect, the hollow interior of the cylinder is under a slight negative pressure created by evacuation through conduit 54. This draws the gas bubbles through the openings 60 and into the hollow interior of the mandrel sizing portion. The gas bubbles then rise and collect within the mandrel adjacent the domed upper end 52. From there, the gas is removed upwardly through the conduit 54 in the mandrel stem. Also, it is preferred that there be an unbroken band 59 about the sizing portion between the conical section 32 and the uppermost hole 60. This band, together with the negative pressure within the hollow cylinder tends to create a seal that prevents gas bubble from migrating upwardly along the surface of the sizing portion 30 and into the volume of the extruded tube above the sizing portion. Occasionally however, a gas bubble may move upward into the volume within the extruded tube above the sizing portion. If need be, the air introduced into this volume can be adjusted to correct for pressure variations that may be caused by the accumulation of gas in this space. In accordance with the present invention, providing the mandrel sizing portion 30 with the openings 60 allows for the gathering of gas bubbles into the hollow interior of the sizing portion. This gathering of gas bubbles removes them to a location where they will not disrupt the formation of the gel tube. Further, it is preferred that the extrusion process be as linear as possible and that there be no moment of force tending to impart a twist of the extrusion about the mandrel sizing portion. Accordingly, the arrangement of the openings 60 along the smooth surface 58 as shown is such that the drag created as the extrusion passes over these opening does not impart such a moment. Another embodiment is illustrated in Figure 3. As shown in Figure 3, the mandrel sizing portion 30 has a plurality of openings 60 arranged so as to form a diamond shape on the mandrel surface 58. However, instead of four longitudinal rows of openings as in the Figure 2 embodiment, there are eight rows shown in Figure 3. The openings 60 in four of the rows A, B, C and D are equally spaced at 90° intervals around the mandrel and are equally spaced in the rows along the mandrel. A second set of openings 70 are arranged in four rows that are offset both radially and longitudinally from the openings 60 so as to produce the diamond shape patterns on the mandrel surface represented by dotted lines 62. In this respect, the openings 70 arranged in the four rows A, B, C and D are radially offset 45° from the openings 60. The openings 70 also are longitudinally offset so that each of the openings 70 A, B, C and D are located about half-way between the openings 60 A, B, C and D. Having one set of the four rows of openings 60 offset 45° from the four rows of openings 70 as shown in Figure 3 minimizes the distance between openings. With this arrangement, gas bubbles have a shorter time to grow in size before being drawn into one of the openings. In the case of a mandrel having a sizing portion 30 about 25 mm in diameter and 152.4 mm long, a suitable arrangement as shown in Figure 3 provided forty-four openings arranged in four rows of six openings each and four rows of five openings each. The unbroken band 59 just below the conical section 52 was about 12.5 mm wide. The openings in each of the four rows are longitudinally spaced about 25 mm apart. Each opening is less than about 3.2 mm in diameter and preferably is about 0.8 to 1.6 mm in diameter. Holes larger than about 3.2 mm were found to create a drag on the extrusion making start-up difficult. Openings smaller than about 0.8 mm in diameter were not as effective in passing the gas bubbles. Thus it should be appreciated that the present invention allows use of a mandrel sizing portion of up to 150 mm or more in length and accommodates the removal of gas bubbles from the surface of the sizing portion without the drawbacks of having a grooved surface to accommodate such removal.

Claims: A method of diametrically sizing a tubular extrusion composed of a thermoplastic cellulose solution comprising: a) forming a seamless cellulose tube (16) by downwardly extruding a tube of thermoplastic solution composed of non derivatized cellulose, a tertiary amine oxide and water through an air gap (24) and into a non solvent bath (20); b) during the course of extruding, causing the extrusion to pass over and about a vertically oriented mandrel (26) having a sizing portion (30) generally in the shape of a hollow cylinder that has a diameter larger than the extruded tube, the mandrel having a lower end disposed below the level (22) of the bath; characterised in that the mandrel has a smooth outer surface (58) provided with a plurality of openings (60) to the interior of the hollow cylinder (56), the openings being arranged in a plurality of circumferentially-spaced rows extending longitudinally of said sizing portion which has a length of at least 60 mm, the openings of each row being offset longitudinally from those of its adjacent rows; and the method further comprising: c) passing gas bubbles evolving from the extruded tube at the interface between the extruded tube and sizing portion through the openings and into the hollow interior; and d) removing the gas from the hollow interior upwardly through the mandrel. A method according to claim 1, wherein said openings are arranged in four rows extending along said sizing portion, said openings being equally spaced apart in said rows and said rows being spaced at 90° intervals around said sizing portion. A method according to claim 2, wherein diametrically opposite rows of said openings are longitudinally offset from openings in adjacent rows by one-half the distance between the openings in a given row. A method according to claim 3, wherein said openings are such that extrusion of the tube along the sizing portion is solely in a linear direction with no angular motion. A method according to any preceding claim, wherein said mandrel sizing portion has a length greater than 150 mm and each of said openings are between 0.8 to 3.2 mm in diameter.

Inventors: ALBERT JAMES JOSEPH, JERANTOWSKI RONALD JOSEPH, LONG LARRY CLYDE, MCGAREL OWEN JOSEPH, WILLIAMS MARK GRIFFITH, ALBERT, JAMES JOSEPH, JERANTOWSKI, RONALD JOSEPH, LONG, LARRY CLYDE, MCGAREL, OWEN JOSEPH, WILLIAMS, MARK GRIFFITH
Applicants: VISKASE COMPANIES INC, VISKASE COMPANIES, INC.
Application year: 2001
Main classification: B29C47
IPC codes: B29C47, B29K1, C08J5, B29L23