WO1996035069A1

WO1996035069A1 - A thin walled tube having an internal rib

Info

Publication number: WO1996035069A1
Application number: PCT/US1996/005650
Authority: WO
Inventors: Zdenek Frank Cerny; Charles Robert Kroeger; Manfred Gerhard Hoffmann; John William Thirlwell
Original assignee: Kimberly-Clark Worldwide, Inc.
Priority date: 1995-05-04
Filing date: 1996-04-19
Publication date: 1996-11-07
Also published as: AR001856A1; CA2217282A1; MX9708499A; AU5569196A

Abstract

A thin walled tube (10) is disclosed having a wall (12) which extends between first (14) and second ends (16). The wall (12) has a thickness (t) of less than about .03 inches. The tube (10) also has a longitudinal extending rib (22) formed on an inner periphery (18) thereof. The rib (22) enables the thin tube (10) to be injection molded from a thermoplastic material using standard temperature ranges and relatively short cycle times.

Description

A THIN WALLED TUBE HAVING AN INTERNAL RIB

FIELD OF THE INVENTION This invention relates to a thin walled tube having an internal rib which can be injection molded from a thermoplastic material. More specifically, this invention relates to a thin walled tube which has a longitudinally extending rib formed on the inner periphery thereof which facilitates distribution of molten plastic to the ends of the tube during the molding process.

BACKGROUND OF THE INVENTION Tubular thermoplastic members have been manufactured by both extrusion molding and injection molding. Such tubes can be utilized in many different products and can vary in design and dimensions. If a tube has a.uniform cross-sectional configuration it may be cost effective to form it by extrusion molding. However, if the tube has a non-uniform cross-sectional configuration it may be advantageous to form it by injection molding. It has been found that when tubes need to be produced having a wall thickness of less than about .03 inches (about .76 mm) that injection molding may be best in yielding consistent quality tubes which exhibit lower standard deviations along the major and/or minor diameters. However, even when tubes are injection molded, difficulties can be encountered during the molding process when the wall thickness is relatively thin and the overall length is equal to or greater than about 2 inches (about 50 mm). Such difficulties increase when one desires to injection mold such tubes using a very short cycle time. Under these conditions, the molten plastic can solidify before the tube is completely molded. The inability to quickly move the molten plastic to the ends of a tube becomes more pronounced when the tube wall tapers in thickness, especially when it tapers downward from one end to the other end. To overcome this problem, one could change the characteristics of the plastics, for example, one could use a plastic exhibiting a higher melting point. Other solutions to the problem include increasing the cycle time, raising the operating temperature of the injection mold, utilizing smaller cavity molds where possible, or using a combination of the above-identified features to injection mold an acceptable tube. Each of these changes will increase the cost of the finished tube. Furthermore, when the temperature of a mold is raised, it is normal to increase the cooling cycle time in order to form a finished article. Operating an injection mold at a higher temperature can also limit the types of thermoplastics which can be molded to those having a relatively high melting point. Plastics with higher melting points may be more expensive.

One use for tubes having a thin wall structure is in the manufacture of tampon applicators. Tampon applicators are used by women to position a catamenial tampon into their vaginas. Such applicators are commercially available in either a single tube design or in a two-piece design. In the two-piece applicator, the absorbent tampon is retained in an outer tube and can be expelled therefrom by an inner tube or plunger which is telescopically assembled therein. The inner tube can be constructed with a very thin wall because most of the applied forces are directed along its longitudinal, central axis. Since millions of such tubes are manufactured yearly, any reduction in the amount of material needed to construct a single tube can result in a substantial cost savings.

For one common style of two-piece applicator, the inner tube or plunger has its ends flared after assembly with the outer tube so that the two members will not separate. It is easy to flare the ends of the inner tube when the wall thickness of the inner tube is relatively thin.

Now a thin walled tube having an internal rib has been invented which can be injection molded from a thermoplastic material at commercial speeds and in a cost effective and efficient manner. SUMMARY OF THE INVENTION Briefly, this invention relates to a tube having a thin wall which can be injection molded. The tube has a first end and a second end and the wall tapers to a smaller thickness adjacent one of the ends. The tube also has a rib formed on its inner periphery which extends along at least a portion of the entire length of the tube. The rib facilitates distribution of molten plastic to both ends of the tube during the molding process.

The general object of this invention is to provide a tube having a thin wall which can be injection molded from a thermoplastic material. A more specific object of this invention is to provide a thin walled tube which has a longitudinally extending rib formed on the inner periphery thereof which facilitates distribution of molten plastic to the ends of the tube during the molding process. Another object of this invention is to provide a tube having a wall thickness of less than about .03 inches (about .76 mm).

A further object of this invention is to provide a thin walled tube having a constant external configuration and having a varying thickness. Still another object of this invention is to provide a thin walled tube which can be injection molded economically and efficiently.

Still further, an object of this invention is to provide a thin walled tube which can be used as the inner tube of a two-piece tampon applicator.

Other objects and advantages of the present invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a planar view of a straight cylindrically-shaped tube. Fig. 2 is a cross-sectional view of the tube shown in Fig. 1 depicting the longitudinally extending rib.

Fig. 3 is an end view of Fig. 2 showing the circular configuration of the tube.

Fig. 4 is another embodiment of a straight cylindrically-shaped tube having a convolutely configured rib. Fig. 5 is an end view of Fig. 4 showing the circular configuration of the tube.

Fig. 6 is a planar view of an arcuately-shaped tube.

Fig. 7 is a sectional view of the arcuately-shaped tube depicted in Fig. 6 showing a tapered wall and a rib formed on the inner periphery of the wall.

Fig. 8 is a right end view of Fig. 7 showing the oval cross-sectional configuration of the arcuately-shaped tube.

Fig. 9 is a planar view of an arcuately-shaped tube having a thick area which serves as a rib which extending from end to end.

Fig. 10 is an end view of the tube shown in Fig. 9 taken along line 10--10 depicting a circular outer configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1-3, a tube 10 is shown which can be injection molded from a thermoplastic material. The tube 10 is a hollow cyl-inder coaxially aligned about a straight, longitudinal central axis A--A. The tube 10 is formed from a oldable thermoplastic material such as polyethylene, polypropylene or some combination thereof. Other kinds of thermoplastic materials can also be used. The material can have a high density or a low density and should have a melting point which is sufficiently high to prevent burning or charring during the injection molding cycle. High density polyethylene is one such material that is desirable for forming a thin walled tube which can be used as the inner tube or plunger of a two-piece tampon applicator. High density polyethylene is available from a number of sources. Two of such sources include Union Carbide Chemicals and Plastics Company, Inc. having a sales office at 3030 Warrenville Road, Suite 870, Lisle, Illinois 60632 and Quantum Chemical Corporation, USI Division, 11500 Northlake Drive, Cincinnati, Ohio 45249.

The tube 10 has a wall 12, see Fig. 2, and a predetermined length which extends from a first end 14 to a second end 16. The length of the tube 10 can vary as can the tube's overall shape and dimension. When the tube 10 is to be used as the plunger member of a two-piece tampon applicator, it should have a length of at least 2 inches (at least 50 mm), preferably at least 2.5 inches (at least 64 mm), and most preferably, about 3 inches (about 76 mm). The tube 10 is depicted as having a circular or round cross-sectional configuration with a constant internal and external radius. It should be noted that the tube can be molded into other shapes if desired. For example, the tube can have a cross-sectional configuration in the form of an ellipse, an oval, a square, a rectangle, a triangle, etc. Since the tube is designed to be injection molded, sharp or pointed corners or surfaces should be rounded or eliminated. The tube could also have a stepped outer configuration or some other non-uniform interior and/or exterior shape. It is also possible to form grooves or indentations in the interior and/or exterior surfaces of the tube to assist in facilitating release of the tube from the injection mold.

Referring to Fig. 3, the tube 10 is shown having a circular or round cross-sectional configuration with an inner periphery 18 formed on a radius r, and an outer periphery 20 formed on a radius r₂. The outer periphery 20 should preferably be smooth.

Even though the tube 10 is depicted as being straight over its entire length, it could also be molded to have an arcuate or curved profile. Any nonlinear, curved or bent shape is within the scope of this invention. The particular curvature of a tube can be varied to suit one's particular needs.

The wall 12 of tube 10 is shown having a thickness "t" which is uniform between the first and second ends, 14 and 16 respectively. For some applications, it may be necessary to utilize a wall of varying or non-uniform thickness. For example, the wall could be formed with a taper wherein the thickness decreases in only one direction, say from the first end 14 to the second end 16. It is also possible to mold the wall such that it has a maximum thickness located at or near the central portion of the tube and have a minimum thickness toward the ends 14 and 16.

The wall 12 is thin having a thickness of less than about .03 inches (about .76 mm). The thickness can range from between about .03 inches (about .76 mm) down to about .005 inches (about .13 mm). Preferably, the thickness is less than about .02 inches (about .51 mm) and, most preferably, is less than about .015 inches (about .38 mm) . Referring again to Fig. 2, the tube 10 also contains a rib 22 which is formed on the inner periphery 18 of the wall 12. In Fig. 2, the rib 22 is shown extending along the full length of the tube 10 from the first end 14 to the second end 16. However, the rib 22 should extend at least along half of the length of the tube 10, and preferably, should extend along a major portion of the length of the tube 10. By "major portion" it is meant a distance equal to at least 75 percent of the length of the tube 10.

The rib 22 is depicted as a straight member formed parallel to the longitudinal, central axis A--A. The rib 22 has a height "h" which is equal to the thickness "t" of the wall 12. However, it is possible to make the height "h" of the rib 22 less than, equal to, or greater than the thickness "t" of the wall 12. Preferably, the height "h" of the rib 22 will be equal to or greater than the minimum thickness "t" of the wall 12. The reason for this is when the wall 12 is very thin, the rib 22 is designed to act as a conduit to move or transport the molten thermoplastic material to both ends, 14 and 16 respectively, so that the tube 10 can be completely formed. Therefore, the rib 22 acts as a channel or pathway for the molten plastic. As mentioned above, the overall shape of the rib 22 can vary but a semi-circular cross-sectional configuration is preferred. Other cross-sectional configurations can include square or rectangular shapes with rounded corners, a teardrop shape, an elliptical shape, etc. Since the dimensions of the rib 22 do not have to conform to the close tolerances which are normally assigned to the outer periphery 20 of the tube 10, the rib 22 can be constructed to almost any desired size and shape. For example, when the tube 10 is designed to serve as the plunger member of a two-piece tampon applicator, the shape and dimension of the exterior of the tube has to be molded to very close tolerances in order to allow the two members to telescopically interact.

The height, width and overall configuration of the rib 22 can vary over its entire length, but preferably, all dimensions will be constant. For best results, the rib 22 will have a height "h" which is at least equal to the minimum thickness of the wall 12. For very thin walls, having a thickness in the range of about .015 inches (about .38 mm) to about .02 inches (about 51 mm), the height "h" of the rib 22 can be increased to be about two to three times the thickness "t" of the wall 12. This will assure proper flow of molten thermoplastic material to both ends 14 and 16 of the tube 10. Referring again to Figs. 1 and 2, the tube 10 has a gate 24 formed in the outer periphery 20. The gate 24 represents the location of the injection nozzle through which the molten thermoplastic material is introduced into the mold cavity. In order to facilitate molding of the tube 10, the gate 24 should be located approximately half way between the first and second ends, 14 and 16 respectively, when the tube 10 is straight and the wall 12 has a constant thickness. For arcuately-shaped tubes and for tubes having odd shapes or characteristics, especially tubes having a varying wall thickness, the gate 24 can be offset toward one end. Preferably, the gate will be located such that an approximately equal volume of molten plastic will flow in opposite directions toward the distally spaced ends 14 and 16. The depth of the gate 24 can vary but normally should be as shallow as possible. Preferably, the depth of the gate 24 will be so shallow that it is not visibly apparent except to those familiar with the injection molding process.

Referring to Figs. 4 and 5, an alternative embodiment of a thin walled tube 26 is shown. The tube 26 is a straight cylindrical member coaxially aligned about a longitudinal central axis B--B. The tube 26 has a wall 28 which extends between a first end 30 and a second end 32. The wall 28 has a circular or round cross-sectional configuration with an inner periphery 34 formed on a radius r₃ and an outer periphery 36 formed on a radius r₄. The tube 26 also contains a rib 38 which is convolutely or spirally formed on the inner periphery 34 of the wall 28. The rib 38 extends along the entire length of the tube 26 from the first end 30 to the second end 32.

The coiled or twisted rib 38 not only facilitates distribution of the molten plastic to both ends 30 and 32 of the tube 26 but also provides structural rigidity to the tube 26 once the material forming the rib 38 solidifies. When injection molding such a tube 26, it may be necessary to rotate either the inner core, the tube, or both in order to free the finished molded tube without damaging the rib 38. The spiral or convolute configuration of the rib 38 tends to provide the tube 26 with a more consistent and uniformly distributed compression strength as compared to the tube 10 shown in Figs. 1-3. Referring to Fig. 5, the tube 26 is depicted having a wall 28 which is thinner than the wall 12, depicted in Fig. 3. The wall 28 is formed on a radius r₃ which is larger than the radius r₁ and has a thickness t₂ which is much less than the thickness t₃ of the rib 38. In fact, the maximum thickness t₃ of the rib 38 is depicted as being approximately three times the thickness t₂ of the wall 28. Another difference in this embodiment is that the spirally formed rib 38 rotates through a circular arc of at least 360° on the inner periphery 34. The actual pitch of the spiral can vary depending upon one's needs.

Referring now to Figs. 6-8, a third embodiment is depicted wherein an arcuately-shaped tube 40 is shown formed on a curved centerline C--C. The arcuately-shaped tube 40 has a wall 42 and a length which extends from a first end 44 to a second end 46. The arcuately-shaped tube 40 can be formed on a curved path of any desired length. The arcuately shaped tube 40 is depicted as having an oval cross-sectional configuration with an inner periphery 48 and an outer periphery 50. It should be noted that other cross-sectional profiles can also be used, for example, a circular or round configuration. The oval cross-sectional configuration will have a major and a minor dimension. The inner periphery 48 and the outer periphery 50 can have either a uniform or a non-uniform profile along the length of the tube 40. Preferably, the outer periphery 50 has an essentially constant profile when the arcuately-shaped tube 40 is to be used as the plunger member of a two-piece tampon applicator. Referring to Fig. 7, the wall 42 tapers in thickness over the length of the tube 40. The wall 42 is thickest adjacent the first end 44 and tapers down to a narrower thickness adjacent the second end 46. This means that the second end 46 has larger inside dimensions than the first end 44. When an arcuate-shaped tube 42 is injection molded and a central mold core is required to form the inner periphery 48, it is advantageous to taper the thickness of the wall 42 in order to facilitate removal of the central mold core. The central mold core could then be retracted from the finished molded tube 40 by way of the second end 46. The thickness of the wall 42 can taper down from a thickness of about .03 inches (about .76 mm) to a thickness of about .005 inches (about .13 mm).

The arcuately-shaped tube 40 also contains a rib 52 which is formed on the inner periphery 48. The rib 52 is shown extending almost the full length of the tube 40. The rib 52 could extend the entire length of the tube 40 from the first end 44 to the second end 46, if desired. The rib 52 stops short of both the first and second ends, 44 and 46 respectively, in order to allow the ends to be flared in a subsequent operation. In order to facilitate the flaring operation, it is beneficial to utilize a thin wall structure. For this reason, the inner periphery 48 is bevelled to form an incline surface 54 approximate the first end 44. The second end 46 already has a tapering wall thickness and therefore does not have to be bevelled. It should be noted that when the tube 40 is to be used as the plunger member of a two-piece tampon applicator, that one way of assuring that the inner tube 40 will stay connected to the outer tube is to flare the ends of the inner tube 40 after the two tubes are assembled. The flared ends will prevent the inner tube 40 from then being pushed or pulled into or out of the outer tube except by the exertion of extreme force. It should also be noted that the rib 52 could be designed to extend to either or both ends, 44 and 46 respectively, if desired.

The rib 52 has a height "h," which is depicted as being constant along the length of the tube 40. The rib 52 could vary in height if desired. The height "h," of the rib 52 can be less than, equal to, or greater than the thickness of the wall 42. Since the thickness of the wall 42 varies, it is likely to assume that at some point, the height "h," of the rib 52 will be less than, equal to or greater than the thickness of the wall 42. Preferably, the height "h," of the rib 52 will be equal to or greater than the thickness of the wall 42 over a length of the tube 40 which represents at least half of the overall tube length. The maximum thickness of the wall 42 should be less than about .03 inches (about .76 mm). The arcuately-shaped tube 40 also contains three grooves 56 formed in the outer periphery 50 which are located adjacent to the first end 44. Three similar grooves 58 are located adjacent to the second end 46. The grooves 56 and 58 are designed to engage with semi-circular rings (not shown) positioned in two mating mold blocks. As the pair of mold blocks come together, the exterior surface of the tube 40 is defined. Although two sets of three grooves 56 and 58 are shown, it is only necessary to have a single groove, either 56 or 58, formed in the outer periphery 50 adjacent to only one of the ends 44 or 46. When only a single groove 56 or 58 is utilized, it is preferable to locate it adjacent to the first end 44. The purpose of the groove(s), 56 and/or 58, is to hold the molded tube 40 stationary as the central mold core (not shown) is removed after the molten material forming the tube 40 solidifies. The presence of more than one groove 56 and/or 58 provides added assurance that the molded tube 40 will be held stationary until the central mold core is fully withdrawn from the inner periphery 52. In the embodiment shown in Fig. 7, two sets of three grooves 56 and 58 are present at the respective ends 44 and 46 because the tube 40 is relatively long and is designed to serve as the plunger member of a two-piece tampon applicator. In this use, the grooves 56 and 58 will be visible to the consumer and therefore it is highly recommended to provide the tube 40 with a balanced aesthetic appearance.

In Figs 6-8, the grooves 56 and 58 are shown as being circular and extending completely around the outer circumference of the tube 40. Each groove 56 and 58 has a depth which is at least 25 percent of the thickness of the wall 42. Preferably, the depth of the grooves 56 and 58 are at least half the thickness of the wall 42 at the location where each groove 56 and 58 is formed^". The cross-sectional profile of each groove 56 and 58 can vary. However, a trapezoidal profile with a flat bottom and opposite sides which slant downward and inward toward the central longitudinal axis C--C works fine and are easy to form.

The arcuately-shaped tube 40 also includes a gate 60 formed in the outer periphery 50. The gate 60 represents the location of the injection nozzle through which the molten thermoplastic material is introduced into the mold cavity. In order to facilitate molding the arcuately-shaped tube 40 having a tapered wall 42, the gate 60 should be located at a point midway between the amount of material needed to form the tube 40. In Fig. 7, it is obvious that the gate 60 is situated closer to the first end 44 since the wall 42 is thicker at this end and requires more material to form it. By locating the gate 60 at a point where half of the volume of the molten material can flow towards each of the ends 44 and 46, one can be assured that the tube 40 can be completely formed and that partially formed tubes are kept to a minimum. Referring to Figs. 9 and 10, a fourth embodiment of a thin walled tube 62 is shown. The tube 62 is arcuately-shaped along a curved centerline D--D. The tube 62 has a wall 64 and a length which extends from a first end 66 to a second end 68. The arcuately-shaped tube 62 can be formed on a curved path of any desired length. The arcuately-shaped tube 62 is depicted as having an inner periphery 70 and a circular or round outer periphery 72. The wall 64 varies in thickness in the cross-sectional plane. However, the inner and outer peripheries, 70 and 72 respectively, have a uniform profile along the length of the tube 62. The wall 64 varies in thickness along it's length. A major portion of the wall 64 can have a thickness of less than about .03 inches (about .76 mm) and preferably, less than about .015 inches (about .38 mm). The tube 62 also contains a thicken portion 74 which is formed on the inner periphery 70. The thicken portion 74 is an alternative configuration for the rib 52 shown in Figs. 6-8. The thicken portion 74 has a constant height "h₂" which is greater than any portion of the wall thickness. The thicken portion 74 spans the full length of the tube 62, extending from the first end 66 to the second end 68. The shape of the thicken portion 74 can vary but should be thick enough to serve as a conduit for the molten thermoplastic material so that the molten material can be routed to the opposite ends 66 and 68.

The arcuately-shaped tube 62 further includes a gate 76 formed in the outer periphery 72. The gate 76 represents the location of the injection nozzle through which the molten thermoplastic material is introduced into the mold cavity. In order to facilitate molding of the arcuately-shaped tube 62, the gate 76 should be situated at a point midway between the first and second ends, 66 and 68 respectively. Since the tube 62 has a constant wall thickness along it's length, the gate 76 can be located an equal distance between the ends 66 and 68.

Lastly, the arcuately-shaped tube 62 contains a single groove 78 formed in the outer periphery 72 which is located adjacent to the first end 66. The purpose of the groove 78 is to hold the molded tube 62 stationary as the central mold core (not shown) is removed after the molten material forming the tube 62 solidifies. The groove 78 has a trapezoidal profile with a flat bottom and opposite sides which slant downward and inward toward the central longitudinal axis D--D. As stated above, another groove could also be formed in the outer periphery 72 of the tube 62 adjacent to the second end 68 if aesthetic appearance is important.

While the invention has been described in conjunction with several specific embodiments, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.

Claims

We claim:

1. A tube comprising a wall and having a length extending from a first end to a second end, said wall having a thickness of less than about .03 inches and a rib formed on an inner periphery of said tube which extends along at least a portion of said length.

2. The tube of claim 1 wherein said rib has a constant height.

3. The tube of claim 1 wherein said rib has a semi-circular cross-sectional configuration.

4. A tube of claim 1 wherein said wall tapers in thickness between said first and second ends.

5. A tube comprising a wall extending from a first end to a second end, said wall having a non-uniform thickness and a longitudinal-extending rib formed on an inner periphery of said tube which extends along at least a portion of said length.

6. The tube of claim 5 wherein said wall has a thickness of less than about .03 inches and said wall tapers downward in thickness as it approaches at least one of said ends.

7. The tube of claim 5 wherein said wall narrows in thickness from said second end to said first end.

8. The tube of claim 5 wherein said rib has a height which is greater than at least a portion of said wall thickness.

9. The tube of claim 5 wherein said rib has a length which is less than the distance between said first and second ends.

10. An arcuately-shaped tube comprising a wall and having a length extending from a first end to a second end, said wall tapering to a smaller thickness adjacent at least one of said ends and a rib formed on an inner periphery of said tube which extends along at least a portion of said length.

11. The arcuately-shaped tube of claim 10 wherein said rib extends from said first end to said second end.

12. The arcuately-shaped tube of claim 10 wherein said rib extends along a major portion of the length of said tube and is spaced away from at least one of said ends.

13. The arcuately-shaped tube of claim 10 wherein said wall has a constant exterior profile and a thickness of less than about .03 inches.

14. The arcuately-shaped tube of claim 10 wherein said tube has an oval cross-sectional configuration.

15. The arcuately-shaped tube of claim 10 wherein said tube has a circular cross-sectional configuration.

16. An arcuately-shaped tube comprising a wall and having a length extending from a first end to a second end, said wall having a non-uniform thickness and a rib formed on an inner periphery of said tube which extends along a major portion of said length.

17. The arcuately-shaped tube of claim 16 wherein said tube is formed from high density polyethylene.

18. The arcuately-shaped tube of claim 16 wherein said tube is formed from low density polyethylene.

19. The arcuately-shaped tube of claim 16 wherein said wall has a thickness of less than about .03 inches.

20. The arcuately-shaped tube of claim 16 wherein said wall has a thickness of less than about .015 inches.

21. The arcuately-shaped tube of claim 16 wherein said wall has an outer periphery and at least one groove is formed in said outer periphery adjacent to an end thereof.

22. The arcuately-shaped tube of claim 16 wherein said wall has an outer periphery and at least one groove is formed in said outer periphery adjacent to each end thereof.

23. A tube comprising a wall and having a length extending from a first end to a second end, said wall having a thickness of less than about .03 inches and a thicken portion formed on an inner periphery of said tube which extends along at least a portion of said length.

24. The tube of claim 23 wherein said thicken portion has a constant height.