Note: Descriptions are shown in the official language in which they were submitted.
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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 inject;on 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 distr;bution 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.
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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.
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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.
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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 cylinder coaxially aligned about a straight, longitudinal
central axis A--A. The tube 10 is formed from a moldable
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
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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 rl and an outer periphery 20 formed on a radius rz. 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).
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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
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(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
1 ength 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.
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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
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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 "h1" which is depicted as being constant
along the length of the tube 40. The rib 52 could vary in height if
desired. The height "h1" 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).
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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
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arcuately-shaped tube 40 having a ~apered wal~ 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
S 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
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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.
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