Background of the Invention
[0001] Plastic closures are presently made for soft-drink containers. ooth plastic and glass.
When they are threaded onto the container and seal. the internal pressure in the container,
which may be in the range of 100-150 psi depending upon the degree of carbonation
and temperature, exerts sufficient force on the underside of the closure to cause
mechanical deflection or "doming" of the center portion of the cap.
[0002] The "doming" of the cap creates problems in maintaining a seal. resulting in loss
of carbonation in some instances and when the closure with a dome is disturbed, there
is even a greater risk in the closure leaking. The domed closure may be disturbed
by the application of the price to the closure by the marking devices in the store,
or by the stacking of several containers on top of each other. Doming which occurs
in some plastic closures due to internal pressure may become more severe with time
due to the tendency for some plastic closure materials to "creep" under stress. As
can be seen, when the closure is first applied, it will seal. and while the pressure
will immediately deflect the center of the cap to some degree, the constant pressure
over a period of time will result in the possible loss of pressure because the seal
may be disturbed.
[0003] Of course. while internal pressure may cause the plastic closure to "dome", a similar
phenomenon will occur when the closure is being used to seal a produce under vacuum.
in which case the closure will deflect inwardly of the container to which it is applied.
This is a common feature of containers that are packed under heat or with steam, such
as baby foods and instant coffee. In the case of containers under vacuum, the diameter
of the closure and degree of vacuum will determine the diameter of the closure and
degree of vacuum will determine the deflection of the closure due to atmospheric pressure
acting on the closure.
[0004] To be competitive with metal closures, the plastic closures must not be overly complicated
because of the tooling costs for the molds and the resin costs for the plastics.
[0005] Obviously, the closure must retain the pressure and therefore cannot be permeable
to the gas that is in the head space.
[0006] In order to minimize this doming effect. some closures have been formed with internal,
radial ribs. While this may be one solution to the "doming" problem, the injection
molds for the closures are expensive to make and maintain. Also. it is possible to
make the closures very thick in section put of course this also runs up the cost per
unit manufactured.
Summary of the Invention
[0007] Plastic closures for pressurized containers are made of the usual matenal but are
molded so as to have a predetermined degree of initial deflection in the top so that
when applied to the container the internal pressure stress will render the closure
top surface essentially flat. In those situations where the span of the closure is
large, such as on a wide mouth jar, a contoured thickness in addition to the premolded
shape is provided to the closure.
[0008] It is therefore an object of the present invention to form closures of plastic for
containers that will be under non- atmospheric internal pressure. where the closure
after ap- phcation will assume a generally horizontal top surface during its useful
period of sealing the container.
[0009] It is a further object of the present invention to form closures of plastic with
a preformed deflection in the closure top to compensate for the deflection that may
be caused by the pressure condition to be encountered when in use.
[0010] Other and further objects will be apparent from the following description taken in
conjunction with the annexed sheets of drawings.
Brief Description of the Drawings
[0011]
FIG. 1 is a cross-sectional view through a plastic closure of the invention for application
to a pressurized container:
FIG. 2 is a vertical cross-section through a plastic closure for an evacuated container:
FIG. 3 is a vertical cross-section through the closures of Figs. 1 and 2 after being
seated to the container finish;
FIG. 4 is a vertical sectional view through a plastic closure adapted for sealing
an evacuated wide mouth container; and.
FIG. 5 is a vertical sectional view through a plastic closure to the invention adapted
for sealing a wide mouth jar under Internal pressure.
Detailed description of the Drawings
[0012] With particular reference to the appended drawings, and in particular Fig. 1, there
is shown a closure 10 formed of a plastic such as polypropylene which has an annular
skirt portion 11 with internal threads 12. The closure 10 is formed with a circular
top 13 joined to the skirt 11 with the central portion formed in a downwardly concave
configuration. The degree of concavity with respect to edges of the closure top is
selected on the basis of the bending modulus of the top panel 13 of the closure, the
thickness of the closure top and the degree of internal pressure within the container
to which the closure will be applied. The closure of Fig. 1 is formed with the curvature
shown, and upon application to a container finish and sealing thereon, the top of
the closure will assume a generally horizontal or flat configuration as shown in Fig.
3.
[0013] Turning now to Fig. 2, there is shown a closure 20 similar to that shown in Fig.
1, except this closure 20 is formed with its central top portion 21 in a "domed" upward
fashion, as shown. The closure 20 has an internally threaded skirt portion 22 to which
the top 2
1 is joined and is adapted to be applied to a container which is intended to hold a
vacuum or pressure less than atmospheric. Here again, when the closure is sealed onto
a container whose contents are under vacuum, the center 21 of the closure will assume
a generally horizontal configuration as illustrated in Fig. 3. The thickness of the
closure top, the degree of premolded "doming" will be based upon the anticipated degree
of vacuum which will be present in the container to which the closure is to be sealed.
[0014] As can be seen. when either the Fig. 1 or Fig. 2 closure is applied to the container
with the appropriate internal pressure condition. the closure will assume the shape
shown in Fig. 3, at which time the tendency of the plastic of the closure to "creep"
under stress is greatly reduced and the possible loss of pressure due to the closure
unsealing because of top loading is greatly reduced.
[0015] Furthermore, there is virtually no possibility that the top of the closure will experience
cracking due to "doming" or sinking as has been experienced in the past with closures
that are not non-premolded with a deflection therein. While the "doming" of present
closures does not always result in cracks, the top loading of the "domed" closure
will usually disturb the seal causing the package to vent or for carbonation leakage
to occur. With the preformed closures of the invention. no "doming" will be present
after sealing and thus nodisturbance of the seal will occur with top loading of the
package as will occur when the sealed bottles, for example. are stacked in cases on
top of each other for shipment or for display in the market.
[0016] The precise amount of premolded deflection in the closures can be specified for different
sizes, pressure. res- ms and top thicknesses using theoretical formulae and/or finite
element stress analysis techniques so that stresses resulting from internal pressure
conditions of the package are within safe limits.
[0017] Turning to Figures 4 and 5, there are disclosed two examples. 40 and 50 respectively,
of the configurations for wide mouth container closures pre-molded for pressure and
vacuum conditions. As shown in Fig. 4, the central portion 41 of the top 42 of the
closure 40 is formed with a significantly thicker section than the outer portions.
This is so take care of the fact that with a wide mouth container the closure will
be subjected to greater overall internal force because the area that will be exposed
to the interior of the container will be significantly larger than in the case of
the closure shown in Fig. 1. The closure of Fig. 4 may be in the range 1.5 to 3 inches
in diameter. In this case the center portion is maoe thicker so as to be able to withstand
the increased forces without "doming" when applied to a pressurized container but
assume a generally flat, top surface configuration.
[0018] In Fig. 5 there is shown a premolded wide mouth closure 50 having a top wall 51 with
a thicker cross-section at the center 52 than at the edges. This closure would. for
example, be suited for application to a baby food jar where the contents will be under
vacuum. Here again. the closure top 51 will assume a generally flat configuration
when applied to a container with internal vacuum conditions of the type experienced
with baby food packages.
[0019] It may also be noted that with the closure as shown in Fig. 5 applied to a baby food
containing jar under vacuum, the flat configuration of the closure top provides an
indication that the package is still sealed. In the event the seal were lost and leakage
may have occurred, the plastic closure will appear "domed" as in Fig. 5 and thus become
a readily apparent tell-tale that the seal has been lost and the contents may not
be sterile. Obviously, in the case where the contents may have spoiled, the center
of the closure may even be "domed" to a greater extent due to the possibility that
pressures will have built up beneath the closure.
[0020] Specific deflections of polypropylene closures for application to containers which
will be under 28 inches of Mercury vacuum are as follows:
70 mm.size with top thickness of .050 in. the deflection will be .200 in.: and,
53 mm. size with top thickness of .050 in. the deflection will be .080 in.
[0021] In addition to the advantages of the closure with premolded deflection to offset
pressure within the container. it should be pointed out that when doming occurs in
the prior art closures. it is necessary that the closure be labeled before application
to the container, thus requinng an inventory of closures for each customer.
[0022] With the closure of the invention. it is possible to make a stock closure which may
be labeled after application without fear of disturbing the seal or the labeling may
occur as part of the filling and sealing. Obviously, a stock closure may result in
manufacturing costs and thus make the sealed package, such as soft drink, less expensive.
[0023] While the foregoing description sets forth the best mode of carrying out the invention,
as presently contemplated, obvious modifications of the concepts may be resorted to
within the scope of the appended claims.
1. A plastic closure for a container having a non- atmospheric pressure therein in
which the closure is molded with internal threads in a cylindrical. depending skirt
portion joined to a top with sealing means formed therein, the improvement in the
closure comprising, molding the top with a predetermined wall thickness and non-linear
contour to compensate for the deflection of the closure when subjected to the prevailing
atmosphere in the container.
2. The closure of Claim 1 wherein the top of the closure is molded with a concave
curvature, whereby a pressurized product in the container will raise the closure top
to a substantially horizontal level.
3. The closure of Claim 1 wherein the top of the closure is molded with a convex curvature
of an amount such than when applied to a container with a vacuum therein the top will
become essentially flat.
4. A plastic closure for a container that is to contain a product under pressure comprising,
a cylindrical skirt, con- tamer attaching means formed internally of said skirt, sealing
means within said closure for sealing cooperation with the finish of the container,
a premolded top on said closure being formed with a concave contour to the outside
top thereof and a thicker wall section at the center than at the edges which join
the skirt. whereby the deflection of the closure is such that virtually no "doming"
takes place.
5. The closure of Claim 4 wherein the center of said top is joined to the side thereof
by a straight taper.
6. The closure of Claim 4 wherein the wall thickness of said top is conical in configuration
with the height of the downwardly extending cone being determined by the degree of
resistance to deflection desired.
7. A method of forming a premolded plastic closure for a pressurized container, comprising
the steps of molding the top of the closure with a concave contour and having a thickness
such that it assumes a generally honzontal attitude when applied to the container
under pressure.
8. The method of Claim 7 further including determining the bending modulus of the
closure material and adjusting the thickness of the closure top panel in accordance
with the pressure to be contained.