One-piece thermoplastic closure having press-on screw off structure including spaced vertical ribs in the skirt of the closure

A closure for a container having generally helical threads on the neck finish is formed from a one-piece cap shell of thermoplastic material including a top wall and a peripheral skirt, the skirt having a plurality of spaced, hard, flexible generally vertical thermoplastic ribs integral with the shell for contacting the threads of the container. Each rib is so constructed and arranged that it has sufficient resistance to cold flow that it only slightly flexes and bends around the thread to form a shallow indentation on the rib when the rib is forced into contact with the thread, the indentation being sufficient to provide purchase on the threads for removing the closure.

The present invention relates to thermoplastic closures and particularly 
one-piece thermoplastic closures including a top and a skirt with means on 
the skirt to engage removably the threads of a container. 
BACKGROUND OF THE INVENTION 
It is desirable to provide a plastic closure with a press-on screw-off 
structure that is adapted for production, for easy use, and adapted for 
vacuum capping operation for liquids such as apple juice that are warm 
when capped, the temperatures being moderately high such as in the range 
of 180.degree.-205.degree. F. It is also desirable to have a simple 
closure that would permit the release of internal pressure in the event of 
product spoilage and one that can be removed very easily with relatively 
small amount of torque. 
It has heretofore been proposed, for example, in U.S. Pat. No. 3,371,813 
(Owen et al) to provide a soft readily deformable gasket material inside 
the skirt of a metal closure, the gasket material being said to deform 
readily and flow around the threads of a container to form cooperating 
thread grooves in the gasket material. The gasket material is soft, 
deformable and a material that sets to retain the grooves. This formation 
of well defined cooperating set grooves for threads in the gasket material 
is said to provide purchase or leverage on the threads to permit opening 
of the container by twisting the cap. The U.S. Pat. No. 3,371,813 suggests 
that the resilient deformable gasket material forming the ribs be at least 
about 20% and generally about 30-70% of the total circumferential surface 
area of the gasket skirt to provide increased package security and greater 
cam off force against the continuous threads. 
U.S. Pat. No. 4,000,825 (Westfall) for a press-on twistoff metal closure 
for oxygen sensitive products describes a top liner having spaced-apart 
vertical ribs inside the skirt of the shell of the closure. The liner is 
formed preferably from a foamed plastisol material that deforms and cold 
flows around the threads of the neck of a container to form a series of 
thread-cooperating cavities in the foamed liner that sets to aid in 
removal of the closure using a twist-off motion. The patent shows a top 
seal in addition to sealing with deformation of the ribs. 
In U.S. Pat. No. 3,270,904 (Foster et al) there is a press-on turn-off 
metal cap, the cap having a gasket material at the top and around the 
bottom of the skirt, the gasket material in the skirt engaging the threads 
of the container. The gasket material is described as plastomeric and 
takes a conformation to form a hermetic seal cooperating with the thread 
means of the container finish. The plastomeric material hardens to form 
rigid thread grooves therein for the camming operation with the raised 
threads of the neck of the container. 
U.S. Pat. No. 3,606,062 (Frisch et al) discloses a crimp-on twist-off metal 
crown cap with top and corner seals. There are deformable metal flutes in 
the metal shell to engage the threads of the container. 
U.S. Pat. No. 3,448,881 (Zipper) discloses a metal closure with a gasket 
for engaging the threads of a container, the metal closure having means to 
prevent an increase in cap removal torque including a ridge on the 
underside of the cap to prevent settling of the cap due to gradual 
deformation of the gasket material. Grooves are formed in the gasket by 
the threads of the container when the metal cap is applied to the threads. 
In general, in the above patents that describe metal caps with soft 
deformable gaskets that set, there is substantial contact between the 
deformable gasket ribs and the container threads such as contact of the 
entire rib or as, for instance, in the Owens et al patent, the ribs are 
shaped for substantial contact with the threads and comprise 20% up to 70% 
of the gasket area. The forcing of the gasket ribs against the threads 
causes the soft deformable gasket ribs to form grooves, the material 
thereafter setting to retain the grooves. 
It is an object of the present invention to provide a closure for a 
container in which there is a one-piece structure so constructed and 
arranged that ribs on the skirt slightly flex and bend around the threads 
of the container to form shallow indentations for easy twist-off of the 
cap requiring less torque than closures of the prior art in which the 
prior art ribs are made of relatively soft deformable gasket materials. 
It is an object of the present invention to provide a plastic closure for a 
container having generally helical threads on the neck finish, the closure 
comprising an easily manufactured one-piece cap shell of thermoplastic 
material including the top wall and a peripheral skirt, the skirt having 
means thereon for engaging threads of the neck of a container, the means 
including a plurality of spaced-apart, flexible generally vertical 
thermoplastic ribs integral with the skirt, each rib being so constructed 
and arranged that it has sufficient resistance to cold flow that the rib 
only slightly flexes and bends around the thread to form a slight 
indentation on the rib when the rib is forced into contact with the 
thread, the slight indentation being sufficient to provide purchase on the 
threads for very easy removal of the closure. 
It is an object of the present invention to provide a cap structure that is 
easy to manufacture and useful in a vacuum capping operation, the cap 
being of a vacuum type push-on twist-off cap which provides greater 
tolerance to wet conditions which generally inhibit tightening screw caps, 
permits release of internal pressure in the event of spoilage, and 
provides just enough purchase on the threads by means of hard, flexible 
thermoplastic ribs to provide enough torque to remove the cap by use of 
far less torque than is required by caps in the prior art.

THE INVENTION 
The present invention provides an economical one-piece closure for a 
container, the closure being easy to manufacture and easy to place on the 
container in a vacuum capping production operation. The closure for the 
container which has generally continuous or discontinuous helical threads 
in the neck finish, is constructed of a one-piece cap shell of 
thermoplastic material that is preferably high density polyethylene, the 
cap shell having a top wall and a peripheral skirt, the skirt having means 
thereon comprising a plurality of spaced-apart generally vertical ribs 
integral with the skirt for engaging the threads of the neck of the 
container, the ribs being hard, flexible and having sufficient resistance 
to cold flow so that each rib only slightly bends around the threads to 
form slight or shallow indentations on the rib when the rib is forced into 
contact with the thread. The shallow indentation provides just enough 
purchase on the threads to remove the same, the amount of torque required 
being substantially less than other cap structures with ribs of deformable 
gasket material that form relatively deep, well defined grooves to 
cooperate with the thread structure. The preferred thermoplastic material 
is one that provides just enough resistance to cold flow when forced 
against the thread to give a slight indentation only on the ribs. Suitable 
thermoplastic materials are those having a combination of properties 
including toughness, hardness, flexibility, resiliency, tensile modulus, 
creep and ease of fabrication at least about equivalent to that of high 
density polyethylene which, based on the technology of today, is the 
preferred material for the one-piece cap shell. The thermoplastic cap is 
preferably made of high density polyethylene having outstanding cap shell 
properties including a specific gravity of about 0.942 to 0.965, a melt 
flow index of about 0.2 to 8, a melt viscosity of about 7,000 to 120,000 
poises at 190.degree. C., a crystallinity generally of about 50-90%, a 
tensile modulus of about 60,000-180,000 psi, a flexural modulus at 
73.degree. C. of about 100,000-180,000 psi and a modulus at 100% 
elongation of about 5,000-15,000 psi. The preferred high density 
polyethylene material has a hardness of Shore A, durometer of about 60-80, 
an impact resistance of about 0.6 to 20 (Izod impact, foot pounds/inch of 
notch-1/4 inch thick specimen), tensile strength at break of about 
3,000-6,000 psi, an elongation at break of about 120-130%, a tensile yield 
strength of about 2,000-4,000 psi and a compressive strength (rupture or 
yield) of about 2,700-3,600 psi. 
Although high density polyethylene is highly preferred because of its 
combination of outstanding properties including toughness, resiliency, 
creep, cold flow and tensile modulus, other polyolefins can be used such 
as low density polyethylene, polypropylene and polybutylene. In some cases 
where lower temperatures in the range of 100.degree.-150.degree. F. are 
involved, low density polyethylene is quite suitable. In case of higher 
temperatures, say, up to 205.degree.-240.degree. F., polypropylene and 
polybutylene are suitable. The creep rupture strength of high density 
polyethylene is generally about 1,000-2,000 psi at temperatures of around 
23.degree. C., this creep strength being sufficient for the closure of the 
present invention. Polypropylene has a creep rupture strength of 3,000 psi 
or more at 23.degree. C. and the creep rupture strength at 23.degree. C. 
for nylon andpolycarbonate being 5,000 psi or more. Although these creep 
rupture strength properties generally are measured at 1,000 hours, the 
short term creep property is probably more important for thermoplastic 
ribs of the present invention than the long term creep property. 
A particularly useful high density polyethylene is one, for instance, 
having a density of 0.95, a crystallinity of about 65-75%, a melt flow 
index of 0.6, and a number average molecular weight of about 
10,000-15,000. In general, number average molecular weights can range from 
about 6,000-100,000 for useful high density polyethylene. 
As shown in FIG. 1, the thermoplastic one-piece closure 20 embodying the 
present invention is adapted to be applied to engaging means such as the 
threads 21 on the neck finish area 22 of a container 23. The one-piece 
closure 20 of the present invention comprises a top wall 33, a peripheral 
skirt 34 having a plurality of spaced-apart generally vertical ribs 40 
adapted to engage the threads 21 on the neck of the container 23. The ribs 
are generally spaced apart and the total width of the ribs is generally 
less than about 10%, say about 4-8%, of the total length of the 
circumference of the skirt. One of the preferred shapes of the rib is the 
long relatively narrow rib 40. The rib 40 has a width that is about the 
same throughout its length, the ribs being generally narrow and the height 
of the ribs (projection from the skirt) is generally about the same from 
top to bottom. 
The one-piece hard plastic shell construction shown in FIGS. 2 and 3 
provide a top seal at 35 with the downward projection 37 from the top 33 
of the shell 20 and a top side seal 38 with the side projection 39 of the 
skirt 34. 
In accordance with the present invention as, for instance, seen in FIGS. 3, 
4 and 5, each of the ribs 40 when forced against the threads 21 slightly 
bends and cold flows around the thread to form a shallow indentation 50 as 
best seen in the enlarged views of FIGS. 4 and 6. This slight indentation 
gives sufficient purchase for removal of the cap, the amount of torque 
being required being much less than the torque required for removal of 
threads from a soft resilient deformed rib. Hence the present invention 
provides an easy-to-manufacture, easy-to-apply vacuum cap which can be 
removed with little torque. 
As seen in FIGS. 6-8, the slight indentation is of a shallow nature formed 
in the crest 51 of the rib, the deepest portion of the indentation at 52 
generally being only about 1 to 10% of the height of the rib (the distance 
the rib projects from the shell skirt). The indentation length indicated 
at 53 generally has a dimension of about 2 to 10 times that of the 
indentation depth depending on the size of the thread of the container. 
As seen in FIG. 7, the configuration of the rib 60 is rectangular in shape, 
the rib 60 projecting from the closure skirt 34 in a manner similar to 
that of rib 40 in FIGS. 2-6. The small indentation 50 is found in the rib 
60 because of the forced engagement with the thread 21. As best seen in 
FIG. 8, the indentation 50 has its depth indicated at 52 and a length 
indicated by 53. The length 53 of the indentation is generally only about 
10 to 20% of the length of the rib. 
FIG. 9 shows another embodiment in which a rib 70 projects from the shell 
skirt 34, the rib 70 being triangular in nature. FIG. 9 shows a round nose 
rib 80 projecting from the skirt 34. Each of the ribs 70 and 80 when 
forced against the thread 21 form a small indentation 50. 
When the closure 20 is pushed on the container 23, the cap skirt is flexed 
and forced outwardly, especially during warm capping operations. The 
plastic memory of the all-plastic shell tends to return the skirt to its 
original dimension and provides some force against the container threads 
to slightly indent the ribs of the skirt. When the closure is removed, the 
slight indentation would tend to smooth out over a long time period, but 
the indentations remain in some form over a short term period of, say, 
several hours. 
As seen in FIG. 11, in another embodiment, a hard plastic shell 60 
comprising a top wall 63 and a peripheral skirt 64 is provided. A 
plurality of spaced-apart vertical ribs 90 are provided for engaging the 
threads 21 in the glass container 23. The toughness, resiliency, and 
resistance to creep of the ribs are the same as those of the ribs 40 of 
the closure of FIG. 2. The area occupied by the ribs 90, just as the ribs 
40, amounts to only about 1 to 8% of the total circumferential area of the 
skirt. The top of the closure has a soft, resilient gasket material 82 
which serves as the top liner for the cap and provides a top seal at 85 
with the projection 87 and a top side seal at 89 with the downwardly 
projecting projection 100 which fits between the top edge of the glass 
container 23 and a hard shell closure 60 near the juncture of the skirt 64 
and the top wall 63. 
In accordance with the present invention, good results have been obtained 
by a vacuum capping operation of warm apple juice at about 200.degree. F. 
With the closure of the present invention, the ribs provide just enough 
purchase with the small indentations to provide for removal of the cap, 
such removal being much easier than when a soft deformable rib is used. 
Advantageously, the plastic closure of the present invention provides an 
outstanding combination of sealing the top seal, the side seal and the 
seal at the ribs. The one-piece plastic closure construction is easy to 
manufacture and economical, it being less costly in the same size as a 
combination of a metal cap with a soft deformable gasket. Also, the 
plastic closure is easy to decorate and generally is more pleasing in 
appearance than the metal cap. 
The gasket liner 80 can be made of foamed polyethylene, foamed 
polypropylene, foamed copolymers of ethylene and vinyl acetate, and foamed 
polyvinyl chloride, which foamed materials are of a closed cell 
construction, durable and yet ideally deformable for a top seal and a top 
side seal.