Metal bottle cap

A metal bottle cap consisting of a central body to close off an opening at the neck of the bottle and an outer crown which is integral with the central body and is made up of a uninterrupted plurality of teeth which may be bent and fixed around the rim over a thread which has a number of starts on a bottleneck, the number of teeth on the outer crown must be different to a whole multiple of the number of starts on the rim of the bottle; and should be between twenty-nine and thirty-three.

BACKGROUND OF THE INVENTION 
The present invention relates to a metal cap which hermetically seals 
bottles, and is particularly aimed at glass bottles containing liquid 
foodstuffs under pressure. 
This crown-cap is already well known and has been used for some time by 
bottling companies to seal many types of glass bottles, above all for 
drinks bottled under pressure, such as beer, Coca-Cola, lemonade, etc. 
The circular central body of the cap has a suitable pressure-tight lining 
(usually made of synthetic, thermoplastic material for example PVC, 
polyethylene derivatives, or natural material such as cork) on the 
internal surface which is placed against a bottle neck opening. the outer 
crown is made up of corrugations which may be bent down to form a 
plurality of teeth which hold the cap firmly on to the rim of the 
bottleneck. 
It is also well known that you need a bottle-opener always in hand to open 
a bottle sealed with the classical crown-cap. 
This particular "problem" with opening has led some drinks manufacturers to 
produce a so-called "hybrid" solution: by closing the bottle with a 
screw-off crown cap. In practice this particular type of cap (in 
commercial jargon called "twist off") is made up of a normal crown-cap 
with twenty one teeth which is firmly placed around the rim of the bottle, 
which is obviously fitted with the necessary thread for unscrewing the 
cap. So the twist off cap has basically the same structure as the 
crown-cap but the teeth are fixed over the external thread which protrudes 
from the bottleneck. 
The main problem with this solution is that it is not very comfortable to 
open by hand. The cap must be fix very firmly to the bottle to maintain 
the pressure of the liquid inside the bottle and to avoid the risk of 
leakage during normal handling. So a certain amount of force which not 
everyone may have, is needed to rotate the cap in order to open the 
bottle. this operation also carries the risk of injuring or cutting the 
hands due to the "sharp edges" of the teeth on the crown. 
With this in mind several devices have been proposed to enable users to 
"twist off" the bottle cap swiftly (see also Canadian patent N. 
1.252.431), but these are obviously considered accessories and must be on 
hand for the user to unscrew the cap. 
SUMMARY OF THE INVENTION 
The stated object is comprehensively realized in the bottle cap disclosed, 
wherein a metal "twist off" cap to seal glass bottles which can be 
comfortably and safely unscrewed by hand with out altering the standards 
of safety and stability of the pressurized seal of the outer cap. This 
result is accomplished by choosing the number of teeth on the outer cap 
crown to be different to a whole number multiple of the number of thread 
starts or ends on the rim of the bottle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to the accompanying drawings, the invention relates to a 
metal bottle cap (denoted 1) is the type of bottle top known as "twist 
off" and is used for bottles containing liquid foodstuffs. 
The cap (1) consists of the central body (2) which covers the opening (3) 
of the rim (4) of the bottle (5) and the outer crown (6) which is integral 
with the central body (2). The central body (2) also has a lining (9) on 
the internal side which faces the bottle opening (5) and enhances closing 
of the bottle. 
The outer crown (6) is an uninterrupted plurality of teeth (7) which may be 
bent and fixed onto the rim (4) over a thread (8) on the rim; the number 
(P) of starts of this thread (8) is variable and depends on the type of 
bottle to be closed. In this case the number four has been chosen purely 
as an example (it is the most frequently used); in this way the cap (1) is 
firmly fixed to the rim (4), or rather with the teeth (7) closed gripping 
the thread (8) a perfect seal is obtained, but at the same time it is 
possible to open the bottle simply by unscrewing the cap (1) by hand. 
The traditional number of teeth (7) for the crown-cap is twenty one. The 
Applicant has discovered that raising this number facilitates opening and 
reduces the possible risk of cuts or injury to the user's hand when 
rotating the cap (1). 
It has been found that the unstableness of the cap or the bottle is 
particularly evident with the cap having a number of teeth that is a 
multiple of the number of starts of the thread on the rim of the bottle. 
Considering that, normally, the number of starts of the thread on the rim 
of the bottle is three or four, and preferably four (because with four 
starts the bottle can be opened with a rotation of about 90 degrees of the 
cap), the number of teeth that are "prohibited" are all the multiples of 
three and four. Considering that the preferred range of the number of 
teeth in the twist-off caps is compromised between 28 and 32, because of a 
comfortable and safe grip of the cap by the hand, the combinations to be 
excluded are 30 teeth if the rim of the bottles has three starts and 28 
and 32 teeth if the rim of the bottle has four starts. 
From a practical aspect, the bottles with three starts in the thread of the 
rim of the bottle is of no interest because of the greater angle (almost 
120 degrees) necessary for the unscrewing action of the cap. Nevertheless, 
if the rim of the Bothell's three starts, the solution with 33 teeth must 
be excluded. 
In other words, the possible combinations of starts versus teeth are 28, 
29, 31, 32 teeth if the rim of the bottle has three starts and 29, 31, 33 
teeth if the rim of the bottle has four starts. Therefore, it is 
preferable to limit the numbers of teeth for the cap of 29, 31, and 33 in 
combination with the four starts four the thread in the rim of the bottle. 
These numbers of the teeth appear to be also suitable for a hypothetical 
new bottle with five starts for the thread in the rim of the bottle. 
The reasons for this propensity of a cap to unscrew itself from the bottle 
when the cap has a number of teeth multiple of number of starts in the rim 
of the bottle can best be understood as follows. 
Considering the rectified rim of the bottle, as shown in FIG. 4, it can be 
seen that, in the case of four starts for the thread and 32 teeth for the 
cap, on each thread are crimped 8 teeth. The resulting rotation forces R1, 
R2, R3, R4, that are transferred from the cap to the threads by means of 
the teeth, are equal and axially symmetrically disposed. Furthermore, for 
a pressure within the bottle, the opening pressure is determined by 
dividing the number of teeth (8), into the pressure. 
On the contrary, with 33 teeth, as shown in FIG. 5, the number teeth per 
thread are not equal and the rotation forces are not equally distributed 
on the four threads (e.g. r1 is different from the other resulting 
rotation forces). Consequently the rotation forces are not axially 
symmetrically disposed, leaving at least one start with higher sealing 
pressure, which is calculated by dividing the number of teeth (7) into the 
same pressure. The conclusion is that the equality and the symmetry of the 
rotation forces, when the number of teeth corresponds to a multiple of the 
number of starts, facilitates the selfrotation of the cap and thus the 
unscrewing of the cap. 
As mentioned, many possible alternatives were studied to find the number of 
teeth (7) which permits comfortable and safe manual opening of a cap (1) 
which is held firmly onto the bottle. Eventually a correlation was found 
between the number of teeth (7) and the number of starts (P) of the thread 
(8) on the rim (4) of the bottle (5). In fact it was ascertained that the 
cap (1) became less stable over time if the number of teeth (7) was equal 
and corresponded to a multiple of the number of starts (P) of the thread 
(8). 
This correlation is clearly visible on the graph in FIG. 3. where several 
caps with different numbers of teeth (7) (from twenty-five to thirty-three 
to be exact, which is clearly labeled on the graph) and relative linings 
(9) made of different material (that is with high 9a, medium 9b and low 9c 
grip) are on the horizontal axis. The number of weeks the cap (1) 
maintained a correct and stable hold on the bottle (5) is plotted on the 
vertical axis. As can be seen from the graph in FIG. 3 the caps with 
twenty-eight and thirty-two teeth have a short seal life, irrespective of 
the material used for the lining (9). while the caps (1) with a number of 
teeth (7) which is not a multiple of the number starts (P) of the thread 
(8) maintain their hold for a considerable number of weeks. 
In particular the optimal "range" for the number of teeth (7) on the outer 
crown (6) proved to be twenty-nine, thirty-one or thirty-three. If the rim 
(4) of the bottle (5) has four starts (P) of the thread (8) the best 
solution is for the cap (1) to have thirty-one teeth or thirty-three (7) 
(which is in fact a prime number and not a whole number to the number of 
thread starts). 
Referring to FIG. 6, it is seen from the prior art arrangement that in the 
situation where a top thread of a first thread start and bottom thread of 
a second and overlapping thread start, a problem is caused due to a 
division of the crimping forces between the thread starts. In that 
instance when one thread begins and the other ends, at least one tooth is 
in a situation where the crimping of the cap tooth on the second thread is 
more difficult and not as strong as against the top thread, thereby 
creating a propensity of the cap to unseat and leak in this area. 
With this invention the objects have been achieved in that the greater 
number of teeth makes fast and safe opening possible by rotation of the 
cap around the rim while ensuring the bottle is sealed as securely as 
bottles sealed with the traditional crown-cap system. 
During the experiments it was found that the above mentioned ideal number 
of teeth is thirty-one, which is also perfectly suitable for bottles with 
threads with three starts, as thirty-one is a prime number; it is not 
divisible by three. 
The invention described can be subject to modifications and variations 
without thereby departing from the scope of the inventive concept. 
Moreover, all details of the invention may be substituted by technical 
equivalent elements.