This invention relates to croquet balls.
In the playing of the game of croquet, there are required four balls each of the same dimensions and characteristics but each differently coloured.
International rules have been accepted for the playing of this game throughout the world which set the diameter, the weight, and the extent of rebound that must be required from each ball as well as the consistency between a set of balls which are used for a particular game.
Any ball must keep a close tolerance to these specifications over a significant range of temperatures both in respect of rebound and in respect of overall size.
The ball that is most conventionally used at the present time is manufactured by having as a composite, a cork innerpart and a surrounding plastic skin.
A number of difficulties arise with these existing style of balls the first of these being that they are found to be extremely expensive to purchase. The manufacturing technique has to be essentially manual including forming cork into a ball shape, and then coating this and having a knurled type finish around the external surface of the coating plastic.
Very significant difficulty lies with the problem that cork being a naturally occurring material is not essentially uniform in providing the required characteristics and there has to be therefore a high degree of selection when using cork for this purpose.
The next problem is that in manufacture, cork has to be cut into a totally round shape and then this has to be centrally located within a surrounding coating plastic.
In practice, it is extremely difficult to centrally locate this cork within the surrounding plastic meaning that some parts of the external plastic are somewhat thinner than others thereby giving an unwanted bias to the ball.
As this effect is essentially hidden to the user, this does not assist expert players in achieving skill based results.
Furthermore, in trials conducting with respect to existing balls, where these have been manufactured according to the technique described when these have been subjected to being immersed in boiling water for a relatively small period, it has been discovered that some of the internal materials expand excessively perhaps because of an infusion of water but the result is that the ball becomes grossly misshapen and, of course, becomes useless.
It has to be recalled that the game of croquet can be played in an outside venue in which even the shade temperatures can be in excess of 40 degrees centigrade so that a ball that cannot withstand such excessive temperatures does provide very significant difficulties to the players.
In an attempt to overcome these problems, it has been known to provide a ball manufactured entirely of a plastics material polypropylene.
The problem that has been experienced with such a ball is that using polypropylene as the material, requires that it be manufactured by injection moulding which means substantial heating of the plastics and this inherently, then, provides an external surface which will be subject to internal pressures which can build up during cooling and therefore the shape is prone to distortion.
Further, however, whereas it is preferred, at least for first class play, that a ball shall have an external knurled surface, experience has shown that if knurling is provided with a polypropylene material, under the normal conditions of playing of the game, the external knurling surface is very quickly flattened with the impact of a croquet mallet causing, therefore, flat spots and a ball which is very quickly, therefore, not uniform.
Accordingly, it has been considered that it is simply not viable to provide a knurled finish on such a ball so that, whereas polypropylene as a plastic is one of the few that would be appropriate from a specific gravity point of view, its impact resistance is not adequate except where it is comprised of a smooth external surface.
Such a ball is, accordingly, in use but only as a ball for less than expert players and where the players are not willing to pay for the better ball.
The problem, therefore, has been to find a croquet ball of a type and technique for manufacture of a croquet ball which will allow a ball to be manufactured which lies within specifications required in terms of bounce over a significant change in temperatures, has dimensional stability over such a range of temperatures, can withstand impact on a knurling external surface without significant long term damage, and finally be relatively economic in manufacture.
Within these criteria the material or materials must also provide for the combination of size and weight so that there must be an average specific gravity which must meet the specification requirement.
This then is the problem.