Home


Figure 1.

Objects Functions And Materials From Which The Object Is Made.
The material that makes up a squash ball is raw butyl rubber, many different natural and synthetic materials and powders (that are added to the rubber in the process explained below) and air that is enclosed inside the rubber ball. Various types of balls are produced for various speeds and playing styles.

Resilience is the most important property of a squash ball. Resilience of a material is the ability of the material to absorb energy and then to release it. Most of the energy turns into thermal energy inside the ball. Air inside the rubber of the ball, as well as the rubber itself, becomes heated and therefore the air inside expands and causes a greater pressure inside the ball; the rubber of the ball becomes more flexible and more resilient as the rubber heats up. Therefore the bounce of the ball becomes better and higher (more bouncy). This is the function of the rubber and air inside. The normal temperature of the ball that players play with is 45˚C. At this temperature the energy lost to the external features is the same as the energy lost in the ball rubber and air inside.

The function of a squash ball is to have a variation of bounces, that increase in height as the temperature increases.

  
Figure 2.

Discussion of the mechanical properties of the materials used and the influence of the objects functionality. 

There are two main elements in a squash ball, a butyl rubber exterior and the air within the ball. (Cunnington, 2008-2009) The type of rubber used is an elastomer which is an elastic polymer that consist of long chain like molecules. Upon stretching, the molecules are forced to take up a more linear shape, restricting their movement and causing the molecules to heat up, as the elastic polymer is released it returns to its original chain like molecular state.(Gent, 2014) When an elastic polymer heats up it becomes more elastic and the air pressure inside increases, making the squash ball more bouncy.(Weeks, 2006) 

Main properties of Butyl Rubber

ElasticityElastic polymers have a very high elasticity, this is the ability to be deformed in any way such as stretched, compressed, twisted or bent and still be able to return to its original shape and size. The elongation at break is around 500% which means that it is able to stretch five times its length before it breaks (Butyl Rubber (IIR, CIIR, BIIR).).The modulus of elasticity varies with the elongation. This property shows that the squash ball can withstand the elongations due to the forces acting on it during a game of squash.

 

HardnessThe hardness of butyl rubber is in the range of 40-90 Shore A which is the same range as the hardness of a car tire( Rubber Material Selection Guide IIR or Butyl Isobutylene Isoprene Rubber).

 

ResilienceThis the ability of the material to return quickly to its original state. It is the ratio of energy recovered to the energy applied. The specifications of squash balls as stated by Specifications for Squash Balls (2013) the rebound resilience at 23 degrees Celsius is minimum 12 % and vulcanized butyl rubber has a rebound resilience of 12% (Haponiuk, Formela, 2014). This resilience is low making it suitable for shock absorbing.

 

Strength: The ultimate tensile strength of vulcanized butyl rubber is 17 MPa making the squash ball strong enough to withstand the tensile forces produced during a game of squash (Butyl Rubber (IIR, CIIR, BIIR).).

 

PermeabilityVulcanized butyl rubber has a very low permeability which means that the air inside of a squash ball is not able to escape, even with a increase in pressure due to the changes in temperature (Weeks, 2006).

 

Wear resistanceThis rubber can be used at temperatures ranging from -50 to 121 degrees Celsius which way exceed the temperatures experienced on a squash court.  

 

From these properties we can see why butyl rubber is used as the material for a squash ball.


Figure 3.

        The Manufacturing Process of                Squash Balls:

           The production of squash balls occurs in the               following processes:

1.Compounding and Mixing of Rubber:  Raw rubber in its natural state is stiff and difficult to process. It is first masticated in a rubber mill to a softer consistency.

The rubber is then compounded with additives to obtain certain strength, resilience, colour and enable it to vulcanise. Various ingredients including polymers, fillers, vulcanising agents, processing aids and reinforcing materials are added according to the secret recipe of the manufacturer and the type of squash ball produced.  Carbon black is used most commonly used as filler. It increases the tensile strength and resistance to abrasion and tearing of the final product.

The additives are then thoroughly mixed to allow to uniform dispersion in material.

2. Shaping and Vulcanization: The compound is then heated and loaded into an extruder which forces the material through a die. A rotating knife then cuts the extruded compound into pellets which are then cooled.

 


                                                         


         Figure 4: Rubber Extrusion through Die                              (Groover,2013,pg 287)

 

The pellets are then placed into a hydraulic press which subjects the pellets to a pressure and temperature of 7584.23 KPa and 150°C for 12 minutes in a half-shell shape mould. This allows the material to cure and therefore vulcanize. Vulcanization allows for the cross-linking of elastomer molecules so that the rubber can become stiffer and stronger but still retain its extensiblity. Long-chain rubber molecules become joined at certain points (crosslinked) which reduce the ability of the elastomer to flow. Vulcanization is accomplished in the mold cavity by maintaining the mold temperature to allow for curing. Various chemicals including zinc oxide and steartic acid are added with small amounts of sulphur to accelerate and strengthen the vulcanisation treatment of the rubber. (Groover,2013)


Figure 5: Vulcanization’s effect on rubber molecules:
1.
Raw rubber;    
2. Vulcanized (cross-linked) rubber 
(Groover,2013,pg 288)

 

50% of the half-shell shaped moulds have a pin at the bottom of the mould to create a small dimple which is painted to indicate the ball speed. The half shells are then removed from press and any flash (excess compound) is cut away.

 

3. Adhesion: The edges of the half-shell are then roughened by buffing by a grinding wheel and coated with a rubber solution containing an adhesive. The half shells are then stuck together by an operation called flapping. The balls are then placed in a second moulding, heating and vulcanising process. The balls are subjected to a pressure of 6894.76 KPa for 15 minutes which allows the adhesive to cure.

4.Surface Finish: The external surface join is then smoothened by buffing to give a matt surface. The balls are then washed, dried and inspected to ensure then conform to the WSF (World Squash Federation) standards.