replied 13 years ago.
This will be my longest post yet, and that's saying something. All this courtesy of www.wilsonsports.com and their FAQ department...
PUTTING IT ALL TOGETHER
Wilson Tennis Ball Manufacturing Fact Sheet
The first step in making a tennis ball is to prepare and mix together the ingredients that make the ball's core. The core of a tennis ball includes approximately 14 different materials. #1 is natural rubber. The
tennis core stock undergoes extensive quality control testing throughout the blending process to ensure consistency.
This rubber is then made into thick sheets, milled, and then a machine punches out "slugs" which are cylindrical shaped chunks of rubber that are all the same size and shape. This "slug" is then molded into a
perfectly shaped hemisphere under controlled curing conditions of time, temperature and pressure (referred to as first cure). These curing conditions are
continuously monitored in order for the half shell to meet our specific requirements.
Each half shell is then buffed to even true the edges and prepare them for the adhesive that is used to bond the two halves together. The half shells are loaded into the top and bottom of a machine that looks like a
waffle iron or an egg carton and they are cured here under controlled time, temperature and pressure. The inner chamber is pressurized so that the air trapped inside as the halves are fused together is at the same pressure (referred to as second cure). The adhesive on the half shell edges fuses the two half shells together for a tight seal. The pressure of each of our second cure presses are constantly monitored to ensure consistency of the core.
QC will then sample cores to test them for weight, size, rebound and deflection. The surface of the cores will then be abraded (roughened)in preparation for adhesion of the felt strips. The core is then dipped
in a high quality adhesive compound and oven dried in preparation for the cover application.
A mix of specifically designed fibers are processed together to form rolls of felt material. These rolls are then "back coated" with a specially designed adhesive. Several rolls of back coated felt are fed into an automated high-speed cutting machine which punches out the figure 8 shaped pieces of felt and packs them together into a bundle. The felt packs
are then dipped into a vat of white seam adhesive that coats only the edges of the felt. The felt packs are dried and the figure eights are then separated. The back coated figure eights are now inserted into the
felt-covering machine and placed on the core. At this point, the product starts to resemble a tennis ball. The final cure insures a perfect bond between the ball and cover. Under conditions of time, temperature and
pressure, the felt is bonded to the core and the seam adhesive is cured (referred to as third cure).
Extensive quality control checks are conducted throughout this entire process to assure a high quality finished product.
After third cure, the balls are steam fluffed to raise the nap on the felt, giving the balls their fuzzy appearance. After the fluffing process, the balls are visually inspected for cosmetic quality. Next comes the
stamping of the company logo and number. The logo operation is also systematically controlled in order to maintain the proper positioning. QC will then sample finished balls and test them to assure that they meet USTA and player specifications. Three balls are sealed in air tight pressurized cans. The pressurized can keeps the ball pressurized for excellent bounce and playability.
DIFFERENCES BETWEEN PRESSURIZED & PRESSURLESS TENNIS BALLS
1. Pressurized balls have traditionally been the ball of choice in this country. This preference for pressurized is based on the following:
They are typically more lively than pressureless and feel lighter off the racquet.
Pressurized balls typically sound a little crisper when hit.
Pressurized balls (in this country) are very inexpensive. In 1930, a can of 3 Wilson tennis balls could be purchased for $1.50 in a Sears & Roebuck catalogue. Over sixty (60) years later, that same can
of balls may be purchased for under $2.00. With this low price, a large majority of players open a new can of tennis balls at every outing.
2. Pressurized balls are packaged in specially designed pressurized containers that are capable of keeping the balls fresh for years in storage. However, once the seal of the can is broken and the pressure is released, the balls will begin to lose air and, therefore, liveliness.
The rate at which this occurs is a function of the following:
TEMPERATURE: The higher the temperature, the faster the balls will lose air (liveliness). For example, at room temperature a ball would typically lose approximately 2 psi of air pressure in a 2-month time period. This would result in a 2 inch loss of rebound height
(liveliness) which a good player could potentially notice.
At elevated temperatures, such as 100° F, this loss of air pressure would occur must faster - probably 2 weeks instead of 2 months. For this reason, it is not a good idea to store opened tennis balls in the trunk
of your car during the hot summer months. We recommend storing the balls at a cool temperature, even a refrigerator, if you have the room.
USAGE: Although we don't have any hard data to substantiate this claim, we do believe balls lose air much faster when they are used in play. The impact with the racket and court during play heat up the balls,
resulting in a higher internal pressure and, consequently, a higher permeation rate.
3. In sharp contract to the above, pressureless tennis balls have no internal pressure inside the core. Therefore, they will not lose liveliness over time. This provides more consistent performance over
time.The most frequent complaint about pressureless balls is that they are slow playing and feel heavy on the racket. The Wilson "advantage" tennis ball is specially formulated to eliminate this heavy feeling on the racquet.
Additionally, its slightly slower playing characteristics make it ideal as a practice ball since it provides a little extra time to prepare for
Pressureless balls are ideal for ball baskets since they don't lose air pressure (liveliness) over time. In areas of the world where tennis balls cost 2-3 times more than in the US, pressureless balls enjoy a
significant market share. They represent an excellent value to the cost conscious consumer.
EFFECTS OF TEMPERATURE ON THE REBOUND HEIGHT OF A TENNIS BALL
6-Wilson T1001 Championship Extra Duty tennis balls
1. Balls were kept overnight at room temperature and measured for 100"rebound height the next day.
2. Balls were placed in a refrigerator overnight at a temperature of 38°F. The next morning, one ball at a time was removed from the refrigerator and measured for rebound height as quickly as possible.
3. Balls were placed in an oven at 100°F for 6 hours. It is important to note that the balls were placed inside a small cardboard box to prevent the hot oven air blast from impinging directly on the balls. After 6
hours, one ball at a time was removed from the oven and measured for rebound height as quickly as possible.
4. Step #3 was repeated at an oven temperature of 130°F.
The results on the 6 tennis balls tested were averaged and plotted as a function of temperature. The results may be found in Graph 1 attached.
DISCUSSION OF RESULTS
1. Temperature was found to have a strong influence on rebound height.
2. Rebound height was most effected at the lower temperature of 38°F. On average, the balls lost 10 inches of rebound in going from 72°F to
38°F. It is important to note that if the balls at low temperatures were used in play, the would quickly increase in rebound height because of the
warming of the balls due to flexing of the core.
3. The balls quickly fell out of the rebound specifications of 53" - 58" when exposed to the 3 test temperatures.
100" REBOUND HEIGHT TEST
1. Balls were dropped from 100" (as measured from the bottom of the ball) onto a solid base, preferably concrete or granite.
2. Rebound height is measured to the bottom of the ball using a large graduated scale mounted behind the ball.
3. Care must be taken to assure that the eyes of the tester are in line with the approximate rebound height of the ball.
4. Three readings must be taken for each ball, and the average of the three is recorded as the rebound height.
USTA TENNIS BALL SPECIFICATIONS
APPENDIX - RULE 3
BALL - SIZE, WEIGHT AND BOUND
1 The ball shall have a uniform outer surface and shall be white or yellow in color. If there are any seams they shall be stichless. The ball shall be more than two and a half inches (6.35 cm) and less than two and five-eighths inches (6.67 cm) in diameter, and more than two ounces (56.7 grams) and less than two and one-sixteenth ounces (58.5 grams) in weight. The ball shall have a bound of more than 53 inches (135 cm) and less than 58 inches (147 cm) when dropped 100 inches (254 cm) upon a concrete base. The ball shall have a forward deformation of more than .220 of an inch (.56 cm) and less than .290 of an inch (.74 cm) and a return deformation of more than .350 of an inch (.89 cm) and less than .425 of an inch (1.08 cm) at 18 lb. (8.165 kg) load. The two deformation figures shall be the averages of three individual readings along three axes of the ball and two individual readings shall differ by more than .030 of an inch (.08 cm) in each case. All tests for bound, size and deformation shall be made in accordance with the regulations in the Appendix hereto.
1The Official USTA Yearbook and Tennis Guide With The Official Rules, H.O. Zimmerman, Inc.,XXXXX, Lynn, MA, 01901, 1977, pp. 415.
HIGH ALTITUDE TENNIS BALLS
Designed to give better performance at altitudes of 3,500 feet or greater.
Core is made of a less resilient rubber compound but has the same pressure as our other tennis balls.
If you use a high altitude tennis ball at heights of less than 3,500, the ball will rebound about 3 inches less and will play more "dead".
PERMEABILITY OF TENNIS BALL CORES
The can is pressurized (not vacuum) with approximately 12 LB/IN2 pressure to maintain the pressure in the ball. Once the can is "popped", and pressure is released, the balls will lose air pressure at a slow rate, similar to a car tire losing air. Below is a test report showing the change in "Rebound Height" and "Deformation" as a function of time (or
days out of the can). As you will notice, the balls lose rebound height (bounce) and become softer. The air pressure leaks through the microscopic pores in the wall of the rubber core.
PERMEABILITY TEST OF TENNIS BALL CORES
Twelve (12) Wilson Extra Duty tennis balls made with the following compounds:
Each group of balls were tested initially right out of the can for rebound and deformation. The balls were then placed in the lab conditioner at
the standard test conditions of 68°F and 60% Relative Humidity. The balls were then re-tested every two (2) days for three (3) weeks. After 27 days, the balls were re-tested every seven (7) days. After 41 days the balls
were re-tested every thirty (30) days. The balls were out of the can for a total of 196 days when the test was discontinued.
TEST RESULTS & CONCLUSIONS
The following table lists the rebound and deformation mean and standard deviation for the various compounds over a number of days out of the can:
This data has been plotted and curve fitted by computer using linear regression formula. The coefficient of determination (R2) and the individual predication equations are given in the following table:
R2 Y=A + Bx R2 Y=A + Bx
0.948 Y=55.81 ± 0.034x 0.984 Y=0.2731 + 0.00031x
0.975 Y=55.31 ± 0.032x 0.979 Y=0.2371 + 0.00028x
0.969 Y=55.06 ± 0.036x 0.984 Y=0.2320 + 0.00028x
0.972 Y=56.36 ± 0.038x 0.974 Y=0.2393 + 0.00026x
What happens to the rebound of tennis balls once they are removed from the pressurized can?
The rebound of all tennis balls decreases over time once they are removed from the can; this is due to pressure loss. The rebound loss, however, is only 0.032" to 0.038" a day.
Based on a 5" difference between the 58" upper spec and 53" lower spec for rebound, it would take approximately 131 to 156 days for these balls to lose 5" in rebound at 68°F and 60% Relative Humidity.
The rate of rebound loss at 68°F and 60% Relative Humidity varies from compound to compound. Based on the slopes of these lines, the compounds can be ranked in order of least amount of loss in rebound:
What happens to the deformation of tennis balls once they are removed from the pressurized can?
All tennis balls soften over time once they are removed from the can;this is due to pressure loss. The amount of softening, however, is only 0.00026" to 0.00031" a day.
Based on a 0.060" range for the deformation specifications of 0.220" to 0.280", it would take 194 to 231 days for these balls to soften 0.060" at 68°F and 60% Relative Humidity.
Based on the slope of the linear regression lines, the compounds may be ranked in order of least amount of softening at 68°F and 60%
It is important to note the initial rebound and deformation of each compound. Compounds that have a rebound near the upper limit (58.0") will remain "in spec" longer than balls that are near the median (55.5") or the lower limit (53.0"). The same discussion applies to deformation; balls that are near the "hard" side of the specification (0.220") will remain
"in spec" longer than balls that are nearer the specification median (0.250").
This test does not accurately measure the life of tennis balls because, in reality, balls are taken out of the can, played and then allowed to sit in non-pressurized cans at various temperatures. The actual play time and amount of hitting will accelerate the changes in rebound and deformation.
At best, XXXXX XXXXX can be used to determine how long a tennis ball is playable once the can has lost its pressure.
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