It is said that the shaft determines 70-80% of the performance of a golf club. What kind of process is that important shaft made through? In the shaft market, he experienced the manufacturing process at Doomina, which is gradually growing into a global brand. Auto Power and Auto Flex shafts produced by Doomina became popular overseas before domestic production, and their names became known. Celebrating its 10th anniversary this year, Doomina has built a new production facility on a large site in Gwangju, Gyeonggi-do, thanks to its growth. After a brief conversation about the shaft with Chairman Park Geon-yul and CEO Doo-na Jeong, who have led Doomina with one perseverance, the experience began in earnest.
Doomina Chairman Park Geon-yul (right) and CEO Jung Doona.
“80~90 ton fabric?… That’s nonsense.”
The first place I found was a low-temperature refrigeration warehouse that stores carbon fabric. Normally, the temperature is maintained at 1 to 3 degrees above zero, and when stored for more than a month, at 1 to 2 degrees below zero. The reason why carbon fabric is stored in such a cool state is that the resin of the fabric dries easily at room temperature. Then, the nature of the fabric changes and the adhesive strength decreases when working later.
Carbon fabric, called prepreg, is a yarn-shaped carbon fiber bonded with an epoxy resin in one direction. One fabric is made with a width of 1 m and a length of 100 m. From 2 million won per roll to 20 million won for expensive ones. Chairman Park said, “Currently, the amount stored in the warehouse is approximately 1 billion won. Sometimes we have more than fabric distributors,” he shrugged. That means there are a lot of shaft orders.
There is a part where Chairman Park raises his voice in relation to fabric. is the tonnage of the fabric. Some brands promote that shafts are made with 80-ton or 90-ton yarn, but Chairman Park insists that that does not make sense. “Tonnage means the modulus of elasticity of the fabric, and if it has that much elasticity, it is like a dry leaf. It is the result obtained through numerous tests that the elastic modulus of the fabric is sufficient at about 40 tons. So I can say with confidence. I don’t know if I cut a little bit of 80-ton or 90-ton fabric and attach it, but it’s a lie to say that it was rolled as the main material.”
Shaft fabric prepreg.
Treat the fabric like a baby…
If there are handprints left , there is a cutting room right next to the warehouse for storing defective fabrics. This is where the carbon fabric is precisely cut according to the shaft design. At one end of a long, large table lay a piece of carbon fabric. Chairman Park, who was explaining the fabric, asked, “Never touch it with your bare hands.” The reason was that the fabric is a very sensitive material, so if a handprint is left, the physical properties change and there is a problem with adhesion during rolling. Chairman Park said, “Fabric should be handled gently like a baby with clean gloved hands.”
Several fabrics are used in the graphite shaft, and the flex (flexibility) of the shaft and the location of the kick point (bending point) vary depending on the shape of each piece and how they are attached. Narrow and long fabric forms a straight layer, and triangular or rhombic small pieces of fabric are used as a reinforcing layer for the tip (head side of the shaft) or butt (grip side of the shaft). There were piles of pre-cut pieces of fabric on one side, but I was told not to take pictures because the design secrets could be leaked.
In the past, workers cut the fabric one by one using a tape measure and a knife. Chairman Park said, “I cut my hand and put a lot of strain on my wrist.” Currently, 400 to 500 sheets are cut at a time using a machine. It was like cutting hundreds of sheets of printing paper at once in the book binding process. However, there was a risk of finger amputation, so I was content with just seeing this work with my own eyes.
A worker cutting fabric.
After winding the carbon fabric on the mandrel and wrapping it with film,
the next place to go was the molding room. This is where the cut fabric is wound around a long iron rod called a mandrel. The diameter of the shaft is determined by the molded mandrel. After attaching the fabric to the mandrel, it is important to roll it with a certain pressure. This rolling work was also done manually in the past, but now it borrows the power of machines. When the mandrel with the fabric attached was placed on the shelf and the switch was pressed, a heavy machine came down and started rolling. One peculiarity was that the machine moved diagonally rather than horizontally. This is because the mandrel has a tapered structure in which the diameter decreases toward the tip. It is for the same reason that the width of the tip part is narrower than that of the butt part when cutting the fabric.
Chairman Park also asked me to touch the workbench where the mandrel is placed. It was very warm. He said it was to increase the adhesion between the mandrel and the fabric. I used to iron it in the past, but since I already warmed up the mandrel, I didn’t need that process. After wrapping the fabric on the mandrel, wrap it again with a film in the form of vinyl tape. The film, which shrinks when heated, is wound from the tip to the butt. The film plays a role in tightly adhering the shaft during heat treatment. I held the film with my left hand and lightly held the shaft with my right hand, and the machine turned the shaft and wound it automatically.안전놀이터
Heat treatment at high temperature like pottery baking… Time and temperature matter
The film-wrapped shaft is baked through a high-temperature heat treatment process, just like pottery is fired. The exact process is a secret because the physical properties of the shaft change depending on the heat treatment time and temperature. It usually takes 4 hours, but sometimes the temperature changes 1 to 3 times.
After the heat treatment, the shaft undergoes a demoulding process. to separate the mandrel. It is said that you should do it right away when it is hot, and it will not come off when it cools down. Every time the workers opened the heavy door of the blast furnace and took out a bundle of shafts, the heat was felt intact. Chairman Park showed scars on his wrist, saying, “I get burned while working.” After demoulding, resin remaining on the mandrel should be removed immediately with a knife and a rag.
The shaft with the mandrel removed is cut to exact length using a high-speed cutting machine. I set the length in advance, but Doomina said that it was cut to 45 inches, which is 1 inch smaller than a normal shaft. Chairman Park said, “When products are shipped in 46 inches, fitting shops often cut off the tip. Then, the overall characteristics such as shaft flex change,” he explained. “We made it 45 inches so that we can maintain the intended characteristics.” The worker who helped the experience said, “Do not push the shaft with strong force to the cutter, but push it gently.” That’s because it cuts smoothly.
The next step is to remove the film wrapped around the shaft. The worker held the razor at an even angle and swiped the film covering the outside of the shaft at high speed, leaving only the top part thinly separated as if a piece of fabric were being drawn, and it came up quickly through the blade and thumb. It was like seeing shavings briskly rising up when you saw fine wood with a planer. After making a couple of mistakes, I felt like I would get a sense of it, but I had to be content with just holding the blade and trying to hold the form because the shaft could be scratched.
the shaft is baked at a high temperature, leaving marks where the film was wound on the surface. This has to be removed by polishing it automatically and manually twice. Polishing is one of the most demanding processes. Chairman Park said, “Just as a person with poor dexterity does not have excellent golf skills, he cannot do it even after 5 years.” After polishing, check the CPM (vibrations per minute) and weight. After that, in one workroom, an employee was checking something by pressing the shaft with strong force to bend it and turning it around. He said it was a job to get rid of spines. Spine, which means ‘vertebral’, is a kind of eccentricity, and if there is a spine in the shaft, it can bend in a certain direction. He said that it is possible to remove the spine through polishing, but it can only be done by skilled workers with excellent hand sense for more than 5 years.
Now it’s time to dress the shaft. it is painted Before that, you need to clean fine dust or carbon residue from the shaft with water and thinner. Thinner is used to remove oil. If oil remains, it cannot be painted.
demoulding process.
the first step of destructive test painting every 10 days is the first step. You can understand it as a background color. It is similar to the concept of applying putty (commonly called ‘pa-da’ in the field) first when painting a car, and mainly uses black or white. Minor weight adjustments are also made at this stage. After pushing the shaft all the way into the paint canister with a hole in one side, the paint was applied by pulling the shaft out while tilting the canister. It was not easy to learn the speed of pulling out the shaft and the tilt of the paint canister by feel. During the trial, I tipped the barrel for too long and spilled some paint.
After the base coat is finished, it’s time to apply the full color. I use an automatic spray, but I do not paint once, but repeat the process about 4 times. So that the color is applied vividly. He said that he did up to 12 times to express the gradation in which the color tone gradually changes naturally. When giving a sparkling effect to the shaft, an ‘ion deposition machine’ is used. It is an expensive equipment that costs 800 million won per unit, and it is said that the shaft is plated by shining light on aluminum or titanium in a vacuum. When the logo is printed on the transfer paper on the painted shaft, the entire manufacturing process is finished. The finished products are then moved to the storage warehouse.
Doomina said that they do a ‘destructive test’ once every 10 days to maintain a certain quality. On the day of the experience, she was also able to observe the destructive test. Press the tip, butt, and center of the shaft with mechanical power to check the pressure until it breaks and the shape of the break. The researcher said, “I cannot give specific figures, but the tip part that receives the most force during impact is about 2 to 3 times stronger than the center or butt.”
A researcher undergoing destructive testing.
◆ Doomina’s main shaft manufacturing process
- Cutting = Cut the carbon fabric according to the designed size and shape. The elongated fabric forms a straight layer, and the triangular or rhombus-shaped pieces are used as a reinforcing layer.
- Forming = Winding the fabric onto a mandrel. It is important to wind while pressing all parts of the shaft with constant pressure. After that, the film is rolled up using a machine to add compression.
- Heat treatment = Bake for about 4 hours at different temperatures in a blast furnace. The resin melts and adheres to the carbon yarn. Shaft properties change with time and temperature.
- Demoulding = Separation of mandrel and shaft. It should be done immediately while hot. Remove the resin residue remaining on the mandrel with a knife and cloth.
- Cutting = Cut both ends of the shaft using a high-speed cutting machine. Also remove the film covering the shaft.
- Grinding = It is an operation to smooth the surface of the shaft. As the film shrinks during the heat treatment process, marks are left on the shaft. These parts are polished. After the work is done, also check the CPM and weight related to shaft strength.
- Spine inspection = If the shaft has a spine (vertebral column), it will be twisted in one direction. Therefore, if there is a spine, it is slightly polished to remove it. A skilled worker bends the shaft and spins it around to inspect it.
- Cleaning = Use water and thinner to remove fine dust or carbon residue from the shaft. If foreign matter or oil remains, the painting work will not work properly.
- Undercoating = Undercoating work prior to painting, mainly using black or white. Minor weight adjustments are also made at this stage.
- Painting = Using automatic spray, repeat about 4 times without finishing once. That way the color is applied well. When I express gradation, I do more.
- Printing = Print logos using transfer paper. After the final inspection, it is stored in the warehouse.
- Destructive test = Check the quality by breaking the tip, center and butt of the shaft every 10 days using a measuring machine.