Coil Build Up Lines – Coil Joining – Seam Welders
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In a typical production facility using coils of cold rolled steel, the downtime for coil changeover can be significant. In fact, coil changeover and rethreading can result in 20 to 30% loss of productivity. Therefore it is worthwhile to insert a machine in line that will join the end of the steel in one coil with the beginning of the steel in the next coil before processing. If the end of each coil is welded to the beginning of the next coil, then productivity will be maximized without increasing the speed of production and taking on the risks associated with faster production speeds.
There are two basic ways to join coils to build up coil lines, with prep-lap seam welders and flash-butt welders.
Prep-lap seam welders use a joining process that’s suitable for several types of coated and un-coated steel. It involves shearing part of the ends off two pieces of steel and then overlapping the ends slightly before welding them together and then planishing the resulting join. A disadvantage of prep-lap seam welding is that the weld may not be the same thickness as the original material.
Flash-butt welding is an excellent method of coil joining. Ends of each strip are set into a copper clamp. Current is passed through the clamps and the strips are pressed together to complete the weld. The one disadvantage is flash build-up, which must be ground down. Flash-butt welding is cost-effective and has high weld integrity. If a TIG welder is used, an electric arc goes from the electrode to where the strips are joined, melting the material. The arc itself is protected by inert gas and the ends of both strips are melted and then solidify instantly. Coil joiners with TIG welding can be used to weld materials including nonferrous materials, copper, brass, aluminum, galvanized stainless steel, pre-painted steel, and mild steel. As long as the carbon content is in the range of low-carbon steel, hardness build-up when joining coil ends should not be a problem. For most metals, a weld cycle will take about two minutes.
In production facilities that include a coil build-up welder, a coil accumulator is often added as well so that there is enough steel readily available to keep the production line running while two coils are joined together. If you have a TIG welder that takes two minutes to join one coil to another, then you’ll need an accumulator that holds at least two minutes’ worth of steel.
Consider a stamping facility that only uses an uncoiler as entry equipment. When the end of a coil is reached, someone has to stop the line while a new coil is put in place and threaded into the machine. A typical running speed for stamping lines is 40 feet per minute. Average downtime for changing out coils in a stamping facility is about 10 minutes. The average steel stamping facility will process 5 to 10 coils per day. The heavier the gauge of the steel, the more coils will be used.
If you know the profit per foot and assume the facility runs 250 days per year, you can easily calculate how much profit is lost due to coil changeover. For the steel stamping facility, assume a profit per foot of $0.05.
40 feet/minute x $0.05/foot = $2.00 profit per minute, or $120.00 per hour. If you were to go from processing 5 coils per day to processing 10 coils per day, profit can double without having to speed up the processing equipment. A coil build-up line and a strip accumulator can be a wise investment that will quickly pay for itself in increased profits.