In a typical production facility using cold rolled steel coil, downtime for coil changeover can be significant. In fact, bobbin changing and rethreading can result in a 20-30% loss in productivity. Therefore, it is worth inserting an in-line machine 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, productivity will be maximized without increasing production speed and taking the risks associated with faster production speeds.
There are two basic ways to join coils to build coil lines, with prep-lap seam welders and butt welders.
Prep-lap seam welders use a joining process that is suitable for various types of coated and uncoated steel. It involves cutting off part of the ends of two pieces of steel and then overlapping the ends slightly before welding and then planning the resulting joint. One 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 joining coils. The ends of each strip are placed in a copper clamp. Current passes through the clamps and the strips are pressed together to complete the weld. The only disadvantage is the accumulation of burrs, which must be removed. Flash butt welding is cost effective and has high weld integrity. If a TIG welder is used, an electric arc travels from the electrode to where the strips meet, melting the material. The arch itself is shielded by inert gas and the ends of both strips melt and then instantly solidify. TIG welded coil joiners can be used to weld materials including non-ferrous 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 buildup when joining the ends of the coil should not be a problem. For most metals, a weld cycle will take about two minutes.
In production facilities that include a coil building welder, a coil accumulator is also often added so that enough steel is available to keep the production line running while two coils are joined. If you have a TIG welder that takes two minutes to join coil to coil, then you’ll need a battery that holds at least two minutes of steel.
Consider a stamping facility that only uses an unwinder as input equipment. When the end of a spool is reached, someone has to stop the line while a new spool is put on and threaded onto the machine. A typical operating speed for stamping lines is 40 feet per minute. The average downtime for changing coils in a stamping plant is about 10 minutes. The average steel stamping plant will process 5 to 10 coils per day. The heavier the gauge of 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 change. 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, the profit can be doubled without having to speed up the processing equipment. A coil building line and strip accumulator can be a good investment that will quickly pay for itself with higher profits.