Flatness requirements for metal strips have increased steadily in recent years. At the same time there is a trend – particularly in the electronics industry – towards increasingly thinner materials. Here, quality improvements through stretch-bend-leveling facilitate further processing considerably. As a result, e.g. dimensional accuracy is increased in the stamping process and thinner surface coatings can be achieved.
Hence, the cutting and levelling machine specialist Burghardt + Schmidt GmbH (B + S) developed an innovative solution for the leveling of extremely thin copper strips for a Chinese manufacturer. The line is suited for all alloys of the material as well as for phosphor bronze, and allows strips just 30 μm thick to be processed with minimal tolerances. Depending on the starting material, flatnesses down to one I-unit can be achieved.
The client in China manufactures parts for the electronics industry that need to satisfy exacting
requirements with regard to surface quality as well as flatness or dimensional accuracy. In this case, extremely thin metal strips are used. At a plant to be redesigned, these are to be prepared for further processing. The aim of the new purchase was to keep the shape tolerance within tight limits during the stamping process. This is vitally important with regard to the automated assembly of electronic components. Besides flatness, the client also defined exclusion criteria relating to the surface quality. This means that the optical density of the end product should not be changed through the processing.
“Given the clearly recognizable trend for ever thinner strips, the B + S lines constantly reach their control limits. Due to customer requirements as well as the results of corresponding market analyses, we have therefore pressed ahead with basic developments for some time. These generally allow the levelling of metal strips that are just 20 μm thick,” explains Volker Lüdecke, technical manager at B + S. The concept presented and the company’s extensive experience in stretch-bendlevelling was ultimately the decisive factors in winning the contract from China.
Cassette with 8 mm levelling rollers
For the build-up of tension, the line has four rollers on the braking side, that is, in front of the leveling machine. Around these rollers runs the metal strip. And together with the decoiler, they generate the back tension. Behind the leveling machine the four tension rollers, together with the recoiler, pull on the strip. By means of force superposition (tension and bending) between the groups of upper and lower levelling rollers, the internal tensions and shape deviations of the strips are reduced. To achieve a sufficient levelling effect in the case of very thin strips with material thicknesses of between 30 and 200 μm, the metal strip specialist has developed a cassette with a leveling roller diameter of 8 mm.
For thicknesses greater than 200 μm, 12 mm leveling rollers are used. Deviations are automatically detected by a Vollmer shapemeter roll for continuous strip shape measurement. Strip zones with waves exert less force on the segmented roller than flat strip zones. A software calculates the mathematical formula for the strip shape with the help of the measured values and uses this to create a 3D model. Based on this, the necessary position of the levelling roller supports can be calculated and automatically adjusted. Manual readjustment is unnecessary.
A special feature are the four adjusting axles at the upper leveling cassette, which ensure the ultra-precise position of the three spatial axes. “Depending on the initial flatness, evennesses down to one I-unit can thus be achieved. This corresponds to a 0.2 mm high and 100 mm long wave in the strip,” explains Lüdecke. Highly dynamic drive
for broad control spectrum
In principle, all copper alloys are processed on the line without restrictions, with the strips having a wide range of cross-sections and the tensile strength of the material varying between 300 and 1,100 N/mm2. The thickness thus lies between 30 and 500 μm and the width is between 100 and 450 mm, which results in an aspect ratio of 1:75. If the yield strength of 200 to 1,050 N/mm2 is added, the ratio to be controlled for is 1:390.
Due to this wide spectrum, the strip tension is measured directly at the decoiler and recoiler as well as in front of the levelling machine and serves as a control value for the drives. These are equipped with highly dynamic AC motors. The installed capacity here amounts to around 550 kW.
“Feed-in is performed with Sinamics S120 Basic Line assemblies, so that the regenerative energy can be reused via an energetic recovery system on the pull side – that is, for the motors after the levelling machine. This leads to significant energy savings,” says Lüdecke, thus pointing out one of the main advantages of the line.
Activation is performed by fully digitalised inverters, which are coupled to the PLC of the line controller via Profibus L2-DP. The actual control functions are controlled via a microprocessor that enables not only considerable ease of operation but also easy start-up and maintenance.