Metal Finishing Guide Book


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Q: We are piston ring platers. I want to know about the ramp limitations/specifications for hard chrome plating of piston rings for both compression & oil rings. I'm finding problems of double plating due to frequent power trippings, especially in India. How many types of rectifiers are available in the industry presently & which one is the best for hard chromium plating over piston rings of cast-iron make? A: Of course it is not possible to avoid the power trips, so to guarantee the quality of your products after power outages you'll have to etch the parts again and re-plate. The best hard chrome rectifiers offer a very efficient DC ripple smoothing filter, and for a number of applications, an etching cycle as well. Q: How well would it work to use a small programmable pulse rectifier to modify the output of a larger (75-volt, 500-amp) rectifier to generate a pulse output for anodizing? I intend to connect them in parallel. A: To start with, NEVER connect rectifiers in parallel unless the rectifiers are specially designed to be connected that way, and always check first with the manufacturer. In your case, do not connect them in parallel, or it will be a very short, smoky event. I assume that you are trying to have the large rectifier 'follow' the pulse pattern generated by the pulse rectifier. This is possible by using the output signal of the pulse generator as a reference signal for the large rectifier. First, you need to know what maximum reference voltage or current level the rectifier accepts on the voltage and current inputs (typically 0–5 V, 4–20 mA or 0–10 V) and then make this value the maximum output value of the pulse generator/rectifier. A limiting factor, however, will be the rate of change that the rectifier can output. A standard rectifier will always need a specified time to get to a certain output level, so if your pulse generator outputs relatively short pulse times, the output of the rectifier will not be able to exactly follow the reference signal, resulting in a distorted output waveform. Q: We plate out copper in a reclaim system for our plate pickling tanks. We are considering the purchase of a new rectifier and I wanted to know how to spec a rectifier for the ripple percentage and how ripple effects plating. Our reclaim tank comprises a series of lead and copper plates. The pickling solution is 10/12% sulfuric at 110 degrees. The material we pickle is copper or copper/nickel alloy. A: For pickling, the output ripple of the rectifier does not really matter. If you know the maximum voltage and current your process needs, you can select a rectifier with a maximum output of around those values. Using a standard thyristor-controlled rectifier or a tap switch rectifier will give you the lowest ripple (5%) and the highest efficiency. Q: We have a cathodic electrocoating paint bath in which two different lines are passing. The conveyor speeds are the same. We are using three sets of electrodes. One at each side & a third one at the middle of the tank. The component loading on both the lines may or may not be similar. For this configuration, should we go with one rectifier or two different rectifiers of the same capacity & why? What is the best alternative? A: The best alternative here would be two rectifiers. With two rectifiers you can individually control the process voltage/current for each line, enabling you to individually control the coating thickness depending on the total surface area per line. 570

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