Metal Finishing Guide Book


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A: Basically, the output voltage of the rectifier depends on the total resistance of the load (including leads, saddles, etc.). On average, for your application, 15VDC should be sufficient (example: a 15V/250A Switch Mode Power Supply). Q: We run our commercial anodizing plant using a thyristor-controlled rectifier supplied by a Spanish company. Since a recent 'upgrade' we are now unable to achieve any of the electrolytic colors we were previously producing regularly. The first prognosis was that our thyristors were unbalanced. However, as these have now both been replaced with no improvement it seems our software provider is at a loss to pinpoint the problem. They have tried to put the blame on the cabling feeding the rectifier, although as this hasn't changed since the upgrade was done we cannot see how this can be the case. The only possible cause we can find is that we are struggling to achieve the correct power output of 10 volts that is required to produce the colors. We do not seem to be able to get above 8.7 volts; is it possible some sort of 'restrictor' has been fitted or can you suggest any other possible cause? A: You are experiencing what many users encounter with a similar upgrade. An upgrade to a thyristor control system for an electrolytic coloring system gives, in most cases, a lower quality, resulting in inconsistency and 'patchy' coloring. We recommend using a variable ratio controlled power supply providing a balanced sine-wave to the to-be-colored load. Q: We are going to install an anodizing process. What kind of material should we use for the cooling coil if I'm going to use freon to cool (the freon presure is 200 psi)? A: You should consider, instead of indirect cooling by cooling pipe, a direct cooling using a water-to-acid heat exchanger. For pipes you can use titanium or lead, however, a solder spot can be a weak part of your cooling system. Q: How much ripple should you have in a rectifier used for chromic anodize? I have always been told that about 5%, or less, is really good. A: We haven't seen any performance differences between high and low ripple for the anodic coating produced in chromic solutions. See: Chromic Acid Anodizing Standard, MIL-A-8625 F TYPE. Q: What is the best way to stop condensation on rectifiers when switched off, for a period of time, due to a supply failure? A: No water should be running through the cooling system of the rectifier when the system is not in use. In colder climates one should avoid pipes freezing up. Contact your rectifier supplier for specific advice. Q: What, if any, effect on type II anodizing do voltages below 5V have on the coating? We ramp our voltages, but the SCR on our rectifier does not allow a response below 5V, so we lose any effect of the ramp from 0–5 V. A: Depending on the material thickness, you might get burning marks caused by the input current rush due to the offset voltage of 5 V. Non-anodized materials act like a short circuit in the tank. It could also trip the rectifier if the load size is big enough and trips the main fuses. I recommend you contact the rectifier manufacturer for service to correct the cause of the offset voltage. 569

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