Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 257 of 707

are left in the oil too long after the coating melts. The size of the oil bath is also important so that the immersion of cold parts does not reduce the oil tempera- ture to the point that heating time must be prolonged. A suitable quench to use is 4 to 6 in. of kerosene (acid-free) over water. The parts should be lowered into the quench slowly so that the molten tin coating solidifies in the kerosene layer. A spangled appearance will result if the molten tin comes in contact with the water. The water further lowers the temperature of the parts and also keeps the kerosene from being heated too near its flash point. The kerosene also serves to remove some of the oil from the tin surface. Small parts generally are immersed in the hot oil in bulk in a suitable basket. They are then separated by pouring them slowly into the quench or by using a breaker, which separates them before entering the quench. For flow-melting large parts, a compartment basket is recommended to prevent disfiguration of parts touching while the tin is molten. The breakdown of free fatty acid content will gradually reduce the effectiveness of the oil, relative to whether it drains freely and evenly from the molten surface or whether a streaked appearance occurs. To maintain the oil suitably for proper draining, part of it (10-25%) must be replaced frequently. Residual oil must be removed from the tin surface after flow melting. Any ordinary degreasing method may be used, but the preferred method is vapor degreasing. There are a number of factors that are critical in successful reflowing. Good cleaning, good plating, and good rinsing practices are vital. Dewetting is more like- ly to occur on large flat areas, rather than with rounded shapes. Troubleshooting hints are offered in Table XIII. DETERMINATION OF ACID NEUTRALIZATION VALUE 1. Weigh 6 to 10 grams of oil in a 250-ml Erlenmeyer flask and add approxi- mately 50 ml of 3A alcohol (a methanol-ethanol mixture) previously neu- tralized to a pH of 7.0. 2. Carefully heat the solution on a hot plate while stirring until one or two bub- bles appear on the surface of the solution. 3. Remove the solution from the hot plate, add five drops phenolphthalein indi- cator, and immediately titrate with 0.1 N sodium hydroxide from clear to a pink endpoint that persists for 10 seconds. Acid Neutralization Value mg KOH/g oil = ml Na/OH X 5.611/weight oil sample, g 256

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