Metal Finishing Guide Book


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sufficient testing beforehand. Surface preparation cycle: 1. Alkaline soak clean. 2. Activate and pickle in a solution consisting of 20-25% v/v hydrofluoric acid 75-80% v/v nitric acid. 3. Etch in solution of sodium dichromate at 30-35 oz/gal (225-263 g/L) and 4-5% v/v hydrofluoric acid for 15-30 minutes. Thorough rinsing between each step. Zinc and Zinc Alloy Die Castings Zinc is molten and cast into many shapes and forms, comprising a wide variety of consumer and industry relegated parts. Just like aluminum, zinc is available in different alloys. (See Table XXII.) The casting operation does result in surface defects, which must be corrected in an appropriate manner either before shipment to the plater or in the surface preparation cycle. Pores, cracks, "cold shut," and roughness are some of these common problems. Mechanical operations, such as buffing and polishing, refine, and smooth the surface but leave accumulated buildup of related soils, grease, compounds, and rouges. The exceptionally high temperature of these mechanical finishing techniques will burn, harden, and drive contaminants into the metal surface. The sooner parts are cleaned the easier the surface preparation cycle becomes. Surface preparation: (refer to specific cleaner baths and operating parameters, as previously given) 1. Soak or ultrasonically clean. Removing buffing and polishing compounds. The cleaner may be wetted with glycol and cyclic pyrollidone type solvents. A combination of high HLB and low HLB surfactants are helpful. Soaps are also an option. The cleaner should be buffered to prevent tarnish and etching of the zinc surface. Many buffing and polishing compounds are effectively softened in the soak cleaner at 175-190°F (79-88°C). Ultrasonic conditioning uses 25-43 KHz/gal of power in the solution to maintain effective standing waves, resulting in bubbles imploding on the surface for cleaning action. Temperature of the ultrasonic cleaner should be in the range of 160-180°F (71-82°C). Somewhat cooler to avoid higher temperatures, which distort the standing waves. 2. Secondary soak clean. Removes residual organic contaminants and any inhibiting films that may have formed on the surface during the step #1 soak cleaning. 3. Electroclean. Moderate alkalinity, inhibited. 4. Acid dip. Zinc die castings may be treated in a specially blended acid solution, commonly referred to as immersion chemical polishing. This process facilitates surface preparation by deburring, smoothening, leveling, and brightening. Common base metal defects, such as nodules and pores, are effectively worked out. A typical solution consists of: 42° Be` nitric acid (20-30%), 66° Be` sulfuric acid (20-25%), ammonium bifluoride (20-40%), and nonionic or amphoteric surfactant (>0.5%). Application: 65-115°F (18-46°C). Maintaining temperature is critical to avoid etching or dulling the surface. Immersion time depends on particular surface requirements. Organic soils (grease, oils, buffing compound, mold release, etc.) should be removed in a suitable soak or ultrasonic cleaner before 154

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