Metal Finishing Guide Book


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Fig. 5. Various types of replaceable tips. and plasticizers. Even before the rack or tip is cemented, it is necessary to rough up the surface for adequate adhesion. This process is called blasting—normally a procedure using some abrasive-type media such as aluminum grit, sand, or metal shot. The plastisol's primary function is to provide a corrosion protective coating, which is impervious to the acid or alkali attack that is prevalent in every plating line. Plastisol racks and tips can be trimmed easily, exposing only the contact area to grip the part to be plated. Large racks will pick up more plastisol than small ones. It is important to try to maintain a consistent thickness, keeping in mind that small wire tips will retain very little heat and, therefore, pick up a lesser amount of coating. Replaceable tips have some advantage by being coated separately and retaining more heat, developing a thicker coating. When plastisol is exposed to a trichloroethylene or perchloroethylene solvent, it will leach out the plasticizer and cause the coating to harden and crack. SPECIAL APPLICATIONS Anodizing Racks Anodizing racks are presently constructed out of two kinds of materials: aluminum or titanium. Generally, anodizing racks are not coated. The main factor, as with copper racks, is that the contact with the anodized part must be positive. Titanium and aluminum do not lend themselves to spring-type tips as they are not spring-tempered materials. Aluminum is a much cheaper material, but it will be chemically attacked and also requires stripping of the anodic film after each cycle. Titanium has excellent corrosion resistance, long life, and maintenance-free operation. Titanium racks can be completely assembled with titanium nuts, bolts, and screws. Printed Circuit Board Plating Racks The requirements for printed circuit board (PCB) rack design for electronic plating of all types and sizes of boards are as complex as the microchip itself. The PCB rack must be designed to hold the board in a locked position with positive contact on the border of the board (see Fig. 6). The board must be held securely because of mechanical or air agitation in the various baths. The contact point should be T316 stainless steel with a thumb screw of T316 with a Teflon tip, thereby creating a positive contact with minimum plating buildup. The most commonly used thumb-screw size is 3/8-16. Some other sizes are 1/4-20, 5/16-18, 3/8-12. 715

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