Metal Finishing Guide Book


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Design of the Spring Tension Tip Questions to be considered in the design of a spring tension tip are: What area of the part is most critical in the plating bath? Where can a rack tip mark be permitted, as it is almost always present? Of what kind of material is the tip fabricated? Referring to the critical area to be plated, thought must be given to the part location relative to anode configuration, drainage of the part, high and low current density areas, and gassing around holes and openings. A part should be held so that the rack mark is in the least critical area. Thought should be given as to what the end result will be with the finished plated part assembled and complete. After looking at this, a more objective rack mark area can be determined. Special care must be taken to make sure that the contact is secure, will hold the part throughout the plating cycle, and will not scratch the part. Materials Used for Tips The two most common materials used in the fabrication of plating rack tips are phosphorus bronze and stainless steel in both gravity- and spring-type tips. Other metals commonly used with a gravity tip are Monel and titanium. In the case of materials used for a spring-type tip, spring tempered or halfhard material is needed either in flat or round stock. Spring tempered is preferred because of the spring action present in the material, reducing metal fatigue. In the fabrication process sharp bends should be avoided as they create a fatigue factor and eventually the tip will break. In recent years, stainless steel has been widely used as a tip material because of the use of proprietary nitric or muriatic acid stripping solutions. Using phosphorus bronze with such strips would cause the tip to dissolve prematurely. It must be stated that phosphorus bronze has a greater current carrying capacity and should be used where current is a factor. Stainless steel tips do create some problems as their conductivity is so low that excess heat is created and could cause a premature breakdown of the plastisol rack coating. Large parts are generally fabricated using flat stock, whereas smaller parts can use round stock. FIXED VERSUS REPLACEABLE TIPS There are two types of racks used in plating: fixed tip and replaceable tip. Each style has its own advantages. Fixed tip racks are generally less expensive on the original outlay, and usually require a very tough tip because of the weight of the part and/or rack. The replaceable rack tip concept offers some advantages over a fixed tip rack: 1. Plating racks can be maintained at 100% capacity. When analyzing how many parts can be put on a rack and knowing what it takes to operate a plating line at a profit, each tip becomes a profit center with this concept. Whenever a tip breaks it can be replaced, thereby creating a consistent production output. 2. The replaceable tip allows many different and varied styles of tips to be used with the same spines, reducing the need to inventory racks for every style of part. 3. Cross bars can be made replaceable instead of every tip, creating some additional cost effectiveness. 4. The repair of the complete rack would be unnecessary as only the tips have to be replaced. 750

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