Improved designs for anodizing jigs and racks.
Improved designs for anodizing jigs and racksSince anodizing became an industrial process in the 1930s, much has been written about the process technology, but the jigging and racking of aluminium parts is a topic that is rarely discussed, yet good jig design is as essential to satisfactory anodizing as is good process technology. Jig design is generally regarded as an art, rather than a science, yet failure to understand the basic physics and chemistry involved can lead to real problems in the form of rejects, such as burning, patchiness, white spots, thin coatings, and so on.
Criteria for the design of anodizing racks
There are five primary criteria which must be fulfilled by anodizing jigs and racks: * They must be capable of ensuring an adequate flow of current to the work. Shape and position of the contacts must ensure good drainage and, as far as possible, eliminate gas pockets or gas streaking. * The contact points must be such that their position is acceptable to the customer, yet providing adequate contact for current flow. * The design must also contribute to good productivity, although sometimes there has to be a trade-off between productivity and providing adequate contact. * Economics have to be taken into account and the relative merits of titanium and aluminium evaluated. * The high initial cost of titanium increases the working capital needed in the business, but can be offset by longer life.
Aluminium racks have specific advantages, in terms of high current carrying capacity, ease of fabrication and freedom from the white spots defect in black dyeing. Life expectancy depends upon the alloy from which they are made, as well as the process and stripping methods to which they are subjected.
Getting current to the parts
The first requirement of a rack is that the stem, or spine, should be capable of carrying the total current needed by the parts to be anodized. Anodizing current densities generally lie between 1.0 and 1.5 A/[dm.sup.2] (or 10 to 15 A/[ft.sup.2]), although much higher current densities are used in hard anodizing. This can involve the passage of several hundred amperes, but on large architectural anodizing plants the current can be several thousand amperes.
Many plants undertaking anodizing for the first time do so with a plating background, so they use rounded copper busbars and hang a hook over them. With the good agitation needed in anodizing it is not uncommon to see the whole rack of work moving. Rectangular aluminium busbars are ideal for securing good clamping and they can carry large currents. Large architectural plants handling 12,000 to 15,000 amperes in a single tank normally use aluminium, yet some plating plants with anodizing tanks carrying only 1000 amperes will insist on using copper because aluminium is not suitable!
There is a major drawback to the use of copper anode and cathode rails on an anodizing tank. Unless they are protected by an acid resistant coating, which requires continued maintenance, copper is dissolved by the sulphuric acid electrolyte. This results in poor corrosion resistance when the coating is exposed outdoors, or when subject to salt spray testing. In addition, use of copper rails or bars on other tanks also results in copper being dissolved, and its presence in rinses and other processes can readily give rise to rinse water corrosion and galvanic pitting.
There is at least one aerospace company where the effect of dissolved copper is taken seriously, to the extent of plating it out regularly and removing all traces of it from the anodizing tanks sides and busbars. Essentially, copper in the electrolyte should not exceed 50 ppm if corrosion resistance is to be maintained and pitting problems avoided. Iron, present as an impurity in some sources of sulphuric acid, or resulting from rust falling into process tanks from structural steelwork, has similar dangers and similar impurity limits should be applied. Heavy metal build-up can also reduce throwing power, especially in hard anodizing.
Spine materials
These are normally made of either aluminium or titanium. The essential requirement is that its cross-section should be sufficient to carry the maximum current available. Of course, with several spines attached to work, this total current is shared between them in proportion to their cross-sectional areas. The amount of current which any metal will carry in air is always significantly lower than in an electrolyte, so that the portion of the spine exposed in air must have a cross-sectional area adequate to carry the current required.
Aluminium spines will readily carry in air 1000 amperes per 625 [mm.sup.2] (1.6 A/[mm.sup.2]). In the electrolyte the same cross-section will carry up top 2000 amperes. On the other hand, titanium will carry only 0.5 A/[mm.sup.2]. This calls for a substantial cross-section to the portion of the titanium spine exposed in the air, with consequent high cost.
One solution to this problem has been to use aluminium for the part of the spine exposed in the air. This has the drawback of attack on the aluminium in the solution, especially around the junction with the titanium. An answer to this problem has been obtained by the Vulcanium Corp USA, with the development of |Ti-Core' spines. These consist of a solid core of aluminium on to which has been drawn down a titanium skin of 0.75mm thickness, thereby combining the conductivity of aluminium with the chemical resistance. This has been tried previously, but the company have been successful in producing this composite at a price comparable with that of titanium.
'Sequel' aluminium alloy racks
The advantages of aluminium are its high current carrying capacity and low initial cost. Further, where work has to be electronically coloured it avoids the discolouration around the contacts which is inevitable with titanium. In addition, it frequently eliminates the white spot problem encountered in dyeing when titanium racks are used.
A major weakness of most aluminium racks in use in the UK at present is that they are made down to a price rather than to meet up with the performance and quality required. This means that they are usually made out of low strength commercial purity aluminium sheet or wire and from low strength HE9 extrusions with a consequent limited life and lack of robustness in handling.
A novel range of aluminium jigs and racks has been developed by the Sequel Corp USA, which is an outstanding example of the application of American production engineering ingenuity to this much neglected area of technology. Hi-Tek Products has been appointed the sole agents for this range of jigs and racks.
The Sequel range of aluminium racks is based on the modular principle. In consequence, the system has a high degree of inter-changeability and low cost due to tooling costs being written off against the high volume demand of the US market. The system consists of three basic elements -- three types of standard spines (stems), a wide range of clips designed to solve any jigging problem, a range of box and umbrella racks, and various clip and twist racks.
The spines available are a plain flat spine, a slotted spine, and a spine with holes drilled at 1in intervals (fig 1). These spines are made from 6063-T5 (HE9P) for maximum current carrying capacity and each type can a be supplied with a flat end, with a twist in the spine and a hook end, or a bent hook end only. All other jig and rack parts are fabricated from the fully heat treated strong alloy 6061-T6. The strength and hardness of this alloy has been found in practice to significantly extend the life of the various types of jigs available in this range and represents a major advance in this respect. For special applications, a number of jigs in the range are also available in the high strength fully heat treated 2024-T6 alloy.
The range available includes a wide variety of clips, of which a small selection is shown in fig 2. These can be bolted on to the drilled hole spines, thus eliminating the need to hold a wide range of jigs, each having a different clip design. The Sequel system enables a stock of different clips to be held which can be assembled on spines. After completion of work requiring one type of spine, the clips can be returned to stock and replaced with a new set of clips.
In addition, there is a wide range of box racks available, of which that shown in fig 3 is an example. A variety of lengths and widths of fingers are offered, with the advantage of readily assembled racks and two, three, or four contact points. The cross-bar and fingers are available ready assembled if required. The notch shapes include an inward and outward pointing vee, rounded notch, and slotted notch.
An ingenious adaption of the adjustable box rack principle is the high density box rack (fig 4). The distance between each finger plate assembly can be adjusted by tightening the bolts at the required spacing. The aluminium centre spline has a cross-section of 0.25in x 1in with a consequent current carrying capacity of 250 amps. It is ideal for carrying a large number of blanked out parts of small to medium surface area.
The principle of self assembled, interchangeable racks has also been used to develop a range of umbrella racks in strong aluminium alloy in which much thought has been given to the detailed design. Sequel Corp offers no less than six different designs of discs for these racks (fig 5). The vee notch discs are conventional for this type of rack. The pointed discs are intended primarily for contacts on short lengths of tubes or with medium diameter holes. Slotted flat or grooved discs provide an ideal way of racking small circle blanks and other flat circular parts. The dimpled hole discs are useful for components with small diameter rounded ends or flat tags. By adding an inner notched disc, three or four point contact can be obtained.
In addition, there is a number of notched formed jigs available (fig 6) for applications where the other types of rack are unsuitable. Overall, the range of types available is extensive.
Vulcanium range of titanium jigs
Many of the designs sold by some manufacturers consist of wire forms secured in a pure aluminium spline. This is an attempt to offset the higher initial cost of titanium. Alternatively, combinations of titanium bar and wire are used. However, the drawbacks of such designs are the low carrying capacity of titanium wire-form contacts and the tendency of drawn wire to become brittle and fracture. Even with fingers blanked out from sheet, which last longer, the fact is ignored that the ductility of titanium across the grain is much less than with the grain.
Hi-Tek Products are UK agents for a range of titanium jigs and racks which are manufactured by Vulcanium Corp USA. Typical of the attention to detail and quality is the fact that the material used is tested to ensure that it will withstand 20,000 openings and closings without fracture. Vulcanium have carefully evaluated titanium, even to the extent of measuring the temperature rise at contacts in the electroylyte so as to establish the limits to current carrying capacity. As a result, a number of specially designed clips have been developed.
A practical guideline for titanium is that it will not readily carry more than 8 amps per contact where there is point contact between the work and the contact clip, or 15 amps per contact where there is line contact. Wire clip contacts invariably result in point minimal contact and a limited life. Contact fingers fabricated from sheet last longer and would be expected to give line contact, but in practice usually behave as points contacts. These clips can either be supplied separately for bolting on to a spine, or welded permanently to it
A range of standard clip shapes, totalling 30 in all, are available as standard products. A representative selection is shown in fig 7. It will be noted that on four of these provision has been made for four-pont contact, whilst ensuring minimal contact marks. There is also available a wide range of finger racks of various sizes and shapes. In addition to the traditional umbrella racks, the Vulcanium range includes more than 50 different types and sizes of box rack, all of which are adjustable in terms of spacing between rows. An example of a box rack is the piano rack (fig 8). An important feature of the design is that most of these are fabricated from sheet and blanked out to ensure that the fingers and clips have flex across the rolling direction of ensuring maximum life.
The white spot problem
One of the most frequent complaints relating to titanium racks is that they appear to promote white spots on dyed work, particularly after black dyeing. The white spots are due to local attack of the anodic coating, usually in the dyebath (but it can happen in sealing). This feature can be erratic in its incidence, but always requires the presence of chloride in the dyebath.
Most dyes contain chloride, whose concentration increases as new dye is added to make up for dye used in colouring work, thereby increasing the probability of the problem arising. Since the stainless steel dyetank provides a galvanic potential of about 1 volt when titanium is coupled with aluminium, pitting can result from the chloride present. However, it usually requires contact between a wet tank rim and an aluminium (or copper) work bar for white spots to appear if aluminium racks are used. Stray currents can also cause this defect to occur erratically.
The problem is always more acute when titanium racks are used because of the greater potential difference which exists between titanium and aluminium. The use of sacrificial magnesium anodes attached to the titanium has been proposed which is not a practical solution. Vulcanium Corp has developed a small unit which protects anodized parts from this galvanic attack by applying a small cathodic potential at a very low current density. Known as Pitstop II this equipment has been successfully used in plants in the USA to overcome the white spot problem in dyeing when using titanium racks. The problem rarely occurs when aluminium racks are used.
PHOTO : Figure 1 (above): the three standard aluminium alloy spines used for 'Sequel' system.
PHOTO : Figure 2 (below): a selection of aluminium alloy 6061-T6 clips used in the 'Sequel' system.
PHOTO : Figure 3: an example of 6061-T6 box rack with provision for three-point contact.
PHOTO : Figure 4: high density box rack in 6061-T6 for easy racking of small parts.
PHOTO : Figure 5: examples of 6061-T6 disc designs for umbrella racks.
PHOTO : Figure 6 (above): a few examples of notched formed 6061-T6 discs for umbrella jigs.
PHOTO : Figure 7 (below): some examples of 'Vulcanium' clip designs in titanium.
PHOTO : Figure 8: a piano rack design in titanium from Vulcanium Corp fabricated from strip.
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| Author: | Brace, Arthur W. |
|---|---|
| Publication: | Finishing |
| Date: | Oct 1, 1991 |
| Words: | 2507 |
| Previous Article: | 'Ioning out' a short life for tooling. |
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