Electroforming is the electrochemical deposition of a metal onto an electrically conductive disposable mandrel, typically for coldshields 6061-T6 aluminum, although for volume production of some simple designs it may be possible to use reusable aluminum or stainless steel mandrels. Metal ions are deposited in intimate contact with the mandrel, so the interior of the deposit will be an exact replica of the mandrel’s size, shape, and surface finish. Scratches, pores, and machine marks on the mandrel will be transferred directly to the electroform. Once the desired thickness has been achieved, the mandrel with its deposit is removed from the electroforming bath and secondary machining operations such as trimming the deposit to size and cutting the entrance aperture, are performed. An aqueous solution of sodium hydroxide is used to completely dissolve the aluminum mandrel, leaving the electroformed part intact; alternatively, the deposit is simply pulled off a reusable aluminum or stainless steel mandrel, likewise leaving a free standing electroformed part.
For commercial electroforming, material selection is limited to those metals that can be deposited electrochemically from aqueous solutions; a further restriction is that the deposit must be relatively free of internal stresses. Because of these limitations the most commonly used metals are nickel and copper, although co-deposits of nickel and cobalt, or rarely, nickel and iron can also be used. The process is also suitable for the manufacture of laminates such as nickel-copper-nickel, or other combinations of metals used for electroforming.
Although nickel and copper are deposited at almost 99.9 percent purity this does not mean that the mechanical properties of the electroform are identical to pure cast or wrought metals of similar composition. To the contrary, the chemistry of the bath can be altered with various components to allow for a wide range of mechanical properties; approximate values for mechanical properties of electroformed metals are tabulated below.
The strength of a coldshield is much more dependent upon its shape than the properties of the material from which it is made, so even low strength deposits are adequate for coldshields. Therefore, mechanical properties are not normally specified for coldshields.
Because the electroforming process is subject to the laws of electrochemistry, especially Faraday's Law, it is possible to control the amount of metal deposited, thereby controlling the thickness of the deposit, simply by measuring both the current and the time the current flows. Like all real world processes this is not exactly the case. In fact, current density (amperes per square decimeter, or per square foot, for example) controls the deposition rate, so if the shape of the mandrel affects current density, the thickness of the deposit will vary accordingly; this is discussed in greater detail in the section on design.
Since current density is a significant factor in manufacturing electroformed parts, a brief discussion of the physical aspects of current density is appropriate before continuing. Consider the case of depositing metal onto a cylindrical mandrel with a single anode positioned alongside the cylinder, where rotation of the cylinder allows uniform deposition around the circumference of the cylinder. Well, not quite. As long as we are considering the area away from either end of the cylinder this is true, but as we approach either end, the edge effects become important. The corner formed by the flat end and the curved surface of the cylinder is a localized region of higher current density. Think in terms of imaginary small square areas on the mandrel; near the center of the cylindrical surface, any small square is surrounded by similar squares, but at the corner the sharp edge represents a discontinuity. A sharp edge with zero radius creates a localized current density that is infinite. As a result of this change in current density there is a change in the deposition rate so the corner will be thicker than the side of the cylinder. Although a corner is an extreme example, other geometric features of the mandrel have a similar effect on the thickness of the deposit.
Repeating the analysis for a frustum of a cone illustrates a less drastic change in thickness. Consider two narrow bands on the conical surface, one with a larger radius than the other, but each with the same width (or height, depending on your perspective). A band with a larger circumference will have a larger area than one with a smaller circumference; accordingly, the current density will be lower at the larger radius, resulting in a thinner deposit than in the band of smaller radius.
Because the level of stress in electrodeposited metal is affected by current density (and several other parameters), bath chemistry is adjusted to provide essentially zero stress for a particular current density, at certain operating conditions such as bath temperature. Just as thickness varies with current density so does the stress. Part of the art of electroforming is knowing how to adjust the bath chemistry to keep the stress within acceptable limits. To design electroformed parts it is not necessary to understand how stresses are controlled during plating, but the designer must be aware that part geometry can influence stress levels in the electroform. Stress is significant because it can cause deformation of the electroformed part which is not usually a problem for smaller coldshields but larger components may require looser dimensional tolerances unless the effect stress is taken into account when designing the coldshield.
The choice of mandrel material is usually dictated by the geometry of the part, but economics also plays a role, especially when few parts are required, since it is usually cheaper to machine aluminum than stainless steel. For this discussion we will assume that permanent mandrels are made of stainless steel, but it is possible and occasionally desirable to use aluminum for reusable mandrels.
Assuming economics will allow the use of a permanent mandrel, the primary design limitation for permanent mandrels is whether or not the electroform can be removed from the mandrel; that is, no reentrant angles (undercuts) may be used in the design. The most common shapes that employ permanent mandrels are surfaces of revolution, such as cylinders or frusta of cones. More complex parts, for example, the classic Coke® bottle shape, cannot be removed from a permanent mandrel. Almost any similarly complex shapes can be electroformed, but the mandrel must be removed by dissolution. Likewise, electroformed coldshields with integral baffles must be made from disposable tooling.
The key to good electroform design is in visualizing the coldshield-mandrel interface; fortunately, the optical designer concentrates on the interior of the coldshield. The relationship between the coldshield and mandrel is too important to be ignored, for failure to do so may result in a design that is simply impossible to manufacture.
The coldshield can be almost any shape if the mandrel is removed by dissolution after plating. However, to be cost effective, some simple rules should be followed. Good optical design is sometimes contrary to good electroforming practice so it is imperative that the optical designer consult the electroformer early enough in the design process to avoid creating a part that is expensive (or impossible) to manufacture.
1. Coldshield shapes that are surfaces of revolution about the optical axis are least expensive to manufacture because the mandrel can be machined on a CNC lathe or automatic screw machine. Non-circular shapes require machining on slower CNC machining centers or turning centers. The worst case for economy is a coldshield with a circular cross-section, a square or rectangular base, and an optical axis that is not coincident with the mechanical axis.
2. The plating thickness at an outside corner is approximately double the nominal plating thickness, while inside corners are typically half the nominal thickness. This is a result of differential plating rates caused by current density variation, and is localized. Avoid sharp corners on the mandrel to minimize, but not eliminate, this effect. Generous corner radii usually facilitate plating and improve yields resulting in lower manufacturing cost. Examples of deposit thickness variation due to mandrel geometry are shown in the following illustration:
(Drawing is not to scale)
3. Most coldshields are designed with .05 to .10 mm (.002 to .004 inch) nominal wall thickness (plating thickness). The part will be sufficiently rigid as long as it is under approximately 25 mm (1 inch) in length and diameter. The minimum practical wall thickness is approximately .025 mm (0.001 inch), but most shapes require greater minimum plating thickness; a wedding cake shape has inside corners that will be so thin that they behave like hinges. Minimum thickness is dictated by the requirement for rigidity and is not a limitation in the electroforming process since plating thickness begins at zero and increases as the current continues to flow.
4. Mandrel dimensional tolerances can be held to normal machining tolerances, typically ±.005 mm (0.0002 inch), with the usual trade-off of cost versus tolerance.
5. Keep secondary machining operations to a minimum by designing important interface features into the coldshield. For example: employ a flat mounting surface such as a flange with a skirt at the base of the coldshield; the flange can be mated to the coldplate, with the skirt extending below it. The mandrel feature for the flat can be machined to close tolerances, but the skirt height can have a generous tolerance that permits plating it to net size, so no secondary machining of the skirt is required.
6. Broad flat surfaces tend to warp slightly but enough to affect the vertical location of the entrance aperture of a large diameter coldshield. In this case, use a long radius spherical surface surrounding a narrow flat zone slightly larger than the aperture. A conical section with a very large, almost flat, cone angle would serve the same purpose.
7. The plating solution temperature is in the range of 40°C to 60°C (104ºF to 140ºF). Therefore, the aluminum mandrel will be slightly larger during the plating operation than when it was machined. This extremely small change in size is inconsequential for most parts but for very large ones with tight tolerances it must not be ignored; rarely will this be a factor for coldshield design.
8. The coldshield detail drawing should specify the dimensions of all important features to the mandrel side of the part, i. e., the inside of the coldshield. A tooling drawing should accompany the part drawing, since most mandrels will be CNC machined, and a tooling drawing in *.dxf format can be converted directly to machine instructions, thus eliminating transcription errors. Dimension the minimum wall thickness, and include a note stating the maximum mass (weight) requirement for the coldshield. Specifying minimum plating thickness assures structural strength, while a maximum weight keeps the mass within acceptable limits.
Two surface coatings are commonly used on coldshields. Gold for high reflectivity on the exterior, and copper oxide for good absorption on the interior.
The electroforming process provides a convenient means of putting these coatings only on the surfaces where they are required. After secondary machining operations, but while the coldshield is still on the mandrel, the outside can be plated with gold. No gold will be deposited on the inner surfaces because these surfaces are still in intimate contact with the mandrel. Likewise, blackening will be applied only to the inside surfaces because that is done after gold plating and dissolution of the mandrel. The gold masks all exterior surfaces so only the interior is blackened. An understanding of the sequence of operations will help the coldshield designer to create a part that is efficiently manufactured.
After the final manufacturing steps, always handle the coldshield with gloves or finger cots to avoid contaminating the surface with fingerprints. A vacuum bakeout (by the customer) is usually required to remove remnants of the plating solutions.