electroformed_parts.pdf

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25. ELECTROFORMED PARTS

25.1. ELECTROFORMING PROCESS

Electroforming is the production of articles by electrodeposition. It is similar

to electroplating, except that the latter involves only a resurfacing of an

existing article, whereas electroforming involves the fabricating of a new

object which did not previously exist. It differs in the extra process time, the

greater thickness of the coating, and the extra controls and process steps

required to ensure that it is uniform, smooth, stress-free. There are three

steps in the process: (1) Prepare a mandrel of the appropriate size, shape,

and finish; (2) electroplate it to the thickness required (much thicker than in

electroplating for surface coating); and (3) separate the mandrel from the

electroplated material. (Sometimes this is not done, or only part of the

mandrel is removed.)

25.2. TYPICAL CHARACTERISTICS AND APPLICATIONS OF

ELECTROFORMED PARTS

Electroforming is versatile. Parts fabricated by electroforming can be simple

or complex and can have walls as thin as 0.025 mm (0.001 in) and as thick as

13 mm (1/2 in) or more. They can range from a very small size, e.g., 10 mm

(3/8 in) in length, to waveguides that weigh up to 220 to 270 kg (500 to 600 lb)

and are 1.5 to 1.8 m (5 to 6 ft) in length.

An outstanding characteristic of electroforms is their near-perfect

reproduction of surface details. This property accounts for a long-standing

application of electroforming: the production of compact audio and video disc

ELECTROFORMED PARTS
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stamper molds. High accuracy of shapes and dimensions is another property

of electroforms.

Holes as small as 0.013 mm (0.0005 in) can be made by attaching

nonconductive filaments to the electroforming mandrel. Larger holes of

various shapes can be made by application of nonconductive materials to the

mandrel. Bosses can be incorporated,

and if collapsible, dissolvable, or flexible mandrels are used, undercuts,

reentrant angles, and reverse tapers are possible. Dissimilar metals can be

laminated and cold-welded, and components such as plates, pins, tubes, etc.,

can be incorporated as inserts in the electroform.

Major applications of electroformed parts are found in the aerospace,

electronics, and electrooptics industries. Common electroformed parts

include duplicating plates, molds, paint masks, surface-finish standards,

waveguides, reflectors, rocket thrust chambers, venturi nozzles, missile nose

cones, medical and prosthetic devices, and holographic masters.

Designers should be aware that most electroformed parts have some degree

of stress that may cause distortion of the part after it is removed from the

mandrel. Special attention to the chemical and electrolytic aspects of the

process can minimize this effect.

Figures 3.14.1 through 3.14.4 illustrate typical parts.

Figure 3.14.1. Large, heavy-walled (1/4-in nickel thickness) waveguide

with grow-on stainless-steel flanges. (Photograph used with

permission of GAR Electroforming Division, Mite Corp., Danbury,

Conn.)
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Figure 3.14.2. Miniature precision electroformed components.

(Photograph used with permission of GAR Electroforming Division,

Mite Corp., Danbury, Conn.)

25.3. ECONOMICS OF ELECTROFORMING

Electroforming is applicable to production levels ranging from one of a kind

to mass production. In many cases in which prototypes or short runs are

involved, mandrels can be provided with short lead times. The production

time in such cases, however, may not be short, particularly if large, thick-

walled parts are to be produced. Several hours or even several days may be

required for the formation of heavy-walled electroforms.

However, the process is employed for high production levels when multiple

mandrels are used. In such cases, one operator can tend as many as 500

parts in process at one time. Unit labor costs, therefore, are low. In other

cases in which fewer multiple mandrels are used, labor costs are more

substantial.


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Figure 3.14.3. Large electroformed nickel vacuum manifold jacket for

a space shuttle. (Photograph used with permission of GAR

Electroforming Division, Mite Corp., Danbury, Conn.)

Figure 3.14.4. Electroformed microfinish comparator blocks.

(Photograph used with permission of GAR Electroforming Division,Mite Corp., Danbury, Conn.)

Electroform production involves skilled labor. At one electroforming company,

it takes an average of 7 years apprenticeship to train an electroforming

technician: 5 years as an apprentice machinist-tool-and-die maker and 2

years electrochemical training.

Little or no material wastage occurs in electroforming. However, the most

commonly used materials are somewhat costly compared with metals

generally used in other forming and fabricating operations.

Tooling costs also may be high if the workpiece is complex or large. Tooling

costs for electroformed components can run from practically negligible

amounts for simple parts to tens of thousands of dollars for steel molds to

cast complex mandrels for aerospace projects. The small components

illustrated in Fig. 3.14.2 can be produced on mandrels made by lathes or

screw machines.

The total effect of these factors is a relatively high cost level for
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electroformed parts. However, when the special attributes of electroforms

precise surface reproduction, accurate dimensions, and complex seamless

shapesare involved, no other method of fabrication is as satisfactory, and

the cost of electroforming becomes a favorable factor.

25.4. SUITABLE MATERIALS

Basically, the prospective customer is limited to two materials for fabricating,

copper and nickel, with possible alternatives of gold and silver. There are

others, such as iron, lead, and cobalt, but for practical consideration there

are only two, copper and nickel, which can be used very successfully in most

applications.

Copper, the most frequently used electroforming material, is inexpensive,

plates easily, and has low residual stresses after plating. Its ultimate tensile

strength is 200 to 325 MPa (30,000 to 55,000 lbf/in ). Nickel has high tensile

strength, 350 to 650 MPa (60,000 to 110,000 lbf/in ), and is the most widely

used material when structural strength is important. It is also used when

corrosion resistance is required. Iron is low in cost but tends to be brittle and

highly stressed when electrodeposited and is subject to corrosion.

25.5. DESIGN RECOMMENDATIONS

One important design parameter of electroforming that must be considered

is inside-corner weakness, as shown in Fig. 3.14.5. Inside corners should be

well rounded to ensure an even deposit of metal. (If sharp inside corners are

unavoidable, they can be made appreciably stronger by the intermediate

added soldering operation shown in Fig. 3.14.6.)

Outside sharp corners cause a buildup of deposited metal, as shown in Fig.

3.14.7. When this is undesirable, outside corners should be well rounded.

Although complex shapes are well within the capabilities of electroforming,

economy is served by minimizing the number and depths of holes and

grooves and by keeping bosses low and wide. Otherwise, mandrels are more

complicated, and shaped anodes may be required for uniform metalthickness.

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Figure 3.14.5. Avoid sharp internal corners on electroformed parts.

Figure 3.14.6. Using solder to augment metal coverage in a sharp

corner. (From Douglas C. Greenwood, Engineering Data for Product

Design, McGraw-Hill, New York, 1961.)

Undercuts, reverse tapers, and reentrant angles especially should be

avoided, if possible, because they necessitate a flexible or dissolvable

mandrel that is less accurate and more costly. Included angles should be as

wide as possible to promote uniform metal coverage. Holes and grooves

should be as wide as possible, at least 1 1/2 times their depth. (See Fig.

3.14.8.)


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Figure 3.14.9 shows the normal thickness distribution of electroformed

nickel, copper, and silver in angles, grooves, and holes. Hole depth and

diameter affect the ratio. The depth of the angle and the radius at the apex

of the angle also affect the ratio. The values shown are approximate and

suitable for design purposes only. The location of the anode in the

electroforming tank and the precise configuration of the electroform itself

may affect the values shown.

Wall thickness should be designed to be essentially uniform. Although it can

be controlled by masking, anode shaping, and anode placement, precise

control is difficult. Sudden or large-magnitude wall-thickness changes should

particularly be avoided.

Electroforms made on permanent metal mandrels should have some internal

draft or taper to facilitate mandrel removal after forming. Taper of 0.1

percent (0.001 in/in) is usually adequate.

Figure 3.14.7. Metal builds up at outside corners. Round such

corners as much as possible.


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Figure 3.14.8. Narrow angled surfaces and narrow, deep grooves lack

an adequate metal deposit and should be avoided.

25.6. TOLERANCES

Tolerances that can be held on electroformed parts depend primarily on theaccuracy of the mandrel. This in turn depends on the skill of the toolmaker

and the capabilities of the equipment and tools. If the mandrel is machined

accurately enough, a tolerance as close as 0.0025 mm (0.0001 in) can be

held. However, such a tolerance would be expensive. A more realistic

allowance for normal everyday production with metal mandrels would be

0.025 mm (0.001 in).

Surface finish tolerances also depend on the finish of the mandrel, and values

as low as 0.05 m (2 in) have been held. Recommended tolerances for

production conditions are shown in Table 3.14.1.

Figure 3.14.9. These diagrams show the distribution of metal deposit

in holes, angles, and grooves. t/T is the ratio of metal thickness in

deep areas to that in prominent, exposed areas. R/L is the ratio ofbottom radius to the groove or hole depth. (From Douglas C.

Greenwood, Engineering Data for Product Design, McGraw-Hill, New

York, 1961.)


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Table 3.14.1. Surface-Finish Tolerance Recommendations for

Production Conditions of Electroformed Parts

Wall-thickness variations within a part can be substantial if the part shape is

complex, as indicated by Fig. 3.14.9. However, repeatability of wall thickness

from part to part at corresponding points can be maintained within 0.025

mm (0.001 in).

Nominal surface roughness Tolerance as a percentage of rated

value

Source:ASA B-46.1 1962, Surface Texture Standard, American Society of Mechanical

Engineers, New York.

0.050.10 m (24 in) +25, 35

0.2 m (8 in) +25, 30

0.4 m (16 in) + 15, 25

0.8 m (32 in) and above + 15, 20

Citation

James G.Bralla: Design for Manufacturability Handbook, Second Edition.

ELECTROFORMED PARTS, Chapter (McGraw-Hill Professional, 1999, 1986),

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