Electroforming mold are special processing methods that use principle of electrolytic deposition of metals to accurately replicate workpieces of certain complex or special shapes. It is a special application for electroplating. Electroforming was invented by Russian scholar Б.С. Jacobi in 1837.

Originally used mainly for reproduction of metal artwork and printing plates, it was used to make record stampers at the end of 19th century, and scope of application has gradually expanded.

 

Original mold prepared in desired shape is used as a cathode, electroforming material is used as an anode. Then they are placed in same metal salt solution as anode material, and are supplied with direct current. Under action of electrolysis, surface of original mold is gradually deposited with a metal electroformed layer, which is taken out from solution after reaching required thickness, and electroformed layer is separated from original mold to obtain a metal replica corresponding to shape of original mold.

Electroformed metals usually have three kinds of metals: copper, nickel and iron, and sometimes alloys such as gold, silver, platinum-nickel-cobalt, cobalt-tungsten, but nickel is the most widely used electroforming. Thickness of electroformed layer is generally 0.02 to 6 mm, and is also as thick as 25 mm. Dimensional error between electroformed part and original mold is only a few microns.

Main purpose of electroforming is to accurately replicate fine-grained, complex and special-shaped workpieces that are difficult to process by other methods, such as printing plates for banknotes and stamps, record stamps, typefaces, metal artwork, mirrors, surfaces roughness samples, microporous filters, dials, electrodes for EDM, high-precision diamond grinding wheel bases, etc.

Material of original mold is gypsum, wax, plastic, low melting point alloy, stainless steel and aluminum. Original mold is generally produced by casting, cutting or engraving. For fine mesh or complex patterns, photolithography can be used. Original mold of non-metallic material shall be subjected to a conductive treatment by applying a conductive powder, an electroless plating film, and a vacuum coating film.
For original mold of metal material, an oxide film or graphite powder is first formed on the surface to facilitate stripping of electroformed layer.

Electroforming equipment consists of electroforming tank, DC power supply (generally 12 volts, several hundred to several thousand amps), device of constant temperature, agitation, circulation, and filtration of electroforming solution. Electroforming solution uses an aqueous solution of a sulfate, a sulfamate, a fluoroborate, and a chloride containing an electroformed metal ion. Main disadvantage of electroforming is low efficiency. Generally, thickness of electroformed metal layer per hour is 0.02 to 0.05 mm. By using a high concentration electroforming solution, and appropriately increasing solution temperature and strengthening agitation, current density can be increased, electroforming time can be shortened, and electroforming efficiency can be improved. This method has found application in nickel electroforming.

 

In 21st century, with informatization and high consumerization of social structure, engineering plastic replicas are flooding. Electroforming technology has been widely used as a replication technology in manufacture of metal film and casting molds. At the same time, as manufacturing technology of engineering plastic molding molds, it has an important position.
21st century is a century of symbiosis and sustainable development with environment. Saving resources and energy is goal pursued by society. Electroforming technology is an effective technology for resource reuse and sustainable development, making it possible to mass produce products by replicating individual products.

Electroforming began in 1841 with electroforming of paraffin or gypsum in Russia to produce a handicraft copper electroforming mold. However, it is widely used in industry from around 1950 to 1960. Early (19th century) electroforming was dominated by copper electroforming, which is limited to reproduction of printed tire plates and production of crafts such as statues and reliefs. In intermediate stage (early 20th century), electroforming of iron gradually replaced copper as mainstream, and application expanded to production of rubber and plastic molds. In later period (second half of 20th century), nickel replaced iron and became mainstream of electroforming. Applications have also expanded to include recording masters, waveguides, casting and injection molds, metal foils, printed circuit boards, arts and crafts.
Copper, nickel, gold, platinum, silver, etc. can be used as materials for electroforming, and have been used in ultra-fine processing such as magnetic information recording, photoelectron, microelectronics, DNA amplification of semiconductors, CDs, and DVDs. Electroforming has been an important processing tool in era of microelectronics technology.

Principle of electroforming: Like electroplating, it is an electrodeposition technique that utilizes electrochemical reactions. By depositing anode in electrolyte containing ions of metal to be obtained, a metal is deposited on parent mold as a cathode to form a metal model capable of replicating prototype, which is principle of electroforming.
The biggest difference between electroforming and electroplating is that electroplated product is a composite of plated part and product. Electroforming is just need for an electrodeposited layer. Coating will eventually be detached from parent.
Quality of electroforming is related to demolding method and metallographic characteristics of electroformed metal. In terms of application, there is a female type of nickel-based optical disc with good ductility, a printed circuit is made of conductive copper, a stamping die is made of iron having good hardness and heat resistance, and a gold wire or decoration is made of a corrosion-resistant and noble metal, a DNA amplification device is produced by sterilization action of silver.

 

Using electroforming technology, it is possible to produce high-precision and meticulous molds that cannot be achieved by other methods, and can replicate high-value and high-complexity shapes. In addition, electroforming can also produce articles that other methods cannot, especially those that are difficult to prototype. In electroforming process shown, when metal deposition layer is peeled off from prototype, it may be disposable or reusable depending on material of prototype. Reusable prototypes can be used multiple times, reducing cost and efficiency of electroforming. However, there is still room for further development in electroforming technology. Level of electroforming technology is closely related to materials science, physical chemistry, advances in electrochemistry and equipment improvement.
Main technical and economic characteristics of electroforming are as follows:
1 Regardless of shape and size of product, it can be made as long as there is an electroforming tank;
2 Physical and chemical properties of electrodeposit can vary over a wide range;
3 Can copy shape of 50~100nm precision;
4 Can produce hollow products and alloy products;
5 Pairs of large-scale reproduction products, compared with other methods in terms of high precision and production costs;
6 Efficiency of production is improved, which is beneficial to environmental protection.

Electroforming process is related to fabrication of prototype, method of obtaining conductive layer (chemical and physics), stripping method using acid and alkali, nature of electrodeposited metal and equipment. Main systems include electrodeposited metals, electrolytes, models, equipment, and so on. Fabrication of system prototype is related to materials used in prototype. Non-conductor electroforming, electroforming of conductors have different electroforming fluids and processes.

Prototype is made of metal, paraffin, gypsum, and resin. Wax processing includes mechanical processing, manual processing, molding, engraving, etc. Metal, resin, and glass have chemical processing methods (etching methods).
Material of prototype is mainly made of metal, resin, glass, etc. Precision and miniaturization of electroformed product can be obtained by optical and chemical methods. This method is mostly used for non-conductor prototypes.
Ground glass plate is coated with a photographic coating. After being photosensitive, a resin pattern is formed, which can be etched to form a prototype. In addition, a plastic pattern is formed on the surface of a metal or glass substrate, and then chemical etching is performed to make a prototype. This photochemically produced prototype is used for electroforming of optical discs (CD, MD, DVD, etc.).
There are also prototypes made by machining methods used to make electroformed mirrors.

 

Where prototype is resin or glass, since prototype is a non-conductor, it is necessary to conduct surface conduction before electroforming. Such electroconductivity includes wet electroless plating and dry vacuum plating, plating layer has a metal film such as silver, copper or nickel. Electroless plating method has a substitution method using a potential difference of different metals and a reduction method using a reducing agent, but most of them use a reducing agent method. Vacuum plating is a method of sputtering a metal to a plated member by impacting a discharge ion to a cathode electrode under vacuum.
After non-conductor surface is electrically conductive, electroforming can be performed according to process flow. If resulting electroforming mold is used as a prototype, an electroforming mold of opposite shape can be electroformed thereon, that is, a male mold is electroformed on female mold, and a female mold is electroformed on male mold.
In the past, electroforming on non-conductors was subjected to electroplating of a first layer of film with low temperature, low concentration, low current density (2 A/dm2). With improvement of power supply and electroforming tanks, as well as electronicization of electroforming process and improvement of surface metallization, it is no longer necessary to carry out electroforming process when first electroplating is performed.

Electroforming process varies depending on metal materials used and electroforming process used. Classification of electroforming is as follows:
Nickel electroforming: nickel sulfamate solution, nickel sulfate solution
Copper electroforming: copper cyanide, copper sulfate plating, copper pyrophosphate plating
Ferroelectric casting: iron sulfamate
Cobalt electroforming: cobalt sulfamate, cobalt sulfate
Gold electroforming: cyanide solution
Silver electroforming: pyrophosphate plating solution, cyanide solution
For electroforming, influence of quality of electroformed metal is crystallization process of electroforming. Ultimately, it is affected by physical and chemical properties of electroforming solution.
Concentration of electroformed metal ions has an effect on maximum current density. Density and viscosity of bath have an effect on efficiency of filtration. Conductivity affects metal ion migration, heat capacity versus bath temperature, ion diffusion coefficient versus limiting current density, electrical conductivity versus current density, surface tension on gas evolution, and pinhole formation. Comprehensive consideration of effects of these factors determines composition of electroforming fluid.

 

Electroforming device is composed of a non-metallic conductor device, a mother-type stripping device, a power source, and an electrolysis tank for electroforming. There are various different devices depending on purpose of electroforming. For example, in order to metallize non-metal surface, electroless plating apparatus required has a sputtering method and a immersion plating apparatus; and power supply also need a switching power supply with high voltage performance; Anode is selected from a spherical shape in consideration of sufficient dissolution. Anode, etc.

Since electroforming requires a stable direct current, a DC power supply device with few AC factors has been used. At the same time, there are new progress in analysis of plating solution, improvement of hangers, and introduction of computing technology. Performance of electroformed metal is further improved. With expansion of field of electroforming applications, gears, golf molds, glasses, etc., which have traditionally been processed by mechanical processing methods, have been manufactured by electroforming. And precision of electroforming has entered era of nanoscale. Microelectroforming techniques are used, for example, in lithographic masters or microelectronics, micros for TEM. This is because electroforming can reproduce model very precisely from prototype and can produce a mirror mold with a roughness of 40 nm.

Although electroforming technology has entered nano age, it is rarely known. Its popularity is very low.
Nano-scale reproduction of electroforming from macroscopic reproduction is based on technology developed based on technology developed 160 years ago. However, great progress has been made in process flow and equipment. In the future, it is necessary to further develop technologies that are free or less from pollution, to develop electroforming technology based on advances in physical chemistry, materials science, electrical engineering, optics, mechanical engineering, economics, and environmental science. Including development of new alloy electroforming technology suitable for electroforming.