Photochemical etching, also known as photochemical machining or metal etching, is a non-traditional, subtractive machining process in which photographic and chemical techniques are used to shape the metal workpiece...
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This article takes an in depth look at metal etching.
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Metal etching is a metal removal process that uses various methods to configure complex, intricate, and highly accurate components and shapes. Its flexibility allows for instantaneous changes during processing. Since there is a limited amount of force or heat during the process, the properties of the etched material remain unaltered, which leaves workpieces free of stress or imperfections.
Etching is an ideal process for producing components with thicknesses of 0.0005 in to 0.05 in or 0.00127 cm to 1.27 cm, which makes it a highly viable method for manufacturing small parts at exceptionally low cost.
The process of metal etching is the removal of excess material from a workpiece using a chemical reaction. It is a method for shaping metals that has been in existence for hundreds of years. Metal etching began as a method for making weapons, household tools, and jewelry. The modern use of etching has paved the way for the creation of precision parts for aircraft, automobiles, and satellites.
Unlike other metal working processes, metal etching is fast, exceptionally accurate, and highly reliable. The tools used for etching have minimal wear and tear due to the low mechanical stress of the process.
The different types of etching include acid, photochemical, laser, and electrochemical. Acid metal etching is done in an acid bath, dipping, or flow coating. Photochemical etching uses light and chemicals to remove material. Laser etching melts the surface of the workpiece. Electrochemical etching uses a sodium based solution with electrical pulses to remove material from the substrate.
The type of metal to be etched influences the acid etching process since some metals etch quicker than others. Nickel and steels take far longer to etch than softer metals such as bronze or copper.
Any metal can be etched with attractive metals being the most popular due to their characteristics and properties. Common metals used in etching are:
For a metal to be acid etched, it must have any contaminants, particles, oil, or chemicals removed from the surface. Solvents, deoxidizing agents, and alkaline solutions are commonly used. Cleaning is a necessary step in the preparation process to ensure that the surface is properly prepared.
The maskant, or masking agent, is applied to the surface of the workpiece. It contains inert substances such as isobutylene isoprene copolymers and neoprene elastomers, which do not lose their structure when exposed to a chemical reaction.
The maskant can be applied by dipping or coating where dipping includes immersing the workpiece and coating flows the maskant over the surface. The success of the application of the maskant is dependent on how well the workpiece has been cleaned.
The application of the image can be completed in several ways. In one process, the pattern for the component is carved into the maskant coating. Some applications use the maskant agent as the method for shaping the image. In either case, the image is placed on the metal sheet.
When the image is carved into the maskant, the image is recessed. When the maskant agent is used to shape the image, the effect is the opposite, and a raised image is produced.
Once the image is emblazoned on the maskant, the etchant or acid is applied. The methods for applying the acid can be by spraying it onto the workpiece or by immersing the workpiece in an acid bath. The acid or chemical that is normally used is ferric chloride, a form of corrosive material. The amount of time that the workpiece is exposed to the etchant is dependent on the type of metal and the intricacies of the piece being produced.
As can be seen in the image below, the image has been carved into the maskant and the workpiece is being lowered into the acid bath where the exposed portions of the metal will be removed.
In the removal process, the maskant and etchant are washed off the workpiece to reveal the final part. There are a variety of solutions and solvents that are used such that the workpiece is not damaged, but the materials are removed. In some cases, depending on the resilience of the workpiece, the maskant can be scraped off.
A common method for removing the etchant is to bath the workpiece in water. As a secondary measure, a deoxidizing bath may be required to remove oxides from the surface. Though deoxidizing may not be necessary, it is a process that should be used with all acid etchings.
The process of photochemical metal etching begins with the creation of a design using CAD. Though the CAD design is the first step in the process, it is not the end of computer calculations. Once the rendering is finished, the thickness of the metal is determined as well as the number of pieces that will fit on a sheet, a necessary factor for lowering the cost of production. A second aspect of the thickness of the sheet is a determination of part tolerances, which hinge on the part dimensions.
As with acid etching, the metal has to be thoroughly cleaned prior to being processed. Each piece of metal is scrubbed, cleaned, and cleansed using water pressure and a mild solvent. The process eliminates oil, contaminants, and tiny particles. This is necessary to provide a smooth clean surface for the application of the photoresist film to securely adhere.
Lamination is the application of the photoresist film. The metal sheets are moved between rollers that coat and evenly apply the lamination. To avoid any undue exposure of the sheets, the process is completed in a room lit with yellow lights to prevent UV light exposure. Proper alignment of the sheets is provided by holes punched in the edges of the sheets. Bubbles in the laminated coating are prevented by vacuum sealing the sheets, which flattens the layers of laminate.
During photoresist processing, the images from the CAD rendering are placed on the layer of photoresist on the metal sheet. The CAD rendering is imprinted on both sides of the metal sheet by sandwiching them over and under the metal. Once the metal sheets have the images applied, they are exposed to UV light that places the images permanently. Where the UV light shines through the clear areas of the laminate, the photoresist becomes firm and hardens. Black areas of the laminate remain soft and uninfluenced by the UV light.
From photoresist processing, the sheets move to the developing machine that applies an acid solution that washes away the soft photoresist film leaving the parts to be etched exposed. The process removes the soft resist and leaves the hardened resist, which is the part to be etched. In the image below, the hardened areas are in blue, and the soft areas are grey. The areas not protected by the hardened laminate are exposed metal that will be removed during etching.
Much like the acid etching process, the developed sheets are placed on a conveyor that moves the sheets through a machine that pours etchant on the sheets. Where the etchant connects with the exposed metal, it dissolves the metal leaving the protected material.
In most photochemical processes, the etchant is ferric chloride, which is sprayed from the bottom and top of the conveyor. Ferric chloride is chosen as an etchant because it is safe to use and recyclable. Cupric chloride is used to etch copper and its alloys.
The etching process has to be carefully timed and is controlled in accordance with the metal that is being etched since some metals take longer to etch than others. For the success of photochemical etching, careful monitoring and control are crucial.
During the stripping process, a resist stripper is applied to the pieces to remove any remaining resist film. Once stripping is completed, the finished part is left, which can be seen in the image below.
Unlike photochemical etching and acid etching, laser etching is a process that places marks, shapes, and images on the surface of parts and products. The process requires a great deal of energy and melts the substrate of the workpiece. A high amount of energy is focused on a small area of the workpiece where it melts and expands the surface metal.
The beam of the laser is pulsed and releases bursts of energy at controlled and monitored intervals. In a second, a 100 W laser releases 100,000 pulses with each pulse containing one millijoule (mJ) of energy that can reach a peak of 10,000 W.
As the beam strikes the surface of the metal, it absorbs the energy and converts it to heat. The amount of energy that is absorbed must only be enough to melt the micro surface, about 0.001 in or 0.00251 cm, and cause it to expand. In the laser etching process, very small amounts of the surface are affected, and a textured impression is left.
The two types of laser etching machines are flatbed or plotters and galvo. The difference between the two methods is the way that the laser is applied.
With a flatbed laser, the laser beam is directed by a mirror that is parallel to the X Y axis of the flatbed to a lens where it is focused. The X and Y move the laser beam and position it correctly. The only limitation to the process is the size of the machine.
The galvo process also uses mirror technology to direct the laser beam. The difference in the processes is how the galvo process directs the lazer, which is through the use of high speed oscillating mirrors that direct the beam in several directions by rotating and adjusting the mirror angles. It is an ideal process for fast speed marking of fine intricate details. Galvo laser beams can mark any type of geometries at speeds of several feet per second.
Prior to the development of laser etching, most etching was done by acid etching. Laser etching replaced acid etching because of its cost effectiveness, the use of only a laser etching machine, and the lack of waste. The list below contains the common metals used for laser etching with their melting points.
The difference between lasers is determined by the type of mediums they use to produce the beam. The various mediums have different wavelength emissions and absorption bands with industrial waves being continuous or pulsing.
Solid state lasers use glass or crystal material as its medium.
A gas laser has an electric current that is discharged by a gas medium.
Liquid lasers use a liquid as the medium, which is mixed with a dye and solvent.
Semiconductor lasers are known as diode lasers and use electric energy.
A metal vapor laser is an ion laser that vaporizes metals.
An excimer laser is an ultraviolet laser that produces powerful pulses.
Electrochemical metal etching is able to etch into any type of material that conducts electricity. It does not require heat, does not change the microstructure of the metal, and does an extraordinary job of bringing out details in the workpiece. Electrochemical metal etching is an inexpensive and quick etching process.
As with photochemical etching, the process of electrochemical etching begins with the creation of a CAD design. Once the design meets the necessary parameters, it is printed on a transparency, which is used to create the stencil of the image to be etched.
The stencil is made of a photosensitive material where the image from the transparency will be exposed and imprinted. Electrochemical stencils are made of highly durable and long lasting materials that can be used for manual operations or electrochemical metal etching production machines. They deliver sharp clear images over and over again for hundreds of impressions.
The transparency is exposed to the stencil and imprinted on it. The stencil is washed out with a developing solution to expose the markings that have been imprinted.
The etching process includes the use of sodium based solutions that are combined with low voltage electrical pulses. The current of the electricity dissolves the metal and extracts it into a cloth pad known as the monopod. As the material is being removed, an oxide forms in the etched area, which gives the etching high contrast, a sharp image for easy detection, and prevents corrosion.
The shallow etching process of electrochemical metal etching happens very fast, in less than a second. The resulting image is dark. The darkness of the image depends on the type of metal being etched.
Deep etching, using electrochemical metal etching, uses direct current (DC) to remove the metal ions. The deep etching process can be used to create parts and components with etching depths being in the range of 0.001 in to 0.003 in or 0.00254 cm to 0.00762 cm or deeper. The power for deep etching is pulsating and may need to be repeated to reach the desired depth.
The process of metal etching is used to shape and form products with intricate and complex designs. It is a method commonly utilized by several industries especially in the areas of computer components and electronics. The versatility of metal etching makes it ideal for products for the home as well as ones for defense and military weapons.
Metal etching is used to produce a wide assortment of parts without damaging the structure or tolerance of the metals being shaped, which is a shortcoming of other metal forming methods. Metals retain all of their characteristics and properties regardless of the piece being produced.
A necessary property of metal covers and lids is that they lay flat such that they can fit tightly and precisely. Since etching does not stress or deform metals, it is the ideal choice for the production of covers and lids, which are made to the required dimensional tolerances.
Electrical connectors are used in a wide variety of environments and must be made of materials capable of enduring and withstanding the conditions. The etching process produces electrical products that are stress and burr free using exceptionally high performance metals that have excellent strength to weight ratios. Connectors can be produced with an accuracy of ±0.025 mm or 0.001 in.
Metal etching is widely used in the medical field for making a variety of prosthetics for implantation and the production of surgical tools. Extremely tiny metal screens that are used in sensors, monitoring devices, and surgical needle threaders are made using the process.
The process of metal etching is ideal for the manufacture of surgical blades that have to be flawless down to the μm. The close dimensional tolerances of the metal etching process create blades that meet the demanding requirements of the medical field.
Unlike the automobiles of old, modern ones require an assortment of electrical components, clutch springs, encoder disks, fuel cell plates, and nameplates that must be produced with exceptionally accurate tolerances to meet weight and noise requirements. Every aspect of an automobile is precision engineered and designed down to the smallest detail, which is why the industry relies so heavily on metal etching.
The diaphragm of a microphone is the device that changes vocal waves into electronic waves. As microphones have gotten smaller so have the diaphragms, which has presented a challenge for manufacturers. Metal etching has become the ideal solution to the conundrum since it is able to print and etch the smallest types of parts.
Modern technologies are getting smaller and smaller creating the need for components that meet the decreasing sizes. Since many of the new products are micro sized, there has been a rising demand for metal etching manufacturers to meet the demand. This has become especially true in the production of micro springs.
Every industry from manufacturers of medical instruments to producers of fire arms depend on micro springs to supply tension and control. As the products have gotten smaller, so have the wires, which have diameters of 0.002 in or 0.05 mm in order to meet the specifications of the miniature applications.
Fuel cells are made by stacking bipolar plates that have complex channels that enable liquids and gases to flow. The plates can be produced using traditional CNC machining, which introduces stress and burrs into the plates. Also, the machining process can be slow, expensive, and inefficient. Metal etching has the ability to produce the plates with their intricate components in one smooth and efficient operation. The process allows designers the flexibility of adjusting and changing the size and shape of the plates.
A common use of metal etching is placing identifying marks on equipment, parts, machinery, products, and components. Since etching can be used on any form of metal and is performed using a variety of processes, placing numbers and descriptions on parts is a minor function of the process.
Metals commonly used for metal etching of name plates are copper, aluminum, brass, stainless steel, and aluminum. Etching is used in place of engraving due to its ability to etch complex and intricate shapes and configurations. Also, metal etching is capable of machining stainless steel. The commonly used process for etching name plates is photochemical etching since it includes the use of computer design.
The brief list above is only a small sampling of the thousands of parts, products, components, and items that are produced using the metal etching process. Modern manufacturing depends on metal etching to provide intricate and complex parts that have the necessary precision and tolerances with on time delivery.
There are very few limitations to the types of metals that can be processed by metal etching. It is a metal machining method that can be applied easily and quickly. As with any form of manufacturing process, some materials are easier to work with than others. This is one of the factors that determines the use of certain materials.
A further restriction on the materials used for metal etching is the purpose of the component being produced. Conditions that place stress and demands on a part will require a different type of material from ones that are needed for less stressful environments.
Titanium has many positive properties that include lightweight, strength, and exceptional fatigue performance. Though these characteristics are ideal for the formation of parts, they prove difficult in the machining process. The difficulties of working titanium using normal machining processes is overcome with metal etching. The high thermal conductivity, chemical reactivity, and strength of titanium makes it an ideal metal for the metal etching process.
Aluminum has many of the positive properties of titanium such as high strength to weight ratio and corrosion resistance. It has an excellent fatigue limit, which makes it good for the production of aeronautical parts.
As with many metals, aluminum is suitable for a select number of etching processes. It is an ideal metal for laser etching because its surface has high thermal conductivity, which allows the machine to etch at very high temperatures. In etching processes that involve heat, the resulting aluminum parts have rough granular surfaces.
Stainless steel can be etched using any of the various processes. It is an excellent material for laser etching since images placed on it meet photo quality standards.
Regardless of the process, the main grades of stainless steel used in metal etching are series 316 and 306.
In photochemical etching, stainless steel easily accepts the photoresist laminate mask as well as the CAD image. The ferric chloride can remove the unnecessary metal from components as it does with other metals. Metal etching is widely used in producing stainless steel parts since burrs are not produced and the metal is not stressed.
Of the various metals, copper alloys are the easiest to work since they etch quickly and can be etched using any of the processes. Metal etching is preferred for working with copper alloys because other metal working methods distort the metal and damage its properties. Copper alloys are conductive, durable, ductile, and malleable, which makes them suitable for two dimensional and three dimensional electronics.
Nickel alloys have resistance to heat and corrosion. As with copper alloys, nickel alloys are easy to etch and are widely used for electrical applications. They are very versatile and can maintain their properties at temperatures up to 500° C or 932° F. A popular use of nickel alloys is as a shielding material for electrical components since they are resistant to electricity.
In the metal etching process, nickel can be etched into any number of designs, shapes, and configurations. Etching is chosen for working nickel alloys because the process does not produce burrs or thermal stress on the metal.
Inconel is a nickel based alloy that has superior resistance to heat, corrosion, pressure, and oxidation. Though these properties are admirable, they also make it difficult to etch and machine Inconel. Special metal etching processing has been developed such that inconel can be used to produce bipolar fuel plates.
There are a wide variety of uses for Inconel, which include extreme conditions where there is tremendous pressure and heat. Its thick layer of oxide offers protection against the elements. The type of Inconel that is used the most in the etching process is series 625.
Modern manufacturing and production demand immediate access to parts and components. Since mechanical metal forming takes time and preparation, it does not meet the needs of complex and involved production methods. For these reasons, metal etching has been rapidly gaining popularity as the go to method of manufacturing.
As product and part designs become more precision planned and computerized, methods of production are continually being adapted and adjusted to meet the new technical requirements. Although metal etching has a long history, it is constantly being improved and modified to meet today‘s manufacturing needs.
Since much of the process of metal etching is digital, it can be quickly generated. The precision of digital tooling ensures that every component created by the process will exactly meet the dimensional requirements of the design.
Of the many aspects of modern production, cost is a major factor when choosing a manufacturing process. The efficiency of metal etching makes it possible to have an accurate product run without errors or the need for finishing. The digital process allows for the adjustment of design errors prior to placing a product in production.
Other machining methods stress metals with heat, force, and mechanical manipulation that change the properties and characteristics of the metals. Metal etching does not do any form of stressing of metals such that they retain their properties and perform according to their characteristics. The metal etching process does not make contact with the metal surface, which allows the metal to be reshaped but have unchanged properties.
An absolute necessity for the production and manufacturing of any metal component or part is that every piece that comes off the assembly process has the same dimensional tolerances. This is an essential part of production repeatability. With metal etching, the digital diagrams and renderings remain unchanged from production run to production run guaranteeing that every workpiece will be exactly the same from the first to the last.
Mechanical metal fabricating processes produce workpieces that have irregularities such as burrs, rough edges, and deformities that are removed during finishing. None of these factors are present in metal etching. Since workpieces are not touched by any form of equipment, they come off the assembly process clean and ready for shipment.
Speed seems to be the key word for every form of manufacturing process in the modern lexicon. Customers place their orders and expect completed parts immediately. Though that type of service is not possible with metal etching, the process has significantly rapid turnaround times. Since there isn‘t any need for finishing, products are processed, produced, and shipped with little delay.
The digital process of metal etching provides the ability to create a prototype of the component for examination before it goes into production. Customers can sit down with a designer or engineer and give them the parameters of the part, which can be fed immediately into a computer for assessment and evaluation. Once agreement is reached on the dimensions, the rendering is transmitted from the computer to production without errors or any need for reprocessing.
Branding has become a central focus in marketing strategies and product development. Metal etching assists in the process by being able to place product identification easily on the items that are produced. This avoids the need for purchasing labels. During the planning process, it is possible to determine where to place any branding symbols or stamps as well as part numbers and contact information.
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