The contents of this article will provide you with everything you need to know about chemical milling and its use.
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Chemical milling is the dissolution of solid material in a targeted area by means of exposing the material to a tightly controlled corrosive environment. Unlike aggressive mechanical milling methods that depend on sharp tools to produce a design, chemical milling erodes sections of a workpiece using chemicals, masks, variations in temperature, and time. An etchant, which is a mixture of acids that react with metal, dissolves portions of the workpiece with precision and accuracy. Portions not to be treated are protected with a coating that resists the effects of the etchant and preserves the surface of the workpiece. The protective coating, referred to as a mask or maskant, is applied to the workpiece prior to its immersion in the etchant solution.
The chemical milling process is used to alter the surface of a workpiece or reduce the weight of a workpiece. The uses for chemical etching include creating contours, spaces, engraving, or openings in a workpiece or for the elimination of bulk materials. Since it is a surface finishing process, it is also used for fine finishing, such as deburring.
The implementation of chemical milling came from difficulties associated with traditional milling that did not produce smooth enough surfaces for the aircraft industry. A chemical engineer experimented with various types of chemicals to produce an etchant capable of producing durable and exceptionally smooth surfaces for aircraft components. The precision of chemical milling has led to its use in several industries that require smooth and even surface to ensure product quality.
The steps for chemical milling are carefully followed to ensure the quality of the final product. The designing, preparation, and engineering of the process is meticulously adhered to down to the smallest detail, which has led to its success. Unlike mechanical milling that involves the use of sharp blades that cut into the surface of a workpiece, chemical milling slowly removes layers. The process is timed, controlled, and monitored such that the workpiece is properly formed.
Although chemical milling is a subtractive process, it does not have the drawbacks of mechanical milling, which produces chips, metal pieces, dust, and particles. Mechanical milling and chemical milling have the goal of subtracting portions of a workpiece. Both processes form components and parts with the main difference between them being how metal portions are removed.
The process of chemical milling is divided into five steps, which are cleaning, masking, scribing, etching, and demasking. Each step is performed with precision and timed to achieve the necessary results. Technicians closely monitor the process and are trained in the reactions of the etchants.
As with all forms of chemical processing, cleaning is an essential step since any stray material, such as dust or chemicals, can negatively impact the quality of the final part. Oils, grease, primer, markings, and residues must be removed using a solvent, alkaline cleaner, or deoxidizing solution. Once the workpiece is cleaned and contaminants are removed, it is handled carefully to prevent the introduction of any materials that can impact the milling process.
Masking takes several forms and is chosen in accordance with the required accuracy, repeatability, processing speed, and cost. The masking process involves the use of an inert substance, known as a maskant, that protects areas of the workpiece that are not to be milled. The process of masking is similar to covering areas when painting. The masking material is resistant to the effects of the acid used to remove material from the workpiece.
The choices of masking processes include using a photoresist, offset printing, scribe and peel, or a mechanical robotic sprayed application. The selection of a maskant is dependent on the chemical resistance of the metal, the configuration of the part being produced, the number of parts, ease of removal of the mask, and the required accuracy.
The chemical milling process involves immersing the workpiece in the etchant, which is normally a form of acid that reacts with metal. The critical aspect of the chemical milling process is the amount of time the workpiece spends in the etchant since the longer the workpiece is immersed the deeper the etchant cuts into the workpiece. The rate of etching varies based on the concentration of the etchant and its composition, the material being etched, and the temperature of the process.
Most etch solutions are comprised of multiple acids for alloys, with the notable exception of aluminum alloys which primarily run in alkaline chemistries. The ratios of acids to one another can significantly impact surface uniformity, surface condition, etch rate, and hydrogen pickup.
The etchant reacts with the exposed area of the workpiece and dissolves the solid metal. The choice of etchant provides an efficient, quick and effective process. The type of etchant varies with the metal being milled with certain etchants designed to shape specific metals. The most common types of etchants are ferric chloride, ferric nitrate, and sodium hydroxide.
Ferric Chloride – Ferric Chloride, also known as iron chloride, is soluble in water. When dissolved in water, it undergoes hydrolysis and becomes a brown corrosive acidic solution. During the process of chemical milling, ferric chloride corrodes the exposed portions of the workpiece.
Ferric Nitrate – Ferric nitrate is an inorganic compound that appears as a violet crystalline solid. It is soluble in water, alcohol, and acetone. For chemical milling, when ferric nitrate is dissolved in water, it becomes corrosive to metals and is commonly used to etch metals.
Of the three etchants, ferric chloride is the most popular and most widely used since it can be used with most metals. It works fast, which requires precision control of the milling process. During processing, the temperature of the chemical milling process rapidly rises and generates heat. Sodium hydroxide works faster than ferric chloride but also produces heat.
Other forms of etchants include potassium hydroxide, hydrochloric acid, sulfuric acid, nitric acid, alkaline potassium ferricyanide solution, and mixtures and combining of etchants. Regardless of the type of etchant, they are regenerated through the use of oxides that return an etchant to its original solution, which is a cost saving and environment saving process.
To achieve etched surface uniformity, the concentration of the etchant is precision controlled and fluctuations in temperature prevented. In order to prevent gas from forming on the surface of the workpiece, it is rotated, overturned, and rocked during chemical milling.
| Metals Used in Chemical Milling | |||||
|---|---|---|---|---|---|
| Metals | |||||
| Etchants | Copper | Aluminium | Steels | Silica | Stainless Steel |
| Cupric Chloride | Sodium Hydroxide | Hydrochloric Acid | Hydrofluoric Acid | Ferric Chloride | |
| Ferric Chloride | Keller's Reagent | Hydrochloric Acid | |||
| Ammonium Persulfate | Nital | ||||
| Ammonia | |||||
| Nitric Acid | |||||
| Hydrochloric Acid | |||||
| Hydrogren Peroxide | |||||
The final step in the chemical milling process is demasking where the maskant is removed from the workpiece. Although it is referred to as demasking, there are two steps to the process with one being the removal of the mask while the other is the removal of any remaining etchant on the workpiece. In some cases, a third step may be necessary, which is a deoxidizing bath to remove the oxide film on the surface of the workpiece.
The processes that are part of demasking include stripping and cleaning that ensure the etchant and maskant have been completely removed. Stripping is a removal of the maskant without damaging the substrate of the workpiece or the pattern. Several chemical strippers are used including acetone or oxygen plasma. The choice of stripper is dependent on the chemical makeup of the maskant.
At the completion of the stripping process, the workpiece has to be meticulously cleaned to remove the stripper and any remaining etchant. The process of cleaning involves a series of rinses using deionized water and various types of cleaning solutions. Since the cleaning process is the final step of production, it is critical in preventing contamination and ensuring the quality of the final product.
Milling is a subtractive shaping process that takes certain forms depending on the desired shape of the final product. While mechanical milling involves using a rotating cutting tool to remove material, chemical milling uses chemicals to erode the surface of a workpiece to achieve a particular shape or thickness. Although the results of chemical milling are the same, the process to get those results are unique and effective.
Perimeter milling is used to reduce the dimensions and weight of a workpiece. Unlike other forms of chemical milling, perimeter milling does not require the use of a maskant, which simplifies the process. As with many chemical milling methods, the purpose of perimeter milling is to reduce the thickness of the workpiece while retaining its strength. During the mill process, the outside perimeter of the workpiece is reduced to meet certain dimensions.
A common use of chemical perimeter milling is with cast or forged parts that require excess material removed to achieve the proper dimensions. The process is also used with parts that have been machine milled where the design or shape has been reached but not to the correct specifications. Chemical milling makes it possible to produce cast parts with a safety margin added to control cast failures. Any excess material can be removed with chemical milling.
There are times in the milling process when features need to be added to a milled piece. With mechanical milling, the additions require the movement of the milling tool into different positions. Use of the chemical milling process makes it possible to add the missing features in one step, which saves time and money. The parts of a workpiece that need to be adjusted are exposed to the etchant, which erodes away the metal.
With step chemical milling, the workpiece is placed in the etchant several times, progressively, to slowly remove layers from the workpiece. Once the first area reaches its proper depth, the workpiece is removed from the etchant, and the maskant is removed or scribed from the next selected area and placed back in the etchant. The number of immersions and withdrawals are controlled in order to achieve the desired form. This is commonly done to achieve tapered cuts.
Tapered chemical milling is similar to step chemical milling in that it is completed in a series of steps where the workpiece is lowered and raised in and out of the etchant at a monitored and controlled rate. The speed of the immersion and removal produces different tapered shapes. Although masking can be used, it is not necessary for tapered chemical milling.
The process for tapering can be completed manually but is more effectively done using a variable speed hoist that can be preset for the length of the part, the degree of the taper, and the milling rate. One cycle is the distance traveled equal to twice the length of the workpiece or twice the length of the section being tapered. Complex tapers can be accomplished using a circular immersion and withdrawal process.
With structural chemical milling, material is removed from the workpiece while maintaining the workpiece’s structural strength. It is a weight reduction process that reduces the weight of a workpiece by 75% or more with a thickness reduction of 0.010 in (0.0254 cm) or less to produce lightweight and strong parts. The use of chemical milling for structural changes makes it possible to lighten the contour of parts to achieve exacting tolerances.
The use of structural chemical milling is an alternative to mechanical milling that produces uneven bending, wrinkling, and unsmooth surfaces. Mechanical milling is time consuming and expensive while chemical milling is less expensive and takes less time. In addition, chemical milling has better lead times, can create complex designs, and has greater accuracy and produces better quality.
There is some confusion regarding chemical milling and chemical etching with many people using the terms interchangeably. Although the processes are similar, they are not totally synonymous and have slight differences.
Chemical milling produces markings, incisions, and deep or shallow cuts in a workpiece using various forms of acids or alkaline solutions. The main purpose of chemical milling is to remove material to reduce the weight of a part while helping maintain the strength of a part. The main components of chemical milling are the etchant and maskant with the etchant used to erode metal from a workpiece while the maskant protects portions of the workpiece from the etchant.
The main use of chemical milling is in the aerospace and aircraft industries where weight is detrimental to the performance of their products. The capabilities of chemical milling to selectively remove aspects of a part to alter the part’s weight while keeping the remaining aspects of a component smooth is an ideal process for aircraft construction.
The process of chemical milling was developed in the 1960s as a solution for machining complex surfaces. Chemical milling, unlike chemical etching, is performed on three dimensional large parts, such as airplane engine housings, cowlings, and wing fairings where the parts are immersed in an etchant for a set period of time.
In many ways, chemical etching is similar to chemical milling in that it uses an etchant. Included in chemical etching is a resistive coating material that protects a workpiece from the etchant. The types of chemical etching are photochemical etching and mechanical etching.
Photochemical etching, like chemical milling, is a metal subtractive process that uses digital tooling and an etchant to produce complex and intricate parts. The process includes applying a light sensitive photoresist material to a workpiece. The pattern for the desired shape is exposed to ultraviolet light with the unexposed areas experiencing a chemical change that protects those areas. The etchant is sprayed on the workpiece and removes or dissolves the unprotected areas leaving the etched shape.
The process of chemical etching is used on thin gauge metals that can be as thin as 0.0005 in (0.0127 mm). The variations in the depths that chemical etching can achieve vary by the type of metal with the thickness of ferrous alloys being 0.04 in (1.016 mm), copper alloys 0.065 in (1.651 mm), and aluminum to .080 in (2.032 mm). Chemical etching is used to produce screens, grids, meshes, and perforated products.
The basic difference between chemical milling and photochemical etching is the size and complexity of the components and parts that they can produce. The working principles of each process are similar, such as using an etchant and chemical removal of material. Ulike chemical etching, chemical milling can be used on all forms of metals regardless of their thicknesses while chemical etching is limited to thicknesses that are less than an inch or 25.4 mm.
| Difference Between Chemical Milling and Photochemical Etching | |
|---|---|
| Chemical Milling | Photochemical Etching |
| Effective on metals of all sizes and thicknesses | Is used for thin material typically to create opening |
| An alteration and removal process | Fabrication process |
| Maskants are elastomer and co-polymer based | Uses a polymeric film or acid resistive called ground |
| Dissolves unwanted areas to reduce weight | Dissolves metals selectively |
| Used to mill three dimensional parts | Used to produce fine meshes, grids, and semiconductors |
Regardless of the complexity of a design or special features, chemical milling always has the same setup procedures and equipment. The process makes it possible to mill different parts at the same time without adding stress to the components. Chemical milling is an alteration and metal removal process that can be performed on any type of metal while chemical etching creates incisions, lines, and designs in the surface of metals.
Chemical milling is performed on any type of metal due to it being a corrosive process that erodes away the surface of a metal. The main use of chemical milling is in the aerospace industry that requires parts to be lightweight but strong with titanium, steel, and Inconels being the primary metals used for the construction of aircraft. Copper is also widely used for chemical milling due to its ductility and ability to be easily shaped.
Titanium is chemically milled to remove its brittle crystalline skin that is formed during the crystallization phase of the casting of parts. The use of titanium for chemical milling is due to its very high strength to weight ratio and its use in the aerospace and defense industries. The high melting point of titanium makes it ideal for chemical milling since it can withstand the repeated stress cycles associated with chemical milling.
Regardless of its density and strength, steel is used for chemical milling of parts and components that need to have pockets or layers removed. Forged, molded, or machine milled steel parts are subjected to chemical milling to remove burrs and deformities in the outer layers of parts. Steel is chemically milled using ferric chloride, hydrochloric acid and nitric acid as well as nital, which is nitric acid mixed with ethanol, methanol, and methylated spirits. Steels used for chemical milling include mild, carbon, tool, and spring steels.
Copper, as with aluminum, is a popular metal for chemical milling due to its many positive properties and the ease at which it can be milled. A long list of chemicals can be used to chemically mill copper, which includes all of the normal chemicals as well as ones that are ideally suited for copper. The adaptability and flexibility of copper makes it the perfect metal for chemical milling since it can be milled to create small details and large peripheral millings. Copper alloys used for chemical milling include brass, phosphor bronze, beryllium copper, and nickel silver.
Stainless steel is one of the steels that is used for chemical milling, especially for products for the medical and food industries. The grades of stainless steel used for chemical milling include austenitic series 300 stainless steels, ferritic stainless steel series 430, martensitic stainless steel series 300 and 400, and duplex and super duplex stainless steels.
Aluminum is an ideal metal for use in chemical milling because of its strength to weight ratio and its low density. It was the first metal used for the chemical milling process during the development of the process. The etchants for chemical milling of aluminum are hydrogen chloride (HCI), sodium hydroxide, and Keller’s agent, which is a mixture of nitric acid, hydrochloric acid, and hydrofluoric acid. All alloys of aluminum are used for chemical milling since each alloy has unique properties suited for the process.
For many years, the chemical milling process has been used for the preparation of printing and engraving plates that have the pages of magazines and newspapers etched into them. Two forms of chemical milling are selective chemical milling and non-selective chemical milling. With selective chemical milling, a specific design, component, or workpiece is created by exposing only portions of a work to the etching acid. When there is no pattern or design to be created, the complete surface of a workpiece is exposed to the etchant, which is a non-selective process.
A significant part of the chemical milling process is the creation of specific types of surface finishes with different textures, which are used to add bond strength to components, like engine blades. In the medical field, texture is added to implants to increase osseointegration. Chemical milling is also used to remove undesirable surface finishes, such as alpha case or non-homogenous conditions. The process can be used for selective removal to create desired or designed surface conditions.
Additive manufacturing is a process that builds components by adding raw materials in layers to form a component or part. It is a process that builds components and parts in layers and is a part of a group of manufacturing methods that relies on computer aided design to construct components.
In order to reach the tolerances of parts and components, various post processing steps are necessary to remove excess material, remove internal and line of sight support structures, prepare workpieces for dye penetration inspection, improvement in long term fatigue performance, or to produce a smooth surface finish. These final steps in the process are completed using chemical milling that smooths surfaces and improves the appearance of additive manufactured parts.
Other post additive manufacturing procedures:
A necessary consideration in regard to the manufacture of airplanes is weight, which has to be kept to a minimum. Chemical milling is widely used in the aerospace industry as a means for reducing the weight of fuselage skins and components to improve the efficiency of aircraft performance. As part of its use in reducing the weight of an aircraft, chemical milling produces blind features such as pockets, channels, spaces, and other special areas.
The use of chemical milling in aerospace is due to it being a clean and scalable procedure capable of producing parts and components with precision and accuracy. Parts are stress burr free without distortion and ready for use. A critical factor in aircraft production is the consistency and reproducibility of parts since uniformity is essential. Chemical milling fulfills this requirement with its exceptional tolerances and identical parts.
In the auto industry, chemical milling is used to produce titanium exhaust components by removing material from their surface area. As with many applications of chemical milling, it is used to change the thickness of the exhaust and its weight to improve exhaust system efficiency. In the process, fuel efficiency and vehicle dynamics are improved while preserving the structural strength of the titanium.
Chemical milling is ideal for working with titanium, which is a very hard metal with high strength that is lightweight and workable. In order to maintain the strength of titanium, chemical milling is used to remove layers from titanium without diminishing its strength.
The chemical milling process is a subtractive process that removes layers of metal from the surface of a workpiece to add various forms and shapes. The process of chemical milling involves the use of carefully selected chemicals, referred to as etchants, to selectively remove portions of a workpiece to achieve design specifications. Since chemical milling does not involve the use of sharp tools or heavy machinery, it is a less intrusive process, which is one of the reasons for its lower cost.
Unlike mechanical milling, which may involve several steps for the completion of the milling of intricate and complex shapes, chemical milling completes the process in a single step by immersing a workpiece in a solution of acids. Damage to the workpiece is prevented by masking portions of the workpiece that are not to be changed by the chemical milling process.
A standard part of the normal mechanical milling process is sharp tools that are made of metals capable of making deep cuts in a workpiece. The manufacture and crafting of milling tools is expensive and labor intensive. Since the metal workpieces are hard and tough, mechanical cutting tools only last for a calculable number of cycles before they have to be replaced.
Chemical milling is more of a chemical engineering process that does not require the use of sharp tools. The process does require careful planning to ensure the proper portions of the workpiece remain after milling. Unlike mechanical milling, the major part of the process is devoted to planning and preparation with a small portion dedicated to the process itself. There are no tools to wear out or expensive pieces of machinery. The main manufacturing tools for chemical milling are the tanks used to immerse workpieces.
Deburring of forged, molded, or mechanically milled parts is time consuming and labor intensive. It takes hours of work to remove minute flakes on the periphery of formed parts. Chemical milling can perform the same function by having a part immersed in an etchant that takes several minutes to produce a burr free part.
One of the factors that is prevalent in many milling processes is the changing of the grain structure of a workpiece due to the stress placed on a workpiece. Since chemical milling removes portions of a workpiece without placing stress on it, the grain structure remains intact and is not altered.
In modern manufacturing, prototyping has become a necessity due to the expense of producing components for large assemblies. The CAD designs for milled parts can easily be reproduced using chemical milling since the process requires a workpiece and the design. This aspect of chemical milling makes it the perfect choice for producing facsimiles of parts to be milled or produced. Engineers can test and alter prototypes to determine their viability and make corrections to the original design.
As with prototyping, adjustments to a design can easily be completed using chemical milling since the features of the design can be changed in CAD and adjusted during the chemical milling process. In other manufacturing processes, there are multiple steps that have to be completed to make any adjustment to a design, which require work stoppages and the waste of time.
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