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 a technique used to selectively remove material from a workpiece by exposing it to a controlled corrosive environment. Unlike mechanical milling, which relies on cutting tools to shape materials, chemical milling involves eroding specific areas with chemicals, protective coatings, and careful management of temperature and exposure time. The process uses an etching solution—a blend of acids designed to react with and dissolve metal. Areas not intended for removal are covered with a protective layer, known as a mask or maskant, which shields these regions from the corrosive effects of the etching solution. This protective layer is applied before the workpiece is submerged in the etching solution.
Chemical milling is employed to modify the surface of a workpiece or to reduce its overall weight. This technique is useful for creating various features such as contours, cavities, engravings, or openings, and it is also effective in removing excess material. Additionally, chemical milling serves as a surface finishing method, often used for tasks like deburring to achieve a finer finish.
The adoption of chemical milling arose from challenges faced with traditional milling methods, which struggled to achieve the smooth surfaces required by the aerospace industry. Chemical engineers developed specialized etchants to produce exceptionally smooth and durable surfaces for aircraft components. The high precision of chemical milling has made it a valuable process in multiple industries where achieving a flawless surface is critical for maintaining product quality.
To ensure high-quality results, chemical milling involves a series of meticulously followed steps. The design, preparation, and execution of the process are carefully managed to maintain precision. Unlike mechanical milling, which uses sharp blades to cut into the material, chemical milling gradually erodes layers from the workpiece. The entire procedure is meticulously timed, controlled, and monitored to ensure that the workpiece achieves the desired shape.
While chemical milling is also a subtractive method, it avoids the common issues associated with mechanical milling, such as the generation of chips, metal fragments, and dust. Both methods aim to remove material from a workpiece, but they differ significantly in their approach to material removal. Chemical milling offers a cleaner alternative to mechanical milling's more abrasive techniques.
The chemical milling process is divided into five key stages: cleaning, masking, scribing, etching, and demasking. Each of these stages is carried out with precision and is carefully timed to achieve optimal results. Technicians overseeing the process are trained to handle the etchants and closely monitor each step to ensure proper execution.
Cleaning is a crucial initial step in chemical milling, as any residual materials like dust, chemicals, or contaminants can adversely affect the final product's quality. To ensure a pristine surface, any oils, grease, primers, markings, and residues must be thoroughly eliminated using appropriate solvents, alkaline cleaners, or deoxidizing agents. After cleaning, the workpiece must be handled with care to avoid reintroducing any materials that could interfere with the milling process.
Masking is a versatile process tailored to the precision, repeatability, speed, and cost requirements of the project. It involves applying an inert material, known as a maskant, to protect specific areas of the workpiece from the milling process. This step is analogous to masking areas before painting, with the maskant being resistant to the corrosive effects of the acid used in material removal.
Various masking techniques are available, including photoresist applications, offset printing, scribe-and-peel methods, and robotic spraying. The choice of masking technique depends on factors such as the chemical resistance of the material, the part’s configuration, the volume of parts being processed, ease of mask removal, and the precision needed.
The chemical milling technique involves submerging the workpiece in an etching solution, typically an acidic compound that reacts with the metal. A crucial factor in this process is the duration of immersion, as extended exposure results in deeper etching. The etching rate is influenced by several factors, including the concentration and composition of the etchant, the type of material being processed, and the temperature of the solution.
Etching solutions usually consist of a blend of acids, though aluminum alloys often use alkaline solutions. The proportions of these acids are critical, as they can affect the uniformity of the surface, the quality of the etch, and the amount of hydrogen absorbed during the process.
The etchant dissolves the metal from the exposed areas of the workpiece, making the choice of etchant crucial for an efficient and precise operation. Different etchants are used depending on the type of metal, with ferric chloride, ferric nitrate, and sodium hydroxide being among the most commonly employed for their effectiveness in shaping various metals.
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.
Among the various etchants, ferric chloride is the most commonly used due to its versatility with different metals. It reacts quickly, which necessitates careful control of the milling process. The chemical milling process with ferric chloride can lead to a significant rise in temperature. Sodium hydroxide, while faster than ferric chloride, also generates heat during its use.
Other etchant options include potassium hydroxide, hydrochloric acid, sulfuric acid, nitric acid, alkaline potassium ferricyanide solutions, and combinations of various etchants. Regardless of the chosen etchant, they can be regenerated using oxides that restore the etchant to its initial state, offering both cost and environmental benefits.
Maintaining consistent etching surface quality requires precise control over the etchant's concentration and temperature stability. To avoid gas buildup on the workpiece surface, it is rotated, flipped, and agitated throughout the chemical milling process.
| 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 concluding stage of the chemical milling process is demasking, which involves removing the maskant from the workpiece. This stage typically comprises two main actions: the removal of the maskant and the elimination of any residual etchant on the workpiece. Occasionally, an additional step may be required, which involves a deoxidizing bath to remove any oxide layer from the workpiece’s surface.
Demasking involves both stripping and cleaning procedures to ensure the complete removal of both maskant and etchant. Stripping entails removing the maskant without harming the substrate or the etched pattern. Various chemical strippers are employed for this purpose, such as acetone or oxygen plasma, with the choice depending on the composition of the maskant used.
After stripping, the workpiece must be thoroughly cleaned to eliminate any remaining stripper and etchant. This cleaning process includes multiple rinses with deionized water and different cleaning agents. As the final step in the process, meticulous cleaning is essential to avoid contamination and ensure the high quality of the finished product.
Milling is a subtractive manufacturing technique used to shape materials, with various methods employed depending on the intended final shape. Mechanical milling utilizes a rotating cutting tool to cut away material, whereas chemical milling employs chemicals to selectively dissolve the surface of a workpiece to achieve the desired shape or thickness. While both methods ultimately produce similar results, their approaches and mechanisms differ significantly, with chemical milling offering a unique and effective alternative.
Perimeter milling focuses on trimming down the dimensions and mass of a workpiece. This technique differs from other chemical milling methods by eliminating the need for a maskant, thus streamlining the process. The goal of perimeter milling is to thin the workpiece while preserving its structural integrity. During this milling operation, the outer edges of the workpiece are shaved to conform to specific dimensions.
Perimeter milling is frequently employed in the refinement of cast or forged components that need excess material removed to achieve precise dimensions. It is also applicable to parts that have undergone machine milling but still do not meet the required specifications. This method allows for the adjustment of cast parts that have been produced with a safety margin to accommodate potential casting imperfections. Excess material can be efficiently removed using chemical milling techniques.
In certain milling applications, it becomes necessary to incorporate additional features into a workpiece. Mechanical milling typically demands repositioning of the milling tool to accommodate these changes. Conversely, chemical milling allows for the addition of these features in a single step, streamlining the process and reducing both time and cost. Areas of the workpiece that require modification are exposed to the etchant, which selectively dissolves the metal to achieve the desired features.
Step chemical milling involves immersing the workpiece in the etchant multiple times in a staged process to gradually remove material in layers. After the initial area reaches the desired depth, the workpiece is withdrawn from the etchant, and the maskant is either removed or adjusted from the next target area before re-immersing it. This process is carefully managed to achieve precise forms, such as tapered cuts, by controlling the number of immersions and withdrawals.
Tapered chemical milling involves a sequential process where the workpiece is carefully lowered into and raised out of the etchant, with the immersion and removal speed precisely controlled to create various tapered shapes. Unlike other methods, masking is optional for this process.
While manual tapering is possible, using a variable speed hoist is more efficient. This equipment can be set to handle the part’s length, desired taper angle, and milling speed. One complete cycle typically equals twice the length of the workpiece or the section being tapered. For more intricate tapers, a circular immersion and withdrawal technique can be employed.
Structural chemical milling focuses on removing material from a workpiece while preserving its overall strength. This process achieves significant weight reduction, often cutting the workpiece’s mass by 75% or more, with thickness reductions as minimal as 0.010 inches (0.0254 cm). This technique is ideal for creating lightweight yet robust components, allowing for precise contour adjustments and meeting strict tolerances.
Structural chemical milling serves as a more efficient alternative to mechanical milling, which can lead to uneven bending, wrinkling, and rough surfaces. Compared to mechanical methods, chemical milling is more cost-effective, quicker, and capable of producing complex designs with higher accuracy and superior quality.
There is often some misunderstanding between chemical milling and chemical etching, with many people using the terms interchangeably. While both processes share similarities, they are not entirely the same and have distinct differences.
Chemical milling involves creating markings, grooves, and varying depths of cuts in a workpiece through the application of acids or alkaline solutions. This technique primarily aims to reduce the weight of a component while preserving its structural integrity. The process relies on two key elements: the etchant, which removes metal from the workpiece, and the maskant, which shields certain areas from the etchant.
This method is predominantly utilized in the aerospace and aviation sectors, where managing weight is crucial for optimal performance. Chemical milling’s ability to precisely eliminate material to adjust a part’s weight, while keeping other sections smooth, makes it especially valuable for aircraft manufacturing.
Developed in the 1960s to address the challenges of machining complex shapes, chemical milling differs from chemical etching in that it is used for large, three-dimensional components such as engine casings, cowlings, and wing fairings. In this process, parts are immersed in an etchant for a predetermined duration to achieve the desired modifications.
Chemical etching shares similarities with chemical milling, particularly in its use of an etchant. However, it employs a resistive coating material to protect certain areas of the workpiece from the etchant. There are two main types of chemical etching: photochemical etching and mechanical etching.
Photochemical etching, like chemical milling, is a subtractive metal processing technique that uses digital tooling and an etchant to create detailed and complex parts. This process involves applying a light-sensitive photoresist to the workpiece. A pattern is then exposed to ultraviolet light, which alters the photoresist in the exposed areas while protecting the unexposed areas. The workpiece is then treated with an etchant that dissolves the unprotected areas, resulting in the desired etched pattern.
Chemical etching is typically applied to thin metals, with thicknesses as small as 0.0005 inches (0.0127 mm). The depth variations achievable with chemical etching depend on the metal type, with ferrous alloys up to 0.04 inches (1.016 mm), copper alloys up to 0.065 inches (1.651 mm), and aluminum up to 0.080 inches (2.032 mm). This process is commonly used to create screens, grids, meshes, and perforated materials.
The primary distinction between chemical milling and photochemical etching lies in the size and complexity of the parts they can handle. Both processes use an etchant to chemically remove material from the workpiece. However, chemical milling is versatile enough to be used with various metals of different thicknesses, while photochemical etching is generally limited to materials thinner than one inch (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 |
No matter how intricate the design or specialized the features, chemical milling follows identical setup steps and utilizes the same machinery. This method allows for the simultaneous milling of various components without imposing any stress on the parts. Chemical milling involves modifying and removing material from metal surfaces, applicable to all metal types, while chemical etching produces grooves, markings, and patterns on metal surfaces.
Chemical milling can be applied to all types of metals, as it is a corrosive technique that gradually wears away the surface material. Its primary application is found in the aerospace sector, where lightweight yet durable components are essential. Metals like titanium, steel, and Inconel alloys are commonly used in the construction of aircraft. Additionally, copper is a popular choice for chemical milling because of its flexibility and ease of shaping.
Titanium undergoes chemical milling to eliminate its brittle crystalline layer, which forms during the casting process. Its application in chemical milling stems from its high strength-to-weight ratio, making it a preferred material in aerospace and defense sectors. Titanium's high melting point allows it to endure the repeated stresses involved in chemical milling procedures.
Despite its strength and density, steel is often used in chemical milling for components requiring material removal in pockets or surface layers. Chemically milling steel parts, whether forged, cast, or machined, helps remove imperfections and surface irregularities. Ferric chloride, hydrochloric acid, nitric acid, and nital (a mix of nitric acid and alcohols) are commonly used chemicals. The steels utilized include mild, carbon, tool, and spring grades.
Similar to aluminum, copper is favored for chemical milling due to its advantageous properties and ease of processing. A broad range of chemicals can be employed for milling copper, in addition to those specifically suited for it. Copper's versatility allows for intricate designs as well as large-scale milling. Common copper alloys in this process include brass, phosphor bronze, beryllium copper, and nickel silver.
Stainless steel is frequently used in chemical milling, especially for products in the medical and food sectors. Grades such as austenitic series 300, ferritic series 430, martensitic series 300 and 400, along with duplex and super duplex stainless steels, are commonly utilized.
Aluminum is ideal for chemical milling due to its excellent strength-to-weight ratio and low density. As the first metal used in the process, aluminum continues to be a key material. Chemical etchants like hydrogen chloride (HCI), sodium hydroxide, and Keller’s agent (a blend of nitric, hydrochloric, and hydrofluoric acids) are used for aluminum. All aluminum alloys are suitable for this process, as each offers unique advantages.
The chemical milling process has long been employed in producing printing and engraving plates for magazines and newspapers, where entire pages are etched into the material. Chemical milling can be categorized into two types: selective and non-selective. In selective chemical milling, only designated areas of a workpiece are exposed to the etching solution to craft a specific design or part. On the other hand, in non-selective milling, the entire surface of the material is subjected to the etchant when no particular pattern or design is required.
An important aspect of the chemical milling process is its ability to create various surface textures and finishes, which can enhance the bond strength of components such as engine blades. In the medical sector, textured surfaces are applied to implants to promote better osseointegration. Additionally, chemical milling is useful for eliminating unwanted surface features, like alpha case or inconsistencies in the material. The process can also be employed for selective removal, allowing for the development of specific surface conditions as needed.
Additive manufacturing involves creating parts by layering raw materials until a component or part takes shape. This method constructs items in layers and belongs to a broader set of manufacturing techniques that rely on computer-aided design (CAD) for the creation of components.
To achieve the required tolerances for parts and components, several post-processing steps are necessary. These include removing excess material, eliminating internal and visible support structures, preparing items for dye penetration inspection, enhancing fatigue resistance over time, or achieving a smooth surface finish. Chemical milling is used in the final stages to smooth surfaces and refine the appearance of additive manufactured components.
Additional post-processing techniques for additive manufacturing include:
Weight is a crucial factor in aircraft manufacturing, and minimizing it is essential. In the aerospace sector, chemical milling is extensively utilized to reduce the weight of fuselage skins and other components, thereby enhancing aircraft performance. This technique also enables the creation of blind features like pockets, channels, and other specialized areas to further reduce weight.
Chemical milling is favored in aerospace applications because it is a precise and scalable method that produces components with high accuracy. The process results in stress-free, burr-free parts without distortion, making them immediately ready for use. Consistency and reproducibility are critical in aircraft production, and chemical milling meets these demands with its exceptional precision and uniformity in parts.
In the automotive sector, chemical milling is employed to manufacture titanium exhaust components by selectively removing material from their surfaces. This process adjusts the thickness and weight of the exhaust components, enhancing the efficiency of the exhaust system. By doing so, it improves both fuel efficiency and vehicle dynamics while maintaining the structural integrity of the titanium.
Chemical milling is particularly suited for titanium due to its hardness, high strength, and lightweight properties. The technique allows for the removal of material from titanium components while preserving their strength, ensuring that the metal remains robust and effective.
Chemical milling is a subtractive method that involves removing metal layers from a workpiece's surface to create various shapes and forms. This process uses specific chemicals known as etchants to selectively dissolve parts of the workpiece, meeting precise design requirements. As it does not require sharp tools or heavy machinery, chemical milling is less invasive and often more cost-effective.
In contrast to mechanical milling, which may require multiple stages to achieve detailed and complex shapes, chemical milling accomplishes the task in a single step by immersing the workpiece in an acid solution. To avoid damaging areas that are not intended to be altered, portions of the workpiece are protected with masking during the chemical milling process.
Mechanical milling typically involves the use of sharp, metal tools designed to make deep cuts into a workpiece. The production and maintenance of these milling tools are costly and labor-intensive. Given the hardness and durability of metal workpieces, these cutting tools have a finite lifespan and must be replaced after a certain number of cycles.
In contrast, chemical milling is a process rooted in chemical engineering that eliminates the need for sharp tools. Instead, the focus is on meticulous planning to ensure that the desired parts of the workpiece are preserved after milling. Unlike mechanical milling, where the majority of effort goes into the milling operation itself, chemical milling dedicates most of its effort to planning and preparation, with the actual milling process taking a smaller role. There are no tools that wear out or costly machinery involved; the primary equipment consists of tanks used for immersing the workpieces.
Removing burrs from forged, molded, or mechanically milled parts can be a laborious and time-consuming task, often requiring hours to eliminate small flakes around the edges. Chemical milling offers an efficient alternative by immersing the part in an etchant, which can achieve a burr-free surface in just a matter of minutes.
In many milling processes, the grain structure of a workpiece can change due to the stress applied during machining. Chemical milling, however, removes material without exerting stress on the workpiece, thereby preserving the original grain structure and preventing any alteration.
In contemporary manufacturing, prototyping is crucial because of the high cost associated with producing components for large assemblies. Chemical milling is well-suited for creating prototypes, as it only requires a workpiece and a CAD design. This capability makes chemical milling an excellent method for generating replicas of parts for testing and evaluation. Engineers can use this process to assess and modify prototypes, ensuring their effectiveness and making necessary adjustments to the original design.
Similar to prototyping, making design adjustments with chemical milling is straightforward. Changes to the design can be made directly in the CAD software and seamlessly incorporated into the milling process. In contrast, other manufacturing methods often involve multiple steps to implement design modifications, which can lead to work stoppages and time wastage.
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