Photochemical Machining: Introduction, History, Working Principle, Applications, Advantages [Notes & PDF]

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Hello, Hope you are well. In this article, we will discuss What is Photochemical Machining? in detail. First, we will see the Introduction, a little bit of history, Working Step by step, Advantages, Disadvantages, and Application in detail.

And at the end of the article, you can easily download a PDF version of this article.

Let’s start to introduction first,

Photochemical Machining Introduction:

Photochemical machining plays a dominant role in the field of manufacturing. This process is the higher resolution of the chemical machining process.

In simple words, the replacement of any maskant by photoresist material in the Chemical machining process turns the process into Photochemical machining.

The photoresist material is made use for ultraviolet exposure before the chemical etching to produce complex geometries on sheet metal components.

This process is also called ‘Photochemical milling’ or ‘Photo etching’. The entities manufactured by this method are stress-free and crack-free.

This process is widely used for prototyping because of its sharp dimensional tolerances.

Photochemical Machining History:

The development of photochemical machining began in the year 1782 when John Senebier noticed that few resins lost their solubility to turpentine and got hardened under sunlight.

The experiments started from then, till 1940. Finally, a machining process with the name Photochemical was evolved by compositing the Chemical process and Photoresist (hardening phenomenon of metal under UV light).

The same has become commercial since the 1960s.

Definition:

Photochemical machining is a process of fabricating desired shapes on sheet metal components by exposing the workpiece to ultraviolet light using a photo-resist material and then removing the metal by the method of etching.

Photochemical machining Working Principle:

The working principle of the Photochemical Machining process is derived from the Chemical machining process. A photoresist film is coated on the clean surface of the workpiece.

The film used is sensitive to ultraviolet light and resistant to the etchant. Then, a series of photos of the design to be fabricated is printed on a film.

The printing has two shades, transparent and black. The photo film is precisely mounted on the workpiece with no errors. The photoresist does the work of differentiating the work surfaces as soft and hard when exposed to ultraviolet light.

Generally, where the light shines through the transparent areas of the film becomes harder and the black remains soft. Then the workpiece with coated resist is put through a developer to remove the soft resist.

This removal of soft resist differentiates the areas to be machined and not to be machined. The exposed area of the metal will be the area to be etched.

The remaining hard resist acts as a maskant. The sheet metal is then passed through an etching machine, where the etchant is sprayed on both sides of the workpiece.

The areas of the metal where there is no resist material get dissolved by the reaction of chemical reagent.

When the sheet is finished etching, the resist is removed by putting it under a spray of resist stripper that dissolves the resist material.

The result will the finished product as same as that designed and printed on the photo film.

Photochemical Machining Working Step by step:

1. Designing & Printing:

The process starts on a computer screen designing the component on any CAD software. The artwork generated has the design structure either black or clear depending on the photoresist being used.

The same is printed on a photo film of a standard size. Either the clear areas or the black areas will be the image of the product to be manufactured. Generally, the artwork is printed twice on two sheets of film.

2. Cutting of sheet:

After a design is made and printed on a photo film, metal is selected in conformity with the details specified in the design. Sheets are then cut from a roll of sheet metal.

The size of the sheet to be cut will be the same as the size of the photo film printed. The sheet metal used in the Photochemical process should have its thickness ranging between 0.013 to 2.032 mm and the metals can be aluminum, brass, copper, nickel, silver, steel, stainless steel, copper, brass, zinc, and titanium.

3. Cleaning of sheet:

The sheet is then cleaned to make sure any oxides, contaminants of oil, or grease aren’t present on the surface of the sheet metal.

Cleaning is usually performed on fully automatic machines by the application of high-pressure water jets, alcoholic solutions, and diluted HCl.

Needs of cleaning:

Improper cleaning can result in poor adhesion of resistance, inaccurate final dimensions.
Improper cleaning can produce undercuts during etching.
Contaminants like oil, grease can lead to oxidation.

Once the sheet is finished washing, it dried under hot air blowers.

4. Applying resist on sheet metal:

Once the sheet has dried fully, it is coated with a resist material. The material used as the resist is sensitive to UV light and resistant to the chemical reagent.

In practice, the photoresist is classified into two types; Positive photoresist and negative photoresist. The resist material is selected conditional to the design printed on the photo film, whether the printed data is clear or black.

Positive photoresist: In a positive photoresist, when the sheet is passed through the UV light, the portion of the resist that is exposed to UV light becomes softer. Then that soft resist is dissolved in the developing process. The unexposed photoresist remains hard.

Negative photoresist: In the case of negative photoresist, the portion subjected to UV light becomes hard and the remaining portion of the resist remains soft for development.

Passing the sheet into resist coat machine

5. Mounting of photo film over the photoresist:

Once the sheet is finished coating with resist material. The printed photo films are carefully and precisely mounted over the resist material on both sides of the sheet. It is always made sure that the alignment of both films is accurate.

6. Passing the sheet through ultraviolet light:

The sheet is then passed through the UV light. In this exposure, the clear areas that were printed on the film allow the UV light to strike the layer of resistance, while the black does not allow it.

Since the resist used is a photo-sensitive material, the resist reacts to the UV light by changing its property to become hard in condition to the type of resist used. This results in parting the resist into hard and soft.

Typical diagram of UV light on photo film

7. Development process:

The sheet is further passed through a developing machine in which a liquid developer is sprayed on both sides of the sheet. The developer dissolves the soft resist and the hard resist survives the developer treatment and continues to be on the sheet metal.

At the end of this process, the sheet has the resist remaining on its surface where the metal should remain after etching while the rest of the sheet has exposed metal where the metal is to be removed.

8. Etching:

The next step is to pass the sheet through an etching machine. Here, the chemical reagent is sprayed on both sides of the sheet.

The areas of the sheet covered with resist materials don’t let the etchant react with the metal while the exposed area of the sheet reacts to the etchant causing the metal to melt due to change in chemical properties.

This metal removal fabricates the designed shape on the sheet. The whole process is driven in a fully automatic closed machine with rolling conveyors to carry sheet metal from one end of the machine to the other.

Factors to be considered in the selection of etchant:

Quality of surface finish required.
Type of base material used.
Type of resist being used.
Depth of etching.
Cost and availability.

The type of etchant used in photochemical milling differs in condition to the base material being used. Few are listed below:

Sheet material	Etchant
Ferrous alloys (carbon steels, alloy steels, stainless steels, etc.)	Ferric Chloride
Non-ferrous alloys (beryllium copper, brass, etc.)	Cupric Chloride
Titanium, Zinc, and their alloys	Copper-based proprietary etchant
Silver and its alloys	Ferric Nitrate
Gold and its alloys	Iodine based etchant

9. Remove resist coat:

The sheet is further passed through a resist stripping machine. The resist that remained on the sheet to save the sheet from the reaction of etchant is removed by spraying the resist strip liquid.

The resist stripper dissolves all the resist material from the workpiece. This is a process similar to developing, while the concentration of the developer used here is greater than that was used in the developing process to dissolve soft resist.

10. Washing and drying:

Once the resist is removed from the sheet, the sheet is washed under pressurized cold water and dried up by hot blowers.

Since this process is widely used for the production of several similar components on a single sheet, each edge is separated by a light push of hand on metal. The result is the finished product.

Yellow Room:

A sequence of operations from the ‘coating of resist’ to the end of the ‘developing’ process is performed in a yellow room.

All of these works are performed under the use of yellow fluorescent light tubes (YFT) to prevent unwanted exposure of photoresists to light of a shorter wavelength.

Units and equipment employed in PCM:

The basic systems required for processing Photochemical machining are drafting units, sheet metal cutting machinery, drawing layout and accessories, CAD platform and developing systems, photographic printing machine and accessories, job cleaning systems, numerically controlled UV light machine, and etching machine, automatic washing units, mask removing units or resist stripper machine and finally drying units.

Applications of Photochemical machining:

Here is the following application of Photochemical machining:

Manufacturing of surgical tools, microscopic components.
Production of metallic gaskets and seals.
Manufacturing of fine screens, fine meshes, electric contacts, and EMI & RFI shielding cans.
Engraving of thin components.
Manufacturing of fuel cell components, pressure membranes and heat sink and flexible heating elements.

Advantages of Photochemical machining:

Following are the advantages of Photochemical machining:

Components that are impossible to manufacture by mechanical systems can be manufactured easily.
Real-time material removal from all surfaces.
Production of components by PCM results in a burr-free surface.
Components manufactured by this method are stress-free and crack-free.
Miniature components of any material can be processed easily.
Extremely thin metals can be machined.
Quick and inexpensive.
Extremely complex contours can be produced with accurate details.

Extremely thin component manufactured in PCM

Photochemical Machining Disadvantages:

Following are the disadvantages of Photochemical machining:

High setup cost.
Not suitable for low volume production.
The fuming associated with the process can cause health issues to the operator.
Higher depth of cuts cannot be achieved because of undercutting.
The thickness of the sheet metal is limited to 2.032 mm.

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Reference [External Links]:

Conclusion:

The photochemical machining process is one of the best methods to fabricate sheet metals with complex, micro, and thin cuts. The process is based on the quality of resist and the principle of UV light, the etching, and the cleaning.

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