Photochemical machining

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2013

This fast and efficient process offers a cheap solution in manufacturing of precise metal parts.

KEY CONCEPTS
• Photochemical machining (PCM) has been used for about 50 years to manufacture precision metal parts.
• PCM is an efficient process that can be used to produce a wide range of products for a variety of different industries.
• One key benefit of PCM is it does not cause any changes in the properties of the metal substrate.

METAL MACHINING IS CONTINUING TO BE MORE CHALLENGING as end-users are looking to manufacture smaller parts to tighter tolerances. They are also demanding that machining be done more efficiently using less energy in order to maximize productivity. Flatness, stress and fatigue strength are very important in machining of intricate parts— hence material properties must not be compromised during part production.

In a previous TLT article, a new technique known as friction-stir was discussed that enables metal to be fabricated below its melting point (1). Friction-stir uses the frictional heat produced by a tool rotating against the work material at high speeds and high pressure conditions. The mechanical energy produced is converted into frictional heat. This process uses less energy as compared to the currently used melting and casting process.

In the preparation of precise metal parts, there are a number of techniques that can be used, including mechanical drilling, electric-discharge machining (EDM) and laser machining. Another option that has been used successfully is photochemical machining (PCM).

Hassan Nojoumi, president of Shimifrez, Inc., in Toronto, Ontario, Canada, says, “PCM represents a number of operations including precise photochemical machining, photochemical machining, photochemical etching and photochemical milling. The procedure essentially creates a specific design on a piece of Mylar mass that is transferred onto a specific metal substrate after etching is done to remove the unwanted metal.” In effect, the Mylar mass acts in a similar fashion to a stencil.

The specific process can be organized into the following five steps:

1. Thoroughly clean the metal substrate.
2. Apply a photoresistant coating to prevent certain areas of the substrate from becoming vulnerable to ultraviolet light.
3. Expose the metal substrate to ultraviolet light to form the specific parts image.
4. Introduce an etching solution to remove the unwanted metal from the substrate.
5. Clean the part and recycle the etching solution.

The important aspect is that the pattern photochemically placed on the metal substrate is not affected by the etching solution used.

Nojoumi says, “The use of etching has been around for about 2,000 years. The actual PCM technology has been in use for about 50 years.”

BENEFITS AND APPLICATIONS
Nojoumi sees PCM as a very fast and efficient process compared to stamping, laser cutting and EDM. He says, “PCM is cheaper and faster with the added benefit of not causing any changes in the properties of the metal substrate.”

PCM will not cause any mechanical or thermal stresses to occur to the metal substrate. Nojoumi says, “In contrast, stamping, punching and die-cutting impart shearing deformation and laser and water-jet cutting can leave ablative deformation.”

No problems with tool wear are seen with PCM. Tolerances are typically a function of the thickness of the metal substrate and are usually +/- 10% of the metal thickness.

Many different types of parts can be prepared using PCM. Nojoumi says, “A wide range of products in a variety of different industries (aerospace, electrical, medical and optical) can be prepared. Some examples include nozzle heads, flow orifices, evaporation masks, fuel cell plates and flat springs/ contacts. In the medical field, PCM is used to prepare medical diagnostic equipment and body implants.”

Another field well suited to using PCM is nanotechnology. Nojoumi indicates that PCM is used to prepare microelectromechanical systems.

Figure 3 shows some of the metal parts that can be prepared using PCM.


Figure 3. Examples of intricate metal parts produced through the use of photochemical machining (PCM) are shown. (Courtesy of Shimifrez, Inc.)

PCM can be utilized on most metal alloys including ferrous alloys and aluminum. Nojoumi says, “The only metals that are a little difficult to work with are titanium and tungsten because both are difficult to etch.”

The main etchants used in most applications are ferric chloride and cupric chloride. Nojoumi says, “We tailor the etchant to meet the specific metal alloy used and the application. In many cases, this requires changing the composition and concentration of the etchant. One other chemical that can be used with ferric chloride is hydrofluoric acid.” The etchant is typically sprayed on the metal part as an aqueous solution.

PCM is mainly used to machine parts with a thickness less than one millimeter. Nojoumi explains, “The reason for the limitation on the thickness is that it takes time for the etching chemical to dissolve the metal. For applications with a greater thickness, another application might be preferred.”

Once the etchant is used, an aqueous cleaning solution is applied to clean the metal substrate.

The entire PCM process can be completed in a matter of a few days. Tool wear is not a problem, so the only factor that needs to be evaluated is whether the process itself needs to be changed. Nojoumi says, “Manufacture of the part takes less than 72 hours from drawing to the actual machining operation.”

Further information on PCM can be found in a recent article in R&D Magazine (2) or by contacting Hassan at photoetch@shimifrez.com.

REFERENCES
1. Canter, N. (2011), “Friction-Stir: Alternative to Melting and Casting Metal,” TLT, 67 (12), pp. 8-9.
2. Nojoumi, H. (2012), “Engineering Intricate Metal Parts,” R&D Magazine, 54 (7), pp. 26-27.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.