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Cranfield University

Machining, moulding, metrology assignment

Comparison and contrasting of laser machining and photochemical machining for the manufacture of complex geometries in sheet metals

M. Alvarez, J. J. Castro, A. Fischer,

N. Macura, L. Petkov, A. Rodriguez,

Abstract

This paper focuses on the comparison of two non-conventional processes which are laser machining and photochemical machining. The purpose is to analyse both processes machining complex geometries in metal sheets. Laser Machining is a non-contact method where the material is heated up by applying high power concentrated beam to part of it where the metal is melted or in some cases evaporated. Photochemical machining combines photoresist image modelling and chemical etching to manufacture the desired finished product by controlled corrosion.

Introduction

The first thinking about machining has been always the process of material being removed by contact between tool and work piece causing a shear stress. Such behaviour is typical from the traditional machining point of view.

However, the term machining applies to a wider variety of concepts. One of them are the non-traditional methods, also known as advanced machining processes (AMPs) where no shear deformation occurs [1].

Non-traditional methods are usually utilised for purposes where traditional machining cannot be applied due to certain constraints such as repeatability or productivity.

There are four different classes of non-traditional methods: mechanical (based on ultrasonic), electrical (based on electrical energy), thermal (based on heat generation), and chemical (based on chemical reactions).

The industrial applications for the latter methods were initially for heavy industry and aerospace as well as ultrathin materials such as copper printed boards and silicon integrated circuits.

Photochemical machining (PCM), also known as photo-etching, is part of the chemical processes and its principles rely on a redox process which causes material dissolution by etchant reduction without affecting the physical or chemical properties of the material.

In the beginning, PCM was seen as an alternative to stamping for 2D complex geometries. Nowadays, after some policy issues, the Photo Chemical Machining Institute (PCMI) provides feedback about the main trends and challenges of the sector within Europe and USA. Moreover, another similar organisation is held in Japan too [2].

Regarding the materials used in the market, the less corrosive a material is, the better, as etching relies on a rapid and controlled corrosion process. Nonetheless, the use of corrosive etchants allows companies to make use of more corrosion resistant components. In terms of material usage within the industry, Stainless Steel is the most employed, followed by copper, nickel and iron alloys [3].

Laser Beam Machining (LBM) belongs to the thermal energy processes as mentioned above. It characterises for melting, vaporising or degrade chemically the undesired part of raw material, which is no longer needed to obtain a finish product. It is one of the most useful non-contact machining processes for its capability to machine complicated profiles and accomplish miniatures in sheet metal in a wide variety of materials [4].

The most worldwide lasers currently employed are those using CO2 and Nd:YAG. The first one seems to have a high number of applications such as cutting, welding or even medical surgery, whereas the second one owns some unique characteristics which makes Nd:YAG more recommended for brittle materials, such as optical materials, silicon and germanium. In spite of having a lower mean beam power, its high beam intensity (because of a lower pulse duration and a better focus), its less kerf width, smaller heat affected zone(HAZ) and better cut edge kerf profile make Nd:YAG more suitable to do the job [5].

The uses of LBM are innumerable, from cutting or drilling to marking or welding although its major function is for cutting metallic and non-metallic sheets [4].

1. The process

photochemical machining laser metal

Laser Machining

Laser is an abbreviation for “Light Amplification by Stimulated Emission of Radiation.” Lasers were initially introduced as welding, bending, cutting tools in the early 70s. To comprehend this tool, the properties of laser light should be known. A laser beam by itself is not doing much. To become a tool, it needs to be guided, shaped, bundled, and integrated into current machines [6].

The specific properties contribute to this [7]:

§ Laser light is monochromatic.

§ The light waves are pulsating at the same frequency, this is known as coherence.

§ The light waves are virtually parallel to each other. Consequently, the laser beam only broadens to a very minor degree.

§ The power concentration of the laser beam is much higher than that of the traditional light source.

§ The power of the laser beam generally used for machining is focussed in the centre of the cross-section and diminishes toward the edges (Gaussian distribution). The brightest light known is the one from a laser. It can burn a hole in steel because of its intense heat [7].

The operation basics of the laser are the following steps (Figure 1) [8]:

(1) In a sealed container is placed a light producing material (liquid or a gas)

(2) The material is agitated by an electrical cause

(3) This makes the atoms or molecules to release light

(4) Different rays of light sent by the atoms are combined together in a bright beam to go through a mirrored opening as a laser light which is a controlled beam.

Figure 1 Principles of a laser [8].

2. Photochemical Machining

Photochemical machining (PCM) is a non-conventional manufacturing process based on the combination of photoresist imaging and chemical etching as a material removal method [3].

Figure 2 PCM process [3].

The PCM process (Figure 2) begins with the design of the photo tool according to the dimensional specifications of the part. On the other hand, the material to be etched often arrives as coils of material that it is then cut into sheets. Once the material is selected, the cleaning process removes the oils and greases from the surface of the sheet ready to be coated with a light-sensitive resist. The metal sheet is covered with a photoresist, which will protect the unexposed metal. Before being exposed to ultra violet light, the workpiece is covered with a stencil from one side (engraving or making recesses) or both sides (producing tapered holes). During the UV exposure the photoresist becomes hardened and the pattern is created. The exposed metal areas are etched (by immersing in a bath or by spraying). After rinsing thoroughly, the photoresist is removed from the already made component and rinsed again [3][9][10].

3. Applications

Laser Machining

This part introduces the applications for the manufacture of complex geometries in sheet metals in one and two dimensions. The applications are various from small series or prototype to larger lots.

Laser machining produces finite items with higher accuracy and surface finishing than conventional machining processes.

However, overall economics and quality requirements must be considered before opting for a laser drilling. Applications in laser machining can be divided into: drilling, cutting, scribing or marking [11].

Laser drilling

Laser drilling is very useful and efficient for high aspect ratio micro drilling applications which traditional machining cannot ensure. For instance, laser drilling can produce 0.05mm diameter holes at a rate of 1ms/hole [11] as well as a 15-90 range angles. The most common applications are in the components of turbine blades, combustion chambers and aerosol nozzles [12].

Laser cutting

Chryssolouri [12] explains laser cutting as a good way to produce complex two dimensional shapes up-to 15mm thick metal sheet with high cutting speed. Nonetheless, it can be associated with defects such as striations, dross or heat-affected zones. Hence, if careful parameters are set for a given application, the quality of the finite parts can be acceptable [11].

Laser scribing and marking

Chryssolouris [12] shows laser scribing is only useful for identification labels of a finish product. It has been created to be economically viable. The main used area is currently in the electronics, cosmetics, food and beverage, and optical industries [12].

Photochemical Machining

Components manufactured within the PCM field have a relatively low thickness (typically under 3 mm), complex shapes and flat surfaces. In contrast to LBM, simultaneously machining of different features is possible. Basically, the corrosion effect provides a wide variety of machining processes.

The typical industrial sectors in which this machining method is applied are electronics, mechanics and aerospace. Some of the main products are the integrated circuit lead frames, components of high value hand-watches and prototyping elements. The last example is well-known because of his high accuracy, reduction of lead time and tool effort [13].

4. Comparison between PCM &LBM

This section shows the differences and similarities of both PCM and LBM technologies.

A range of different parameters are taking into account in order to know which technology is more suitable for a specific application.

Table 1 Process comparison of PCM & LBM [14].

Another point of view is establishing a comparison between volume quantity and thickness.

Figure 3 Batch size VS Complexity diagram 0.5mm thickness [14]

Figure 4 Batch size VS Complexity 2mm thickness [14]

In Figure 3, PCM and LBM cover the same volume quantities, but PCM can accomplish more complex items. On the other hand, as the thickness increases laser cutting cope a much ampler area in terms of complexity and volume quantity leaving only place for photochemical machining in complex products due to the long-time processing. (Figure 4).

In addition to that, PCM takes less time to manufacture the components apart from their complexity. The process allows producing components simultaneously by applying a mask for a certain lot size whereas LBM is a step by step process.

5. Limitations & challenges

The photochemical and laser machining are surely one of the most novel manufacturing methods. Although, among many advantages, there are still some issues that have to be targeted in order to enhance the sustainability of manufactured products.

One of the main problems of the PCM is its negative environmental impact, that should be reduced by the use of the organic solvents [15].

What is more, in order to allow manufacturing more complex shapes (3D), complementary processes like Laser Direct Image Technology are being introduced [16].

On the other hand, one of the challenges that laser machining faces is the damage of the shaped material that may be caused by high temperature (HAZ) [17]. In fact, more research is needed on the optimization of process variables to avoid the damage of the surrounding material during the cut of the workpiece. Moreover, during laser cutting, the sides of the material edges as well as the bottom may be deposited with a layer of metal that has been previously re-melted. Dubey and Yadava [4] call it a recast layer. The deposition of this material is an stress source that may act as a crack concentrator reducing the quality of the part [4].

While designing the final part, there are certain limitations that have to be taken under consideration. Main ones are: thickness and composition of the material, geometry of the part, finish of the sheet surface and properties of the material like heat conductivity or vaporization point. All of the characteristics mentioned affect the cutting speed and have an impact on the overheating of the surrounding material and on the deterioration of the edge quality. To provide more selective heating and higher efficiency of the cut, the direct-diode laser technology is being developed [18].

In terms of future development of LBM technology, 3D machining can be pointed out [4]. The problem that must be faced in this advanced method is the simultaneous control of two or more laser beams at different angles [4].

Conclusions

To conclude, PCM and LBM are a great alternative to traditional methods for manufacturing complex geometries in sheet metals. In order to assess which method is more suitable for a certain process, the analysis of different parameters has to be taken into account.

Although both can deal with the same range of hardness, the production costs may differ depending on the batch size. For high manufactured volumes PCM has an advantage over the LBM. The cost associated with the process relies fundamentally on the complexity of the product. PCM has an economic advantage over LBM as the parts can be more complex and machined simultaneously [19].

On the other hand, LBM presents better tolerances, less environmental impact, faces a wider range of thickness and has a potential advantage in using 3D technology.

In contrast, PCM parts are not subjected to stress due to HAZ because it operates at low temperatures, but it can present a `bevelled' profile on the machined edge.

Finally, in order to become more competitive on the global market, improving productivity and quality of the product, some companies are trying to mix PCM and LBM together.

References

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