optimization of photochemical machining

OPTIMIZATION OF PHOTOCHEMICAL

MACHINING

ATUL R. SARAF

Department of Mechanical Engineering,Dr.Babasaheb Ambedkar Technological University, Lonere, Raigad, Maharashtra State, India

[email protected]*

Dr. M.SADAIAH


[email protected]

SANTOSH DEVKARE


[email protected]

Abstract :

This paper presents the machining of Oxygen-Free High Conductivity (OFHC) copper using

Photochemical Machining (PCM). Twenty-seven experimental runs base on full factorial (33) Design of

Experiments (DoE) technique were performed. The control parameters considered were the etchant

concentration, etching temperature and etching time. The response parameters were Undercut (Uc) and the

Surface roughness (Ra). The effects of control parameters on undercut and surface roughness were analyzed

using Analysis of Variance (ANOVA) technique and their optimal conditions were evaluated. An optimum

surface roughness was observed at an etching temperature of 55 °c, an etching concentration of 600 gm/litre and

15 minutes etchings time. The minimum undercut (Uc) was observed at the etching temperature of 45 °c, etching

concentration of 600 gm/litre and 15 minutes etching time. It was found that etchant temperature and etching

time are the most significant factors for undercut.

Keywords: PCM, DoE , ANOVA, Surface Roughness, Undercut.

Atul R. Saraf et al. / International Journal of Engineering Science and Technology (IJEST)

ISSN : 0975-5462 Vol. 3 No. 9 September 2011 7108

1. Introduction

The PCM is one of the non-conventional machining processes that produces a burr free and stress free

flat complex metal components. The machining takes place using a controlled dissolution of work-piece

material by contact of the strong chemical solution. The PCM industry currently plays a vital role in the

production of a variety of precision parts viz. microfluidic channels, silicon integrated circuits, copper printed

circuit boards and decorative items. It is mainly used for manufacturing of micro-components in various fields

such as electronics, aerospace and medical. It employs chemical etching through a photo-resist stencil as a

method of material removal over selected areas. This technique is relatively modern and became established as a

manufacturing process. The newly-developed products made from PCM especially relevant to Micro-

engineering, Micro-fluidics and Microsystems Technology.

OFHC copper is an important material for various engineering applications, as because of its excellent

electrical and thermal conductivity, easy fabrication, and good strength and fatigue properties. An aqueous

solution of ferric chloride (FeCl3) is the most commonly used etchant. Less literature is available on the

photochemical machining of OFHC copper. In the literature, Cakir (2006) studied that the copper etching with

CuCl2 etchant and a suitable regeneration process of waste etchant simultaneously. This would reduce the

overall manufacturing cost and environmentally friendly etching process which ultimately enhances the

productivity. David et al. (2004) studied the characterization of aqueous ferric chloride etchant used in industrial

photochemical machining. He found that FeCl3 is most commonly used etchant with a wide variety of grades.

Rajkumar et al. (2004) investigated the cost model for PCM which defines standards for industrial etchants and

methods to analyse and monitor them. Cakir (2004) stated that the etch rate of FeCl3 is higher than CuCl2 but

CuCl2 gives a better surface finish than FeCl3 during machining of Cu-ETP copper.

From literature, it is observed that no statistical study has been reported to analyse the interaction effects of

process parameters on etching process of OFHC Copper. Till date an accuracy of PCM depends only on the skill

and experience of the operator. Work to date an optimal set of process parameters is not calculated. The

statistical study is necessary to investigate the performance in different ranges as well as to find the global

optimum process parameters. It is also necessary to find out the single optimum process parameters setting to

satisfy the requirements of excellent etching quality. In this work, an attempt is made to study the optimum

process parameters by using full factorial design method and an ANOVA technique.



2. Experimental work

In this work, Ferric Chloride (FeCl3) was used as an etchant for machining of OFHC copper. The chemical

composition of OFHC Copper is shown in Table 1. The etchant, FeCl3 was prepared in 600,700, and 800

gm/litre respectively. For each experiment 5000 ml. of etchant was used.

Table 1. Chemical composition of OFHC copper.

Grade Type Cu P Tellurium Cr Ni P S

Copper OFHC 99.99 0.0003 0.0010 - - - -

Etchant was poured into etching bath that is coated by insulator jacket to control chemical etching temperature

and also partially air tight is shown in Fig. 1. Photochemical machining tests were carried out at 45 ◦C, 50 ◦C

and 55 ◦C. The temperature was maintained within a range of ±1 ◦C. Single-sided chemical etching process was

used. The etching times were 15, 20, and 25 minutes.

Fig. 1. Experimental set-up for etching.

The DOE technique was used to design the experiments to know the interaction of process parameters. Each

parameter was designed to have three levels namely low, medium and high respectively (Number of treatment

conditions = 3k = 33 = 27) using a full factorial DoE technique. The experimental design matrix was a simplified

method of putting together an experiment (Table 2).



Table 2 . Experimental design matrix.

Sr.No. Temperature

(Co) Concentration

(gm/litre) Time

(minute)

1 45 600 15 2 45 600 20 3 45 600 25 4 50 600 15 5 50 600 20 6 50 600 25 7 55 600 15 8 55 600 20 9 55 600 25

10 45 700 15 11 45 700 20 12 45 700 25 13 50 700 15 14 50 700 20 15 50 700 25 16 55 700 15 17 55 700 20 18 55 700 25 19 45 800 15 20 45 800 20 21 45 800 25 22 50 800 20 23 50 800 25 24 55 800 15 25 55 800 20 26 50 800 15 27 55 800 25

3. Results and discussion

The analysis of experimental data was performed in order to determine the effect of temperature, time

and concentration on the magnitude of undercut and surface roughness. The analysed results are presented using

main effects and interaction plots.

3.1 Statistical analysis of the undercut (Uc)

The summary of analysis of variance (ANOVA) is shown in Table 3. It is observed that the factor undercut has a

significant effect at 95% confidence interval as evident from ANOVA Table 3. It is observed that etching time

and etchant temperature has a large contribution on the variability of undercut among the selected parameters.

However, the concentration has negligible effect on undercut. The ANOVA for undercut shows that the time

and temperature are the most significant parameters for undercut.



Table 3. ANOVA for undercut (mm).

Control factor Degree of freedom

Sequential sum of squares

Adjusted sum of squares

Adjusted sum of squares F-ratio p-value

Temperature (0C) 2 0.0060512 0.0060512 0.0030256 193.35 0.000

Concentration(gm/litre) 2 0.0000223 0.0000223 0.0000111 0.71 0.519

Time (Minute) 2 0.0262614 0.0262614 0.0131307 839.12 0.000

Temperature* Concentration

4 0.0004097 0.00040970.0001024

6.55 0.012

Temperature*Time 4 0.0007259 0.0007259 0.0001815 11.60 0.002

Concentration*Time 4 0.0011228 0.0011228 0.0002807 17.94 0.000

Error 8 0.0001252 0.0001252 0.0000156

Total 26 0.0347185

S=0.00395577 R-sq.=99.64% R-sq. (Adj.) = 98.83%

Etching process if prolonged, undercut was always going to be more, as etching starts in sidewise with

downward action also. The evaluation of undercut as a function of temperature, concentration and time is shown

in Fig. 2.

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0.16

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0.12

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0.18

0.16

0.14

0.12

0.10

Tepmerature

Mea

n

Time

Concentration

Main effects plot for undercut Uc (mm)Fitted Means

Fig. 2. Main effect plots for undercut Uc (mm).

The main effects plots show that the response undercut was increased with increasing in etch time and

temperature. But initially at lower level undercut was very small and it was very high at higher levels. The

parameters that have greater influence on undercut in the decreasing order of importance are temperature, time



and concentration as shown in Fig. 2. The temperature has the most dominant effect amongst all the main

parameters. Further, it is better to set the concentration, time and temperature at a lower level.

It is observed from the main effects plots (Fig. 2) that the optimum etching performance parameters for

the undercut were observed at a temperature of 45 °c, a concentration of 600 gm/litre and etching time of 15

minutes. Fig. 3 shows the interaction plot for undercuts, with each plot exhibits the interaction between two

different machining parameters.

0.20

0.15

0.10

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0.15

0.10

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0.20

0.15

0.10

T epmerature

Concentration

T ime

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(°c)Tepmerature

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(gm/litre)Concentration

152025

(minutes)Time

Interaction plot for undercut Uc (mm)Data Means

Fig. 3. Interaction plots for undercut Uc (mm).

An interaction between factors takes place when the variation in response from a level of a factor to another

level varies from the change in response in the same two levels of a another factor; this implies that the effect of

one factor is dependent upon another factor.

3.2 Statistical analysis of surface roughness value (Ra)

The summary of analysis of variance (ANOVA) is shown in Table 4. It is observed that the factor Ra has a

significant effect at 95% confidence interval as evident from ANOVA Table 4. It is also observed that etching

time and the enchant concentration have a large contribution on the variability of Ra among the selected

parameters. However the temperature has negligible effect on surface roughness value.



Table 4. ANOVA for surface roughness Ra (µm).

Control factor Degree of freedom

Sequential

Sum of squares

Adjusted sum of squares

Adjusted mean of squares

F-ratio p- value Significant (Yes / No)

Temperature (0C) 2 0.07509 0.07509 0.03754 0.41 0.677 NO

Concentration (gm / litre)

2 1.54596 1.54596

0.77298 8.43 0.011 YES

Time (Minute) 2

7.51387

7.51387

3.75693

40.98

0.000 YES

Temperature * Concentration

4 3.06522 3.06522

0.76631 8.36 0.006

YES

Temperature* Time

4 0.04231 0.04231 0.01058

0.12 0.973 NO

Concentration* Time

4 1.99484 1.99484

0.49871 5.44 0.020

YES

Error 8 0.73338 0.73338 0.09167 Total 26 14.97067

S = 0.302774

R-Sq = 95.10%

R-Sq(adj.) = 84.08%

It is observed etching time is the most significant parameter which affects the surface roughness (Ra).

Etching process if prolonged; Ra was always going to be higher. Fig. 4 shows the evaluation of surface

roughness as a function of temperature, concentration and time. According to the main effects plot, we can

evidence that response Ra was increased with increasing in the etch time.

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1.0

Temperature

Mea

n

Concentration

Time

Main effects plot for surface roughness Ra (µm)Fitted Means

Fig. 4. Main effect plots for surface roughness value Ra (µm).



Initially, Ra was very small. It increases as time increases and remains approximately constant for an increase in

the temperature. The effect of concentration on the surface roughness shows different trends than temperature

and time. The value of Ra increases initially and then starts reducing. The time has the most dominant effect

amongst all the main parameters. After analysing, it is advisable to set the temperature at a higher level, time

and concentration at lower levels.

It is found from the main effects plots (Fig. 4) that the optimum etching performance parameters for the

Ra value were observed at a temperature of 55 °c, a concentration of 600 gm/litre and time of 15 minutes. Fig. 5

shows the interaction plots of Ra in the PCM process. Two way interactions among the control parameters were

assessed for their effects on the surface roughness.

3

2

1

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2

1

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2

1

T emperature

Concentration

T ime

455055

(°c)Temperature

600700800

(gm/liter)Concentration

152025

(minutes)Time

Interaction plot for surface roughness Ra (µm)Data Means

Fig. 5. Interaction plots for surface roughness Ra (µm).

Fig.5 shows the interaction plots among the three process parameters for their effect on surface roughness.

4. Conclusions

From the experimental investigations based on full factorial method and the analysis of the results, the

following conclusions are drawn.

It is observed from the ANOVA, the input variables time and temperature both have statistically

significant effects on the undercut. It is also observed from the ANOVA that the input variables

time and concentration both have statistically significant effect on the surface roughness value Ra.



The above discussion confirmed the validity of the full factorial methodology for enhancing the

etching performance and optimizing the etching parameters. The undercut and surface roughness

are greatly improved by this approach.

An optimum surface roughness (Ra) was observed at 55 °c of etchant temperature, 600 gm/litre of

etchant concentration and 15 minutes etching time. For minimum Undercut (Uc), the temperature

was 45 °c, etchant concentration 600 gm/litre and etching time 15 minutes.

References

[1] O. Cakır, 2006, “Copper etching with cupric chloride and regeneration of waste etchant”, journal of materials processing technology, 175, 63–68.

[2] David M. A., 2004, “Photochemical Machining: From Manufacturing’s Best Kept secret to a $6 Billion per annum”, Rapid Manufacturing Process, CIRP Journal of Manufacturing Systems,53, 559-573.

[3] David M. A., Heather, J.A. Heather J.A., 2004, “Characterization of aqueous ferric chloride etchants used in industrial photochemical machining”, Journal of Materials Processing Technology, 149, 238–245.

[4] Davis, P.J., Overturf, G.E., 1986, ”Chemical machining as a precision material removal process”, precision engineering , 67-71. [5] Douglas, C. M.,1997, Design and Analysis of Experiments. Fifth Edition, John wiley and sons, INC. [6] Rajkumar, R., Heather J.A., Oscar Zamora., 2004, “Cost of photochemical machining”, Journal of Materials Processing Technology,

149, 460–465.



optimization of photochemical machining

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