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Measuring the solidification time of silicone rubber using optical inspection technology
Optik 125 (2014) 196–199
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Measuring the solidification time of silicone rubber using optical inspection technology Chil-Chyuan Kuo ∗ , Yu-Teng Siao Department of Mechanical Engineering and Graduate Institute of Electro-Mechanical Engineering, Ming Chi University of Technology, No. 84, Gungjuan Road, Taishan District, New Taipei City 24301, Taiwan
a r t i c l e
i n f o
Article history: Received 1 February 2013 Accepted 10 June 2013
Keywords: Silicone rubber mold Solidification time Temperature Solidification mechanism
a b s t r a c t A room temperature vulcanization silicone rubber was widely used as the mold making material due to its high elasticity, good heat-resistance and low surface energy. To enhance the efficiency of making the silicone rubber mold, accurately measuring the solidification time is an important issue. This study demonstrated a non-invasive measurement system to measure the solidification time of silicone rubber. The solidification time can be determined rapidly from the thickness of silicone rubber according to the predicted equation. The maximum relative error of the predicted equation is about 8.26%. The temperature rise of the silicone during the solidification process is an important phenomenon to determine the solidification behavior of silicone rubber. The solidification mechanism of silicone rubber mold is demonstrated. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction As the manufacturing industry grow, the demand for precision meal working prototypes increases. To reduce the time and cost of product development, rapid prototyping (RP) was developed [1,2], which offers the potential to completely revolutionize the process of manufacture. However, the features of the prototype do not usually meet the need of the end product with the required material. Rapid tooling (RT) technologies are then developed because it is the technology that uses RP technologies and applies them to the manufacturing of mold inserts [3]. Since the importance of RT goes far beyond component performance testing, RT is regarded as an important method of reducing the cost and time to market in a new product development process. Several RT technologies are commonly available in industry now. RT is divided into direct tooling and indirect tooling [4]. Direct tooling means fabricating mold inserts, core and cavity, directly on a RP machine such as selective laser melting technology [5]. Indirect tooling means fabricating mold insert by master pattern fabricated by various RP technologies. Soft tooling is widely used for low volume production. Materials used for soft tooling own low hardness levels such as silicone rubber and epoxy resins. Conversely, hard tooling is associated with higher volume of production. Materials used for hard tooling often have high hardness levels. Soft tooling is easier to work with than tooling steels because these tools
∗ Corresponding author. Tel.: +886 2 29089899x4524; fax: +886 2 29063269. E-mail address: [email protected] (C.-C. Kuo). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.06.021
are created from materials such as epoxy-based composites with aluminum particles, silicone rubber or low-melting-point alloys. It is well known that RT is capable of replacing conventional steel tooling, saving the cost and time in the manufacturing process [6]. Indirect soft tooling is used more frequently in the development of new products than direct tooling because it is fast, simple and costeffective. Especially, the silicone rubber mold is widely employed since it owns flexible and elastic characteristics so that parts with sophisticated geometry can be fabricated [7–9]. Quick response to the market was considered as one of the important factors to ensure company competitiveness. Thus, new prototypes produced by silicone rubber mold must be swiftly and cost-effectively manufactured and introduced to the market. To enhance the efficiency of making the silicone rubber mold, accurate measuring the solidification time is an important issue. Although the solidification time can be determined by manual measurement, it includes certain disadvantages including human error, time consuming and exhaustive. This study demonstrated a non-invasive measurement system to determine the solidification time of silicone rubber. Based on the manual measurement result, the criterion for determining the solidification time of silicone rubber was proposed according to the changes both in the transmitted power and temperature obtained. Predicted equation for predicting the solidification time of silicone rubber mold was investigated. The applicability of the solidification time obtained using the predicted equation was investigated and compared with manual measurement. The relative error of the predicted equation was also discussed. The solidification mechanism of silicone rubber mold was finally proposed.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199
197
Transmitted power (mW)
4.7 4.5 4.3 4.1 3.9
Stabilization
3.7 3.5 0
10
20
30
40
50
60
70
Time (min.) Fig. 1. Schematic illustration of experimental set-up for measuring the solidification time of silicone rubber.
Fig. 2. Variation in transmitted power of probe laser during a warm-up.
Liquid silicone rubber
1.6
Solid silicone rubber 1.4 1.2 1 0.8 0.6 0.4 30
20
40
50
60
70
80
Thickness of silicone rubber (mm) Fig. 3. Transmitted power as a function of thickness of silicone rubber.
molecular structures of silicone rubber becomes compactly after solidification. Besides, the transmitted power density decreased with increasing the thickness of both solid and liquid silicone rubber [18]. Fig. 4 shows the criterion for determining the solidification time of silicone rubber. The transmitted power spectrum is divided into three stages. Stages A, B and C stand for liquid silicone rubber, semiliquid silicone rubber and solidified silicone rubber, respectively. The temperature spectrum is also divided into three stages. Stages D, E and F stand for semi-liquid state of silicone rubber, solidified silicone rubber and room temperature of silicone rubber, respectively. The results reveal that the temperature of silicone increase slightly during the solidification process, which is an important phenomenon to determine the solidification behavior of silicone rubber. Fig. 5 shows the temperature rise of silicone rubber as a function of thickness of silicone rubber. As can be seen, the gradual increase in the temperature of silicone rubber is due to an increase in the volume of silicone rubber. Fig. 6 shows the effect of probe laser on the temperature of silicone rubber. As can be seen, the temperature of silicone rubber for the experiment with the use of probe laser is slightly higher than that of silicone rubber for the experiment without the use of probe laser. This means that the 1.6
24.6 Solidification
It is a well-known fact that optical measurement method has the main advantage of reliability [15,16]. Fig. 2 shows the variation in transmitted power of probe laser during a warm-up. As can be seen, a warm-up time of at least 30 min for probe laser is required [17]. After warm-up of the probe laser, standard deviation of the transmitted power is reduced significantly to 0.000732 mW. Fig. 3 shows the transmitted power as a function of thickness of silicone rubber. As can be seen, the transmitted power density of solid silicone rubber is higher than that of silicone rubber because
)
1.55
3. Results and discussion
24.2 1.5
D
1.45
F
E
23.8
A
1.4
B
C
1.35 0
150
300
450
600
750
900
1050
Temperature (
Transmitted power (mW)
Fig. 1 shows the schematic illustration of experimental set-up for measuring the solidification time of silicone rubber. The optical measuring system consists of a He–Ne laser, an aperture [10–13], two focusing lenses, a rotary stage and a photodetector. He–Ne laser (NT 61-337, JSD Uniphase) emitting at a wavelength of 632.8 nm was used as a probe laser. The temperature measuring system consists of a k-type thermocouple (C071009-079, Cheng Tay) and a data acquisition system (MRD-8002L, Acqview). A room temperature vulcanization silicone rubber (KE-1310ST, Shin Etsu) was used as the mold making material. The amount of silicone rubber required was calculated by multiplying the desired volume of the silicone mold to be made by the density of silicone rubber. Depending on the extent of air bubbles in the mixture, the degassing process can range from 25 to 60 min. In general, the curing agent and silicone rubber in weight ratio of 10:1 was mixed thoroughly with a stirrer. The features of the silicone rubber mold such as durability and mold life are significantly affected by the relative amounts of curing agent and silicone rubber. Thus, calculating the weight of base and curing agent precisely is crucial before mixing. A homemade vacuum machine was used to extract the air-bubbles derived from the mixing process of the silicone rubber and curing agent under vacuum conditions to obtain bubble-free silicone rubber mold [14]. In general, the curing agent and silicone rubber in weight ratio of 10:1 was mixed thoroughly with a stirrer. Polymethylmethacrylate was chosen as the mold frame material for making silicone rubber mold. The height and wide of silicone rubber mold were fixed at 30 mm and 50 mm, respectively. Five different thicknesses of silicone rubber (30 mm, 40 mm, 50 mm, 60 mm and 70 mm) were first employed to investigate the correlations between the transmitted power and both the thicknesses of liquid and solid silicone rubber. Predicted trend equations for predicting the solidification time of silicone rubber mold was investigated. Three different thicknesses of silicone rubber (20 mm, 55 mm and 80 mm) were then employed to investigate the relative error of the predicted equation. Finally, the solidification mechanism of silicone rubber mold was proposed in this work.
Transmitted power (mW)
1.8
2. Experimental
23.4 1200
Time (min.) Fig. 4. Criterion for determining the solidification time of silicone rubber. Red dotted line stands for the solidification time obtained by manual measurement.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199 1.6
) 23.8
1.3 1.25
First trial
Second trial
Third trial
Temperature
1.2
0.4 30
40
50
60
70
0
80
150
300
450
26 With laser
)
25.8
Without laser
25.6
900
1050
Transmitted power (mW)
1.4
24.6
1.3 1.25
24.4
1.2
24.2
1.15 1.1
First trial
Second trial
Tird trial
Temperature
24
1.05 150
0
300
450
600
750
900
1050
23.8 1200
Time (min.) 1.1
Transmitted power (mW)
(c)
25.8
1.05
25.6 1
25.4
0.95
25.2
0.9
First trial
Second trial
Third trial
Temperature
25
0.85 0
150
300
450
600
750
900
1050
24.8 1200
Time (min.) 23
(d)
22.8
0.8
)
Transmitted power (mW)
0.9 0.85
22.6
0.75
22.4
0.7 22.2
0.65 0.6 0.55
First trial
Second trial
22
Third trial
Temperature
21.8
0.5 0
150
300
450
600
750
900
1050
21.6 1200
Time (min.) 0.7
23
(e)
22.8
)
0.65
22.6
0.6
22.4 0.55 22.2 0.5
First trial
Second trial
0.45
Third trial
Temperature
22 21.8
0.4 0
25.4
23.4 1200
24.8
(b)
1.35
Transitted power (mW)
effect of probe laser on the temperature of silicone rubber can be neglected. Fig. 7 shows the transmitted power and temperature as a function of measurement time. The solidification time of silicone rubber can be determined based on the criteria described above. Fig. 8 shows the solidification time as a function of five thicknesses of silicone rubber. The predicted equation, y = 9.43x + 215, can be obtained according to the curve fitting method. This means that the solidification time of silicone rubber (y) can be predicted from the predicted equation of y = 9.43x + 215 by the thickness of silicone rubber (x). It is noteworthy that the R2 stands for correlation coefficient. In general, a higher R2 value (maximum value = 1) means a better accuracy of the trend equation [19]. The R2 value is 0.9929, showing the predicted equation obtained is a good candidate for predicting solidification time of silicone rubber. Five solidified silicone rubber molds are shown in Fig. 9. To evaluate the relative error of the predicted equation, three different thicknesses of silicone rubber (20 mm, 55 mm and 80 mm) were further investigated in this work. Fig. 10 shows the solidification time obtained by predicted equation and manual measurement. As can be seen, there is an excellent agreement on the solidification time determined by predicted equation and manual measurement. The maximum discrepancy, however, is about 8.26%. The average relative error is 6.87%. This means that the non-invasive measurement system is robust and owns high accuracy for measuring the solidification time of silicone rubber mold. According to results described above, the solidification mechanism of silicone rubber is shown in Fig. 11. The surface of the liquid silicone rubber was solidified firstly. The inner liquid silicone rubber was then solidified progressively. It can be concluded that the advantages of this measurement system include cost-effective of hardware, simplicity in experimental setup, fast measurement speed and high measurement accuracy. A simple scheme for stabilizing commercially available He–Ne lasers oscillating has been presented Makinen and Stahlberg [20]. According to the normal
750
Time (min.)
Thickness of silicone rubber (mm) Fig. 5. Temperature rise of silicone rubber as a function of thickness of silicone rubber.
600
)
20
Temperature (
23.6
Temperature (
0.5
24
1.4 1.35
)
0.6
24.2
1.45
Temperature (
0.7
24.4
1.5
Temperature (
0.8
24.6
(a)
1.55
Temperature (
Transmitted power (mW)
Temperature rise (
)
0.9
150
300
450
600
750
900
Temperature (
198
1050
21.6 1200
Time (min.)
25.2
Fig. 7. Transmitted power and temperature as a function of measurement time for thickness of silicone rubber of (a) 30 mm, (b)40 mm, (c)50 mm, (d)60 mm and (e) 70 mm.
25 24.8 24.6
0
200
400
600
800
1000
1200
Time (min.)
Fig. 6. Effect of probe laser on the temperature of silicone rubber.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199
has broad application prospects in the development of new products using silicone rubber mold. The solidification time can be determined rapidly from the thickness of silicone rubber according to the predicted equation with the maximum discrepancy about 8.26%. The temperature rise of the silicone during the solidification process has been observed experimentally, which is an important phenomenon for determining the solidification behavior of silicone rubber. The center of the liquid silicone rubber started to solidify firstly and the surface of the silicone rubber was then solidified gradually.
1000
Solidification time (min.)
900
y=9.43x+215 R =0.9929
800 700 600 500 400 300 200 100 0 20
30
199
40
50
60
Thickness of silicone rubber (mm)
70
80
Acknowledgement
Fig. 8. Solidification time as a function of five thicknesses of silicone rubber.
The authors gratefully acknowledge the financial support of the National Science Council of Taiwan under contracts nos. NSC1012221-E-131-007 and NSC101-2815-C-131-001-E. References
Fig. 9. Five solidified silicone rubber molds. 1100 Predicted equation
Solidification time (min.)
1000
Manual measurement
900 800 700 600 500 400 300 0
10
20
30
40
50
60
70
80
90
Thickness of silicone rubber (mm) Fig. 10. Solidification time obtained by predicted equation and manual measurement.
Fig. 11. Schematic illustration of solidification mechanism of silicone rubber.
operating life of He-Ne laser, this system could be operated around 0.5–1.2 years by switching the probe laser every 18 h [17,21]. 4. Conclusions A non-invasive measurement system to determine the solidification time of silicone rubber has been developed. This system
[1] Y. Gao, J. Xing, J. Zhang, N. Luo, H. Zheng, Research on measurement method of selective laser sintering (SLS) transient temperature, Optik Int. J. Light Electron Opt. 119 (13) (2008) 618–623. [2] C.C. Kuo, Y.C. Tsou, B.C. Chen, Enhancing the efficiency of removing support material from rapid prototype parts, Materialwiss. Werkstofftech. 43 (3) (2012) 234–240. [3] C.C. Kuo, Y.R. Chen, Rapid optical inspection of bubbles in the silicone rubber, Optik Int. J. Light Electron Opt. 124 (13) (2013) 1480–1485. [4] G.R. Link, J. Fessler, A. Nickel, F. Prinz, Rapid tooling die cast inserts using shape deposition manufacturing, Mater. Manuf. Processes 13 (2) (1998) 263– 274. [5] H. Meier, Ch. Haberland, Experimental studies on selective laser melting of metallic parts, Materialwiss. Werkstofftech. 39 (9) (2008) 665–670. [6] C.C. Kuo, A simple and cost-effective method for fabricating epoxy-based composites mold inserts, Mater. Manuf. Processes 27 (4) (2012) 383–388. [7] C.C. Kuo, Z.Y. Lin, Development of bridge tooling for fabricating mold inserts of aspheric optical lens, Materialwiss. Werkstofftech. 42 (11) (2011) 1019– 1024. [8] C.C. Kuo, Z.Y. Lin, Rapid manufacturing of plastic aspheric optical lens, Materialwiss. Werkstofftech. 43 (6) (2012) 495–502. [9] C.C. Kuo, Z.S. Shi, Fabrication of a high precision silicone rubber mold for replicating wax patterns of cylinder heads, Indian J. Eng. Mater. Sci. 19 (3) (2012) 157–162. [10] C.C. Kuo, P.J. Huang, Rapid surface roughness measurements of silicone thin films with different thicknesses, Optik Int. J. Light Electron Opt. 123 (19) (2012) 1755–1760. [11] C.C. Kuo, J.H. Lee, Y.R. Chen, Effects of pinhole diameters on beam characteristics for silicon thin film optical inspection, Optik Int. J. Light Electron Opt. 123 (7) (2012) 641–644. [12] C.C. Kuo, Y.R. Chen, Rapid optical measurement of surface texturing result of crystalline silicon wafers for high efficiency solar cells application, Optik Int. J. Light Electron Opt. 123 (4) (2012) 310–313. [13] C.C. Kuo, In situ time-resolved optical measurements of a-Si thin films during excimer laser crystallization, Optik Int. J. Light Electron Opt. 122 (8) (2011) 655–659. [14] C.C. Kuo, M.Y. Lai, Development of an automatic vacuum degassing system and parameters optimization for degassing process, Indian J. Eng. Mater. Sci. 18 (6) (2011) 405–410. [15] I. Martincek, D. Pudis, D. Kacik, K. Schuster, Investigation of intermodal interference of LP01 and LP11 modes in the liquid-core optical fiber for temperature measurements, Optik Int. J. Light Electron Opt. 122 (8) (2011) 707– 710. [16] H. Jia, G. Xia, B. Wu, T. Jin, H. Lu, A novel optical polarimeter based on the signal width measurement of the waveform, Optik Int. J. Light Electron Opt. 122 (23) (2011) 2107–2109. [17] C.C. Kuo, C.S. Chao, Characterization of probe lasers for thin film optical measurements, J. Russ. Laser Res. 31 (1) (2010) 22–31. [18] P.K. Choudhury, Wong Keng Soon, On the transmission by liquid crystal tapered optical fibers, Optik Int. J. Light Electron Opt. 122 (12) (2011) 1061– 1068. [19] C.C. Kuo, Solidification velocity in liquid silicon during excimer laser crystallization, Appl. Phys. A Mater. Sci. Process. 95 (2) (2009) 573–578. [20] J. Makinen, B. Stahlberg, Long-term frequency stability and temperature response of a polarization-stabilized He–Ne laser, Measurement 24 (3) (1998) 179–185. [21] C.C. Kuo, C.S. Chao, Rapid optical measurement of surface roughness of BiFeO3 films for nonvolatile memory application, J. Russ. Laser Res. 31 (3) (2010) 239–244.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Measuring the solidification time of silicone rubber using optical inspection technology Chil-Chyuan Kuo ∗ , Yu-Teng Siao Department of Mechanical Engineering and Graduate Institute of Electro-Mechanical Engineering, Ming Chi University of Technology, No. 84, Gungjuan Road, Taishan District, New Taipei City 24301, Taiwan
a r t i c l e
i n f o
Article history: Received 1 February 2013 Accepted 10 June 2013
Keywords: Silicone rubber mold Solidification time Temperature Solidification mechanism
a b s t r a c t A room temperature vulcanization silicone rubber was widely used as the mold making material due to its high elasticity, good heat-resistance and low surface energy. To enhance the efficiency of making the silicone rubber mold, accurately measuring the solidification time is an important issue. This study demonstrated a non-invasive measurement system to measure the solidification time of silicone rubber. The solidification time can be determined rapidly from the thickness of silicone rubber according to the predicted equation. The maximum relative error of the predicted equation is about 8.26%. The temperature rise of the silicone during the solidification process is an important phenomenon to determine the solidification behavior of silicone rubber. The solidification mechanism of silicone rubber mold is demonstrated. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction As the manufacturing industry grow, the demand for precision meal working prototypes increases. To reduce the time and cost of product development, rapid prototyping (RP) was developed [1,2], which offers the potential to completely revolutionize the process of manufacture. However, the features of the prototype do not usually meet the need of the end product with the required material. Rapid tooling (RT) technologies are then developed because it is the technology that uses RP technologies and applies them to the manufacturing of mold inserts [3]. Since the importance of RT goes far beyond component performance testing, RT is regarded as an important method of reducing the cost and time to market in a new product development process. Several RT technologies are commonly available in industry now. RT is divided into direct tooling and indirect tooling [4]. Direct tooling means fabricating mold inserts, core and cavity, directly on a RP machine such as selective laser melting technology [5]. Indirect tooling means fabricating mold insert by master pattern fabricated by various RP technologies. Soft tooling is widely used for low volume production. Materials used for soft tooling own low hardness levels such as silicone rubber and epoxy resins. Conversely, hard tooling is associated with higher volume of production. Materials used for hard tooling often have high hardness levels. Soft tooling is easier to work with than tooling steels because these tools
∗ Corresponding author. Tel.: +886 2 29089899x4524; fax: +886 2 29063269. E-mail address: [email protected] (C.-C. Kuo). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.06.021
are created from materials such as epoxy-based composites with aluminum particles, silicone rubber or low-melting-point alloys. It is well known that RT is capable of replacing conventional steel tooling, saving the cost and time in the manufacturing process [6]. Indirect soft tooling is used more frequently in the development of new products than direct tooling because it is fast, simple and costeffective. Especially, the silicone rubber mold is widely employed since it owns flexible and elastic characteristics so that parts with sophisticated geometry can be fabricated [7–9]. Quick response to the market was considered as one of the important factors to ensure company competitiveness. Thus, new prototypes produced by silicone rubber mold must be swiftly and cost-effectively manufactured and introduced to the market. To enhance the efficiency of making the silicone rubber mold, accurate measuring the solidification time is an important issue. Although the solidification time can be determined by manual measurement, it includes certain disadvantages including human error, time consuming and exhaustive. This study demonstrated a non-invasive measurement system to determine the solidification time of silicone rubber. Based on the manual measurement result, the criterion for determining the solidification time of silicone rubber was proposed according to the changes both in the transmitted power and temperature obtained. Predicted equation for predicting the solidification time of silicone rubber mold was investigated. The applicability of the solidification time obtained using the predicted equation was investigated and compared with manual measurement. The relative error of the predicted equation was also discussed. The solidification mechanism of silicone rubber mold was finally proposed.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199
197
Transmitted power (mW)
4.7 4.5 4.3 4.1 3.9
Stabilization
3.7 3.5 0
10
20
30
40
50
60
70
Time (min.) Fig. 1. Schematic illustration of experimental set-up for measuring the solidification time of silicone rubber.
Fig. 2. Variation in transmitted power of probe laser during a warm-up.
Liquid silicone rubber
1.6
Solid silicone rubber 1.4 1.2 1 0.8 0.6 0.4 30
20
40
50
60
70
80
Thickness of silicone rubber (mm) Fig. 3. Transmitted power as a function of thickness of silicone rubber.
molecular structures of silicone rubber becomes compactly after solidification. Besides, the transmitted power density decreased with increasing the thickness of both solid and liquid silicone rubber [18]. Fig. 4 shows the criterion for determining the solidification time of silicone rubber. The transmitted power spectrum is divided into three stages. Stages A, B and C stand for liquid silicone rubber, semiliquid silicone rubber and solidified silicone rubber, respectively. The temperature spectrum is also divided into three stages. Stages D, E and F stand for semi-liquid state of silicone rubber, solidified silicone rubber and room temperature of silicone rubber, respectively. The results reveal that the temperature of silicone increase slightly during the solidification process, which is an important phenomenon to determine the solidification behavior of silicone rubber. Fig. 5 shows the temperature rise of silicone rubber as a function of thickness of silicone rubber. As can be seen, the gradual increase in the temperature of silicone rubber is due to an increase in the volume of silicone rubber. Fig. 6 shows the effect of probe laser on the temperature of silicone rubber. As can be seen, the temperature of silicone rubber for the experiment with the use of probe laser is slightly higher than that of silicone rubber for the experiment without the use of probe laser. This means that the 1.6
24.6 Solidification
It is a well-known fact that optical measurement method has the main advantage of reliability [15,16]. Fig. 2 shows the variation in transmitted power of probe laser during a warm-up. As can be seen, a warm-up time of at least 30 min for probe laser is required [17]. After warm-up of the probe laser, standard deviation of the transmitted power is reduced significantly to 0.000732 mW. Fig. 3 shows the transmitted power as a function of thickness of silicone rubber. As can be seen, the transmitted power density of solid silicone rubber is higher than that of silicone rubber because
)
1.55
3. Results and discussion
24.2 1.5
D
1.45
F
E
23.8
A
1.4
B
C
1.35 0
150
300
450
600
750
900
1050
Temperature (
Transmitted power (mW)
Fig. 1 shows the schematic illustration of experimental set-up for measuring the solidification time of silicone rubber. The optical measuring system consists of a He–Ne laser, an aperture [10–13], two focusing lenses, a rotary stage and a photodetector. He–Ne laser (NT 61-337, JSD Uniphase) emitting at a wavelength of 632.8 nm was used as a probe laser. The temperature measuring system consists of a k-type thermocouple (C071009-079, Cheng Tay) and a data acquisition system (MRD-8002L, Acqview). A room temperature vulcanization silicone rubber (KE-1310ST, Shin Etsu) was used as the mold making material. The amount of silicone rubber required was calculated by multiplying the desired volume of the silicone mold to be made by the density of silicone rubber. Depending on the extent of air bubbles in the mixture, the degassing process can range from 25 to 60 min. In general, the curing agent and silicone rubber in weight ratio of 10:1 was mixed thoroughly with a stirrer. The features of the silicone rubber mold such as durability and mold life are significantly affected by the relative amounts of curing agent and silicone rubber. Thus, calculating the weight of base and curing agent precisely is crucial before mixing. A homemade vacuum machine was used to extract the air-bubbles derived from the mixing process of the silicone rubber and curing agent under vacuum conditions to obtain bubble-free silicone rubber mold [14]. In general, the curing agent and silicone rubber in weight ratio of 10:1 was mixed thoroughly with a stirrer. Polymethylmethacrylate was chosen as the mold frame material for making silicone rubber mold. The height and wide of silicone rubber mold were fixed at 30 mm and 50 mm, respectively. Five different thicknesses of silicone rubber (30 mm, 40 mm, 50 mm, 60 mm and 70 mm) were first employed to investigate the correlations between the transmitted power and both the thicknesses of liquid and solid silicone rubber. Predicted trend equations for predicting the solidification time of silicone rubber mold was investigated. Three different thicknesses of silicone rubber (20 mm, 55 mm and 80 mm) were then employed to investigate the relative error of the predicted equation. Finally, the solidification mechanism of silicone rubber mold was proposed in this work.
Transmitted power (mW)
1.8
2. Experimental
23.4 1200
Time (min.) Fig. 4. Criterion for determining the solidification time of silicone rubber. Red dotted line stands for the solidification time obtained by manual measurement.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199 1.6
) 23.8
1.3 1.25
First trial
Second trial
Third trial
Temperature
1.2
0.4 30
40
50
60
70
0
80
150
300
450
26 With laser
)
25.8
Without laser
25.6
900
1050
Transmitted power (mW)
1.4
24.6
1.3 1.25
24.4
1.2
24.2
1.15 1.1
First trial
Second trial
Tird trial
Temperature
24
1.05 150
0
300
450
600
750
900
1050
23.8 1200
Time (min.) 1.1
Transmitted power (mW)
(c)
25.8
1.05
25.6 1
25.4
0.95
25.2
0.9
First trial
Second trial
Third trial
Temperature
25
0.85 0
150
300
450
600
750
900
1050
24.8 1200
Time (min.) 23
(d)
22.8
0.8
)
Transmitted power (mW)
0.9 0.85
22.6
0.75
22.4
0.7 22.2
0.65 0.6 0.55
First trial
Second trial
22
Third trial
Temperature
21.8
0.5 0
150
300
450
600
750
900
1050
21.6 1200
Time (min.) 0.7
23
(e)
22.8
)
0.65
22.6
0.6
22.4 0.55 22.2 0.5
First trial
Second trial
0.45
Third trial
Temperature
22 21.8
0.4 0
25.4
23.4 1200
24.8
(b)
1.35
Transitted power (mW)
effect of probe laser on the temperature of silicone rubber can be neglected. Fig. 7 shows the transmitted power and temperature as a function of measurement time. The solidification time of silicone rubber can be determined based on the criteria described above. Fig. 8 shows the solidification time as a function of five thicknesses of silicone rubber. The predicted equation, y = 9.43x + 215, can be obtained according to the curve fitting method. This means that the solidification time of silicone rubber (y) can be predicted from the predicted equation of y = 9.43x + 215 by the thickness of silicone rubber (x). It is noteworthy that the R2 stands for correlation coefficient. In general, a higher R2 value (maximum value = 1) means a better accuracy of the trend equation [19]. The R2 value is 0.9929, showing the predicted equation obtained is a good candidate for predicting solidification time of silicone rubber. Five solidified silicone rubber molds are shown in Fig. 9. To evaluate the relative error of the predicted equation, three different thicknesses of silicone rubber (20 mm, 55 mm and 80 mm) were further investigated in this work. Fig. 10 shows the solidification time obtained by predicted equation and manual measurement. As can be seen, there is an excellent agreement on the solidification time determined by predicted equation and manual measurement. The maximum discrepancy, however, is about 8.26%. The average relative error is 6.87%. This means that the non-invasive measurement system is robust and owns high accuracy for measuring the solidification time of silicone rubber mold. According to results described above, the solidification mechanism of silicone rubber is shown in Fig. 11. The surface of the liquid silicone rubber was solidified firstly. The inner liquid silicone rubber was then solidified progressively. It can be concluded that the advantages of this measurement system include cost-effective of hardware, simplicity in experimental setup, fast measurement speed and high measurement accuracy. A simple scheme for stabilizing commercially available He–Ne lasers oscillating has been presented Makinen and Stahlberg [20]. According to the normal
750
Time (min.)
Thickness of silicone rubber (mm) Fig. 5. Temperature rise of silicone rubber as a function of thickness of silicone rubber.
600
)
20
Temperature (
23.6
Temperature (
0.5
24
1.4 1.35
)
0.6
24.2
1.45
Temperature (
0.7
24.4
1.5
Temperature (
0.8
24.6
(a)
1.55
Temperature (
Transmitted power (mW)
Temperature rise (
)
0.9
150
300
450
600
750
900
Temperature (
198
1050
21.6 1200
Time (min.)
25.2
Fig. 7. Transmitted power and temperature as a function of measurement time for thickness of silicone rubber of (a) 30 mm, (b)40 mm, (c)50 mm, (d)60 mm and (e) 70 mm.
25 24.8 24.6
0
200
400
600
800
1000
1200
Time (min.)
Fig. 6. Effect of probe laser on the temperature of silicone rubber.
C.-C. Kuo, Y.-T. Siao / Optik 125 (2014) 196–199
has broad application prospects in the development of new products using silicone rubber mold. The solidification time can be determined rapidly from the thickness of silicone rubber according to the predicted equation with the maximum discrepancy about 8.26%. The temperature rise of the silicone during the solidification process has been observed experimentally, which is an important phenomenon for determining the solidification behavior of silicone rubber. The center of the liquid silicone rubber started to solidify firstly and the surface of the silicone rubber was then solidified gradually.
1000
Solidification time (min.)
900
y=9.43x+215 R =0.9929
800 700 600 500 400 300 200 100 0 20
30
199
40
50
60
Thickness of silicone rubber (mm)
70
80
Acknowledgement
Fig. 8. Solidification time as a function of five thicknesses of silicone rubber.
The authors gratefully acknowledge the financial support of the National Science Council of Taiwan under contracts nos. NSC1012221-E-131-007 and NSC101-2815-C-131-001-E. References
Fig. 9. Five solidified silicone rubber molds. 1100 Predicted equation
Solidification time (min.)
1000
Manual measurement
900 800 700 600 500 400 300 0
10
20
30
40
50
60
70
80
90
Thickness of silicone rubber (mm) Fig. 10. Solidification time obtained by predicted equation and manual measurement.
Fig. 11. Schematic illustration of solidification mechanism of silicone rubber.
operating life of He-Ne laser, this system could be operated around 0.5–1.2 years by switching the probe laser every 18 h [17,21]. 4. Conclusions A non-invasive measurement system to determine the solidification time of silicone rubber has been developed. This system
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