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Manufacturing of the composite screw rotors by resin transfer molding
Journal of
Materials Processing Technology ELSEVIER
Journal of Materials Processing Technology 48 (1995) 641-647
Manufacturing of the composite screw rotors by resin transfer molding Young Goo Kim a, Dai Gil Leea and Park Kyoun Ohb
a Department of Precision Engineering andMechatronics, Korea Advancedlnstitute of Science and Technology, Taejon, Korea 305-701 b KoreaAutomotive Technologylnstitute, Seocho-dong, Seocho-ku, Seoul, Korea 137-070
Industrial Summary A screw-type pump with at least two rotors that are composed of male and female rotors with helical extending lands and grooves is increasingly used because it has a smaller weight to power ratio and produces less noise and vibration compared to conventional reciprocating pumps. Most male and female rotors of the screw type pump have been manufactured by ma~hiuing. The manufacturing time is long because the amount of material cut-away is considerable and the manufacturing cost is high because the screw rotors have a complicated shape and require high degree of accuracy in machining. Therefore, there are many attempts to manufacture the screw rotors with plastics and ceramics using different manufacturing methods such as injection molding and casting. Since the lands of rotors manufactured with polymer composite materials have resilient deflocting characteristics when clashed with other rotors, in this paper, the composite screw rotor was manufactured with chopped carbon fiber reinforced epoxy composite materials by resin transfer molding. The interfacial strength between the screw and the aluminum core shaft was tested with respect to the knurling size of the surface of the aluminum core shaft and the surface treatment. Also, the flexural strength and coefficient uf thermal expansion were tested. Using the measured data, the composite screw rotors were manufactured by resin transfer molding.
1. Introduction The continuous fiber reinforced resin matrix composite materials have been used in the manufacture of aircraft and spacecraft structures because of their high specific moduli(E/p), high specific strengths(S/p) and high material dampings, and recently these materials are extensively used in sports and leisure goods such as tennis rackets, fishing rods and golf clubs as the prices of these materials become lower[I,2]. Also, these materials have been used in the manufacture of the machine elements such as robot arms[3,4], automotive power transmission shafts[5] and machine tools[6]. As the demands for the composite materials increase, many processing methods of composite materials have been developed and refined in order to meet the requirements of quality and productivity. Recently RTM(resin transfer molding) is extensively studied and employed to produce
commercial products because this process uses very cheap molds to fabricate very complicated components and can manufacture a wide variety of articles ranging from small articles to large process plants components[7]. Unlike processes such as compression molding and injection molding, which requires rigid tools and equipment, RTM process can be carried out at low pressure and in some cases pressures below atmospheric pressure because RTM process uses very low viscosity resins in the range of 100 ~ 1000 cPs. Low pressures reduce the cost and complexity of tooling required. Low cost epoxy tooling can be used and is in fact used for the majority of today's low volume RTM production. Therefore, RTM has the potential of becoming a dominant low cost process for the fabrication of large, integrated, high performance products for the consumer segment of the economy and ultimately for segments now dominated by the higher precision laminated fabrication teclmiques[8].
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 7 0 4 - 5
642
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
TABLE 1 Properties of the IPCO 410/183 and the CIBA-GEIGY LY564/HY2954 epoxy resin systems 410 (Resin) 183 (Hardener) LY564 (Resin) HY2954 (Hardener) Specific Gravity 1.165 1.098 1.15 0.94 Mixed Ratio (wt%) 100 44 100 35 Viscosity (cPs) 1500 + 300 1300 + 200 1000 - 1400 70 Pot Life (min.) 120 480 ~ 580 C.T.E. 30 x 10-6/°C 72.5 x 10-6/°C The process of RTM starts when a dry reinforcement material that has been cut and/or shaped into a preformed piece, generally called a preform, is placed in a prepared mold cavity. Once the mold has been closed and clamped shut, resin is injected into the mold cavity, where it flows through the reinforcement preform, expelling the air in the cavity and wetting out or impregnating the reinforcement. When excess resin begins to flow from the vent areas of the mold, the resin flow is stopped and the molded component begins to cure. When cure is completed, which can take from several minutes to hours, it is removed from the mold and the process can begin again to form additional parts. The molded components may require a postcure to further complete the resin reaction. In order to reduce time and cost for the design and process of RTM, several researchers studied analytically and experimentally the resin flow in the RTM molds[9-12]. Coulter[13] used the Darcy's law to simulate the resin flow in the mold of RTM. Cai[14] suggested that the vent of the mold should be designed for the resin to travel the shortest distance and the outside line gate should be used from which the resin flows into the center of the mold. The screw rotors are employed in compressor, turbo-chargers of automobiles and pumps because they produce less noise and vibration with small size, compact shape and easy maintenance. The capacity of screw rotors can be varied continuously from 10 ~ 100 % capacity with high volumetric and compressional efficiencies. Even though, the screw rotors have several beneficial properties, the manufacturing of the screw rotors with conventional materials such as steel and aluminum, requires expensive CNC machines and the very long production time.
The metallic screw rotors must be machined very accurately lest the lands of the rotors should be damaged by the collision with other lands of the rotors in operation. These manufacturing difficulties can be solved if the screw rotors are manufactured with composite materials by RTM process. If the screw rotor is designed to have moderate value of modulus by adjusting the fiber orientation and volume, then the probability of damage of the screw rotor by collision can be reduced because of the resilience characteristics of the composite materials. Therefore, the screw rotors were manufactured with the chopped carbon fiber epoxy composite materials by RTM in this work. Since the screw rotor has the helical shape and the unloading operation requires releasing and turning of the screw from the mold, the interfacial strength between the outer composite part and the inner aluminum core was measured after knurling and surface treating the surface of the aluminum. Also, the flexural strength and thermal expansion coefficient of the carbon fiber reinforced epoxy composite material used in RTM were measured. Based on the experimental test results, finally, the screw rotors were manufactured with the chopped carbon fiber(length : 6 ram) epoxy composite materials by RTM process. 2. Materials for the mold and the screw rotor
Since the mold for the RTM should have the low coefficient of thermal expansion for dimensional stability and easy demolding of parts manufactured, the long chopped carbon fiber epoxy material was used. The carbon fiber was T-300 whose length was 20 nun. The epoxy was IPCO 410/183 whose properties are shown in Table 1. Aluminum 2024 alloy was selected for the core material of the screw
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
TABLE 2 Properties of the ASHLAND carbon fiber Electric Resistivity Density Diameter Tensile Modulus Tensile Strength Fiber Len~da
6 x 10-4 n - m 1.57 g/ml 12 ~ 14 ~tm 35 GPa 500 MPa 6 mm
rotor and 6 mm chopped carbon fiber was selected for the reinforcement for the outer composite part. The resin system for the RTM should have minimum 2 hour pot life and very low viscosity of the range of 100 ~ 1000 cPs at the injection temperature because the resin must be filled every inch of space of the mold after traveling the long porous medium of the reinforcing fibers. Another requirement of the resin in this work was that the screw rotor should have little distortion and thermal degradation until 120 °C. Since the LY564/HY2954 epoxy resin system of Ciba-Geigy[15] whose properties are shown in Table 1 satisfies the requirements of RTM, this material was selected as the material for the composite screw rotor. In order to know the thermo-mechanical properties of the resin, the DMA(dynamic mechanical analysis) test was performed. Figure 1 shows the results of DMA of the LY564/HY2954 epoxy resin system. Since the glass transition temp-
3
643
i i i i i i i i i
erature was 140 °C, it was satisfactory to be used under 120 °C. The mixed viscosity of the resin at 25 °C is 500 - 700 cPs and can be reduced further if the resin temperature goes up. Since the screw rotor is operated in very high rotational speed, the heat produced in the area of mating lands of the screw rotor must be dissipated easily. Therefore, the thermal conductivity of the material must be large. If we used ordinary carbon fibers for the reinforcement to increase the thermal conductivity, the modulus of the screw rotor would be too high for the resilience movement of the lands. Therefore, in this work, the carbon fiber manufactured by Ashland Corparation[16] was selected. Since the carbon fiber was not graphitized, it had moderate values of moduins and thermal conductivity. Table 2 shows the properties of the fiber. The tensile modulus of the fiber was about 30 GPa that was one order small compared to that of ordinary carbon fiber and its electrical resistivity was 6 x 10-4 ~.m which was much lower than that of the glass fiber. 3. Measurement of the thermal properties of the composite material
! ~
i ~
i ~
i
!
i
i
i
i
.
0.15
Temperature ( °C ) Fig. 1. DMA (dynamic mechanical analysis) test results of the epoxy resin and hardener (LY564/ HY2954 of CIBA-GEIGY) for RTM
The flcxural strength and coefficient of thermal expansion of the composite material were measured using the cylindrical specimens manufactured by RTM. The mold for manufacturing of test specimens and RTM equipment are shown in Figure 2 and Figure 3, respectively. The sequence of manufacturing of test specimens is as follows. After coating mold release on the surface of the mold and filling the carbon fiber 30 grams, the resin was injected into the mold cavity, The resin was kept for 10 minutes under vacuum before injecting to remove air bubbles in the resin.
644
Y.G. Kim et al. /Journal of Materials Processing Technology 48 (1995) 641-647
Maintaining the mold under vacuum state, 0.2 MPa pressure was applied to the resin bath which made the resin flow from the resin bath to the mold. Then the resin filled mold was moved to an autoclave and cured under the cure cycle as shown in Figure 4.
was 53.2 MPa and the maximum deflection was 1.86 ram. Because the resilient deflection of the composite material was very large compared to metal, the damage of the lands of the screw rotors by the collision with other lands of the rotors in operation can be reduced.
0-ring
0.1
1
150 140 130 120 110 100 90 80
Ii
v
70.
Fig. 2. Mold for manufacture of test specimens
6@
E
40-
302010O0
6
Time Excess
[0omp.ossor : II II
Resin
Bath
/
~
1
~
Resin
Bo,
Mold
(hour)
Fig. 4. Cure cycle for the epoxy resin and hardener (LY564/HY2954 of CIBA-GEIGY) for the RTM The coefficients of thermal expansion of the screw rotor and the mold were 70.7 x 10. 6 / K and 11.1 x 10-6 /K, respectively. Since the coefficient of thermal expansion of the mold was smaller than that of the screw rotor, it was easy to demold the screw rotor from the mold. 4. Interfacial strength between the aluminum core and the composite material
Fig. 3. RTM equipment for the manufacture of the composite screw rotors The modulus of the specimens was calculated after measuring the fundamental natural frequency using the laser vihrometer and the FFT(fast Fourier transform) signal analyzer. Since the lands of the screw rotor was subjected to large bending moment, the flexural strength of the composite material was measured by the three point bending test. The test length was 110 nun and strain rate was 0.5 ram/rain. From the test results, it was found that the flexural strength of the specimens
The composite screw rotors consist of the inside aluminum core and the outside composite material. If the interfacial strength between the aluminum core and the composite material is small, the interface will be fractured either at the moment that the screw rotor is demolded from the mold by turning action or during operation alter demolding. Therefore, in order to improve the interracial strength, the surface of the aluminum was treated with surface treatment chemical for 10 minutes. The chemical whose volume was 1,000 ml contained sulfuric acid 304 grams and sodium dichromate 34 grams[17]. Also, the surface of the aluminum was knurled to improve the interfacial strength by mechanical interlocking.
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
The test specimens were manufactured with the same mold used to manufacture the specimens for the flexural strength measurement. The aluminum core was inserted in the mold and 15 grams of the chopped carbon fiber was inserted in the gap between the core and the mold. Figure 5 shows the specimens manufactured by RTM. The interfacial joining length of the specimens was 20 % of the real length of the screw rotor.
-Compomit,e -\
AI ~
T ~ O U Tape
645
Howler, specimens with knarling size 1.5 mm and 2.0 nun that was surface-treated bad smaller torque transmission capabilities than the specimens without surface treatment. This might come from the fact that the shear stresses in the surface of the specimens with larger knurling size easily broke the knurled surface weakened by surface treatment. The angle of twist before fracture had the similar trend. Since the aluminum core with 1.5 mm knnrling size bad the largest torque transmission capability when it was not surface treated, this condition was used to manufacaae the composite screw rotors.
5. Manufacturing of the composite molds and the screw rotors by RTM 80 trio
Fig. 5. Specimen for the measurement of the interfacial strength The torque transmission capabilities of the specimens and twisting angles before fracture were measured by MTS axial torsion tester. From the test results of Figure 6, it was found that the torque transmission capabilities of the specimen which was surface treated with knurling size less than 1.5 nun was larger than that of the specimen without surface treatment. 220200" u
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IlC
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KnurlingSize (mm) Fig. 6. Torque transmission capabilities between the outer composite material and the inner aluminum core
The RTM apparatus consisted of the resin bath, the transfer equipment and the mold. The epoxy and the hardener were mixed in the resin bath. The resin bath was equipped with a vacuum system and a pressurizing equipment to be used when the resin conld not fill completely the mold and wet reinforcements with the vacuum pressure only. The vacuum pressure also belps void elimination in the resin. In this work, the pressurizing equipment was installed whose maximum pressure was 0.6 MPa because the resin flow path in the manufacture of the screw rotor was long. The vacuum system was able to produce 1 tort of vacuum pressure. The transfer equipment composed of the air compressor and the flow line which transfers the resin from the resin bath to the mold. In this work, the molds for RTM were manufactured with chopped carbon fiber epoxy composite material. The cavity of the mold was made using the metal screw rotor. The mold had the cavity and the steel tool part that has the inlet for the resin injection, the resin flow area and the void reduction part. The inlet pan has two ports for resin injection and the resin flow area had the outside line gate from which the resin flew into the inside small area. The recommended design for the line gate as shown in Figure 7 was used in this work[18]. A small slot was made in the cover plate of the mold to be used as the air vent. The surface of the steel screw rotor was coated by mold release agent to ease the demolding operation.
646
I~.G. l~m et al. / Journal of Materials Processing Technology 48 (1995) 641-647
Inlet Fig. 7. The recommended gate suggested in Reference[18] Then, the surface of the screw rotor was coated with gel coat material to improve the surface finish quality ufthe mold. The materials for the mold were IPCO 410/183 whose properties are shown in Table 1 and T-300 carbon fiber. The length of the carbon fiber was about 20 nun. The male and female steel screw rotors coated with mold release agent and gel coat were placed inside of the steel tools. The chopped carbon fiber was filled into the gap between the steel screw and the steel tool. After sealing the steel tool, the resin was injected by imposing atmospheric pressure in the resin bath while maintaining the vacuum in the steel tool. From several experiments, it was found that the resin bath should be put under vacuum for about 10 minutes to remove air bubbles in the resin. The mold filled with resin and fibers was maintained for 1 hour at room temperature to improve the wetting of fiber with resin and cured in the autoclave under 0.6 MPa pressure at 80 °C. Then the steel screw rotor was demolded by turning. This resulted the composite mold whose cavity was the same shape of the screw rotor. Figure 8 shows the composite mold manufactured by this method.
The coefficient of thermal expansion of composite material for the screw rotor was 70.7x10 "6/K and that of the composite mold was l l . l x l 0 "6 /K, which made the demold of the composite screw rotor easy. Since the composite molds had enough surface quality, they were used to manufacture the composite screw rotors. The sequence of operation to manufacture the composite screw rotors were similar to that of the composite mold except that the aluminum core was inserted in the cavity of the composite mold. After the surface of the aluminum core was knurlcd to improve the interfacial strength, it was fixed in the center of the cavity of the mold using the clamping apparatus of the cover plates of the mold. Figure 9 shows the composite screw rotors manufactured by RTM.
Figure 9. Photograph of the composite screw rotors manufactured by RTM 6. Conclusions
Figure 8. Composite mold manufactured by RTM
In this study, the RTM(Resin Transfer Molding) apparatus for the manufacturing of the screw rotor was developed and the manufacturing method of the screw rotor with carbon fiber reinforced resin matrix composite materials was experimentally investigated. The cavity of the mold for the screw rotor was successfully manufactured using the steel screw rotor. From the results of the experimental investigations, the following conclusions were drawn :
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
(1) The knurling size of 1.5 mm on the surface of the aluminum core without surface treatment produced the best torque transmission capability. (2) It was found that the mold fabricated by RTM and coated with gel coat could be used in the manufacture of the composite screw rotor by RTM. The composite screw rotor was easily released and demoldod by turning the aluminum core. (3) The screw rotor was successfully manufactur~ by RTM with the chopped carbon fiber reinforced resin matrix composite materials.
7.
8. 9.
10.
Acknowledgement The authors wish to thank Dr. Ki Soo Kim for helping experiments.
11.
References
12.
1. 2.
3.
4.
5.
6.
P.K. MallicL Fiber-Reinforced Composites, Marcel Dekker, Inc., New York, (1988) 3-4. M. M. Schwartz, Composite Materials Handbook, McGraw-Hill, New York, (1984) Ch. 7. D . G . Lee, K. S. Kim and Y. K. Kwak, Manufacturing of a Scara Type Direct-Drive Robot with Graphite Fiber Epoxy Composite Material, Robotica, (1991) 219-229. D.G. Lee, K. S. Jeong, K. S. Kim and Y. K. Kwak, Development of the Anthropomorphic Robot with Carbon Fiber Epoxy Composite Materials, Composite Structures, (1993) 313324. K.S. Kim, W. T. Kim, D. G. Lee and E. J. Jun, Optimal Tubular Adhesive-Bonded Lap Joint of the Carbon Fiber Epoxy Composite Shaft, Composite Structures, (1992) 163-176. D . G . Lee, H. C. Sin and N. P. Suh, Manufacturing of a Graphite Epoxy Composite Spindle for a Machine Tool, Annals of the CIRP, 27(1), (1985) 365-369.
13.
14.
15. 16. 17. 18.
647
C. F. Johnson, Engineered Materials Handbook Vol. 1, ASM International, Ohio, (1987) Ch. 8. Mallick and Newman, Composite Materials Technology, Hanser Publishers, New York, (1990) Ch. 5. M . K . Um and W. I. Lee, A Study on the Mold Filling Process in Resin Transfer Molding, Polymer Engineering and Science, Vol. 31, No. 11, (1991) 765-771 Aoyagi and M. Uenoyama, Analysis and Simulation of Structural Reaction Injection Molding(SRIM), Intern. Polymer Processing V/I, (1992) 71-83. IL Dave, A Unified Approach to Modeling Resin Flow during Composite Processing, J. Composite Materials, Vol. 24, (1990) 22-41 Zhong Cai, Simplified M old Filling Simulation in Resin Transfer Molding, J. Composite Materials, Vol. 26, (1992) 26062630. J.P. Coulter, Resin Impregnation During the Manufacturing of Composite Materials Subject to Prescribed Injection Rate, J. of Reinforced Plastics and Composites, Vol. 7, (1988) 200-219 Zhong Cai, Analysis of Mold Filling in RTM Process, J. ComposRe Materials, Vol. 26, (1992) 1310-1338. CIBA-GEIGY, Catalog of Araldlte Laminating Resin Systems, Switzerland. Ashland Carbon Fibers Manual, Ashland, Kentucky 41114, P. O. Box 391, U.S.A. CINAMID FM 123-2 Adhesive Film Manual, Wayne, New Jersey 07470, U.S.A. 3M Aerospace Materials Department Technical Service Bulletin, Miunessota, U.S.A.
Materials Processing Technology ELSEVIER
Journal of Materials Processing Technology 48 (1995) 641-647
Manufacturing of the composite screw rotors by resin transfer molding Young Goo Kim a, Dai Gil Leea and Park Kyoun Ohb
a Department of Precision Engineering andMechatronics, Korea Advancedlnstitute of Science and Technology, Taejon, Korea 305-701 b KoreaAutomotive Technologylnstitute, Seocho-dong, Seocho-ku, Seoul, Korea 137-070
Industrial Summary A screw-type pump with at least two rotors that are composed of male and female rotors with helical extending lands and grooves is increasingly used because it has a smaller weight to power ratio and produces less noise and vibration compared to conventional reciprocating pumps. Most male and female rotors of the screw type pump have been manufactured by ma~hiuing. The manufacturing time is long because the amount of material cut-away is considerable and the manufacturing cost is high because the screw rotors have a complicated shape and require high degree of accuracy in machining. Therefore, there are many attempts to manufacture the screw rotors with plastics and ceramics using different manufacturing methods such as injection molding and casting. Since the lands of rotors manufactured with polymer composite materials have resilient deflocting characteristics when clashed with other rotors, in this paper, the composite screw rotor was manufactured with chopped carbon fiber reinforced epoxy composite materials by resin transfer molding. The interfacial strength between the screw and the aluminum core shaft was tested with respect to the knurling size of the surface of the aluminum core shaft and the surface treatment. Also, the flexural strength and coefficient uf thermal expansion were tested. Using the measured data, the composite screw rotors were manufactured by resin transfer molding.
1. Introduction The continuous fiber reinforced resin matrix composite materials have been used in the manufacture of aircraft and spacecraft structures because of their high specific moduli(E/p), high specific strengths(S/p) and high material dampings, and recently these materials are extensively used in sports and leisure goods such as tennis rackets, fishing rods and golf clubs as the prices of these materials become lower[I,2]. Also, these materials have been used in the manufacture of the machine elements such as robot arms[3,4], automotive power transmission shafts[5] and machine tools[6]. As the demands for the composite materials increase, many processing methods of composite materials have been developed and refined in order to meet the requirements of quality and productivity. Recently RTM(resin transfer molding) is extensively studied and employed to produce
commercial products because this process uses very cheap molds to fabricate very complicated components and can manufacture a wide variety of articles ranging from small articles to large process plants components[7]. Unlike processes such as compression molding and injection molding, which requires rigid tools and equipment, RTM process can be carried out at low pressure and in some cases pressures below atmospheric pressure because RTM process uses very low viscosity resins in the range of 100 ~ 1000 cPs. Low pressures reduce the cost and complexity of tooling required. Low cost epoxy tooling can be used and is in fact used for the majority of today's low volume RTM production. Therefore, RTM has the potential of becoming a dominant low cost process for the fabrication of large, integrated, high performance products for the consumer segment of the economy and ultimately for segments now dominated by the higher precision laminated fabrication teclmiques[8].
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 7 0 4 - 5
642
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
TABLE 1 Properties of the IPCO 410/183 and the CIBA-GEIGY LY564/HY2954 epoxy resin systems 410 (Resin) 183 (Hardener) LY564 (Resin) HY2954 (Hardener) Specific Gravity 1.165 1.098 1.15 0.94 Mixed Ratio (wt%) 100 44 100 35 Viscosity (cPs) 1500 + 300 1300 + 200 1000 - 1400 70 Pot Life (min.) 120 480 ~ 580 C.T.E. 30 x 10-6/°C 72.5 x 10-6/°C The process of RTM starts when a dry reinforcement material that has been cut and/or shaped into a preformed piece, generally called a preform, is placed in a prepared mold cavity. Once the mold has been closed and clamped shut, resin is injected into the mold cavity, where it flows through the reinforcement preform, expelling the air in the cavity and wetting out or impregnating the reinforcement. When excess resin begins to flow from the vent areas of the mold, the resin flow is stopped and the molded component begins to cure. When cure is completed, which can take from several minutes to hours, it is removed from the mold and the process can begin again to form additional parts. The molded components may require a postcure to further complete the resin reaction. In order to reduce time and cost for the design and process of RTM, several researchers studied analytically and experimentally the resin flow in the RTM molds[9-12]. Coulter[13] used the Darcy's law to simulate the resin flow in the mold of RTM. Cai[14] suggested that the vent of the mold should be designed for the resin to travel the shortest distance and the outside line gate should be used from which the resin flows into the center of the mold. The screw rotors are employed in compressor, turbo-chargers of automobiles and pumps because they produce less noise and vibration with small size, compact shape and easy maintenance. The capacity of screw rotors can be varied continuously from 10 ~ 100 % capacity with high volumetric and compressional efficiencies. Even though, the screw rotors have several beneficial properties, the manufacturing of the screw rotors with conventional materials such as steel and aluminum, requires expensive CNC machines and the very long production time.
The metallic screw rotors must be machined very accurately lest the lands of the rotors should be damaged by the collision with other lands of the rotors in operation. These manufacturing difficulties can be solved if the screw rotors are manufactured with composite materials by RTM process. If the screw rotor is designed to have moderate value of modulus by adjusting the fiber orientation and volume, then the probability of damage of the screw rotor by collision can be reduced because of the resilience characteristics of the composite materials. Therefore, the screw rotors were manufactured with the chopped carbon fiber epoxy composite materials by RTM in this work. Since the screw rotor has the helical shape and the unloading operation requires releasing and turning of the screw from the mold, the interfacial strength between the outer composite part and the inner aluminum core was measured after knurling and surface treating the surface of the aluminum. Also, the flexural strength and thermal expansion coefficient of the carbon fiber reinforced epoxy composite material used in RTM were measured. Based on the experimental test results, finally, the screw rotors were manufactured with the chopped carbon fiber(length : 6 ram) epoxy composite materials by RTM process. 2. Materials for the mold and the screw rotor
Since the mold for the RTM should have the low coefficient of thermal expansion for dimensional stability and easy demolding of parts manufactured, the long chopped carbon fiber epoxy material was used. The carbon fiber was T-300 whose length was 20 nun. The epoxy was IPCO 410/183 whose properties are shown in Table 1. Aluminum 2024 alloy was selected for the core material of the screw
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
TABLE 2 Properties of the ASHLAND carbon fiber Electric Resistivity Density Diameter Tensile Modulus Tensile Strength Fiber Len~da
6 x 10-4 n - m 1.57 g/ml 12 ~ 14 ~tm 35 GPa 500 MPa 6 mm
rotor and 6 mm chopped carbon fiber was selected for the reinforcement for the outer composite part. The resin system for the RTM should have minimum 2 hour pot life and very low viscosity of the range of 100 ~ 1000 cPs at the injection temperature because the resin must be filled every inch of space of the mold after traveling the long porous medium of the reinforcing fibers. Another requirement of the resin in this work was that the screw rotor should have little distortion and thermal degradation until 120 °C. Since the LY564/HY2954 epoxy resin system of Ciba-Geigy[15] whose properties are shown in Table 1 satisfies the requirements of RTM, this material was selected as the material for the composite screw rotor. In order to know the thermo-mechanical properties of the resin, the DMA(dynamic mechanical analysis) test was performed. Figure 1 shows the results of DMA of the LY564/HY2954 epoxy resin system. Since the glass transition temp-
3
643
i i i i i i i i i
erature was 140 °C, it was satisfactory to be used under 120 °C. The mixed viscosity of the resin at 25 °C is 500 - 700 cPs and can be reduced further if the resin temperature goes up. Since the screw rotor is operated in very high rotational speed, the heat produced in the area of mating lands of the screw rotor must be dissipated easily. Therefore, the thermal conductivity of the material must be large. If we used ordinary carbon fibers for the reinforcement to increase the thermal conductivity, the modulus of the screw rotor would be too high for the resilience movement of the lands. Therefore, in this work, the carbon fiber manufactured by Ashland Corparation[16] was selected. Since the carbon fiber was not graphitized, it had moderate values of moduins and thermal conductivity. Table 2 shows the properties of the fiber. The tensile modulus of the fiber was about 30 GPa that was one order small compared to that of ordinary carbon fiber and its electrical resistivity was 6 x 10-4 ~.m which was much lower than that of the glass fiber. 3. Measurement of the thermal properties of the composite material
! ~
i ~
i ~
i
!
i
i
i
i
.
0.15
Temperature ( °C ) Fig. 1. DMA (dynamic mechanical analysis) test results of the epoxy resin and hardener (LY564/ HY2954 of CIBA-GEIGY) for RTM
The flcxural strength and coefficient of thermal expansion of the composite material were measured using the cylindrical specimens manufactured by RTM. The mold for manufacturing of test specimens and RTM equipment are shown in Figure 2 and Figure 3, respectively. The sequence of manufacturing of test specimens is as follows. After coating mold release on the surface of the mold and filling the carbon fiber 30 grams, the resin was injected into the mold cavity, The resin was kept for 10 minutes under vacuum before injecting to remove air bubbles in the resin.
644
Y.G. Kim et al. /Journal of Materials Processing Technology 48 (1995) 641-647
Maintaining the mold under vacuum state, 0.2 MPa pressure was applied to the resin bath which made the resin flow from the resin bath to the mold. Then the resin filled mold was moved to an autoclave and cured under the cure cycle as shown in Figure 4.
was 53.2 MPa and the maximum deflection was 1.86 ram. Because the resilient deflection of the composite material was very large compared to metal, the damage of the lands of the screw rotors by the collision with other lands of the rotors in operation can be reduced.
0-ring
0.1
1
150 140 130 120 110 100 90 80
Ii
v
70.
Fig. 2. Mold for manufacture of test specimens
6@
E
40-
302010O0
6
Time Excess
[0omp.ossor : II II
Resin
Bath
/
~
1
~
Resin
Bo,
Mold
(hour)
Fig. 4. Cure cycle for the epoxy resin and hardener (LY564/HY2954 of CIBA-GEIGY) for the RTM The coefficients of thermal expansion of the screw rotor and the mold were 70.7 x 10. 6 / K and 11.1 x 10-6 /K, respectively. Since the coefficient of thermal expansion of the mold was smaller than that of the screw rotor, it was easy to demold the screw rotor from the mold. 4. Interfacial strength between the aluminum core and the composite material
Fig. 3. RTM equipment for the manufacture of the composite screw rotors The modulus of the specimens was calculated after measuring the fundamental natural frequency using the laser vihrometer and the FFT(fast Fourier transform) signal analyzer. Since the lands of the screw rotor was subjected to large bending moment, the flexural strength of the composite material was measured by the three point bending test. The test length was 110 nun and strain rate was 0.5 ram/rain. From the test results, it was found that the flexural strength of the specimens
The composite screw rotors consist of the inside aluminum core and the outside composite material. If the interfacial strength between the aluminum core and the composite material is small, the interface will be fractured either at the moment that the screw rotor is demolded from the mold by turning action or during operation alter demolding. Therefore, in order to improve the interracial strength, the surface of the aluminum was treated with surface treatment chemical for 10 minutes. The chemical whose volume was 1,000 ml contained sulfuric acid 304 grams and sodium dichromate 34 grams[17]. Also, the surface of the aluminum was knurled to improve the interfacial strength by mechanical interlocking.
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
The test specimens were manufactured with the same mold used to manufacture the specimens for the flexural strength measurement. The aluminum core was inserted in the mold and 15 grams of the chopped carbon fiber was inserted in the gap between the core and the mold. Figure 5 shows the specimens manufactured by RTM. The interfacial joining length of the specimens was 20 % of the real length of the screw rotor.
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645
Howler, specimens with knarling size 1.5 mm and 2.0 nun that was surface-treated bad smaller torque transmission capabilities than the specimens without surface treatment. This might come from the fact that the shear stresses in the surface of the specimens with larger knurling size easily broke the knurled surface weakened by surface treatment. The angle of twist before fracture had the similar trend. Since the aluminum core with 1.5 mm knnrling size bad the largest torque transmission capability when it was not surface treated, this condition was used to manufacaae the composite screw rotors.
5. Manufacturing of the composite molds and the screw rotors by RTM 80 trio
Fig. 5. Specimen for the measurement of the interfacial strength The torque transmission capabilities of the specimens and twisting angles before fracture were measured by MTS axial torsion tester. From the test results of Figure 6, it was found that the torque transmission capabilities of the specimen which was surface treated with knurling size less than 1.5 nun was larger than that of the specimen without surface treatment. 220200" u
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The RTM apparatus consisted of the resin bath, the transfer equipment and the mold. The epoxy and the hardener were mixed in the resin bath. The resin bath was equipped with a vacuum system and a pressurizing equipment to be used when the resin conld not fill completely the mold and wet reinforcements with the vacuum pressure only. The vacuum pressure also belps void elimination in the resin. In this work, the pressurizing equipment was installed whose maximum pressure was 0.6 MPa because the resin flow path in the manufacture of the screw rotor was long. The vacuum system was able to produce 1 tort of vacuum pressure. The transfer equipment composed of the air compressor and the flow line which transfers the resin from the resin bath to the mold. In this work, the molds for RTM were manufactured with chopped carbon fiber epoxy composite material. The cavity of the mold was made using the metal screw rotor. The mold had the cavity and the steel tool part that has the inlet for the resin injection, the resin flow area and the void reduction part. The inlet pan has two ports for resin injection and the resin flow area had the outside line gate from which the resin flew into the inside small area. The recommended design for the line gate as shown in Figure 7 was used in this work[18]. A small slot was made in the cover plate of the mold to be used as the air vent. The surface of the steel screw rotor was coated by mold release agent to ease the demolding operation.
646
I~.G. l~m et al. / Journal of Materials Processing Technology 48 (1995) 641-647
Inlet Fig. 7. The recommended gate suggested in Reference[18] Then, the surface of the screw rotor was coated with gel coat material to improve the surface finish quality ufthe mold. The materials for the mold were IPCO 410/183 whose properties are shown in Table 1 and T-300 carbon fiber. The length of the carbon fiber was about 20 nun. The male and female steel screw rotors coated with mold release agent and gel coat were placed inside of the steel tools. The chopped carbon fiber was filled into the gap between the steel screw and the steel tool. After sealing the steel tool, the resin was injected by imposing atmospheric pressure in the resin bath while maintaining the vacuum in the steel tool. From several experiments, it was found that the resin bath should be put under vacuum for about 10 minutes to remove air bubbles in the resin. The mold filled with resin and fibers was maintained for 1 hour at room temperature to improve the wetting of fiber with resin and cured in the autoclave under 0.6 MPa pressure at 80 °C. Then the steel screw rotor was demolded by turning. This resulted the composite mold whose cavity was the same shape of the screw rotor. Figure 8 shows the composite mold manufactured by this method.
The coefficient of thermal expansion of composite material for the screw rotor was 70.7x10 "6/K and that of the composite mold was l l . l x l 0 "6 /K, which made the demold of the composite screw rotor easy. Since the composite molds had enough surface quality, they were used to manufacture the composite screw rotors. The sequence of operation to manufacture the composite screw rotors were similar to that of the composite mold except that the aluminum core was inserted in the cavity of the composite mold. After the surface of the aluminum core was knurlcd to improve the interfacial strength, it was fixed in the center of the cavity of the mold using the clamping apparatus of the cover plates of the mold. Figure 9 shows the composite screw rotors manufactured by RTM.
Figure 9. Photograph of the composite screw rotors manufactured by RTM 6. Conclusions
Figure 8. Composite mold manufactured by RTM
In this study, the RTM(Resin Transfer Molding) apparatus for the manufacturing of the screw rotor was developed and the manufacturing method of the screw rotor with carbon fiber reinforced resin matrix composite materials was experimentally investigated. The cavity of the mold for the screw rotor was successfully manufactured using the steel screw rotor. From the results of the experimental investigations, the following conclusions were drawn :
Y.G. Kim et al. / Journal of Materials Processing Technology 48 (1995) 641--647
(1) The knurling size of 1.5 mm on the surface of the aluminum core without surface treatment produced the best torque transmission capability. (2) It was found that the mold fabricated by RTM and coated with gel coat could be used in the manufacture of the composite screw rotor by RTM. The composite screw rotor was easily released and demoldod by turning the aluminum core. (3) The screw rotor was successfully manufactur~ by RTM with the chopped carbon fiber reinforced resin matrix composite materials.
7.
8. 9.
10.
Acknowledgement The authors wish to thank Dr. Ki Soo Kim for helping experiments.
11.
References
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4.
5.
6.
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