한국정밀공학회지제 33 권제 1 호 pp. 31-35 J. Korean Soc. Precis. Eng., Vol. 33, No. 1, pp. 31-35 ISSN 1225-9071(Print), ISSN 2287-8769(Online) January 2016 / 31 http://dx.doi.org/10.7736/kspe.2016.33.1.31 폴리우레탄발포노즐형상이혼합성능에미치는영향 Influences of Polyurethane Nozzle Shape on Mixing Efficiency 김도연 1, 이태경 1, 정해도 2, 김형재 1, Do Yeon Kim 1, Tae Kyung Lee 1, Hae Do Jeong 2, and Hyoung Jae Kim 1, 1 한국생산기술연구원정밀가공제어그룹 (Precision Manufacturing & Control R&D Group, Korea Institute of Industrial Technology) 2 부산대학교기계공학부 (Department of Mechanical Engineering, Pusan National University) Corresponding author: hyjakim@kitech.re.kr, Tel: +82-51-974-9257 Manuscript received: 2015.9.22. / Revised: 2015.10.5. / Accepted: 2015.10.8. For reaction injection molding (RIM) polyurethane was mixed in the mixing head by impingement mixing, injected into the mold, and cured quickly, as soon as the mold is filled. The shape of the nozzle in the mixing head is critical to improve the quality of polyurethane. To achieve homogeneous mixing, an intensive turbulence energy in the mixing nozzle is essential. In this study, a mixing nozzle for RIM was designed, and mixing efficiency was investigated based on experiment. Experiments were conducted with different combinations of nozzle tips and exit diameter to measure the mixing efficiency by measuring jet force and investigating mixing image with high speed camera. Jet force increased gradually and reaches steady state conditions. The jet force depended on shape of nozzle tip and outlet sizes. These results suggest that optimized nozzle configurations are necessary for high efficiency mixing with RIM. KEYWORDS: Reaction injection molding ( 반응사출성형 ), Mixing head ( 믹싱헤드 ), Mixing nozzle ( 믹싱노즐 ) 1. 서론 반응사출성형 (Reaction Injection Molding, RIM) 은반응물또는폴리머를챔버 (Chamber) 내에서고속충돌시켜짧은시간내에혼합반응시키는공정으로대표적적용분야는폴리우레탄발포이다. 폴리우레탄은폴리올 (Polyol) 과이소시아네이트 (Isocyanate) 를서로다른노즐에서고속으로분사하여혼합용기내에서충돌혼합후발포하여경화과정을통해성형된다. 1 폴리우레탄성형을위한반응사출성형은수분이내의짧은혼합과정을걸쳐금형내부로발포하기때문에혼합성능및발포성능이 중요하다. 2 특히, 폴리우레탄품질은공정변수인온도, 압력, 유량등의영향과믹싱헤드 (Mixing head) 의혼합및발포성능에따라결정된다. 3 폴리우레탄발포에사용되는믹싱헤드구조는 Fig. 1과같은구조로폴리올과이소시아네이트를분사할수있는두개의노즐부와충돌혼합공정이발생되는혼합챔버 (Mixing chamber), 그리고혼합물이발포되는출구부로구성되어있다. 노즐부의성능을결정하는인자는노즐홀더 (Nozzle holder) 형상, 노즐니들팁 (Nozzle needle tip) 직경, 노즐콘 (Nozzle cone) 출구직경등을들수있다. 4 Copyright C The Korean Society for Precision Engineering This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
한국정밀공학회지제 33 권제 1 호 pp. 31-35 January 2016 / 32 (a) Mixing head (b) Cross section Fig. 1 3D & cross section geometry of mixing head 반응사출성형에관한연구는공정조건변화에따른혼합성능과, 충돌혼합후혼합균일도분석에대한연구들이진행되었으며, 노즐형상에따른분사및혼합특성에대한연구는미비한실정이다. Kolodziej 5 은반응사출성형의혼합균일도를분석하는연구에서혼합챔버내부의유체충돌후발생되는유동장특성과혼합균일도와의상관계를제시하였다. Erkoc 6 은 CFD 를이용하여혼합균일도를분석하였으며, 이연구로부터혼합균일도분석을위해유동자의압력편차가주요변수임을밝혔다. Santos 7,8 는 PIV(Particle Image Velcocimetry) 와 LDA(Laser Doppler Anemometry) 을이용하여유동장거동을분석하고, CFD 를이용하여물질이동과화학반응분석을통해유체혼합에영향을미치는인자를제시하였다. Tucker 9,10 는믹싱헤드구조를설계하고혼합과정을예측하고분석하는연구를진행하였으며, 혼합효율을높이기위해애프터믹서 (aftermixer) 사용방법을제시하였다. 본연구에서는반응사출성형에서사용되는노즐의혼합성능향상을위한분사노즐형상을설계하였다. 또한설계된노즐형상의혼합성능을가시화실험을통해분석하였다. 2. 실험조건 본실험에서는믹싱헤드의분사력과혼합성능을가시화할수있도록실험장치를구성하였다. 이를위하여 Fig. 2 의노즐형상에따라분사되는유체의분사력측정을위해힘센서를장착하였으며, 이때분사유량도같이측정하였다. 또한초고속카메라를이용하여노즐형상에따른혼합챔버 (Mixing Chamber) 내에서혼합현상및혼합효율을측정하였다. A B C (a) Cross section of nozzle (b) Shape of needle, cone Fig. 2 Nozzle of mixing head design parameters Table 1 Experimental conditions Needle distance from outlet (A) 1, 2, 3mm Diameter of nozzle needle tip (B) Ø1, 1.5, 1.8, 2.0mm Nozzle exit diameter (C) Ø1.5, 2.0, 2.5, 3.0, 3.5, 4.0mm Density of mixing fluid 0.864kg/l Viscosity of mixing fluid 40cp Pumping pressure 40bar Fig. 3 Jet force measurement setup with nozzle 2.1 노즐분사력측정 Fig. 3 에나타난것과같이힘센서를이용하여노즐의분사력을측정하였다. 이실험에사용된힘센서는 Kistler 사의 9147B 를사용하였으며, 노즐에서유체분사시발생되는힘을 Fig. 4 와같은지렛대원리를이용하여측정하였다. 이러한방법은노즐분사력을직접적으로측정하는방식에비해힘을증폭하여측정할수있어외력및실험장비구조에의한측정오차를줄일수있을것으로판단된다. 또한실험에사용한노즐의형상및다른실험조건은 Table 1 에나타내었다. 실험에사용한유체는실제공정에서사용되는유체와유사한점도를가진유체를사용하였다.
한국정밀공학회지제 33 권제 1 호 pp. 31-35 January 2016 / 33 600 500 Fig. 4 System configuration of measuring jet force Nozzle jet force [N] 400 300 200 600 500 400 Ø1.0mm Ø1.5mm 300 Ø1.8mm Ø2.0mm 200 1.5 2.0 2.5 3.0 3.5 4.0 1.5 2.0 2.5 3.0 3.5 4.0 Nozzle exit diameter [mm] Fig. 6 Nozzle jet force characteristic with nozzle exit diameter and needle diameter 3. 실험결과및고찰 Fig. 5 System configuration of mixing efficiency measurement 2.2 혼합성능측정믹싱헤드에서유체혼합효율에영향을미치는물성은유체의온도, 점도, 압력등매우다양하다. 11 그러나이것은공정설계에서고려해야할인자들이며, 일반적으로믹싱헤드에서혼합효율은노즐에서분사되는유체가혼합챔버내에서균일하게혼합되는지가중요한부분이며, 이때노즐니들직경과출구직경이혼합효율에영향을미치는중요한변수로작용한다. 12 따라서본실험에서는노즐니들과출구직경에따른믹싱챔버내에서혼합과정을측정하였다. 각노즐에서분사되는유체의혼합현상을측정하기위해동일한점도의무색인두유체중한유체에는색을첨가해혼합과정을관찰하였다. 특히, 혼합과정은수초내의짧은시간에서일어나는현상으로육안관찰이어려워 Optronis사의 CR3000x2 초고속카메라를이용하여관찰하였다. 각실험은분사가균일하게이루어지는시점부터측정을진행하였다. 3.1 노즐형상에따른분사력측정반응사출성형에사용되는믹싱헤드의혼합성능에가장큰영향을미치는인자는레이놀즈수 (Reynolds numbers) 로레이놀즈수는 Re = ρvd/μ로정의할수있으며, d는노즐직경, v는노즐에서분사는유체유속, ρ는유체밀도, 그리고 μ는유체의점도를나타낸다. 13 레이놀즈수가낮을경우난류가형성되지않아유체의혼합이제대로이루어지지않는다. 14 이때레이놀즈수는유체속도에비례하며실험에서는노즐형상에따른분사속도를상대비교하기위해분사력을측정하였다. Fig. 6은노즐의형상에따른분사력을나타낸것으로노즐출구직경이커질수록분사력이커지는경향을보인다. 하지만출구직경이 3mm 이상결과에서는분사력이일정한결과를보이고있다. 이러한노즐출구직경에따른분사력변화는출구직경이작은초기구간에서는레이놀즈수감소로혼합효율이떨어질수있음을보이며혼합효율을높이기위해서는 3mm 이상의출구직경을가져야하는것을알수있다. 실험에사용한오리피스형태의노즐에서는출구의면적이작아지면서압력감소가발생한다. 이때노즐내부공급압력과분사압력이평형을이루는시점에서분사력편차폭이작아지게된다. 이러한특성은 Fig. 7의유량결과에서도동일한현상을보인다. 노즐출구직경이커지면분사유량도함께증가하지만, 출구직경이 3mm, 3.5mm 에서는더이상의유량증가없이일정한
한국정밀공학회지제 33 권제 1 호 pp. 31-35 January 2016 / 34 Flow rate [l/min] 24 22 20 18 16 14 12 Ø1.0mm Ø1.5mm 10 24 22 20 18 16 14 12 Ø1.8mm Ø2.0mm 10 1.5 2.0 2.5 3.0 3.5 4.0 1.5 2.0 2.5 3.0 3.5 4.0 Nozzle exit diameter [mm] Fig. 7 Flow rate characteristic with nozzle exit diameter 값을보인다. 이러한노즐니들과출구직경변화에따른유체분사력과유량결과는니들직경과출구직경에비례한결과를보인다. 하지만, 분사력과유량증가는특정노즐형상크기에서변화폭이작아지기때문에형상변화에의한혼합효율증가는한계성을가지는것으로판단된다. 따라서혼합효율을증가시키기위해서는노즐성능뿐만아니라공정조건의최적화도필요할것으로판단된다. 3.2 노즐형상에따른혼합성능믹싱헤드의가장중요한역할은각노즐에서분사되는유체를혼합챔버내에서균일혼합하여발포하는것으로노즐의분사력과챔버내에서의혼합과정이중요하다. Santos 15 의해알려진바와같이혼합성능은믹싱노즐뿐만아니라챔버에의한영향도크게받는다. 본실험에서는실험을통해설계한노즐을이용하여실제챔버내에서의혼합성능을측정하였다. 노즐출구직경은실험결과에서가장높은분사력을보인 3mm로고정하였다. Figs. 8과 9는노즐니들직경이 1.5mm, 2.0mm 일때노즐니들위치에따른혼합성능을측정한것이다. 니들의위치가출구와가까운 Fig. 8(a) 경우혼합현상이유체가충돌하는부분에서이루어지지만, Figs. 8(b) 와 8(c) 는챔버전구간에서혼합이일어난다. 그러나, 노즐니들직경이 2.0mm인 Fig. 9에서는모든위치에서균일혼합이일어나는것을볼수있다. 혼합챔버내혼합균일도를분석하기위해포토샵 (Photoshop) 을이용하여혼합영역부분측정이미지의그레이스케일히스토그램 (Histogram) 을분석하여표준편차를계산하였다. (a) 1mm (b) 2mm (c) 3mm Fig. 8 Mixed flow image with diameter of nozzle needle tip 1.5mm and nozzle exit diameter 3.0mm at nozzle needle distance from outlet 1, 2 and 3mm (a) 1mm (b) 2mm (c) 3mm Fig. 9 Mixed flow image with diameter of nozzle needle tip 2.0mm and nozzle exit diameter 3.0mm at nozzle needle distance from outlet 1, 2 and 3mm Standard deviation of mixing efficiency 50 40 30 20 10 0 1 2 3 Position of nozzle needle from outlet [mm] Ø1.5mm Ø2.0mm Fig. 10 Standard deviation of mixing efficiency at nozzle distance from outlet 1, 2 and 3mm Fig. 10 나타낸것과같이노즐니들직경 1.5mm 에서는혼합면적이가장넓은니들위치 3mm 에서가장낮은균일도편차값을보였다. 노즐니들직경이 2.0mm 에서는모든노즐니들위치에서균일한혼합특성을보인다. 따라서
한국정밀공학회지제 33 권제 1 호 pp. 31-35 January 2016 / 35 혼합챔버내넓은영역에서혼합이일어날때균일도가향상되는것으로판단된다. 4. 결론 반응사출성형에사용하는믹싱헤드의혼합효율을높이기위해노즐을설계하였다. 설계한노즐의성능을평가하기위해분사력과유량측정하였다. 노즐의니들팁직경과출구직경이커질수록분사력은증가하였으나, 노즐출구직경이 3mm 이상에서는증가폭이급격히작아지면서이후일정한분사력과유량결과를보였다. 이러한결과는믹싱헤드의혼합효율이레이놀즈수에비례하는선행연구결과에서와같이분사력증가로인해점차혼합효율이좋아지지만노즐출구직경이 3mm 이후부터는큰편차가없다는것을알수있다. 설계된노즐의혼합효율가시화실험에서는분사력이가장큰노즐니들팁직경 2mm 에서혼합효율이좋은것으로관찰되었다. 두유체가혼합챔버내에서충돌혼합하는과정에서높은분사력에의한충돌로넓은영역으로혼합과정이이루어지면균일혼합이이루어지는것으로판단된다. REFERENCES 1. Youn, J. W. and Kim, H. S., A Study on Foaming Characteristics of Polyurethane Reaction Injection Molding Using Cup Foam Test, Proc. of KSTP Autumn Conference, pp. 106-109, 2008. 2. Kwon, J. W. and Lee, D. G. Cure Monitoring for Prototyping of Reaction Injection Molding, Proc. of KSPE Spring Conference, pp. 32-36, 2001. 3. Lee, H. S. and Kim, D. M., A Study on Mixing Characteristics of Two-Component Polyurethane for In-Mold Coating, J. Korean Soc. Precis. Eng., Vol. 30, No. 3, pp. 317-323, 2013. 4. Rho, B. J., Jeung, W. T., Lee, S. J., and Kim, S. M., Flow Characteristics of High Pressurized Jet with Aspect Ratio, Proc. of KSME Autumn Conference, pp. 717-722, 2003. 5. Kolodziej, P., Macosko, C. W., and Ranz, W. E., The Influence of Impingement Mixing on Striation Thickness Distribution and Properties in Fast Polyurethane Polymerization, Polymer Engineering & Science, Vol. 22, No. 6, pp. 388-392, 1982. 6. Erkoç, E., Santos, R. J., Nunes, M. I., Dias, M., and Lopes, J. C. B., Mixing Dynamics Control in Rim Machines, Chemical Engineering Science, Vol. 62, No. 18, pp. 5276-5281, 2007. 7. Santos, R. J., Teixeira, A. M., and Lopes, J. C. B., Study of Mixing and Chemical Reaction in Rim, Chemical Engineering Science, Vol. 60, No. 8, pp. 2381-2398, 2005. 8. Santos, R., Teixeira, A., Costa, M., and Lopes, J., Operational and Design Study of Rim Machines, International Polymer Processing, Vol. 17, No. 4, pp. 387-394, 2002. 9. Santos, R. J., Erkoç, E., Dias, M. M., Teixeira, A. M., and Lopes, J. C. B., Hydrodynamics of the Mixing Chamber in Rim: Piv Flow-Field Characterization, AIChE Journal, Vol. 54, No. 5, pp. 1153-1163, 2008. 10. Tucker, C. L. and Suh, N. P., Mixing for Reaction Injection Molding. I. Impingement Mixing of Liquids, Polymer Engineering & Science, Vol. 20, No. 13, pp. 875-886, 1980. 11. Tucker, C. L. and Suh, N. P., Mixing for Reaction Injection Molding. II. Impingement Mixing of Fiber Suspensions, Polymer Engineering & Science, Vol. 20, No. 13, pp. 887-898, 1980. 12. Nguyen, L. T. and Suh, N. P., Processing of Polyurethane/Polyester Interpenetrating Polymer Networks by Reaction Injection Molding (Rim). Part I: Design of a High Pressure Rim System, Polymer Engineering & Science, Vol. 26, No. 12, pp. 781-798, 1986. 13. Nguyen, L. T. and Suh, N. P., Processing of Polyurethane/Polyester Interpenetrating Polymer Networks by Reaction Injection Molding. Part Ii: Mixing at High Reynolds Numbers and Impingement Pressures, Polymer Engineering & Science, Vol. 26, No. 12, pp. 799-842, 1986. 14. Nguyen, L. T. and Suh, N. P., Processing of Polyurethane/Polyester Interpenetrating Polymer Networks by Reaction Injection Molding. Part III: Flow Reorientation through Multiple Impingement, Polymer Engineering & Science, Vol. 26, No. 12, pp. 843-853, 1986. 15. Santos, R. J., Erkoç, E., Dias, M. M., and Lopes, J. C. B., Dynamic Behavior of the Flow Field in a Rim Machine Mixing Chamber, AIChE Journal, Vol. 55, No. 6, pp. 1338-1351, 2009.
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