tain temperature. Tus, the solidification time will be shorter. Higher thermal conductivity will allow the heat of the liquid metal to flow through the shell more quickly, which also shortens the solidification time and increases the maximum temperature of the shell. Sensitivity testing by modeling also showed the external heat transfer coefficient mainly effected the shell and casting cooling rates after solidification. To evaluate the shell density and porosity, whole pieces of the shell containing all layers were examined and the overall bulk density and open porosity accessible for water were calculated. Seven industrial ceramic shells (Tables 1 & 2) were


evaluated in the study and a thermal property database was developed. According to the results of the tests, temperature- dependent specific heat capacities in all shells had a similar trend, but the average and maximum values mainly depended on the phase of the starting materials and the reactions and transformations during the thermal processing, which were not readily predictable (Fig. 2). Generally, at above room temperature, the thermal con-


ductivity of the most dense ceramics decreased with increas- ing temperature because phonon scattering is more intense from the vibrating lattice at a higher temperature. However, the investment casting shells, where the colloidal silica is used as a binder in most cases and a significant amount of fused silica is utilized as flour and stucco, more often showed an increasing thermal conductivity at higher temperatures due to the photon radiation becoming dominant at higher temperature in semi-transparent silica. Porosity has a significant influence on the thermal


conductivity. Between the two aluminosilicate shells (#4 and #6), #6 with higher total porosity (37.65%) exhibited lower thermal conductivity values throughout the measured temperature range compared to shell #4 having lower total porosity (33.52%). Another good finding is the weak temperature depen- dence of conductivity in the alumina-based shell (#5). Since the photon radiation in alumina is not significant until 1,832F (1,000C), this radiation compensates phonon scatter- ing in alumina and the porosity effects and consequently the thermal conductivity didn’t change much over the elevated temperature range. Te thermal conductivity and specific heat capacity values measured from laser flash for the shells studied are listed in Fig. 3. Shell #7 (rapid shelling technique) was highly porous and broke apart when being surface ground during laser flash sample preparation. Effective density calculated from sample surface topography was used to calculate these values. It was found that laser flash showed a similar trend to the inverse method on both thermal conductivity and heat capacity values.


Using the Data


When putting thermal property data from the inverse method and laser flash method together, as shown in Fig. 4., the thermal conductivity values were fairly close, yet the inverse method presented higher specific heat capacity values than the laser flash method. Because many thermal reac-


य़工Џ陶瓷型 ث，ИՇ开९ूङ库据数ਈ性热֨


ד型有۱，果ৈડ测据根。ડ测੧进） չ ੮（ד ם最չқ֮平Ѹ，ѷ近ब势趋ङ化Պچ温随容热比ङ қП要ՈӐйԽ材料չ加热过३ङՆ应չबՊ，这Љ ）。 ֣（测预易容


动振格晶Јچ温高较֨йं，时温室й高，常通


随率ح热ङד型瓷陶ङם最چإ，烈强更ذ散子גङ ە数ךם֨，ד型造铸模熔，৲然。Ѻ降৲高升چ温 ҁ用ੴਸ਼ी融熔ङ量ם，剂ৈডОҁਇ溶硅用҅Јӑ ПО成ذ辐子Ұйं，Јچ温高较֨，泥灰չঞূ 。升Ї率ح热现ӟ常经，状明透半是ਸ਼ी，ح （ד型ठ酸铝硅З 。ր影ੋ显有性热حث隙孔


ङ高较有Ӏ，ӄ֠ਸچ温ङ量测֨，） ＃ չ＃ Ѻ较率隙孔总比，ד型 总孔隙率（ ％）ङ＃ 。Ѻ率ح热ङד型 （ ％）ङ＃ ）电 ＃（ד型ׂ铝化氧是果ৈ९ूङञ有З一Ր


йѺ֨ذ辐子Ұङ铝化氧йं。Ѻ性赖依چ温ङ率ح 散子גङ铝化氧йं，显明Љ时）℃ q,（


ਸچ温过超҅即此֜，应效率隙孔չҤ੭ذ辐ङذ 。םЉЭ化Պङ性热ح，֠ 比չ率ح热ङד型ङ得测法ذ照Ұ激是，ॐ۱ ֣ ，高率隙孔）术技ד脱速快（ד型 热容ङ数қ。＃ 样据根。开ӣ੮֪֨，И३过ս样法ذ照Ұ激וӲ֨ 容热比չ率ح热，现Շچإ效有ङ算ઋ貌形面੮ङս ङѷঝӟ现੮法方է逆չ法ذ照Ұ激֨қ数य़Д这 趋势。


数据的҄用 放据数ਈ性热ङ得测法ذ照Ұ激չ法方է逆把当


է逆Ѹ，近接常非қङ率ح热，ॐ۱ ֣ײ，起一֨ О֜。қ数ङ法ذ照Ұ激й高қ数容热比ङॐ显法方 չ应Ն热ךકङ硅化氧и形定无ङИ料材成构ד型֨ 显ر率速չ量数ङ应Նп这，生ՇЈ温高֨是都Պब ，И法方է逆֨。қ数ङ容热比ङד型用҅۱ր影ੋ چ速却Ӓङ३过֡凝，热加速快ੴד型时液ً金注浇 Иҿ，联Ҽब量热时हךકЊ३过п这。慢较ثब却 激用چ温境环ङ验ડ֨，৲然。ր影ङ热潜Պब括包 ֨，此֜。ङ੫平是果ৈ量测样ડ量批ش，法ذ照Ұ Ф量测法ذ照Ұ激֨经已ਈՕՊबङ生фИ法方է逆 ӹՇ生。


算ઋખ理ङ法ذ照Ұ激չ法方է逆较比，֪样գ March 2016 FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 53


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