A method for determining the drying effect of a silicon sol gel shell and its application in energy saving and efficiency improvement

　　A method for determining the drying effect of silica sol shell molds and its application in efficiency enhancement and energy saving

　　Jiaxing Yaiida Special Steel Casting Co., Ltd.

　　Wang Yan, Xiang Guangkun, Jiang Yafei, Ma Cai

　　Ji Junhao

　　Abstract: This paper analyzes the drawbacks and difficulties in detecting the interlayer drying degree of investment casting shell molds using the silica sol process, and provides a new method for determining the drying effect of silica sol shell molds. It aims to provide a reference solution for improving the quality of precision casting products, shortening the product production cycle, and reducing energy consumption by utilizing a new method for measuring the drying effect of silica sol shell molds.

　　Keywords: precision casting, shell drying, energy saving and consumption reduction, temperature difference measurement method

　　I. Overview

　　1. When producing precision castings using the silica sol shell process, "excessively long shell making cycle" is a common understanding among us precision casting practitioners. Typically, it takes more than 12 hours to dry and harden one layer, and even longer for difficult-to-dry parts such as deep holes. At the same time, because investment casting shells need to be manufactured in layers, each layer needs to be fully dried to ensure that the next layer does not cause re-dissolution and detachment when the shell is dipped in slurry. The slurry itself contains moisture, which can penetrate into the already dried shell, leading to a long overall drying cycle. (As shown in the figure below) [1]

　　A long shell making cycle not only affects the casting delivery time and increases electricity consumption and labor costs, but also leads to a large drying room area, resulting in increased construction costs or limited production scale.

　　2. From the figure above, it can be analyzed that the shell making time accounts for more than 50% of the entire casting production cycle. To shorten product delivery time, shortening the shell making cycle is the core link of the problem. The key factors for shortening the shell making cycle can be divided into internal and external factors: internal factors are mainly binder characteristics, and external factors are drying conditions. The main factors affecting shell drying speed are: room temperature (t℃), relative humidity (RH%), wind speed (m/s), air volume, and air flow conditions in the drying room. Currently, most precision casting factories in China can control the above process parameters and meet process requirements. For example, using air conditioning units, humidifiers or dehumidifiers, fans, etc., to achieve constant temperature, constant humidity, and wind speed/air volume requirements. However, the interlayer drying time of the shell is difficult to determine reasonably. Due to the different complexity of casting structures, different parts of the same set of shells, such as inner cavities and outer surfaces, have different drying degrees under the same drying conditions. There is a lack of quantitative measurement methods to determine whether the most difficult-to-dry areas (inner cavities, deep holes, pipe grooves, etc.) are completely dry and whether the next shell operation can be performed in time. To prevent "re-dissolution" of silica sol due to incomplete drying of the coating in complex cavities, leading to defects such as local cracking, steel leakage, and steel penetration in the shell, most factories adopt the "rather long than short" measure for shell drying time. This is one of the main reasons for shell (casting) quality fluctuations and long production cycles. However, excessive drying (especially of the surface layer) can cause "shell cracks" because the shrinkage stress of silica sol due to water loss is greater than the shell strength when the glue is cold or frozen. Even the surface layer may delaminate or peel off, leading to common casting surface defects such as "sand dropping," "fins," "flow lines," "delamination," or "bulging." How to accurately determine the complete drying time of the shell (referring to the most difficult-to-dry part) is a key measure for formulating reasonable drying processes for each shell layer.

　　II. Current methods for detecting shell drying degree and their drawbacks:

　　According to literature [2][3], common methods for detecting the drying degree of shell layers include:

　　1) Resistance method: Using a "shell desiccant tester," the relationship between the moisture content (water loss rate) in the shell and its resistance value is utilized to determine whether the shell is thoroughly dry by the degree of increase in resistance value (HB5352-1986).

　　Disadvantage: The first three layers are relatively accurate, but the resistance value of the back layer (or middle layer) shell is difficult to determine [2].

　　2) Colorimetric method: By adding a color developer to the slurry, the change in moisture content in the slurry causes the shell layer color to change accordingly, thus qualitatively determining whether the shell is thoroughly dry.

　　Disadvantage: Qualitative, not quantitative, and high cost.

　　The above two methods are not widely used in actual production due to their limited versatility (it is difficult to accurately measure and observe small holes, pipe grooves, etc.).

　　III. A New Method for Detecting Shell Mold Drying Degree

　　Basic Principle:

　　Evaporation is an endothermic process—meaning the process of evaporation itself must absorb thermal energy; otherwise, evaporation cannot occur. Under conditions of equal mass, temperature, and pressure, gaseous water contains more energy than liquid water, so when liquid water turns into gaseous water, it must absorb energy from the outside (i.e., absorb heat); otherwise, energy conservation cannot be achieved.

　　Applying this principle to our precision casting shell drying, we find that when the shell humidity is high, its temperature is low; conversely, as the shell humidity decreases, its temperature increases. Drawing upon foreign testing experience, we found that using the "temperature difference measurement method" to determine the temperature difference between the most difficult-to-dry areas and the easiest-to-dry areas can assess the shell's drying degree.

　　Test Equipment:

　　1) Infrared thermometer

　　2) Anemometer

　　3) Temperature and humidity recorder

　　4) Industrial temperature and humidity sensor

　　5) Dual-channel paperless recorder

　　Test Procedure and Conclusion:

　　1. Connect an industrial moisture (temperature) sensor to a dual-channel paperless recorder to test and record the temperature change as a freshly coated and sanded shell mold, initially containing 10-20% free water, dries to less than 1% free water. Due to the precision limitations of the testing equipment, this test only yielded relative data. Under relatively identical environmental parameters, with room temperature at 25.2-25.5°C and humidity at RH 83-86%, we can conclude from the test comparison and analysis (as shown in Table 1) using the figures below (Figure 2 and Figure 3) that, under the same environmental conditions, a decrease in the free water content of a wet shell mold (i.e., during the drying process) is accompanied by an increase in temperature.

　　2. Under the same environmental parameters, we use a thoroughly dried mold shell as a standard comparison block for measuring the drying temperature. An infrared thermometer is used to measure the temperature difference between the mold shell test block, the wet mold shell, and the dry mold shell. The test results (Figure 4) show that an infrared thermometer can determine the temperature difference between the wet and dry mold shells, thus determining the drying effect of the mold shell.

　　Figure 4: From left to right, the temperatures of the comparison block, the undried mold shell, and the dried mold shell, respectively.

　　3. Under the same temperature and humidity conditions, a wind speed meter and an infrared thermometer are used to measure the influence of wind speed on the surface temperature and measurement accuracy of the mold shell. Through the data analysis of Table 2 and Figure 5 below, it can be verified that the influence of wind speed on the measured temperature of the mold shell can be ignored;

　　Figure 5: Comparison of wind speed and measured temperature.

　　4. After drying for a certain period of time, the temperature difference between the outer surface (cup mouth or mold head) and the deep hole and narrow groove of the same group of mold shells is tested to determine the drying effect of the mold shell. The data comparison in Figure 6 shows that on the undried mold shell, there is a certain temperature difference between the easiest-to-dry mold shell gate cup position and the most difficult-to-dry deep holes and narrow grooves, but the temperature tends to be the same after drying is complete.

　　Figure 6: Temperature test at different positions of the same group of mold shells.

　　Based on the above experimental process and data, we have obtained the following information:

　　After each layer of mold shell is coated, sanded, and dried for a certain period of time, an infrared thermometer can be used to determine the surface temperature of each layer. A completely dried mold shell is used as a standard comparison block for reference, and then the surface temperature of the mold shell gate cup and the most difficult-to-dry inner cavity, deep holes, and grooves of the mold shell are measured. There will be a certain temperature difference between the two before drying (generally 1-3℃), with the outer surface being higher and the inner cavity temperature being lower, but both are slightly lower than room temperature (generally 2-4℃ lower). As the drying time of the mold shell increases, the water evaporates as the silicone sol dries, the humidity on the inner and outer surfaces of the mold shell decreases, and the temperature increases accordingly. The temperature difference between the inner and outer surfaces is generally (1.5-2.5℃). When the surface temperature of the mold shell reaches room temperature (±1℃), and the temperature difference between the inside and outside drops to within 0.5℃, it indicates that the mold shell is completely dry.

　　IV. Conclusions and Practical Applications:

　　1. Using the "temperature difference measurement method", the temperature and humidity are exponentially related to the drying time of the mold shell [2]. Using an infrared thermometer with a smaller temperature range, higher resolution, and smaller error can quickly and accurately determine whether the mold shell layer is completely dry by measuring the temperature (difference) of the outer surface (easiest-to-dry part) and inner surface (most difficult-to-dry part) of the mold shell and room temperature, that is, the silicone sol dehydration drying has completed the transition from sol to gel—an insoluble hard gel. At this time, the next layer of mold shell immersion coating operation can be performed.

　　2. Through production practice, accurately judging the drying degree of each layer of the mold shell can achieve "rapid shell making", greatly shortening the drying time between mold shell layers. The total drying time of a typical six-layer mold shell can be shortened from ≥63 hours to ≈30 hours (saving 50%), and the entire shell making production cycle can be shortened from 5-7 days (castings within 10 kg) to 2-4 days (from the first immersion coating to dewaxing).

　　3. In our company's current production, the average drying time between shell making layers is 5 hours, and the average shell making production cycle is 2-3 days, which is half the time compared to the past. The energy consumption of shell making has been reduced by 60%, and the production area has also been reduced by 50% (the original back-layer drying room was 2*130 m², now only one room is used). Shell making production efficiency has increased, energy consumption has decreased, costs have decreased, and delivery times have shortened.

　　V. References:

　　[1] Investment Casting Shell Manufacturing and Enhanced Fast-Drying Silica Sol, Lü Zhigang, Yan Shuangjing, Cui Xulong, Jiang Buju

　　[2] Practical Investment Casting Technology (Jiang Buju, 2008).

　　[3] Chen Bing Collection: Control of Silica Sol Shell Drying Process.