yvonne@nydcasting.com 
For the sand sticking problem of clutch shell casting in the workshop, the possible causes of the problem were analyzed according to the production situation of the workshop. The cause-effect diagram of sand sticking problem of clutch shell casting was drawn by using quality analysis method. Measures were formulated and verified in production to determine the key factors leading to sand sticking of castings, improve the production process, and reduce the scrap rate of castings.
1. Introduction
The clutch shell in the workshop is cast by green sand process, and the sand core is made by coated sand process. The casting return rate is as high as 30%, and 10% of the sand sticking castings are scrapped because of severe sand sticking and cannot be removed. The sand sticking parts of the scrapped sand sticking castings include the inner cavity and the outer surface of the casting. The conventional measures of brushing paint and increasing the coal powder content have not improved the sand sticking condition.
2. Sand sticking mechanism
Casting sand sticking can be categorized into mechanical, chemical, explosive, and hot sand sticking. Mechanical sand sticking, also called metal liquid penetration bonding, occurs when liquid metal infiltrates sand gaps via capillary or gas phase penetration, forming a mixed adhesion layer. The tendency depends on opposing forces: penetration power (metal’s dynamic and static pressure) and penetration resistance (sand pore resistance and gas back pressure).
Chemical sand sticking results from high-temperature oxidation of metal, forming FeO, which reacts with SiO₂ in the mold to produce low-melting ferrous silicate that adheres to castings. Explosive sand sticking happens when gas-producing substances in sand rapidly gasify, creating pressure that forces metal into gaps. Hot sand sticking occurs when low SiO₂ content in the sand causes surface sintering due to high metal temperatures.
3. Analysis of the reasons for sticky sand on the clutch housing
Through statistical analysis of the sand sticking castings of the clutch housing, it was found that the sand sticking parts of the castings that were not seriously sand sticking and could be cleaned up were concentrated in the pits, bosses, and corners on the outer surface. However, the outer surface of the scrapped castings had sand sticking except for the large arc surface and the top surface, and the inner cavity had sand sticking except for the sand core brushing parts, and the sand sticking was dense and the sand sticking layer was thick. Since the workshop uses wet sand process to produce clutch housing, and the raw sand has high SiO2, chemical and hot sand can be excluded.
The factors that lead to mechanical sand sticking of castings are:
3.1. The molding and core sand have coarse particles, low mud and ash content, large gaps, and weak metal penetration resistance.
Workshop molding sand has a particle size of 50/100 or 100/50, with an average fineness of 50–57, while coated sand ranges from 49 to 55. The raw sand is coarser than that used in wet mold processes, with air permeability between 130 and 170. However, insufficient mud and ash content prevent proper gap filling, affecting sand compactness. Additionally, the workshop produces clutch housings, cylinder blocks, and cylinder heads, leading to variations in molding sand performance due to product changes.
3.2. The compactness of the sand mold and sand core is low or uneven, and the parts are loose.
The molding mold is complex, and the pits and corners are not easy to compact; the molding equipment parameters are improperly selected, and the molding pressure is low; the molding sand has poor fluidity and is not easy to fill. Factors such as low compactness of the sand mold may cause the compactness of the sand mold. The low compactness of the sand core may be caused by poor exhaust effect of the core box and low sand injection pressure during the core making process.
3.3. The coating quality of the mold and core is poor, the coating thickness is uneven, and the coating peels off.
The workshop sand mold is sprayed with alcohol-based coating. Because the compactness of the upper mold near the parting surface is low, it is easy to cause sand collapse after spraying and drying. Therefore, the upper mold is sprayed less in the original process and the parts prone to sand collapse are not sprayed. The sand core is composed of two sand cores, 1# and 2#. During the core making process, the 1# sand core is brushed with water-based coating as a whole, and the 2# is brushed locally. The sand core is used to preheat and dry the coating. During the spraying and brushing process, the operation may not be in place, resulting in missing coating and sticking sand.
3.4. High pouring temperature, good fluidity, low surface tension, and high metal liquid dynamic pressure.
Because the shell casting is a thin-walled part, the thinnest part has a wall thickness of 8mm. In order to avoid cold shut waste of castings, the pouring temperature in the original process was relatively high.
3.5. The coal powder content of the molding sand is low and the quality of the coal powder is poor.
The main molding sand additive to prevent sand sticking and improve the surface finish of wet mold iron castings is coal powder. The quality of coal powder affects the anti-sand sticking of the molding sand. The effective coal powder content of the workshop molding sand is between 3.5% and 4%. The gas emission of the molding sand is between 17 and 20ml/g.
3.6. The height of the upper box or the pouring cup is too high, and the static pressure of the metal liquid is large; the pouring system and the riser are improperly set, resulting in local overheating of the mold and casting, causing sand sticking.
Factors causing explosion and sand sticking include high mud and moisture content in molding sand. High coal powder content increases gas emission. Poor ventilation worsens the issue. Fast pouring speed contributes to defects. Sand cores made of coated sand emit more gas than resin sand cores. Sand core curing affects gas emission. The coating on the sand core surface also emits gas. Brushing a hot sand core can cause cracking. The sand core must cool before brushing. This delays drying and affects coating quality.
4. Improvement measures
Based on the above analysis, combined with the actual situation of the workshop and the sand sticking of other models of castings, the non-critical factors affecting the sand sticking of the clutch housing casting are eliminated, and corresponding measures are taken, and the operability is analyzed and verified in production:
1）. Reduce the gap of the molding sand.
Because the original sand for core making and the original sand for molding in the workshop are supplied by the same system, and the molding line produces three products: cylinder block, cylinder head, and clutch housing, adjusting the original sand particle size will affect the production of core making and the other two products, so the original sand particle size remains unchanged. In the process of producing clutch housing, the amount of new sand added is reduced or no new sand is added, and the mud content and ash content of the molding sand are appropriately increased. Considering that the increase in the content of the two will increase the tendency of the casting to have pores, the increase is controlled within 0.5%. After taking measures, the sand sticking of the casting is slightly reduced, but the effect is not obvious.
2）. Improve the compactness of the sand mold.
Since the casting structure cannot be changed, the molding process was optimized to enhance sand mold compactness. The foundry uses a German KW high-pressure molding line with adjustable pressure. The clutch housing’s upper molding pressure was increased from 79 to 100, improving compactness. More alcohol-based coatings were applied to areas prone to sand sticking, and the molding sand’s water content was reduced for better fluidity. Verification showed no increase in casting pores, and the return rate dropped to 15%.
For sand cores, quality was improved by adding more exhaust grooves and increasing their depth. Coated sand, with excellent fluidity and dense structure, ensured good core quality even with minimal coating. However, production tests showed no significant reduction in sand sticking within the casting’s inner cavity.
3）. The workshop coating is commonly used for three products.
There is no abnormal sand sticking phenomenon in the cylinder block and cylinder head, so the coating quality problem can be ruled out. After the molding pressure is increased, the alcohol coating spraying on the outer mold has been incorporated into the process. At the same time, strengthen the training of operators to ensure the quality of operation. The 2# sand core was brushed with water-based coating as a whole and verified. It was found that although the sand sticking in the inner cavity of the casting was solved, the scrap rate of the casting pores increased significantly, so the brushing method was not feasible.
4）. Reduce the casting pouring temperature.
Although the clutch housing is a thin-walled part and is prone to cold shut waste, the casting is small in size and the runner diameter is large enough, so that a box of 2 pieces can be filled in only about 12 seconds. To a certain extent, it makes up for the problem of cold shut waste in castings. Therefore, the pouring temperature was lowered by 10℃ for verification, and the verification results showed that the sand sticking of the castings was improved.
5）. Because there was no abnormal sand sticking phenomenon in the cylinder block and cylinder head.
The coal powder quality problem was also ruled out. The gas emission of the workshop sand was not high, and there was room for increase. The effective coal powder content of the sand was increased by 0.3%~0.5% for verification, and no significant improvement was observed.
6）. The casting structure determines the casting system of the casting to a certain extent, which has been verified in the mold verification.
7）. Reduce the gas emission of the sand and sand core.
Since the workshop sand has low coal powder content, gas emission cannot be further reduced. For the sand core, curing time was extended by 10 seconds and temperature increased by 10℃ for better curing. As the core coating relies on residual heat for drying, complete drying was not guaranteed. The 1# core, with a simple flat-plate structure, was tempered after coating. To prevent deformation during secondary curing, the tempering temperature was set to 180℃. Additionally, instead of brushing, the 1# core was dipped, tempered, and dried after cooling. Verification showed that simultaneous sand sticking inside and outside the casting was nearly eliminated, reducing the casting return rate to around 10%.
Through the above verification, it can be determined that the molding pressure, pouring temperature, sand core solidification degree and sand core gas emission are the key factors affecting the sand sticking of the clutch housing. Other factors can be abandoned due to low operability or insignificant effect. By improving the key factors, the problem of sand sticking and scrapping of the clutch housing is solved. At the same time, the casting return rate is reduced, saving costs.
5. Summary
There are mechanical sand sticking and explosive sand sticking in the clutch housing. The production mechanisms are different, and some are even opposite. There are many factors affecting sand sticking. Only through rigorous analysis, formulation of corresponding measures and verification, determination of key factors, and improvement can the surface quality of the clutch housing be better controlled.