How to solve the sand sticking defect in the production of gray cast iron parts using film coated sand technology?

The serious sand sticking problem in the production of 27 kg gray cast iron belt pulley parts using film coated sand technology is a common defect in the casting process. Sticky sand can significantly increase cleaning workload, damage the surface quality of castings, increase costs, and may even lead to the scrapping of castings. To solve this problem, a systematic investigation and optimization need to be carried out from multiple aspects such as the coated sand itself, process parameters, molten iron characteristics, mold design and operation. The following are detailed solutions and measures:

1. Quality issues with laminated sand - core reasons:

a. Poor quality of raw sand: low SiO ₂ content/high impurities: Raw sand with low SiO ₂ content (such as<90%), or excessive impurities such as feldspar, mica, alkali metal oxides, etc., will significantly reduce the refractoriness and easily sinter and melt under the action of high-temperature molten iron, leading to chemical sand sticking. Unreasonable particle size distribution: The sand particles are too coarse (such as>70 mesh), the gaps between the sand particles are large, and the molten iron is prone to infiltrate and form mechanical sand sticking. If the sand particles are too fine (such as>140 mesh), although the surface is denser, the permeability is poor, and the gas pressure inside the sand core/mold increases, which may actually exacerbate the infiltration of molten iron or cause porosity. Poor particle shape: Sand with high angular coefficient (polygonal) has lower packing density and higher porosity than circular sand, making it more prone to mechanical infiltration and sand sticking.

b. Insufficient resin film performance: Insufficient resin content or poor quality: resin addition is too low (<1.8-2.2%), or the resin itself has low thermal strength and poor high-temperature resistance, unable to form a sufficiently strong and dense coking layer under the action of high-temperature molten iron to effectively isolate the molten iron. Incomplete curing: Insufficient mold temperature or curing time during core making can result in incomplete cross-linking and curing of the resin, low strength of the sand mold/core, and easy disintegration and failure at high temperatures.

How to solve it in actual production

Select high-quality raw sand: Priority should be given to raw sand with high SiO ₂ content (≥ 97%), low impurities, low angular coefficient (round or semi-circular), and moderate particle size (recommended mix of 70/140 mesh or 50/100 mesh). Improve the performance of laminated sand: increase the resin content: appropriately increase the amount of phenolic resin added (e.g. to 2.3-2.8%) to ensure the formation of a sufficiently thick and continuous resin film. Adding refractory filler: Adding additives that can improve refractoriness and reduce sintering tendency to the coated sand: Zirconia powder: the best effect, extremely high refractoriness (>2000 ℃), but the highest cost. It can partially replace the original sand or be used as an additive (5-20%). Chromite powder: high refractoriness, low coefficient of thermal expansion, and good resistance to metal penetration. Olivine powder: Good high-temperature stability and resistance to alkaline slag erosion. High alumina alumina powder/mullite powder: improves high-temperature strength. Use high-performance resin: Select resin specifically designed for laminated sand with high fire resistance, high strength, and low gas generation (such as modified phenolic resin). Ensure sufficient curing: Strictly control the core making process parameters (the mold temperature is usually between 220-260 ℃, and the curing time is adjusted according to the size of the sand core) to ensure that the resin is fully cured.

2. Reasons for pouring system and process parameters

a. Excessive pouring temperature: Grey cast iron has good fluidity, and excessive pouring temperature (such as>1450 ℃) will significantly enhance the permeability of molten iron to sand particles, exacerbating sand adhesion. High temperatures are also more likely to damage the resin film.

b. Excessive pouring speed: Excessive pouring speed increases the flushing force of the molten metal on the cavity wall, damages the integrity of the sand mold/core surface, increases the risk of molten iron infiltration, and gas pressure may also press molten iron into the gaps between sand particles.

c. Excessive head (pouring height): Excessive metal static pressure will force molten iron to more easily penetrate the pores between sand particles.

d. The influence of molten iron composition: high carbon equivalent (CE): high carbon silicon content (CE>4.3-4.5) will significantly improve the fluidity of molten iron and increase its permeability tendency. Low Mn/S ratio: When the sulfur content is too high or the manganese content is insufficient, less MnS is formed, which is not conducive to forming a dense oxide film/sulfide film on the surface of the casting to prevent infiltration. It is recommended to control the Mn/S ratio between 8-12. Phosphorus content: High phosphorus (P>0.1%) will reduce the surface tension of molten iron, increase wettability, and promote infiltration. Oxidation: Excessive oxidation of molten iron may produce more oxide inclusions, affecting the formation of a protective film on the surface.

Solution - Strictly control the pouring temperature: while ensuring complete filling and avoiding cold insulation, try to reduce the pouring temperature as much as possible. For a 27 kilogram gray cast iron pulley, controlling the pouring temperature between 1360-1400 ℃ (adjusted according to the wall thickness, with the lower limit for thick walled parts) is usually a feasible goal. The thermometer must be calibrated accurately! Optimize pouring speed: Adopt a smooth and moderate pouring speed. Consider using a pouring system with buffer bags or bottom injection to reduce direct impact on the mold cavity. Reduce the height of the indenter: While ensuring filling, try to reduce the height of the sprue cup or use a stepped sprue. Optimize the composition of molten iron: While meeting the mechanical properties (mainly strength) of castings, appropriately reduce the carbon equivalent (CE) (such as target CE=4.0-4.2), especially the silicon content. Ensure sufficient Mn/S ratio (≥ 10): By adjusting the scrap/pig iron ratio or adding manganese iron, ensure that the manganese content is sufficient to neutralize sulfur and form MnS. Control the oxidation of molten iron: carry out pre furnace inoculation treatment (improve graphite morphology, indirectly affecting the surface), avoid excessive stirring or prolonged high-temperature insulation that may cause oxidation.

3. Mold/sand core design and operation issues

a. Insufficient compactness of sand mold/core: Insufficient core injection pressure, poor mold exhaust, or short sand injection time result in low local compactness of sand mold/core, high porosity, and easy infiltration by molten iron.

b. Mold temperature too high: During continuous production, the mold temperature accumulates too high (>280 ℃), causing local pre solidification of the coated sand before or during sand blasting, affecting the overall strength and uniformity of the sand mold.

c. Unused (or improperly used) casting coating: Unpainted coating: For castings with high requirements or areas prone to sand adhesion, not applying fire-resistant coating is highly risky. Poor coating quality: low fire resistance, poor suspension stability, thin coating or uneven coating. Insufficient drying: The coating is poured before it is completely dried, and the water evaporates at high temperatures, generating pressure that may push the molten iron into the gaps between the sand particles or cause the coating to peel off.

d. Mold removal/handling damage: The sand mold/core is bumped during the process of mold removal, handling, and core assembly, resulting in local looseness or damage to the surface.

e. Improper design of sprue position: The sprue is directly facing the cavity wall or thin wall, causing high-speed molten iron to directly flush the surface of the sand mold/core.

Solution - Optimize the core making process: Ensure sufficient sand injection pressure and holding time to ensure that the sand mold/core is compact and uniform. Regularly clean and maintain the mold to ensure smooth exhaust plugs. Control the mold temperature, and if necessary, add mold temperature cooling devices (such as water cooling channels) or extend the production cycle. Mandatory use of high-quality refractory coatings and correct application.