Rational Design Concept and Proportioning of Weight Reduction Holes for Large Cast Iron Platforms with T-Slots
The sand core of a large cast iron platform is composed of two combined sand cores. During the core-making process, the entire sand core is coated with water-based paint, applied locally. The sand core is preheated to dry the coating. Spraying and brushing may not be fully executed, leading to missed coatings and sand adhesion. The pouring temperature is high, with good fluidity, low surface tension, and high dynamic pressure of the molten metal. Due to the thin-walled structure of the shell casting, the wall thickness at thin sections of the large cast iron platform is designed to avoid cold shut defects. In the original process, the pouring temperature was relatively high.

The pouring speed for large cast iron platforms is slow, and the pouring cup is not fully filled. The sprue entrains air and inhales impurities, forming entrapped gas holes and slag holes. The small capacity of the pouring cup causes the molten metal in the large cast iron platform to form vortices, generating gas holes. The connection between the pouring cup and the pouring system of the large cast iron platform is poorly sealed, especially between the sprue and the pouring cup. Under negative pressure, sand inclusions and gas holes are easily formed. The high dust content in the molding sand and poor permeability, combined with internal blockages in the negative pressure pipeline, cause distortion of the negative pressure level. This results in the negative pressure value around the mold cavity being much lower than the indicated negative pressure, preventing gaseous products from being discharged in time, leading to gas holes or surface wrinkles.
Some researchers have studied the temperature range for crack formation in carbon steel castings. Using X-ray imaging, the temperature range for crack formation in large cast iron platforms was recorded. Regardless of the carbon content in the casting, the temperature range for hot crack formation is near the solidus line. The strength and plasticity of the casting at high temperatures are low, and the casting stress of large cast iron platforms is high.
Reasonable pouring process and negative pressure level. The lost foam casting process follows the principle of filling the closed sprue. The pouring should not be too fast or too slow, turbulent, or interrupted, and the sprue must not be exposed. The pouring speed for large cast iron platforms, especially during the pause in hoist lifting, should strive for balance without interruption. Towards the end, the ladle should be slowly withdrawn to allow sufficient time for slag, gas, and vaporized residues to escape. Excessive negative pressure intensifies metal penetration and sand adhesion, causing a wall-adhesion effect that hinders the adsorption of liquid foam by the coating, resulting in many gas holes. Appropriate negative pressure is both a measure for discharging gas and preventing sand adhesion. Causes of gas holes in large cast iron platforms: excessive resin and hardener in resin sand, high resin content, high angularity coefficient of raw sand and sand, fine particle size, high ignition loss and micro-powder content, and high gas generation of the sand with low permeability. Due to damp, rusty, or oily furnace charge, humid climate, un-dried melting tools and ladles, and improper metal composition,
Pouring process: The influence of pouring temperature and pouring speed on hot crack formation in castings is complex. Generally, for thin-walled sections of large cast iron platforms, higher pouring temperatures and speeds are advisable. This allows the casting temperature to quickly become uniform, preventing local overheating, and enables slower condensation of the casting, reducing shrinkage stress in large cast iron platforms, thereby minimizing or preventing hot cracks. For thick-walled sections, lower pouring temperatures and slower pouring speeds are recommended. If high pouring temperatures and fast pouring speeds are used for thick-walled sections, the shrinkage of the molten metal can easily cause hot cracks in large cast iron platforms. In severe cases, the casting may simultaneously develop hot cracks and shrinkage porosity (if both issues occur at the same location, it is called shrinkage cracking).
Additionally, low clay and ash content in the molding sand can prevent the filling of sand gaps, affecting the sand間隙. Meanwhile, besides producing large cast iron platforms, the workshop also manufactures cylinder heads. The performance of the molding sand for large cast iron platforms is affected by product changes. The low content of coal dust in the sand and poor coal dust quality impact the sand's anti-adhesion properties. The main additive for preventing sand adhesion and improving surface finish in green sand iron castings is coal dust. The workshop's molding sand has a coal dust content between 3.5~4%. The gas generation of the molding sand ranges between 17~20 ml/g.
Weiyue Machinery, Ms. Xie, 15350773479