In casting production, urea resin sand's collapsibility and strength are two mutually restrictive factors. Balancing these two is crucial to ensuring casting quality. Insufficient strength can easily cause the sand mold to deform or even collapse under the impact and pressure of molten metal, leading to problems such as sand sticking and dimensional deviation. Poor collapsibility can make it difficult to clean the sand mold after cooling, increasing the workload in subsequent steps and potentially damaging the casting surface due to mechanical forces during cleaning. Therefore, finding the right balance between these two factors requires a comprehensive approach, focusing on both material properties and process control.
The amount of resin added is fundamental to regulating this relationship. Urea resin sand, as a binder, directly affects the strength of the sand mold. Increasing the amount of resin enhances the bond between sand particles and consequently improves mold strength. However, excessive resin can cause tough residues to form after high temperatures, hindering the collapsibility of the sand particles. Conversely, reducing the resin content can improve collapsibility but may result in insufficient strength. Therefore, the appropriate resin ratio must be determined based on the structural complexity and weight of the casting. While ensuring the sand mold can withstand the pressure of the molten metal, the resin dosage should be minimized to allow for proper collapsibility.
The selection and ratio of the curing agent also play a crucial role. Different curing agents affect the resin's curing speed and structure after curing, which in turn alters the sand mold's strength and its collapsibility after high temperatures. Some curing agents create a brittle solid that easily breaks after cooling, favoring collapsibility. Others, however, make the solid tougher, increasing strength while reducing collapsibility. By adjusting the type and ratio of the curing agent, the final properties of the sand mold can be adjusted within a certain range, achieving a balance between strength and collapsibility.
The characteristics of the urea resin sand grains are also crucial to balancing these two aspects. The grain size, shape, and surface condition of the sand grains influence the distribution of the resin and the bond strength between the sand grains. Coarser sand grains have larger gaps between them, making it difficult for the resin film to fully cover them. While this reduces the bonding area and facilitates collapse, it may also reduce overall strength. Fine sand grains create a denser structure and improve strength, but the larger contact area between the grains may hinder collapse. Selecting sand grains of moderate size and relatively rounded shape ensures even distribution of the resin, providing sufficient strength, while also reducing intergranular interlocking, creating conditions for subsequent collapse.
Controlling process parameters is crucial for balancing these two aspects. Too short a mixing time will result in uneven mixing of the resin and sand, leading to excessive strength in some areas and insufficient strength in others, and also affecting the consistency of collapse. Too long a mixing time may weaken the resin's bonding properties, reducing overall strength. Furthermore, the curing temperature and time must be precisely controlled. Excessively high temperatures or prolonged times can overcure the resin, making the sand mold too hard and difficult to collapse. Conversely, incomplete curing can result, resulting in insufficient strength. By optimizing these process parameters, the sand mold can achieve both sufficient strength after curing and smooth collapse after the casting cools.
The cooling process of a casting indirectly affects its collapsibility. After the molten metal is poured into the sand mold, the high temperature gradually decomposes the resin. Different cooling rates in different areas of the sand mold can lead to varying degrees of collapsibility. For castings with uneven wall thickness, the thicker walls cool more slowly, leaving the surrounding sand mold exposed to high temperatures for a longer period of time. This allows for more complete resin decomposition and better collapsibility. However, thinner walls cool more quickly, leaving a significant amount of incompletely decomposed resin in the sand mold, which can affect collapsibility. Therefore, when designing the sand mold, the resin-to-hardener ratio can be adjusted based on the cooling characteristics of the casting to ensure that the collapsibility and strength requirements are met in each area of the sand mold.
Post-processing can further optimize the balance between these two factors. For castings with complex structures where it's difficult to balance strength and collapsibility, a small amount of collapsing agent can be added to the sand mold. These agents decompose at high temperatures, producing gases or forming low-melting-point compounds that help break down the structure of residual resin and promote sand grain collapse without significantly affecting the strength of the sand mold. In addition, proper vibration treatment of the sand mold after pouring can also accelerate the separation of sand particles, improve the collapse efficiency without damaging the casting, and make up for any deficiencies that may exist in the early process control.
