Cause Analysis: The deformation observed in the spring cover is primarily caused by two main factors. First, the wax pattern itself may be deformed. Since the cylinder and body are pressed separately and then manually bonded together, weak bonding can lead to misalignment or tilting of the cylinder. This type of deformation can be effectively prevented through enhanced on-site operational management. Second, deformation can occur during the solidification and cooling process. As the casting cools from its solidification temperature to room temperature, shrinkage is restricted, leading to shrinkage stress. If the shrinkage stress is too high and the structural rigidity of the casting is insufficient, especially in the intermediate cylinder, deformation may occur. Additionally, excessively high casting temperatures or premature shell formation can also contribute to this issue.
Process and Tooling Shrinkage Solution: The four corners of the blue area are thick sections in the spring cover. Although there is a through hole, the hot spot is not properly decomposed, forming an isolated heat source that does not allow for smooth filling. This leads to a loose channel. Moreover, if the pouring temperature is too high or the molten metal contains excessive impurities, it can affect the flow and result in shrinkage defects.
Process Improvement: Based on the above analysis, several improvement measures have been proposed. These include degassing and slag removal to purify the molten steel, ensuring the quality of the molten metal meets all required standards. The pouring temperature must also be strictly controlled. To address the deformation issue, reinforcing ribs are used under the condition of controlling the pouring temperature and speed. The size of the reinforcing rib depends on the wall thickness at the joint, typically using three or four small round ribs.
The most common method to solve shrinkage is the use of risers, internal and external chill methods, or the so-called "meat adding" technique. However, using a blind riser here is inconvenient during both mold making and waxing. Since investment casting is a hot-shell process, cold iron cannot be used to increase the wall thickness. Therefore, the casting is designed to solidify sequentially by increasing the wall thickness, which helps eliminate shrinkage. However, this changes the original structure, requiring mechanical machining to achieve the desired dimensions. Given the deep inner cavity of the spring cover and internal obstacles, machining becomes difficult and costly, making this approach less desirable.
The four corners of the blue area represent isolated hot spots that need individual supplementation. Therefore, supplementary ribs were considered as a solution to the shrinkage problem. In addition, reinforcing ribs are necessary to prevent deformation. Thus, a combined approach was adopted—using one rib to address both deformation and shrinkage. The calculation formula for the supplementary rib is based on the wall thickness at the rib location. After calculations, it was decided to use four small round reinforcing ribs to fill each of the four corners.
Process Verification and Conclusion: During the trial production, strict control over all processes was implemented to minimize human-induced defects. Joint trials were conducted, and after cleaning, no surface shrinkage was observed. Radiographic testing in the inspection room showed only minor looseness in some castings, achieving an acceptable pass rate. When cutting the reinforcing ribs from the middle position, obvious shrinkage was observed, indicating that the ribs functioned effectively as feeding elements. To further confirm the reliability of the process, multiple batches were tested, maintaining a yield rate above expectations.
There are various ways to solve casting defects, but this study achieved unexpected results by cleverly applying reinforcing ribs, successfully addressing both deformation and shrinkage issues in the spring cover.
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