Mold design is a rather complex task, as both the flow of molten metal and the sequence of metal solidification must be considered during the design process. In addition, stress deformation caused by temperature heat transfer in the mold may also lead to other problems. CM Taricco is an Italian mold manufacturer that discovered a problem of metal liquid leakage at the bottom of the mold during each casting process while developing new molds. At the beginning of mold production, this phenomenon only occurs in a few cases; As the number of production cycles increases, this phenomenon will become increasingly severe. Once a problem occurs, production personnel must immediately troubleshoot it, as this can result in unpredictable production time and increased casting costs.

Research on the causes of problem occurrence

    Due to the evaluation and numerical simulation of the casting system and slag package position during the initial development of this gravity casting mold, it is determined that the problem is not caused by the flow of molten metal. Gabriele Taricco (founder of CM Taricco) raised concerns that the leakage issue may stem from thermal stress during the casting process. Due to the lack of simultaneous evaluation of the mold cooling path during mold design, severe heat accumulation occurred in the mold. And this problem caused unexpected and severe deformation at the bottom of the mold, resulting in leakage of molten metal from there. In order to confirm whether this phenomenon is truly occurring, the company used LOW-3D for numerical simulation, hoping to confirm whether this problem can be reproduced in the simulation process and solve it from the perspective of mold design.

Numerical Setting and Problem Determination
     Using LOW-3D, the entire numerical model can be set and calculated in a short amount of time. Flow-3D's simple and efficient processing of grids can reduce numerical grids and improve execution efficiency. Compared with the traditional grid setting method of general software, LOW-3D can provide high-precision analysis results in a shorter running time.
   In this analysis case, considering the design of the parting surface position, the first technique adopted was to rotate the drawing surface before dividing the mesh, which could control the mesh as close to the casting position as possible. Even if there are small details on the casting, LOW-3D can still fully describe them.

   The second technique is to use a new conformal mesh for the inner cavity (thin wall) while retaining the traditional larger mesh blocks for the entire area. Random grids are suitable for open volumes and are limited to filling cavities with very small gaps.

Global view of molds and cores and their alignment with grid blocks

   In addition, to reduce the number of grid calculations, after rotating the mold surface by 20 degrees, the "domain removing" component of LOW-3D is used to remove the grid areas that do not require calculations.

    The remaining settings follow the traditional approach, utilizing most of the software's recommended default values. Due to these characteristics and the new region decomposition feature in FlOW-3D, the originally planned 9 million grids can be reduced to only 1.84 million grids in the fluid grid region and only 2.43 million grids in the solid grid region.

    By using the above grid setting techniques, the number of analysis grids was successfully reduced by about 80%, which can significantly shorten the required computation time.

Forming analysis

    After filling the simulation, to ensure good filling, the focus of the simulation is shifted to thermal cycle analysis. In this case, the setup is fast and simple, and it only takes 1 hour to replicate 10 production cycles on a regular desktop computer (i7 5930K, commercial value of $1500). The results confirmed the initial hypothesis of CM: by observing the temperature field from different angles and cross-sections in a single image, using FlowSight, it is clear that the temperature distribution of the mold can easily lead to expected deformation and metal liquid leakage. 

Simulation of mold temperature during mold thermal cycling process

    By utilizing the LOW-3D mold thermal cycle analysis technology, it is possible to fully simulate the temperature distribution of the mold caused by heat accumulation after undergoing filling, solidification, and cooling. At the initial setting, the preset mold undergoes ten production processes of switching the mold on and off. Using the FlowSight post-processing module of Flow-3D to analyze and output results, CM Taricco confirmed that there was indeed heat accumulation caused by uneven temperature distribution at the bottom of the mold. And these areas should be the main locations causing leakage problems.

Mold deformation analysis

     After determining that the problem may be caused by mold deformation due to mold temperature, resulting in metal liquid leakage, CM Taricco company hopes to confirm the amount of mold deformation in order to make mold design changes. XC Engineering assisted CM Taricco in setting up and performing calculations for LOW-3D, using FLOW-3D to directly calculate the deformation of the mold caused by uneven temperature distribution. These deformation results can be directly confirmed in the Flowsight post-processing module of FLOW-3D, and the deformation dimensions can be measured simultaneously.

Mold design changes

 
      After confirming that the accumulation of heat in the mold would indeed cause deformation and lead to leakage of molten metal, CM Taricco Company made the following mold design changes. By adjusting the parting surface design, excess support positions are added to the mold body during mold closing to reduce the possibility of mold deformation. In addition, during the design process, the thermal deformation temperature caused by uneven temperature distribution was also considered, and adaptability adjustments were made to the thickness of the mold.

conclusion

    After making changes to the mold design, CM Taricco company once again verified it using LOW-3D and confirmed that the mold deformation was within an acceptable range before making mold modifications. The new mold can evenly distribute heat to the mold body, avoiding uneven temperature distribution that causes local thermal stress concentration in the mold, which reflects perfect casting forming. During subsequent mass production molding, the mold will no longer experience mold leakage issues. Not only does it significantly improve production efficiency, but it also greatly reduces the cost of casting forming.

Author:
Gabriele Taricco, CM Taricco;
Stefano Mascetti, XC Engineering