Computer simulation aids V-process steel casting.
Since 1980, Svedala-Arbra's steel foundry, located in southern Sweden, has been developing methods to produce heavy steel castings using the vacuum molding method - the V-process. The steel foundry produces about 6500 metric tons (7150 tons) annually - mainly machine and wear castings for crushers.
Compared to conventional sand molding, there are several benefits to using the V-process that result in a lower total production cost for the foundry and a high quality casting.
However, to obtain the desired casting results with the V-process, several precautions must be taken. The major cause of scrap is "mold collapses." As a mold fills with metal, the plastic melts and opens the mold up to the vacuum. If the air lost through the vacuum is not replaced with air from the atmosphere to equalize the pressure, the sand in the cope may fall in, causing a mold collapse. To prevent this from happening, air is drawn into the mold through a communicator, which could be a riser from the outside.
To minimize the risk of these defects, computer simulation can be used to analyze the filling sequences and optimize the casting and gating design. In this article, some examples of simulated and cast results of vacuum molded steel castings are presented.
Simulation and calculation methods are used at Svedala for several different purposes. The development process can be described as a kind of simultaneous engineering. The design department is responsible for the coordination of the different activities during the process. The development work is carried out parallel in respect to time with the engineers from the involved departments. To avoid suboptimizing any part of the manufacturing process, engineers collaborate throughout the product development process.
Finite element-based software systems are used to analyze general solid mechanics and dynamic problems. When preparing the cast part for optimum castability, foundry engineers use a computer simulation program (MagmaSoft) based on the finite difference/finite volume method.
Simulation of Filling and Solidification
As a first step, a 3-D model of the cast part with gating and feeding system is created in a 3-D computer aided design (CAD) environment [ILLUSTRATION FOR FIGURE 1 OMITTED]. The CAD-model is then divided into elements [ILLUSTRATION FOR FIGURE 2 OMITTED]. The element mesh is generated automatically after setting a few parameters describing the mesh accuracy. Before starting the calculations, the operator sets different boundary conditions like heat transfer values, and initial conditions such as start temperatures for the appropriate materials, including the type of alloy and mold and core sands.
This article focuses mainly on the filling calculations needed to optimize the gating system to obtain calm filling. Filling calculations are often used primarily to find the right temperature distribution as start values to the solidification simulation. But with the V-process, a more detailed analysis of the filling calculations must be carried out to minimize the risk of the casting defects that might occur due to uncontrolled filling.
When producing a vacuum mold, the pattern plate is mounted on a hollow patternbox connected to a vacuum system. A sheet of thin plastic is preheated and vacuum-formed to the contour of the pattern. After applying the mold coating, a special flask is positioned on the pattern plate and filled with dry, unbonded sand while vibrating the mold. The flask is then connected to the vacuum system. Following the separation of the pattern from the mold, the two half molds are set together with cores, insulating sleeves, etc. and transferred to the pouring station.
During filling of the mold cavity, three different "filling steps" occur with respect to the air flow in the cavity.
* When pouring begins, the air in the cavity is heated and escapes out of the cavity through the vents.
* After the filling gets more stable, air is drawn into the cavity through the vents as shown in Fig. 3. Because the plastic film becomes moist a few millimeters above the melt front, the underpressure causes the air to flow out of the cavity and into the mold. Air is drawn into the cavity through vents to prevent an underpressure in the cavity that could cause a mold to collapse.
* At the end of pouring, the airflow turns and the remaining air in the cavity and vents is pressed out through the vents. This part of the filling is critical and demands careful design of the venting system.
If the gating and venting design is incorrect, the scrap due to mold collapses and sand inclusions will increase. This unforgiving design is the major potential disadvantage with the V-process and must be treated with care.
All filling of V-process molds should be done smoothly. The metal should enter the lower part of the casting with a laminar flow, but with a higher flow rate than used in green sand molds. Three different castings of moderate complexity will be examined below. The examples focus mainly on how to analyze different types of filling problems.
Example 1: Pressurized or Unpressurized Gating System? - In general, an unpressurized gating system is preferable when using a bottom-pour ladle. However, for a vacuum-molded steel casting, the metal flow must never be turbulent. The casting in Fig. 4 is machined on all surfaces and experienced initial sand inclusions in its upper region. During filling with the original unpressurized gating system, the metal was flowing uncontrollably in a "wave-like fashion" next to the gate due to a pressure drop when the metal entered the cavity. By changing to a pressurized gating system, the flow is controlled and sand inclusions in the top of the casting were eliminated. Several hundred filler rings have been produced this way without any weld repair, resulting in shorter lead times and lower manufacturing costs.
Example 2: Multiple Filling - By controlling the pressure in the gating system for a multicavity mold, all the castings are filled simultaneously. This is necessary to prevent a mold collapse in the last-filled detailed. Figure 5 shows an example of a computer simulation designed to the dimensions of the different gates. The parting of the mold and other practical considerations determine the positions of the gates. Multiple-casting patterns are of course beneficial for the total capability of the vacuum molding plant.
Example 3: Velocity Drop in Curved Gating System - The often-used rule that a decreasing cross-section in a gating causes a proportionally related pressure drop is not necessarily true. For instance, a gating system with curved gatings [ILLUSTRATION FOR FIGURE 6 OMITTED] leads to frictional losses at the mold/metal interfaces. These losses cause a velocity drop that results in a longer filling time for the casting.
Originally, straight gates were used, but severe erosion at the steps of the mantle demanded a better solution. To prevent mold erosion or collapse due to high pressure, a refractory impact plate was placed at the lower end of the downspine. The new and improved design resulted in better surfaces and wear characteristics of the mantle.
The simulation of casting solidification is an aid for foundryman to predict shrinkage and porosity in castings. Following are two examples of how solidification simulation can be used.
A simple, but important result of the collaboration between the design and foundry engineers is shown in Fig. 7. Computer simulated results of the original design indicated porosity in the area where the mantle is exposed to wear. Design changes were made, which allowed the part to be converted from hand to V-process molding. With the final design, no porosity was found.
In Fig. 8, a frame end with a casting weight of about 2.5 metric tons (2.75 tons), is shown at 50% solidification. When examining the internal cores (made from olivine sand) in Fig. 9, it can be seen that sintering occurs at the surfaces of the casting's center with accompanying surface defects on the casting. For this reason, chromite sand replaced olivine for the internal cores.
Computer simulation of the casting process gives the foundryman information on how to design the casting and gating system properly. The principle of the calculations can be generalized to more complex castings without any restrictions. A proper casting design results in lower scrap and less repair welding for the foundry. Using simulation, development times are decreased and the foundry gets as close as possible to a "right the first time solution."
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|Date:||Feb 1, 1996|
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