Riser Design Basics for Shrinking Alloys

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RISER DESIGN BASICS FOR SHRINKING ALLOYS DAVID C. SCHMIDT Vice President FINITE SOLUTIONS, INC. ARTICLE TAKEAWAYS: Figure 1. Directional Solidification, from • Riser sizes are determined based on thermal modulus and available feeding volume Low Modulus to High Modulus. • Risers are positioned to provide feed metal to distinct feeding areas The following example was created • Riser design can be automated using casting simulation tools using the Riser Design Wizard, a part of the SOLIDCast simulation software. Much of the data input BASIC THEORY OF RISER DESIGN needed for riser size calculation Riser design for shrinking alloys is based on a simple premise; as metal can be extracted from simulation cools and solidifies, it shrinks, or contracts. Risers must be large enough models. For detailed descriptions to solidify after the casting, or casting section, being fed, and must have of complete design of risering enough volume to provide enough ‘feed metal’ to offset casting shrinkage. components and systems, see the One starts by estimating the Solidification Time of a casting, based on AFS Handbook on Basic Principles Chvorinov’s Rule: of Risering. t = B (V/A)2 Unrigged Simulation The first step is to run a simulation where of the part without rigging, to t = Time to complete solidification determine the ‘natural’ pattern of B = Mold Constant solidification. Gate locations and V = Volume of a section of the casting chills may be added at this stage, A = Surface area of the same section of the casting if you know the locations. Typical This can be simplified, so that solidification time is proportional to the results are shown in Figure 2. ratio of V/A, commonly known as the geometric modulus, or just modulus. Before computers, the modulus was a fairly easy value to estimate, and a riser calculation method was developed, known as the Modulus Technique. The two key features of this technique are: 1) The modulus of the riser should 2) The riser should have enough be larger than the modulus of volume to provide the required the casting, which encourages feed metal to the casting. directional solidification, insuring Figure 1 illustrates the concept of that feed metal will be available directional solidification. to counteract shrinkage in the casting throughout solidification Figure 2. Unrigged Simulation Results. 36.

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