Simple Solutions That Work! Issue 9

BACK-2-BASICS Prior to development of computers and software, calculation of Mc was tedious and time-consuming; it required the foundry engineer to estimate volumes and surface areas by approximating various parts of the casting to relatively simple shapes. With modern casting simulation software, solidification of a casting can be simulated, often in a matter of minutes. The result data from this simulation can be converted to Modulus values within the casting. This means that Modulus data is now available at every point within a 3D representation of the casting; this also means that the Modulus data is more accurate, as effects such as local superheating of the mold material are accurately taken into account by the simulation, which is not possible with manual methods. With the Modulus data for the casting, as well as the chemistry and temperature data, the point at which expansion begins can be calculated. Castings which have a higher Modulus (heavy section castings) will begin to expand earlier and will undergo more expansion than castings with low Modulus (light section castings). This point at which expansion begins is expressed as a percent of full solidification and is often referred to as the Shrinkage Time (ST) point. Knowing the ST point for the iron in a casting, it is possible to calculate an equivalent Modulus value which then corresponds to the Modulus at which contraction of the iron stops and expansion begins. This Modulus value is known as the Transfer Modulus (MTR), because it defines for us the areas of the casting where liquid metal transfer is possible. The calculation of MTR is as follows: MTR = SQR ( ST /100) * MC By plotting the value of MTR we are able to visualize the feed zone(s) in the casting. This allows us to determine the number of required feeders, using the rule of one feeder per feed zone. The value of MTR can be understood as representing the Modulus value below which feeding of the casting from risers is no longer effective and the iron becomes self-feeding due to expansion. MTR is critical in designing the feeding system. The basic premise in all design work for feeding iron castings is that the expansion pressure must be controlled. This means that, assuming the mold is rigid enough, all contacts with the casting (gates and riser contacts) should be solid enough to ensure that the expansion pressure is contained in the casting after the onset of the graphite expansion. This leads to another simple rule: The Modulus of the feeder contact neck should be equal to MTR. This ensures that feeding of the liquid contraction will be able to occur, and also that the expansion pressure will be contained within the casting due to freezing of the feeder contact at just the correct point in solidification. CASE STUDY As an example of both the incorrect and the correct feeding approach, we consider first of the all the ductile iron control arm as shown in Figure 1. The foundry originally approached the feeding design for this iron casting by placing two symmetrical feeders as shown in Figure 2. This was, perhaps, understandable as the two sections to which these feeders were attached are the heaviest sections of the casting. 33 Figure 1. Ductile iron control arm casting. Figure 2. Original pattern layout and feeder design. During initial production of this casting, it was found that porosity occurred at one feeder contact on a consistent basis, as shown in Figure 3. The porosity was not always at the same contact, but on all castings one contact showed evidence of porosity and the other did not. No acceptable castings were produced with this pattern design. Continued on page 34