feeding calculations could be per- formed. It is tempting to conclude the original feeder design was correct, because the two areas of high modu- lus value in the casting were in close proximity to the feeder contacts in the original design. However, it is necessary to analyze
this casting further to determine the shrinkage time and MTR
to under-
stand the location and size of the feeding zone(s). Analysis of the iron characteristics indicates the value of the MTR
is 0.254 in. (0.645 cm). Fig. 6. The revised pattern features a single feeder attached to the center of the arm.
zones then determines the number of required feeders, using one feeder per zone.
Te value of transfer modulus can be understood as representing the casting modulus value below which feeding from risers is no longer effec- tive and the iron becomes self-feeding due to expansion. Te expansion pres- sure must be controlled, which means, assuming the mold is rigid enough, all contacts with the casting (gates and riser contacts) should be solid enough to ensure the expansion pressure is contained within the casting after the onset of the graphite expansion. Tis also means the modulus of the feeder contact neck should be equal
to transfer modulus to ensure the 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 the correct point in solidification. To resolve this problem in the
ductile iron control arm example, the casting was analyzed to deter- mine feeding requirements. First, a solidification simulation of the casting without gating or feeders was per- formed. Te results of this simulation are shown in the plot of solidification time (in minutes) in Figure 3. Te data from the simulation was converted to modulus data so the
Creating a plot of this value within the casting will indicate the location of feed zone(s), shown in Figure 4. Te entire casting is actually a
single feed zone. Te areas of higher modulus are connected by a section in which the modulus is above the transfer modulus value, thus allowing liquid transport for feeding through- out the casting. Only a single feeder should be used to avoid potential pososity at a non-piping feeder.
Troubleshoot Defects Te computer simulation in this
case took 16 minutes to perform, and after calculating the shrinkage time and transfer modulus, the plot was created in 5 minutes. After about 20 minutes of analysis, the correct feeder design was determined. Had this been done before the original pattern equipment was created, several months dealing with defective castings could
Figs. 5a-b. There was no porosity at the feeder contact, and no porosity elsewhere. 36 | MODERN CASTING October 2013
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