What is in this article?:
- Computer-Designed Feeding Systems for Iron Castings
- Case study: Ductile iron control arm
- Graphite expansion during solidification
- Determining Casting Modulus
- Understanding Transfer Modulus
- Single-feeder success
A cast ductile iron part for control arm. The foundry originally approached the feeding design for this casting by placing two symmetrical feeders, but that arrangement produced excessive porosity in the part, and no acceptable castings were produced with that arrangement.
Many iron foundries do not take into account the solidification characteristics of iron when designing feeding systems; feeders for iron castings are designed essentially as feeders for steel castings, resulting in defects in production castings. Often the suggested remedies for these defects worsen the situation, due to the same lack of understanding.
The design methods are generally quite easy to implement and require only a minimal investment of time. Most of these problems could have been prevented had the foundry engineers applied correct design methodology from the beginning. Spending a short amount of time up front to prevent ongoing problems in foundry production over months or years results in an extremely high return on investment.
Design principles for cast iron
The biggest difference between iron and other alloys is graphite expansion during solidification. This is significant because in most situations the casting becomes “self-feeding” after the onset of expansion, and no further feeding is required. The objects of feeding systems for iron castings are 1) to provide feed metal only for contraction of the liquid alloy and 2) the contraction of the solidifying iron prior to the start of expansion. Once expansion begins, a feeding system should control the expansion pressure to ensure that the casting is self-feeding from that point forward. With shrinking alloys, feed metal must be supplied during the entire solidification time.
Another difference has to do with the “piping” mechanism in the feeder. Cast irons (particularly ductile iron) do not readily form a solid skin during solidification; the freezing mechanism is often described as “mushy” or “pasty”. This renders atmospheric cores ineffective with these alloys.
For blind feeders to pipe effectively, atmospheric pressure must be able to collapse the weak plastic skin after the internal pressure drops below atmospheric. Once one riser punctures, the pressure is equalized so there is no longer a higher external pressure to cause other feeders to pipe. This means that only one feeder should be used on each “feeding zone” in an iron casting; if multiple feeders are used, one feeder will begin piping while the others will not. Often, porosity will be seen at the contact point of non-piping feeders.
The requirement for a single feeder within a single zone of the casting is the rule that is violated most often in iron foundries. Designs where two or more feeders are feeding the same zone a casting results in porosity, often at the contact point of one of the feeders. The tendency of many foundry engineers to add more feeders to try to resolve the porosity issue is, in fact, exactly the wrong approach and will worsen the situation.
To correctly design a feeder system for iron castings, it is necessary to analyze the cast shape and determine the location and size of feed zones in the casting. To make this determination, we use the Transfer Modulus.
Feed zones are defined by knowing where it is possible for liquid metal to flow from one point to another in response to expansion pressures. If metal cannot flow from one area of the casting to another, then each of these areas forms a separate feed zone and will require its own feeder (but no more than one.)
Begin with M<sub>c</sub>
Analysis of a casting begins with the Casting Modulus (Mc). This is defined as the volume-to-surface area ratio for various areas of the casting, and has been used for many years to estimate the order of solidification of different parts of the casting. The Casting Modulus allows us to estimate which part of the casting will solidify first, and which will solidify last. In steel castings, the modulus of the feeder should be greater than the modulus of the casting. In iron castings, the Casting Modulus is used to estimate when expansion will begin.
Prior to development of computers and software, calculating Mc was time-consuming and not very accurate; it required the foundry engineer to estimate volumes and surface areas by approximating various parts of the casting with relatively simple shapes. With casting simulation software, solidification of a casting can be predicted in a matter of minutes. The data from this simulation can be converted to Modulus values in the casting. Modulus data is available at every point in a the casting and the data is more accurate, as time-related effects such as local superheating of the mold material are effectively 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. The point at which expansion begins is expressed as a percent of full solidification and is referred to as the Shrinkage Time (ST).
Knowing the ST point for a casting, you can calculate the Modulus value at which contraction stops and expansion begins. This is known as the Transfer Modulus (MTR), because it defines the areas of the casting where liquid metal transfer is possible. MTR is calculated as:
MTR = SQR ( ST /100) * MC
By plotting the value of MTR in our simulation, we can determine whether the entire casting is a single feed zone (MTR is continuous throughout the casting) or whether there are multiple zones (MTR is discontinuous). This allows us to determine the number of required feeders, using the rule of one feeder per feed zone.
MTR can be understood as the point at which the iron becomes self-feeding due to expansion. MTR is critical in designing the feeding system for the casting. The basic premise for feeding iron castings is that the expansion pressure must be controlled. 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. This leads to another 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 that expansion pressure will be contained in the casting due to freezing of the feeder contact at just the correct point in solidification. In-gates should be thin so that they freeze off shortly after filling.