Gating ductile iron hubs.

Gating Ductile Iron Hubs

It would difficult to find a ductile iron foundry that has not produced a hub-shaped casting. It also would come as no surprise to learn that many of these foundries have had problems relating to shrink when designing gating and rise-ring systems for them.

Producing a part that fits a given foundry's production system ultimately begins at the design stage. Design engineers who are not fully versant with foundry practices, such as molten metal handling and solidifcation principles, fall victim to potential feeding problems because of design restrictions.

Unfortunately, by the time a new part reaches the foundry engineer, dimensions have been set, patterns produced and the opportunity for design changes are severely limited or not possible. In addition, today's highly competitive market exerts a constat pressure to add one more job, or to exceed maximum allowable pattern dimensions by just one more inch. All of these factors hinder the foundry engineer's options when it comes to designing proper gating and risering systems for any new part.

The primary step in analyzing the casting design is to determine where risers will be necessary, and to assess the effect that attaching a riser will have on solidifcation. Minimizing the disruption of cooling surfaces can greatly reduce potential shrink problems.

Figure 1 makes it apparent that, because of the thickness of the outside flange in relation to the intersecting "L" junction, a side riser would not produce a sound casting. In this example, a ram-up-type exothermic riser sleeve was placed over the shrinkage area resulting in a 184 lb casting being fed with a 6.3 lb riser. However, as the flange thickness more closely approaches or exceeds the thickness of the inner hub, the decision is not so obvious.

Numerous studies have been conducted using similar "T" plate test patterns to determine relationships necessary to feed a plate section of a given thickness to an adjoining T intersecting plate. But how does the foundry engineer make these not-so-clear decisions short of trial and error on the foundry floor?

Modulus Values

One of the simplest procedures is the use of modulus values for various casting sections. Modulus is the ratio of casting volume to cooling surface area. The higher the modulus value, the longer it takes a section to solidify. By calculating values of adjoining sections, it can be determined whether or not one section can be fed through another.

It should be noted that modulus relationships vary from foundry to foundry depending on treatment practices, mold hardness, etc.

Figure 2 represents a drawing generated by an AFS riser program that very rapidly can calculate modulus values for different sections of a casting. The o.d. of the flange is input as a hot surface to simulate an attached riser, but this still will not produce a sound casting in the intersecting junction with the modulus value of 0.801. This particular casting was rotated 90 degrees, pared lengthwise and gated directly into the innerhub. This solution required a new pattern and considerable lost time, all of which might have been avoided with more pre-engineering.

The number of computerized solidification programs available to the foundryman is increasing rapidly, each presenting its own benefits and limitations. Though they speed the engineer's work and allow him to do his job better, these programs are only tools, and cannot solve all problems.

Figures 3 and 4 represent examples of two different systems. Figure 3 is an example of a cross section of a hub showing solidification wave fronts generated from the Unity of Wisconsin's SWIFT (Solidification Waves in Foundry Technology) program, a geeometry-based system for simulating solidification. Figure 4 is an analysis by Foseco's Solstar program of a hub with the riser attached showing where shrinkage will occur.

Combining computer technology with the ever-expanding selection of feeding aids greatly increases the success rate for producing sound castings. Advancements in feeding aids, such as exothermic riser sleeves, enable the foundry engineer to riser isolated metal masses which, in the past, could only be reached by expensive cover cores. The flexibility and efficiency of exothermic sleeves also contribute to reducing costs and improving yields.