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Sand Casting Design Engineering Considerations and Principles

THE DESIGN ENGINEER

The design engineer must consider function, stress, strain, fatigue, environment, corrosion, service temperature, conductivity, fabrication vs. casting, rough shape vs. net shape, machining, cosmetics etc.

THE FOUNDRY ENGINEER

The foundry engineer considers molten metals flowing into and through shapes, heat transfer, solidification patterns, section sizes, junctions between sections, castability, fixturing points, machined surfaces vs. as cast surfaces, pattern design and construction, heat treatment, surface finish, and infinite variability in shape.

GETTING IT TOGETHER

The design engineer who understands the issues the foundryman is dealing with will get a more successful outcome from his design and will save time and money in the process. Often a foundryman is forced to work with a bad design only to be blamed later when the customer discovers defects during machining operations. By that point everyone is upset because they all have much time and money in the part. Much grief can be avoided by careful design.

Every alloy exhibits different characteristics during melting, pouring, solidification and cooling. By considering these characteristics early in the design process, the component can be made in a way that minimizes problems. It is very important to understand that one set of rules that works for a given alloy may not work for another.

There are no universal rules that one can learn. Instead one must learn the effect of different molten alloy characteristics on a desired outcome.

THE FOUR METALCASTING FACTORS THAT AFFECT CASTING DESIGN.

There are four major factors that will affect the outcome of a casting design:
Fluidity
Solidification Shrinkage
Slag/Dross Formation
Temperature

These factors vary from alloy to alloy. Even very similar alloys, such as class 30 and class 40 gray iron, can have differences.

An engineer must take an approach to casting design that considers everything including structural function, molten metal flowing into a shape and solidifying, machining methods, assembly, testing, final use and abuse.

FLUIDITY

Fluidity is the ease with which a metal flows. Each metal flows with greater or lesser ease into and through cavities. Fluidity determines how well thin sections and fine detail will be filled.

Each alloy exhibits a different degree of fluidity at its normal pouring temperature. Superheat of a molten bath (raising the temperature above the liquidus) will increase fluidity some. But, some steels that are poured at 3000 degrees F are less fluid than some aluminums poured at 1300 degrees F. The fluidity of metals within a given family can be changed by small changes in chemistry.

When designing castings, consider fluidity to know how fine a detail you can expect to get. Do not try to get tiny letters or very thin fins from a metal that has poor fluidity. Soft, rounded shapes are essential. The metal will not flow into sharp corners in the same way a more fluid metal would. Low fluidity metals will soften a shape whether you want them to or not.


SOLIDIFICATION SHRINKAGE

LIQUID SHRINKAGE


Liquid shrinkage is the contraction of the molten metal as it cools from its superheat temperature to the point at which it starts to solidify (the liquidus). This shrinkage does not normally cause the foundryman any trouble provided he has sufficient feed metal in the risers.


SOLIDIFICATION (LIQUID TO SOLID) SHRINKAGE

Most metal alloys are heterogeneous mixtures of molecular compounds. These compounds solidify as crystalline structures called dendrites. Different molecular compounds solidify at different times and tend to group together in phases and constituents. The difference in the solidification temperatures of the various constituents determines how wide the freezing range will be. Thus, for design purposes, we have grouped alloys into three groups of solidification ranges. Narrow, medium and wide. Each group presents different behaviors during solidification. The design considerations vary greatly for each group as do the tricks that a foundryman may employ in his efforts to overcome the shortcomings of a casting's design.

Solidification shrinkage is a critical factor. It has a large influence on the quality and soundness of castings. Within each solidification range different alloys shrink to greater or lesser degrees.

NARROW FREEZING RANGE ALLOYS

Narrow range alloys solidify in two ways. First, progressively from the mold walls inward, and second directionally from lowest to highest thermal mass. If progressive solidification closes off the feed path to the riser, which is a thermal mass, then shrinkage will occur. If directional solidification passes the progressive front then superior soundness will result.

The metal and mechanical properties obtainable by promoting directional solidification are excellent. Directional solidification must be designed in. The way to do this is to taper sections so they become progressively thicker as they get closer to the riser. The other option is for the foundryman to simulate a taper by using chills on the end of the casting. Chills set up an end effect. An end effect forces solidification to "flow" from the area of fastest energy removal to the area of slowest energy removal. Foundrymen have many other tricks to create directional solidification including the use of insulating pads on casting surfaces and on risers and runners. Tricks are effective but generally more expensive than designing directional solidification into the casting.

Narrow range alloys require very large risers because the riser also must contend with progressive solidification trying to choke off its directional solidification. The riser must be large enough to stay liquid long enough to feed the casting's shrinkage.

Junctions are a particular problem with narrow freezing range alloys because the feeding distance of the alloy is so short. Often this requires a riser unless one follows the guidelines for junctions with utmost care. See the section on junctions.

MEDIUM FREEZING RANGE ALLOYS

Medium range alloys are the most forgiving. Some medium range alloys can be poured without the use of risers. The risers with medium range alloys are typically small. This can be an advantage in cosmetic appearance, dimensional accuracy and fixturing for machining because the riser contact leaves so much less evidence of its having been there.

The thing that distinguishes medium freezing range alloys is that the liquid metal in the center of the casting freezes off so that the last portions to freeze are fed until the point at which they solidify. This results in a sound casting. The prevailing consideration in risering medium range alloys is to allow enough feed metal to avoid shrinkage depressions on the outside of the casting. This results from the contraction of the metal while it is still liquid.

WIDE FREEZING RANGE ALLOYS

Wide freezing range alloys are difficult to get completely sound. They are often called mush type alloys because the whole casting becomes partially solidified or "mushy" at about the same time. There is little direction to the solidification. Grains of solidification begin to form in the center of the casting at almost the same time they form on the walls of the mold.

It is very difficult to use risers, no matter how large, to solve this problem. Adding taper is of little use. The necessary amount of taper would completely obliterate the desired shape. The key to getting acceptable results from wide range alloys is to keep the thermal sections as uniform as possible. In this way the casting solidifies uniformly. The shrinkage that occurs is microscopic and evenly distributed throughout the inside of the casting. These tiny voids have a minimal effect on mechanical properties of the casting because they are so small and evenly dispersed.

This is where some confusion has occurred because most design doctrines recommend keeping sections uniform just as we have here. The problem is what works for wide range alloys will not work for narrow range alloys. You must design for the alloy you are going to use. There are situations where uniform sections are the best choice, particularly junction design on page 10.

The best solution for junctions in any freezing range, however, is the solution that must be used for wide freezing range alloys. See the section on junctions.

SOLID SHRINKAGE

After the metal has solidified it will continue to shrink in a mostly linear fashion. This is often called patternmakers shrinkage. A patternmaker compensates for this shrinkage by making a pattern oversized so that as the casting cools in the mold it will shrink to the correct dimension. Different metals exhibit greater or lesser degrees of solid shrinkage.

Across linear dimensions the amount of shrinkage is easily predictable. As the casting becomes more complex such as across cores the amount of shrinkage becomes less predictable. Because of this unpredictability it is a good idea to run a first article to find out specifically how much and where a casting will shrink. The pattern can then be adjusted, if necessary, for critical dimensions.

CAD systems that allow for different shrinkage at different places on a casting can be very useful. A knowledgeable patternmaker can predict with confidence the amount that should be allowed across various dimensions. For difficult designs, consult with your patternmaker in the early stages.

The table below shows the amounts of shrinkage patternmakers typically allow for different alloy families.



DROSS/SLAG FORMATION

Dross is the non-metallic compound formed by the metal reacting with air and especially oxygen. Some molten metals are more susceptible to dross formation than others. Dross is much lighter than metal and so floats on the surface of the molten metal like a cork on water. Once the casting has solidified the dross is knocked away by shot blasting leaving voids. If the dross is subsurface it can be detrimental to machineability and reduce tool life.

Because dross usually floats, it is good practice to design a casting so that machined surfaces and those that are cosmetically critical can be placed in the drag (the bottom of the mold). This means designing a part so it can be parted across a plane parallel to the lowest portion of the casting in the mold. This is especially important with alloys that have a high tendency for dross.

POURING TEMPERATURE

Different alloys are poured at different temperatures. The higher the temperature the more consideration must be given to refractories used and to the transfer of the heat of the metal through the refractory. Hot spots can develop in confined areas that can change the behavior of the metal and the mold. This is especially true in sharp internal corners. The mass of metal surrounding the sand is so concentrated that it heats the sand to almost the same temperature as the metal. This keeps the metal liquid longer creating the effect of a thicker section.

Very hot metals also require soft shapes with few small, internal cavities. One cannot place a small diameter core in a high temperature alloy as the heat of the metal will break down the core and cause metal penetration into the core.

OTHER METALCASTING CHARACTERISTICS

GASSING


Like shrinkage, different alloys retain gas to greater or lesser degrees. This is important to be aware of and given the choice between two alloys that are otherwise satisfactory for an application, it is always wise to choose the one that gasses less. Otherwise one cannot do much to design out gas. It is more a function of good foundry practice.

CASTING YIELD

In addition to the metal that is required to make a casting foundry practice requires an additional amount of metal for risers, gates, filter systems, runners, sprues etc. Some alloys require far more than others. The ratio between saleable metal to non-saleable metal is called yield.

The foundry can re-melt the non-saleable metal (which is called returns) but it must charge you for the cost of melting all the metal that goes into a mold whether they sell it to you or re-melt it. Like gassing, if there is an alternative that meets the requirements of an application that has a better yield it is always better to choose that alloy (assuming an approximately equal alloy cost) as it will save money.

COMPUTER DESIGN

The power of computer aided design can profoundly affect casting design. One of the most powerful tools is Finite Element Analysis. FEA permits the simulation, on computer, of stress loading, thermal dynamics and fluid flow. It can eliminate the trial and error and guesswork previously used in design. If you have a difficult application and do not have access to an FEA system, we highly recommend that you hire an engineering firm that does.

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Saint Paul, Minnesota 55104
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