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by: James D. Mullins
Mar. 2001
In general, shrinkage is one of the most troubling of all foundry problems. Most often it is not seen until after the casting is machined and then found in critical junction areas of the casting causing it to be scrap and the customer to be unhappy.
Shrinkage happens when the molten metal cools, it contracts and requires additional feed metal to replace or fill in the contracted area. If this feed metal is not available, either a large gassy looking type hole (primary shrinkage) or a smaller spongy and dendritic porosity (secondary shrinkage) occurs. When ductile iron is poured into strong molds, with heavier sections, shrinkage is often minimized or eliminated due to the expansion of graphite forming from the liquid. However, most engineered castings have sections that are less than 1-inch thickness, which causes more problems with solidification and feeding out possible shrinkage prone areas. This is a result of mold walls cooling and reducing feed metal passage or transfer.
Continuous shrinkage problems in a specific part can be a result of a poor part design or incorrect set up and /or processing in the foundry. These type problems can be addressed and solved. However, this is not what this article is going to address.
If the shrinkage is of the sporadic type it is usually a result of a number of variables probably only slightly out of control. The usual production variables are: Green sand strength and mold density (ramming), weighting and clamping of molds, pouring temperature variation, overall chemistry, but primarily carbon, silicon and magnesium levels, changes in nodule count, graphitic carbon content and carbide content, and type and/or functioning of risers (piping). Each of these items are covered in detail below:
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Mold strength and density - changes from day to day in clay and moisture levels and mold ramming will allow the expansion pressure from the graphite growth in the metal to dilate the mold wall more or less depending on the strength and overall density of the mold.
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Weighting and clamping of molds - will have a similar effect as previously mentioned allowing mold wall movement.
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Pouring temperature variation - each part number will have its own special pouring temperature requirements, because of casting section sizes, gating system design and riser functioning. Testing should be done to determine what this range is, so that it is checked frequently and controlled properly. Higher pouring temperatures are usually beneficial to casting quality, such as less gas defects, but increase primary shrinkage amounts that must be compensated for by the risering system.
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Chemistry is important. Having the correct carbon for the section size (without carbides) usually reduces the amount of shrinkage formed. Higher silicon levels will usually increase shrink, but reduce carbides. Excessive amounts of magnesium and magnesium plus rare earths can increase shrinkage. Other elements such as chromium that promote carbides usually increase shrinkage.
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Nodule count (good nucleation) - having the necessary amount of nodules for a given section modulus is important as it determines the amount of pressure generated during graphite growth. Too few nodules may not cause much expansion pressure leading to excessive metal shrinkage. Inoculant fading or under inoculation may cause this. Too many nodules (excessive inoculation) can create excessive metal pressure and again shrinkage. Over-inoculation especially with strong inoculants and faster cooling of the liquid can produce too many nodules.
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Higher graphitic carbon contents generally reduce shrinkage, because of more expansion produced on nodule formation whereas larger amounts of carbides reduce the available amount of graphitic carbon and increase shrinkage. The overall volume of graphite in ductile iron can vary from 10 to 12 %.
Remember that trying to follow the rules of risering and proper feeding will not always guarantee success, but ignoring them will often result in failure. In this regard we offer a few risering suggestions to help reduce shrinkage problems:
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Bottle-shaped risers are advantageous as they work at lower and wider range of pouring temperatures than other riser types and can significantly improve casting yield.
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Small exothermic ram-up core type risers strategically placed work well and can also improve yield, but the iron needs to be hot to make them work as they are usually placed in areas of the mold where cold metal occurs (top of cope).
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Blind & hot risers are preferred to open & cold risers, because they generally work better. Insulating and exothermic sleeves work well but also need higher temperature to function.
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All risers function better (pipe better) at higher metal temperatures. This is especially true of top (cold) risers but also is appropriate for hot (gated) risers.
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Using chills in isolated sections can often give better results than risers can, when used properly. Often chills are less costly to use and remove than risers are.
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When examining risers in the finishing room they should have a characteristic shrinkage hole or pipe in or near the top. Oftentimes if there is no hole, the riser probably did not work. Risers with large flat areas on the top are very susceptible to freezing off (forming a surface skin). This is why bottle shaped (also called Heine) risers work well - they begin to form a shrinkage hole very quickly, allowing wider pouring temperature ranges and continue to supply feed metal through solidification. When new parts are sampled it is often wise to saw up the risers (vertically) and through the neck (contact) to see how well they have or have not worked.
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Often when there are two or more risers in close proximity - one will have a shrinkage hole and another does not. This can mean that one riser is feeding the other(s) and the other(s) often can be removed.
Improving casting yield by improving risering and eliminating scrap is one of the
best sources of cost reduction in the foundry. Keep this listing handy to help solve shrinkage problems.
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