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AFS Technical Department


ANSWERS & ADVICE


Q A


Green Sand, Metallic Dust Make Mischief the squeeze head you move.


Can green sand properties affect casting dimen- sions and infl u- ence shrink in


ductile iron castings?


Shrink in ductile iron castings is typi- cally caused by metal chemistry, pattern gating and riser


design, or pouring and feeding issues. However, mold wall movement can play a role in some instances. Graphite expansion during ductile iron solidification can apply greater pressure inside the mold than its strength can withstand. Some metalcasters have reported green sand molding conditions can lead to mold swell, or soft mold conditions. This can affect the cast- ing wall thickness dimensions and potentially contribute to shrink, depending on the severity of the di- mensional variation and how much feed metal is available. Mold density also can aff ect con-


ductivity and heat removal during solidifi cation and have a correspond- ing eff ect on shrinkage behavior. T e conditions that contribute to varia- tions in mold density and hardness that can lead to mold wall move- ment are not only related to green sand composition and properties, but also include molding process and equipment parameters. T e following recommendations


can help prevent mold wall movement


and casting dimensional variations: • Mold fi ll. If the sand is not uni- formly distributed in the fl ask prior to squeezing, the compensating squeeze head can “bottom-out” and create soft spots.


• Aeration. Even if molds fi ll uni- formly, squeezing a large mold is more diffi cult if the sand is not well aerated. T e sand will be more prone to soft areas or voids, especially the further away from


• Compactibility and clay levels. Higher compactibility will aff ect squeeze properties. Particularly with elevated moisture levels, the higher the clay content, the more resistant to the squeeze it will be.


• Seacoal. The swelling action of seacoal can reduce mold wall movement and be beneficial in heavier castings.


• Soft molds or pockets. Soft areas in the mold are related to mold fi lling and can be exacerbated by mold geometries and pattern features, such as the proximity of cavities to the fl ask wall.


bleed air out of hydraulic lines.


• Molding equipment hydraulics. If any small leaks in the hydrau- lic system affect the hydraulic squeeze and compensating squeeze heads, air can eventually be introduced into the oil res- ervoir and decrease the squeeze effectiveness. Molding equipment should be regularly inspected to identify and repair leaks and


Graphite expansion during ductile iron solidifi cation can apply greater


pressure inside the mold than its strength can withstand.


Q A


Is there a way to monitor and measure metal dust (e.g. from shot blast or grinding) in a


green sand system? We have been told that 1-3% is acceptable. Is this correct?


Little research has


been conducted in this area, so no clear answer is available. However, metal dust


in sand typically causes rough surface fi nish casting defects, such as pen- etration and burn-on, at fairly low addition levels. Most metalcasting sand experts


do not recommend introducing any amount of metallic material into the green sand system. All metallic dust collection should be isolated from the sand system dust collection. If ferrous metal dust has been


Metal dust in sand typically causes rough surface fi nish casting defects.


introduced into the sand system in error, the amount of material can be measured through magnetic separa- tion. The AFS Mold and Core Test Handbook procedure AFS 5101- 00-S defines a procedure for passing a magnet over a measured, cleaned and dried sand sample to assess the quantity of metallic material. This procedure is not effective if the metallic material is nonmagnetic. In this case, dilution is the best remedy.


Recommendations are the opinion of the AFS Technical Department. If you need assistance with a technical issue, email moderncasting@afsinc.org.


October 2011 MODERN CASTING | 51


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