casting Q&A C Green Sand Affects Final Cast Product
Q A
Can green sand properties affect casting dimensions and infl uence shrink
in ductile iron castings?
Shrink in ductile iron castings is typically caused by metal chem- istry, 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 solidifi cation can apply greater pressure inside the mold than its strength can withstand. Some metal- casters have reported that green sand molding conditions can lead to mold swell, or soft mold conditions. T is can aff ect the casting wall thickness dimensions and potentially contribute to shrink, depending on the severity of the dimensional variation and how much feed metal is available. Mold density also can aff ect conduc-
tivity and heat removal during solidifi ca- tion and have an eff ect on shrinkage. T e conditions that contribute to variations in mold density and hardness that can lead to mold wall movement 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 uniformly distributed in the fl ask prior to squeezing, the compen- sating squeeze head can “bottom- out” and create soft spots.
• Aeration. Even if molds fi ll uniformly, 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 the squeeze head you move.
• Molding equipment hydrau- lics. If any small leaks in the hydraulic system affect the hy- draulic squeeze and compensat- ing squeeze heads, air eventu- ally can be introduced into the oil reservoir and decrease the squeeze effectiveness. Molding equipment should be inspected regularly to identify and repair leaks and bleed air out of hy- draulic lines.
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.
• 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. • Soft molds or pockets. Soft areas
“ 48 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2011
• Seacoal. The swelling action of seacoal can reduce mold wall movement and be beneficial in heavier castings.
Recommendations are the opinion of the AFS Technical Department. If you need assistance with a metalcasting issue, email
aspada@afsinc.org.
Graphite expansion during ductile iron solidifi cation can apply greater pressure inside the mold than its strength can withstand.
Simulation showed shrink in this green sand copper casting. Because the porosity did not lead to leakage, no design change was necessary.
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