Shrinkage cavities are hollow voids formed inside machine tool gray iron castings during solidification, mainly appearing at thick bosses, rib junctions, wall-thickness mutation areas and isolated heavy sections. They are generated fundamentally by volume shrinkage of molten iron, aggravated by unreasonable casting structure, gating system and pouring process factors.
1. Material volume shrinkage of molten cast iron
Liquid molten iron will produce three stages of shrinkage during cooling: liquid shrinkage, solidification shrinkage and solid shrinkage. The solidification shrinkage of gray cast iron still accounts for an obvious volume reduction. When the metal liquid around thick sections solidifies first and loses fluidity, the residual molten iron inside thick parts cannot supplement the shrinkage volume, leaving concentrated hollow shrinkage cavities.
2. Unreasonable casting structural design
This is the most common root cause for machine tool frames, worktables and column castings:
Excessively thick local wall thickness or large isolated thick bosses without thin-wall transition;
Abrupt wall thickness changes and dense intersecting ribs forming hot spots;
Lack of fillet transition at internal corners, leading to local heat accumulation and delayed solidification.
Hot spots cool much slower than surrounding thin walls, forming a closed molten pool with no feeding channel, which directly forms large shrinkage cavities.
3. Improper feeding system (riser design defects)
Risers are designed to supply supplementary molten iron for hot spot shrinkage. Defective riser layout fails to eliminate shrinkage cavities:
Risers are too small, with insufficient molten iron storage capacity;
Risers are placed far away from thick hot spot areas, blocked by thin walls during feeding;
No open feeding channels between risers and hot zones, blocked early solidified metal;
Insufficient riser height and low feeding pressure, unable to fill shrinkage voids under gravity.
4. Defective gating and pouring process
Low pouring temperature: molten iron loses fluidity rapidly, feeding stops before thick sections finish shrinking;
Too fast or discontinuous pouring: unstable liquid level entraps air and blocks feeding channels prematurely;
Improper pouring position: molten iron fills thin walls first and solidifies in advance, cutting off feeding paths for thick hot zones.
5. Uneven cooling speed of sand molds
Excessively thick sand layers around thin walls accelerate their solidification, forming a solid barrier surrounding the unsolidified thick core;
Local chilling blocks are misarranged, causing inconsistent solidification sequence and obstructing feeding;
Slow cooling of internal sand cores leads to long retention of liquid iron inside thick parts without timely supplement.
6. Improper chemical composition of gray cast iron
Low carbon equivalent reduces graphite expansion during solidification. Graphite precipitation can offset partial solidification shrinkage. If C and Si content is too low, graphite expansion is weak, and shrinkage cavity tendency rises sharply. Excess sulfur also hinders graphite precipitation and worsens shrinkage defects.
7. Chilling effect from cold mold or cold core
Cold sand molds, cold internal cores or cold chills absorb heat quickly. The outer layer of thick sections solidifies instantly to form a hard shell, while the internal liquid iron shrinks without replenishment, producing large concentrated shrinkage cavities inside the casting body.
Hazards of shrinkage cavities for machine tool castings
Shrinkage cavities reduce structural rigidity and vibration damping performance. Stress concentrates around voids under long-term cutting vibration, easily inducing fatigue cracks. Cutting fluid and oil will penetrate internal cavities, triggering hidden corrosion and accelerating long-term deformation of machine tool beds.
References
GB/T 7714
Yang X, Liu Z. Generation and control technology of shrinkage cavity defects in heavy machine tool gray iron castings[J]. Foundry, 2020,69(7):732-736.
MLA
Yang, Xu, and Zeng Liu. "Generation and Control Technology of Shrinkage Cavity Defects in Heavy Machine Tool Gray Iron Castings." Foundry, vol. 69, no. 7, 2020, pp. 732-736.
APA 7th
Yang, X., & Liu, Z. (2020). Generation and control technology of shrinkage cavity defects in heavy machine tool gray iron castings. Foundry, 69(7), 732–736.
