Sand inclusion is one of the most frequent surface and internal defects for large gray iron machine tool castings including beds, columns and worktables. It arises when broken molding sand is washed into molten iron during pouring, resulting in hard embedded sand particles. These particles damage cutting tools and reduce casting rigidity. Defect control covers molding sand preparation, mold making, pouring operation and casting structure optimization.
1. Optimize the performance of molding sand
Strictly control sand moisture content
Excess water weakens sand bond strength and leads to mold surface peeling. Keep water content within the standard range to maintain intact mold surface.
Improve binder addition and uniformity
Add enough bentonite or resin binder and conduct full sand mixing. Uniform bonding enhances the tensile strength and surface strength of sand molds, preventing sand from flaking under molten iron impact.
Control sand grain fineness and dust content
Excessive fine dust reduces sand permeability and surface hardness. Circulate and clean recovered sand regularly to remove ultra-fine powder. Appropriate sand grain size improves mold surface compactness.
Add anti-stripping auxiliary additives
Mix special anti-sand-scouring additives into molding sand to boost high-temperature resistance against molten iron erosion.
2. Standardize mold making and coating process
Increase mold compaction density
Hand-molded and machine-molded sand molds must be compacted evenly, especially at runners, ingates, hot spot bosses and thin rib corners. Loose sand layers easily peel off when impacted by high-temperature molten iron.
Apply uniform and complete refractory coating
Brush or spray refractory coating evenly on all mold cavity surfaces, runners and gating channels. The coating forms a dense protective film to isolate molten iron from raw sand and avoid sand washing. Ensure no missing coating, thin coating or coating peeling.
Proper mold drying
Thoroughly dry sand molds and sand cores before closing the mold to eliminate internal moisture. Water vapor will break the mold surface at high pouring temperature and carry loose sand into molten metal.
Smooth mold fillet transition
Remove sharp edges and right-angle corners of mold cavities by adding large fillets. Sharp corners suffer concentrated scouring force from molten iron and are prone to sand loss.
3. Rational design of gating and pouring system
Adopt bottom or stepped pouring instead of top pouring
Top pouring allows high-speed vertical molten iron impact to directly wash the bottom sand mold. Bottom gating makes molten iron fill the mold steadily from low to high, greatly lowering scouring force on mold walls.
Set up filter screens and slag traps
Install ceramic filter screens at the sprue or runner to intercept washed sand particles and oxidation slag before molten iron enters the mold cavity. Add slag traps at the bend of runners to settle sand debris.
Reduce molten iron flow velocity
Enlarge cross-sectional area of runners and ingates to slow down molten iron flow speed, weaken impact erosion on mold surfaces, and prevent massive sand stripping.
Avoid direct impact on mold wall
Guide the molten iron flow to buffer with a sand boss or buffer area, instead of directly hitting vertical mold walls and core surfaces.
4. Regulate pouring operation specifications
Stable pouring speed without interruption
Maintain continuous and steady molten iron flow during pouring. Intermittent pouring causes fluctuating liquid level and repeated impact on mold surface.
Control pouring temperature reasonably
Over-high molten iron temperature strengthens thermal erosion on sand molds; too low temperature reduces fluidity and entraps washed sand easily. Strictly follow the process temperature range for gray cast iron.
Clear ladle slag and sand residue
Skim floating slag and sand particles on molten iron surface completely before pouring, to avoid bringing foreign sand into the mold cavity along with liquid iron.
5. Optimize casting structure and sand core fixing
Reinforce thin sand cores with support ribs and core prints
Thin suspended sand cores are easily washed away by molten iron. Increase core print contact area and add metal core supports to fix cores firmly and prevent core displacement and breakage.
Reduce complex narrow deep cavities
Too narrow and deep mold spaces are hard to compact and coat uniformly, forming weak sand layers prone to sand inclusion. Simplify local structures if process permits.
6. Post-molding inspection before mold closing
Carry out full visual inspection before closing molds: clean loose sand, broken sand pieces and floating sand in the cavity; repair damaged coating and loose mold surfaces immediately to avoid residual loose sand entering the casting.
Hazards of sand inclusion if not controlled
Embedded sand particles will cause severe tool chipping and grinding wheel wear during machining. Internal sand inclusions form weak structural points, triggering stress concentration and fatigue cracks under long-term cutting vibration, reducing the rigidity and service life of machine tool castings.
References
GB/T 7714
Qin H, Zhou M. Formation causes and full-process control of sand inclusion defects in heavy machine tool castings[J]. Foundry Technology, 2021,42(3):267-271.
MLA
Qin, Hao, and Ming Zhou. "Formation Causes and Full-Process Control of Sand Inclusion Defects in Heavy Machine Tool Castings." Foundry Technology, vol. 42, no. 3, 2021, pp. 267-271.
APA 7th
Qin, H., & Zhou, M. (2021). Formation causes and full-process control of sand inclusion defects in heavy machine tool castings. Foundry Technology, 42(3), 267–271.
