Machine tool castings require high dimensional stability, vibration damping performance and surface precision. Various casting defects such as shrinkage cavities, sand holes and cracks will seriously reduce the rigidity and machining accuracy of the base, resulting in tool vibration and workpiece scrapping during long-term processing. This article sorts out typical defects in the casting production process, analyzes the root causes, and puts forward targeted preventive and remedial measures to help foundries and machine tool manufacturers control casting quality.
Sand Holes & Scabs
Cause
1. The molding sand has poor compactness, loose sand or sand drop during mold closing and pouring.
2. The gating system is unreasonable, high pouring speed washes the sand surface.
3. The mold surface coating is uneven or partially missing.
Preventive Solutions
1. Strictly control molding sand moisture, compact the sand mold uniformly, clean loose sand before closing the mold.
2. Optimize the runner structure to slow down the molten iron filling speed and avoid direct impact on the sand wall.
3. Brush refractory coating evenly and ensure full coverage of all inner surfaces.
Shrinkage Cavity & Shrinkage Porosity
Cause
1. Improper casting structure design with hot spots and thick and uneven wall thickness.
2. Insufficient feeding riser setting or unreasonable riser position.
3. Too fast cooling speed of local thick sections of castings.
Preventive Solutions
1. Optimize structural design to avoid abrupt wall thickness changes, use rounded transitions.
2. Set feeding risers, cold iron and chiller reasonably at hot spots to realize sequential solidification.
3. Control pouring temperature properly, avoid excessively high molten iron temperature.
Cold Shut & Misrun
Cause
1. Low pouring temperature, poor fluidity of molten gray cast iron.
2. Overly thin casting wall, too long flow distance of molten iron.
3. Improper gating layout leading to multiple molten iron streams converging with low temperature.
Preventive Solutions
1. Raise the pouring temperature appropriately and ensure good fluidity of molten iron.
2. Optimize runner layout to shorten the filling distance of molten iron.
3. Avoid excessively thin structural design on non-key stress parts.
Hot Crack & Cold Crack
Cause
1. Sharp corners cause stress concentration during solidification shrinkage.
2. Excessive sand mold resistance restricts free shrinkage of castings.
3. Uneven cooling speed leads to huge internal thermal stress.
Preventive Solutions
1. Adopt rounded transition for all sharp corners to reduce stress concentration.
2. Improve mold sand collapsibility to reduce shrinkage resistance.
3. Arrange annealing stress relief treatment after rough machining to eliminate internal stress.
Distortion & Deformation
Cause
1. Unsymmetrical casting structure causes inconsistent shrinkage stress distribution.
2. Improper stacking and cooling placement after pouring.
3. Insufficient stress relief treatment, residual stress releases slowly during long-term use.
Preventive Solutions
1. Optimize structural symmetry, set reasonable reinforcing ribs to enhance overall rigidity.
2. Place castings flatly during natural cooling to avoid one-side force extrusion.
3. Carry out twice stress relief annealing: once after casting cooling, once after rough machining.
Gas Hole
Cause
1. Excessive moisture in molding sand, binder or coating decomposes to generate gas.
2. Insufficient vent holes arranged on sand mold, gas cannot be discharged smoothly.
3. Molten iron is not sufficiently degassed before pouring.
Preventive Solutions
1. Strictly control the moisture content of molding sand and refractory coating.
2. Open enough vent holes at the highest position of the mold cavity.
3. Keep the ladle and pouring tools dry to avoid contact with water.
Most defects of machine tool castings originate from unreasonable structural design, unstable molding sand parameters, improper pouring process and incomplete stress relief treatment. By optimizing casting structure, standardizing molding and pouring procedures, matching reasonable risers and cold iron, plus standardized annealing treatment, the reject rate can be greatly reduced, ensuring the high rigidity, high precision and long-term dimensional stability of machine tool bases.
