Cold Shut
Causes:
The molten metal has a low pouring temperature and poor fluidity; the gating system design is unreasonable — the number of ingates is too small, the cross-sectional area is insufficient, and the sprue height is too low, resulting in inadequate metal head pressure; the molten metal flow in the mold cavity is obstructed.
Prevention Methods:
Increase the pouring temperature and improve the melting process to prevent metal oxidation and enhance fluidity; improve pouring operations to prevent large slags from blocking the gates, and ensure the metal flow is continuous during pouring; design the gating system reasonably by increasing the cross-sectional area and quantity of ingates or adjusting their locations; use a higher cope or pouring basin; strengthen inspection of mold matching and fastening to prevent molten metal leakage through parting lines or core vents; adjust the pouring position of the casting — for thin-walled or flat sections, place them downward or use inclined pouring; ensure sufficient wall thickness; improve mold sand permeability and provide adequate venting risers.
Sand Inclusion and Scab
Causes:
Uneven compaction during molding; low anti-sand-inclusion ability of molding sand; improper pouring position. This defect is the most common one in sand casting.
Prevention Methods:
Reduce the expansion pressure of the mold by adding coal dust, asphalt, heavy oil, or sawdust to provide a cushioning effect during sand expansion; use high-quality bentonite to improve wet strength for green sand; use coarser sand grains to increase permeability and punch more vent holes in the upper mold; ensure uniform compaction during molding to avoid localized over-compaction; strictly control sand moisture — avoid excessive water content; reinforce areas prone to defects with nails to prevent surface peeling; avoid excessively high pouring temperatures and shorten pouring time to allow molten metal to fill the cavity quickly and uniformly.
Burn-on (Metal Penetration)
Burn-on can be divided into mechanical and chemical types according to the mechanism of formation.
Causes:
Metal oxidation on the casting surface produces oxides that react with the molding materials to form low-melting compounds; excessive metal head pressure during pouring forces molten metal into sand pores; when the pouring temperature is too high and the molten metal remains liquid in the mold for a long time, it more easily penetrates the sand and reacts chemically with the molding material, causing burn-on; the refractory level of the molding materials is too low.
Prevention Methods:
For green sand molds, under the condition of maintaining sufficient permeability, use fine-grained base sand whenever possible; increase the compaction degree of the mold, especially at the lower parts of large molds; add coal dust, heavy oil, or asphalt to green sand used for cast iron; lower the pouring temperature appropriately; reduce the sand-metal contact area to increase cooling rate; avoid local overheating of molds or cores; select molding materials with high refractoriness or high cooling capacity.
Inclusion (Slag Entrapment)
Causes:
The slag floating on the molten metal is not removed completely before pouring; slag blocking measures during pouring are inadequate, allowing slag to flow into the mold cavity with the molten metal; unreasonable gating system design with poor slag-trapping performance, so that slag entering the gating system is not separated or removed. This defect may occur in all liquid metal casting processes, including sand casting, investment casting, permanent mold casting, and die casting.
Prevention Methods:
Design the gating system to ensure smooth metal flow and include slag traps and slag baffles; reduce sulfur content in the molten metal as much as possible; increase tapping temperature appropriately; keep ladles clean and use teapot-type ladles when possible; add slag-removing agents such as rice-husk ash or cryolite before pouring.
Erosion, Sand Drop, and Sand Holes
Causes:
Low strength of the mold or core, or excessive baking of mold/core; excessively high flow rate of molten metal, causing prolonged erosion of local surfaces; uneven parting surfaces, small core prints, or local crushing of mold/core during core setting or closing; impact and vibration during mold fastening cause local sand loss; excessive moisture and poor permeability of sand lead to boiling and gas-related defects during pouring; loose sand left in the mold cavity forms sand holes in the casting. This defect may occur in both sand casting and shell molding.
Prevention Methods:
Increase the strength of molds and cores; avoid overbaking molds or cores; prevent ingates from directly facing mold walls or sharp corners; use special refractory coatings or materials in areas subject to strong erosion; for large dry molds, provide adequate parting draft; handle molds carefully during assembly, fastening, weighting, and transportation to prevent impact; reinforce repaired or weak areas of molds and cores; carefully inspect and clean the mold cavity before core setting or closing to remove loose sand.
Post time: Oct-31-2025