Slag inclusion and sand inclusion defects have long been major challenges in lost foam casting production. Currently, lost foam casting has achieved great success in three main types of products: wear-resistant parts, pipe fittings, and box-type castings, all of which require little or no post-processing. However, for castings with multiple high-precision machined surfaces, addressing slag inclusion defects is critical. Based on practical experience, the following measures can effectively reduce or eliminate such defects:
The role of lost foam coatings is multifaceted:
Improving casting surface finish by reducing roughness by 2-3 grades, thereby enhancing surface quality and usability.
Minimizing sand adhesion and sand hole defects.
Facilitating sand removal and cleaning.
Allowing molten liquids and gases from the foam pattern to escape through the coating into the mold sand during pouring, while preventing metal penetration into the sand mold and avoiding gas holes, metal infiltration, and carbon defects.
Enhancing the strength and rigidity of the pattern to prevent deformation or damage during transportation, sand filling, and vibration molding, thus improving casting dimensional accuracy and yield.
To prevent slag inclusions, coatings must exhibit high strength and refractoriness. The coating layer applied to the foam pattern must resist cracking or peeling during drying and transportation (requiring sufficient room-temperature strength) and withstand prolonged scouring by high-temperature metal without failing (requiring high-temperature strength). A tightly sealed sprue and intact coating on the casting and gating system are primary safeguards—any looseness, cracks, or peeling can allow sand, coating debris, or impurities to enter the molten metal, causing slag inclusions.
Strength and permeability are key coating properties; gating system coatings often require higher refractoriness than those for castings to resist prolonged high-temperature metal scouring. Operators must ensure uniform coating application.
During mold assembly, the coating on the pattern cluster (pattern + gating system) must be free of peeling, cracks, or splits—especially at joints between sprue and runner, runner and ingate, and ingate and casting. Weak or poorly coated joints risk sand infiltration, so these areas require higher strength, thicker coatings, and sufficiently rigid gating systems (with reinforcing ribs or sleeves if necessary).
The pattern cluster must rest stably on the sand box’s bottom sand; suspended placement during sand filling and vibration can crack the coating. Sand should be added gently via hoses initially, with 雨淋 - style sanding used only during vibration compaction. Vibration should start with low amplitude until the pattern is fully covered, then increase gradually. The gating system, particularly the sprue, must not be bent or twisted during vibration to avoid coating damage, and the sprue must be tightly sealed to prevent sand entry.
Throughout assembly, sand filling, and vibration, strict care is required to ensure the coating remains intact. Immediately before pouring, the pouring cup must be cleaned of floating sand, dust, and debris.
Higher pouring heads increase scouring on the gating system and mold, raising the risk of coating damage and sand inclusion. Pouring head height should be adjusted for casting size, using appropriately sized ladles to minimize pouring height and keep the ladle nozzle close to the pouring cup—avoid using large ladles for small castings.
Elevated pouring temperatures intensify demands on coating performance and increase risks of sand adhesion and slag inclusions. Optimal temperatures vary by material:
Gray iron: 1380–1420°C (with tapping temperature ~1480°C).
Ductile iron: 1420–1450°C (tapping temperature ≥1500°C).
Steel castings: 1480–1560°C.
For iron castings requiring 300–500 kg of molten metal per mold, pouring duration should be controlled at 10–20 seconds.
Lost foam casting typically uses vacuum conditions during pouring to compact dry sand, accelerate gas evacuation, improve filling capacity, and enhance workplace safety. However, excessive negative pressure increases the risk of drawing dry sand and impurities into the molten metal through coating cracks, while also promoting sand adhesion. Rapid filling exacerbates coating scouring and peeling. For iron castings, optimal negative pressure is generally 0.025–0.04 MPa.
Including slag deflectors, skimmers in the gating system, and slag collection risers in castings helps trap and remove slag, mitigating sand and slag inclusion defects.
Sand grain size affects slag and sand adhesion—excessively coarse grains increase defects. For iron castings, dry quartz sand (washed sand) with a grain size of 30/50 is typically suitable.
Purifying molten metal throughout the casting process—from melting and superheating to pouring—is critical in lost foam casting. Filtration technology is one effective method to achieve this.
By systematically implementing these measures, slag inclusion defects in lost foam casting can be significantly reduced, improving overall casting quality and yield.
