Carbon buildup refers to the black, adherent residue formed on mold surfaces when die casting release agents and plunger lubricants undergo pyrolysis and carbonization upon contact with high-temperature molten aluminum. It manifests as dark deposits on mold surfaces—mild cases can be wiped off, while severe buildup requires scraping. Concurrently, aluminum castings exhibit rough, uneven surfaces with sticking or tearing defects.
Based on Kezhiying’s expertise and industry experience, carbon buildup frequently occurs in thick-walled sections of castings or areas with elevated mold temperatures.
Understanding that carbon buildup preferentially forms in high-temperature zones (thick sections, hot spots) allows us to identify key contributing factors:
(1) Elevated Molten Aluminum and Mold Temperatures: Normal pre-release-agent mold temperatures range from 200–280°C; post-application, they drop to 180–250°C. If mold temperatures exceed 350°C (with poor cooling), release agents with insufficient heat resistance carbonize under intense thermal exposure and high-velocity molten aluminum scouring. Excessively high aluminum temperatures exacerbate this issue.
(2) Thick-Walled Castings: Thick sections require more high-temperature molten aluminum, generating greater heat and localized high temperatures. This demands higher thermal resistance from release agents, making carbon buildup more likely.
(3) Rough or Uneven Mold Surfaces: On uneven surfaces, release agents pool in recesses. Under high heat, some agents carbonize, while others form thin films. High-velocity molten aluminum then flushes carbonized residues into these recesses, accelerating buildup.
(4) Excessive Plunger Stroke Time: Slow molten aluminum flow generates excess heat, degrading release agent performance.
(5) Suboptimal Mold Design: Poor runner/gate flow or inadequate venting causes localized overheating (stagnant flow) or heat accumulation, promoting carbonization.
(6) Over-Application of Release Agents: Excessive concentration, low dilution ratios, or prolonged spraying result in thick film layers. Only a thin layer is needed; surplus agents carbonize over cycles.
(7) Excessive Plunger Oil Usage: Leads to blackening of casting surfaces, particularly near inlets.
(1) For temperature-induced buildup: Adjust molten aluminum and mold temperatures to optimal ranges. This reduces buildup and extends mold life.
(2) For thick-walled castings: Add vent holes or enhance cooling systems to reduce localized heat, ensuring smooth, bright castings.
(3) For rough molds: Nitride new molds to improve hardness; regularly maintain molds to prevent rust. Avoid corrosive release agents.
(4) For excessive plunger stroke time: Optimize plunger speed to balance flow and heat generation.
(5) For poor mold design: Simulate molten aluminum flow and optimize vent placement to prevent buildup, cold shuts, or porosity.
(6) For over-applied release agents: Test varying dilution ratios under consistent conditions to determine the minimal effective dosage.
(7) For excess plunger oil: Use appropriate volumes to minimize residue.
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