Veining: Defect Pattern & Causes

Veining is a typical form of defect that occurs in organically bonded mold parts, i.e. a defect that is found in cores very frequently. It results from the formation of a shell, which forms a vein-like crust. The defect appears as thin, irregular metallic protuberances.

Core with veining

Veining forms as a result of the silica expanding into the mold cavities, which causes the casting metal to fill in the resulting gap and form a vein-like crust.

Veining can occur in all foundry materials. Cast iron with nodular graphite, malleable cast iron and bronzes containing lead are particularly prone to veining, whereas aluminum and magnesium alloys are less prone to veining.
Veining that is caused by cores appears as irregular, fine, thin metallic protuberances in angles or at corners and edges of the cast parts. Veining leads to an increased level of rework and, in some cases, to rejections.

Veining is likely to occur on the inner contours (cores) of the cast parts when chemically hardened mold materials are used. Its formation is caused by cracks in the surface of the mold part that can be penetrated by the liquid metal.

Binders and additives are crucial

Since veining must primarily be attributed to the heat-induced expansion of the mold materials, mold parts made of silica sand are particularly at risk. The liquid metal that surrounds the core causes a temperature gradient between the surface and the center of the core, which leads to stress.
Up to approx. 400°C, silica sand expands linearly and then displays an abrupt increase in the temperature range of the reversible ß-conversion (Figure 3), which increases the stress.
Veining is also influenced by the high-temperature behavior of the binder (hot tensile strength, thermoplasticity) and possible additions and also by the wettability and thermal conductivity of the mold material.

Factors that influence the formation of veining

  • Grain size and grain particle size distribution (a higher degree of uniformity leads to higher stress, since all silica grains pass through the conversion temperature simultaneously.)
  • Intensity of compaction (the higher the intensity of compaction, the higher the packing density, which increases the stress.)
  • Molding base material (influences the tendency toward defects via the height and uniformity of expansion and the thermal conductivity.)
  • Casting temperature (high casting temperatures cause a quick silica conversion and thus lead to increased stress in the mold part.)