Our global experts recommend:
The casting defect you describe, bubbles, mainly occurs as a result of a poor degassing process in the core or mould and is more common in lamellar graphite iron castings (GJL) than in spheroidal graphite iron castings (GJS). Cavities with round, smooth walls then usually occur over large areas. The reason for the rounded or elongated bubbles are gases enclosed by the solidifying metal on the surface of the casting, often associated with slag or oxides. Such defects usually occur in the upper box of a mould, in poorly vented pockets and/or undercuts.
We recommend that you check the causes of gas formation separately.
The release of core gases can promote the formation of bubbles. Binder reduction or the use of gas shock retardant binders can be advantageous.
Note: the lower the binder content, the lower the gas potential.
Depending on the speed at which the mould is filled with the casting metal, the gas permeability of the moulding material coatings (coating) must be taken into account. In general: fast mould filling = low gas permeability, slow mold filling = high gas permeability.
Always ensure that the cores are well dried after the coating process!
When storing cores, care should be taken to ensure a dry environment (low relative humidity) so that moisture cannot be absorbed. Warm cores or cores stored hot tend to absorb more moisture!
Insufficient core ventilation also plays a major role. When using coatings, please ensure that the core marks are free of coating materials. In some cases it is advisable to subsequently drill core vent holes or connect core vent holes using ceramic inserts.
When producing molds, especially on automatic green sand molding machines, the compacting of the molding material must not be too high. A too low gas permeability of the molding material (sand-binder mixture) or a too high gas release, e.g. from the lustrous carbon binder used in bentonite-bonded molding material (green sand / wet casting), lead to an increased gas risk.
If sand that is too moist and/or too warm is used in the production of the mold, this can cause the mold to "boil" and thus lead to an increased steam pressure in the mold. You can also improve degassing from the mould side by reducing the fine dust content, using coarser sand grains, reducing carbon carrier content or using slow-reacting lustrous carbon formers, and using bentonite with a high montmorillonite content (high specific binding capacity) and high thermal stability. A continuous control of the molding material preparation is absolutely necessary. A reduction in compaction force and the resulting reduction in mold hardness also yields results; the molding sand should be uniformly compacted.
When checking gating and casting techniques, it is important to ensure sufficient mold ventilation (air whistles). Improvements can be achieved by increasing the casting height and extending casting times.
Make sure that the melt is sufficiently degassed, in particular the specific boiling temperature and the holding time (standing of the Fe-melt) must be observed. Use clean materials, e.g. stainless steels and broken cast iron, to reduce the oxides directly at the beginning of melting activities. When melting, the temperature range in which the melt absorbs more gas must be passed through quickly.
If the problem still persists after checking all the above points, please contact our ASK-Tech Service.