Solid thickening on internal and external surfaces as well as edges of castings. Frequently appearing in association with rough casting surface.
Incidence of the defect
The defect may occur principally in areas which have not been optimally compacted. Also, it can often appear in lower sections of the mould. In the case of grey cast iron, swelling may occur as a result of expansion pressure. The defect frequently appears in conjunction with “roughness” and “penetration” defects.
If the mould and the core are insufficiently rigid, the mould cavity is expanded by metallostatic pressure. In the case of grey cast iron, the volume is enlarged during solidification. If castings are poured without feeders, the expansion pressure causes an en-largement in volume. The defect may also be caused by move-ment of the mould side walls (formation of hot crusts).
- Uneven or poor compaction
- Compactability too high
- Poor flowability of the sand
- Insufficient compaction
- In the case of grey cast iron, expansion pressure too high
- Improve distribution of sand prior to compaction; if neces-sary, increase pressure; increase tapers on patterns
- Reduce sand compactability
- Improve flowability of the sand during compaction by using products containing graphite
- Improve compaction of cores
If necessary, reduce the degree of saturation of the iron in order to reduce graphite expansion
Changes in the dimensions of the mould cavity can be attributed to the pressure of the inflowing metal, the expansion forces of the formed sand crust and, in the case of iron-carbon alloys, the expansion pressure of the graphite. The higher the rigidity of a bentonite-bonded mould and the lower its humidity for a similar compacting energy, the more accurate is its cavity.1 An authorita-tive publication confirms that the influence of metallostatic pres-sure is considerably less than that of graphitization pressure and the expansion force of the moulding material crust.2 The higher the casting temperature, the greater the pressure on the mould wall. Adding quartz powder has a similar effect, thereby in-creasing the movement of the mould wall. Circulating sands usu-ally exhibit a greater mould wall movement than new sands. This is probably caused by the higher proportion of quartz dust.2 Dimensional stability increases with improved compaction and higher strength of binder bridges.3 Higher bentonite contents lead to increased mould wall movement,4 the reason for which lies in the lower packing density of the quartz grains.
Additions of organic substances have varying effects on dimen-sional stability. Wood flour and powdered peat as well as hydro-carbons with a low softening point reduce mould wall movement. This is also true of pit coal if the proportion of volatiles is greater than 20 %. Pit coal dust with 10 – 16 % volatiles increases compressive stress.2 This is probably because such coals have a minimal softening range. Higher dimensional stability is attained by using various pit coals with differing degrees of milling.5
Apart from sand crust expansion leading to mould swelling, poor-ly compacted areas of the mould are particularly susceptible to expansion defects. The use of sands with high compactability causes risk to cods and sand areas in the vicinity of the flask wall. The use of products containing graphite can markedly improve flowability whilst maintaining compactability. Mould hardness be-comes more uniform and improves,6 thereby reducing swelling.
Swelling may also occur through defects in the mould construc-tion (uneven filling of the sand, poor or uneven compaction).When using core binders with higher viscosity and higher soft-ening properties, particular attention should be paid to ensuring proper compaction.