The clay level necessary to run a green sand system not only depends on making good molds, but also good castings, and to do so with the least amount of defects attributable to the sand. In addition, minimizing sand carryout to maintain return sand silo levels without adding excessive new material.
For mold production, sand should have enough clay to provide:
Adequate strength – green compression strength, green tensile strength.
Enough plasticity – to minimize tear-up, brittleness and friability, and rough edges.
The capacity to absorb swings in sand-to-metal ratio during job changes.
For minimizing casting defects, aim for:
High dry and hot strengths to minimize erosion and mold wall movement.
High wet tensile strength to minimize scabs – both expansion and explosion type scabs.
For easy shakeout, aim for:
- Low enough dry and hot strength to minimize sand carryout.
Some of these requirements seem contradictory and may be satisfied by using a blend of different clays if necessary.
Clay level depends on many factors:
Increased clay increases strength up to a point, depending on mulling time.
For “cushion”, or extra-activated clay in the system, to assimilate inputs changes due to job changes (sand to metal ratio, core sand input, new sand) and outputs (dust collection).
The length of mold cooling line, silo size, rat tailing (amount of useable sand in the system) – where changes in the job mix affect the system quickly and cumulatively (effect of low sand-to-metal ratio or high core sand input manifest quickly).
Mold line; mold handling after molds are poured (rigid flask or flaskless system).
Type of clay used (sodium or calcium – affects dry and hot strength).
Temperature of metal poured (amount of clay deactivated by metal poured depends on temperature –due to burn out, sand used for iron and steel require increased replacement rate than aluminum alloys).
Method of addition – addition prior to storage silos, pre-activated clays using advanced oxidation methods, and addition of recycled fines from dust collectors.
Shakeout system (higher clay level will cause increased sand carryout, especially with sodium bentonite).
The type of molding system affects the clay level:
a) Higher clay level for flaskless molding compared to rigid flasks.
b) Higher clay level for higher temperature melts poured into molds.
c) Higher clay level when calcium bentonite is used rather than sodium bentonite.
d) Higher clay level if the sand-to-metal ratio is lower.
e) Higher clay level if more new material is added every time, even when the sand-to-metal ratio is lower.
f) Higher clay level when increased core usage (core sand digestion in to the system sand).
Clay, when mulled with moisture, develops green strength. Green strength can be increased by mulling longer. Hence, 30psi green compression strength can be obtained with 5% clay or 10% clay. Strength does not increase necessarily with increasing clay.
Higher strength is required for iron castings and heavier metals compared to light weight metals.
Higher strength requires lower moisture, which can be attained by:
Reducing moisture absorbing materials, such as cereals, cellulose and sea coal.
Removing inert fines from the system through dust collection.
Why do we need higher clay with lower sand-to-metal ratio and high material input (new sand and core sand)?
In low sand-to-metal ratio sand systems, in each mulling cycle increased amount of clay needs to be added to replace deactivated clay and losses. This in effect reduces the average mulling time in the system. To have an adequate amount activated clay in the system, it is necessary to have higher levels.
Range of clay levels (MB clay) used in iron foundries:
Cope and drag systems 6.75 to 8%
Disamatic 7.5 to 11.0%
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