The Importance of Mixer Calibration and Sand Analysis

Q: What are the best operating practices for sand handling and sand preparation, specifically for foundries trying to ensure cost-efficiency and product quality in the molding stage?  

A: Cost-conscious foundries, large and small, are looking for every advantage to reduce core, mold, and casting scrap. Most foundries also are concerned about binder consumption. All these things can be related: Sand fines, material on or passing a 200-mesh screen, can weaken tensile strengths and influence mold permeability.

Too little or too much binder can affect mold and core strengths in different ways that can contribute to scrap on the molding line and in the cleaning room.

Sand fines may be a concern for foundries using new and reclaimed sand, mechanical or thermal. Sand fines above 1% increase the amount of surface area the binder has to cover: Unless binder level is increased to compensate, cores and molds can be weak or friable. This may lead to casting defects such as burn-in, penetration, erosion, inclusions, and warpage. Depending on the sand-to-metal ratio, weak or friable molds could lead to run outs.

In addition to reducing tensile strengths, fines above 1% can make molds less permeable. Molds need adequate atmospheric permeability for two reasons: So that gases from binder decomposition can escape; and so that blind risers can pull air in as the metal moves through its liquid contraction phase during cooling.

While sand fines can contribute to reduced tensile strengths, simply not having a properly calibrated mixer can mean too little binder is being added to the sand. Too much binder may cause molds and cores to be too hard, which can make shakeout more difficult. Too-high tensile strengths can lead to hot tearing in certain cases. Also, high binder levels can contribute to gas issues, particularly when using mechanically reclaimed sand. Excess binder and/or catalyst can change the sands’ pH and ADV levels.  

In acid-cured systems like furan no-bakes, low pH and low ADV can shorten work time/strip time below a foundry’s needed open time. The same is true for alkaline systems like phenolic urethane no-bake binders, except that pH and ADV could be raised in the latter instance. Uncontrolled work time and strip time could cause tensile-strength loss, friability, increased cycle time, or issues with removing molds from patterns.

What should a foundry do? — Good foundry practice regarding new and reclaimed sand starts with understanding and addressing sand segregation. Anti-segregation devices should be placed in new and reclaimed sand silos to minimize variations in sand grain size distribution at the mixer. Proper pressure on the dust collection will help to remove fines from the system as castings go through shakeout and sand passes through the reclaimer or sand heater. Too much pressure will remove some of the desirable sand grains and decrease the GFN. Too little pressure will cause fines to build up within the sand system and increase GFN.

To ensure a foundry‘s sand system is in good order, periodic sieve analysis should be done for reclaimed sand. Dry sand samples should be taken from the mixer head; all chemical pumps including sand additive feeders should be turned off during sample collection.  Depending on how often a foundry‘s sand system is turned over, these checks might be done daily, weekly, or monthly. Mechanical adjustments to the reclamation system or increasing the addition of new sand can correct over-large deviations from new foundry sand.

Mixer calibration should be completed on a frequency that makes sense for the foundry’s volume of work. This might mean calibrating once per shift or once per week. Mixer calibration checks should include measuring sand flow, sand additive level, binder flow, and catalyst flow. Changes in binder temperature can affect calibration levels, too. These periodic checks can ensure proper binder and catalyst ratios, as well as binder levels, to avoid the types of defects and molding/coremaking issues mentioned above.

Following these recommendations for periodic sand and mixer checks will help cost-conscious foundries to ensure processes stay under control, or pinpoint when and where they are not so corrective actions can be taken.
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