Casting answers and advice.

Q We seem to have a lot of clay balls in our melding sand system. What causes these clay balls and how can we eliminate them?

Background - As long as mullers prepare a molding mixture of sand, clay and water, the resulting sand will contain clay balls. If additives such as seacoal or seacoal substitutes are added to the sand-clay-water mixture in their respective finenesses, clay balls will increase. With advances in technology to increase mold productivity, one of the most important fundamentals of craftsmanship molding has been forgotten - riddling of sand. Skilled molders recognize that the most important sand in a mold is at the face of the pattern - the location of the mold/metal interface. Riddling system sand filters out clay balls as well as other undesirable "debris" materials (pieces Of metal, filter ceramics, shell core residuals, etc.) that came through the screening system.

A good casting surface is the direct result of a properly formulated sand mixture free of such "debris." Each clay ball (which is an encapsulated concentration of moisture) at the mold/metal interface will leave its "imprint" on the surface of a casting as a result of the miniature explosions from the contained water concentration of the clay ball, guaranteeing a surface defect.

With today's molding machine technology, sand riddling no longer has a place in the mold production cycle. But aerating the sand prior to compaction is another guarantee for a quality mold and should always be included in the molding cycle, although it cannot completely eliminate surface defects caused by clay balls.

Recommendations - Here are some rules to follow:

* If sand fiddling is practiced, the residual clay balls remaining in the riddle must be discarded - not added to the backup sand!

* Check the sequence of material additions in the muller. In particular, add water after the clay has been dry mulled with the sand.

* Once clay balls exist in system sand, repeated recycling and mulling increases the size of the clay ball. Clay balls must be "screened" or "riddled out" from the system sand in molding operations that will allow manual fiddling.

* Remember - clay balls are casting, quality problem sources.

Q Despite passing through visual inspections prior to shipping, an aluminum bronze bracket casting broke during installation. What caused this failure and how can it be prevented?

Investigation - The bracket casting was sand cast in aluminum bronze [poured at 2150F (1177C)]. The casting fracture surface (darkened and discolored, as shown in the photograph) was viewed under a microscope at varying magnifications, and spectrometer analysis verified that the alloy chemistry is well within the Copper Development Assn.'s aluminum bronze specifications. Because aluminum bronze is prone to the formation of oxide films, if the molten metal is subjected to excessive turbulence during pouring, these films can enter the mold cavity and form a barrier between two fronts of liquid metal, preventing those fronts from joining or "knitting" together. The gating diagrams and location of the casting within the mold were carefully considered and microscopic examination did not reveal any oxide film or folds.

Recommendations - The darkened appearance indicated that this surface was subjected to atmospheric conditions and/or liquid at some point, so the casting probably had a hairline crack prior to shipping that was not visible from the surface. The casting broke on the fracture line of this crack, which was probably caused by improper handling during solidification (the mold was jarred during travel on the rollers or shaken out too soon). The crack was then subjected to air and cutting and cleaning fluids. (Editor's Note: During its own examinations and testing, the foundry duplicated this failure through premature shakeout.)

To prevent further occurrence of this defect, the jarring during solidification must be corrected. All rollers, conveyors and carts used for mold transfer must be in good repair so that the solidifying molds travel smoothly and without jarring or abrupt halting. All foundry personnel should be aware of the solidification times for each casting, and molds shouldn't be shaken out until solidification is complete. Solidification time varies with casting thickness, pouring temperature and alloy. Once established, the pouring temperature must remain consistent for this solidification time to remain constant.