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The Cast Oil-Field Fitting

1. The binder for the no-bake sand is a polymerizable alkyd-oil/urethane material. Gases can be evolved

from the binder when it is heated and the polymer material begins to depolymerize. In fact, there are

two possibilities for gas problems with this material. If the binder had been completely polymerized

during the manufacture of the core, the high temperature of the cast iron could break down the binder

into small fragments having low molecular weight and low boiling point, thus producing the bubbles. In

addition, this particular type of binder has a long curing time --12 to 24 hours are required for the

polymerization to complete at room temperature. If the core or the mold were not completely cured,

there would already be low molecular weight, low boiling point, constituents present that could form

gases as soon as the liquid iron entered the mold cavity.

2. The gases are located near a surface, just beneath the core. It appears that the gas bubbles formed,

started to float, and were trapped by the core.

3. Vents could be added to the core and/or mold to give the gases an easier path to escape through the

sand, rather than becoming trapped in the liquid metal. In addition, we want to make sure that the

binder is completely cured prior to pouring. A coarser grained sand with a narrow distribution of sand

grain sizes will provide higher permeability and permit easier gas removal. Finally, a switch to a

different type of binder could reduce the amount of gas produced from that of the oil/urethane.

4. Penetration occurred by liquid metal flowing between the sand grains of the core. It appears that the

core was not properly compacted, with relatively large voids between the sand grains. The core may

have also had very large sand grains with a very narrow distribution of sizes (although this is contrary to

the conclusion of question 1. The core also gets hotter than the mold, since the core is completely

surrounded by liquid metal. In addition, the region showing the penetration is adjacent to the gate

where it will have received the molten metal first and would have been hotter longer than the

remainder of the mold. The long exposure to high heat may have led to the breakdown of the binder

and helped the liquid metal penetrate the sand. Finally, the defect was only noted near the bottom of

the casting because of the higher metallostatic pressure head (the pressure of the column of molten

metal) helping to force the metal between the sand grains.

5. The enlargement could have occurred because the mold was weak and the high metallostatic

pressure crushed the sand, thus enlarging the mold cavity. Better compaction during mold making

would produce denser, and stronger, sand. Using a larger amount of binder might also help, but gas

problems would tend to become more severe. Another possible cause would be erosion, because the

enlargement occurred next to the gate where all of the liquid metal entered the mold cavity. The sand

near the gate becomes the hottest, and the binder may have decomposed prematurely. The use of

several gates, rather than just one, might help reduce the problem.

6. Both the molds and the cores could be reclaimed. The binders are organic, and, with luck, most of the

organic material will have broken down during the casting and cooling process. If the organic

breakdown is not sufficient, some form of reclamation process can be used. A mechanical reclamation

system would perhaps fire the sand grains at a hard metal plate, where the impact would break the

brittle polymer binder off of the sand grain surface. A thermal reclamation system, in which the sand is

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heated to a high temperature (usually above 10000F), will burn off any residual binder. The processed

sand is then carefully screened to assure the proper size and distribution of sizes prior to rebonding and

reuse.