Thursday, April 8, 2010

Sand Core Basics

3.0  SAND CORE Basics
Making a solid rectangular cube casting is pretty straight forward.  We simply place the solid cube face down on the mold board, shake on some parting dust, riddle some sand over the rectangular pattern, and ram the sand.  Next, turn over the drag, attach the cope to the drag and repeat the process.  Removing the cope, we lightly tap the pattern and lift it out of the drag.  Add some sprue and vent holes and we're ready to pour our casting.
However, when we add holes, slots, and protrusions to our rectangular cube, things get a bit difficult when we try to make a mold for our cube.  Holes, hollows, and undercuts in castings present unusual challenges in that the mold will be destroyed if we attempt to remove the pattern.

In order to overcome this problem, we have to modify the pattern.
Remember that molten metal will flow into all voids in the mold.  Whenever we want to create holes, slots, etc, we have to fill those voids in the mold.  We do this through the use of SAND CORES.  As defined by the American Foundryman's Association, a SAND CORE is simply "...a bonded sand insert that is placed in the mold to form an undercut or hollow section in the casting which cannot be shaped by the pattern.  A core is frequently used to create openings and various shaped cavities in the casting. The shaped body of sand which forms interior of casting and also selected external features".

The SAND CORE is made in a specially prepared CORE MOLD that resembles the shape of the hole slot, etc that we want to create.  The CORE MOLD is filled with a sand and glue mixture, baked to form a solid mass, and then inserted into the larger MOLD.
In order to hold the CORE in place in the middle of the hollow mold, we have to make the SAND CORE longer than the hole we want to create and make depressions called CORE PRINTS in the casting sand.  To make these CORE PRINT depressions, the PATTERN is modified so that the CORE PRINT is an integral part of the PATTERN and is created when we make the mold. In this case, we add two pieces of dowel to the solid cube of our pattern to accommodate the longer length of the sand core (the second dowel piece is hidden in the back of our pattern).
The sand core will be lowered into the voids created by the two dowels. However, if we try to pull our pattern out of the sand, we will destroy the mold.  We have to further modify our pattern to account for the two dowels that are attached to our basic solid core pattern as shown below (the back part of the modification is hidden).
However, when we remove the pattern, we will have two "V-shaped" voids because we modified our pattern so that we could remove the dowel part of the pattern whereas we want a cylindrical void.  In order to create this cylindrical void and fill in the V-shape, we have to make a V-shaped SAND CORE with a circular bottom.  This will fit over our cylindrical SAND CORE and fill the V-shaped void in the mold.
 If we place our two sand cores together - the cylindrical sand core and the V-shaped sand core - we thus create a hole through the middle of our rectangular casting.
In creating any pattern, we always have to be aware of those places on the pattern where SAND CORES will be required and design our pattern accordingly.

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