Thursday, April 8, 2010

Sand Core Basics - Part II

In our previous blog on sand core basics, we were dealing with a solid pattern.  Wherever possible, patterns with an irregular shape are made in two halves called a SPLIT PATTERN.  With a SPLIT PATTERN, we place the bottom part of the pattern face-down in the drag.  We then ram green sand around the pattern, flip the drag, pattern, and rammed sand over.  We then place the top-half of the pattern on top of the bottom-half, ram green sand around the pattern, insert some risers and sprues.  We then lift the cope off of the drag, remove the patterns and complete the rest of the mold (SPRUES, GATES, and RISERS) so that it is ready for pouring.

In the graphic below, we have the SPLIT PATTERN for a hollow pipe with flanges on each end.  The pattern can be split in half along a "PARTING LINE".  The two halves of the pattern are kept aligned by holes in the bottom half of the pattern and wooden dowels that protrude from the top half.  The wooden dowels align with the holes in the bottom half of the pattern.  Here's what our pattern would look like.
However, our pattern doesn't account for the hollow centre of the pipe.  To create this hollow centre, we make a cylindrical SAND CORE that we insert into the mold to create the hollow centre.  In order to ensure that the SAND CORE is properly placed in the mold and is kept there, we add CORE PRINTS to each end of our SPLIT PATTERN.

Now that we have added CORE PRINTS to the PATTERN, we place the bottom half of the pattern face-down in the drag, and ram the drag with green sand.
 We then flip the drag and bottom half pattern over on the mold board, place the cope on top of the drag and set the top-half of the pattern on the bottom half, using the alignment pins and holes that we have built into the SPLIT PATTERN.  We then ram the cope with green sand.
We lift the cope off the drag and remove the patterns from the mold.  We now have the external shape of the casting we want to make, including two round voids at each end that will hold our SAND CORE in place.
We now insert our cylindrical SAND CORE which will give us a hollow centre when we pour our molten metal into the sand mold.  Notice how the SAND CORE fills the voids made by the CORE PRINTS in the green sand.
Wherever possible, when we are making patterns, we will use a split pattern and sand cores to create the holes that are required to complete the casting.

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