Mold Making 101 - Sand Core Basics

Simple Patterns
Just to recap on our previous post we placed our PATTERN on a MOLD BOARD in the middle of the bottom half of a FLASK which is called the DRAG.  We then RAMMED green sand around the PATTERN, levelled or STRUCK off the excess sand and then flipped the DRAG over.  We next put on the top half of the FLASK (which is called the COPE), dusted on some PARTING POWDER, inserted a SPRUE PIN, filled the COPE up with green sand and RAMMED the green sand.  We then removed the COPE off of the DRAG, made some GATES and VENTS so that the molten metal would flow into and out of the hollow MOLD.  We next removed the PATTTERN, producing a reverse image of our pattern in the form of a hollow MOLD. 

Complex Patterns In our previous post, we were molding a simple pattern.  However, very few patterns are that simple.  In the majority of cases, patterns are usually irregularly shaped with hollows and cavities in the middle.  We therefore have limitations when we want to make castings that have hollows and cavities in the middle.  

We can usually get around this problem by inserting a core of solid sand into the mold.  If we can make this core of solid sand into the shape of our hollow or cavity, when the molten metal is poured into our mold, it will flow around the sand core.  When the metal solidifies, we simply break out the solid sand core and......  voila!..... we have our hollow or cavity. 

Split Patterns
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, place the COPE on top of the DRAG, ram green sand around the pattern, insert/cut in some SPRUES, GATES, and RISERS.  We then lift the COPE off 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.

Split Patterns & "Core Prints"
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 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 of the 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 add the sprues, gates and risers (not shown).  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.

Adding the "Sand Core"
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'll use a SPLIT PATTERN and SAND CORES to create the holes that are required to complete the casting.