Thursday, April 8, 2010

Mold Making 101 - Very Basic Instructions

1.1 Parts of the Mold.
Castings are made in a 2-piece wooden box called a FLASK.  The bottom part is called the DRAG.  The top piece is called the COPE.  The DRAG does not have a bottom.  Instead, it is placed on a MOLD BOARD which is an oversized piece of 3/4" plywood on two skids.  LOCKING PINS allow the two sections to be separated and then later re-joined in perfect alignment.  The joint and surface separating the COPE from the DRAG is called the PARTING LINE. 

To recap.
  • FLASK – a 2-piece wooden box. 
  • DRAG – bottom half of the FLASK
  • COPE – top half of the FLASK
  • MOLD BOARD – oversized thick plywood on skids that the DRAG rests on. 
  • PARTING LINE – joint and surface which separates the COPE from the DRAG.
1.2  Making A MOLD
A CASTING is made from a MOLD.  The MOLD is made in the FLASK from a PATTERN using GREEN SAND to create a hollow cavity that is in the shape of the PATTERN.  To make a mold:

The pattern is placed top-down in the DRAG on a MOLD BOARD.  The DRAG is filled up and tightly packed or RAMMED to the top of the DRAG with GREEN SAND.  The excess sand is struck off with a straight edge which  produces a flat surface on the DRAG.

A second MOLD BOARD is placed on top of the leveled sand.  The DRAG, along with both MOLD BOARDS, is flipped over.  The top MOLD BOARD is removed so that the top surface of the pattern is now exposed.  This surface now forms the PARTING LINE.  

The PARTING LINE is next dusted with TALC so as to allow the COPE to be separated from the DRAG when it comes time to remove the PATTERN. 

The COPE is then fitted to the DRAG using the LOCKING PINS. 

Tapered SPRUE and VENT PINS are then pressed about 1"-2"into the green sand of the DRAG.  The SPRUE and VENT PINS will form openings through the sand in which to pour the hot metal and to allow the hot gasses to escape.
 The COPE is filled and rammed to the top with green sand.  The excess sand is struck off with a straight edge.

With the COPE and DRAG tightly packed with green sand, the PATTERN next has to be removed or DRAWN from the MOLD.  The SPRUE and VENT PINS are removed from the COPE, the LOCKING PINS removed, and the COPE lifted off the DRAG.  The PATTERN is lightly RAPPED so that it can be loosened from the green sand and lifted out. 

The green sand in the DRAG between the SPRUE and VENT holes and the hollow impression left by the PATTERN is removed using a spoon-like instrument called a SLICK to form channels called GATES which will allow the molten metal to flow into the MOLD and the hot gasses to escape.
The COPE is then re-placed on top of the DRAG using the LOCKING PINS. 

Hot molten metal is then poured into the SPRUE hole until the metal returns to the surface by the VENT hole. 

When the metal has solidified, the FLASK is upturned off the MOLD BOARD.  The green sand is then knocked out of the FLASK to expose the CASTING.  The SPRUE and VENT holes, which are now filled with metal, are next cut off of the CASTING.

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