How to Make a Sand CastingSand Casting
involves four basic steps: assemble the sand mold, pour liquid metal into the mold, allow the metal to cool, then break away the sand and remove the casting. Of course, the process is more complex than it sounds.
The first step of mold assembly is to partially fill the drag with sand. The pattern, core print, cores, and gating system are placed near the parting line. The cope is then assembled to the drag. Additional sand is poured on the cope half until the pattern, core, and gating system are covered, and then the sand is compacted by vibration or mechanical means. Excess sand is removed with a strike off bar.
Now that the mold is formed, the cope is removed from the drag so the pattern can be extracted from the mold.
Pattern extraction is undertaken carefully to avoid breaking or distoring the newly formed mold cavity. This is facilitated by designing a draft: a vertical taper perpendicular to the parting line. The draft taper is usually at least 1°. The rougher the surface of the pattern, the more the draft to be provided.
Before it can be filled with liquid metal, the complete mold needs to be prepared; the mold cavity is often lubricated with mold washes to facilitate casting removal. Then the cores are positioned, additional mold materials such as cope rope are added to help prevent run-outs, and the mold halves are closed and securely clamped together; the cope and drag sections maintain proper alignment with the aid of pins and guides.
The mold halves must remain secure so liquid metal can’t leak through the parting line. Before pouring flaskless molds, wood or metal pouring jackets are often placed around the mold and weights are set on the top to keep the cope from lifting.
Molten metal enters the mold cavity through a gating system: molten metal is poured into a mold via the pouring cup, continues down the sprue (the vertical part of the gating system), and then through the runners (horizontal portions). Built up gases and displaced air escape through vents. The multiple points where metal is introduced into the mold cavity from the runners are called the gates.
After the casting has cooled, it is broken out of the sand mold. The process of removing sand from the castings is called shakeout. The casting can be removed manually or by automated machinery. Agitations tables and rotary tumblers are commonly used.