Draft is the amount
of taper or slope given to cores or other parts of the die cavity to
permit easy ejection of the casting from the die casting die.
All die cast
surfaces which are parallel with the opening direction of the die
require draft (taper) for proper ejection of the casting from the
die. This draft requirement, expressed as an angle, is not constant.
It will vary with the type of wall or surface specified, the depth
of the surface and the alloy selected.
The animation to the
right shows the exaggerated application of draft to surfaces
parallel with the opening direction of the die. The amount of draft
required for holes, inside and out walls is different. Untapped
holes require the most draft because the casting shrinks (while
cooling) with great force around the die steel while forming the
inside surface of the hole. Inside walls also are subjected to the
casting shrinking onto the die steel that creates the surfaces of
inside walls. Outside walls require the least amount of draft
because the casting shrinks away from the die steel forming outside
surfaces.
When proper draft is
applied to a die casting it is much easier to open the die and also
eject the casting from the die surfaces. A die that is easy to open
and eject castings from will result in a more precise die casting
with a higher surface quality.
