Designing
a die cast part is different from designing a screw machined part. The
following items are the most common characteristics for a die cast part
that a designer need to consider:
-
Flash
-A thin web or fin of metal on a casting which occurs at die partings,
vents and around moveable cores. This excess metal is due to working and
operating clearances in a die.
-
Gate
vestige -- Passage for molten metal which connects runner with die
cavity. Also, the entire ejected content of a die, including castings,
gates, runners, sprue (or biscuit) and flash. After cast, the cast part
is cut off from the runner but a protruded part (vestige) will be left
on the part. This required a secondary operations such as hammering, saw
cutting, sander or milling to make the gate vestige smaller or flush with
the adjacent part. To completely make the mark invisible requires sand
blasting or painting or anodizing.
-
Draft
-- The taper given to walls, cores and other parts of the die cavity to
permit easy ejection of the casting. Typical draft angle is 2 degree per
side. When designing a part please please allow draft angle at the mold
separating direction.
-
Ejector
marks -- Marks left on castings by ejector pins. Designer must first
allow a supporting spot for the ejector pins to eject the casting, then
allow a round pin mark on the surface.
-
Porosity
-- Voids or pores resulting from trapped gas, or shrinkage during solidification.
Porosity is inevitable but the degree can be controlled.
-
Shrink
mark -- A surface depression which sometimes occurs next to a heavy
section that cools more slowly than adjacent areas. Try to design with
uniform thin wall and avoid any heavy section. Heavy section would also
result in residual stress in the casting which is a contributor to
the future in-service cracking and failure.
-
Surface
finish -the as cast surface finish is smooth in general but cosmetically
not appealing due to the existence of flow line, gas mark, heat mark and
lubricant residue, etc. However these cosmetic appearance issues can be
easily minimized or eliminated or by cleansing, sand blasting, anodizing
or powder coating. Other surface defects include soldering due to the adherence
of molten metal to portions of the die.
-
Dimensional
Tolerance - Compared to other casting method die casting can achieve
the best precision. When the dies are properly made, casting may be accurate
within 0.001 inch or even less and a limit of 0.002 or 0.003 inch per inch
of casting dimension can be maintained on many classes of work. However
try to design a less tight casting will lower the manufacturing cost, especially
in mass production, tighter tolerance will result in more scraps and more
frequently tool maintenance. So be careful of the tolerance you choose.
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