User manual

Appendix I
ZCast Design Guide - 22
6.5.2 Venting the Mold
Vented molds facilitate the safe passage of gas generated as a result of molten metal coming into
direct contact with the ZCast material. The design and adequacy of the mold vents is a
contributing factor that strongly influences whether quality castings are made or the mold possibly
fails releasing molten metal. Therefore, when designing mold vents, careful consideration is
placed on their overall design, capacity, placement, and number. Here are the more common
vent design issues a mold designer must consider:
 Large Surface-Area-To-Volume Ratios - As depicted in the example below, closely
spaced, thin walled mold geometries can lead to gas entrapment, because there is a
large area of interaction between the molten metal and the resins in the ZCast powder.
In some situations, such geometric conditions can impede proper ventilation. Mold
geometries with thin walls, such as those used to cast heat sinks, illustrate this issue well.
They require careful design and placement of vents to prevent unexpected mold failure
and yield quality castings.
 Excessive mold wall thickness can prevent adequate curing of the binder resins during
the mold baking process. Inadequately baked molds, when in contact with molten metal,
will create excessive quantities of gas and an increased possibility of unexpected mold
failure during casting.
 Poor ventilation can cause the entrapment of gas during the pouring process resulting in
the unexpected release of molten metal through the gating system.
 Gas entrapment within the mold cavity walls will result in an infiltration of gas in the
molten metal. This entrapment will show up in the solidified casting resulting in porosity –
a quality control issue.