Excerpt from the Jan. 2013 Foundry & Management Technology article by John Hall.
Starting a new die Die coating application Die heating, coating
Die coating in gravity diecasting is one of the operational parameters of the casting process that is often overlooked or misunderstood. Die coatings are necessary for three basic reasons.
Coatings provide a protective barrier between the die and the casting to prevent die erosion and wear. Coatings provide some degree of control over the solidification rate and direction. Coatings provide a barrier between the die and casting so that the casting will release from the die.
With proper use, a die coating can be used to control the thermal gradients such that directional solidification can be achieved. This allows a pathway for feed metal to flow into the solidifying structure and compensate for normal metal shrinkage during solidification. This is particularly important in castings with thin sections that expand to thicker sections: The thin areas must remain open to ensure that shrinkage will not occur in the adjacent thick section.
Starting a New Die
In some casting designs there may be two or more characteristics working against each another. For example, a design might have a thin-walled section in need of additional insulation to prolong solidification, yet it is also in an area that is difficult to release form the die. In this case a compromise must be reached.
By their nature release coatings are not isolative, and isolative coatings will not aid in the release of tight or difficult geometries. In such cases a choice must be made as to which of the operational characteristics is most important. One choice may be to use a combination coating, which will allow for some insulation and some release. Another option is to use an insulating coating as a base coat and a release topcoat.
Insulating coatings vary greatly in insulating qualities as well as the surface finishes they will impart to the casting. The insulating qualities of a coating are a function of the type of refractory filler that is used and their thermal conductivity and heat capacity.
Also contributing to a coating’s isolating capabilities, as well as surface finish, is the amount of binder and the dilution rate. Typically, binders are a sodium silicate. Typical refractory materials found in die coatings include: vermiculite, bentonite, talc, titanium dioxide, alumina, olivine, and graphite
Both release and chill coatings contain materials that act as heat conductors to allow for more rapid solidification while protecting the die against wear. Release coatings typically contain graphite as the lubricant, which is non-wetting by aluminum.