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 isolative 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.

Die Coating Application

As in any coating application, surface preparation is critical.  A new die should arrive from the pattern shop clean of all oils and cutting fluids.  The parting line should be clean and flat, with return and ejector pins flush.  The casting cavity must be sand blasted with 80 grit coal slag to ensure that the cavity surfaces have a “tooth” or texture that will allow the coating to stick to the die.  Sand may be used in place of the coal slag if necessary. 

Dies that have been in service must have all the old coating completely removed.  The types of cleaning media used vary, and may include blasting with coal slag, sand, metal shot, grit, glass beads and dry ice (CO2).  The choice depends on availability as well as how difficult the coating is to remove.  In most cases, dry ice blasting is recommended for routine cleaning with periodic coal slag blasting to restore the die surface finish for die coating.  Over-blasting, especially with sand, shot, or grit, erodes die details and shortens die life.

Die coatings should be sprayed on to the die surface with an airless spray gun.  Spraying equipment may be any one of many different styles and types of spray guns.  Using a paint gun is not recommended as the heavy materials in the die coating easily clog the small ports.  A siphon type gun, which has one straight fluid tube with replaceable fluid tips, works well. 

Some siphon guns are available with interchangeable pots.  With extra pots, two or three different types of coating may be kept on hand, mixed and ready.  As the need arises for a particular coating, it may be snapped on the spray head and used immediately. 

In areas where a high degree of insulation is required, such as gates, runners, risers, and pouring cups, brushing the coating on provides more insulating capability.   In addition to the insulating properties of the coating itself, brushing will trap air bubbles, which enhance insulation.  Additionally, the rough surface caused by brushing can aid molten metal flow through the gating system by continuously disrupting the oxide skin as the metal flows.

Proper safety gear should be used when coating a die, including gloves, protective sleeves, boots, and eye and ear protection. Thoroughly mix the coating in its original container before diluting or using only a portion of the can weight.  This will alleviate any settling problems that occur during transit and storage.  When diluting, soft warm water works best, but cold water is acceptable.  In either case, adequate mixing with any equipment such as a lighting mixer or a bent rod in a hand drill is required.  Excessive shear should be avoided.