Controlling permanent mold coatings application parameters.

A major variable in permanent mold foundries, coatings often don't perform well due to a lack of understanding, and process controls.

Ever since the introduction of gravity, low-pressure and counter-pressure permanent mold casting for aluminum, foundrymen have recognized the necessity of using mold coatings. The mold coating prevents the liquid metal from soldering to the ferrous mold material and increases mold life.

Today, however, the coating is considered indispensable in improving casting quality by controlling heat transfer to avoid misruns and cold-shuts, as well as to promote directional solidification to enhance internal soundness. Despite this recognition, wide and sometimes contradictory differences in coating practice exist within the permanent mold community. The only consensus seems to be that mold coating is a "necessary evil."

In application, the coating--a water-based suspension of ceramic particles--is sprayed on the metal surface of the mold while hot so that drying is almost instantaneous. The sodium silicate contained in this slurry acts as a binder. For a given slurry, the working properties of the dry coating layer deposited on the mold will depend to a great extent on the conditions of application, and because mold spraying is often a manual process, human factors play a significant role. This is particularly true when measurable, controlling factors are not monitored. Thus, application is widely acknowledged as a contributor to quality variations in the permanent mold industry.

Coating Selection

The primary working properties of a coating are adhesion, insulating capability, wear-resistance and surface texture. Ease of removal, "all-purpose" characteristics and low cost are secondary--but desirable--features. No single coating can provide optimal results on all scores. Coating selection is therefore really based on compromise, and requires the judgment and experience of the foundryman.

The two properties most often considered in the choosing of a coating are insulating capability and smoothness (important for surface finish). Once the proper coating with consistent properties has been selected and obtained from the manufacturer, the success of the operation rests with the mold coating applicator.

Application Parameters

Most permanent mold foundries use compressed air spray guns at reduced pressure (about 30 psi) to atomize the slurry as droplets of 100-200 micrometers ([micro]) in diameter. During application, spraying distance should not exceed 9-12 in. to avoid partial drying of the slurry before contact with the mold surface. Longer spraying distances of up to 4 ft result in a "dry spray" condition, and are sometimes used as a top spraying pass to deposit a speckled, granular overcoat (Fig. 1). This coat has high insulating properties, but almost no adhesion. The dry spray will remain embedded in the outer skin of the casting. To maintain this condition, spraying should be repeated on nearly every cycle.

[Figure 1 ILLUSTRATION OMITTED]

The rate of deposition, expressed in [micro]m of dry coating deposited per second of spraying, will depend on three parameters: the flow rate (in milliliters per second) of the slurry coming out of the gun nozzle; the surface area of the spray halo; and the solid content of the slurry which is directly related to its Baume density, as well as the theoretical density of the ceramic charge. Figure 2 provides curves for this data.

[Figure 2 ILLUSTRATION OMITTED]

Spray Gun Motion

Once the rate of deposition is given, the operator's motion with the gun will determine the evenness and final thickness of the coating on the mold. This is the only totally operator-dependent factor in coating application.

To obtain optimal results, the gun should be moved with a constant velocity; that is, with a motion of translation imparted to the gun, wrist and forearm alike, rather than just a rotating wrist motion. Figure 3 represents the coating distribution that would result from a perfect sinusoidal rotation of the spray gun by the wrist, and shows a heavy buildup of up to 160% of the average deposit, formed near the extremities of the moving spray halo course.

[Figure 3 ILLUSTRATION OMITTED]

Even in the best conditions of the translating movement recommended above, a similar buildup is certain to occur. It is advisable that the operator change the direction of the spraying motion. Also, generously overlapping the passes by 50% or more will considerably improve the evenness at right angles to the travel of the spray gun. The sketch at right of Fig. 3 shows how crossing the passes can improve coating evenness. This practice should be the rule when possible (on flat surfaces, for instance).

Rate of Drying

Within the usual spraying conditions, each individual droplet of slurry ejected from the gun nozzle impinges a dry coating substrate. The final special arrangement of these dried droplets will to a great extent affect all the working properties of the deposited layer. For the mechanical spraying conditions given above, the morphology of a coating will depend on the rate of drying in [micro]m of dried coatings per second ([micro]m/s). A rapid rate of water evaporation will result in a more porous, and hence a more insulating (though less durable) coating.

This rate of drying depends on three factors: it will increase with the mold temperature; it will decrease with slurry dilution; and it is affected by the thickness of the already deposited coating.

The photomicrographs in Fig. 4 illustrate the morphology of a 15 Baume density talc-based coating applied on a mold at 365F (185C) in thicknesses of 45 and 95 [micro]m respectively. The rate of deposition was the same at 4 [micro]m/s for both cases. Because of the higher rate of drying at 45 [micro]m, the lighter coating is less dense--at 69% average porosity--than the 95 micrometer coating, which has an average porosity of 62%.

[Figure 4 ILLUSTRATION OMITTED]

Figure 5 correlates mold temperature, slurry dilution and rate of drying for a talc-based coating. The top graph shows the rate of drying for a 20 [micro]m primer coat, while the bottom is for a 100 [micro]m main coat or retouch. The graphs provide an easy way to determine whether mold temperature or slurry dilution need to be adjusted when coating application is done at mold cycling temperature.

[Figure 5 ILLUSTRATION OMITTED]

Process Control

Once an appropriate and consistent coating has been selected, application repeatability can be achieved by calibrating and carefully maintaining the spraying devices, as well as by monitoring mold temperature, slurry density and coating thickness. This can be easily done, once current operations have been changed and simplified.

It is a fact that many foundries use too many different coatings for reasons that are rarely clear and often contradictory. Once everyone understands the reasons for using a particular coating in a consistent manner, the spray gun motion becomes the only variable subject to human error. This will be a considerable improvement for many permanent mold foundries.