Understanding insulating properties of permanent mold coatings; although mold coating application involves many complex factors, recent research has produced information that may make process control feasible in this area.
Although mold coating application involves many complex factors, recent research has produced information that may make process control feasible in this area.
Along with their role as mold protectors, mold coatings also serve to control the heat flow from the metal to the mold. Heat flow control helps ensure filling of the mold cavity and promotes directional solidification toward an adequately fed section of the casting. Understanding how coating application procedures affect heat transfer at the mold/casting interface is vital if the optimal use of a mold coating is desired.
Centre de Metallurgie du Quebec has recently conducted experiments that measure the thermal conductivity of coatings applied to cast iron molds. These thermal conductivity values were deduced from the measurement of the temperature drop experienced by liquid aluminum (A356) as it filled, from the bottom, a vertical plate 2 1/4 x 16 x 1/8 in. Pouring rates and mold and metal temperatures were strictly controlled.
The experiments first focused on different thicknesses of commonly used, white insulating coatings of 20 Be density that were sprayed on a 400F mold. These coatings consisted of a suspension of ceramic material (mica, vermiculite) in a water base solution with a sodium silicate binder.
Examination of the results in Table 1 shows that, for insulating coatings applied by spray gun, thermal conductivity decreases substantially as coating thickness increases up to 200 [Mu] m (0.008 in.). However, building up the ceramic layer above 200 [Mu] m only provides a marginal increase in thermal resistance.
For example, a 400 [Mu] m thick coating would only be 20% more insulating than a coating half that thickness. Consequently, increasing the coating thickness beyond 200 [Mu] m does not significantly affect heat transfer, and it may be detrimental to dimensional accuracy of the casting and mold coating adherence.
Centre de Metallurgie's study of coatings of different thicknesses has shown that coatings exhibit much higher porosity close to the mold surface, with a resulting higher thermal resistance. Under the experiments' coating application conditions, porosity was 52% at 120 [Mu] m away from the mold surface, but only 17% at 430 [Mu] m, a phenomenon that is attributed to the high rate of evaporation that takes place when the first layers of coating are sprayed. Figure 1 illustrates a 120 [Mu] m coating's higher porosity compared to that of a 240 [Mu] m coating.
Because the greatest porosity was found near the high temperature mold/metal interface, it follows that a coating sprayed on a hotter mold will result in a less dense, more insulating dressing.
The influence of a polished coating on the heat transfer rate was then investigated. The surface aspect of a polished coating, seen in Fig. 2, differs considerably from that of the nonpolished coatings (Fig. 1). For the conditions investigated, the polished surface's reduced roughness produced a 40% increase in heat transfer across the coating. Consequently, polishing the mold coating to control hot spots should be quite effective.
To examine the effects of applying a thicker coating by brush, a half dilution slurry was brushed onto a 400F mold. This resulted in a rough, 275 [Mu] m thick coating, the surface of which is shown in Fig. 3. It can be seen that this coating was much denser than an equivalent thickness coating applied by spray gun. This greater density is a result of the thicker individual coats produced by brush application. In contrast, each spray gun pass produced a coating of approximately five [Mu] m.
Although surface roughness significantly affects conductivity (see Fig. 4), the rough coatings were not more insulating than equivalent thickness sprayed coatings because of the denser ceramic shell that brush application produces.
The conductivity values of black and white mold coatings were also evaluated and compared. As indicated in Table 1, a typical black coating is two to three times more conductive than a standard white coating of 100-200 [Mu] m thickness. In addition, previous results have shown that a black coating's thermal conductivity is about five times less dependent on thickness than a white coating.
Mold coating application is the most important area where process control has not yet been fully implemented in permanent mold foundries, probably because of the complexity of factors involved. This forces great reliance on the skill of experienced personnel, with all the attached inconveniences.
Understanding the heat transfer process across the mold/metal interface makes it conceivable that application procedures could be established that would ensure consistency and reproducibility of mold coating conditions while reducing dependence on human factors. [Figures 1 to 4 Omitted] [Tabular Data Omitted]
Franco Chiesa Centre de Metallurgie du Quebec Trois-Rivieres, Quebec