This paper concerns the development of Wso, an indicator of the efficient protection against humidity, as a new parameter characterizing protective coatings. This indicator determines to what degree the covering of cores and sand molds influences, advantageously or disadvantageously, the humidity absorption from surroundings into surface layers, in comparison with layers without covering, which absorb the moisture at high-humidity conditions of atmospheric air. Simultaneously, this indicator will allow for an efficient comparison of tendencies of individual coatings for moisture sorption depending on the number of their layers. Thus, it can serve as a guideline in selecting the coating type for the molding sand on which it will be deposited, while taking into account the time of waiting—of molds and cores covered with coatings—on pouring with liquid metal and the atmospheric conditions to which they will be exposed.
Hinweise
This article is an invited submission to JMEP selected from presentations at the 73rd World Foundry Congress and has been expanded from the original presentation. 73WFC was held in Krakow, Poland, September 23-27, 2018, and was organized by the World Foundry Organization and Polish Foundrymen’s Association.
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Introduction
Protective coatings applied to cores and sand molds are, in foundry practice, widely distributed materials. In accordance with environmental protection, only the water-based coatings, which are environmentally friendly, should be used. Nevertheless, the alcohol-based coatings are common as well. The coatings are mainly used for protection against the liquid metal penetration into deeper layers of sand molds and for decreasing the tendency of molding sands to stick to the cast’s surface (Ref 1-4). Despite that, the protection against humidity as a result of coating is not considered by either the literature or material manufacturers. However, since there is a chance of an occurrence of cast surface defects, it seems to be important, especially regarding defects of gaseous origin (Ref 5, 6). Even more so, because the authors proved in the previous studies (Ref 7-9) that in conditions of high relative humidity of air and temperature, the water from surroundings can be absorbed into surface layers of sand molds elements (including those covered by protective coatings). Therefore, it was decided to develop the indicator of the efficient protection Wso, which will allow estimating whether covering the cores and molds with coatings influences, advantageously or disadvantageously, the moisture sorption from surroundings.
Materials and Methods
The determination of the indicator of the efficient protection against humidity Wso was performed on cuboidal shaped elements (Fig. 1) of dimensions 50 × 30 × 5 mm, made of molding sands with organic (alkyd resin and furan resin) and inorganic (hydrated sodium silicate—water glass) binders (see Table 1).
Fig. 1
Shaped elements for testing: (a) without coating, (b) with one layer of zirconium coating, (c) with two layers of zirconium coating
Table 1
Molding sands used in tests
Molding sand
Component
Amount
With alkyd resin
Sand grains
Silica sand
100 p. p. w.
Binder
Resin SL 2002
1,3 p. p. w.
Hardener
KL
25% to the binder
With furan resin
Sand grains
silica sand
100 p. p. w.
Binder
Resin Furanol FR75
1,2 p. p. w.
Hardener
PU6
60% to the binder
With hydrated sodium silicate
Sand grains
Silica sand
100 p. p. w.
Binder
Sodium water glass
3,5 p. p. w.
Hardener
Flodur FM5
10% to the binder
×
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The 5 mm thickness of the shaped element, at double-sided moisture exchanges (presented schematically in Fig. 2) which occurs during tests, reflects conditions in surface layers of molds and cores down to the depth of 2.5 mm. Alcohol-based and water-based zirconium coatings were applied as the protective coatings.
Fig. 2
Scheme of moisture exchange between surroundings and the shaped element
×
The investigation method was quantitative measurements of the moisture sorption based on the gravimetric method (described in detail in Ref 7), with simultaneous consideration of the modification allowing to deposit the protective coating (Ref 8). The immersion time was 5 s at the relative viscosity of coatings. It was measured by a Ford Cup with an outflow hole diameter equal to 4 mm. The prepared samples were placed on the measuring stand for 12 h under the thermal humid conditions as follows:
27-32 °C—air temperature,
Above 95% of the relative humidity of the atmospheric air.
Changes in the sample weight, due to the moisture absorbed from surroundings, were continuously measured and recorded every 30 s with a computer.
Determination of the Indicator
The determination of the indicator of the efficient protection against humidity—Wso—is done based on the collected data concerning increases in the sample weight due to the moisture sorption from the surroundings in time. The indicator Wso is expressed by Eq 1:
where \(W_{{{\mathrm{so}}}}\) is an indicator of the efficient protection against humidity (%), \(I_{{w_{0} }}\) the humidity amount of the shaped element without the protective coating (g/cm2) and \(I_{{w_{{\mathrm{p}}} }}\) the humidity amount of the shaped element covered by the protective coating (g/cm2).
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The positive indicator value shows that the amount of the moisture absorbed by surface layers covered by coatings was smaller (in a percentage formulation) than by layers not covered. In other words, it shows that these coatings provide an efficient protection against humidity. In a similar fashion, the negative value indicates that layers covered with coatings absorb more moisture than layers not covered, which means that the protective function is not provided.
The Wso indicator can be determined for the arbitrary period during which cores or sand molds are exposed to the atmospheric conditions influence.
Analysis of the Results
Figure 3, 4 and 5 represents the course of the moisture sorption into surface layers of cores made from molding sands with different types of binders. The collected data are a basis to determine the Wso indicator. Figure 6, 7 and 8 shows the graphic interpretation. Another aspect, which must be mentioned, is the fact that the analysis of the sorption process, and in consequence of the Wso indicator, has been presented here only for the relatively high air humidity (over 95%) and temperature (27-32 °C). Moreover, it should be mentioned that these analyses can be done for any ambient conditions.
Fig. 3
Process course of the moisture sorption (an increase in the moisture weight Δm and kinetics Δm/Δt) into surface layers of the sand mold made of molding sand with alkyd resin
Fig. 4
Process course of the moisture sorption (an increase in the moisture weight Δm and kinetics Δm/Δt) into surface layers of the sand mold made of molding sand with furan resin
Fig. 5
Process course of the moisture sorption (an increase in the moisture weight Δm and kinetics Δm/Δt) into surface layers of the sand mold made of molding sand with hydrated sodium silicate (water glass)
Fig. 6
Indicator of effective protection against humidity for surface layers with protective coatings exposed for 1-h atmospheric conditions influence
Fig. 7
Indicator of effective protection against humidity for surface layers with protective coatings exposed for 6-h atmospheric conditions influence
Fig. 8
Indicator of effective protection against humidity for surface layers with protective coatings exposed for 12-h atmospheric conditions influence
×
×
×
×
×
×
The course of the moisture sorption varies in terms of the sorption intensity depending on the kind of molding sand, particularly on the type of the binder. Molding sands with organic binder (alkyd resin—Fig. 3) are characterized by lower moisture sorption ability, which is equal to approximately 0.0075 g/cm2 in the case of samples without protective coatings after 12-h exposure to atmospheric condition. This means that during that time (12 h), 0.0075 g of water from surroundings is penetrating each unit of the core’s surface.
According to the diagram, the second molding sand with organic binder (furan resin—Fig. 4) shows a tendency to much higher weight increase as a result of absorbed moisture. In comparison with the alkyd molding sand, a 70% increase is noted. Notwithstanding, the most susceptible to humid air is molding sand with inorganic binder (water glass—Fig. 5). The diagram depicts that during the 12-h exposure to the ambient condition influence, 0.024 g/cm2 moisture was absorbed into the surface layers of that molding sand.
However, application of zirconium water-based and alcohol-based coatings can significantly change the character of the moisture sorption into surface layers of sand cores in the case of each analyzed molding sand. The effectiveness of the protective function changes over time. To observe these changes and for the comparison of efficiency of selected coatings and number of their layers in moisture sorption, the Wso indicator was developed.
According to Fig. 6, 1-h exposure to the atmospheric conditions negatively affects the cores made of molding sands with resins, especially in the case of using the water-based coating. For the alkyd molding sand with a single layer of water-based coating, the Wso indicator is equal Wso1 = − 85%, whereas for double layer it is Wso1 = − 125%. Similarly, the protective action of this coating, applied on furan cores, is ineffective. Only in combination with molding sand with water glass, the water-based coating generally does not affect molding sand sorption ability, which is indeed high. The alcohol-based coating shows greater efficiency in protection. Furthermore, a single layer of this coating’s type protects sand cores with inorganic binder significantly well. In the case of furan molding sand, alcohol-based coating’s efficient protection against humidity oscillates around 20%. The same coating negatively affects surface layers of molding sands with alkyd resin.
Increasing the exposure time up to 6 h (Fig. 7) decreases the effectiveness of protection against the humid air of the alcohol-based coatings applied on cores made of molding sand with furan resin and water glass. Simultaneously, the water-based coatings have a negative impact on moisture sorption in molding sands with resins and have no influence on molding sands with inorganic binder.
Figure 8 shows that a single layer of alcohol-based coating applied on cores, independently of the molding sand’s type they are made from, is the best choice in terms of protection. Nevertheless, the efficient protection of this coating is not too high. Its maximum value of the Wso indicator was round 20%, which is characteristic for application on cores made from molding sands with furan resin.
On the other hand, the water-based zirconium coatings increase the molding sand’s tendency to moisture sorption processes. The efficient protective indicator Wso holds negative values for both molding sands with resins during the entire analyzed period (12 h). For the molding sands with water glass, the Wso indicator shows a negative value for double layers and oscillates around 0 for a single one.
It should be considered that the alcohol-based coatings protect the surface layers of elements of sand molds against humidity much better if they are applied in a single layer. In other words, if in the foundry’s work schedule, the time between the application of the protective coatings on cores and molds, made with a different type of molding sands with chemical binders (alkyd resin, furan resin and water glass), and pouring out the mold with the liquid metal is 12 h, in terms of moisture sorption under the humid air conditions, the deposition of double-layer coating (regardless of the coating type) seems to be pointless. Moreover, if the intended aims of applying coatings, such as protection against the liquid metal penetration into the deeper layers of sand mold or decreasing tendency to stick the molding sand to the surface of cast, are accomplished by using a single layer of protective coating, then the application of larger numbers of layers would be uneconomic as well as risky because of the greater possibility of defects occurring in the casts, especially of a gaseous origin.
Conclusions
The Wso indicator presents effectiveness of applying protective coatings for cores and molds against humid air. It can be determined for the arbitrary period during which elements of cast molds are exposed to the atmospheric condition as well as various ambient conditions, for example, the conditions which are characteristic during the season. The indicator can be easily adjusted to the work schedule of foundries. Thus, it can have practical implication as a pointer or a guideline in the selection of protective coatings (the type of coating and the number of layers) depending on molding sand the elements of sand mold were made of, atmospheric conditions the elements are exposed to and the length of the exposure. In this way, the process of excessive moisture sorption from surroundings through the surface layers of sand molds can be minimized, which means that the amount of accumulated water in the very surface layers is decreased, while consequently decreasing the possibility of defects occurring in the casts, especially of a gaseous origin.
Acknowledgments
This research received funding from the project NCBiR No. POIR.01.01.01-00-0042/17.
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