Foundry Technology: Summary of Discrimination Method of Coatings for Foundry

The main technical indexes of Foundry coatings

1. density

Density of foundry coatings reflects the amount of solid particles in coatings. If the density of casting coating is too small, the thickness of coating layer formed on the surface of sand mould and sand core is not enough to protect it. Therefore, generally speaking, the density of casting coating is good; but the density of coating is not the higher, the better. If the density is too high, the difficulty of coating brushing will result in uneven coating surface, local accumulation and other problems, which will also give the casting quality. Quantity has adverse effects. There is a certain relationship between the density and concentration of coatings. The density of casting coatings can be measured by barrel weighing method or by Baume meter, but the reading of Baume meter is greatly influenced by the viscosity of casting coatings.

2. conditional viscosity

Viscosity cup No.4 and No.1 commonly used in foundry production are used to determine the conditional viscosity of coatings. The purpose of measuring viscosity is to control the brushability of coatings, the depth of penetration into the surface of sand moulds and cores, and the thickness of coatings. Generally, foundry coatings manufacturers have recommended conditional viscosities for brushing.

3. suspensibility

Suspensibility is an important performance of foundry coatings. General determination methods include relative height sedimentation method (barrel method), sedimentation meter method and sedimentation rate method. Among them, the barrel method is the simplest and most practical one.

4. brushability

Brushability is generally judged by the experience of operators. A more objective method is to determine the apparent viscosity of casting coatings at different shear rates. The ratio of apparent viscosity of coatings at low speed (6r/min) and high speed (60r/min) is regarded as brushing index M.

5. leveling

When brushing or flowing coating on the surface of sand mould or core, a coating is established after brushing or flowing, and grooves and brush marks often appear on its surface. These grooves and brush marks may disappear in a short time or may be retained. The performance of automatically eliminating brush marks or grooves on the surface of wet coatings is called leveling.

6. flowability

Due to the influence of gravity, the casting coatings tend to flow downward on the vertical surface of the casting mould (core), which results in the thickness of the lower coatings being larger than that of the upper coatings, and even the phenomenon of accumulation at the bottom of the casting mould (core), which is called flowability.

7. permeability

Casting coating permeability refers to the ability of the coating to penetrate into the pore of sand mould. Casting coatings with large amount of penetration can enhance the adhesion of coatings to sand moulds, reinforce sand moulds and improve the anti-sticking ability of sand moulds. The penetration depth of the coating can be measured by dipping standard sand blocks.

8. paint PH value

Usually the pH value of casting coatings varies within 4-11, but alkaline casting coatings are commonly used, usually within the range of 8-10. PH value is not only used as an index of the properties of foundry coatings, but also as a method to monitor the changes of the properties of coatings during use and storage. It is also a process monitoring index of the production and use of coatings. Generally, paints can be determined by colorimetry and potentiometer.

9. coating permeability

Coating forms an isolation layer between metal and sand mold (core) surface, requiring compact coating, except EPC coating, the permeability should be low. The air permeability can be measured by a certain amount of air under a certain pressure through a standard cylindrical sample method.

10. Coatings' anti-abrasion strength

Sand moulds and sand cores coated with coatings should be transported, dried, cleared of surface floating ash and matching boxes without damage. Therefore, the coatings cured by drying must have a certain surface strength-anti-abrasion strength. The more accurate methods to determine the anti-friction strength of coatings are scratching method, pressure method, mechanical brushing method, sand drop method, vibration method and so on. Paint manufacturers can use hand scratching method to determine the anti-abrasion strength of coatings. 

11. coating hygroscopicity

Water-based coating sand mold and sand core will absorb moisture from the air after drying, which will deteriorate its performance, reduce its strength and increase its gas emission sharply. In serious cases, casting defects such as sand sticking, porosity and porosity can be caused. The hygroscopicity of coatings is mainly related to binders, and water-soluble binders have strong hygroscopicity. The hygroscopicity of coated sand mold and core must be tested before casting. The basic method for determining the moisture absorption of coatings is to keep the coatings sample in a constant humidity box with a certain humidity for a certain period of time, and then weigh the samples before and after the retention.

12. coatings thermal conductivity

Foundry coatings thermal insulation and chilling properties are particularly important for the selection of metal mold coatings. Chilling coatings are used in the thick wall of the castings to accelerate the cooling of the castings, and thermal insulation coatings are used in the thin wall to slow down the cooling of the castings.