Functions and Performance of Coatings in the Lost Foam Casting Process

I. Functions, Basic Composition, and Properties of Lost Foam Coatings

(I) Main Functions of Lost Foam Coatings

Enhancing foam pattern strength and rigidity

The coating layer improves the strength and stiffness of the foam pattern, preventing damage or deformation during handling, coating application, sand filling, and vibration compaction.

Acting as a barrier between molten metal and dry sand

During pouring, the coating serves as an isolation layer between molten metal and dry sand. It prevents molten metal from penetrating into the sand, ensuring smooth casting surfaces and eliminating sand adhesion defects. At the same time, it prevents dry sand from flowing into the gap between the molten metal and foam pattern, avoiding mold collapse.

Facilitating the discharge of foam decomposition products

The coating allows the thermal decomposition products of the foam pattern (large amounts of gas and/or liquid) to escape smoothly into the surrounding sand and be instantaneously extracted, preventing defects such as gas porosity, folds, carburization, and residue.

Due to different pouring temperatures for different alloys, foam decomposition products vary significantly.

For cast iron and cast steel (ferrous metals), pouring temperatures are relatively high (above 1350–1600 °C), and decomposition products are mainly gaseous, requiring coatings with excellent permeability.

For aluminum alloys, pouring temperatures are lower (approximately 760–780 °C), and decomposition products are mainly liquid. In this case, the liquid products must be able to wet the coating, penetrate into it smoothly, be absorbed by the coating, and be discharged from the mold cavity.

Providing thermal insulation

The coating reduces heat loss of molten metal during mold filling, improving mold filling completeness, especially for thin-walled castings.

(II) Basic Composition of Lost Foam Coatings

Lost foam coatings generally consist of refractory materials, binders, carriers (water or ethanol), surfactants, suspending agents, thixotropic agents, and other additives. These components are uniformly mixed and function together during coating application and pouring.

Refractory materials (aggregates)

These are the backbone of the coating and determine refractoriness, chemical stability, adsorption capacity, and thermal insulation. Particle size distribution and particle shape have a significant influence on permeability. Particles should not be too fine; columnar or spherical particles are preferred, followed by flaky shapes.

Binders

Essential additives to ensure both sufficient coating strength and good permeability.

Organic binders (syrup, starch, dextrin, carboxymethyl cellulose—CMC) enhance coating strength at room temperature and burn out during pouring, improving permeability.

Inorganic binders (sodium bentonite, sodium silicate, silica sol) ensure both room-temperature and high-temperature strength.

Proper combination of multiple binders is usually required to optimize coating performance.

Carriers

Water-based or alcohol-based (ethanol) systems.

Surfactants (wetting agents)

Mainly used to improve the coating ability of water-based coatings. These amphiphilic molecules have both hydrophilic and lipophilic ends: the hydrophilic end bonds with water, while the lipophilic end is attracted to the foam pattern, forming a “bridge” between the coating and the foam surface.

Suspending agents

Added to prevent sedimentation of refractory particles and to regulate rheology and process performance. Selection depends on refractory type and carrier.

Common for water-based coatings: bentonite, attapulgite clay, sodium carboxymethyl cellulose

Common for organic solvent coatings: organic bentonite, lithium bentonite, attapulgite clay, polyvinyl butyral (PVB)

Thixotropic agents

Typically attapulgite clay. Thixotropy refers to the property where viscosity decreases under constant shear and gradually recovers when shear stops.

Other additives

Defoamers: eliminate bubbles (e.g., n-butanol, n-amyl alcohol, n-octyl alcohol), typical addition 0.02%

Preservatives: prevent fermentation and deterioration of water-based coatings (e.g., sodium benzoate), typical addition 0.02%–0.04%

Surfactants, defoamers, and preservatives should be added proportionally during coating preparation.

(III) Performance Requirements of Lost Foam Coatings

Lost foam coatings should exhibit: strength, permeability, refractoriness, thermal insulation, resistance to thermal shock, hygroscopicity, adsorption capacity, easy cleanability, coatability, flow leveling, and suspension stability.

These properties can be categorized into:

Functional (working) properties

Including strength, permeability, refractoriness, insulation, thermal shock resistance, hygroscopicity, adsorption, and cleanability.

The most critical properties are strength, permeability, and refractoriness.

Process properties

Including coatability, flow leveling (low dripping tendency), and suspension stability.

The most important are coatability and flow leveling, as foam patterns are inherently non-wettable.

An ideal coating should be “thick but not sticky, smooth but not dripping.”

Methods to improve coating performance

(Section heading retained for extension or practical guidance.)

II. Selection of Lost Foam Coatings

(I) Chemical Properties (Acidity/Alkalinity)

Acidic

Cast iron and cast steel (carbon steel, low-alloy steel): kyanite, flake graphite, silica sand (acidic or neutral refractories)

Neutral

High-alloy steel: zircon kyanite, corundum, zircon sand, flake graphite (weakly acidic or neutral refractories)

Basic

High-manganese steel: magnesia sand, magnesia-olivine (basic refractories)

Aluminum alloys

Corresponding suitable refractory materials should be selected

(II) Physical Properties (Pouring Temperature)

Selection should consider gating system design, process parameters, mold assembly method, operator habits and proficiency, and site conditions.

III. Preparation and Storage of Coatings

(I) Coating Preparation Process

Coating preparation equipment includes colloid mills, ball mills, low-speed mixers, and high-speed mixers.

Colloid mills and ball mills provide excellent wetting and low air entrainment but have disadvantages such as long preparation time and high noise.

High-speed mixers are currently the mainstream solution.

If unavailable, extended low-speed mixing can also achieve acceptable results.

High-speed mixing

Purpose: thoroughly mix powders and water into a uniform slurry and disperse binder fibers.

Mixing time: ≥ 2 hours

Low-speed mixing

Purpose: remove entrained air introduced during high-speed mixing and improve coating strength and casting surface quality.

Mixing time: 2 hours or continuous slow stirring

(II) Quality Control of Coatings

Density

Indicates viscosity and coating thickness. Measured using a hydrometer (Baumé meter).

pH value

Controls chemical stability and compatibility with molten metal. Measured with pH paper or pH meter.

Coating weight

Determined by weighing the pattern before and after coating to estimate thickness.

(III) Storage of Coatings

Coatings should ideally be prepared fresh and used promptly. Remaining coatings should be stored in a cool place and not kept for extended periods.

Summer: 2–5 days

Winter: 5–10 days

Avoid fermentation and freezing.

IV. Coating Application and Precautions

(I) Application Methods and Scope

Brushing: medium and large patterns, small-batch production

Dipping / pouring: small, complex patterns, large batches

Spraying: thin-walled or easily deformed patterns

(II) Selection of Coating Thickness

Pattern body: 0.3–3.5 mm, prioritizing permeability

Gating system: 3.5–6.0 mm, prioritizing strength and erosion resistance

(III) Precautions

Utilize thixotropic properties fully during continuous stirring

Stirring speed: 10–20 rpm

Control immersion position, angle, speed, and force

Ensure uniform, complete coverage

Prevent deformation and damage throughout the process

(IV) Typical Improper Practices

Shaking

Causes uneven thickness and poor leveling

Exposure (“bare spots”)

Incomplete coating coverage without repair, reducing strength and quality

V. Drying of Coatings

(I) Drying Methods

Natural drying: outdoor air drying, solar rooms

Heated dehumidification: drying rooms using coal, gas, electricity, geothermal energy, or steam

(II) Drying Quality Control

Drying temperature: 35–50 °C

Heating rate: 5–10 °C/hour

Drying time:

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