Prevention of Collapse Defects in Investment Casting (Lost Foam Casting)

1. Prevention Methods

(1).Increase Sand at the Top of the Mold

Add sufficient dry sand to the top of the mold before pouring, and place a weight if necessary to support the sand mold and prevent localized collapse.

(2).Use Low-Density Foam Patterns

Use lightweight foam materials to reduce gas generation during molten metal pouring, minimizing the buoyant force that can destabilize the mold.

(3).Optimize Vacuum Parameters and Pouring Speed

For vacuum-assisted pouring, select reasonable vacuum levels and pumping parameters, and match them with the pouring speed to ensure uniform and stable mold compaction.

(4).Use High-Strength, High-Refractory Coatings

Select coatings with high strength, high refractoriness, high-temperature hardness, and good permeability to enhance the mold's support, especially on thin walls.

(5)Select Appropriate Sand Grain Size

Optimize sand particle size and distribution to improve mold permeability, facilitating gas escape and reducing the risk of local collapse.

(6)Optimize Gating System Design

Ensure molten metal flows smoothly into the mold cavity, avoiding turbulence or stagnation that can cause localized sand mold failure.

(7)Control Pouring Process Parameters

Lower the pouring temperature (while maintaining adequate alloy fluidity), control pouring speed, and avoid interruptions to reduce pressure fluctuations that can destabilize the mold.

2. Causes of Collapse Defects

(1).Excessive Buoyancy of Molten Metal

The upper part of the sand mold cannot withstand the impact of molten metal, causing localized collapse. Insufficient top sand or vacuum, as well as improper pouring speed, can lead to uneven forces on the mold.

(2).Insufficient Gas Escape

Gas generated by the thermal decomposition of foam patterns during pouring is too rapid or too large, and mold venting is inadequate, leading to mold collapse.

(3).Insufficient Coating Strength

Coatings with low refractoriness or high-temperature strength cannot support the mold during metal filling, resulting in sand collapse.

(4).Improper Gating System Design

Turbulent metal flow may prevent complete replacement of the mold cavity space, leading to localized sand mold failure.

Methods to Prevent Metal Spitting (Back-Splash) in Lost Foam Casting

1.Use Low-Density Foam Patterns

Keep foam lightweight while maintaining surface quality and pattern strength to minimize gas generation during pouring and reduce spitting.

2.Ensure Foam Patterns are Fully Dried

Foam patterns must be completely dried before coating. Weigh samples from each batch and check drying curves. Only patterns with stable weight should be coated to prevent localized vaporization that causes spitting.

3.Optimize Pouring System and Local Protection

Do not coat straight runners or gates with refractory coating to allow gases to escape smoothly.

Install baffles on pouring cups to block splashing molten metal, protecting operators and reducing coating consumption.