What Causes the Dull Surface on Gold Bars in Vacuum bar Casting Machine

What Causes the Dull Surface on

Gold Bars in Vacuum bar Casting Machine

Vacuum melting is typically employed to minimize oxidation, so a dull surface on gold bars in vacuum bar casting machine may not be caused by oxidation. What are the possible reasons?

(Silver bars with smooth surface)

1. Mold Problems 

If there are impurities on the mold surface or it is not smooth enough, the surface of the gold bar will naturally be rough. Or the mold temperature is not appropriate, resulting in an uneven surface when the metal solidifies. In addition, improper use of mold release agents may also affect the surface gloss. For example, too much or uneven mold release agents may leave residues, making the surface look dull. The specific manifestations are as follows:

— Rough mold surface: If there are scratches, oxide layers or residues on the inner wall of the mold, the roughness of the mold will be replicated on the surface of the gold bar.

— Improper mold temperature: Insufficient mold preheating may cause the molten metal to solidify too quickly, the fluidity will deteriorate, and the mold details cannot be filled; too high a temperature may cause the metal to react with the mold.

— Release agent residue: Excessive or unevenly applied release agent may form spots or mist residue on the surface of the gold bar.

Therefore, the selection of graphite molds is crucial for casting gold bars. If the quality of the graphite mold is not good, a layer of graphite ash will easily appear on the gold bar, and the service life of the graphite mold will be greatly reduced.

2. GoldPurity and Impurities

Additional Factors: Material Properties & Contamination

The inherent properties of the material also play a critical role. Higher-purity gold tends to form smoother surfaces during casting due to its uniform solidification behavior. In contrast, K-gold or other alloys may develop uneven surfaces because of differing solidification shrinkage rates among constituent metals.

-Impurities in Raw Material

If the gold contains other metals (e.g., copper, silver) or non-metallic inclusions, microstructural heterogeneity may form during solidification, reducing surface reflectivity.

Example: Copper impurities can create oxide microphases, scattering light and causing a matte appearance.

-Melting Process Contamination

Although vacuum melting suppresses oxidation, residual contaminants (e.g., furnace lining debris, tool oxidation flakes) may introduce defects.

Critical Note: Even trace carbon from graphite crucibles can react with gold at high temperatures, forming carbides that degrade surface quality.

3. Improper Process Parameters

– Inaccurate Casting Temperature

Too low: Poor metal fluidity → incomplete mold filling (e.g., surface voids).

Too high: Metal vaporization or reaction with crucible/mold (e.g., coil burnout in melting furnaces if overheated).

– Suboptimal Cooling Rate

Too fast: Thermal stress cracks or rough surfaces (e.g., “orange peel” texture).

Too slow: Coarse grain growth → reduced reflectivity.

Critical Note: Water chiller temperature must be precisely calibrated—excessive deviation disrupts solidification control.

-Pouring Speed Issues

  Too fast: Gas entrapment → porosity (e.g., pinholes).

  Too slow: Premature solidification → uneven surfaces (e.g., flow marks).

– Water pressure problem: The vacuum bar casting machine needs to work with the water chiller. The cooling water needs to flow from the water chiller to the machine. If the distance is far, the time for the water flow to arrive may become longer, resulting in untimely temperature control. For example, when the melting machine needs to cool down quickly, the water cooler may not respond quickly, causing temperature fluctuations and affecting the quality of the gold bars.

4. Insufficient vacuum

– Influence of residual gas: If there is a leak in the vacuum system or the vacuuming efficiency is low, residual oxygen or other gases will form an oxide layer or pores on the metal surface, resulting in matte finish.

5. Lack of subsequent processing

– Not polished or cleaned: After casting, gold bars usually need electrolytic polishing, mechanical polishing or pickling to remove the surface oxide film and microscopic unevenness. If this step is skipped, the surface will appear dull.

6. Shrinkage and defects

– Uneven solidification shrinkage: If the volume shrinkage of the metal during cooling is not effectively compensated by the mold design (such as shrinkage feeder), shrinkage holes or depressions may be formed, affecting the appearance. In addition, unreasonable water cooling temperature setting will also cause severe shrinkage of the edge of the gold bar.

7. The influence of purification solution

• Although the gold purity meets the standard (99.99%), the residual purification solution may affect the fluidity and solidification behavior of the molten metal through chemical pollution or microscopic inclusions, and finally lead to a dull surface.
• Even if the gold purity reaches 99.99%, if the chemical solution (such as acid, reducing agent, complexing agent, etc.) used in the purification process is not thoroughly cleaned, the residual trace substances may cause the following problems in the subsequent smelting process:
• uHigh-temperature volatile pollution: Residual acidic substances (such as nitric acid, hydrochloric acid) or organic matter (such as oxalic acid, thiourea) may be decomposed by heat during vacuum smelting, producing gas (such as CO₂, SO₂) or carbide, polluting the metal surface, forming an oxide layer or tiny pores.
• uMetal surface passivation: Certain chemical solutions (such as strong oxidants or passivators) may form an extremely thin compound film (such as oxides, sulfides) on the gold surface, resulting in a decrease in the fluidity of the molten metal and affecting the smoothness of mold filling.
• uImpurities: If the potion contains metal ions (such as iron and copper), they may remain in the gold through substitution reaction or adsorption, forming microscopic inclusions, which will destroy the continuity of grain boundaries during solidification and cause surface roughness.

In summary, the surface quality of gold bars is mainly affected by three core process parameters: raw material quality, cooling temperature control accuracy and smelting aging control. In order to achieve the best surface finish, it is recommended to implement the following systematic improvements: raw material quality control, select high-purity raw materials, use the selected high-purity raw materials to adjust the machine’s process parameters to the most appropriate, and then use the process parameters to cast gold materials from different origins. 

🛠 Ready to Supercharge Your Output? 🛠

👉 CLICK the YouTube link for more details:https://youtube.com/@cdocast?si=O04gWHlOBBJORes7

👇Click the link below now to get exclusive quotes and technical solutions!

WA:‪+86 158 6960 2281‬

Email: [email protected]

Website: www.cdocast.com