How to Cast Perfect Mirror Surface Gold Silver Bars

1. Why Don’t the Results Meet Expectations Even When Following the Manual?

Causes:

● Environmental variations: Parameters provided in the manual are typically based on laboratory or ideal conditions. Real-world production environments—such as gold purity, stability of factory power supply, equipment differences, and ambient temperature—can significantly impact casting outcomes.

● Gold purity and composition: Materials with different purities (e.g., 99.99% Au vs. 99.9% Au) have different melting points and flow characteristics, making standard parameters potentially ineffective.

Solutions:

● Keep detailed records of each debug session, including power settings, heating time, vacuum duration, cooling time, and delayed heating time. Adjust these parameters incrementally for optimal results.

● Consult the equipment manufacturer for model-specific recommendations.

2. Why Should New Graphite Molds Be Preheated?

Causes:

● Removing moisture and impurities: Graphite molds can absorb moisture or volatile substances during production, transportation, or storage. Preheating (typically to 200–300°C) evaporates these residues, preventing mold cracking or molten metal splatter.

● Ensuring consistent molding quality: A cold mold may cause premature solidification, leading to incomplete filling, flow marks, or porosity. Preheating ensures uniform mold temperature, improves metal fluidity, and enhances surface finish and dimensional accuracy.

Solutions:

● Preheat the mold for approximately 1minute before beginning the casting process.

3. Why Are There Graphite Residues on the Gold Bar Surface?

Causes:

● Graphite mold properties: Graphite is self-lubricating and may shed slightly at high temperatures, especially when the mold is new.

● New mold “break-in”: The first 1–5casts may produce more graphite dust until a dense oxide layer forms on the mold surface.

● Mold wear or damage: Micro-cracks from extended use can accelerate graphite particle shedding.

● Poor temperature control: Temperatures exceeding 1100°C can intensify graphite oxidation and shedding.

Solutions:

● Use the initial few casts for trial runs to “clean” the mold.

● Replace the mold once it reaches the end of its service life.

● Control the melting temperature between 1064°C and 1080°C to minimize graphite wear.

4. What Causes Bubbles on the Gold Bar Surface?

Causes:

● Prolonged heating causing localized overheating and gas release.

● Insufficient vacuum time or reduced vacuum pump efficiency, failing to remove all air from the mold.

● Unpreheated mold: Moisture or oil on a cold mold vaporizes upon contact with molten metal, forming bubbles.

Solutions:

● Reduce heating time appropriately.

● Extend vacuum time to ensure the mold reaches the required pressure before casting.

● Preheat the graphite mold for about 1 minute before adding material.

5. What Causes Dents on the Gold Bar Surface?

Causes:

● Slow cooling prolongs solidification, and excessive vacuum pressure may draw incompletely solidified metal, creating voids at the bottom.

● Inadequate vacuum time traps gas within the molten metal, resulting in porosity during solidification.

Solutions:

● Lower the chiller temperature to accelerate cooling.

● If cooling water temperature is adequate, extend the vacuum time.

6. What Causes Bulges on the Gold Bar Surface?

Causes:

● If bulges appear on the inner side of the bar, heating time may be insufficient, leaving unmelted gold.

● If bulges are on the outer side, excessive vacuum pressure may be pulling unsolidified metal upward.

● Slow cooling combined with high vacuum can extend solidification and cause top-side bulging due to suction.

Solutions:

● For inner bulges: Increase heating time.

● For outer bulges: Reduce vacuum pressure to below –20 kPa.

● Lower chiller temperature to speed up cooling.

7. Why Are the Two Ends of the Gold Bar Different Thicknesses?

Causes:

● Uneven machine base:Improperly adjusted feet or ground settlement (>0.5° tilt) can cause uneven flow of molten metal.

● Inconsistent gold material:Mixing gold from different batches may lead to variations in fluidity.

Solutions:

● Use a precision level (accuracy 0.02 mm/m) to check the machine base and adjust the feet until the deviation at all four corners is ≤0.1 mm.

● Perform daily checks with a level before operation.

● Pre-melt and homogenize gold from different batches before casting.

8. Why Does the Gold Bar Surface Have Ripples?

Causes:

● Uneven mold temperature:Localized cold zones (e.g., mold edges) cause premature solidification and shrinkage patterns.

● Mixed gold materials:Using gold from different batches or purities can result in viscosity variations and flow layering.

● High impurity content: Oxidized impurities such as copper or silver can suspend in the melt and disrupt flow.

Solutions:

● Optimize the layout inside the mold to ensure even heating of graphite slots.

● Standardize raw materials: Use the same batch of gold for each cast and avoid mixing. Pre-purify recycled material (e.g., via electrolytic refining).

● Add a small amount of flux (e.g., ≤0.1% borax) for gold with impurities to improve fluidity.

9. What is the cause of this phenomenon on the surface of the gold bar?

Causes:

Gold bars may contain impurities such as lanthanum or titanium. These metals have a higher melting point and a lower density than gold. Therefore, during melting, they float to the surface, resulting in a non-lustrous appearance of the gold.

Solutions:

Identify impurities (e.g., La, Ti) via spectrometry.

For La: Treat with dilute H₂SO₄ or HCl before re-casting.

For Ti: Treat with HF solution before re-casting.

Alternative approach: Use new 9999 purity raw material for casting.