Come risolvere i difetti di colata?

27 settembre 2024

Investment casting, also known as lost-wax casting, is a widely used process for creating precise metal castings. However, several defects may occur during the investment casting process which may seriously effect the quality of the products. Solving defects in investment casting process requires understanding the root causes and applying corrective measures at various stages of the process. Here we list out the common investment casting defects and the corresponding solutions for your reference.

1. Shrinkage Defects

Causes: Poor solidification control, improper gating system design, insufficient feeding of molten metal.

Solutions:

• Optimize the Gating System: Design the gating and riser system to ensure even metal flow and minimize shrinkage. Use chills or insulated risers to control the cooling rate.
• Increase Feeding: Use larger or additional feeders to ensure that the metal can compensate for shrinkage during solidification.
• Control Cooling Rate: Ensure a uniform cooling rate across the part, especially in thicker sections.

2. Porosity 

Causes: Gas entrapment during pouring, improper degassing, or poor mold permeability.

Solutions:

• Degas Molten Metal: Degas the molten metal properly before pouring to remove dissolved gases.
• Improve Mold Venting: Design the mold to allow trapped air and gases to escape during pouring.
• Use Vacuum Casting: Employ vacuum or low-pressure casting to minimize gas entrapment.
• Control Pouring Temperature: Ensure the molten metal is at the right temperature to reduce gas absorption.

3. Cold Shuts

Causes: Two streams of molten metal failing to fuse properly, often due to low temperatures or poor fluidity.

Solutions:

• Preheat the Mold: Preheat the mold to an appropriate temperature to ensure proper metal flow.
• Increase Pouring Temperature: Ensure the molten metal is poured at a temperature that maintains fluidity.
• Redesign Gating System: Use a gating system that directs metal flow more efficiently and reduces turbulence.

4. Misruns

Causes: Insufficient filling of the mold due to low metal temperature, poor flow characteristics, or cooling before the mold is fully filled.

Solutions:

• Increase Pouring Temperature: Pour molten metal at a higher temperature to maintain fluidity.
• Redesign the Mold and Gating System: Ensure the gating system allows for quick and complete filling of the mold.
• Use Better Wax Patterns: Ensure that the wax pattern is designed to avoid thin or sharp sections that are hard to fill.
• Preheat the shell mold: Preheat the shell mold to an appropriate temperature to prevent premature cooling; after the burning of the shell mold, pouring when the shell mold is still hot.

5. Inclusions

Causes: Foreign materials, such as slag, sand, or oxides, getting trapped in the metal during casting.

Solutions:

• Filter Molten Metal: Use filters to trap impurities before the metal enters the mold.
• Clean the Mold: Ensure the mold is free of debris or sand that can be entrained in the metal.
• Use Cleaner Melting Practices: Minimize slag formation during melting by using fluxes and keeping melting equipment clean.
• Employ Slag Traps in the Gating System: Design the gating system to trap any slag or inclusions before they enter the mold cavity.

6. Cracking or Hot Tears

Causes: High internal stresses as the metal cools and contracts, often due to uneven solidification or mold expansion.

Solutions:

• Use Controlled Cooling: Implement controlled cooling strategies to ensure even solidification, especially in thicker sections.
• Improve Mold Design: Use a mold design that reduces stress concentrations, especially around sharp corners and thick sections.
• Apply Insulation: Use insulated risers or chills to ensure even cooling and reduce internal stresses.
• Stress Relief Post-Treatment: Perform stress-relief heat treatment after casting to reduce the risk of cracking.

7. Surface Roughness or Finish Defects

Causes: Poor-quality molds or wax patterns, inadequate coating, or mold breakdown.

Solutions:

• Use High-Quality Patterns: Ensure that the wax patterns are smooth and free from defects before investment.
• Improve Slurry Coating Process: Use a high-quality ceramic slurry to create a smooth mold surface.
• Maintain Mold Integrity: Ensure proper curing and drying of the ceramic shell to avoid mold cracking or breakdown.
• Polish or Treat the Surface Post-Casting: Apply surface treatments like grinding or polishing after casting to improve the finish.

8. Mold Expansion (Shell Cracking)

Causes: Expansion of the mold due to high temperatures, which can lead to cracks or distortion.

Solutions:

• Use High-Strength Shell Materials: Use high-quality refractory materials for the shell to withstand expansion stresses.
• Applicare un rivestimento uniforme: Assicurare uno spessore uniforme del rivestimento ceramico per evitare tensioni localizzate e fessurazioni.
• Migliorare l'asciugatura e la polimerizzazione: Assicurarsi che il guscio sia completamente essiccato e indurito prima di versare il metallo fuso per ridurre al minimo il rischio di crepe da espansione.
• Utilizzare modelli espandibili: Utilizzare modelli di cera che possono espandersi leggermente durante il processo di riscaldamento per alleviare le sollecitazioni sul guscio.
• In sintesi, possiamo adottare le seguenti misure preventive generali per evitare i difetti di fusione durante il processo di produzione:
• Controllo dei parametri di processo: Monitorare e controllare regolarmente le temperature, le portate e altri parametri chiave del processo per mantenere la coerenza.
• Utilizzare un software di simulazione: Il software di simulazione della colata può essere utilizzato per prevedere e correggere potenziali difetti prima della produzione effettiva.
• Controllo qualità e ispezione: Eseguire ispezioni approfondite in ogni fase, dalla creazione del modello in cera al riempimento dello stampo, per identificare tempestivamente i difetti.
• Ottimizzare la selezione dei materiali: Utilizzare le leghe e i materiali di stampo giusti per ottenere le proprietà e le prestazioni di colata desiderate.
• Combinando queste soluzioni con test sistematici e perfezionamento del processo, è possibile ridurre al minimo i difetti della microfusione, ottenendo getti di qualità superiore.