fabriquer un moule pour couler de l'aluminium en fusion

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Casting Molten Aluminum: A Comprehensive Guide to Mold Making

In foundry work, precision and durability are paramount. Casting molten aluminum requires a sturdy and expertly crafted mold to produce high-quality castings without imperfections or defects. This article provides a comprehensive guide on how to make a mold for casting molten aluminum, covering every step from materials selection to final casting.

Materials Required

To construct a durable and effective mold, you will need the following materials:

Sand: Choose fine-grained, silica-based sand with a grain size between 60 and 100 microns.
Binder: Use sodium silicate or furan resin as the binder to hold the sand particles together.
Pattern: The master shape or model that will be replicated in the casting.
Release agent: Apply a thin layer of a release agent, such as wax or silicone, to prevent the casting from sticking to the mold.
Gating system: This includes the sprue, runners, and gates, which allow the molten aluminum to flow into the mold cavity.
Vent holes: Small holes in the mold allow air and gases to escape during pouring.

Step-by-Step Mold Making Process

1. Create the Pattern

The pattern should be slightly larger than the desired casting to allow for shrinkage during cooling. It should be made of a material that can withstand the heat of molten aluminum, such as wood, metal, or plastic.

2. Prepare the Sand

Mix the sand and binder according to the manufacturer's instructions. The mixture should be moist enough to hold its shape when packed, but not so wet that it slumps.

3. Create the Mold Cavity

Place the pattern on a bed of sand in the molding box. Pack the sand around the pattern, creating a mold cavity that is slightly larger than the pattern. Create vent holes to allow for air and gas escape.

4. Remove the Pattern

Carefully lift the pattern out of the mold cavity. Touch up any imperfections in the mold surface.

5. Apply Release Agent

Apply a thin layer of release agent to the mold surface to prevent the casting from sticking.

6. Create the Gating System

Cut channels into the mold surface to create the sprue, runners, and gates. These channels will allow the molten aluminum to flow into the mold cavity.

7. Dry the Mold

Allow the mold to dry completely before pouring. This can be done by heating it in an oven or by exposing it to warm air.

8. Cast the Aluminum

Melt the aluminum in a crucible and superheat it to a temperature slightly above its melting point. Pour the molten aluminum into the sprue, filling the mold cavity.

9. Allow the Aluminum to Cool

Let the casting cool slowly to prevent cracking or warping. The cooling time will depend on the size and complexity of the casting.

Tables

Property	Typical Value
Melting point (pure aluminum)	660.32 °C (1220.58 °F)
Density (pure aluminum)	2.70 g/cm³ (0.0975 lb/in³)
Tensile strength (pure aluminum)	70-110 MPa (10,000-16,000 psi)
Thermal conductivity (pure aluminum)	237 W/m·K (138 Btu/ft·h·°F)

Binder	Advantages	Disadvantages
Sodium silicate	Strong, heat-resistant	Hygroscopic, requires thorough drying
Furan resin	High strength, excellent surface finish	Expensive, requires special curing process

Mold Type	Advantages	Disadvantages
Sand casting	Inexpensive, versatile, suitable for large castings	Poor surface finish, low dimensional accuracy
Die casting	High dimensional accuracy, good surface finish	Expensive, limited size and shape

Stories and Lessons Learned

Story 1:

A foundry worker accidentally poured molten aluminum into a mold that was not completely dry. The resulting casting contained cracks and imperfections due to the steam generated by the moisture in the mold.

Lesson: Ensure that the mold is completely dry before pouring molten aluminum to prevent defects.

Story 2:

A manufacturer attempted to cast a large and complex part using a sand mold. The part was warped due to uneven cooling.

Lesson: Consider using a more sophisticated mold type, such as die casting, for large or complex castings.

Story 3:

A company used a release agent that was not compatible with the aluminum alloy being cast. The casting stuck to the mold, resulting in a damaged surface.

Lesson: Select a release agent that is specifically designed for the type of aluminum alloy being cast.

Pros and Cons of Aluminum Casting

Pros:

Excellent strength-to-weight ratio
High thermal conductivity
Corrosion resistance
Relatively low cost
Recyclable

Cons:

May require special heat treatment for strength
Can be difficult to cast complex shapes
Tendency to oxidize at high temperatures
Prone to porosity if not properly degassed

FAQs

Q: What is the best binder for aluminum casting molds?
A: Sodium silicate or furan resin are commonly used binders that provide strength and heat resistance.

Q: How do I prevent the casting from sticking to the mold?
A: Apply a thin layer of release agent to the mold surface.

Q: What is the best way to cool an aluminum casting?
A: Allow the casting to cool slowly to prevent cracking or warping.

Q: Can I cast other metals in an aluminum casting mold?
A: Yes, but you may need to adjust the mold design and casting conditions to accommodate the different metal properties.

Q: How can I improve the surface finish of an aluminum casting?
A: Use a mold with a smooth surface and consider polishing the casting after cooling.

Q: What are the common defects in aluminum castings?
A: Cracks, porosity, inclusions, and warpage are some common defects that can occur during aluminum casting.