Gravity casting vs pressure casting: which is better?

Introduction of gravity casting

Gravity casting is the process of injecting molten metal into a mold under the force of gravity, also known simply as casting.

The broader definition of gravity casting encompasses several methods, including sand mold casting, metal mold casting, grain mold casting, disappearing mold casting, and clay mold casting.

However, in a narrower sense, gravity casting specifically refers to metal mold casting.

Metal die casting is a modern technique that uses heat-resistant alloy steel to create molds.

The liquid metal is normally manually poured into the gate and the final product is produced by filling the mold cavity, ventilating, cooling and opening the mold through the force of the weight of the liquid metal.

The process involves the following steps: casting, pouring the material into the mold, ventilation, cooling, opening the mold, heat treatment and post-processing.

Following are the characteristics of gravity casting:

• The casting has few pores, allowing heat treatment. This results in a significant improvement in strength, surpassing that of die-cast products.
• The product has low density. Due to its self-weight molding, the density is lower than that of die casting, resulting in slightly lower strength but greater elongation.
• The surface finish of the product is not ideal. Cooling and shrinkage can cause the formation of craters similar to those produced by sandblasting.
• The filling process is slow and production efficiency is low. With an efficiency of only 1/4 of high pressure casting, the product cost is lower.
• The die has a longer lifespan and lower cost compared to die casting.
• The process is simple.
• Gravity casting is not suitable for producing thin-walled parts.

Introduction of high pressure casting

Die casting is divided into two categories: high pressure casting and low pressure casting.

High pressure casting is a casting technique in which molten alloy is quickly poured into the cavity of a steel mold within a pressure chamber, solidifying under high pressure.

(1) Metallic liquid is filled into the cavity under pressure and solidifies through crystallization, typically with a pressure range of 15-100 MPa.

(2) Metallic liquid is filled into the cavity at high speed, with velocities ranging from 10-50 m/s and in some cases, even exceeding 80 m/s, which is introduced through the internal gate. This results in a very short filling time of the liquid metal, typically just 0.01-0.2 seconds depending on the size of the casting.

The characteristics of high pressure casting include:

• Fast filling and efficient production.
• High pressure molding results in a product with high density, strength and surface hardness, but low elongation.
• An excellent surface finish, typically up to Ra6.3 and in some cases up to Ra1.6.
• Ability to produce parts with thin wall thicknesses, with aluminum casting capable of reaching a wall thickness of 0.5 mm.
• Rapid filling eliminates the involvement of internal gases, resulting in a product with low porosity.
• High pressure casting does not require heat treatment, as the internal gas will expand during heat treatment, causing defects such as bulges or cracks.
• Small machining tolerances (ideally less than 0.25 mm) are required, as excessive processing can damage the dense surface layer and reveal internal pores, reducing the strength of the product.

However, high pressure casting has high mold costs and low mold life.

Introduction of low pressure casting

The process of low pressure casting technology is as follows: Dry compressed air is introduced into the crucible or sealed tank. Under the influence of gas pressure, the liquid metal rises along the riser and enters the sprue, smoothly entering the mold cavity. Gas pressure is maintained at the level of the liquid in the crucible until the casting is fully solidified.

After solidification, the gas pressure on the surface of the liquid is released, causing the liquid metal in the riser to flow back into the crucible.

Finally, the mold is opened and the casting is removed.

Low pressure casting differs significantly from high pressure casting in terms of equipment, process and product performance.

The pressure applied in low pressure casting is relatively low, typically ranging from 20 to 60 kPa, and the pressure source is supplied by gas.

Due to the low pressure, the filling of the product is slower and the production efficiency is lower, however, the porosity of the product is improved, allowing it to undergo heat treatment.

In general, low pressure casting is a balance between the advantages of high pressure casting and gravity casting.

Comparison of advantages and disadvantages

The strength of aluminum alloys can vary greatly due to differences in raw materials. Die casting products, on the other hand, have high density and strength due to the fact that they are formed under high pressure. For example, A380 die-cast products have a strength of 360MPa and an elongation of only 3.7%.

Gravity cast products generally have lower strength compared to pressure cast products. For example, ZL101A has a strength of 310MPa after heat treatment, but with an elongation of 5-6%. There are also materials, such as ZL201a and ZL204a, with strength above 380MPa.

When choosing a production process, the wall thickness of the product is the main consideration. High pressure casting is a highly efficient process and should be the first choice as long as the production process and product performance meet the requirements. However, if the wall thickness of the product is greater than 8mm, high pressure casting may not meet the desired strength requirements due to internal pores, in which case gravity casting or low pressure casting may be considered. .