Gravity Casting Vs Pressure Casting: What’s Their Difference

Introduction of gravity casting

Gravity casting refers to the process that molten metal is injected into the mold under the gravity of the earth, also known as casting.

Generalized gravity casting includes sand mold casting, metal mold casting, lump mold casting, disappearing mold casting, clay mold casting, etc;

The narrow sense of gravity casting specifically refers to metal mold casting.

Metal mold casting is a modern process of making mould with heat-resistant alloy steel.

The liquid metal is usually poured into the gate by hand, and the product is obtained by filling the cavity, exhausting, cooling and opening the mold with the self weight of the liquid metal.

The technological process is: smelting, pouring material filling, exhaust, cooling, mold opening, heat treatment and post-processing.

The characteristics of gravity casting are as follows:

1. There are few pores in the casting, so it can be heat treated.

After heat treatment, the strength of the product can be greatly improved, even more than that of die-casting products.

2. The density of the product is low.

Depending on liquid self weight molding, the density of the product is lower than that of die casting, its strength is also slightly worse, but its elongation is higher.

3. The surface finish of the product is not high.

It is easy to form craters similar to shot blasting on the surface of casting products after cooling and shrinkage.

4. Slow filling and low production efficiency.

Its production efficiency is only about 1 / 4 of that of high-pressure casting, which leads to low product cost.

5. The service life of the die is longer than that of the die casting, and the cost of the die is lower.

6. The process is simple.

7. It is not suitable to produce thin wall parts.

Introduction of high pressure casting

Pressure casting is divided into high pressure casting and low pressure casting.

High pressure casting is a casting method in which molten alloy is poured into the pressure chamber to fill the cavity of steel mould with high speed and solidify the molten alloy under pressure.

(1) The metal liquid fills the cavity under pressure and crystallizes and solidifies under higher pressure, commonly 15-100 MPa.

(2) The metal liquid fills the cavity at high speed, usually at 10-50 m/s, some can exceed 80 m/s (the linear speed of the cavity is introduced through the internal gate – the speed of the internal gate), so the filling time of the metal liquid is very short, about 0.01-0.2 seconds (depending on the size of the casting) can fill the cavity.

The characteristics of high pressure casting are:

1. Fast filling and high production efficiency.

2. High pressure molding, high denseness, high strength and surface hardness, but low elongation of the product.

3. Good surface finish, generally up to Ra6.3, even up to Ra1.6.

4. Can produce parts with thin wall thickness, the thinnest wall thickness of aluminum die-casting can reach 0.5mm.

5. Fast filling, internal involvement in the gas, the product porosity is poor.

6. Not heat treatment. When heat treatment, the internal gas will expand, resulting in defects such as bulging or cracking of the product.

7. Small machining allowance (as much as possible less than 0.25mm), too much processing will damage the dense layer on the surface and reveal the internal pores, resulting in lower strength of the product.

High mold cost and low mold life.

Introduction of low pressure casting

The technological process of low pressure casting is as follows: dry compressed air is introduced into the sealed crucible (or sealed tank), the liquid metal rises along the riser into the sprue under the action of gas pressure, and enters the mold cavity smoothly through the sprue, and the gas pressure on the liquid level in the crucible is maintained.

Until the casting is completely solidified.

Then, the gas pressure on the liquid surface is released to make the liquid metal in the riser flow back to the crucible. Then open the mold and take out the casting.

Low pressure casting is completely different from high pressure casting in terms of equipment, process and product performance

The pressure used in low pressure casting is low, generally 20 ~ 60kpa, and the pressure source is gas pressure.

Because of the low pressure, the product filling is slow and the production efficiency is low, but the porosity of the product is better, so the product can be heat treated.

Generally speaking, low pressure casting is a compromise between high pressure casting and gravity casting.

Comparison of advantages and disadvantages

 TecniquesMold costDie lifePorosityHeat treatmentMachining allowanceSurface finishSuitable productsProduction efficiency
High pressure castingcomplexhighshortbadnosmallhighThin walled partshigh
Low pressure castingsimplelowlonggoodyesbighighMiddle wall partsmedium
Gravity castingsimplelowlonggoodyesbiglowThick wall partslow

In terms of strength, due to the different raw materials of aluminum alloy, the strength difference is also large.

Die casting products have high density and high strength because they are formed under high pressure.

For example, A380 die casting products, its strength can reach 360MPa, and its elongation can only reach about 3.7%.

Generally, the strength of gravity casting products is lower than that of die casting products.

The better ZL101A can only reach 310mpa after heat treatment, but its elongation is 5-6%. There are also materials with strength above 380MPa, such as ZL201a and ZL204a.

It can be seen from the above that when we choose the production process, we mainly choose according to the wall thickness of the product

Because of the high production efficiency of high pressure casting, high pressure casting should be the first choice on the premise that the production process and product performance can meet the requirements.

When the product wall thickness is greater than 8 mm, the product strength of high-pressure casting may not meet the expected requirements due to too many internal pores, so gravity casting or low-pressure casting can be considered.

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