What is Gravity Casting?
For high volume runs of small parts, permanent mold casting (gravity casting) is an option that's both cost effective and produces the quality our customers require.
Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as zinc alloys, aluminum alloys, magnesium and copper alloys (brass alloys). However, irons and steel alloys can also be cast in this method using graphite molds.
What can be made with Gravity Die Casting?
- Gear Housings
- Pipe Fittings
- Fuel injection housings
- Valve and pump parts
- Glass manufacturing equipment parts
- much more
Advantages of Reusable Metal Casting Molds
- Reusable molds
- High production rates
- Good part-to-part dimensional consistency (near net shape) and excellent surface finishes. Any traces of metal flow lines on the casting surface are cosmetic rather than functional defects.
- Permanent mold castings exhibit good soundness with low porosity and metal folds. There may be some micro shrinkage, but mechanical properties are favorably influenced by the castings characteristically fine grain.
- Permanent mold castings cool faster, which results in a finer grain size than sand cast parts.
- Directional solidification is easily induced.
- Permanent mold castings have good dimensional accuracy.
What is the Gravity Casting Process?
The gravity casting process consists of the following steps:
1. Mold preparation
First, the mold is preheated to around 300-500°F (150-260°C) to allow better metal flow and reduce defects. Then, a release agent (refractory material or ceramic coating) is applied to the mold cavity surfaces to facilitate part removal and increase the mold life.
2. Mold assembly
The mold consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made of iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together.
The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the mold cavity.
The molten metal is allowed to cool and solidify in the mold.
5. Mold opening
After the metal has solidified, the two mold halves are opened and the casting is removed.
Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting, is cut off and this excess material is now removed.
What is a Gravity Die-Casting Mold?
A reusable metal mold is used and the most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used in this process. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminium alloys, magnesium, and copper alloys. Other materials include tin, lead, and zinc alloys and iron and steel alloys are also cast in graphite molds. The gravity process begins by preheating the mold to 150–200 °C (300–400 °F) to ease the flow of metal and reduce thermal damage to the casting, when metal is poured into the mold cavity.
Prior to the hot metal entering the mold cavity, it is coated with a refractory material or a mold wash, which prevents the casting from sticking to the mold and also prolongs the mold life. Any sand or metal cores or components are then installed into the die and the mold is clamped shut. Molten metal is then poured into the mold and soon after solidification, the mold is opened and the casting removed to reduce chances of hot tears. The process is then started all over again, but preheating is not required because the heat from the previous casting is adequate and the refractory coating should last several castings. Because this process is usually carried out on large production run work-pieces, sometimes automated equipment is used to coat the mold, pour the metal, and remove the casting.
Designing Dies for Permanent Mold Casting
When dies are designed, they vary from part to part and alloy to alloy, however when designing a mold, one can allow for some venting usually through the slight crack between the two mold halves, but if this is not enough then very small vent holes are used. They are small enough to let the air escape but not the molten metal. A riser must also be included to compensate for shrinkage. This usually limits the yield to less than 60%.
Sometimes, mechanical ejector pins are included in the mold when coatings are not enough to remove casts from the molds. These pins are placed throughout the mold and usually leave small round impressions on the casting.
What is the Difference Between Gravity Casting and Sand Casting?
There are a wide range of key differences regarding gravity casting and sand casting. The main differences lie in the mold and how it is structured. Sand casting is accomplished by shaping a mold from a sand mixture and pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified. It is essentially an expendable mold casting process since the mold is destroyed after its initial use. Additionally, sand casting allows for smaller batches to be made compared to permanent mold casting, which is designed for larger runs and is costlier.
Conversely, gravity casts can be used repeatedly to make identical parts. This type of reusable mold casting is highly useful for applications that require numerous production runs of the same part. The initial investment costs are a bit higher than sand casts, but the costs generally make up for themselves due to the extended usage factor.
Ensuring a High-Quality Surface Finish
The metal is poured at the lowest practical temperature in order to minimize surface cracks and porosity. The pouring temperature can range greatly depending on the casting material; for instance, zinc alloys are poured at approximately 370 °C (698 °F), while Gray iron is poured at approximately 1,370 °C (2,500 °F).
We take all these things into consideration when designing molds for this process. For more information about permanent mold castings, contact Deeco Metals.