Metal casting can be achieved in different ways, and gravity casting is one of the simplest forms of casting operation.
Due to its simple operation and cheap equipment, it is the most favorable casting technique for DIY projects.
Gravity casting is the process of manufacturing intricate metal parts by pouring molten metal into a mold that comprises a cavity with geometry similar to the desired part. The molten metal flows under the action of gravity, taking the shape of the desired part, and is then removed from the mold.
This article discusses gravity casting in detail by going through its process, applications, and disadvantages.
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What is Gravity Casting? How it Works
Gravity casting is a type of casting in which no external force is applied to the casting. Instead, the molten metal is allowed to flow into the mold, under the action of gravity.
To carry through a gravity casting process, we first need to convert the solid metal ingots into their molten state by placing them in a crucible and heating them in a furnace.
Crucibles are usually made of materials like silicon carbide or clay graphite that are good at withstanding high temperatures developed in the furnace while melting the ingots.
For DIY applications, you can make your own gas-fired furnace and use refractory cement to line its interior walls, providing the ability to withstand the high temperatures involved in casting.
After completing the melting process, the molten metal is poured into the mold or die. A pair of tongs is used to handle the hot crucible.
The molds used in gravity casting can be characterized into two types based on their design: open-type molds and closed-type molds.
An open-type mold consists of an open cavity and the metal is directly poured until filled completely.
While a closed-type mold is completely enclosed and consists of a gating system for pouring the molten metal into the mold.
The cavity of the mold is generally larger than the required dimension of the part to compensate for the shrinkage that takes place during the solidification of the metal, also known as shrinkage allowance.
After the solidification of the metal, the mold is either opened or destroyed to get the final casting out of it.
Based on the type of mold used, gravity casting can be further classified into expendable mold gravity casting and permanent mold gravity casting.
Expendable Mold Gravity Casting
In expendable mold gravity casting, the mold is broken to remove the solidified cast. Sand casting is an example of expendable mold gravity casting.
This is generally done because of the complex geometry of the part, which makes it impossible to retrieve without breaking the mold.
As a result, expendable mold gravity casting is best suited for applications where unique parts are to be made, allowing for one-time use of the mold.
Permanent Mold Gravity Casting
Permanent mold gravity casting consists of molds made of durable material that can be reused again and again.
These molds are usually made of metals, such as steel, capable of sustaining high temperatures developed during the casting process.
Generally, permanent molds are constructed using two or more sections that can be taken apart to retrieve the final casting, without damaging the mold.
However, the metal molds are only suitable for casting parts with simple shapes and metals having a comparatively lower melting point than the molds.
The ability to reuse molds provides the ability to produce identical parts every single time, making it suitable for high-volume production applications.
Apart from that, the smooth surface of metal molds produces parts with a better surface finish than sand molding.
Gravity die casting is one of the most commonly used permanent mold gravity casting techniques that is suitable for DIY projects and industrial applications.
Gravity Die Casting-The Process
Gravity die casting is a type of permanent mold casting in which the molten metal is poured into a die and allowed to fill the die cavity, under the action of gravity.
It does not involve the use of external pressure to force the molten metal, instead, the filling of the die cavity is controlled by varying the tilting angle of the mold.
The dies are generally divided into two or more sections for the ease of retrieving the casting, without damaging the die.
This process is also suitable for alloy casting applications, where different alloying metals are melted and pored together to produce an alloy workpiece.
People often confuse die casting with molding, but it must be noted that the difference between casting and molding lies in their workpiece material.
While die casting is used for metal workpieces, molding is concerned with plastic workpieces.
Gravity die casting consists of 5 easy steps to get the perfect casting.
Step 1: Die preparation
Preheating the mold and spraying of coating (refractory or lubricant) on the mold cavity.
Preheating allows the molten metal to flow easily to the cavity through the gating system, whereas, the coating facilitates heat dissipation and easy ejection of the cast product.
Step 2: Pouring the molten metal
The die halves are clamped together and the molten metal is poured via sprue (a funnel-like structure) into the die.
The molten metal flows through the gateway channel and into the cavity by the virtue of the gravitational pull and no external pressure or forces are applied.
Avoiding the external pressure eliminates the risk of turbulence, reducing the porosity in the casting, and resulting in a smoother surface finish.
Step 3: Cooling and solidification of the melt.
The molten metal is allowed to cool and solidify within the mold for the desired amount of time.
The solidification time depends upon the type of metal and the size of the casting.
Generally, the solidification time for medium-size castings of aluminum alloys varies between 1.6-2.4 seconds/cm2.
Opening the die prematurely can result in various defects within the cast parts, which can render the casting useless or require extensive reworking to rectify them.
Step 4: Removal of the cast part.
To remove the solidified cast part, the dies are unclamped, following which the part is ejected using the ejector pins or by hand.
It must be ensured that you wear safety gear such as gloves when retrieving the hot casting to avoid any risk of skin burns.
Step 5: Machining and finishing
The excess metals that get solidified in the gates, runners, and sprues are machined along with the sharp edges from the cast part to obtain the final finished product.
Factors to Consider During Gravity Casting
Although gravity casting is comparatively easy and having a good know-how of the process can get you started with your DIY casting projects, there are various factors to consider to produce quality castings.
Cautiously Designed Dies
Avoid designing parts with sharp corners or with fine angles and instead select the shape that can be cast easily.
This is because sharp corners and fine edges make the part difficult to retrieve from the mold after casting.
Apart from that, sharp corners lead to uneven cooling and can result in various casting defects.
Incorporate Requisite Allowances
Always incorporate machining and shrinkage allowances while designing the part to obtain the dimensions close to the desired part.
Apart from that, the molten metal undergoes shrinkage upon solidification, resulting in smaller dimensions of the part than the mold.
Therefore, it is important to consider the shrinkage and machining allowances when designing the mold.
Generally, outer dimensions are maintained larger than the required dimensions, whereas internal dimensions, such as holes, are maintained at slightly smaller than the required dimension of the part.
The amount of contraction of various gravity die-cast materials along with commonly used shrinkage allowances are given below:
| Alloy | Shrinkage allowance (%) |
|---|---|
| Aluminum | 1.3 |
| Brass | 1.3-1.6 |
| Magnesium | 1.3 |
| Zinc | 1.3 |
Superior Quality Ingots
The raw material used for casting can consist of impurities such as rust and other suspended impurities (non-metallic inclusions).
These impurities can affect the surface finish of the casting and also result in various casting defects, degrading the overall quality of the casting.
Therefore, it is important to use superior-quality ingots to avoid the inclusion of impurities.
Controlled Pouring of the Melt
The pouring of the molten metal into the cavity plays an important role in determining the quality of the casting.
Pouring the molten metal abruptly can cause turbulence in the flow of molten metal, causing air bubbles and other similar casting defects.
On the other hand, an extremely slow pouring rate will result in non-uniform cooling of the casting, leading to the solidification of casting without filling the mold completely.
Therefore, the pouring of molten metal should be steady and uninterrupted to avoid casting defects such as entrapment of air bubbles, which lead to porous casting.v
Common Casting Defects and Their Remedy
Misruns
Misrun is a casting defect in which the casting solidifies before the complete filling of the mold cavity.
This kind of defect can be usually eliminated by avoiding very slow pouring speed.
Cold Shots
The defect usually occurs due to the liquid metal splattering that can lead to the formation of metallic solid globules which remain entrapped in the cast product.
Designing a proper gating system and following ideal pouring procedures can avoid this type of defect.
Hot Cracking
Hot cracking refers to cracks in the casting due to obstruction in shrinkage after the solidification of the melt in the mold cavity.
It can also occur due to uneven cooling, leading to non-uniform shrinkage of the casting.
Generally, hot cracking can be avoided by improving the design of the mold, uniform heating of the casting during melting, and using chillers to facilitate uniform cooling rates through the casting.
Apart from that, the chances of hot cracking can also be reduced if the casting is removed immediately after the solidification of the melt.
Metals Suitable for Gravity Casting
Permanent mold die casting is limited by the type of material used to make the die.
The metal to be cast should have a melting temperature significantly lower than the die material.
| Suitable Metal | Applications |
|---|---|
| Aluminum | Automobile parts such as pistons, housings of clutch or intake manifolds, electrical appliances, frames, and housing of the motor. |
| Brass | Fixtures of plumbing, the hardware involved in a lock, and ornament casting. |
| Magnesium | Parts of automobiles such as crankcases, transmission housings, power tools, and sports equipment. |
| Zinc | Equipment for household or office, toys, utensils, automotive parts, door handles, and radiator grills. |
On the other hand, expendable mold gravity casting processes like sand casting are suitable for casting ferrous as well as non-ferrous alloys.
This is because the refractory material, such as sand, used for making their molds can withstand high temperatures without failing.
Expendable mold gravity casting is capable of casting all the metals mentioned above, along with the following ferrous materials:
| Suitable Metals | Their Applications |
|---|---|
| Iron (Ductile, gray, malleable, white) | Gears, crankshafts, engine blocks, brakes, and construction machinery. |
| Steel (carbon, low alloy, high alloy) | Die blocks, railroad wheels, gas turbine housings, and pump components. |
Out of all the alloys, aluminum is most commonly used with the gravity casting process because of its high stability and better strength-to-weight ratio.
Moreover, aluminum also provides the luxury of corrosion resistance with superior mechanical and electrical properties.
Advantages of Gravity Casting
Intricate Parts with Thin Walls
Gravity die-casting is capable of casting intricate parts with very thin walls and an excellent strength-to-weight ratio.
Aluminum die-cast parts can have a thickness as low as 0.5 mm, whereas for zinc a thickness of around 0.3 mm can be obtained.
Superior Mechanical Properties
Owing to the metallic molds, the solidification is swift in the case of gravity die casting leading to the formation of a finer grain structure of the casted part with superior strengths.
Minimal Porosity
Unlike the casting processes involving external pressure, gravity casting produces parts with less porosity and can also undergo heat treatment to refine their grain structure.
Better Ductility
The gravity-cast products exhibit better ductility or elongation as compared to the other casting processes.
Suitable for Mass Production
Mass production of cast parts makes the gravity die-casting process substantially economical as compared to the other processes.
Although the cost of dies can be high, the ability to reuse them to produce a large number of products overcomes the high initial cost, thereby increasing the overall productivity of the process.
Frequently Asked Questions (FAQ)
Can gravity casting be used for casting ferrous metal alloys?
Yes, gravity casting can be used to cast ferrous metal alloys. However, their high melting temperature limits them to be cast by expendable mold gravity casting process only.
Why gravity die casting is not recommended for ferrous metals?
Gravity die casting is not recommended for ferrous metals because the molds used in this process are made of iron or steel. Making it suitable to cast metals that have a comparatively lower melting temperature. However, you can use molds made of refractory materials such as graphite to cast some non-ferrous metals, but the cost of the process will increase substantially.
What is the purpose of a riser in the gravity-casting process?
The riser acts as a reservoir of molten metal provided in the gating system to compensate for the shrinkage of the metal volume in the cavity due to solidification.
Which type of gravity-casting process can create highly complex parts and why?
The expendable mold gravity casting process is capable of creating more complex parts because the molds don’t need to be opened post-casting. On the contrary, permanent mold is required to be opened after the casting and thus limiting its degree of intricacy in the cast parts.
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