Casting

Article Tour:

1. What is casting?

2. Types of casting:

Sand Casting

Investment Casting (Lost Wax Casting)

Lost Foam Casting

Die Casting

Centrifugal Casting

3. Raw materials for casting:

4. The process of casting:

5. Advantages and disadvantages of casting:

1. What is casting:

Casting is a metal manufacturing process: it is meaning that when we want to produce metal parts, we design the corresponding shape, make the corresponding mold, and then heat the solid metal (at room temperature) to a molten state through high temperature, and pour the molten metal into a prepared mold and wait for the metal to cool to get the tool we want. This process is casting.

2. Types of casting

There are many types of casting processes, the main categories are sand casting and special casting. Among them, there are five common casting processes, namely: sand casting, investment casting (lost wax casting), lost foam casting, metal mold casting, and pressure casting.

Almost all types of casting will be described below.

Sand Casting

Sand casting is one of the most widely used casting methods. It uses molding sand and core sand as molding materials to make a mold. After heating the solid metal to a molten state, the liquid metal is filled into the mold (mold) made of foundry sand and molding sand binder by gravity to produce castings.

Foundry sand is one of the raw materials for making sand molds: the most commonly used foundry sand is siliceous sand. When the high-temperature performance of silica sand cannot meet the requirements, we can use special sands such as zircon sand, chromite sand, and corundum sand.

The role of the molding sand binder is to make the sand mold and core have a certain strength, and will not be deformed or damaged during handling, molding, and pouring of liquid metal.

According to the difference of the original sand, the types of sand casting can be divided into:

Clay Sand Casting:

Clay sand means natural raw sand with a mud content of less than or equal to 50%. The method of using sand molds and sand cores made of this material to produce castings is called clay sand casting.

Resin Sand Casting:

Resin sand casting means that synthetic resin is used as sand grains and binders to make sand molds and sand cores, or cores with the necessary strength. Casting done using this sand mold and core is called resin sand casting.

Water Glass Sand Casting:

Water glass is a general term for various polysilicate aqueous solutions. Commonly used in casting is sodium (Na2O·mSiO2) water glass, followed by potassium (K2O·mSiO2) water glass, in addition to lithium (Li2O·mSiO2) water glass, potassium sodium (mK2O·Na2O·mSiO2) water glass quaternary ammonia Salt (quaternary ammonium salt) water glass, etc. The method of using such materials to make sand molds and complete the production of castings is water glass sand casting.

Silica Sol Sand Casting:

Silica sol is a dispersion of nano-sized silica particles in water or solvent. Since the SiO2 in the silica sol contains a large amount of water and hydroxyl groups, the silica sol can also be expressed as mSiO2.nH2O. Casting done using this type of sand is known as silica sol sand casting.

Coated Sand Casting:

The coated sand is mainly made of high-quality natural quartz sand as raw sand, thermoplastic phenolic resin, urotropine and reinforcing agent as raw materials. The surface of this sand is covered with a layer of solid resin film before molding. The casting method that uses this kind of sand to make sand molds and cores is called coated sand casting.

Advantages And Disadvantages Of Sand Casting:

Advantages：

Lower cost because the sand used in the molds can be reused;

Disadvantages：

The mold making is time-consuming, the mold itself cannot be reused, and the finished product can only be obtained after being destroyed.

Special Casting

With the development of science and technology, higher requirements are put forward for casting, and it is required to produce castings with more precision, better performance, and lower cost. To meet these requirements, foundry workers have invented many new casting methods, which are collectively referred to as special casting methods, that is, special casting.

Commonly used special casting methods include:

Investment Casting (Lost Wax Casting):

The main reason it is called investment casting is that the mold will melt. The casting process is: first use wax to make a wax mold of the part to be cast, and then apply (spray) mud on the wax mold to make a mud mold. After drying the clay mold, heat the clay mold, and the wax mold will melt and evaporate as the temperature rises. The clay mold without wax mold is taken out and fired into a pottery mold. The molten metal is poured into the mud mold along the sprue, and the parts are made after cooling. This process is known as investment casting (lost wax casting).

Plaster Mold Casting

Assemble the investment mold and fix it on the sandbox plate specially used for grouting. The gypsum slurry is poured in under a vacuum. After the slurry is coagulated and dried, the investment mold can be removed. Gypsum type, cast under vacuum to obtain castings. This process is known as plaster casting.

Ceramic mold casting:

It means a casting method that uses ceramic slurry to make a casting mold to produce castings.

Lost Foam Casting:

Lost foam casting (also known as solid casting) is to bond and combine paraffin or foam models similar in size and shape to castings into model clusters. After brushing refractory paint and drying, they are buried in dry quartz sand and vibrated for modeling. The new casting method is a new casting method in which the model is gasified by pouring down, and the liquid metal occupies the position of the model and solidifies and cools to form a casting.

Metal Mold Casting:

A casting method in which liquid metal is poured into a metal mold to obtain a casting. Molds are made of metal and can be used many times (hundreds to thousands).

Die Casting:

Die casting is a casting method. Its process is: filling liquid or semi-solid metal or alloy, or liquid metal or an alloy containing reinforcing phase, into the cavity of the die-casting mold at high speed under high pressure, Solidification of metals or alloys under pressure to form castings. The pressure commonly used in die casting is 4-500MPa, and the metal filling speed is 0.5-120m/s.

Low-Pressure Casting:

Low-pressure casting means that the casting mold is generally placed above a sealed crucible, and compressed air is introduced into the crucible to cause a low pressure (0.06-0.15MPa) on the surface of the molten metal so that the molten metal rises from the riser to fill the mold and control Solidified casting method.

Differential Pressure Casting:

The mold is covered with a sealed cover, which is filled with compressed gas so that the mold is under certain pressure of the gas. When the molten metal is filled, the pressure of the gas in the holding furnace is greater than the pressure of the gas in the mold, to realize the filling, pressure maintaining, and pressurization of the molten metal as in low-pressure casting. But at this time, the casting is crystallized and solidified under the action of higher pressure, so it can ensure that the casting with higher density can be obtained.

Vacuum Suction Casting:

Vacuum suction casting is a casting method in which a vacuum is created in the cavity, and the metal is sucked into the cavity from bottom to top for solidification and forming.

Squeeze Casting:

It is to pour a certain amount of cast metal liquid directly into the cavity coated with lubricant, continue to apply mechanical static pressure, and use the easy flow and forging technology to make the solidified hard shell plastically deform when the metal is hardened and formed. The metal crystallizes and solidifies under pressure and forcibly eliminates shrinkage cavities and shrinkage porosity formed by solidification shrinkage to obtain liquid die forgings without casting defects.

Centrifugal Casting:

Centrifugal casting is a technology and method of injecting liquid metal into a high-speed rotating mold so that the liquid metal can be filled with a centrifugal motion to form a casting.

Continuous Casting:

Continuous casting is an advanced casting method whose principle is to pour molten metal into a special metal mold called a mold, and the solidified (crust) castings are continuously poured from the other end of the mold. Pulled out, it can obtain castings of any length or specific length.

Semi-Continuous Casting:

Semi-continuous casting means that a process in which molten steel is solidified into a billet through a crystallizer and then passed through the second cooling zone by a billet drawing machine without continuous cutting.

Shell Type Casting:

The use of resin sand to make thin shell molds or shell cores can significantly reduce the amount of molding sand used, resulting in castings with clear contours, smooth surfaces, and precise dimensions that can be machined without or only in small quantities. Therefore, shell casting is especially suitable for castings of various alloys with large batches, high dimensional accuracy requirements, thin walls, and complex shapes.

Graphite Mold Casting:

Graphite mold casting is a casting method for pouring metal in a mold made of graphite material. The graphite type is usually made of graphite rods and blocks by cutting, and it is a semi-permanent type.

Electroslag Casting:

Electroslag casting is an electro-slag metallurgical process that completes metal refining and casting at one time and produces high-quality alloy castings. It uses the resistance heat generated by the current passing through the liquid slag to continuously melt the metal electrode, and the molten metal gathers into droplets, which drop into the metal molten pool through the slag layer, and at the same time solidify into casting in a special-shaped water-cooling mold.

Advantages of special casting:

(1) The size of the casting is accurate, the surface roughness value is low, and it is closer to the final size of the part, so it is easy to achieve less or no cutting.

(2) The internal quality of the casting is good, the mechanical properties are high, and the wall thickness of the casting can be reduced.

(3) Reduce metal consumption and casting rejection rate.

(4) Simplify the casting process (except investment casting) to facilitate the mechanization and automation of the production process.

(5) Improve labor conditions and increase labor productivity.

3. Raw materials for casting:

Gray Cast Iron

It means that the cast iron with flake graphite, because the fracture is dark gray when broken, so it is called gray cast iron. The main components are iron, carbon, silicon, manganese, sulfur, and phosphorus. It is the most widely used cast iron, and its output accounts for more than 80% of the total output of cast iron.

Ductile Iron

Ductile iron is obtained by spheroidization and inoculation treatment to obtain spheroidal graphite, which effectively improves the mechanical properties of cast iron, especially the plasticity and toughness, so as to obtain higher strength than carbon steel.

Malleable Cast Iron

Malleable cast iron is a high-strength and tough cast iron obtained by graphitizing and annealing white cast iron.

Casting Steel

Cast steel refers to the steel specially used for the manufacture of steel castings. Cast steel should be used when the strength requirements of castings are high and cast iron cannot meet the requirements.

Aluminum Alloy

Cast aluminum alloy is an aluminum alloy that fills the mold with molten metal and obtains blanks of various shapes. It has the advantages of low density, high specific strength, good corrosion resistance, and casting manufacture ability, and is less restricted by the structural design of parts. Divided into Al-Si and Al-Si-Mg-Cu-based medium-strength alloys; Al-Cu-based high-strength alloys; Al-Mg-based corrosion-resistant alloys; Al-Re-based heat-strength alloys. Most require heat treatment to strengthen the alloy, eliminate internal stress in the casting, and stabilize the structure and size of the parts.

Other Cast-able Raw Materials

Brass, Tin Bronze, no-Tin Bronze

4. The process of casting:

Sand casting process:

Drawing

Molding

Core Making

Forming

Cleaning

Investment casting process:

Drawing

Molding

Pressed Wax

Repair Wax

Group Tree

Dipping

Melting Wax

Casting Molten Metal

Forming

Post-Processing

Gypsum Mold Casting Process:

Investment Mold Assembly

Fixed

Pour In The Gypsum Slurry

Dry

Remove The Investment

Drying

Roasting

Pouring Under Vacuum

Post-Processing

Ceramic Mold Casting Process:

Preparation And Grouting Of Ceramic Slurry

Molding

Burning

Roasting

Pouring And Cleaning

Loss Foam Casting Process:

1. Making foam gasification molds

2. Foam gasification, drying after mold assembly

3. Brush and spray refractory paint on the surface of the foam plastic gasification mold and then dry it again

4. Place the special sandbox on the 3D vibrating table

5. Fill in the bottom sand (dry sand), tap, and level

6. Put the dried foam plastic gasification mold on the bottom sand, divide it into sand filling according to the process requirements, automatically vibrate for a certain period of time, and then scrape the opening of the box.

7. Cover the mouth of the flask with plastic film, put the pouring cup, and connect it to the negative pressure system. After the compaction, the molten steel is cast, the foam-plastic gasification mold disappears, and the molten metal takes its place.

8. After the casting is condensed, the vacuum is released and the box is turned over, the casting is taken out, and the next cycle is carried out

Metal Mold Casting Process:

Metal Mold Making

Preheating Of Metal Molds

Pouring

Molding And Core Pulling

Die Casting Process:

Die Casting Machine Debugging

Die-Casting Mold Design And Manufacture

Die Casting Mold Installation

Coating Configuring

Mold Cleaning

Mold Preheating, Coating

Forming (Clamping)

Alloy Smelting And Heat Preservation

Casting Injection

Insert Preparation

Pressure Holding

Mold Opening And Core Pulling

Surface Quality Inspection

Clean Up (Refurbishment)

Aging Treatment

Casting Infiltration Shot Peening

Final Inspection

Low-Pressure Casting Process:

Prepare Metal Smelting And Molds Or Castings.

Preparation Before Pouring: Including Crucible Sealing (Fitting Sealing Cover), Slag Removal In The Liquid Riser, Measuring Liquid Level, Tightness Test, Mold Matching, Fastening Mold Or Casting Mold, Etc.

Pouring: Including Liquid Liter, Filling, Pressurization, Solidification, Pressure Relief, Cooling, Etc.

Demoulding: Including Loose Demoulding And Removal Of Castings.

Differential Pressure Casting Process:

On the basis of low-pressure casting, the casting mold is covered with a sealing sleeve, and compressed air (generally about 0.2~0.5MPa) is introduced into the crucible and the cover at the same time, but the pressure in the crucible is slightly higher, which makes the liquid in the crucible The metal enters the mold through the riser under the action of the pressure difference and crystallizes under the pressure.

Vacuum Suction Casting Process:

Direct Suction Casting Method：

First, the shell made by the ordinary investment process is placed in the sealing chamber, the sealing chamber is lowered, and the sprue is inserted into the liquid metal.

Start the vacuum pump to evacuate the sealed chamber, and the liquid metal is filled at the same time.

After the molten metal in the shell is solidified, the sealed chamber is connected to the atmosphere to eliminate the vacuum, and the unsolidified liquid metal in the runner flows back into the crucible.

Finally, take out the shell and clean the sand to get the casting.

Tilting And Inversion Method Vacuum Suction Casting：

After the vacuum suction casting and filling are completed, the casting mold is inverted, and the remaining molten metal in the sprue provides feeding for the solidification of the casting.

Special Vacuum Suction Casting Method：

After completing the vacuuming of the upper sealing chamber; after the molten metal fills the cavity,

Compressed gas is then introduced into the lower sealing chamber to increase the pressure difference between the upper and lower sealing chambers and enhance the metal liquid feeding ability during the crystallization and solidification of the casting.

Vacuum Suction Casting With Inert Gas Protection：

melt metal under vacuum

Inert gas is introduced into the vacuum melting chamber and suction casting chamber at the same time, and they are kept at the same pressure.

Insert the shell sprue or riser into the molten metal, and then reduce the pressure of the suction casting chamber to perform suction casting.

After a certain period of time, the molten metal in the sprue flows back to the crucible after the pressure is released.

Squeeze Casting Process:

Material Preparation → Metal Smelting

The Hotter The Mold Is → Spraying And Demoulding

Pouring

Clamp The Mold

Pressure

Pressure Holding

Demoulding → Liquid Forging

Mold Cleaning And Reset

Spray Mold Release Agent

Back To Step 3

Centrifugal Casting Process:

Mold Assembly

Boot

Preheat The Mold

Coating

Alloy Quantitation

Pouring

Water Cooling

Casting

Clean Up The Mold

Shell Type Casting Process:

Cover the metal template heated to 180-280°C with a heat-hardening molding sand (phenolic resin-coated sand) to harden it into a thin shell (the thickness of the thin shell is generally 6-12 mm),

Then heat and solidify the thin shell to achieve sufficient strength and rigidity. Therefore, after the upper and lower shells are clamped with clamps or glued with resin, the mold can be formed without a sandbox.

The heating temperature of the casting metal template is generally about 300 ℃, and the molding sand used is resin sand, that is, resin sand with phenolic resin as the binder.

5. Advantages And Disadvantages Of Casting:

Advantages:

1. It can produce parts with complex shapes, especially blanks with complex inner cavities;

2. Wide adaptability, metal materials commonly used in industry can be cast, ranging from a few grams to several hundred tons;

3. Wide source of raw materials and low prices, such as scrap steel, scrap parts, chips, etc.;

4. The shape and size of the casting are very close to the parts, which reduces the amount of cutting and belongs to no-cutting processing;

5. Widely used, 40%~70% of agricultural machinery and 70%~80% of the weight of machine tools are castings.

Disadvantages:

1. The mechanical properties are not as good as forgings, such as coarse structure and many defects;

2. In sand casting, single piece and small batch production, the labor intensity of workers is high;

3. The quality of castings is unstable, there are many processes, and the influencing factors are complex, which are prone to many defects.