Assignment # 2

Q1) Design various core making processes?

Q2) Compare Molasses sand with silicate/CO2 sand process?

Assignment # 3

Q3) Discuss various sand molding processes?

Q4) Explain Compactibility, mould hardness, green compression strength and AFS no. of green molding sand.

Core Making Processes:A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting.There are many types of cores available. The selection of the correct type of core depends on production quantity, production rate, required

precision, required surface finish, and the type of metal being used. Some core making processes are defined as under:

Sodium Silicate/ CO2 Process Molasses Process Shell process Green sand Cold-box

1. Sodium Silicate/CO2

Core-making Process

It is one of the easiest modern core-making processes. In this process, sodium silicate (4 -6%) is mixed with silica sand by either Batch type mixer or Continuous type screw mixer. After mixing coating of sodium silicate takes place on silica sand. The sand is rammed into a core-box and cured by passing CO2 through the core. CO2 dissolves in the water

of sodium silicate and forms carbonic acid. SiO2 + Na2O + H2O (l) + CO2 (g) → H2CO3 + Na2O + SiO2

Carbonic acid reacts with sodium silicate and forms silica jel. H2CO3 + Na2O + SiO2 → SiO2 (gel) + Na2 CO3 • H2O (glass)

The silica gel that is formed binds individual sand grains together. Sand temperature is critical in this process. The core should be between 25ºC to 30ºC (75ºF to 85ºF). Below 15ºC (60ºF) the reaction proceeds very slowly, and more CO2 or gassing time is required to fully

cure the core. Above 30ºC, excessive amounts of moisture evaporate during the curing process, resulting in a very weak and brittle bond. It should also be noted that the gel tends to hydrate, which causes a reduction in binder strength. This limits core shelf life to about one month.

2. Molasses Process7-10 % molasses is mixed with silica sand and filled in mould or core boxes then put them in ovens to dry at about 200oC. It takes time and is not suitable for mass production.

3. Shell ProcessIn shell process Silica and is coated with Phenol formaldehyde. The color would become light brown. This sand is Thermoplastic.Then it is filled in core box and heat at 200 oC, which allows the chemical components in the sand to bond together and form the shape within the core box.

There are two processes of coating of Phenol formaldehyde.

1) Warm at 80-100 oC: Phenol formaldehyde is added in liquid form about 3-3.5%

2) Hot at 120-130oC: Phenol formaldehyde is added in form of solid flakes about 3-3.5%.

4. Cold box ProcessCold box process consists of two parts:

1. Polyol Phenolic Resins (1%)2. Isocynate (0.4%)

The "polyol" representing one of the components is a phenol-formaldehyde resin exhibiting benzyl ether character. These resins display the general formula:

in which the sum of m and n is at least two, and the ratio m:n is at least 1:1

The polyisocyanate is an oligomeric product of 2,4'- and 4,4' -diphenylmethanediisocyanate, and exhibits the following structure:

However, the difference in polarity of the polyisocyanate and phenolic resin limits the choice of appropriate solvents that are compatible with both components. This "compatibility" is nonetheless necessary to achieve complete reaction and curing of the binder.Polar solvents are, for example, very appropriate for phenolic resins, but less so forpolyisocyanates. The situation is exactly the reverse when nonpolar solvents are used.The preferred nonpolar solvents are high-boiling aromatic hydrocarbons (generally in the form of mixtures).

Silica sand represents the bulk of the sand grades used for the cold box process and also the particle size of the sand has a major effect on the bending and tensile strength of the core produced using the cold box method. The fact must also be considered that the required binder level is directly related to the particle size.

Comparison between Molasses Sand & Sodium Silicate Sand Process

Sodium Silicate/CO2

Process

One of the easiest modern core making processes for instructional and small foundries to use is the sodium silicate/ CO2 process.

Liquid sodium silicate is mixed with the silica sand.

The sand is rammed into a core box and cured by passing CO2 through

the core. Cores made from this process

produce less gas than other processes. Cleanup is also easy since water can dissolve the sodium silicate. The environmental friendliness, ease of cleanup, and simplicity makes the process very simple to conduct in the teaching foundry.

The sodium silicate/CO2 process

hardens through the following reactions: SiO2 + Na2O + H2O (l) + CO2 (g) → H2CO3 + Na2O+ SiO2

H2CO3 + Na2O + SiO2 → SiO2 (gel) + Na2CO3 • H2O (glass)

Molasses ProcessConventional and small foundries are using Molasses Sand for making cores and moulds.These are all manual processes or for Non-Ferrous industry (Brass and Bronze)

Mix the Molasses with Sand and bake up to 2000C.

7 to 10% Molasses added in sand and fill in mould then put in ovens up to temperature 2000C.

Due to the high hygroscopicity of the mix prepared with molasses, its use is not much favoured for good-quality castings.

Molding ProcessesAccording to different standard and requirement, considering features of the castings and actual technical condition, we adopt different processes to produce the castings. For example: Green Sand molding, Resin sand molding, Lost foam and coating sand molding.

1. Green Sand MoldingThis process derives its name from the presence of moisture in molding sand. The sand undergoes a "mulling" process in which various clay and chemical additives that act as binders such as pitch, cellulose, and silica flour are blended with the sand, which results in a compound which is suitable for the sand molding process.This prepared sand mixture is then compressed around the pattern at specific pressures and temperatures, to ensure it will maintain its shape throughout the remainder of the casting process. The blended sand and binders are compacted around the pattern, taking on the shape of the desired casting.Sometimes the design of the casting entails internal passageways being formed into the mold. This is done by using sand cores which are made of a similar sand mixture. The cores are strategically placed to form the necessary passageways in the casting. The two halves of the mold are subsequently closed and metal is poured into the cavity and left to solidify.After solidification has taken place, the sand is vibrated until it is released from the casting. The finishing process can then be completed by grinding, machining, plating and painting.

Features Low material costs Reclaimable mould material. Environmentally friendly. Ensures strong and rigid moulds.

Advantages These molds are relatively inexpensive to produce, since the basic

material is readily available. Complex patterns can be accommodated in the in the mold design,

at affordable costs. Easily adapts to automated production methods.

2. No Bake MoldingNo Bake is a casting process that uses chemical binders to bond the molding sand. The sand is then transported to the mold fill station in preparation for filling of the mold. A mixer is then used to blend sand with the chemical binder and the catalyst. When the sand exits the mixer, the binder begins the hardening process. After the compaction process, a rollover process is used to remove the mold from the pattern box. The mould is then readied for handling the molten metal. After a shakeout process, the molded sand is taken away from the casting. Then various procedures follow including the finishing and the sand can be reclaimed by thermal means.The chemical nature of the binders makes this a highly specialized process that has to been handled with considerable expertise and knowledge.

Features Chemical binders are used to create high strength moulds. When the temperature is brought to normal levels, the molds turn rigid.

Advantages It is adaptable to any quantity It creates high strength moulds & improves dimensional repeatability Requires low skill and labor requirements Provides better dimensional control.

Applications Ideal for high value and critical parts Desired for Complicated Profiles

3. CO2 MoldingCarbon dioxide molding is a sand casting process that employs a molding mixture of sand and liquid silicate binder. The molding mixture is then hardened by blowing carbon dioxide gas through it. This method offers a great deal of advantages over other forms of sand molding. It reduces production time as well as fuel costs and reduces the number of mould boxes required for making moulds. This process also offers a great deal of accuracy in production.

Features

High accuracy molding systems incorporating the gas carbon dioxide as a catalyst.

Advantages Provides good dimensional tolerances through strong core and mold Provides excellent casting surface finishes Generally used for high-production runs Accommodates a wide range of core and mold sizes. When used for making cores, the CO2 process can be automated for

long durations & speedy production runs.

Applications Ideal for casting applications where speed and flexibility is

paramount.

4. Resin Sand CastingSand molds often use resin based chemical binders that possess high dimensional accuracy and high hardness. Such resin-bonded sand molds take somewhat longer to manufacture than green sand molds because a curing reaction must take place for the binder to become effective and allow formation of the mold. As in clay-bonded molds, the sand can often be recycled, although with some treatment to remove the resin. In Resin Sand Casting, resin-coated sand is packed tightly around a pattern. The pattern is removed, and the molten metal is poured into the corresponding cavity. The sand casting process is one of the most affordable methods of creating a metal casting. Various high quality castings are made of gray iron, ductile iron, aluminum, steel, brass etc.

5. Mold Shell CastingA heated metal pattern is covered with a mixture of sand and thermoset plastic. This causes a skin of sand mixture to adhere to the pattern. This skin is removed from the pattern to form the "shell mold". The two halves of the shell mold are secured together and the metal is poured in the shell to form the part. Once the metal solidifies, the shell is broken.The materials that can be used with this process are cast irons, and aluminum and copper alloys. Typical parts made with this process are connecting rods, gear housings, lever arms etc.

The basic process for these molds is1. Create two mating patterns of desired shape.

2. Coat the molds with a shell (sand and binders, such as a resin) until desired thickness and other properties are obtained.3. Cure the molds and remove the patterns.4. The mold halves are mated and held firm while metal is poured.5. The final part is removed.

This technique can be very economical. Special care must be taken to assure venting for gasses, as the

mold media is less porous. This method can easily use cores and chills to make complex molds. Graphite molds can be used for materials that would normally react

with other materials used for the molds.

Advantages Shell molding process offers better surface finish. Shell molding process offers better dimensional tolerances Shell Molding Process offers higher throughput due to reduced cycle

times.

CompactibilityCompactibility is directly related to the performance of the sand in the molding operation and reflects the degree of temper of the sand mix.

Procedure The test is run by filling a standard specimen tube with riddled sand through a screen mounted at a constant height above the tube. The excess sand is struck off the top of the tube, and the sand is rammed three times. The distance from the top of the tube to the surface of the sand is read as percent compactibility.

SignificanceBecause the test is independent of the specific gravity of the sand, it is superior to the bulk density test for measuring the water requirement of the sand mix. The presence of water in extra amount of what is required to establish the minimum density point of the molding sand results in free water within the sand mass. As the moisture decreases, the water-clay coating thickness decreases, and more sand can be riddled into the specimen tube.Compactibility duplicates how a fixed volume of sand will react to a fixed input of energy and is useful in controlling the clay-to-water ratio. This test, in conjunction with green compression, can be used to determine the working bond or effective clay present in a sand mix.

Effect of Compactability

Mold HardnessThe mold hardness test indicates the resistance of the mold-to-metal damage as the metal contacts the mold surface.

ProcedureMold hardness is measured by the resistance offered by the mold surface to a spring-loaded plunger. When the tester is placed base down on the molds surface, the plunger gets pressed and forced into the sand and gives reading. Both "B" and "C" scale hardness testers are available, but the "C" scale tester is more accurate at the high end of the hardness scale.

SignificanceProper mold hardness will give castings a better finish, more accurate dimensions and reduced penetration, drops and swells. Excessive hardness, meanwhile, can cause cracks, scabs, blows, pinholes and penetration.

Green CompressionGreen compression has been the most widely used control tool to measure the rate of clay addition to a sand molding system.

Procedure

The green compressive strength of green sand is the maximum compressive stress that a mixture is capable of sustaining when prepared, rammed and broken under standard conditions. The rammed cylindrical specimen is formed by placing a weighed amount of sand in a tube and ramming the sand three times. The instrument used for breaking the specimen must continuously register the increasing load until the specimen fractures.

SignificanceThe degree of mulling, sand-to-metal ratio, clay content, compactibility range and type of additives have a significant effect on green compression. The compression reading should be read at comparable compactibility ranges. Molding sand at higher or lower compactibility will produce varying green strengths. Green compression in conjunction with moisture can be used to determine the available bond.

AFS NumberAFS Number of green molding sand indicates the amount of fines and water-absorbing material in the sample.

ProcedureA known amount of dried molding sand mixed with a pyrophosphate solution is stirred with a high-speed mixer for 5 min. Water is added to the top level line, and the mixture is allowed to settle for 5 min. before the top of the water is siphoned off. The procedure is repeated until the water above the sample is clear. The sand then is dried, and the weight loss is recorded as AFS Number.

SignificanceAFS Number of green sand may contain active clay, dead clay, silt, seacoal, cellulose, cereal, ash, fines and all materials that float in water. Only the active clay gives active bonding capacity to the system.