SHELL MOLDING

Presented By: Suraj Kute. Ambarish Bhoga.

INDEX1) Molding definition2) Types of molding3) What is Shell molding?4) Manufacturing of shell sand.5) Shell molding raw materials 6) Molding procedure7) Defects in shell molding8) Advantages9) Disadvantages10)Recent advancement in shell technology11)Applications

1)1) Molding DefinitionMolding Definition

Molding is the process of Molding is the process of manufacturing by shaping manufacturing by shaping pliable raw material using a pliable raw material using a rigid frame or model called a rigid frame or model called a pattern.pattern.

2) Types of Molding2) Types of Molding Injection moldingInjection molding Compression molding Compression molding Transfer moldingTransfer molding Extrusion molding Extrusion molding Blow molding Blow molding Rotational molding Rotational molding Shell moldingShell molding

3) The Shell Moulding Process: A German Innovation

Shell moulding processes was invented by Johannes Croning.

The process was developed The process was developed and patented by Croning in and patented by Croning in Germany during World War Germany during World War II and is sometimes referred II and is sometimes referred to as the Croning shell to as the Croning shell process.process.

3) What is Shell molding?3) What is Shell molding? Shell molding is a process for producing Shell molding is a process for producing

simple or complex near net shape simple or complex near net shape castings, maintaining tight tolerances castings, maintaining tight tolerances and a high degree of dimensional and a high degree of dimensional stability. stability.

Shell moulding is a method for making Shell moulding is a method for making high quality castings. These qualities of high quality castings. These qualities of precision can be obtained in a wider range precision can be obtained in a wider range of alloys and with greater flexibility in of alloys and with greater flexibility in design than die-casting and at a lower cost design than die-casting and at a lower cost than investment castingthan investment casting. .

It is a process well suited to rapid, automated, repetitive and high-volume production. The most common method for producing shell moulds is to use a dump box.

Why SHELL MOULDING ?

SAND CASTING IS MOST COMMONLY USED CASTING PROCESS.

BUT NOT SUITABLE FOR ECONOMICAL IN MANY APPLICATION.

SO WE MAKE USE OF SPECIAL CASTING PROCESSES AND SHELL MOULDING IS ONE OF IT .

Machine tools used

1. Conventional dump-box method

2. Modern automated method

4) Manufacturing of shell sand.

The various methods used are as follows.

1. Warm air process

2. hot coating process.

Warm air process Pre weighted sand along with hexa catalyst

is mixed in mixer. Phenolic resins are then added (liquid resins ) alcohol or water is then removed by passing

hot air. During coating process there can be lump

formation. But they can be easily disintegrated . After this sand is send to cyclone for removal

of the siliceous particles.

Hot coating process Sand is heated between temperature of 125-150

deg. Resins are added which melts at such high

temperature and sand gets coated. Hexamine is then added which is aqueous

solution. Lubricant is then added to avoid lump formation. Water is then removed by means of hot air. By disintegrator the sand lumps are broken At the last the sand is sent to cyclone.

5) Shell molding raw materials

Silica sand. Zircon sand . Chromite sand. Aluminium silicate sand . Phenol formaldehyde resins. Hexa catalyst with lubricants. Additives.

Silica sand Basic raw material required. Ideal silica sand should have following

composition. 1) chlorides 0% 2) carbonates 0% 3) Acid demand value <6 ml of KOH/100

gm of sand. 4) Silica content 98% minimum. 5) Loss on ignition (max. 0.2% ) 6) Some amount of clay content.

Zircon sand

Much costlier than silica sand. Following are its advantages. 1) Low thermal expansion. 2) high sintering point (2150 -2250

deg) generally not blended with silica sand

Aluminium Silicate and Chromite Sand

Has got same advantages of that of zircon sand.

It is rarely used.

Phenol formaldehyde Resins.

Used as binder. form of the resin may be liquid or

flake. Liquid resins are resins

dissolved in alcohol. mostly liquid resins are used

due to their easiness.

Properties to be checked for resins

Clarity viscosity Specific gravity. Solid content . PH values.

Properties to be checked for solid resins

Softening point .

Flow rate.

Particle size

Hexa catalyst It is nothing but blend of

hexamethylene tetramine and lubricant.

It is supplied in powder form. used mainly to develop the

thermosetting characteristics in the shell sand .

Added in the proportion of 14 to 16 %.

Additives The function of the additives are. 1) To improve surface finish. 2) To avoid thermal cracking .

The commonly used additives are . 1)Coal dust. 2)pulverised slag. 3)manganese dioxide. 4)Calcium carbonate.

6) Shell Molding process

1. A match‑plate or cope‑and‑drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

2. Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell;

3. Box is repositioned so that loose uncured particles drop away;

4. Sand shell is heated in oven for several minutes to complete curing;

5. Shell mold is stripped from the pattern;

6. Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished;

7. The finished casting with sprue removed.

Figure shows that typical shell thicknesses that can be obtained with various pattern temperatures and contact times.

VIDEO

Real time automated process

7) Defects in shell molding

1. Bonding defects 2. Bubble on back of

shell 3. Distortion 4. Drop off 5. Lack of uniformly in

shell 6. peel back

7. Seizure 8. Sticking 9. Thermal shock 10. Thick shell 11. Thin Shell 12. Warping of

mold 13. Weak shells

1. Bonding defects Appearance : 1. Casting fins, possible run out of metal

when weakness is extreme.

Possible causes:1. Too viscous a bonding resin can cause the mold to close improperly.2. A resin much too fluid to have good surface finish.3. Delay in bonding can also cause the mold to close improperly.

2. Bubble on back of shell Appearance :

1. Resin ‘bubbles’ or ‘beads’ appear on the back of moulds.

Possible causes:1. Poor distribution of resin in sand.2. Inadequate melt of resin on hot coated sand.3. Additives not properly dispersed.4. Over mulling of coated sand causing resin to break loose.

3. Distortion Appearance :

1. Cores or moulds out of shape. Possible causes:

1. Too much plasticity due to insufficient care.

2. Too little hexamthylene tetramine.3. Ejection or moulding pins may be out of adjustment causing uneven pressure with resulting distortion on removal.

4. Drop off Appearance :

1. Sections of shell wall break off Possible causes:

1. Over lubrication preventing proper adhesiveness of resin-sand mix to pattern.

2. Too much resin3. Pattern temperatures below those needed to

bond resin and sand properly.

5. Lack of uniformly in shell Appearance :

1. Low density ares.2. solid resin chunks.

Possible causes:1. Moist sand restricting flowability.2. Low stick point of coated sand.3. Corebox below pressure too low or to high.

6. peel back Appearance :1.Pattern set sand brakes off leaving weak spots.2. Moulds break and cores collapse when poured Possible causes:

1. Too much resin.2. Time in contact with pattern too long.3. Pattern temperature too high or too low.4. Too much vibration.5. Wrong resin for the job.6. Hot or cold spots on plate or core box.

7. Seizure Appearance :

1. Mould difficult to remove though there is no sand adhesion to the pattern.

Possible causes:1. Insufficient or improper lubicant.2. temperature too high.3. cure time excessive.4. Excessive resin shrinkage during cure.

8. Sticking

Appearance :1.Moule will not remove easily from pattern or core plate.

Possible causes:1. Rough surfaces of pattern or corebox.2. Insufficient or improper lubrication.

9. Thermal shock

Appearance :1. Cracks in mould before metal solidifies causing run-out.

Possible causes:1. Pattern design may contribute to problem.2. Sand grains expansion during pouring causes thermal shock and cracks which stem from it.

10. Thick shell

Appearance :1. Excessive wall thickness.

Possible causes:1. Dwell time excessive.2. Temperature too high.3. Resin stick point too low.

11. Thin Shell

Appearance :1.Shell thinner than normal in given dwell time.

Possible causes:1. Dwell cycle too short.2. Pattern temperature too low.3. Resin flow may be too short for the job..

12. Warping of mold

Appearance :1. Moulds distorted when compared to patterns they are made from.

Possible causes:1. Temperature differential between pattern and oven may be too great.2. Resin may be too slow or may not cure

rigid enough.

13. Weak shells Appearance :

1.Lack of strength

Possible causes:1. Not enough binder.2. Too much moisture.3. Uneven distribution of resin.4. Undercure.5. Overcure.6.Resin melt point too high.

8) Advantages Excellent Surface Finish: Shell sand

process has ability to produce casting with excellent surface finish & capacity to produce fine detail.

Dimensional Accuracy: The process has an ability to produce casting to tight dimensional tolerance. Due to this characteristic machining allowance can be reduced which ultimately helps for reduction in fettling and finishing cost.

Sand to Metal Ratio: This is the only process which given hollow cores & thin walled mould, which results in substantial weight reduction and material saving. Normally sand to metal ration is 1:1 which is much lower than other processes.

Ease of Handling: Shell Sand moulds and cores are exceptionally resistance to damage through handling and storage. This happens due to high resistance to humidly. The shell cores and moulds can be stored for month without difficulty.

Resistant to Moisture Pickup: The shell process is resistant to moisture pick up. The shell cores and moulds can be stored for long period even in high humid conditions. The resin used for shell process is stable and moisture resistant.

No Skill is Required: Since every thing is ready to use, with minimum training any worker can produce, repeatedly, precision moulds and cores.

Little or No Pattern Wear: Since patterns are made up of cast iron, very little or no wear is observed resulting longer pattern life. This helps to produce “N” number of casting without any dimensional problems.

Longer Shelf Life: Shell sand has indefinite shelf life, if properly stored. Thus shell sand can be stored and used as needed by the foundry.

Low Capital Investments: Molds and cores making equipments is of moderate cost and where necessary can be added as cells. In shell process, bases sand can be easily changed to suit specific requirement of the casting.

Economics: Though the basic of shell sand is more than any other processes hollow cores, contoured shells, back up systems low fettling and cleaning, with good dimensional accuracy, makes process economically viable.

High rate of production is possible with limited floor space.

9) Disadvantages:

High Cost of The Process: Phenolic resins used for the shell process are costly & at the same time % usage of resins are high compared to other processes. Hence process requires a tight control of the shell thickness otherwise competitiveness of the process will be sacrificed.

High Tooling Cost: Shell process is thermoset & requires high temperature to cure, hence patterns used are of cast iron with smooth surface have very low expansion coefficient. All these results in costly affair compared to other processes.

High Energy Cost: Shell process operates at around 250 C toding temperature which requires energy either by the way of electricity or L.P.G. The cost of energy is very high, when compared with other processes.

Cycle Time: Comparatively cycle time required for shell process in more than either cold box or CO processes.

Limited Casting Weight: Shell process in best for small intricate light weight casting. With shell process casting weight up to 200kg is produced.

Highly complicated shapes cannot be obtained.

More sophisticated equipment is needed for handling the shell moulding such as those required for heated metal patterns.

Sand inputs need to be of higher quality than traditional sand casting.

Emissions from the process are noxious, so effective extraction systems are required.

10) Recent advancement in shell technology

Application oriented special sand is available.

Faster curing resins are available. Faster builtups. Faster shakeout shell sand . All these things have greatly

reduced casting cost and improved productivity.

One of the latest technology in shell molding is new formulation which gives great help for better shakeout for aluminium casting.

Many time the casting needs to be baked at 400-500deg to brake down cores and remove the casting.This is an expensive process.

By means of new shake out shell this problem can be easily solved.

11) 11) ApplicationsApplications

Crankshaft Crankshaft fabricationfabrication

Molded tubing Molded tubing fabrication fabrication

Automotive Automotive castings (cylinder castings (cylinder head fabrication).head fabrication).

Hydraulic control Hydraulic control housing housing fabrication fabrication

cylinders and cylinder heads for air cooled IC engines,

automobile transmission parts,

cast tooth bevel gears, brake beam

track rollers for crawler tractors,

transmission planet carrier,

steel eyes, gear blanks, refrigerator valve plates are some of the common applications of shell mould casting.

Thank you