1. SAND MOULDING Sand types, Cores, Machines
Hareesha N G
Lecturer, Dept. of Aeronautical Engg, DSCE, Bangalore-78

2. Syllabus Sand Moulding : Sand moulds: Cores: Moulding machines :
Types of base sand, requirement of base sand, Types of sand moulds.
Sand moulds:
Moulding sand mixture, ingredients (base sand, binder & additives) for different sand mixtures, Method used for sand moulding.
Cores:
Definition, Need, Types.
Method of making cores, Binders used.
Concept of Gating & Risering, Principle involved and types.
Fettling and cleaning of castings, Basic steps involved.
Casting defects, causes, features and remedies.
Moulding machines :
Jolt type, squeeze type, Jolt & Squeeze type and Sand Slinger.

3. MOLD MATERIALS A mold material is one, out of which the mold is made.
A mold material should be such that the mold cavity retains its shape till the molten metal has solidified.
Castings can be made in:
Permanent molds—made of ferrous metals and alloys (steel, Grey C.I. etc.).
Temporary refractory molds — made up of refractory sands and resins.
Permanent molds are normally employed for casting low melting point materials. Permanent molds are too costly.
For the above mentioned reasons, most of the foundry industry has its castings produced using refractory mold materials like Refractory Sands.
As compared to permanent molds, the refractory sand molds can cast high melting point materials and bigger objects, whereas permanent molds produce small castings of better quality and dimensional accuracy.

4. TYPES OF BASE SAND
The primary and basic material used for preparing moulds is sand, due to its high refractoriness.
Sand usually referred to as 'base sand‘
Nearly % of the total moulding sand is occupied by sand and the remaining is binder and additives.
Basic types of base sand are given below
Silica Sand
Chromite sand
Zircon
Olivine sand

5. I. Silica Sand
Silica sand is essentially silicon dioxide (Si02) found in nature on the bottoms and banks of rivers, lakes and seashore.
Silica deposits tend to have varying degree of organic and contaminants like limestone, magnesia, soda and potash that must be removed prior to its use, otherwise which affects castings in numerous ways.
Silica sand is available in plenty, less expensive and possess favorable properties.
Thermal expansion leads to certain casting defects; the reason for which not being used in steel foundries.
However, silica sand when mixed with certain additives like wood flour, (corn flour), saw dust etc., defects can be eliminated.
These additives burn by the heat of the molten metal thereby creating voids that can accommodate the sand expansion.

6. IV. Zircon or Zirconium silicate
II. Olivine sand
Olivine sand is typically used in non-ferrous foundries.
With its thermal expansion about half of that of silica sand, makes it suitable for production steel castings .
But the high cost restricts its wide use
III. Chromite sand
This is African sand with cost being much higher compared to other sands.
Due to its superior thermal characteristics, it is generally used in steel foundries for both mould and core making.
IV. Zircon or Zirconium silicate
This sand possesses most stable thermal properties of all the above sands.
The choice for this type of sand arises when very high temperatures are encountered and refractoriness becomes a consideration.
But the major disadvantage is that, zircon has trace elements of Uranium and Thorium which is hazardous in nature thereby restricting its use in foundries.

7. PROPERTIES OF MOLDING SANDS
The very important characteristic of a molding sand is that it should produce sound castings.
For doing so, the molding sand should possess certain desirable properties and they are:
Flowability
Green Strength
Dry Strength
Hot Strength
Permeability or Porousness
Refractoriness
Adhesiveness
Collapsibility
Fineness
Bench Life
Coefficient of expansion
Durability

8. Flowability 2. Green Strength 3. Dry Strength
Flowability is the ability of the molding sand to get compacted to a uniform density.
Flowability assists molding sand to flow and pack all-around the pattern and take up the required shape.
Flowability increases as clay and water contents increase.
2. Green Strength
It is the strength of the sand in the green or moist state
A mold having adequate green strength will retain its shape, Will not distort, Will not collapse, even after the pattern has been removed from the molding box.
Green strength helps in making and handling the molds.
3. Dry Strength
It is the strength of the molding sand in the dry condition.
A mold may either intentionally be dried or a green sand mold may lose its moisture and get dried while waiting for getting poured or when it comes in contact with molten metal being poured.
The sand (of molding cavity) thus dried must have (dry) strength to
withstand erosive forces due to molten metal,
withstand pressure of molten metal, and
retain its shape.
There should be an optimum balance between dry strength, and collapsibility of the molding sand.

9. 5. Permeability or Porousness
4. Hot Strength
It is the strength of the sand (of mold cavity) above 212°F.
In the absence of adequate hot strength, the mold may Enlarge, break, erode or, get cracked.
5. Permeability or Porousness
The moisture, binders (organic compounds) and additives present in mould sand core produce steam and other gases.
Though much of these gases escape through vents and open feeder heads, yet a good amount of the same tends to pass off through the pore spaces of the molding sand.
Thus to provide a path for free escape of the gases, the molding sand should be permeable or porous.
Sands which are coarse (Bigger in size) or have rounded grains exhibit more permeability.
Soft ramming and clay addition in lesser amounts also improves permeability.
In the absence of adequate permeability, defects like surface blows, gas holes, mold blast etc. may be experienced.
6. Refractoriness
It is the ability of molding sand to withstand high temperatures (experienced during pouring) without
fusion,
Cracking
Buckling
experiencing any major physical change.
As compared to castings of low melting point alloys, refractoriness is much more essential in the production of high melting point alloy castings (e.g. steel etc.).

10. 7. Collapsibility 8. Fineness 9. Bench Life
Collapsibility is that property of the molding sand which determines the readiness with which the molding sand or mold,
automatically gets collapsed after the casting solidifies, and
breaks down in knock out and cleaning operations.
If the mold or core does not collapse, it may restrict free contraction of the solidifying metal and cause the same to tear or crack.
8. Fineness
Finer sand molds resist metal penetration and produce smooth casting surfaces.
Fineness and permeability are in conflict with each other and hence they must be balanced for optimum results.
Fineness and permeability, both the properties of the molding sand can be maintained by using mold coating on highly permeable mold cavity walls.
9. Bench Life
It is the ability of the molding sand to retain its properties during storage or while standing (i.e., in case of any delay).

11. 10. Coefficient of expansion
Molding sands should possess low coefficient of expansion.
11. Durability
The molding sand should possess the capacity to withstand repeated cycles of heating and cooling during casting operations.
Molding sand should be chemically immune to molten metals.
Molding sand should be reusable.

12. TYPES OF SAND MOULDS
Moulds prepared with sand are called 'sand moulds' or 'temporary moulds', as they are broken for removing the casting.
The different types of sand moulds are:
Green sand mould
Dry sand mould and
No-bake sand mould
1. Green sand mould
The word 'green‘ signifies that the moulding sand is in the moist state at the time of metal pouring.
The main ingredients of green sand are silica sand, clay and moisture (water).
Additives may be added in small amounts to obtain desired properties of mould/casting.
Nearly 60 % of the total castings are prepared from green sand moulds.

13. Advantages of green sand moulds
Preferred for simple, small and medium size castings.
Suitable for mass production
Least expensive
Sand can be reused many times after reconditioning with clay and moisture
Disadvantages
Moulds/cores prepared by this process lack in permeability, strength and stability.
They give rise to many defects like porosity, blow holes etc., because of low permeability and lot of steam formation due to their moisture content.
Moulds/cores cannot be stored for appreciable length of time.
Not suitable for very large size castings.
Surface finish and dimensional accuracy of castings produced are not satisfactory.
Difficult to cast thin and intricate shapes.
Mould erosion which is common in green sand moulds is another disadvantage.

14. 2. Dry sand mould
The word 'dry' signifies that the mould is dry or free from moisture at the time of metal pouring.
The absence of moisture makes dry sand moulds to overcome most of the disadvantages of green sand moulds.
A dry sand mould is prepared in the same manner as that of green sand mould, i.e., by mixing silica sand, clay and water.
The entire mould/core is dried (baked) in ovens to remove the moisture present in them.
Baking hardens the binder thereby increasing the strength of moulds/cores.
The temperature and duration of baking ranges from °F and from a few minutes to hours respectively depending on the type of metal being poured and size of the casting.

15. 2. Dry sand mould Advantages Disadvantages
Strength and stability of moulds is high when compared to green sand moulds.
Baking removes moisture and hence, defects related to moisture are eliminated.
Better surface finish and dimensional tolerance of castings.
Disadvantages
Consumes more time, labor and cost due to baking process. Hence, not suitable for mass production.
Not suitable for large and heavy size castings, as they are difficult to bake.
Capital cost of bake ovens.
Under baked or over baked moulds/cores is another disadvantage.

16. 3. No-Bake sand moulds
A no-bake or self-setting sand mould is one that does not require baking.
The main ingredients of no-bake sand are silica sand, binder (resin type), hardener and a catalyst or accelerator (if necessary).
The bonding strength developed in moulds/cores is by means of a self-setting chemical reaction between the binder and the hardener.
In some cases, a catalyst or an accelerator is added to speed up the chemical reaction.

17. 3. No-Bake sand moulds Advantages Disadvantages
Higher strength - about 50 to 100 times that of green sand moulds.
Patterns can be stripped within a few minutes after ramming which is not possible in both green and dry sand moulds.
Moulds/cores can be stored for longer periods.
Highly simplified moulding. Hence, reduced need for skilled labour.
Better dimensional accuracy and stability.
Improved casting quality with increased freedom from defects.
Surface finish is excellent. In many cases, castings can be used in as-cast condition without machining.
Disadvantages
Use of resins and catalysts causes lot of environmental problems both within (i.e., during mixing and pouring) and outside (dumped sand) the foundries.
Resins and catalysts are expensive.
Unsafe to human operators.
Due to high strength and hardness of moulds/cores, sand reuse is a slightly difficult process.

18. Skin-dried molds
Sands used for making skin dried molds contain certain binders like linseed oil which harden when heated.
The mold is made with the molding sand in the green condition and then the skin of the mold cavity is dried with the help of gas torches or radiant heating lamps.
Unlike dry mold, a skin dried mold is dried only up to a depth varying from 6 mm to 25 mm.
A skin-dried mold possesses strength and other characteristics in between green and dry sand molds.
If a skin-dried mold is not poured immediately after drying, moisture from green backing sand may penetrate the dried skin and make the same ineffective.

19. MOULDING SAND MIXTURE- INGREDIENS FOR DIFFERENT SAND MIXTURES
A moulding sand is a mixture of base sand, binder and additives.
ingredients of
green sand
no-bake sand mixture
dry sand mixture

20. Ingredients for Green Sand Mixture
Green sand mixture is composed of base sand, binder, moisture and additives.
Base sand
Silica sand is used as the base sand.
It possesses favorable properties, inexpensive and can be reused many number of times.
The amount of silica sand added may vary from % depending on the requirements.
Binder
Bentonite (clay binder) is the widely used binder for bonding sand particles.
It is activated in the presence of water.
A best bond between the sand particles can be obtained with Bentonite varying from % and water %.
Additives
Additives are added in small quantities to develop certain new properties, or to enhance the existing properties of moulding sand.
Sea coal, silica flour, wood flour and iron oxide are a few commonly used additives.

21. Ingredients for No-bake sand mixture
Ingredients of 'alkyd binder system' which is one of the most widely used binder system in Indian foundries is discussed below.
Base sand
Silica sand is used as the base sand.
Binder
The alkyd binder system consists of three parts: Part A (binder), Part B (hardener) and Part C (catalyst).
Part A (Binder):
The binder is an alkyd resin which is obtained by reacting linseed oil with a polybasic acid like isopthalic and solvents like turpentine, kerosene or mineral spirit to improve flowability.
Its addition ranges from % based on weight of sand.
Part B (Hardener):
The hardener is a reacted product between cobalt/lead salts and napthanic acid.
Its addition ranges from % based on weight of binder.
Part C (Catalyst):
Methylene-diphenyl-Di-isocyanate commonly known as MDI is used as catalyst to speed up the chemical reaction.
Its addition ranges from % based on weight of binder.

22. Ingredients for Dry Sand Mixture
Ingredients for dry sand mixture is similar to that of green sand.

23. Loam sand ingredients
Loam sand contains much more clay as compared to ordinary molding sand. The clay content is of the order of 50% or so.
The ingredients of loam sand may be fine sands, finely ground refractories, clays, graphite and fibrous reinforcement.
A typical loam sand mixture contains silica sand 20 volumes, clay 5 vols, and moisture 20%.
Molds for casting large bells etc., are made up of brick framework and lined with loam sand and dried.
Sweep or skeleton patterns may be used for loam molding.

24. MOLDING METHODS Various molding methods are: a) Bench molding
Floor molding
Pit molding
Machine molding
a) Bench molding
Molding is carried out on a bench of convenient height.
Small and light molds are prepared on benches.
The molder makes the mold while standing.
Both green and dry sand molds can be made by bench molding,
Molds, both for ferrous and (especially) non-ferrous castings are made on bench molds.
Both cope and drag are rammed on the bench.
b) Floor molding
Molding work is carried out on foundry floor when mold size is large and molding cannot be carried out on a bench.
Medium and large-sized castings are made by floor molding.
The mold has its drag portion in the floor and cope portion may be rammed in a flask and inverted on the drag.
Both green and dry sand moulds can be made by floor molding

25. c) Pit molding
Very big castings which cannot be made in flasks are molded in pits dug on the floor.
Very large jobs can be handled and cast easily through pit molding.
The mold has its drag part in the pit and a separate cope is rammed and used above the (pit) drag.
The depth of the drag in pit molding is much more than that in floor molding.
In pit molding, the molder may enter the drag and prepare it.
A pit is of square or rectangular shape.
The sides of the (pit) drag are lined with brick and the bottom is covered with molding sand .
The cope (a separate flask) is rammed over the pit (drag) with pattern in position.
Gates, runner, pouring basin, sprue etc. are made in the cope.
The mold is dried by means of a stove(heater) placed in the pit.
Cope and drag are then assembled. A crane may be used for lifting and positioning the cope over drag.
Cope can be clamped in position.
Mold is ready for being poured.

26. d) Machine molding
In bench, floor and pit molding, the different molding operations are carried out manually by the hands of the molder, whereas in machine molding, various molding operations like sand ramming, rolling the mold over, withdrawing the pattern etc. are done by machines.
Machines perform these operations much faster, more efficiently and in a much better way.
Molding machines produce identical and consistent castings.
Molding machines produce castings of better quality and at lower costs.
Molding machines are preferred for mass production of the castings whereas hand molding (bench, pit and floor) is used for limited production.
Machine molding is not a fully automatic process; many operations can though be performed by machines, yet some others have to be carried out by hands.
A few different types of molding machines are listed below:
Jolt machine
Squeeze machine
Jolt-squeeze machine
Sand Slinger

27. CORE
Introduction
Core is an obstruction-which when positioned in the mold, naturally does not permit the molten metal to fill up the space occupied by the core. In this way a core produces hollow castings.
Cores are required to create the recesses, undercuts and interior cavities that are often a part of castings.
A core may be defined as a sand shape or form which makes the contour of a casting for which no provision has been made in the pattern for molding.
core as a sand shape is generally produced separate from the sand mold and is then baked (hardened) to facilitate handling and setting into the mold.
Cores may be made up of sand, metal, plaster or ceramics.

28. Different Functions (Purposes) of Cores
For hollow castings, cores provide the means of forming the main internal cavities.
Cores may provide external undercut features
Cores may be employed to improve the mold surface
Cores may be inserted to achieve deep recesses in the castings.
Cores may be used to strengthen the molds
Cores may be used to form the gating system of large size molds.
Essential Characteristics of (dry sand) Cores
A Core must possess
Sufficient strength to support itself and to get handled without breaking.
High permeability to let the mold gases escape through the mold walls.
Smooth surface to ensure a smooth casting.
High refractoriness to withstand the action of hot molten metal (metal penetration etc.).
High collapsibility in order to assist the free contraction of the
solidifying metal.
Those ingredients which do not generate mold gases.

29. TYPES OF CORES Cores may be classified according to
The state or condition of core
Green sand core
Dry sand core
No bake sand core
The nature of core materials employed
Oil bonded cores
Resin bonded cores
Shell cores
Sodium silicate cores
The type of core hardening process employed
C02 process
The hot box process
The cold set process
Fluid or castable sand process
Furan-No-Bake system
Oil-No-Bake process
D. The shape and position of the core
Horizontal core
Vertical core
Hanging or cover core
Balanced core
Drop core or stop off core
Ram up core
Kiss core.

30. A. The state or condition of core
1. Green sand cores
Green sand cores are formed by the pattern itself.
A green sand core is a part of the mold.
A green sand core is made out of the same sand from which the rest of the mold has been made i.e., the molding sand.
2. Dry sand cores
Dry sand cores (unlike green sand cores )are not produced as a part of the mold.
Dry sand cores are made separately and independent of the mold.
A dry sand core is made up of core sand which differs very much from the sand out of which the mold is constructed.
A dry sand core is made in a core box and it is baked after ramming.
A dry sand core is positioned in the mold on core-seats formed by core-prints on the patterns.
A dry sand core is inserted in the mold before closing the same.
3. No-bake sand cores
The sand used for preparing no-bake core is similar to that used for making no-bake sand moulds.
Synthetic resins like phenol or urea formaldehyde are used as binder for bonding silica sand.
Certain chemicals are used as hardeners and catalysts to bring about a chemical reaction with the binder due to which bonding of sand grains takes place.

31. B. The nature of core materials employed
1. Oil bonded cores
Conventional sand cores are produced by mixing silica sand with a small percentage of linseed oil.
2. Resin-bonded cores
Phenol resin bonded sand is rammed in a core box.
The core is removed from the core box and baked in a core oven at 375 to 450°F to harden the core.
3. Sodium Silicate cores
These cores use a core material consisting of clean, dry sand mixed with a solution of sodium silicate

32. C. The type of core hardening process employed
1. hot box process
It uses heated core boxes for the production of cores.
The core box is made up of cast iron, steel or aluminium and possesses vents and ejectors for removing core gases and stripping core from the core box respectively.
Core box is heated from 350 to 500°F.
Heated core boxes are employed for making shell cores from dry resin bonded mixtures.
2. The cold set process
While mixing the core-sand, an accelerator is added to the binder.
Curing begins immediately with the addition of accelerator and continues until the core is strong to be removed from the core box.
Cold set process is employed for making large cores.
3. Castable sand process
A setting or hardening agent such as dicalcium silicate is added to sodium silicate at the time of core sand mixing.
The sand mixture possesses high flowability and after being poured in the core box, it chemically hardens after a short interval of time.

33. 2. Vertical core D. The shape and position of the core
1. Horizontal core
Fig. shows horizontal core.
A horizontal core is positioned horizontally in the mold.
A horizontal core may have any shape, circular or of some other section depending upon the shape of the cavity required in the casting.
2. Vertical core
Fig. shows a vertical core.
On the cope side, a vertical core needs more taper so as not to tear the sand in the cope while assembling cope and drag.
A vertical core is named so because it is positioned in the mold cavity with its axis vertical.
A horizontal core is supported in core seats at both ends.
Uniform sectioned horizontal cores are generally placed at the parting line.
A horizontal core is very commonly used in foundries.
The two ends of a vertical core are supported in core seats in cope and drag respectively.
A big portion of the vertical core usually remains in the drag
A vertical core is very frequently used in foundries.

34. 3. Hanging or cover core Fig. shows a hanging (cover) core
It is known as hanging core because it hangs; it is also called cover core if it covers the mold and rests on a seat made in the drag.
A simple hanging core is one which is not supported on any seat rather it hangs from the cope with the help of wires, etc.
A hanging core is supported from above and it hangs vertically in the mold cavity.
A hanging core has no support from bottom.
A hanging core is provided with a hole through which molten metal reaches the mold cavity.
Hanging cores can be made up of either green or dry sand.

35. 4. Balanced core Fig. shows a balanced core.
A balanced core is one which is supported and balanced from its one end only.
A balanced core requires a long core seat so that the core does not sag or fall into the mold.
A balanced core is used when a casting does not want a through cavity.

36. 5. Drop or stop off core Fig. shows a Drop or stop off core.
A stop off core is employed to make a cavity (in the casting) which cannot be made with other types of cores.
A stop off core is used when a hole, recess or cavity, required in a casting is not in line with the parting surface, rather it is above or below the parting line of the casting.
Depending upon its shape and use, a stop off core may also be known as tail core, saddle core, chair core, etc.

37. 6. Ram-up core A ram-up core is shown in Fig.
A ram-up core is one which is placed in the sand along with pattern before ramming the mold.
A ram-up core cannot be placed in the mold after the mold has been rammed.
A ram-up core is used to make internal or external (surface) details of a casting.

38. 7. Kiss core Kiss core is shown in Fig.
A kiss core does not require core seats for getting supported.
A kiss core is held in position between drag and cope due to the pressure exerted by cope on the drag.
A number of kiss cores can be simultaneously positioned in order to obtain a number of holes in a casting.

39. Method of making the cores
Core Making (Preparation) Procedure
Steps involved:
Core Sand Preparation
Making the Cores
Baking the Cores.
Finishing of Cores.
Setting the Cores.
1. Core Sand Preparation
The core sand of desired type (dry sand, no-bake etc.,) and composition along with additives is mixed manually or using Muller of suitable type.
2. Making The Cores
Cores are prepared manually or using machines depending on the needs.
Machines like jolt machine, sand slinger, core blower etc., are used for large scale continuous production, while small sized cores for limited production are manually made in hand filled core boxes.
A core box is similar to a pattern that gives a suitable shape to the core.
Figure shows a core box used to produce rectangular shaped cores with procedure.

40. Steps Involved in making the core
Core box is usually placed on work-bench; it is filled with already mixed and prepared core sand, is rammed by hand and the extra sand is removed from the core box.
Weak cores may be reinforced with steel wires to strengthen them.
Core box is inverted over the core plate and this transfers the core from the core box to core plate which (i.e., core) is then baked in the oven (over the core plate itself).
Larger cores can also be made manually but on the floor (and not on bench). It needs more than one man to work and the cranes may also be used, if necessary
3. Core Baking
Cores are baked in ovens in order to drive away the moisture in them and also to harden the binder thereby imparting strength to the core.
The temperature and duration for baking may vary from °F and from a few minutes to hours respectively depending on the size of the core and type of binder used.
4. Core finishing
The baked cores are finished by rubbing or filing with special tools to remove any fins, bumps, lose sand or other sand projections from its surface.
The cores are also checked for dimensions and cleanliness.
Finally, if cores are made in parts, they are assembled by using suitable pastes, pressed and dried in air before placing them in the mould cavity.

41. Core binders A core binder, core binders are of the following types
holds sand grains together
gives strength to cores
makes cores to resist erosion and breaking,
imparts adequate collapsibility to cores.
core binders are of the following types
Organic binders
Inorganic binders
Other binders.

42. Vegetable (i.e., linseed oil) Marine animal (i.e., whale oil), and
Organic Binders
1. Core oil.
They may be
Vegetable (i.e., linseed oil)
Marine animal (i.e., whale oil), and
mineral oil (used for diluting vegetable and marine animal oils)
2. Cereal binders
They are
Gelatinized starch. It is made by wet milling and contains starch and gluten.
Gelatinized corn flour.
Cereal binders contribute to green strength.
3. Water soluble binders
Dextrin, made from starch.
Molasses, etc.
4. Wood product binders
Natural resins (i.e., rosin, thermoplastic).
Sulfite binders. They contain Lignin, are water soluble compounds of wood sugars produced in the paper pulp process i.e., as a by-product of paper making.

43. Note: Inorganic binders have been discussed under 1st chapter
B. Inorganic Binders
They are
Fire clay
Bentonite
Silica flour
Iron oxide, etc.
These binders develop green strength, baked strength, hot strength and impart smooth surface finish.
They are finely pulverized materials.
They greatly increase the amount of oil necessary in oil sand mixes.
Note: Inorganic binders have been discussed under 1st chapter
C. Other Binders
Portland cement. It hardens at room temperature.
Cements (i.e., rubber cements). They harden at room temperature
Sodium silicate. The core hardens as carbon-di-oxide gas is passed through it.

44. PRINCIPLES OF GATING SYSTEM
The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity.
The gating system is composed of
Pouring cups and basins
Sprue
Runner
Gates
Risers.
Fig. shows the various components of the gating system.
Since the way in which liquid metal enters the mold has a decided influence upon the quality and soundness of a casting, the different passages for the molten metal are carefully designed and produced.
A gating system should avoid sudden or right angle changes in direction.
Sudden change in direction causes mold erosion, turbulence and gas pick-up.
If possible the gating system should form a part of the pattern

45. REQUIREMENTS, PURPOSES OR FUNCTIONS OF THE GATING SYSTEM
A Gating system should,
fill the mold cavity completely before freezing;
introduce the liquid metal into the mold cavity with low velocity and little turbulence, so that mold erosion, metal oxidation and gas pickup is prevented;
incorporate traps for the separation of non metallic inclusions which are either introduced with the molten metal or are dislodged in the gating system;
regulate the rate at which liquid metal enters into the mold;
be practicable and economical to make and;
consume least metal. In other words, the metal solidified in sprue, runner, gates and risers should be minimum because gates, risers etc., are removed from the final casting; the gating system should provide for the maximum yield.

46. GATES Characteristics
A gate is a channel which connects runner with the mold cavity and through which molten metal flows to fill the mold cavity.
A gate should feed liquid metal to the casting at a rate consistent with the rate of solidification.
The size of the gate depends upon the rate of solidification.
A small gate is used for a casting which solidifies slowly and vice-versa.
More than one gates may be used to feed a fast freezing casting.
A gate should not have sharp edges as they (i.e., edges) may break during pouring and (sand pieces) thus be carried with the molten metal into the mold cavity..
Moreover, sharp edges may cause localized delay in freezing, thus resulting in the formation of voids and inclusions in the cast objects.
A gate may be built as a part of the pattern or it may be cut in the mold with the help of a gate cutter.

47. Types of Gates The major types of gates are, Top Gate Bottom gate
Parting line side gate
1. Top gate
Fig. shows a few types of top gates.
A top gate is sometimes also called as Drop gate because the molten metal just drops on the sand in the bottom of the mold
In top gate, a stream of liquid metal impinges against the bottom of mold cavity until a pool is formed and this is kept in a state of agitation until the mold is filled.

48. Advantages of Top Gating
Simplicity for moulding.
Low consumption of additional metal.
Generation of favourable temperature gradients to enable directional solidification from the casting towards the gate which serves as riser too.
Disadvantages of Top Gating
The dropping liquid metal stream erodes the mold surface.
Dropping metal does cutting action, lifts portions of the surface and causes scab (Skin).
Splashing of molten metal associated with the liquid metal stream increase chances of oxidation.
There is lot of turbulence and pick-up of air and other gases.

49. Fig. shows a few types of bottom gates.
A bottom gate is made in the drag portion of the mold.
In a bottom gate, liquid metal fills rapidly the bottom portion of the mold cavity and rises steadily and gently up the mold walls.
Types of bottom gate are
Simple bottom gate
A horn gate
Simple bottom gate
A horn gate

50. Advantages of Bottom Gating
There is no scouring(Rubbing) and splashing in the bottom gate.
As compared to top gate, a bottom gate involves little turbulence and metal erosion.
Bottom gate produces good casting surfaces.
Disadvantages of Bottom Gating
In bottom gates, liquid metal enters the mold cavity at the bottom. If freezing takes place at the bottom, it could choke off the metal flow before the mold is full.
A bottom gate creates an unfavorable temperature gradient and makes it difficult to achieve directional solidification especially when the bottom gate has a riser at the top of the casting.
A bottom gate involves greater complexity of molding.
The liquid metal cools as it rises the mold walls and results in cold metal and cold mold near the (top) riser and hot metal and hot mold near the gate.