Q.6. Explain the following with neat sketches :
(i) Extrusion moulding
(ii) Blow moulding (AKTU - 2012 - 13)
Sol. Extrusion moulding :- Practically all thermoplastic materials can be extruded into various shapes like tubes, rods, sheets, films, pipes, ropes and other profiles. Thermosetting plastics are, generally, not suitable for extrusion. A sectional view showing the extrusion process in shown in figure. The process consists of feeding the powdered plastic from the hopper into the heated chamber. A rotating screw called feed screw, carries this material forward and forces it out through the heated orifice of the die. The orifice carries the shape corresponding to the shape desired of the product. After leaving the die, the product is suitably cooled by water or air - blast and carried away by a running belt.
Blow Moulding :- It is used for producing narrow neck plastic containers, like bottles and similar other articles. An example showing the production of a screwed neck plastic bottle is illustrated in figure. In this process a heated closed end Thermoplastic Tube is placed in the mould and air pressure is applied to inflate it to acquire the shape of mould cavity. After the plastic is cooled the mould is opened and the product taken out.
Q.7 What do you mean by core and define its requirement.
Ans. Core: - (AKTU - 2008 - 09)
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, but are also used in injection molding.
An example of the use of cores is in the casting of engine blocks. For example, one of the GM V-8 engines requires 5 dry-sand cores for every casting.
Requirements: -
There are seven requirements for core:
1. In the green condition there must be adequate strength for handling.
2. In the hardened state it must be strong enough to handle the forces of casting; therefore the compression strength should be 100 to 300 psi (0.69 to 2.1 MPa).
3. Permeability must be very high to allow for the escape of gases.
4. As the casting or molding cools the core must be weak enough to break down as the material shrinks. Moreover, they must be easy to remove during shakeout.
5. Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.
6. A smooth surface finish.
7. A minimum generation of gases during metal pouring.
Q.8 How the mould is made with the use of a core? Explain in brief. (AKTU - 2008-09)
Ans. Cores are used as part of the mould for getting holes and cavities in the final casting. There are a large number of products in which holes and cavities can not be produced by other methods except casting with cores. Following figure shows the various methods of fabrication.
Dry and green sand cores are used for producing holes and cavities in the casting. Cores are made independent from the mould and then inserted into core prints to hold them in proper position. Dry sand cores can be made in several ways. In each way, sand, mixed with binder is packed in a wood or metal core box which contains the hole or cavity of desired shape. A dump core box gives a simple approach. Sand is packed in the box and scraped level with the top surface. A plate, wooden or metallic, is then placed on the top of box and the box is then turned over and lifted is upward direction, leaving the moulded sand resting on plate. After baking process, core segments are joined with glue or other bonding material. Irregular and rough spots along parting line are removed with files and the final products is given a thin coating to provide a smooth surface and resistance to heat.
Q.9 Describe gating system in brief.
Related Questions -
Q. What is the function of riser in casting process? (AKTU - 2011 - 12)
Ans. The term gate is defined as a channel which leads into mould cavity. A gating system involves all the passages through which the molten metal enters the mould cavity.
A gating system is made up of :
(i) Pouring basin (ii) Sprue
(iii) Runner (iv) Gate
The functions of a gating system are :
(i) It is used to provide continuous and uniform supply of molten metal into mould cavity with least turbulence. Excessive heads to “aspiration of air” and “dross formation”.
(ii) It supplies the casting with molten metal at best location to achieve proper directional solidification and optimum feeding of shrinkage cavity.
(iii) It fills the mould cavity with liquid metal in minimum time to avoid temperature gradient.
(iv) It prevents erosion of mould walls.
(v) It prevents slag, sand and other foreign matters from entering the mould.
Description of Components of a Gating System: -
Pouring Basin [or cup]: -
This component of gating system is made on the top of mould. It has a funnel like shape. The purpose of pouring cup is to direct the flow of metal from ladle to sprue in order to maintain the required rate of molten metal flow and also to reduce turbulence and vortex at the entrance of sprue.
Sprue: -
The vertical or tapered passage between the pouring cup and runner is known as sprue. It facilitates the feeding of metal to runner which finally reaches the cavity through gate. Sprue can have a square, rectangular or circular cross section. Sprue is generally tapered downward to avoid aspiration of air and destruction of metal.
Sprue Base: -
The lower most portion of spure is called sprue base or sprue well.
Runner: -
In larger castings, liquid metal is generally carried from sprue base to several gates around the cavity by means of a passage way called runner. A runner is generally preferred in drag but it can sometimes be located in cope also depending upon the shape and size of casting. It should be well streamlined to avoid aspiration of air and excess turbulence.
Gate: -
It is a channel through which the liquid metal flows from runner to the mould cavity. Gates should be located where they can be easily removed without causing any damage to the casting.
Riser: -
A riser, also known as feeder head, is a channel of sand made in the cope to allow the liquid metal to rise above the highest point in the casting after the mould cavity is filled up. A riser compensates for solidification shrinkage and also promote directional solidification. It indicates that the mould cavity is filled with the liquid metal.
Main requisites of a good riser are as follows :
(i) It serves as a feeder to feed the molten metal into the main casting to compensate for its volumetric shrinkage during solidification.
(ii) It facilitates escaping of the steam, gas and air from the mould cavity while filling the mould with the molten metal.
(iii) It enables the pourer to see the metal as it falls into mould cavity. If the metal does not appear in the riser it indicates that either the metal is insufficient to fill the mould cavity or there is some obstruction to the metal flow between the sprue and riser.
(a) Time for solidification of metal in riser should be greater than in mould cavity.
(b) A riser should derive adequate feeding pressure by atmospheric pressure or metallostatic pressure.
(c) Design of riser should be such that it any establish temperature gradient suitable for directional solidification.
(i) Extrusion moulding
(ii) Blow moulding (AKTU - 2012 - 13)
Sol. Extrusion moulding :- Practically all thermoplastic materials can be extruded into various shapes like tubes, rods, sheets, films, pipes, ropes and other profiles. Thermosetting plastics are, generally, not suitable for extrusion. A sectional view showing the extrusion process in shown in figure. The process consists of feeding the powdered plastic from the hopper into the heated chamber. A rotating screw called feed screw, carries this material forward and forces it out through the heated orifice of the die. The orifice carries the shape corresponding to the shape desired of the product. After leaving the die, the product is suitably cooled by water or air - blast and carried away by a running belt.
Blow Moulding :- It is used for producing narrow neck plastic containers, like bottles and similar other articles. An example showing the production of a screwed neck plastic bottle is illustrated in figure. In this process a heated closed end Thermoplastic Tube is placed in the mould and air pressure is applied to inflate it to acquire the shape of mould cavity. After the plastic is cooled the mould is opened and the product taken out.
Q.7 What do you mean by core and define its requirement.
Ans. Core: - (AKTU - 2008 - 09)
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, but are also used in injection molding.
An example of the use of cores is in the casting of engine blocks. For example, one of the GM V-8 engines requires 5 dry-sand cores for every casting.
Requirements: -
There are seven requirements for core:
1. In the green condition there must be adequate strength for handling.
2. In the hardened state it must be strong enough to handle the forces of casting; therefore the compression strength should be 100 to 300 psi (0.69 to 2.1 MPa).
3. Permeability must be very high to allow for the escape of gases.
4. As the casting or molding cools the core must be weak enough to break down as the material shrinks. Moreover, they must be easy to remove during shakeout.
5. Good refractoriness is required as the core is usually surrounded by hot metal during casting or molding.
6. A smooth surface finish.
7. A minimum generation of gases during metal pouring.
Q.8 How the mould is made with the use of a core? Explain in brief. (AKTU - 2008-09)
Ans. Cores are used as part of the mould for getting holes and cavities in the final casting. There are a large number of products in which holes and cavities can not be produced by other methods except casting with cores. Following figure shows the various methods of fabrication.
Dry and green sand cores are used for producing holes and cavities in the casting. Cores are made independent from the mould and then inserted into core prints to hold them in proper position. Dry sand cores can be made in several ways. In each way, sand, mixed with binder is packed in a wood or metal core box which contains the hole or cavity of desired shape. A dump core box gives a simple approach. Sand is packed in the box and scraped level with the top surface. A plate, wooden or metallic, is then placed on the top of box and the box is then turned over and lifted is upward direction, leaving the moulded sand resting on plate. After baking process, core segments are joined with glue or other bonding material. Irregular and rough spots along parting line are removed with files and the final products is given a thin coating to provide a smooth surface and resistance to heat.
Q.9 Describe gating system in brief.
Related Questions -
Q. What is the function of riser in casting process? (AKTU - 2011 - 12)
Ans. The term gate is defined as a channel which leads into mould cavity. A gating system involves all the passages through which the molten metal enters the mould cavity.
A gating system is made up of :
(i) Pouring basin (ii) Sprue
(iii) Runner (iv) Gate
The functions of a gating system are :
(i) It is used to provide continuous and uniform supply of molten metal into mould cavity with least turbulence. Excessive heads to “aspiration of air” and “dross formation”.
(ii) It supplies the casting with molten metal at best location to achieve proper directional solidification and optimum feeding of shrinkage cavity.
(iii) It fills the mould cavity with liquid metal in minimum time to avoid temperature gradient.
(iv) It prevents erosion of mould walls.
(v) It prevents slag, sand and other foreign matters from entering the mould.
Description of Components of a Gating System: -
Pouring Basin [or cup]: -
This component of gating system is made on the top of mould. It has a funnel like shape. The purpose of pouring cup is to direct the flow of metal from ladle to sprue in order to maintain the required rate of molten metal flow and also to reduce turbulence and vortex at the entrance of sprue.
Sprue: -
The vertical or tapered passage between the pouring cup and runner is known as sprue. It facilitates the feeding of metal to runner which finally reaches the cavity through gate. Sprue can have a square, rectangular or circular cross section. Sprue is generally tapered downward to avoid aspiration of air and destruction of metal.
Sprue Base: -
The lower most portion of spure is called sprue base or sprue well.
Runner: -
In larger castings, liquid metal is generally carried from sprue base to several gates around the cavity by means of a passage way called runner. A runner is generally preferred in drag but it can sometimes be located in cope also depending upon the shape and size of casting. It should be well streamlined to avoid aspiration of air and excess turbulence.
Gate: -
It is a channel through which the liquid metal flows from runner to the mould cavity. Gates should be located where they can be easily removed without causing any damage to the casting.
Riser: -
A riser, also known as feeder head, is a channel of sand made in the cope to allow the liquid metal to rise above the highest point in the casting after the mould cavity is filled up. A riser compensates for solidification shrinkage and also promote directional solidification. It indicates that the mould cavity is filled with the liquid metal.
Main requisites of a good riser are as follows :
(i) It serves as a feeder to feed the molten metal into the main casting to compensate for its volumetric shrinkage during solidification.
(ii) It facilitates escaping of the steam, gas and air from the mould cavity while filling the mould with the molten metal.
(iii) It enables the pourer to see the metal as it falls into mould cavity. If the metal does not appear in the riser it indicates that either the metal is insufficient to fill the mould cavity or there is some obstruction to the metal flow between the sprue and riser.
(a) Time for solidification of metal in riser should be greater than in mould cavity.
(b) A riser should derive adequate feeding pressure by atmospheric pressure or metallostatic pressure.
(c) Design of riser should be such that it any establish temperature gradient suitable for directional solidification.