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What are the three stages of shrinkage during casting?

There are three distinct stages of shrinkage as molten metal alloys solidify:

  • liquid shrinkage.
  • liquid-to-solid shrinkage.
  • solid shrinkage.

Why do most metals shrink when they are cast?

Solidification shrinkage occurs because metals are less dense as liquids than solids. Risers and chills are used to address this problem. Risers feed a continuous supply of molten metal to the casting as it solidifies. They help to equalise the rate of solidification in thicker areas of the metal casting.

Is there any difference in the tendency for shrinkage void formation in metals with short and long freezing ranges respectively explain?

If the metal has a short freezing range, the part is more likely to have large shrinkage cavities in the are near the solidification center. If gates and risers are properly placed, the metal with the shorter freezing range will be more controllable to prevent shrinkage.

What are the benefits and drawbacks to having a pouring temperature that is much higher than the metal’s melting temperature?

A pouring temperature that is much higher than the metal’s melting temperature significantly reduces the likelihood that the metal will solidify in the mold. This makes molds easier to design because even small cross sections in detailed molds can be completely filled.

Why can blind risers be smaller than open top risers?

A blind riser is less prone to this phenomenon, as it is in contact with the mold on all surfaces. Thus, it is slower to cool since the mold increases in temperature and the riser can be located in an area that will cool more slowly; thus, a blind riser may be made smaller.

What is the ideal shape of sprue?

parabolic taper

How can we eliminate the shrinkage cavity?

Prevent shrinkage cavities by improving casting structure

  1. Design a running (gate) system with risers that ensure a continuous flow of molten metal.
  2. Increase local heat dissipation by inserting internal chills, cooling ribs or cooling coils.
  3. Reduce casting temperature to limit the total volume deficit.

What is the function of riser and which shrinkage does riser take care of?

A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Most metals are less dense as a liquid than as a solid so castings shrink upon cooling, which can leave a void at the last point to solidify.

What is the difference between runner and riser?

In a casting both runner and riser is used to pass the molten metal into the mould cavity. The main difference is that runner is a horizontal pathway into the mould cavity whereas riser is a vertical pathway . Riser is of two types open riser and blind riser .

How do you calculate riser size?

Sample calculation of a staircase that should be 2.60 meters high

  1. Calculate the number of steps that will be needed. Considering an ideal riser of 18 cm, the height of the space is divided by the height of each step.
  2. Calculate the height of each riser.
  3. Calculate the width of the tread.

What is the general shape of an open riser?

cylindrical

Which material required largest size of riser?

Steel

What is the function of runner and riser?

Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern. Patterns properly made and having finished and smooth surfaces reduce casting defects.

Why riser is not used in die casting?

Using risers would of course slow the cooling time, and therefore they are economically undesirable. Further, the metals that are used in die casting will therefore be ones that develop internal shrinkage porosity, but do not separate from the mold wall, so that risers are not as necessary.

What is a gate in casting?

Gate – The portion of the runner where the molten metal enters the mold cavity. Gates are formed into a mold extraneous of the desired final product and must be removed by machining once casting has cooled. Gates are arranged so molten metal is fed to the casting at a rate consistent with the rate of solidification.

Why the sprue hole is tapered?

Sprue – a hole where the metal enters the mold. It is ideally tapered to prevent air from entering the casting. Because the riser is larger, it will cool slower and feed liquid metal to the shrinking casting, thus preventing a number of defects.

Why is sprue tapered?

In many cases it controls the flow of material into the mold. During casting or molding, the material in the sprue will solidify and need to be removed from the finished part. It is usually tapered downwards to minimize turbulence and formation of air bubbles.

Why sprue design is made tapered instead of straight?

Explanation: Pressure balance is maintained, to avoid the negative pressure (to ensure positive pressure anywhere in the liquid column), the sprue should be tapered. The ideal shape of the sprue is parabola but to reduce the manufacturing difficulty shape of the sprue is considered as tapered cylinder.

What is sprue puller?

[′sprü ‚pu̇l·ər] (engineering) A pin with a Z-shaped slot to pull the sprue out of the sprue bushing in an injection mold.

What is ejector pin?

Ejector pins are the “bouncers” of the injection molding world. They apply a force to eject a part from the mold, and in some cases can leave marks. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, allowing the mold to close and be refilled.

Which of the following is casting defect?

If the pouring temperature is too high or a sand of low melting point is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance. A run out occurs when the liquid metal leaks out of the mould because of a faulty mould or flask.

Which of the following statement is true casting is the replica of the object to be cast?

Which of the following statement is true? Explanation: Pattern is the replica of the object. Except for the core prints and allowances, the pattern exactly resembles the casting. 2.

Which of the following pattern is cheapest?

Right Answer is: D Skeleton pattern uses the least amount of material for the operations on pattern and therefore, it does not need a lot of investment on its materials, making it inexpensive.