TYPES OF CASTING DEFECTS

          Casting defects are the unwanted irregularities that appear in the casting during metal casting process. Casting process involves a number of variables and a loss of control in any of these variables can cause defects under certain circumstances. Some of the common defects, their features and remedies to prevent such defects are described below:

1. Porosity (Blow-holes and pin holes):

          They appear as small holes in the casting. They may be open to surface (or) they may be below the surface of the casting. They are caused due to entrapped bubbles of gases. Molten metal absorb gases from various sources such as fluxes, moisture in sand , additives and normal atmosphere gasses and nitrogen. They may be causes by excessively hard ramming, improper venting, excessive moisture (or) lack of permeability in the sand.

Remedies:

1) Avoid excess ramming of mould.

2) Provide proper vent.

3) Avoid use of excess carbonaceous (or) other organic material in the sand/core binders, because these materials react with the molten metal producing large amount of gasses.  

2. Shrinkage cavity:

Some times due to faulty design of casting consisting of very thick and thin sections, a shrinkage cavity may be caused at the junction of such sections. Shrinkage cavity is totally internal. It is illustrated in Fig., It is caused due to shrinkage of molten metal. It is sometimes called as 'Pipe'.

Remedies: 

1) Use large sprue and riser to promote directional solidification.

2) Use either a chill (or) relocation of risers. (Chill is an object used to promote solidification in a specific portion of a metal casting).

3) Gates to be cut as as wide as possible.

3. Misrun:

This denotes incomplete filling of mould cavity. It may be caused by bleeding of molten metal at the parting of cope and drag, inadequate metal supply (or) improper design of gating.

Remedies:

1) Fluidity of the metal should be high.

2) Pouring rate and time should be controlled.

3) Thin sections should be suitable designed.

4) Pouring temperature should be high.

4. Cold shut:

A cold shut is formed within a casting, when molten metal from two different streams meets without complete fusion. Low pouring temperatures may be the primary cause of this defect.

Remedies:

1) Place gates and risers at proper locations.

2) Metal fluidity should be high.

3) Proper pouring temperature of metal should be maintained.

 5. Mismatch:

This defect takes place when the mould impression in the cope and drag do not sit exactly on one another but are shifted a little bit. This happens due to mismatch of the split pattern (dowel pin may have become loose) (or) due to defective clamping of cope and drag boxes.

This defect is of two types (1) Core shift, (2) Mould shift.

Remedies:

1) Proper alignment of cope and drag box.

2) Proper handling of assembled cope and drag boxes during operations.

3) Proper dowel pins are made in split pattern ( they not to be loose).

6. Drop:

This happens when a portion of the mould sand falls into the molten metal. Loose sand inadequately rammed (or) lack of binder may cause this defect.

7. Scab:

This defect occur when a portion of the face of a mould lifts (or) breaks down and the recess is filled up by molten metal.

8. Hot tear:

These cracks are caused in thin long sections of the casting, if the part of the casting cannot shrink freely on colling due to intervening sand being too tightly packed, offers resistance to such shrinking. The tear (or) crack usually takes place when the part is red hot and has not developed full strength, hence the defect is called "Hot Tear". Reason may be excessively tight ramming of sand.

Remedies:

1) Provide adequate fillets at sharp corners.

2) Proper metallurgical and pouring temperature.

3) Place gates and risers at proper locations.

4) Proper mold design to be maintained.

9.Penetration:

When fluidity of liquid metal is high, it may be penetrated into the sand mould/core (into the voids between the sand particles) causing a fused aggregate of metal and sand on the surface of the casting leading to defect.

Remedies:

1) Sand should be properly rammed.

2) Mould sand/core sand should not be too coarse to promote metal penetration.

3) Control proper metal temperature.

10.Sand wash:

Surface dip that results from erosion of the sand mold during pouring. This counter is formed in the surface of the final cast part.

11) Other defects include scars, blisters, sponginess (due to a mass of pin holes at one location) and slag inclusions etc.,

   

MOULDING MACHINES

          Moulding processes may be classified as Hand moulding (or) Machine moulding moulding according to whether the mould is prepared by hand tools (or) with the aid of some moulding machines. When large number of castings to be produced, hand moulding consumes more time, labour and also accuracy and uniformity in moulding varies. To overcome this difficulty, machines are used for Moulding.

Based on Ramming methods Moulding machines are classified as follows:

(1) Jolt Machine,

(2) Squeeze machine,

(3) Jolt-Squeeze machine,  and 

(4) Sand slinger.

 

(1) Jolt machine:

          A Jolt machine consists of a flat table mounted on a piston cylinder arrangement and can be raised (or) lowered by means of compressed air.

          In operation, the mould box with the pattern and sand is placed on the table. The table is raised to a short distance and then dropped down under the influence of gravity against a solid bed plate. The action of raising and dropping is called Jolting .

          Jolting causes the sand particles to get packed tightly above and around the pattern. The number of jolts may vary depending on the size and hardness of the mould required. Usually, less than 20 jolts are sufficient for a good moulding.

          The disadvantage of this type is that, the density and hardness of the rammed sand at the top the mould box is less when compared to its bottom portion. 

(2) Squeeze machine:

          A Squeeze machine is very useful for Shallow patterns. In squeeze machine, the mould box with pattern and sand in it is placed on a fixed table as shown in fig., A flat plate (or) rubber diapharm is brought in contact with the upper surface of the loose sand and pressure is applied by a Pneumatically operated piston. The squeezing action of the plate causes the sand particles to get packed tightly above and around the pattern.

          Squeezing is continued until the mould attains the desired density. In some machines, the squeeze plate may be stationary with the mould box moving upwards.

Moulding force (Mf) = P (π. d²/4)-W

Where P - Pressure in squeezing cylinder,

              d - Piston diameter

              w - Weight of flask pattern and sand.

          The disadvantage of squeeze machine is that, the density and hardness of the rammed sand at the bottom of the mould box is less when compared to its top portion.

(3) Jolt squeeze machine:

          Jolt-Squeeze machine combines the operating principles of 'Jolt' and 'Squeeze' machines resulting in uniform ramming of the sand in all portions of the moulds.

          The machine makes use of a match plate pattern placed between the cope and drag box. The whole assembly is placed on the table with the drag box on it. The table is actuated by two pistons in air cylinders, one inside and the other. One piston called as Jolt piston rises and drops the table repeatedly for a predetermined number of times, while the other piston called Squeeze piston pushes the table upwards to squeeze the sand in the flask against the squeeze plate. In operation, sand is filled in the drag box and jolted repeatedly by operating the Jolt piston.

          After jolting, the complete mould assembly is rolled over by hand. The cope is now filled with sand and by operating the squeeze piston, the mould assembly is raised against the squeeze plate. By the end of this operation, the sand in the mould box is uniformly packed. The match plate is now vibrated and removed. The mould is finished and made ready for pouring.

(4) Sand slinger:

          A Sand slinger is an automatic machine equipped with a unit that throws sand rapidly and with great force into the mould box. The figure shows a sand slinger. The sand slinger consists of a rigid base, a sand bin, a bucket elevator, a swinging (or) movable arm, a belt conveyor and the sand impellor (ramming head). Prepared sand lying in the sand bin is packed up by the elevator buckets and is dropped on to the belt conveyor which takes the same to the impellor head.

          Inside the impellor head, rapidly rotating cup shaped blade picks up the sand and throws it downwards into the moulding box as a continues stream of sand with machine gun rapidly and great force.

          The sand is discharged into the moulding box at a rate of 300 to 2000 kg/min. This force is great enough to ram the mould satisfactorily. The density of the ramming sand can be controlled by varying the speed of the impellor. Rest of the operations, viz., removal of pattern, cutting gates etc., are done manually.

       In moulding boxes, sand is filled and rammed at the same time. The density of sand which is the result of sand's inertia is uniform throughout the mould. In the initial stages of ramming, the blades are rotated at slow speeds, around 1000-2000 rpm to avoid damage to the pattern due to the abrasive action of the high velocity sand particles. 

SHAFT SLIDING SCREW PRESS DEHYDRATOR

This dehydrator, developed by integrating a  shaft sliding mechanism with a screen type thickener and a traditional dewatering screw press.

PROBLEMS ON AVERAGE - 1

Examples 1: what will be the average of 11, 12, 13, 14, 15, 16, 17 series?

Solution: The common difference is same i.e., 1, so the series is in A.P. Average is the middle term when the number of terms is odd, So the middle term is 14 which is our average of the series.

Example 2: What will be the average of 11, 12, 13, 14, 15, 16, 17, 18?

Solution: Since the common difference is same for each of the term we can say that the series is in A.P. So we have discussed that when the number of terms are even then the average will be the average of two middle terms.

Now the two middle terms are 14 and 15, and the average is (14+15)/2 = 14.5

Example 3: The average of five numbers is 29. If one number is excluded the average becomes 27. What is the excluded number ?

Solution: let the excluded number is

= (29 x 5) – ( 27 x 4 )

= 145 – 108

= 37 .

Example 4: Find the average of first 20 natural numbers?

Solution: 

Sum of first n natural numbers = n ( n + 1 ) /2

So, we can find easily average of first 20 natural numbers 20 x 21 / 2 = 210 

So, then Required average is = 210 / 20 = 10.5.

Example 5 : Find the average of first 20 multiplies of 5 .

Solution: 

Required average = 5 ( 1 + 2 + 3 +……………….. + 20) /20

= ( 5 x 20 x 21 / 20 x 2) = 2100 / 40 = 52.5 .

So the Required average is 52.5.