Expanded Polystyrene Process

• In this process, a mold of sand packed around a polystyrene foam pattern is used. This pattern will vaporize when the molten metal is poured into the mould.

• The refractory compound will provide a smoother surface on the pattern and to improve its high temperature resistance.

• Moulding sands usually include bonding agents.

• This process is also called as Lost-Foam process (or) Lost Pattern Process (or) Evaporative-Foam Process.

• The foam pattern includes risers, sprue, gating system, internal core.

• Parting lines and draft considerations are reduced.

VACCUM MOULDING

          In this process, a sand mold is held together by vacuum pressure and not by a chemical binder.

          The term vacuum in this process refers to the making of the mould, rather than the casting operation. Casting operation is same as any other process.

Advantages :

(1) No binders are used and hence sand is readily recovered in vacuum molding.

(2) Mechanical ramming is not required.

(3) Since no water is mixed with the sand, moisture related defects are absent from the product.

Disadvantages :

(1) Relatively slow and not readily adaptable to mechanization.

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.,