Sand Casting Defects:
• Production of castings involves a large number of steps including casting design, pattern making, moulding, melting, pouring, shake out, fettling, inspection and finishing.
• It is not uncommon for one or more of these steps to be performed unsatisfactorily due to use of defective material or equipment, carelessness of the operator or lack of skill.
• Such unsatisfactory operations result in a defective casting which may be rejected at the final stage.
• Since reclamation of defective castings is often costly and sometimes outright impossible, care should be taken to avoid the occurrence of the defects in the first instance.
• It is therefore necessary to understand the various defects that occur in sand castings and the main factors that are responsible for their occurrence.
• Some of the common defects are described below.
1. Open Blows and Blow Holes
2. Pin Hole Porosity
3. Entrapped Air and other gases
4. Cracked Casting
5. Bent or Twisted Casting
6. Dropped Mould
9. Run out
11. Mis-run and Cold Shut
13. Rat Tail and Buckles
14. Core Shift
16. Cuts and Washes
17. Metal penetration
18. Hard Spots
20. Hot tears
Open Blows and Blow Holes:
• Open blows are smooth cavities or voids on the surface of the casting. Blow holes are bubbles of gas entrapped inside the casting. Both are caused by gases carried by hot metal.
• Blow holes are also caused by poor permeability of sand, too fine sand grains, over rammed moulds or insufficient venting.
Blow holes can be controlled by controlling moisture content, providing proper venting, not too hard ramming of the mould and controlling binder content.
Pin Hole Porosity:
• This is porosity caused by gases absorbed by the molten metal.
• Practically all metals absorb oxygen, hydrogen and nitrogen.
• Oxygen and nitrogen form oxides and nitrides respectively. Hydrogen is responsible for causing pin hole porosity.
• Molten metal picks up hydrogen from fuel, moisture in air and moulds.
• As the metal solidifies solubility of hydrogen decreases considerably. The hydrogen thus comes out forming a number of small holes distributed throughout the metal.
• Pin hole porosity is quite common in aluminium alloys.
• Pin hole porosity can be minimized by using improved melting and pouring methods are by prompting a rapid rate of solidification.
Entrapped Air and other gases:
• Air or other mould gases are sometimes entrapped within the metal due to turbulent flow of metal during pouring.
• When the metal solidifies this air or gas remains in the metal as a void.
• This defect can be eliminated by providing proper gating systems and avoiding turbulence.
• These are cracks caused by the inability of the casting to cope up with internal stresses resulting from solid shrinkage of the metal.
• Immediately after solidification some metals have a low strength and if stresses set up are high, cracks develop in the casting.
• Cracks can be avoided by proper design of the casting, avoiding joining of unequal sections, providing round corners etc.
Bent or Twisted Casting:
• When a casting has wide variation in its sections, internal stresses are often set up in it due to differential cooling in the thick and thin areas. This leads to casting warpage.
• The remedy is to provide suitable allowances on the pattern, controlling deformation by providing suitable ribs etc. and plan for uniform cooling rate.
• This defect occurs due to a portion of the mould breaking and dropping into the metal.
• This may occur when the sand mix has a low green strength or it has not been properly rammed or reinforced.
• It appears as an irregular deformation of the casting.
• It can be avoided by proper ramming and reinforcements of the mould.
• When the mould sand does not have enough refractoriness or the metal is poured at very high temperature or the facing sand is of poor quality, the sand may melt and fuse with casting surface.
• This makes it difficult to clean the castings and gives them a rough glossy appearance.
• Using sand of enough refractoriness and pouring at suitable temperature can eliminate this defect.
• A swell is an enlargement or bulging of the casting surface resulting from liquid metal pressure.
• It occurs due to poor ramming of the mould or not properly reinforcing deep moulds.
• Swells can be avoided by proper ramming of the sand and providing adequate support to the mould.
• A run out occurs when the molten metal leaks out of the mould during pouring. This is caused by defective moulding boxes, faulty clamping and weighting and by casting too near the edge of the box.
• Run out is avoided by modifying the moulding system, providing proper parting line and providing proper gating system.
• A mismatch is caused by the cope and drag parts of the mould not remaining in their proper position.
• This is caused by loose box pins, inaccurate pattern dowel pins or carelessness in placing the cope on the drag.
• The deficiencies mentioned above must be attended to if mismatch is to be avoided.