INDUSTRIAL
TRAINING REPORT
Training
Taken at:
LAKSHMI
MACHINE WORKS Ltd, COIMBATORE
&
SIMTA Ltd,
COIMBATORE
SUBMITTED
BY
SATHYA PRAKASH.G
2K2-177
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
KURUKSHETRA
CLEANING:
The final step during metal casting is cleaning. Cleaning generally refers to the removal of all materials that are not part of the finished casting. Rough cleaning is the removal of the gating systems from the casting. This operation is called shakeout. This shakeout can be affected either manually or mechanically, but generally mechanical shakeouts are used for large-scale work here. After that, initial finishing removes any residual mould or core sand that remains on the piece after it is free of the mould. Trimming removes any superfluous metal, and in the last stages of finishing the surface of the casting is cleaned for improved appearance. This operation of cutting off the unwanted parts, cleaning and finishing the casting is known as fettling.
Some of the cleaning processes are:
v 4MT/3MT/1.5MT automatic temperature controlled heat treatment furnaces with water quenching facilities.
v 1000 kgs/ 500 kgs hook hanger automatic shot blasting machines.
v Pickling/passivation facilities
v Electrically operated high speed grinders
v Welding rectifiers.
This process consists of the following steps.
1.Removal of cores from the castings.
2.Removal of gates, risers, runners, etc. from the castings.
3.Removal of fins, and other unwanted projections from the castings.
4.Removal of adhering sand and oxide scale from the surface of the castings (surface cleaning)
5.Repairing castings to fill up blowholes, straightening the warped or deformed castings.
Defects in the castings:
Sand castings particularly are subject to certain defects, which in a well designed casting, are controllable by proper foundry technique, but are not wholly preventable. However the common types of defects found in the castings, their causes and remedies are explained here under:
Shifts:
This is an external defect in a casting caused due to core misplacement or mis- matching of top and bottom parts of the casting usually at a parting line. Mis-alignment of flasks is another likely cause of shift. The defect can be prevented by ensuring proper alignment of the pattern or die part, moulding boxes, correct mounting of the patterns on pattern plates and checking the flasks, locating pins, etc. before use.
Warpage:
Warpage is unintentional and undesirable deformation in a casting that occurs during or after solidification. Due to different rates of solidification in different sections of a castings, stresses are set up in adjoining walls resulting in warpage in these areas. Large and flat sections or intersecting sections such as ribs are particularly prone to warpage. The remedy is to produce large areas with wavy, corrugated construction, or add sufficient ribs or rib-like shapes, to provide equal cooling rates in all areas. A proper casting design can go a long way in reducing the warpage of the casting.
Fin:
A thin projection of metal not intended as a part of the casting is called the fin. Fins usually occur at the parting of the mould or core sections. Moulds and cores incorrectly assembled with cause fins. Insufficient weighting of the moulds, or improper clamping of flasks may again produce the fin.
Blow Holes:
Blow Holes are smooth, round holes appearing in the form of a cluster of a large number of small holes below the surface of a casting. These are entrapped bubbles of gases with smooth walls. Blow Holes are caused by excessive moisture in the sand, or when permeability of sand is low, sand grains are too fine, sand is rammed too hard or when venting is insufficient. To prevent blow holes, the moisture content in sand must be well adjusted, sand of proper grain size should be used, ramming should not be too hard, and venting should be adequate.
Sand Holes:
Sand holes are found on external surface or inside the casting. They are caused by loose sand washing into the mould cavity and fusing into the interior of the casting or rapid pouring of the molten metal. Proper cleaning of the mould and careful pouring of the molten metal prevent sand holes.
Pin holes:
Pinholes are numerous small holes, usually less than 2mm, visible on the surface of the casting cleaned by shot blasting. They are caused by sand with high moisture content, absorption of hydrogen or carbon monoxide gas or when steel is poured from wet ladles or is not sufficiently gasified. Using good melting and fluxing practices, by reducing the moisture content of moulding sand and increasing its permeability, and by promoting a rapid rate of solidification can minimize the defect.
Hot Tears:
They are internal or external cracks having ragged edges occurring immediately after the metal has solidified. Hot tears may be produced if the casting is poorly designed and abrupt sectional changes take place, no proper fillets and corner radii are provided, and chills are wrongly placed. Incorrect pouring temperatures and improper placement of gates and risers and hard ramming can also create hot tears. Improved design, proper directional solidification, even rate of cooling, correct pouring temperatures, and control of mould hardness eliminate hot tears.
Cold Shut and Misrun:
A cold shut is an external defect formed due to imperfect fusion of two steams of metal in the mould cavity or unequal sections of pattern assembled together. The defect may appear like a crack or seam with rounded edges. A misrun casting is one, which lacks completeness due to the failure of the metal to fill the mould cavity. The reasons for cold shut or misrun may be too thin sections and wall thicknesses, improper gating system, damaged patterns, slow and intermittent pouring, poor fluidity of metal caused by low pouring temperature, improper alloy composition, etc. Use of hotter metal, frequent inspection and replacement of patterns and core boxes and proper design of the casting keeping in mind the fundamental principles of gating and risering are some of the steps that may be used to eliminate cold shut and misrun.