CASTING AND WELDING
Manufacturing process :-It is process of converting raw material into a finished product It is a process of value addition to the raw material Classification :-1) casting -- zero process ( casting ), 2) Forming - Additive process ( Rapid prototype ), 3) Fabrication process -- subtractive process, 4) material removal process -- subtractive process. Casting :-It is process in which molten liquid metal will be allow to solidify in a pre defined mould cavity. ORE = RAW material of casting1) pattern 2) mounding sand 3) tools Mould box :-Two box, Three box Advantage 1) complex shape of object produce ,2) It is simple and leas expensive process ,3) Ductile and brittle material can be produced, 4) large size object can produce by casting only, Ex. Machine tool beds , engine blocks, gear box housing , Road rollers. Limitations :- Poor surface finish due to liquid metal is contact with grains of mounding sand Ra = 10.15 um Due to permeability of mounding bond (press different ) gas defects are more so casting is not used for domestic cylinder Bcs of non - uniform condition. 1) casting objects are not having smooth surface finish 2) it is laborious and time consuming process 3) there is a possibility of gas defect in the casting 4) casting object are not having uniform mechanical properties due to non uniform cooling.
Selection of the manufacturing process will depends on :- 1) shape and size of the object 2) accuracy and surface finish required 3) properties required by the object 4) cost of the object Pattern :- It is the replica of casting to be produce with some allowance Difference allowances, Shrinkage and contraction ,Draft or taper allowance , Shake or rapping ,Finish or machining allowance, Distortion or camber Riser :- It is also called reservoir of liquid molten metal Surface area of riser is minimum
1)Shrinkage allowance :- When the liquid metal will be allow to solidly in cavity there is a possibility of Shrinkage of the material due to this size of the casting will be decrease to overcome this Shrinkage allowance can be provided. When the liquid metal is cooled from pouring to freezing temperature the Shrinkage is liquid Shrinkage During phase transformation Shrinkage of the material is solidification Shrinkage When the solid casting is cooled from freezing to ambient temperature Shrinkage is solid Shrinkage Liquid and solidification Shrinkage can be compensated by providing a riser , this value are expressed as % of Shrinkage volume. Solid Shrinkage will be compensated by providing Shrinkage allowance on patter this values are expressed in term of linear dimension ( mm/m length ) Shrinkage values for different material :-Aluminum:- 65% ( liquid and solidification Shrinkage is maximum ) which required more volume of the riser Solid Shrinkage are maximum for steel Special case :- Grey cast iron ( Graphite fluxes ) In case of Grey cast iron during the solidification process liquid and solidification phase there is a possibility of expansion of material due to this size of casting will be increase . There is no need of riser . This is because of the carbon is present in the form of graphite flux In solid state there is a possibility of contraction of the material to overcome these size of the pattern will be increase by providing Shrinkage allowance.
Gate book mistake Problem on Grey CI always more than 1 Calculation the dimension of pattern only considering the dimension of cooling. 2) Draft or Taper allowance: - For easy removal of the pattern from the mould for vertical surface of the pattern draft or Taper allowance is provided. This value will depend on vertical height of the pattern. 3) Machining or finish allowance: - Casting object is not having smooth surface finish machine is required. Due to machining size other casting will be decrease to overcome this size of the pattern can be increase by providing machininor finish allowance. Problem ) calculate the dimensions of the pattern for the casting showing below by considering machining allowance of 1min on each surface Machining allowance are always positive allowance we are adding on pattern . For external as well as internal dimension the machining allowance are always positive allowances. 4) Shake or rapping allowance :- During remaining process mounding sand will be stacked to the surface of the pattern. For easy removal some clearance is required between pattern and mould surface. This can be produce by shaking the pattern. Due to shaking size of the cavity will be increase. To overcome this dimension of the pattern can be reduce by providing shake allowance. It is negative allowance provided on the pattern
Distortion or camber allowance Depending on shape and size of the casting. Due to difference in linear dimension there is a possibility of distortion of the casting to overcome these distortion allowance is provided on pattern This value is depends on L/ t ratio Pattern material 1) wood 2) metals and alloy AL, brass, CI, steel etc To produce no. Of casting in mass production metallic pattern can be used. These are produce by wooden pattern. To produce wooden pattern Shrinkage of casting and Shrinkage of metallic pattern both can be added. This is known as double Shrinkage allowance and wooden pattern is known as master pattern 3) plastic as a pattern material :- Polystyrene , pvc , thermocouple etc ( This can easily evaporated when in contact with liquid metals ) ( this material is converted into gas directly heated Pattern is light weight, Easily available, converted into complex shape and non required draft and shake allowance.
TYPES of pattern :- 1) solid or single piece pattern If the object to be produce is simple in shape and size and one of the surface is flat then solid or single piece pattern can be used 2) spit piece pattern :- when complexity is more If the object to be produced is complex in shape and size, pattern can be removed from cope and drag boxes separately 3) loose piece pattern :- If the pattern are having internal projection or undercuts it is difficult to remove the pattern by assuming split piece. In this case loose piece pattern can be used. After removing the main part of the pattern loose piece are removed to get the required cavity 4) gated pattern:- To produce small size object in mass production producing of getting elements annually will take more time to overcome this number of pattern along with getting element will produce a single pattern known as gated pattern 5) match plate pattern To produce small size Complex shape of the object in mass production this pattern can be used. No. Of pattern can be spit along the parting line and they will be added on both sides of match plate along with parting line with gating element 6) sweep pattern :- To produce 3- dimensional complex mould cavities, 2- dimensional plane pattern will be rotated on the surface of mould These are generally used for axis- symmetric object which are produced by surface of revolution 7) Follow board pattern :- If the pattern are structure weak due to ramming force there is a possibility of breaking of the pattern to overcome this the pattern are supported by providing follow boards