MANUFACTURING PROCESS - DRAFT OR TAPER ALLOWANCE

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DRAFT OR TAPER ALLOWANCE

BY DRAFT IS MEANT THE TAPER PROVIDED BY THE PATTERN MAKER ON ALL VERTICAL SURFACES OF THE PATTERN SO THAT IT CAN BE REMOVED FROM THE SAND WITHOUT TEARING AWAY THE SIDES OF THE SAND MOLD AND WITHOUT EXCESSIVE RAPPING BY THE MOLDER. FIGURE SHOWS A PATTERN HAVING NO DRAFT ALLOWANCE BEING REMOVED FROM THE PATTERN. IN THIS CASE, TILL THE PATTERN IS COMPLETELY LIFTED OUT, ITS SIDES WILL REMAIN IN CONTACT WITH THE WALLS OF THE MOLD, THUS TENDING TO BREAK IT. FIGURE IS AN ILLUSTRATION OF A PATTERN HAVING PROPER DRAFT ALLOWANCE. HERE, THE MOMENT THE PATTERN LIFTING COMMENCES, ALL OF ITS SURFACES ARE WELL AWAY FROM THE SAND SURFACE. THUS THE PATTERN CAN BE REMOVED WITHOUT DAMAGING THE MOLD CAVITY. 


PATTERN HAVING DRAFT ON VERTICAL EDGES
DRAFT ALLOWANCE VARIES WITH THE COMPLEXITY OF THE SAND JOB. BUT IN GENERAL INNER DETAILS OF THE PATTERN REQUIRE HIGHER DRAFT THAN OUTER SURFACES. THE AMOUNT OF DRAFT DEPENDS UPON THE LENGTH OF THE VERTICAL SIDE OF THE PATTERN TO BE EXTRACTED; THE INTRICACY OF THE PATTERN; THE METHOD OF MOLDING; AND PATTERN MATERIAL. TABLE 2 PROVIDES A GENERAL GUIDE LINES FOR THE DRAFT ALLOWANCE.

TABLE 2 : DRAFT ALLOWANCES OF VARIOUS METALS

PATTERN MATERIAL
HEIGHT OF THE GIVEN SURFACE (INCH)
DRAFT ANGLE
(EXTERNAL SURFACE)
DRAFT ANGLE
(INTERNAL SURFACE)
WOOD
1
1 TO 2
2 TO 4
4 TO 8
8 TO 32
3.00
1.50
1.00
0.75
0.50
3.00
2.50
1.50
1.00
1.00
METAL AND PLASTIC
1
1 TO 2
2 TO 4
4 TO 8
8 TO 32
1.50
1.00
0.75
0.50
0.50
3.00
2.00
1.00
1.00
0.75


MACHINING OR FINISH ALLOWANCE
THE FINISH AND ACCURACY ACHIEVED IN SAND CASTING ARE GENERALLY POOR AND THEREFORE WHEN THE CASTING IS FUNCTIONALLY REQUIRED TO BE OF GOOD SURFACE FINISH OR DIMENSIONALLY ACCURATE, IT IS GENERALLY ACHIEVED BY SUBSEQUENT MACHINING. MACHINING OR FINISH ALLOWANCES ARE THEREFORE ADDED IN THE PATTERN DIMENSION. THE AMOUNT OF MACHINING ALLOWANCE TO BE PROVIDED FOR IS AFFECTED BY THE METHOD OF MOLDING AND CASTING USED VIZ. HAND MOLDING OR MACHINE MOLDING, SAND CASTING OR METAL MOLD CASTING. THE AMOUNT OF MACHINING ALLOWANCE IS ALSO AFFECTED BY THE SIZE AND SHAPE OF THE CASTING; THE CASTING ORIENTATION; THE METAL; AND THE DEGREE OF ACCURACY AND FINISH REQUIRED. THE MACHINING ALLOWANCES RECOMMENDED FOR DIFFERENT METAL IS GIVEN IN TABLE 3.

TABLE 3 : MACHINING ALLOWANCES OF VARIOUS METALS

METAL
DIMENSION (INCH)
ALLOWANCE (INCH)
CAST IRON
UP TO 12
12 TO 20
20 TO 40
0.12
0.20
0.25
CAST STEEL
UP TO 6
6 TO 20
20 TO 40
0.12
0.25
0.30
NON FERROUS
UP TO 8
8 TO 12
12 TO 40
0.09
0.12
0.16


DISTORTION OR CAMBER ALLOWANCE

SOMETIMES CASTINGS GET DISTORTED, DURING SOLIDIFICATION, DUE TO THEIR TYPICAL SHAPE. FOR EXAMPLE, IF THE CASTING HAS THE FORM OF THE LETTER U, V, T, OR L ETC. IT WILL TEND TO CONTRACT AT THE CLOSED END CAUSING THE VERTICAL LEGS TO LOOK SLIGHTLY INCLINED. THIS CAN BE PREVENTED BY MAKING THE LEGS OF THE U, V, T, OR L SHAPED PATTERN CONVERGE SLIGHTLY (INWARD) SO THAT THE CASTING AFTER DISTORTION WILL HAVE ITS SIDES VERTICAL THE DISTORTION IN CASTING MAY OCCUR DUE TO INTERNAL STRESSES. THESE INTERNAL STRESSES ARE CAUSED ON ACCOUNT OF UNEQUAL COOLING OF DIFFERENT SECTION OF THE CASTING AND HINDERED CONTRACTION. MEASURE TAKEN TO PREVENT THE DISTORTION IN CASTING INCLUDE:

  1. MODIFICATION OF CASTING DESIGN
  2. PROVIDING SUFFICIENT MACHINING ALLOWANCE TO COVER THE DISTORTION AFFECT
  3. PROVIDING SUITABLE ALLOWANCE ON THE PATTERN, CALLED CAMBER OR DISTORTION ALLOWANCE (INVERSE REFLECTION). 

RAPPING ALLOWANCE
BEFORE THE WITHDRAWAL FROM THE SAND MOLD, THE PATTERN IS RAPPED ALL AROUND THE VERTICAL FACES TO ENLARGE THE MOLD CAVITY SLIGHTLY, WHICH FACILITATE ITS REMOVAL. SINCE IT ENLARGES THE FINAL CASTING MADE, IT IS DESIRABLE THAT THE ORIGINAL PATTERN DIMENSION SHOULD BE REDUCED TO ACCOUNT FOR THIS INCREASE. THERE IS NO SURE WAY OF QUANTIFYING THIS ALLOWANCE, SINCE IT IS HIGHLY DEPENDENT ON THE FOUNDRY PERSONNEL PRACTICE INVOLVED. IT IS A NEGATIVE ALLOWANCE AND IS TO BE APPLIED ONLY TO THOSE DIMENSIONS THAT ARE PARALLEL TO THE PARTING PLANE.

CORE AND CORE PRINTS
CASTINGS ARE OFTEN REQUIRED TO HAVE HOLES, RECESSES, ETC. OF VARIOUS SIZES AND SHAPES. THESE IMPRESSIONS CAN BE OBTAINED BY USING CORES. SO WHERE CORING IS REQUIRED, PROVISION SHOULD BE MADE TO SUPPORT THE CORE INSIDE THE MOLD CAVITY. CORE PRINTS ARE USED TO SERVE THIS PURPOSE. THE CORE PRINT IS AN ADDED PROJECTION ON THE PATTERN AND IT FORMS A SEAT IN THE MOLD ON WHICH THE SAND CORE RESTS DURING POURING OF THE MOLD. THE CORE PRINT MUST BE OF ADEQUATE SIZE AND SHAPE SO THAT IT CAN SUPPORT THE WEIGHT OF THE CORE DURING THE CASTING OPERATION. DEPENDING UPON THE REQUIREMENT A CORE CAN BE PLACED HORIZONTAL, VERTICAL AND CAN BE HANGED INSIDE THE MOLD CAVITY. A TYPICAL JOB, ITS PATTERN AND THE MOLD CAVITY WITH CORE AND CORE PRINT 

TYPES OF PATTERN
PATTERNS ARE OF VARIOUS TYPES, EACH SATISFYING CERTAIN CASTING REQUIREMENTS.
  1. SINGLE PIECE PATTERN
  2. SPLIT OR TWO PIECE PATTERN
  3. MATCH PLATE PATTERN
SINGLE PIECE PATTERN
THE ONE PIECE OR SINGLE PATTERN IS THE MOST INEXPENSIVE OF ALL TYPES OF PATTERNS. THIS TYPE OF PATTERN IS USED ONLY IN CASES WHERE THE JOB IS VERY SIMPLE AND DOES NOT CREATE ANY WITHDRAWAL PROBLEMS. IT IS ALSO USED FOR APPLICATION IN VERY SMALL-SCALE PRODUCTION OR IN PROTOTYPE DEVELOPMENT. THIS TYPE OF PATTERN IS EXPECTED TO BE ENTIRELY IN THE DRAG AND ONE OF THE SURFACE IS IS EXPECTED TO BE FLAT WHICH IS USED AS THE PARTING PLANE. A GATING SYSTEM IS MADE IN THE MOLD BY CUTTING SAND WITH THE HELP OF SAND TOOLS. IF NO SUCH FLAT SURFACE EXISTS, THE MOLDING BECOMES COMPLICATED.

SPLIT OR TWO PIECE PATTERN
SPLIT OR TWO PIECE PATTERN IS MOST WIDELY USED TYPE OF PATTERN FOR INTRICATE CASTINGS. IT IS SPLIT ALONG THE PARTING SURFACE, THE POSITION OF WHICH IS DETERMINED BY THE SHAPE OF THE CASTING. ONE HALF OF THE PATTERN IS MOLDED IN DRAG AND THE OTHER HALF IN COPE. THE TWO HALVES OF THE PATTERN MUST BE ALIGNED PROPERLY BY MAKING USE OF THE DOWEL PINS, WHICH ARE FITTED, TO THE COPE HALF OF THE PATTERN. THESE DOWEL PINS MATCH WITH THE PRECISELY MADE HOLES IN THE DRAG HALF OF THE PATTERN.


CLASSIFICATION OF CASTING PROCESSES
CASTING PROCESSES CAN BE CLASSIFIED INTO FOLLOWING FOUR CATEGORIES:

1. CONVENTIONAL MOLDING PROCESSES
  1. GREEN SAND MOLDING
  2. DRY SAND MOLDING
  3. FLASK LESS MOLDING
2. CHEMICAL SAND MOLDING PROCESSES
  1. SHELL MOLDING
  2. SODIUM SILICATE MOLDING
  3. NO-BAKE MOLDING
3. PERMANENT MOLD PROCESSES
  1. GRAVITY DIE CASTING
  2. LOW AND HIGH PRESSURE DIE CASTING
4. SPECIAL CASTING PROCESSES
  1. LOST WAX
  2. CERAMICS SHELL MOLDING
  3. EVAPORATIVE PATTERN CASTING
  4. VACUUM SEALED MOLDING
  5. CENTRIFUGAL CASTING
GREEN SAND MOLDING
GREEN SAND IS THE MOST DIVERSIFIED MOLDING METHOD USED IN METAL CASTING OPERATIONS. THE PROCESS UTILIZES A MOLD MADE OF COMPRESSED OR COMPACTED MOIST SAND. THE TERM "GREEN" DENOTES THE PRESENCE OF MOISTURE IN THE MOLDING SAND. THE MOLD MATERIAL CONSISTS OF SILICA SAND MIXED WITH A SUITABLE BONDING AGENT (USUALLY CLAY) AND MOISTURE.

ADVANTAGES
  1. MOST METALS CAN BE CAST BY THIS METHOD.
  2. PATTERN COSTS AND MATERIAL COSTS ARE RELATIVELY LOW.
  3. NO LIMITATION WITH RESPECT TO SIZE OF CASTING AND TYPE OF METAL OR ALLOY USED
DISADVANTAGES

SURFACE FINISH OF THE CASTINGS OBTAINED BY THIS PROCESS IS NOT GOOD AND MACHINING IS OFTEN REQUIRED TO ACHIEVE THE FINISHED PRODUCT.

SAND MOLD MAKING PROCEDURE
THE PROCEDURE FOR MAKING MOLD OF A CAST IRON WHEEL IS 
  • THE FIRST STEP IN MAKING MOLD IS TO PLACE THE PATTERN ON THE MOLDING BOARD.
  • THE DRAG IS PLACED ON THE BOARD 
  • DRY FACING SAND IS SPRINKLED OVER THE BOARD AND PATTERN TO PROVIDE A NON STICKY LAYER.
  • MOLDING SAND IS THEN RIDDLED IN TO COVER THE PATTERN WITH THE FINGERS; THEN THE DRAG IS COMPLETELY FILLED.
  • THE SAND IS THEN FIRMLY PACKED IN THE DRAG BY MEANS OF HAND RAMMERS. THE RAMMING MUST BE PROPER I.E. IT MUST  NEITHER BE TOO HARD OR SOFT.
  • AFTER THE RAMMING IS OVER, THE EXCESS SAND IS LEVELED OFF WITH A STRAIGHT BAR KNOWN AS A STRIKE ROD.
  • WITH THE HELP OF VENT ROD, VENT HOLES ARE MADE IN THE DRAG TO THE FULL DEPTH OF THE FLASK AS WELL AS TO THE PATTERN TO FACILITATE THE REMOVAL OF GASES DURING POURING AND SOLIDIFICATION.
  • THE FINISHED DRAG FLASK IS NOW ROLLED OVER TO THE BOTTOM BOARD EXPOSING THE PATTERN.
  • COPE HALF OF THE PATTERN IS THEN PLACED OVER THE DRAG PATTERN WITH THE HELP OF LOCATING PINS. THE COPE FLASK ON THE DRAG IS LOCATED ALIGNING AGAIN WITH THE HELP OF PINS 
  • THE DRY PARTING SAND IS SPRINKLED ALL OVER THE DRAG AND ON THE PATTERN.
  • A SPRUE PIN FOR MAKING THE SPRUE PASSAGE IS LOCATED AT A SMALL DISTANCE FROM THE PATTERN. ALSO, RISER PIN, IF REQUIRED, IS PLACED AT AN APPROPRIATE PLACE.
  • THE OPERATION OF FILLING, RAMMING AND VENTING OF THE COPE PROCEED IN THE SAME MANNER AS PERFORMED IN THE DRAG.
  • THE SPRUE AND RISER PINS ARE REMOVED FIRST AND A POURING BASIN IS SCOOPED OUT AT THE TOP TO POUR THE LIQUID METAL.
  • THEN PATTERN FROM THE COPE AND DRAG IS REMOVED AND FACING SAND IN THE FORM OF PASTE IS APPLIED ALL OVER THE MOLD CAVITY AND RUNNERS WHICH WOULD GIVE THE FINISHED CASTING A GOOD SURFACE FINISH.
  • THE MOLD IS NOW ASSEMBLED. THE MOLD NOW IS READY FOR POURING .

MOLDING MATERIAL AND PROPERTIES
A LARGE VARIETY OF MOLDING MATERIALS IS USED IN FOUNDRIES FOR MANUFACTURING MOLDS AND CORES. THEY INCLUDE MOLDING SAND, SYSTEM SAND OR BACKING SAND, FACING SAND, PARTING SAND, AND CORE SAND. THE CHOICE OF MOLDING MATERIALS IS BASED ON THEIR PROCESSING PROPERTIES. THE PROPERTIES THAT ARE GENERALLY REQUIRED IN MOLDING MATERIALS ARE:

REFRACTORINESS
IT IS THE ABILITY OF THE MOLDING MATERIAL TO RESIST THE TEMPERATURE OF THE LIQUID METAL TO BE POURED SO THAT IT DOES NOT GET FUSED WITH THE METAL. THE REFRACTORINESS OF THE SILICA SAND IS HIGHEST.

PERMEABILITY
DURING POURING AND SUBSEQUENT SOLIDIFICATION OF A CASTING, A LARGE AMOUNT OF GASES AND STEAM IS GENERATED. THESE GASES ARE THOSE THAT HAVE BEEN ABSORBED BY THE METAL DURING MELTING, AIR ABSORBED FROM THE ATMOSPHERE AND THE STEAM GENERATED BY THE MOLDING AND CORE SAND. IF THESE GASES ARE NOT ALLOWED TO ESCAPE FROM THE MOLD, THEY WOULD BE ENTRAPPED INSIDE THE CASTING AND CAUSE CASTING DEFECTS. TO OVERCOME THIS PROBLEM THE MOLDING MATERIAL MUST BE POROUS. PROPER VENTING OF THE MOLD ALSO HELPS IN ESCAPING THE GASES THAT ARE GENERATED INSIDE THE MOLD CAVITY.

GREEN STRENGTH
THE MOLDING SAND THAT CONTAINS MOISTURE IS TERMED AS GREEN SAND. THE GREEN SAND PARTICLES MUST HAVE THE ABILITY TO CLING TO EACH OTHER TO IMPART SUFFICIENT STRENGTH TO THE MOLD. THE GREEN SAND MUST HAVE ENOUGH STRENGTH SO THAT THE CONSTRUCTED MOLD RETAINS ITS SHAPE.

DRY STRENGTH
WHEN THE MOLTEN METAL IS POURED IN THE MOLD, THE SAND AROUND THE MOLD CAVITY IS QUICKLY CONVERTED INTO DRY SAND AS THE MOISTURE IN THE SAND EVAPORATES DUE TO THE HEAT OF THE MOLTEN METAL. AT THIS STAGE THE MOLDING SAND MUST POSSES THE SUFFICIENT STRENGTH TO RETAIN THE EXACT SHAPE OF THE MOLD CAVITY AND AT THE SAME TIME IT MUST BE ABLE TO WITHSTAND THE METALLOSTATIC PRESSURE OF THE LIQUID MATERIAL.

HOT STRENGTH
AS SOON AS THE MOISTURE IS ELIMINATED, THE SAND WOULD REACH AT A HIGH TEMPERATURE WHEN THE METAL IN THE MOLD IS STILL IN LIQUID STATE. THE STRENGTH OF THE SAND THAT IS REQUIRED TO HOLD THE SHAPE OF THE CAVITY IS CALLED HOT STRENGTH.

COLLAPSIBILITY
THE MOLDING SAND SHOULD ALSO HAVE COLLAPSIBILITY SO THAT DURING THE CONTRACTION OF THE SOLIDIFIED CASTING IT DOES NOT PROVIDE ANY RESISTANCE, WHICH MAY RESULT IN CRACKS IN THE CASTINGS.BESIDES THESE SPECIFIC PROPERTIES THE MOLDING MATERIAL SHOULD BE CHEAP, REUSABLE AND SHOULD HAVE GOOD THERMAL CONDUCTIVITY.

MOLDING SAND COMPOSITION
THE MAIN INGREDIENTS OF ANY MOLDING SAND ARE:
  • BASE SAND,
  • BINDER, AND
  • MOISTURE
BASE SAND
SILICA SAND IS MOST COMMONLY USED BASE SAND. OTHER BASE SANDS THAT ARE ALSO USED FOR MAKING MOLD ARE ZIRCON SAND, CHROMITE SAND, AND OLIVINE SAND. SILICA SAND IS CHEAPEST AMONG ALL TYPES OF BASE SAND AND IT IS EASILY AVAILABLE.

BINDER
BINDERS ARE OF MANY TYPES SUCH AS:
  1. CLAY BINDERS,
  2. ORGANIC BINDERS AND
  3. INORGANIC BINDERS
CLAY BINDERS ARE MOST COMMONLY USED BINDING AGENTS MIXED WITH THE MOLDING SANDS TO PROVIDE THE STRENGTH. THE MOST POPULAR CLAY TYPES ARE:
KAOLINITE OR FIRE CLAY (AL2O3 2 SIO2 2 H2O) AND BENTONITE (AL2O3 4 SIO2 NH2O)
OF THE TWO THE BENTONITE CAN ABSORB MORE WATER WHICH INCREASES ITS BONDING POWER.

MOISTURE
CLAY ACQUIRES ITS BONDING ACTION ONLY IN THE PRESENCE OF THE REQUIRED AMOUNT OF MOISTURE. WHEN WATER IS ADDED TO CLAY, IT PENETRATES THE MIXTURE AND FORMS A MICROFILM, WHICH COATS THE SURFACE OF EACH FLAKE OF THE CLAY. THE AMOUNT OF WATER USED SHOULD BE PROPERLY CONTROLLED. THIS IS BECAUSE A PART OF THE WATER, WHICH COATS THE SURFACE OF THE CLAY FLAKES, HELPS IN BONDING, WHILE THE REMAINDER HELPS IN IMPROVING THE PLASTICITY. A TYPICAL COMPOSITION OF MOLDING SAND IS GIVEN IN (TABLE 4).

TABLE 4 : A TYPICAL COMPOSITION OF MOLDING SAND

MOLDING SAND CONSTITUENT
WEIGHT PERCENT
SILICA SAND
92
CLAY (SODIUM BENTONITE)
8
WATER
4












2 comments:

  1. I actually enjoyed reading through this posting.Many thanks.





    Pressure Die Casting Manufacturers

    ReplyDelete
  2. actually titz naptol's artikle... huehue
    thanks to them
    http://nptel.ac.in/courses/112107144/metalcasting/lecture4.htm

    ReplyDelete