Pattern allowances in metal casting


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Pattern allowances in metal casting

  1. 1. Pattern allowances in metal casting Presentado por: Diana Gasca Andrés Escárraga Fundición de Metales Profesor: Dr. Héctor Sánchez S. Universidad del Valle
  2. 2. Pattern is a model or the replica of the object to be cast. It is a larger in size as compared to the final casting, because it carries certain allowances due to metallurgical and mechanical reasons for example, shrinkage allowance is the result of metallurgical phenomenon where as machining, draft, distortion, shale, and other allowances are provided on the patterns because of mechanical reasons. Top center is the clay original, then the two part plaster mold used for casting the lead at above, and wax cast from mold, sprued for better brass casting, not yet cast. 2008-01-12. 1b.jpg
  3. 3. Functions of Patterns: • A Pattern prepares a mould cavity for the purpose of making a casting. • A Pattern may contain projections known as core prints (corazón) if the casting requires a core and need to be made hollow. • Patterns properly made and having finished and smooth surfaces reduce casting defects. • Properly constructed patterns minimize overall cost of the casting.
  4. 4. The pattern material should be: 1. 2. 3. 4. 5. 6. 7. Easily worked,shaped and joined. Light in weight. Strong,hard and durable. Resistant to wear and abrasion . Resistant to corrosion,and to chemical reactions. Dimensionally stable and unaffected by variations in temperature and humidity. Available at low cost.
  5. 5. Pattern Material Characteristics
  6. 6. (a) Split pattern (b) Follow-board (c) Match Plate (d) Loose-piece (e) Sweep (f) Skeleton pattern Types of Patterns:
  7. 7. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Types of Patterns Figure 11.3 Types of patterns used in sand casting: (a) solid pattern (b) split pattern (c) match-plate pattern (d) cope and drag pattern
  8. 8. Fig:Single piece pattern
  9. 9. castings Gating system 6.Gated pattern:
  10. 10. Fig:Cope and drag pattern
  11. 11. Reason for allowances: Solidification Shrinkage  Most metals undergo noticeable volumetric contraction when cooled  Three principle stages of shrinkage:  Shrinkage of liquid as it cools from the solidification temperature  Solidification shrinkage as the liquid turns into solid  Solid metal contraction as the solidified metal cools to room temperature Figure Dimensional changes experienced by a metal column as the material cools from a superheated liquid to a room-temperature solid. Note the significant shrinkage that occurs upon solidification.
  12. 12. Prediction of porosity after casting Minuto 1:12. Se observa la contracción del metal, no hay tolerancia para compensarla, genera porosidad residual.
  13. 13. Volumetric Shrinkage 2% Residual Shrinkage after casting
  14. 14. Dimensional Allowances  Typical allowances  Cast iron 0.8-1.0%  Steel 1.5-2.0%  Aluminum 1.0-1.3%  Magnesium 1.0-1.3%  Brass 1.5%  Shrinkage allowances are incorporated into the pattern using shrink rules  Thermal contraction might not be the only factor for determining pattern size  Surface finishing operations (machining, etc.) should be taken into consideration
  15. 15. Solidification Shrinkage  Amount of liquid metal contraction depends on the coefficient of thermal contraction and the amount of superheat  As the liquid metal solidifies, the atomic structure normally becomes more efficient and significant amounts of shrinkage can occur  Cavities and voids can be prevented by designing the casting to have directional solidification  Hot tears can occur when there is significant tensile stress on the surface of the casting material
  16. 16. Types of Pattern Allowances: THE VARIOUS PATTERN ALLOWANCES ARE: 1. 2. 3. 4. Shrinkage or contraction allowance. Machining or finish allowance. Draft of tapper allowances. Distortion or chamber allowance. 5. Shake or rapping allowance.
  17. 17. 1.ShrinkageAllowance: All most all cast metals shrink or contract volumetrically on cooling. 1.Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature.To account for this shrinkage;riser,which feed the liquid metal to the casting,are provided in the mold. 2.Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state.To account for this,shrinkage allowance is provided on the patterns.
  18. 18. Almost all cast metals shrink or contract volumetrically after solidification and therefore the pattern to obtain a particular sized casting is made oversize by an amount equal to that of shrinkage or contraction. Different metals shrink at different rates because shrinkage is the property of the cast metal/alloy. The metal shrinkage depends upon: 1. The cast metal or alloy. 2. Solidification temp.of the metal/alloy. 3. Casted dimensions(size). 4. Casting design aspects. 5. Molding conditions(i.e.,mould materials and molding methods employed)
  19. 19. Material Dimension Shrinkageallowance (inch/ft) Grey Cast Iron Up to 2 feet 2 feet to 4feet Over 4feet 0.125 0.105 0.083 CastSteel Upto2feet 2feetto6feet over6feet 0.251 0.191 0.155 Aluminum Upto4feet 4feetto6feet over6feet 0.155 0.143 0.125 Magnesium Upto4feet Over4feet 0.173 0.155 RATE OF CONTRACTION OF VARIOUS METALS :
  20. 20. i. ii. iii. iv. i. ii. iii. 2.MachiningAllowance: A CASTING IS GIVEN AN ALLOWANCE FOR MACHINING, BECAUSE: Castings get oxidized in the mold and during heat treatment;scales etc.,thus formed need to be removed. It is the intended to remove surface roughness and other imperfections from the castings. It is required to achieve exact casting dimensions. Surface finish is required on the casting. HOW MUCH EXTRA METAL OR HOW MUCH MACHINING ALLOWANCE SHOULD BE PROVIDED, DEPENDS ON THE FACTORS LISTED BELOW: Nature of metals. Size and shape of casting. The type of machining operations to be employed for
  21. 21. Metal Dimension(inch) Allowance(inch) Castiron Upto12 12to20 20to40 0.12 0.20 0.25 Caststeel Upto6 6to20 20to40 0.12 0.25 0.30 Nonferrous Upto8 8to12 12to40 0.09 0.12 0.16 MACHINING ALLOWANCES OF VARIOUS METALS:
  22. 22. 3.Draft or TaperAllowance: It is given to all surfaces perpendicular to parting line. Draft allowance is given so that the pattern can be easily removed from the molding material tightly packed around it with out damaging the mould cavity. The amount of taper depends upon: i. Shape and size of pattern in the depth direction in contact with the mould cavity. ii. Moulding methods. iii. Mould materials. iv. Draft allowance is imparted on internal as well as external surfaces;of course it is more on internal surfaces.
  23. 23. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Core Full-scale model of interior surfaces of part  It is inserted into the mold cavity prior to pouring  The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces  May require supports to hold it in position in the mold cavit during pouring, called chaplets Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.
  24. 24. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Draft  Minor changes in part design can reduce need for coring Figure 11.25 Design change to eliminate the need for using a core: (a) original design, and (b) redesign.
  25. 25. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Product Design Considerations  Draft Guidelines:  In expendable mold casting, draft facilitates removal of pattern from mold  Draft = 1 for sand casting  In permanent mold casting, purpose is to aid in removal of the part from the mold  Draft = 2 to 3 for permanent mold processes  Similar tapers should be allowed if solid cores are used
  26. 26. Pattern material Heightofthe givensurface (inch) Draftangle (External surface) Draftangle (Internal surface) Wood 1 1to2 2to4 4to8 8to32 3.00 1.50 1.00 0.75 0.50 3.00 2.50 1.50 1.00 1.00 Metal and plastic 1 1to2 2to4 4to8 8to32 1.50 1.00 0.75 0.50 0.50 3.00 2.00 1.00 1.00 0.75 Table 2 : Draft Allowances of Various Metals:
  27. 27. Fig:taper in design
  28. 28. 4. Distortion or cambered allowance: A CASTING WILL DISTORT OR WRAP IF : i. It is of irregular shape, ii. All it parts do not shrink uniformly i.e.,some parts shrinks while others are restricted from during so, iii. It is u or v-shape, iv. The arms possess unequal thickness, v. It has long,rangy arms as those of propeller strut for the ship, vi. It is a long flat casting, vii. One portion of the casting cools at a faster rate
  29. 29. 5.Shake allowance: A patter is shaken or rapped by striking the same with a wooden piece from side to side.This is done so that the pattern a little is loosened in the mold cavity and can be easily removed. In turn,therefore,rapping enlarges the mould cavity which results in a bigger sized casting. Hence,a ²ve allowance is provided on the pattern i.e.,the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping. The magnitude of shake allowance can be reduced by increasing the tapper.
  30. 30. Pattern Layout: Steps involved: Get the working drawing of the part for which the pattern is to be made. Make two views of the part drawing on a sheet,using a shrink rule. A shrink rule is modified form of an ordinary scale which has already taken care of shrinkage allowance for a particular metal to be cast. Add machining allowances as per the requirements. Depending upon the method of molding, provide the draft allowance.
  31. 31. Pattern Construction: Study the pattern layout carefully and establish, a. Location of parting surface. b. No.of parts in which the pattern will be made. Using the various hand tools and pattern making machines fabricate the different parts of the pattern. Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy. Fill wax in all the fillets in order to remove sharp corners. Give a shellac coatings(3 coats) to pattern. impart suitable colors to the pattern for identification purposes and for other informations.
  32. 32. Design Considerations in Castings  Location and orientation of the parting line is important to castings  Parting line can affect:  Number of cores  Method of supporting cores  Use of effective and economical gating  Weight of the final casting  Final dimensional accuracy  Ease of molding
  33. 33. Design Considerations Various allowances incorporated into a casting pattern. Two-part mold showing the parting line and the incorporation of a draft allowance on vertical surfaces.
  34. 34. Design Considerations Figure 11-16 (Left) Elimination of a core by changing the location or orientation of the parting plane. Figure 11-17 (Right) Elimination of a dry- sand core by a change in part design.
  35. 35. Design Considerations  It is often desirable to minimize the use of cores  Controlling the solidification process is important to producing quality castings  Thicker or heavier sections will cool more slowly, so chills should be used  If section thicknesses must change, gradual is better  If they are not gradual, stress concentration points can be created  Fillets or radii can be used to minimize stress concentration points  Risers can also be used
  36. 36. Parting Line and Drafts Figure 11-18 (Top left) Design where the location of the parting plane is specified by the draft. (Top right) Part with draft unspecified. (Bottom) Various options to produce the top- right part, including a no-draft design.
  37. 37. Section Thicknesses (Above) Typical guidelines for section change transitions in castings. Figure a) The “hot spot” at section r2 is cause by intersecting sections. B) An interior fillet and exterior radius lead to more uniform thickness and more uniform cooling.
  38. 38. Design Modifications  Hot spots are areas of the material that cool more slowly than other locations  Function of part geometry  Localized shrinkage may occur Hot spots often result from intersecting sections of various thickness.
  39. 39. Design Modifications  Parts that have ribs may experience cracking due to contraction  Ribs may be staggered to prevent cracking  An excess of material may appear around the parting line  The parting line may be moved to improve appearance  Thin-walled castings should be designed with extra caution to prevent cracking
  40. 40. Design Modifications Figure 11-23 Using staggered ribs to prevent cracking during cooling.
  41. 41. References  Rao, P.N. (2003). Manufacturing Technology. New Delhi: Tata McGraw-Hill.  Pattern Allowances in casting, Vikrant Sharma, MITS Lakshmangarh 2008  Foundry technology. Peter Beeley, 2th edition, Reed Elsevier, 2001  Fundamentals of metal casting. Richard A. Flinn. Addison- Wesley, 1963