1. Casting & Welding Engineering (IE 203) Second Year, Industrial Engineering Dept., Faculty of Engineering, Fayoum University Dr. Ahmed Salah Abou Taleb 1

2. Patterns Is the replica of the final object to be made with some modifications. The mould cavity is made with the help of the pattern. 2

3. 3 Patterns

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5. Variety of patters are used in casting and the choice depends on the configuration of casting and number of casting required. Question: You are required to design a suitable pattern to produce the following product. 5

6. Pattern materials Typically, materials used for pattern making are wood, metal or plastics. Wax and Plaster of Paris are also used, but only for specialized applications. Mahogany is the most commonly used material for patterns, primarily because it is soft, light, and easy to work. The downside is that it wears out fast, and is prone to moisture attack. Metal patterns are more long lasting, and do not succumb to moisture, but they are heavier and difficult to repair once damaged. Plastic pattern are used due to their low weight,easier formability, smooth surface, durability, and succumb to moisture. 6

7. Pattern Requirements of a good pattern: – Secure the desired shape and size of the casting. – Simple in design, for ease of manufacture. – Cheap and readily available Light in mass and convenient to handle. – Have high strength. 7

8. Patterns Variety of patters are used in casting and the choice depends on the configuration of casting and number of casting required – Single-piece pattern. – Split pattern. – Follow board pattern. – Cope and drag pattern. – Match plate pattern. – Loose-piece pattern. – Sweep pattern. – Skeleton pattern. 8

9. (a)Split pattern (b) Follow-board (c) Match Plate (d) Loose-piece (e) Sweep (f) Skeleton pattern Types of Pattern 9

10. Pattern Design Location of parting line. Allowances: – Shrinkage allowance. – Draft: To minimize the damage of the mold when the pattern is withdrawn. Is incorporated on all surfaces parallel to the direction of withdrawal – Machining allowance: Is added when machined surfaces must be provided on castings. - Shake allowance. - Distortion allowance. 10

11. Shrinkage during Solidification & Cooling (0) starting level of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction during cooling 11

12. (2) reduction in height and formation of shrinkage cavity caused by solidification; (3) further reduction in volume due to thermal contraction during cooling of solid metal Shrinkage during Solidification & Cooling 12

13. Solidification Shrinkage Occurs in nearly all metals because the solid phase has a higher density than the liquid phase. Thus, solidification causes a reduction in volume per unit weight of metal. Exception: cast iron with high C content – Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change. 13

14. Shrinkage Allowance Patternmakers correct for solidification shrinkage and thermal contraction by making the mould cavity oversized. Amount by which mould is made larger relative to final casting size is called pattern shrinkage allowance. Casting dimensions are expressed linearly, so allowances are applied accordingly. 14

15. Shrinkage Allowance MaterialShrinkage allowance (mm/mm) Grey cast iron0.0105 White cast iron0.0160 – 0.0230 Plain carbon steel0.0201 Chromium steel0.0200 Aluminum0.0130 Aluminum bronze0.0200 – 0.0230 Brass0.0155 Bronze0.0155 – 0.0220 Copper0.0160 Lead0.0260 Magnesium0.0130 15

16. Draft Allowance When the pattern is to be removed from the sand mould, there is a possibility that any leading edges may break off, or get damaged in the process. To avoid this, a taper is provided on the pattern, so as to facilitate easy removal of the pattern from the mould, and hence reduce damage to edges. The taper angle provided is called the Draft angle. The value of the draft angle depends upon the complexity of the pattern, the type of moulding (hand molding or machine molding), height of the surface, etc. Draft provided on the casting 1 to 3 degrees on external surface ( 5 to 8 internal castings) 16

17. Draft Allowance Draft angle (deg.) Pattern MaterialOuterInner Wood0.25 to 3.000.50 to 3.00 Metal0.35 to 1.500.50 to 3.00 Plastic0.25 to 1.000.35 to 2.25 17

18. Finish or Machining Allowance The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning or grinding in order to improve the surface finish. During machining processes, some metal is removed from the piece. To compensate for this, a machining allowance should be given in the casting. 18

19. Finish or Machining Allowance Allowance (mm) DimensionBoreSurfaceCope Side Cast iron Up to 3003.0 5.5 300 to 5005.04.06.0 500 to 9006.05.06.0 Cast steel Up to 1503.0 6.0 150 to 5006.05.57.0 500 to 9007.06.09.0 Non ferrous Up to 2002.01.52.0 200 to 3002.51.53.0 300 to 9003.02.53.0 19

20. Shake Allowance Usually during removal of the pattern from the mold cavity, the pattern is rapped all around the faces, in order to facilitate easy removal. In this process, the final cavity is enlarged. To compensate for this, the pattern dimensions need to be reduced. There are no standard values for this allowance, as it is heavily dependent on the personnel. This allowance is a negative allowance, and a common way of going around this allowance is to increase the draft allowance. 20

21. Distortion Allowance During cooling of the mould, stresses developed in the solid metal may induce distortions in the cast. This is more evident when the mould is thinner in width as compared to its length. This can be eliminated by initially distorting the pattern in the opposite direction 21

22. Questions 1.What is a riser mainly used for? 2.What is fluidity? What is it depended on? 3.What are the three stages of shrinkages as a liquid is converted into a finished casing? 4.What types of allowances are generally incorporated into a casting pattern? 5.What is the purpose of a draft on pattern surfaces? 22

23. Problem 23 The figure shows a steel part is to be obtained by sand casting. Design the requisite pattern.