CASTING Fundamentals. Prepared by Associate Prof. Mohamed Ahmed Awad Cairo, 2014

Transcription

1 CASTING Fundamentals Prepared by Associate Prof. Mohamed Ahmed Awad Cairo, 2014

2 Casting Definition Casting is the process of pouring molten metal into a mould containing a cavity, which represents the required product shape. 2

3 Casting materials 3

4 Casting steps Melt the metal Pour it into a mould Freezing 4

5 Casting Advantages Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can produce very large parts Some casting methods are suited to mass production. It is economical to use. 5

6 Casting Advantages Complex Shape Complex Small 6 Simple Big

7 Sand Casting Products 7

8 Disadvantages of casting Limitations on mechanical properties Poor dimensional accuracy and surface finish for some processes; e.g., sand casting Safety hazards to workers due to hot molten metals Environmental problems 8

9 Casting mold: Open and closed Open mold is a container in the shape of the desire product Closed mold has a complex shape 9

10 Types of molds Expandable molds After the casting has solidifies, the molds in these molds are broken up to remove the casting. Permanent molds The casting can be easily removed and the mold used for the next casting. 10

11 Expendable versus permanent mold 11

12 Solidification of Metals Solidification is the transformation of molten metal back into solid state 12

13 Solidification shrinkage 13 Prepared by dr. Mohmaed Ahmed Awad

14 Sand Casting Advantages: Almost no limit on size, shape, weight or complexity; low cost; almost any metal Limitations: Relatively poor tolerances and surface finish; machining often required; low production rate Common metals: Cast irons, steel, stainless steel, casting alloys of aluminum and copper, magnesium and nickel 14

15 Sand Casting Properties required in mould sand Permeability Strength Refractoriness Grain size and shape 15

16 Desirable Mold Properties and Characteristics Permeability - to allow hot air and gases to pass through voids in sand Strength - to maintain shape and resist erosion Refractoriness- to resist cracking on contact with molten metal Collapsibility - ability to give way and allow casting to shrink without cracking the casting 16

17 Casting Casting = Pattern making + Mold making + Pouring Molten Metal + Cooling + Removal + Inspection Types of Casting : Expendable Mold, Multiple Use Mold Sand Casting : Expendable Mold casting process uses sand as molding material 17

18 Elements of Sand mould 18

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20 Flask 20

21 Mould 21

22 Pouring Basin This is where the molten metal employed to enter the mould 22

23 Main Cavity This is the impression of the actual part to be cast 23

24 Riser Riser is reservoir of molten metal. It feed this material to sections of the mould to compensate for shrinkage as the casting solidifies. 24

25 Vent Vent is made to assist in the escape of gases that are expelled from the molten metal during solidification 25

26 Pattern The pattern is a geometric replica of the metal casting to be produced. It is made slightly oversize to compensate for the shrinkage that will occur in the metal during solidification. Also, increasing the machining allowances to compensate volume decrease due to chip formation. 26

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28 Pattern Material Pattern can be made from wood, various plastics or metal, like aluminum Pattern is usually coated with parting agent to ease their removal from the mould. 28

29 Pattern Making One piece or Solid Pattern Match Plate Pattern Split Pattern Cope & Drag Pattern 29

30 Core A core is a replica, of the internal features of the part to be cast. Like a patter, the size of the core is designed to accommodate for shrinkage and machining. 30

31 Core Core is used to make internal Cavity inside the product. The core is placed in the mould during pouring 31

32 Cast Part

33 Mould Parts Pattern Cope Drag Riser Cavity An exact copy of the required casting The upper part of the mould The lower part of the mould Extra material added to feed the molten metal during solidification Formed in sand using pattern Flask 33

34 Pouring Stage 34

35 Pattern Allowances 1 Shrinkage 2 Machining (Finish) 3 Draft 35

36 Shrinkage

37 Shrinkage Allowance Shrinkage allowance is used to compensate the loss volume during solidification 37

38 Machining Allowances Machining Allowance is added to compensate the loss of material as chip during machining process 38

39 Draft Allowance Draft allowance to facilitate withdraw of pattern from mould before solidification 39

40 Allowances Shrinkage allowances Machining allowances Final Product 40

41 Other Expendable Mold Casting Processes Shell Molding Vacuum Molding Expanded Polystyrene Process Investment Casting Plaster Mold and Ceramic Mold Casting 41

42 Die casting Stationary Moving die die Molten metal Plunger Ejector pins Sprue Parting line 42

43 Die Casting 43

44 Die Casting Steps 44