Lecture W2 - Casting & Molding

Page 1

METAL CASTING PROCESSES

Page 2

LESSON OBJECTIVES

  • Fundamentals of casting process

  • Applications, advantages, and limitations of casting

  • Patterns in casting

  • Cores and core prints in casting

Page 3

CASTING (ALSO CALLED FOUNDRY)

  • Definition: A process involving pouring molten metal into a refractory mould with a cavity shaped to form the final product, which is then allowed to solidify.

  • Mould cavity creation: Often done using a pattern around which the mould is formed.

  • Process: Pattern is removed before introducing molten metal to the mould.

Page 4

FOUNDRY (CONT.)

  • Heat extraction: Once poured, the mould material extracts heat from the liquid, causing the metal to solidify in the mould's shape.

  • Post-solidification: The solidified metal shape (the casting) is removed from the mould, along with excess metal (gates, runners, risers, etc.).

Page 5

ADVANTAGES OF CASTING

  • Capability: Allows for making intricate shapes, both internal and external.

  • Material versatility: Any material can be cast.

  • Cost-effectiveness: Simple tools lead to inexpensive operations.

  • Production method: Ideal for trial production or small lot manufacturing.

  • Design efficiency: Can reduce weight in design.

Page 6

ADVANTAGES OF CASTING (CONT.)

  • Uniform cooling: Castings generally cool uniformly, leading to no directional properties.

  • Size flexibility: Capable of producing castings of any size.

Page 7

LIMITATIONS OF CASTING

  • Accuracy issues: Dimensional accuracy and surface finish may not meet final application standards in many cases.

  • Labor intensity: The process can be labor-intensive.

  • Defect management: Difficult to remove defects caused by moisture in sand castings.

Page 8

FOUNDRY PROCESSES

  • Key steps:

    • Moulding sand

    • Mould pouring

    • Melting of metal

    • Pattern making

    • Core making

    • Gating system

    • Casting

    • Heat treatment

    • Cleaning and finishing

  • Solidification

  • Shakeout

  • Inspection

  • Removal of risers and gates

  • Additional heat treatment

  • Defect analysis (pressure tightness, dimensions)

Page 9

BASIC REQUIREMENTS OF FOUNDRY PROCESSES

  1. A mould cavity

  2. A melting process

  3. A pouring technique

  4. The solidification process

  5. Mould removal

  6. Finishing (cleaning, inspection etc.)

Page 10

PATTERNS

  • Definition: A pattern is an exact replica of the part intended for creation.

  • Purpose: Used to create the cavity with the same size and dimensions as the final casting.

Page 11

PATTERN ALLOWANCES

  • Definition: Modifications made to a pattern.

  • Types of allowances:

    1. Shrinkage allowance: Patterns are larger to compensate for liquid contraction during freezing (most important).

    2. Machining allowance: Required to remove rough cast surface (2 to 25 mm / 0.1 to 1 inch).

Page 12

SOLIDIFICATION CONTRACTION FOR VARIOUS CAST METALS

Metal or alloy

Volumetric solidification contraction (%)

Aluminum

6.6

Al-4.5%Cu

6.3

Al-12%Si

3.8

Gray iron

Expansion to 2.5

Carbon steel

2.5-3

Magnesium

4.2

1% carbon steel

4

White iron

4-5.5

Copper

4.9

Zinc

6.5

Page 13

PATTERN ALLOWANCES (CONT.)

  1. Draft allowance: A slight taper to facilitate removal, usually from 1/2 to 2°.

Page 14

PATTERNS FOR SAND CASTING

  • Importance of draft angle for ease of removal from the sand mould.

  • Quality of patterns can affect sand casting outcomes:

    • Damage (poor) or good conditions.

Page 15

PATTERN MATERIALS

  • Types of materials:

    • Wood: Pine, Teak, Mahogany

    • Metal: Cast iron, Brass, Bronze, Aluminum alloys

    • Plastic and Rubber: Quick setting compounds, polystyrene, silicone rubber

    • Plaster: Gypsum plaster, plaster of Paris

    • Other: Wax and mercury

Page 16

CHARACTERISTICS OF PATTERNS

  • Lightweight: For easy handling.

  • Durability: Strong, hard, and durable.

  • Workability: Easy to work, shape, and join.

  • Resistance: To wear, abrasion, corrosion, and chemical attack.

  • Availability: Easily available at low cost.

  • Repair: Easy to repair.

  • Surface finish: Ability to provide a good finish.

Page 17

PATTERN MATERIAL CHARACTERISTICS

Rating

Characteristic

Wood

Aluminum

Steel

Plastic

Cast Iron

Machinability

E, G, F, W

E

G

F

G

G

Wear resistance

P, G, E, F

G

E

F

E

G

Strength

F, G

E

G

G

G

G

Weight

E, G, P

E

G

P

G

P

Repairability

E, G

E

P

G

F

G

Resistance to corrosion

E

E

P

E

P

Resistance to swelling

P, E

E

E

E

E

E

Page 18

TYPES OF PATTERN

  • Types:

    • Single piece or solid pattern

    • Two piece or split pattern

    • Multiple piece or loose pattern

    • Match plate pattern

    • Gated pattern

    • Skeleton pattern

    • Cope and drag pattern

Page 19

CORES: WHAT ARE CORES?

  • Definition: Materials used for making cavities and hollow projections not produced by patterns alone.

  • Capability: Allows for intricate shapes, typically made of sand, and require higher strength than moulding sand due to thermal and mechanical conditions.

Page 20

CORES AND CORE PRINTS

  • Creation: Cores produced separately by pressing sand in core boxes.

  • Support mechanism: Core prints created on the pattern to support the core in the mould.

Page 21

CORE PRINT EXAMPLE

  • Visual representation of core prints relative to the pattern.

Page 22

EXAMPLES OF SAND CORES AND CHAPLETS

  • Illustration showing examples of sand cores, their prints, and chaplets used to support cores in the mould.

Page 23

CHARACTERISTICS OF A CORE

  • Key attributes:

    • Green strength

    • Dry strength

    • Refractoriness

    • Permeability

    • Collapsibility

    • Friability

    • Smoothness

    • Low gas emission

Page 24

TYPE OF CORES

  1. CORE GREEN SAND CORES

    • Made by the pattern itself during moulding.

  2. DRY SAND CORES

    • Made from special core sands in separate core boxes, baked, and set in the mould before pouring.

Page 25

METAL CASTING PROCESSES (CONT.)

Page 26

LESSON OBJECTIVES

  • Differences between two casting processes (expandable and permanent mould casting)

  • Focus on sand casting

Page 27

2 CATEGORIES OF CASTING PROCESS

  1. Expendable mould processes:

    • Uses expendable moulds that must be destroyed to remove casting.

    • Common mould materials include sand and plaster.

  2. Permanent mould processes:

    • Uses permanent moulds that can be reused.

    • Allows for more intricate geometries and is typically made of metal or ceramic refractory material.

Page 28

2 CATEGORIES OF CASTING PROCESS (CONT.)

  • Advantages of permanent mould processes:

    • More economical for high production operations.

    • However, part shapes are limited by the need to open the mould.

Page 29

SUMMARY OF CASTING PROCESSES

Process

Advantages

Limitations

Sand

Almost any metal can be cast; no limit to part size; low tooling cost

Some finishing required; wide tolerances

Shell mold

Good dimensional accuracy; high production rate

Part size limited; expensive patterns

Evaporative

No limit to size; complex part shapes

Patterns have low strength; costly for low quantities

Plaster mold

Intricate shapes; good finish

Limited to nonferrous metals; longer mold-making time

Ceramic mold

Intricate shapes; close tolerance parts

Limited part size

Investment

Intricate shapes; excellent surface finish

Expensive patterns; limited part size

Permanent

Good surface finish; high production rate

High mold cost; complexity; not suitable for high melting-point metals

Die

Excellent dimensional accuracy; high production rate

High die cost; limited to nonferrous metals

Centrifugal

Large parts with good quality; high production rate

Expensive equipment; limited part shape

Page 30

SAND CASTING

  • Definition: Traditional method used in metal casting, primarily utilizing silica sand (SiO2).

  • Sand selection considerations:

    • Fine and round grains contribute to effective packing, strength, and gas permeability.

Page 31

SAND CASTING (CONT.)

  • Mulling machines:

    • Utilized for uniform mixing of sand with additives.

    • Clay (bentonite) acts as a cohesive agent for sand particles.

    • Other common additives include zircon, olivine, iron silicate, and chromite.

Page 32

STEPS IN SAND CASTING

  • Process Overview:

    1. Pattern making

    2. Moulding

    3. Core making

    4. Gating system setup

    5. Sand mould cleaning

    6. Melting of metal

    7. Pouring into mould

    8. Solidification

    9. Shakeout

    10. Inspection

    11. Heat treatment

    12. Finishing (removal of defects, risers, gates, dimensions)

Page 33

SAND MOULD FEATURES

  • Key features:

    • Open riser

    • Vent

    • Pouring basin (cup)

    • Blind flask riser

    • Sprue

    • Cope and drag

    • Choke

    • Runner

    • Gate

    • Mould cavity

Page 34

SAND MOULD

  • An illustrative schematic showing the various features of a sand mold, similar to the previous page.

Page 35

CASTING TERMINOLOGY

  • A pattern: Approximate duplicate of the final casting.

  • Moulding material: Used to pack around the pattern (e.g., sand).

  • Flask: The frame that holds the moulding material (e.g., metals or woods).

  • Cope: The top half of a horizontal-parted two-part mould; Drag: The bottom half of it.

Page 36

CASTING TERMINOLOGY (CONT.)

  1. Core: Inserts into a mould to produce internal features (holes, passages).

  2. Core print: Locates and supports the core within the mould.

  3. Mould cavity: Hole shaped for pouring molten metal.

  4. Riser: Reservoir void compensating for shrinkage during solidification.

Page 37

CASTING TERMINOLOGY (CONT.)

  1. Gating system: Network of channels delivering molten metal to the mould.

  2. Pouring cup: Receives the molten metal first in the gating system.

  3. Sprue: Vertical channel in the gating system for the molten metals.

  4. Runner: A horizontal channel used to transport metal.

  5. Gate: Controls the entrance of molten metal into the mould cavity.

  6. Vent: Provides an escape for gases.

Page 38

RISER-GATED CASTING

  • Schematic illustration of a typical riser-gated casting.

  • Purpose of risers is to supply molten metal as the casting shrinks during solidification.

Page 39

CASTING TERMINOLOGY (CONT.)

  • Additional terms:

    1. Parting line: Interface separating cope and drag.

    2. Draft: Taper facilitating withdrawal from the mould.

    3. Core box: Mould for producing cores.

    4. Casting: Term for both the process and the solidified product.

Page 40

SEQUENCE OF OPERATIONS FOR SAND CASTING

  • Steps in operation: a. Mechanical drawing is used for pattern design, incorporating shrinkage and draft considerations. b-c. Patterns mounted on aligned plates. d-e. Core boxes produce core halves, assembled together for hollow areas in the part.

Page 41

SEQUENCE OF OPERATIONS FOR SAND CASTING (CONT.)

  • Steps in operation: f. Cope mould half is secured to the flask, alignments are established. g. Flask is rammed with sand, removing plates and inserts. h. Drag mould created similarly, with the pattern inserted. i. Inversion of the mould allows pattern removal, leaving impressions.

Page 42

SEQUENCE OF OPERATIONS FOR SAND CASTING (CONT.)

  • Continued operations: j. Cores set in the drag cavity. k. Mould closed by placing cope on top of drag, and molten metal poured through sprue. l. After solidification, casting is removed, and excess material (sprues, risers) recycled. m. Final steps include cleaning, inspection, and necessary heat treatment.

Page 43

SAND CASTING

  • Schematic showing cope & drag molds with cores placed in the drag.

Page 44

SAND CASTING

  • Two sets of castings (bronze and aluminum) resulting from the aforementioned sand mold.

Page 45

DESIRABLE MOULD PROPERTIES AND CHARACTERISTICS

  1. Strength: To maintain shape and resist erosion.

  2. Permeability: Allowing passage of hot air and gases through voids in sand.

  3. Thermal stability: Resisting cracking on contact with molten metal.

  4. Collapsibility: Allowing casting shrinkage without cracking.

  5. Reusability: Can the sand be reused from broken moulds?