Casting Processes Contents General Overview Molten Metal Mould

Casting Processes Contents • General Overview • Molten Metal • Mould Design • Solidification and Inspection

History of Casting

Advantages of Castings • Complex forms, low cost • Certain shapes cannot be machined • One piece parts vs. multiple piece parts • Design changes are easily incorporated • High volume, low skilled labor • Large, heavy parts can be made easily

Disadvantages of Castings • Problems with internal porosity • Dimensional variations due to shrinkage • Trapped impurities, solids and gasses • High-tolerance, smooth surfaces not possible • More costly than stamping or extruding in some cases

Expendable Moulds Green Sand Dry Sand Shell Investment Plaster

Expendable Moulds Pattern must be produced New mould for every cycle

Permanent Moulds Die Centrifugal Pressure Injection

Permanent Molds Made of durable material Multiple use Built-in Alignment

MELTING

Cupola Furnaces Five Zones Stack Charging Zone Melting Zone Tuyers (Air Intake) Crucible or Hearth

Induction Furnaces Magnetic field Rapid melting rate Low pollution

Foundries

Foundry Automation • • • Pouring into Moulds Cleaning Heat Treatment Inspection Automated Guided Vehicles – Automatic Storage – Moving Cores and patterns – Etc.

Reasons For Automation • Increased Manufacturing Efficiency • Reducing Costs • Reliability in Harsh Environments • Release of Skilled Man Power • Maximize Space • Improved Quality

Why Fluid Flow? • When Heated Metal Becomes Liquid • Important in Cast Design - Sprue - Well - Runner - Gate

Fluidity of Metals • • • Viscosity Surface Tension Inclusions Mould Design Degree of Super Heat Mould Material - Thermal Conductivity - Surface Roughness • Heat Transfer

• Design Consideration of Cast Parts Corners, angles, section thickness – Avoid sharp corners angles and fillets – Stress raisers, cracking, and tearing – Sections should be blended smoothly

Design Consideration of Cast Parts

Design Consideration of Cast Parts • Flat Areas – Large flat areas may warp during cooling – Poor surface finish from uneven flow in pouring – Resolve with ribs, and serrations • Circles and Rounds – Lower cooling rate in circles induces hot spots – Shrinkage cavities and porosity

Design Consideration of Cast Parts • Shrinkage – to avoid cracking during the cooling process there should be shrinkage allowance – Staggered ribs, change the intersection geometry • Patternmaker’s Shrinkage Allowance – 10 -20 mm/m

Design Consideration of Cast Parts • Draft – Used to enable easy removal of pattern without damage to mold – Range from 5 to 15 mm/m – Angles of 0. 5° to 2° • Dimensional Tolerances – As wide as possible within limits of good performance – Letters and markings

Locating the Part Line • Part line – separates upper and lower mold – Flat plane, along corners or edges – This will avoid flash at the parting lines

Locating and Designing Gates • Gates – Connections between runners and the part – Multiple gates preferred – Feed into thick sections of the castings • Runners – Distribution channel for molten metal from the sprue into the gates

Casting Alloys • Nonferrous Alloys – Aluminum, magnesium, copper, zinc, tin, lead – Good electrical conductivity and resistant to corrosion – Most of these alloys share characteristics of good machinability and can all be welded

Casting Alloys • Ferrous alloys – Cast irons • Largest quantity of all metals cast • Good wear resistance, hardness and machinability – Cast Steels • Material used under extreme heat conditions • Railroad, mining and construction

Casting Economics • Cost – Depends on materials, equipment, and labor – Preparations require raw materials, time and effort – Melting, pouring, heating, cleaning, inspections – Equipment cost lowers with large # of castings – Solutions to alleviate unneeded cost are being looked into • Rapid prototyping

Molten Metal Pour

Solidification

Solidification Temperatures

Casting Defects

Dimensional Change

Pinholes

Blowhole

Scab

Pinholes

Rough Surface