Casting Processes

Tim Goldmann – General Overview Caleb Hanson – Molten Metal Phillip Pinsonneault – Mold Design Rey Jordan – Solidification and Inspection

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History of Casting

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Advantages • 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

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Disadvantages • 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 Molds • Green Sand • Dry Sand • Shell • Investment • Plaster

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Expendable Molds • Pattern must be produced • New mold for every cycle

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• Die • Centrifugal • Pressure • Injection Permanent Molds

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

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

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Induction Furnaces • Magnetic field • Rapid melting rate • Low pollution

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Foundries • • Pattern and Mold Making – Computer Aided Design – Manufacturing – Rapid Prototyping The Casting Process – Metal Melting – Metal Compositions – Impurities – Pouring into molds

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Foundries

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

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

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Why Fluid Flow?

• When Heated Metal Becomes Liquid • Important in Cast Design - Sprue - Well - Runner - Gate

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

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Tests for Fluidity • There are several different fluidity tests • All use the same principle • Useful in simulating how metal will react • See how far the metal will travel before solidification

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Heat Transfer • Heat transfer is important in many aspects of casting - Pouring - Solidification - Cooling rate - Rate is a function of Volume and surface area

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Design, Materials and Economics

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Design Considerations in Casting • Design of the part • Casting process and material • Locate parting line • Locate design gates • Locate mold features such as sprue, screens and risers

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

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Design Consideration of Cast Parts

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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

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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

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Design Consideration of Cast Parts • Draft – Used to enable easy removal of pattern without damage to mold – Range from 5 to 15mm/m – Angles of 0.5° to 2° • Dimensional Tolerances – As wide as possible within limits of good performance – Letters and markings

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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

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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

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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

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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

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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

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Molten Metal Pour

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Solidification

Solidification Temperatures

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Dimensional Change

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Pinholes

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Blowhole

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Scab

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Pinholes

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Rough Surface

References • ABB, Automation Solutions, 2005, 10/01/2005, www.abb.com

• Brown Jerry, Primary Metals, 10/01/2005, www.p2pays.org

• ECO BRASS workability, 10/01/2005, www.ecobrass.com

• 2.008 Casting, 10/01/2005, www.ocw.mit.edu

• Wright, Thomas, Processes of Manufacturing, Goodheart Wilcox Company, Inc., 1990 • Lindbeck, John R., Product Design and Manufacture, Prentice Hall, 1995 • Kalpakjian & Schmid, Manufacturing Engineering and Technology, Prentice-Hall, Fifth Ed.

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