Expanded Polystyrene Lost Foam Casting—Modeling Bead Steaming Operations

[+] Author and Article Information
Douglas M. Matson

Mechanical Engineering Department, Tufts University, Medford, MA 02155Douglas.matson@tufts.edu

Rakesh Venkatesh

Mechanical Engineering Department, Tufts University, Medford, MA 02155

Scott Biederman

 GM Powertrain/Metal Casting Technology, Milford NH 03055

J. Manuf. Sci. Eng 129(2), 425-434 (Sep 13, 2006) (10 pages) doi:10.1115/1.2540608 History: Received December 20, 2005; Revised September 13, 2006

The retention of pattern voids observed in the production of expandable polystyrene patterns for lost foam casting can be traced to conditions developed during mold filling and subsequent steaming. Void formation and closure, or healing, was observed using high-speed video imaging through a clear acrylic sheet cut to match one-half of a test pattern mold. Two processing conditions, i.e., the initial bead packing density and the velocity of steam as it passes between beads, were shown to significantly impact the ability of a void to heal during steaming. A model is proposed to predict conditions where voids will heal based on three criteria that relate to a limitation of the processing window, the void size, and the ability of the bead to swell.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

The acrylic mold cavity used to simulate the moving tool of an injection molding pattern; contains ninety-five 0.5cm diameter vents spaced 2.5cm apart.

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Figure 2

Digital video images taken 0.3s apart showing void formation during the initial stages of steaming. In each image, the leg shown is 2.5cm wide. (a) before steaming, (b) during steaming, and (c) after steaming.

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Figure 3

Digital video images taken 4ms apart showing void healing during steaming operations. The circular object in each image is a 0.5cm diameter slotted vent where steam is exhausted to ambient from the mold cavity. The void is formed by a similar vent pressurized from below with 10kPa(14psig) steam.

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Figure 4

Mold geometry for venting operations. (a) volume element for gas flow through packed beads, (b) developing void in think section of mold, and (c) developing void in thick section of mold. White is void, black is compacted bead region, gray is uncompacted.

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Figure 5

Schematic of the simplified volume element changes occurring during void formation: Phase I of the void life cycle model

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Figure 6

Schematic of the simplified volume element changes occurring during void healing: Phase II of the life-cycle model

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Figure 7

Typical extent of compacted region during Stage I void formation. (a) Uncompacted regions prior to steaming (before image). (b) Same areas following steaming operations (after image). (c) Difference contrast created by subtracting before and after images Each of the five vented legs measures 2.5cm wide.

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Figure 8

Influence of void shape and packing density on void formation for thin and thick geometry molds. We assume that interface growth stops when the critical fluidization velocity decreases to below a critical value when the force ratio decreases below F∕Fo=1.



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