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TECHNICAL PAPERS

A Strategy for Rapid Thermal Cycling of Molds in Thermoplastic Processing

[+] Author and Article Information
Donggang Yao

School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332dong.yao@ptfe.gatech.edu

Pratapkumar Nagarajan

School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332

Lei Li, Allen Y. Yi

Department of Industrial, Welding and Systems Engineering, The Ohio State University, Columbus, OH 43210

J. Manuf. Sci. Eng 128(4), 837-843 (Mar 23, 2006) (7 pages) doi:10.1115/1.2335855 History: Received July 19, 2005; Revised March 23, 2006

Thermal cycling of molds is frequently desired in thermoplastic processing. Thermal cycling of the entire mold with a large mass, however, requires an exceedingly long cycle time. A processing strategy for mold rapid heating and cooling, involving a thin-shell mold and two thermal stations (one hot and one cold), was investigated. Because of its low thermal mass, the shell mold can be rapidly heated and cooled through heat conduction by selectively contacting with the two stations. Numerical simulations were performed to study the effect of different design parameters, including thermal contact resistance, shell material, and shell thickness, on the thermal response at the mold surface. Experimental studies showed aluminum shell molds with a thickness of 1.4mm can be rapidly heated from room temperature to 200°C in about 3s using a hot station at 250°C. The method was used for thermal cycling of embossing tools. Surface microfeatures can be rapidly transferred from thin metallic stamps to polymer substrates with cycle times less than 10s.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 2

Three slabs of materials involved in the two station process

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Figure 3

Rapid embossing of microchannels on HDPE substrates: (a) an embossing master with protruded microfeatures, (b) replicated microchannels, and (c) replication at the edge of the microchannel

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Figure 5

Predicted effect of shell thickness on the surface thermal response: (a) regular plot; (b) normalized plot with constant interfacial conductance; and (c) master plot with adjusted interfacial conductance. The materials for the shell and the hot station are aluminum and stainless steel, respectively. In (a) and (b), the interfacial conductance is set to 3000W∕m2K. In (c), the interfacial conductance is 6000, 3000, 1500W∕m2K, respectively, for shell thicknesses of 0.5, 1, and 2mm.

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Figure 6

Predicted surface thermal response of stainless steel shells with varied thickness in contact with a stainless steel hot station. The interfacial conductance is set to 2500W∕m2K. An experimental thermal response with a 1-mm-thick stainless steel shell is included for comparison.

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Figure 10

Experimental heating and cooling response for a 1.4-mm-thick aluminum shell during thermal cycling. The hot station temperature is 250°C and the cold station temperature is 25°C. Predicted thermal response with an interfacial thermal conductance of 3000W∕m2K is given for comparison purposes.

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Figure 11

Surface topology of HDPE sheet (a) before and (b) after embossing

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Figure 1

Schematic principle of the two-station process for rapid thermal cycling of thin-shell molds

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Figure 4

Predicted surface thermal response of a 1.4-mm-thick aluminum shell in contact with a stainless steel hot station. The dimensionless temperature, T̃s, is defined as T̃s(t)=[Ts(t)−Ti]∕(Th−Ti), where Ti and Th are the initial temperatures of the shell mold and the hot station, respectively. The interfacial conductance, h, varies from 500W∕m2K to infinity. An experimental thermal response with an aluminum shell at the same thickness is included for comparison. The simulation result agrees well with the experimental one with h≈3000W∕m2K.

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Figure 7

Predicted heating and cooling response during thermal cycling of a 1.4-mm-thick aluminum shell. The interfacial thermal conductance is set to 3000W∕m2K. Thermal responses with different heating times are compared.

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Figure 8

Experimental heating response of a 1.4-mm-thick aluminum shell in contact with a stainless steel hot station at 250°C. Predicted heating response from numerical simulation with an interfacial thermal conductance of 3000W∕m2K is given for comparison purposes.

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Figure 9

Experimental heating response of a 1-mm-thick stainless steel shell in contact with a stainless steel hot station at 250°C. Predicted heating response from numerical simulation with an interfacial thermal conductance of 2500W∕m2K is given for comparison purposes.

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