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Research Papers

Effect of Micro-Injection Molding Processing Conditions on the Replication and Consistency of a Dense Network of High Aspect Ratio Microstructures

[+] Author and Article Information
John W. Rodgers

Mechanical Engineering and Mechanics,
Lehigh University,
Bethlehem, PA 18015
e-mail: johnwilliamrodgers@gmail.com

Meghan E. Casey

Bioengineering,
Lehigh University,
Bethlehem, PA 18015

Sabrina S. Jedlicka

Materials Science and Engineering,
Bioengineering,
Lehigh University,
Bethlehem, PA 18015

John P. Coulter

Mechanical Engineering and Mechanics,
Lehigh University,
Bethlehem, PA 18015

1Corresponding author.

Contributed by the Manufacturing Engineering of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received April 29, 2013; final manuscript received January 15, 2014; published online February 20, 2014. Assoc. Editor: Ashutosh Sharma.

J. Micro Nano-Manuf 2(1), 011006 (Feb 20, 2014) (8 pages) Paper No: JMNM-13-1023; doi: 10.1115/1.4026606 History: Received April 29, 2013; Revised January 15, 2014

When molding macroscale polymer parts with a high density of microfeatures (>1 × 106/cm2), a concern that presents itself is the ability to achieve uniform replication across the entire domain. In the given study, micro-injection molding was used to manufacture microfeatured polymer substrates containing over 10 × 106 microfeatures per cm2. Polystyrene (PS) plates containing microtopography were molded using different processing parameters to study the effect of flow rate and mold temperature on replication quality and uniformity. Flow rate was found to significantly affect replication at mold temperatures above the glass transition temperature (Tg) of PS while having no significant effect on filling at mold temperatures below Tg. Moreover, replication was dependent on distance from the main cavity entrance, with increased flow rate facilitating higher replication differentials and higher replication near the gate. Simulation of the molding process was used to corroborate experimental trials. A deeper understanding of polymer fluid behavior associated with micro-injection molding is vital to reliably manufacture parts containing consistent microtopography (Note: Values are expressed in average ± standard error).

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References

Figures

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Fig. 1

Supported (a) versus Unsupported and (b) high aspect ratio microfeature

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Fig. 2

Schematic of silicon wafer surface prior to etching (a) and dimensions of photoresist windows on silicon substrate with the dimensions of the tooling inset (b)

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Fig. 3

Micromold aluminum insert assembly exploded (a) and assembled (b). TC = thermocouple.

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Fig. 4

Full mold assembly schematic with aluminum insert fixed to moving mold half

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Fig. 5

Schematic of molded part (a), locations of measured regions (b), actual molded part (c), and meshed model containing three higher mesh density locations (d)

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Fig. 6

Microfeatured silicon mold insert micrograph (a) and evidence of slight microchannel widening (inset; indicated with arrow). Bar indicates 2 μm.

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Fig. 7

Trial 1 micrographs of topography near gate of Tmold = 65.6 °C (a) versus Tmold = 76.7 °C at identical flow rate of 7.54 cm3/s

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Fig. 8

Cross-WLF plot of viscosity versus shear rate (a) and viscosity versus melt temperature for different melt temperatures (b) of 666D

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Fig. 9

Trial 1 micrographs of topography near gate showing the effect of flow rate on replication at Tmold = 76.7 °C. Substrates were molded with Qinj = 2.51 cm3/s (a), 5.03 cm3/s (b), and 7.54 cm3/s (c).

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Fig. 10

Trial 2 micrographs of 9 regions of PS molded with flow rate of 2.01 cm3/s

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Fig. 11

Trial 2 micrographs of 9 regions of PS molded with flow rate of 4.02 cm3/s

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Fig. 12

Trial 2 micrographs of 9 regions of PS molded with flow rate of 6.03 cm3/s

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Fig. 13

Pillar heights along the part for different flow rates at a mold temperature of 104.4 °C

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Fig. 14

Velocity streamlines showing outward flow near tip and slight backward flow after initial filling

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Fig. 15

Micrograph of pillars with slightly stretched scallop marks closer to base of pillar

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