| Melt temperature, mold temperature, pack pressure, and switchover pressure were varied to study their effects on replication of nanoscale features injection molded using three semi-crystalline materials, polypropylene, polyoxymethylene, and polyethylene terephthalate. Atomic force microscopy showed that depth ratio (i.e., the ratio of feature depth to tooling projection) and surface roughness improved with increases in melt temperature, mold temperature and pack pressure, but that switchover pressure did not significantly affect feature replication. Feature depth in semi-crystalline polymers did not correlate with melt viscosity or the activation energy for flow, but depended on the cooling time, solidification range---as measured using the deflection temperature under load (DTUL), and the temperature range associated with crystallization. In contrast, surface roughness depended on surface crystallinity, DTUL, and cooling time. The semicrystalline polymers provided deeper features, but rougher surfaces, than similar features molded using amorphous polymers. Moreover, the processing window shifted to moderate melt temperatures and lower mold temperatures (in contrast to amorphous materials which preferred high mold and melt temperatures). |
