Influence of temperature and polymer/mold surface interactions on micro-feature replication at ambient pressure in micro injection molding

Microinjection molding is based on the concept of vario-thermal processing in which the injection unit heats the polymer and presses it into a micro featured mold. After the unit cools, the featured part is de-molded. An inherent problem with microinjection molding is poor feature replication. The polymer in the micro-cavity instantaneously freezes when it comes in contact with the mold wall thus, limiting the achievable aspect ratio of the features in the part.; This study assesses micro-feature replication at elevated mold temperature and ambient pressure using a variety of polymers and commonly used mold surfaces. In order to more fully explore the micro injection molding processing window, a better understanding of the interaction of polymer melt with the mold surface is needed. These interactions can be partially determined by measuring the contact angle of polymer melt directly onto the mold surface which can subsequently be correlated to the wetting and surface tension. Viscosity measurements provided a comparison of the behavior of different polymers to varying shear rates. Molding trials were performed on micro and nano featured mold surfaces at elevated mold temperatures and ambient pressure. Feature replication was analyzed quantitatively using an atomic force microscope, comparing the attained depth of the polymers for different aspect ratio features. A qualitative and dimensional analysis was also performed by field-emission scanning electron microscope. Crystallinity of the polymers in the molded parts was attained by X-ray diffractometer. While feature detail was well replicated for all the polymers, the moldings exhibited poor dimensional accuracy due to high shrinkage in the parts. In general, polymers with low viscosity and crystallinity and a surface tension comparable to that of the mold material showed the best feature replication.