Single thermoplastic pellet molding by means of diode laser for micromolding application

Polymer Engineering and Science, Feb, 2007 by Isabella Citrea, Fabrizio Quadrini

INTRODUCTION

Polymer micromolding is one of the most promising fabrication techniques for non-electronic microdevices, depending on the negligible material cost and productivity. Moreover, plastic materials are a very large material class, which allows one to find a suitable polymer for nearly every application. Micromolding has been employed for the fabrication of a variety of polymer components. Most applications are in the field of micro-optics (CD ad DVD for data storage, spectrometers, lenses, optical switches and fiber connectors, and waveguides) and micro-fluidic devices (pumps, nebulizers, ink jets, capillary systems, and flow sensors). There are also some examples of micro-electrical and mechanical devices. Several commercial machines are available for polymer micromolding processes, but they were all originated adapting the systems for macroscopic processes to the microscale. There are four processes which are employed for micromolding of thermoplastic materials (injection molding, hot embossing, injection compression molding, and thermoforming) and one for thermosetting materials (reaction injection molding). In microinjection molding and hot embossing the mold cavity is equipped with a microstructured tool (mold insert). Differently from injection molding, during hot embossing, the polymer flows through a very short way and as a result very little stress is produced in the molded parts. Thus more delicate microstructures with higher aspect ratios are fabricated by hot embossing. Very short set-up times are required for hot embossing but very high process times are necessary compared to injection molding. In fact, in injection molding the molten polymer is filled into a mold insert which is colder than the polymer softening point, while in hot embossing the polymer is heated up by the same mold insert. A combination of injection molding and hot embossing is the injection compression molding, which overcomes the problem of heating the polymer by the tool. The material is injected from a screw in a semi-closed tool and is subsequently pressed by closing the tool. Time cycles are strongly reduced even if machine complexity increases and flow effect on the material are not completely eliminated because of the presence of screw [1].

Recently, Su et al. [2] have studied the microstructure replication by microinjection molding for microdevice fabrication. They adapt a conventional injection molding machine to the mass replication of polymeric microstructures starting from an appropriate mold design and process control. Wet-etched silicon wafers are used in this case as mold inserts. At the end they observe that melt temperature is the key factor among all the process parameters, depending on the quality of microinjection-molded structures. However, quality is defined only in terms of mold filling and no mention is given to polymer stability. Melt temperature is used principally to compensate insufficient machine pressure by means of melt viscosity reduction. It is not considered that combining high pressure (resulting from the restricted flow path) and high temperature could produce high material degradation [2].

Zhao et al. [3] defined a new micromolding machine where the system is composed of a screw plastification barrel, a plunger injection system, and a melt dosage control. A commercial injection microextruder was used to fill a small diameter plunger, which was in turn employed for mold filling. The proposed system appears to be more complex than the simple injection molding and a comparison is not performed between the two different processes. They fabricated a series of micro-gears using POM and statistically evaluated the effect of process parameters on part quality. Also in this case part quality was evaluated not considering process effects on material but examining part weight, dimensions, and surface. At the end they observed that holding pressure time and metering size (and not melt temperature as in Ref. 2) were the most significant factors affecting part weight and diameter [3].

Moreover, Yu et al. [4] also studied injection molding with microfeatures for bioMEMS applications. They tested various types of mold inserts (one CNC machined, one photoresist, and several nickel molds) by means of an ordinary injection molding machine. They really obtained that melt temperature on microchannel filling was relatively insignificant [4].

It is not clear from scientific literature the influence of process parameters on micromolding, as the aforementioned studies show. In such cases melt temperature is indicated as the most important factor while in other cases it is considered almost insignificant. Probably this strangeness is due to the great effect that the single injection machine can determine on the micromolding process. In any case it is well know that from the material point of view macro- and microinjection molding are strongly different. A lot of microscale effects must be considered in microinjection molding optimization.