Injection molding of polymeric LIGA HARMs

Abstract

The primary goal of an ongoing research effort at LSU is to develop the three-step LIGA process to inexpensively manufacture high aspect ratio microstructures (HARMs). The ®rst two steps of the process (lithography and electroplating) produce a metallic mold insert that can be used as a template for molding microstructures. The ®nal step of LIGA is molding. This paper focuses on injection molding of thermoplastics to produce surfaces covered with HARMs. The resulting microstructures are hundreds of micrometers in height, tens of micrometers in width, and separated by gaps on the order of tens of micrometers. Injection molding experiments using high density polyethylene were performed using a commercially available injection molding machine. Experimental variables included injection speed, the tool temperature, and air pressure in the mold cavity. Elevating the tool temperature above the melting point ensured that the polymer completely ®lled the mold, producing microstructures with the desired geometry. As the temperature of the mold was reduced, higher injection speeds did not necessarily ensure ®lling of the mold cavity. The cycle time is shorter than the values previously reported in the literature [Madou (1996)].

Microinjection Molding of Enhanced Thermoplastics

“…Since the start of the micro technology development, is has been referred as a suitable technique for fabrication of the high aspect ratio microstructures with surface roughness down to 30 nm and such a low lateral resolution as 200 nm (Despa et al, 1999;Hormes et al, 2003;Munnik et al 2003). Being a multi stage process, LIGA may be divided in three main steps: lithography, electroforming and plastic molding (Fig.…”

“…Although with deep ray X LIGA it is possible to obtain very accurate patterns of micro features with high aspect ratios up to 100 and micro structures with size less than 250 nm, it is still not a widespread commercial technique for micro replication, being time consuming and costly (Despa et al, 1999;Meyer et al, 2008). UV-LIGA and IB (Ion Beam) LIGA technologies are less complex and costly comparing to the X-ray LIGA process.…”

“…Moreover, high injection speed and complex geometry of micro features may contribute for air entrapment and could eventually lead to incomplete filling, especially in case of the blind hole features with high aspect ratios (Heckele & Schomburg, 2004). In a number of studies, evacuation of air from the cavity prior to injection is referred as an efficient method for improvement of the microparts replication (Despa et al, 1999;Heckele & Schomburg, 2004;Sha et al, 2005;Liou & Chen, 2006;Chang et al, 2007;Sha et al, 2007b). A typical layout of the mould evacuation system is shown on Fig.…”

“…For example, it could be attributed to the fact that micro cavities are aligned with the parting line of the mould and therefore air can escape easily through the partition (Sha et al, 2005). To assist more efficient air evacuation, the mould platens must be properly adjusted for hermetic sealing of the cavity (Despa et al, 1999). Fig.…”

“…Moreover, at higher mould temperature viscosity of the polymer melt significantly decreases and therefore requires less injection pressure and speed (Despa et al, 1999). Even in the absence of the variotherm cycling, higher mould temperature was reported to promote better cavity filling for a wide range of the tested polymers (Zhao et al, 2003;Sha et al, 2005;Michaeli et al, 2007b;Sha et al, 2007a;Sha et al, 2007b;Tofteberg & Andreassen, 2008).…”