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Lab-on-a-chip (LOC) and other microfluidic devices for medical applications need to be mass produced at a low fabrication cost because the disposable device is destroyed after a single use to avoid sample contamination. In this paper, a new method for rapidly fabricating metallic micromold masters for manufacturing large volumes of polymeric microfluidic devices is presented. Polymers are preferred over silicon as the device material due to their better compatibility with biological and chemical substances. The manufacturing method involves laser micromachining of the desired imprint features from thin metallic sheets and then microwelding them onto a substrate to form the final mold master. The polydimethylsiloxane (PDMS) elastomer is then poured over the mold and cured to produce the microfluidic device. The proposed method involves fewer processing steps than the soft lithography, electroplating and molding (LIGA) process. To verify the method, a metallic mold for a passive Y-channel microfluidic mixer was fabricated. The mold master was made from low-cost steel and the mold manufacturing process can be completed within an hour. PDMS elastomer is then poured over the mold and cured to produce the mixer. The channels of the mixer were 75 micrometers wide and 50 micrometers high. The mixer created from the mold was tested by mixing two streams of colored water in it. The maximum flow rate achieved by the prototype was 6.4 microlitres per minute. The experimental results confirm that a viable metallic mold master for microfluidic devices can be created by combining laser micromachining and microwelding processes. Finally, the limitations of the proposed rapid fabrication method are discussed.