•HDPE polymer HX is fabricated using layer-by-layer line welding of plastic sheets.•Experimental testing of the HX has been successfully performed.•The polymer-based wall thermal resistance is no longer the limiting factor.•The polymer HX shows superior air-side performance over plane plate fin surface. In addition to their low cost and weight, polymer heat exchangers offer good anticorrosion and antifouling properties. In this work, a cost effective air-water polymer heat exchanger made of thin polymer sheets using layer-by-layer line welding with a laser through an additive manufacturing process was fabricated and experimentally tested. The flow channels were made of 150μm-thick high density polyethylene sheets, which were 15.5cm wide and 29cm long. The experimental results show that the overall heat transfer coefficient of 35–120W/m2K is achievable for an air-water fluid combination for air-side flow rate of 3–24L/s and water-side flow rate of 12.5mL/s. In addition, by fabricating a very thin wall heat exchanger (150μm), the wall thermal resistance, which usually becomes the limiting factor on polymer heat exchangers, was calculated to account for only 3% of the total thermal resistance. A comparison of the air-side heat transfer coefficient of the present polymer heat exchanger with some of the commercially available plain plate fin heat exchanger surfaces suggests that its performance in general is superior to that of common plain plate fin surfaces.