The mid-plane model for warpage simulation of injection-molded parts requires a mid-plane mesh whose transformation is considerably time consuming. To overcome this drawback, a surface model-based warpage simulation is presented, in which the part is represented as a perfect bonding of two half-thickness plates with their reference surfaces at the outer boundary of the part. The. plates over the surface mesh are modeled as flat shell elements, and a new triangular flat shell element is developed which combines an Assumed Natural DEviatoric Strain (ANDES) based membrane component and a Refined Nonconforming Element Method (RNEM) based bending component. The bonding is accomplished by multipoint constraints and a Lagrange multiplier based elimination method is proposed for constraint application. The results show that compared with some popular shell elements, ANSYS, Moldflow and the experiments, the presented model exhibits a high performance in computation accuracy. POLYM. ENG. SCI., 51:785-794, 2011. (C) 2011 Society of Plastics Engineers