Influence of Single Point Incremental Forming on Mechanical Properties and Chain Orientation in Thermoplastic Polymers

Abstract

Incremental forming of thermoplastic surfaces has recently received significant interest due to the potential for simultaneous reduction in thermal energy consumption and in part-shape specific tooling. This paper examines the mechanical properties and the chain orientation of the formed material in single point incremental forming (SPIF) of amorphous polyvinyl chloride (PVC) and semicrystalline polyamide sheets. Tensile and stress relaxation properties of the formed polymers are compared to those of the unformed polymer. The effect of incremental depth and tool rotation speed on the above properties, and on the temperature rise of the sheet during SPIF, is quantified. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) are used to compare the chain orientation and crystallinity of the formed and the unformed polymers. It is observed that the formed material has greater toughness and ductility, but lower yield stress and reduced Young's modulus, as compared to the unformed material. We also observe deformation-induced chain reorientation in the formed polymer, with minimal change in the degree of crystallinity. The link between the SPIF process parameters, temperature rise of the polymer during SPIF, change in chain orientation, and change in mechanical properties of the polymer is discussed.

Post-Forming Mechanical Properties of a Polymer Sheet Processed by Incremental Sheet Forming: Insights into Effects of Plastic Strain, and Orientation and Size of Specimen

“…The plastic forming processes affect the post-forming properties of the thermoplastics and thus, influence the service performance of the formed components [ 20 ]. The early investigation on this topic in ISF was undertaken by Devarpanah et al [ 21 ]. They, keeping the strain to a fixed value, reported that the mechanical deformation causes a reduction in the tensile yield strength and tensile modulus, while an increase in the tensile ductility and ultimate tensile strength.…”

“…As regard the correlation between the process parameters and post-forming properties, the nature and degree of their effects are found to be material dependent [ 18 ]. From the structural analyses, they revealed that the deformation reorients the molecular chains, and further, voids are formed whose density increases with increasing of the step size, thereby causing a loss of post-forming yield strength [ 18 , 21 ]. Lozano-Sánchez [ 22 ] further studied the orientation of molecular chains in the formed polymers and clarified that the chains on the inner surface are preferentially aligned in the horizontal feed direction, reasoning to tool’s shearing action, while those on the outer surface are pulled downwards representing the dominant effect of sheet stretching.…”

“…Although, as discussed in the last paragraph, this subject has received attention in ISF of the polymers, the reported studies are fundamental and further work is required to comprehend the knowledge. The deformation effects on the post-ISF tensile properties have been undoubtedly investigated [ 18 , 21 , 23 ]; however, the strain level was kept constant throughout the investigations. In fact, according to the literature published on traditional cold forming of polymers, the degree of plastic strain substantially impacts the chain orientation and thus, the post-forming mechanical properties of polymers [ 20 ].…”

“…As discussed above, the ISF process affects the chain orientation and further the orientations on the inner and outer surfaces are different [ 21 , 22 ] thereby signifying inducement of directional properties in a formed polymer component. Therefore, the dependence of resulting properties of a formed polymer on the direction or orientation of a specimen (say tensile specimen), with respect to the tool travel, should be investigated.…”

Incremental sheet forming of thermoplastics: a review

“…Davarpanah et al [67] found the increase of void density in the formed PLA material can lead to the enhanced formability. In other studies [81,82], the authors observed there is a change in orientation of crystalline lamella and molecular chain orientation in thermoplastics due to the deformation of ISF process with a slight change in the total degree of crystallinity. The schematic for the orientation change in ISF could be presented as Fig.…”