Stamp Thermo-Hydroforming: A New Method for Processing Fiber-Reinforced Thermoplastic Composite Sheets

Abstract

The stamp thermo-hydroforming process involves supporting the thermoplastic sheet with a bed of heated viscous fluid that applies a hydrostatic pressure across the part throughout the forming process. This hydrostatic pressure produces a through-thickness compressive stress that delays the onset of delamination, reduces the formation of wrinkles due to frictional traction forces, and results in better-formed parts. The overall goal of this research is to verify, through experimentation and numerical modeling, that the stamp thermo-hydroforming process provides a suitable alternative to conventional thermoforming methods, such as bag molding and stamp forming, as a means for processing glass-mat fiber-reinforced thermoplastic materials. The goal of this paper is to solely address whether the use of a counteracting pressure can be beneficial during the forming of fiber-reinforced thermoplastic sheets more specifically; this paper addresses the experimental setup that was developed and built to conduct small-scale stamp thermo-hydroforming experiments with a hemispherical punch, and discusses some results obtained using this setup. Experiments were performed on a random oriented continuous glass-mat fiber-reinforced polypropylene with hydrostatic pressure applied locally on the top surface of the composite sheet, as it was being stretched by a hemispherical punch. An increase of up to 10% in draw depth was achieved by applying local hydrostatic pressures up to 207 kPa (30 psi). In addition to the draw depth improvements, the application of the counteracting hydrostatic pressure also delayed the onset of delamination.