Hot Isostatic Pressing to Enhance Inter-Laminar Tensile Strength in Additively Manufactured Carbon Fiber-PEEK Parts

Abstract

Additive manufacturing (AM) of composites is a rapidly expanding technology with an increasing focus on printing end use parts rather than prototyping. Fused filament fabrication (FFF) is relatively inexpensive and fast compared to other AM technologies. The resulting FFF parts, however, suffer low interlaminar mechanical properties due to poor layer-to-layer adhesion and voids. A possible solution to this would be post processing using hot isostatic pressing (HIP). HIP utilizes high temperatures and pressures to consolidate parts in turn reducing void content, enhancing inter-layer adhesion, and increasing the crystallinity of the parts. This paper seeks to investigate the effects of HIP on structure and interlaminar strength of short carbon fiber reinforced polyetherketoneketone (PEKK) parts manufactured via FFF. Coupons were printed vertically (normal to the print bed and along the Z-direction) and post processed for one hour in an in-house HIP device. The HIP pressure was set to 0, 20, and 200 psig and a constant temperature of 165 °C (slightly above its glass transition temperature) was used. HIP treated samples were characterized via tensile tests to evaluate their tensile strength and modulus. Additionally, the samples were analyzed using X-ray micro computed tomography (μCT) device to assess their void content before and after the HIP processing. Structure, morphology, and properties for all tested samples were compared and correlated to the HIP treatment conditions. It was concluded that HIP can effectively increase the inter-layer mechanical properties of FFF parts by up to 50% in strength and 30% in modulus, rendering them more suitable for end use applications. Based on the new understanding of failure resulted from this study, post-treatment strategies to further increase the Z tensile properties in FFF are suggested.