Material Extrusion-Based Additive Manufacturing with Blends of Polypropylene and Hydrocarbon Resins

Abstract

Filament-based material extrusion additive manufacturing (MEAM) is one of the most commonly used techniques in additive manufacturing (AM). In spite of recent notable development in the MEAM process, there is still a need to develop more materials that can be printed consistently using this technique. Isotactic polypropylene (PP), a popular thermoplastic material, undergoes rapid crystallization and subsequent volume contraction. This can lead to residual stress buildup in PP parts when processed using MEAM, resulting in poor adhesion to the printing platform, poor geometric tolerance, and mechanical performance. In this work, the effects of varying composition of low molecular weight hydrocarbon resins incorporated to PP are investigated. Specifically, the thermal behavior, crystallization, morphology, and printability of the blends are studied. The rapid crystallization of PP has been delayed by the addition of hydrocarbon resins that provided a larger time window for the residual stresses to relax. The addition of the resins to the pure PP matrix lowers the crystallization temperature of PP from 121.8 to 116.0 °C, which further enables additional diffusion during the solidification process. Polarized optical microscopy demonstrates the differences in crystalline morphology, which is expected to impact the structure at the interlayer boundaries between deposited layers. The combination of modifications in crystallization rate, time, and morphology significantly affects the interlayer adhesion and residual stress state, which directly controls the mechanical properties and part warpage of printed parts. Tensile bars of the different blends were printed in two different orientations to analyze the mechanical performance and study part anisotropy. The maximum tensile stress of pure PP (26.8 ± 2.1 MPa) printed at a ±45° raster angle increased with addition of 20 wt % hydrocarbon resin (32.4 ± 3.0 MPa) when printed under the same conditions. The improvement in the tensile strength is due to a combination of changes in crystallinity, morphology, and improved interlayer adhesion during printing. The parts were annealed postprinting to improve polymer chain diffusion across the layers, thereby improving interlayer adhesion and resulting in tensile modulus and strength values in excess of 90% of injection-molded PP parts.

Effect of extrusion and fused filament fabrication processing parameters of recycled poly(ethylene terephthalate) on the crystallinity and mechanical properties

“…printing temperature and build plate temperature), the level of interlayer diffusion and void fraction. [52][53][54][55] This section focuses on these factors and how they affect the mechanical performance of the printed rPET, compared to printed vPET. Firstly, only the Ultimaker 3, requiring filaments with a diameter of 2.85 mm, was used to investigate the influence of the different printing parameters.…”

Rheological investigation of nylon‐carbon fiber composites fabricated using material extrusion‐based additive manufacturing

“…Changes in the total crystalline content of semicrystalline polymers play a major role in dictating the mechanical properties of the 3D printed parts. [13,14] Weighed amounts (5-8 mg) of samples were loaded and pressed into a TA Instruments aluminum T zero pan, and DSC was carried out using a Q2000 Series TA Instruments differential scanning calorimeter. The tests were performed using a 50 ml min À1 nitrogen flow in order to minimize polymer degradation.…”

“…The broader crystallization window ensures the polymer chains stay in the melt state for a longer time prior to coalescence (weld formation typically occurs above glass transition temperature for amorphous polymers and above the T c for semicrystalline polymers), which results in better residual stress dissipation due to increased chain mobility and therefore ensures printed parts with minimal warpage and improved dimensional stability. [14,19] The total crystalline content in the samples is an important factor in controlling the mechanical properties of the final printed part. Any adverse effect on the degree of crystallinity needs to be minimized in order to maintain properties in the printed parts comparable to traditionally processed parts.…”

Advances in modeling transport phenomena in material-extrusion additive manufacturing: Coupling momentum, heat, and mass transfer

“…The fundamental mechanisms of polymer crystallization are detailed in a review article elsewhere [81]. The use of semi-crystalline polymers in MatEx has sparked significant interest but is largely relegated to academic studies [82][83][84][85][86]. Briefly, the crystalline regions in these polymers act as physical crosslinks that restrict movement of the polymer chains, thereby strengthening the material [19,20].…”