Experimental Investigation of Mechanical Properties of 3D-Printed Components Fabricated from Waste Plastic Extruded Filaments

Abstract

Abstract The exponential global growth of post-consumer plastic waste poses severe environmental and socio-ecological challenges, necessitating innovative, decentralized recycling strategies. Simultaneously, the rapid industrial and domestic proliferation of Fused Deposition Modeling (FDM) 3D printing has dramatically increased the consumption of virgin thermoplastic filaments such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG) thereby compounding virgin petrochemical resource depletion, supply chain vulnerabilities, and escalating material costs for educational institutions and small-scale makerspaces. This study explores a robust, sustainable alternative by converting municipal post-consumer plastic waste specifically high-density polyethylene (HDPE) milk jugs and recycled polypropylene (PP) packaging containers into usable, high-performance 3D printing filaments utilizing an optimized desktop single-screw extrusion system. Standardized tensile test specimens were fabricated via FDM in strict accordance with ASTM D638 Type IV dimensional specifications to evaluate critical mechanical properties, including ultimate tensile strength, elastic modulus, and elongation at break. The mechanical performance of the recycled waste-derived plastic parts was systematically characterized and benchmarked against standard commercial virgin PLA filaments. Comprehensive experimental results indicate that while recycled polyolefin filaments exhibit a moderate reduction in tensile strength and stiffness averaging approximately 18% to 25% lower than virgin PLA counterparts due to cumulative thermal-mechanical degradation and polymer chain scission during multiple processing lifecycles they concurrently display exceptionally superior impact resistance, high toughness, and remarkable ductility. This research demonstrates the absolute technical and economic viability of closed-loop recycling at a local, institutional, and diploma-laboratory scale, offering an economical, highly sustainable, and environmentally benign material stream suitable for prototyping, functional mechanical testing, and small-scale manufacturing applications. Keywords: Additive Manufacturing, Plastic Recycling, Fused Deposition Modeling, Mechanical Properties, Sustainable Engineering, Filament Extrusion, Polymer Degradation. 1. Introduction Additive manufacturing (AM), most notably Fused Deposition Modeling (FDM), has fundamentally transformed rapid prototyping, tooling fabrication, and small-batch production paradigms across diverse engineering domains owing to its design complexity freedom, low barrier to entry, and capital cost-effectiveness (Gibson, Rosen and Stucker, 2021). However, the vast majority of commercial FDM printing ecosystems rely heavily on virgin thermoplastic polymers derived from petrochemical or specialized bio-derived sources, including Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS), and Polyethylene Terephthalate Glycol (PETG). The continuous, unmitigated reliance on virgin feedstocks stands in direct conflict with emerging global circular economy mandates, carbon neutrality goals, and municipal waste reduction objectives (Nizami et al., 2020). Concurrently, municipal solid waste (MSW) streams globally are heavily saturated with post-consumer packaging materials, with High-Density Polyethylene (HDPE) and Polypropylene (PP) constituting a dominant fraction of discarded consumer goods. Diverting these abundant polymer streams from landfills, incineration plants, or natural ecosystems through systematic sorting, cleaning, mechanical shredding, re-melting, and precise desktop extrusion into custom 3D printing filaments presents a profound dual benefit: it drastically curtails raw material procurement costs for hobbyists, technical high schools, diploma-level engineering laboratories, and small enterprises, while simultaneously mitigating environmental pollution and lowering the carbon footprint of additive manufacturing (Kreiger et al., 2014). Despite the compelling economic and ecological appeal, recycled polymer filaments frequently suffer from structural inconsistencies, localized void formation, moisture entrapment, and polymer chain scission induced by cumulative thermal-mechanical degradation during repeated melting and extrusion cycles (Cruz Sanchez et al., 2020). Understanding the precise mechanical behavior, stress-strain response, and anisotropic failure mechanisms of parts printed from waste-derived plastics is an essential prerequisite before these secondary materials can be reliably deployed in functional mechanical elements, load-bearing fixtures, or structural prototypes. This comprehensive study investigates the tensile, structural, and physical characteristics of mechanical test specimens printed from waste-derived HDPE and PP filaments, establishing a rigorous baseline comparison with commercial virgin PLA. 2. Methodology The experimental methodology adopted for this study was structured around a systematic, closed-loop processing workflow designed to transform raw municipal plastic waste into standardized, mechanically testable FDM specimens. 2.1 Materials Preparation and Cleaning Protocols Post-consumer packaging waste, specifically comprising rigid HDPE milk jugs and injection-molded PP food containers, was collected from local institutional recycling bins. The collected waste streams were meticulously sorted by resin identification code (SPI codes 2 for HDPE and 5 for PP) to prevent polymer incompatibility and phase separation during thermal processing. The sorted items were stripped of paper labels, adhesive residue, and metallic caps. Subsequently, the plastic components were subjected to an intensive cleaning protocol involving hot water immersion (60∘C) infused with industrial surfactant detergent for 45 minutes to remove organic contaminants, grease, and dust. Following washing, the materials were rinsed thoroughly with distilled water and air-dried for 24 hours. The cleaned polymers were then fed into a heavy-duty mechanical rotary shredder equipped with hardened steel blades, reducing the material into uniform flake sizes ranging strictly between 3 mm and 5 mm to ensure consistent feed rates during subsequent extrusion. 2.2 Experimental Procedure: Filament Extrusion and Thermal Control The shredded polymer flakes were transferred to a laboratory convection drying oven and maintained at a constant temperature of 70∘C for a duration of 6 hours to eliminate all residual moisture, thereby preventing hydrolytic degradation, steam bubble generation, and micro-void nucleation during melting.Filament extrusion was executed using a specialized desktop single-screw filament extruder equipped with a 25 mm diameter extrusion screw possessing a length-to-diameter (L/D) ratio of 24:1 and a compression ratio of 3:1. The extruder operated at a constant, optimized screw rotational speed of 20 rpm. The barrel was divided into three distinct thermal zones feed zone, compression zone, and metering zone with temperatures precisely maintained between 180∘C and 210∘C tailored to the specific melting point and rheological profile of the polymer (lower for HDPE, slightly higher for PP).The molten polymer was forced through a precision-machined 2.0 mm steel nozzle die. As the extrudate emerged, it was immediately passed through a temperature-controlled, 1.5-meter chilled water-cooling bath maintained at 20∘C to arrest polymer crystallization and stabilize diameter shrinkage. The extruded filament diameter was continuously monitored in real-time using a dual-axis digital laser micrometer and maintained at a stringent target diameter of 1.75±0.05 mm by balancing extrusion output velocity with a synchronized, variable-speed motorized spooler. 2.3 Specimen Design and FDM Additive Manufacturing Procedure Tensile test specimens were digitally modeled using Computer-Aided Design (CAD) software and exported as stereolithography (.STL) files. The specimen geometry was strictly designed in accordance with ASTM D638 (Type IV) standards, featuring a gauge length of 25mm, an overall length of 115mm, and a thickness of 4.0mm. Slicing was executed using professional slicing software to generate G-code with tightly controlled, constant printing parameters, thereby isolating material performance from slicing artifacts. Table 1 outlines the primary FDM process parameters utilized during the layer-by-layer fabrication of both recycled polyolefin and control virgin specimens. Table 1: FDM Process Parameters for Recycled and Virgin Specimen Fabrication Parameter Setting / Value Nozzle Diameter 0.4mm (Hardened Steel) Layer Height 0.2mm Infill Density 100% (Full Rectilinear Solid Infill) Raster Orientation 45 alternating cross-hatch Printing Temperature 2100C(HDPE)/220∘C (PP) / 200∘C (Virgin PLA) (Virgin PLA) Build Plate Temperature 60∘C (PEI Coated Spring Steel with PVA adhesive) Printing Speed 50 mm/s (Constant contour and infill speed) Cooling Fan Speed 100% (Active layer cooling after layer 3) 2.4 Mechanical Testing Protocols Tensile mechanical testing was conducted using a high-precision, computer-controlled Universal Testing Machine (UTM) equipped with a calibrated 5 kN piezoresistive load cell and pneumatic grip fixtures. Testing was performed at standard ambient laboratory conditions (23±2∘C and 50±5% relative humidity). The crosshead displacement rate was maintained at a constant speed of 5 mm/min throughout all trials.For each material category (Virgin PLA control, Recycled HDPE, Recycled PP, and a 50/50 Recycled Blend), exactly five valid, defect-free samples were tested to ensure statistical repeatability. From the resulting load-displacement and engineering stress-strain curves, critical mechanical properties namely Ultimate Tensile Strength (σu), Elastic Modulus (E) calc