Rotational molding is a shear-free technology to produce hollow plastic parts. Rotomolded poly(lactic acid) (PLA) and its biocomposites remain in a largely amorphous state, which limits their stiffness, toughness, and thermal resistance. Post-processing thermal annealing develops crystallinity without additives, but its combined effect with natural fibers has rarely been quantified. This work evaluates annealing at 100 °C for 1 h on rotomolded PLA biocomposites reinforced with 10, 20, and 30 wt.% of agave, coir, or pine fibers. Crystallinity (DSC, XRD), density and porosity, morphology (SEM), water absorption, mechanical properties (tensile, flexural, Charpy impact, Shore D hardness), and 28-day disintegration under lab-scale composting conditions were measured for treated and untreated samples. Annealing raised the matrix crystallinity from below 21% to 41–56% and produced predominantly α crystals with a nearly constant average size of about 20 nm. The treatment improved the matrix-dominated properties for every formulation: for neat PLA, Charpy impact strength increased by 120% (28.1 to 61.7 J/m), flexural strength by 53% (61.1 to 93.4 MPa), and flexural modulus from 3264 to 4511 MPa. Tensile strength and modulus, in contrast, remained unchanged or decreased. Porosity, set by fiber content, was unaffected by annealing; at 30 wt.%, the matrix barrier nonetheless reversed, and annealed samples absorbed more water and disintegrated faster than untreated ones. Fiber content sets the balance between matrix crystallinity and the interfacial damage caused by crystallization-induced contraction.

Mechanical recycling of post-consumer poly(ethylene terephthalate)/high-density polyethylene (PET/HDPE) streams is limited by immiscibility and processing-sensitive behavior. A nominal 30 wt% PET/70 wt% HDPE blend was evaluated using a 2 × 3 factorial matrix combining maximum extrusion setpoints (T_e: 235 and 260 °C) and maximum injection setpoints (T_inj: 200, 230, and 260 °C). Each processing combination was represented by one run from common bulk feedstocks. Tensile (n = 54) and flexural (n = 53) responses were compared using within-campaign specimen-level contrasts and descriptive multi-response scoring. The T_e= 260 °C/T_inj= 200 °C condition produced the highest observed strength–stiffness response, with tensile strength of 32.5 ± 2.23 MPa, flexural strength of 49.2 ± 6.35 MPa, and flexural modulus of 1550 ± 143 MPa. At T_inj= 200 °C, phase-agnostic SEM analysis showed finer fracture-surface texture after extrusion at 260 °C. Charpy impact followed a different trend, with T_e= 235 °C/T_inj= 200 °C giving the highest observed blend impact response. These results indicate distinct processing–performance relationships for static and impact behavior in the uncompatibilized post-consumer PET/HDPE blend and establish a baseline for further processing and compatibilization studies.