Defect and Diffusion Forum Vol. 451

Abstract: In this study, the thermomechanical behavior of PMMA(poly-methyl methacrylate) during high-temperature vacuum forming was analyzed through both experimental and computational approaches. The material behavior of PMMA was modeled as a temperature and strain-rate dependent viscoplastic response, coupled with time-dependent creep deformation. The creep behavior was represented by the Norton–Bailey power law (Eq. 1), while the constitutive model for the strain rate and temperature-dependent stress-strain behavior was implemented in ABAQUS via a user subroutine (UHARD). The forming process was simulated by using ABAQUS/Standard VISCO solver, incorporating vacuum pressure loading and clamping conditions. The numerical framework enables effective analysis of deformation behavior under thermomechanical forming conditions and provides a basis for process-oriented modeling of PMMA vacuum forming.

Abstract: Polymethylmetharylate (PMMA) has been widely used for aircraft canopies and transparent structural components, and processed into various parts through vacuum forming. In this study, the effects of forming speed and deformation characteristics on thickness uniformity during high-temperature vacuum forming of PMMA were analyzed. First, creep tests and high-temperature tensile tests were conducted at the specimen level to quantitatively distinguish between creep deformation and plastic deformation. Creep tests were performed under constant temperature and load conditions, and strain was measured through Digital Image Correlation. For plastic deformation analysis, tensile tests at room temperature and elevated temperatures were carried out to compare yield strength and elongation changes. To analyze thickness uniformity during the forming process, rectangular-shaped parts were fabricated using vacuum forming under various conditions where temperature and forming speed are key variables. After forming, thickness uniformity and surface transparency of the products were measured. Additionally, internal structural changes according to forming speed and temperature conditions were analyzed, and a comprehensive evaluation of material stability was performed.

Abstract: The deposition sequence and the resulting thermal history govern the development of distortions and residual stresses in components made by material extrusion, ultimately affecting their structural performance. This work presents a process-aware thermomechanical simulation framework that reproduces the actual deposition directly from the G-Code. The nozzle trajectory is processed to reconstruct the bead order and timing and to automatically generate a voxel-based finite element mesh suitable for progressive activation. A transient thermal analysis is then performed with incremental element activation, while the thermal effect of extrusion is prescribed through a temperature predefined field applied to the newly activated elements, together with convective-radiative heat losses to the environment. The resulting temperature history is subsequently transferred as a time-dependent temperature field to a quasi-static mechanical analysis to predict residual distortions and stresses after cooling. Finally, a linear elastic virtual tensile test is carried out on the final, deformed configuration, accounting for the residual stress state. The framework is applied to PLA ASTM D638 specimens manufactured at different extrusion temperatures and validated against experimental tests, showing that extrusion temperature governs thermal gradients, residual stress distributions and the resulting macroscopic elastic response.

Abstract: The objective of the present research was to identify the mechanical properties of 3D-printed biocomposite parts and their variation with different natural fillers (olive wood and almond shell). The materials were produced by filament extrusion with 5% fiber content in the polylactic acid matrix, and the samples were fabricated using the Material Extrusion Additive Manufacturing process. 3D printed specimens underwent tensile and flexural tests to assess their mechanical properties. The results showed reductions of 5%-18% in the tensile modulus and 10%-38% in the tensile strength for olive wood-and almond shell-based PLA, respectively. The same trend was detected for the flexural properties, with a slight reduction of 2%-3% in the flexural modulus and 3%-5% in flexural strength.

Abstract: Warpage in injection-molded thin-walled box-shaped parts is primarily caused by non-uniform cooling and differential shrinkage. This study proposes a two-step, multi-objective optimization strategy to reduce part warpage by addressing both thermal and geometric factors. In the first step, the mold cooling system is optimized through a bi-objective formulation that simultaneously minimizes (i) the temperature standard deviation within the part and (ii) the total cooling channel length. The optimization is carried out using a coupled workflow involving parametric CAD modeling, Autodesk Moldflow simulations, and a genetic algorithm. The optimized cooling design reduces temperature non-uniformity by 44% compared to a conventional cooling layout. In the second step, a geometric optimization is performed through the addition of a reinforcing border, where maximum deflection and total part volume are minimized simultaneously. The combined optimization leads to a reduction in maximum warpage from 14.5 mm in the reference configuration to 2.06 mm in the final design. The results demonstrate the effectiveness of a sequential optimization approach in achieving significant warpage reduction while maintaining material and manufacturing efficiency.

Abstract: Process speed in pultrusion is significantly influenced by the exothermic reactions of the matrix materials used. The main reaction zone (gel zone) is a key indicator to describe and interpret the reaction behavior in pultrusion. It can be easily observed by elevated temperatures in the die, particularly for highly exothermic thermoset matrices like vinyl ester, epoxy, and polyurethane. However, this effect is not as pronounced in reactive thermoplastics. The exothermic reactions contribute to a reduction in power consumption of the heating plates within the different heating zones, each with its individual temperature. Analyzing the power consumption of the individual heating zones across different process parameter settings allows to determine the position of the gel zone. This information is foundational for pultrusion process optimization, as it allows for more efficient utilization of the die length, ultimately increasing the pull speeds and enabling higher production rates. In this study, a comparative analysis of the power consumption across the heating zones was performed. To validate the findings obtained from the power measurements, thermocouples were drawn through the die at the same process parameters to accurately measure the temperature evolution within the pultruded profile throughout the die length.

Abstract: Efficient thermal management is a key factor in improving the sustainability and productivity of injection moulding processes, particularly at the micro-scale where thermal transients strongly affect part quality and cycle stability. This work investigates the thermal behaviour of hybrid moulds composed of polymeric support plates manufactured in Precision Resin V01 and stainless-steel inserts manufactured by additive manufacturing. An experimental campaign was carried out on a micro-injection moulding machine to characterize the intrinsic thermal response of the mould under uncooled conditions. Temperatures were monitored through embedded thermocouples and used to develop and calibrate a three-dimensional transient numerical model in COMSOL Multiphysics. Particular attention was devoted to the identification and calibration of heat transfer coefficients at the injection and extraction interfaces, which were found to play a dominant role in governing insert temperature evolution. The calibrated model accurately reproduces the experimental thermal transients, with deviations below 10%, demonstrating its reliability as a predictive tool for analysing mould thermal behaviour and supporting early-stage design and process optimization. The results highlight the advantages of hybrid architectures in promoting thermal stability and provide a robust methodology for modelling heat exchange in unconventional mould configurations.