Optimizing anisotropy in injection-moulded poly(methyl methacrylate) parts using DOE and simulation

Abstract The injection moulding of Polymethyl Methacrylate (PMMA) is crucial for producing high-precision optical components where isotropic material properties are essential for uniform optical performance. Anisotropy, manifested as variations in volumetric shrinkage (Vs), residual stress (Rs), warpage, and density difference ( δ d), can compromise dimensional stability and optical clarity. This study investigates the influence of key injection moulding parameters on the anisotropy of 3 mm and 6 mm thick PMMA disks. A novel sectional probing technique was developed to assess core-to-surface variations along the z -axis, offering unique insights into internal anisotropy gradients that conventional methods often overlook. We employed a synergistic approach combining Design of Experiments (DOE) with Taguchi’s method, finite element simulation using Moldflow, and multi-objective optimization via genetic algorithms to identify optimal process conditions. Analysis of Variance (ANOVA) revealed that melt temperature and mould temperature are the most significant factors, accounting for 60 % and 36 % of the variance in Vs and Rs, respectively. A highly predictive linear regression model ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:msubsup> <m:mi>R</m:mi> <m:mrow> <m:mtext>Pred</m:mtext> <m:mo>.</m:mo> </m:mrow> <m:mn>2</m:mn> </m:msubsup> </m:mrow> </m:math> ${R}_{\text{Pred}.}^{2}$ &gt;0.95) was developed, demonstrating that optimization reduced Vs by 4 % and Rs by 9 % compared to the initial trials. The study also found that part thickness significantly amplifies anisotropy, with 6 mm parts exhibiting variations of 54 % increase in Vs, 54 % decrease in Rs and a 43 % rise in δ d compared to 3 mm parts. Experimental validation using a linear polarized microscope and dial gauge measurements confirmed these simulation results, showing reduced birefringence and shrinkage in the optimized parts. The experimental shrinkage values were within 0.02 mm and 0.06 mm of the simulated values for the 3 mm and 6 mm disks, respectively. This work not only demonstrates the efficacy of integrating DOE, simulation, and optimization but also highlights the critical role of part thickness and the novelty of the sectional probing method in elucidating anisotropy mechanisms, providing a robust methodology for producing isotropic PMMA parts for advanced optical applications such as lenses and light guides.