Thermo-elastomechanics with a liquid-solid phase transition

Turk, Janez (author), Svenšek, Daniel (mentor)

Plastic injection molding is a manufacturing process for producing complexly shaped plastic components, widely used in mass production industries. The process involves injecting molten polymeric material into a mold cavity, where it solidifies and takes the desired shape. During cooling, the solidified plastic continuously deforms due to thermal shrinkage, which can lead to warping of the final product or the appearance of surface defects, such as sink marks. Precise prediction of these deformations is essential for optimizing the injection molding process and ensuring the quality of the final product. In this study, we present a novel thermoelastic simulation method that predicts local deformation in injection molded parts by continuously calculating the displacement and stress fields of the solidifying plastic throughout the production process. Our approach accounts for the intricacies of part shape and enables the prediction of surface defects without requiring any specific material property measurement or calibration of the method. The method relies on material characterization data typically used in standard injection molding simulation analyses. The plastic injection molding process can be divided into three phases: the filling phase, the packing phase, and the cooling phase. Each of these phases has its pressure behavior regime in the liquid core of the product, which represents, along with the temperature change, an elastomechanical load for the plastic product. The pressure behavior in each of the phases is explained in this work, with a novel addition being a simple model of pressure behavior in the packing phase and the calculation of equilibrium pressure in the molten core during the cooling phase, which is crucial for predicting part deformation and ultimately for assessing the potential for void formation. We verified the accuracy of the thermoelastic method by comparing the results with measurements of products we injected using different process parameters. We also tested the method's performance with the result of a profilometry measurement published in the literature.

• Language: English
• Keywords: Residual stress, Molding defects, Sink marks, Injection molding, Warpage
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-154030
• COBISS.SI-ID: 181785091
• Publication date in RUL: 20.01.2024

TURK, Janez, 2024, Thermo-elastomechanics with a liquid-solid phase transition [online]. Doctoral dissertation. [Accessed 11 April 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=154030

Secondary language

• Language: Slovenian
• Title: Termoelastomehanika s faznim prehodom iz tekočine v trdnino

Abstract

Brizganje plastike je proizvodni postopek za izdelavo plastičnih komponent zapletenih oblik, močno razširjen v masovni industriji. Postopek vključuje vbrizgavanje staljene polimerne mase v votlino kalupa, kjer se strdi in dobi želeno obliko. Med hlajenjem se strjena plastika zaradi termalnih skrčkov neprekinjeno deformira, kar lahko privede do zvijanja končnega izdelka ali nastanka površinskih napak, kot je posedanje. Natančna napoved teh deformacij je bistvena za optimizacijo postopka brizganja in zagotavljanje kakovosti končnega izdelka. V tej študiji predstavljamo termoelastično simulacijsko metodo za računanje deformiranosti brizganih izdelkov z neprekinjenim izračunavanjem deformacijskega in napetostnega polja strjevane plastike skozi vse faze proizvodnega procesa. Tak pristop omogoča napovedovanje površinskih napak, ne da bi bilo treba določiti kakšne posebne materialne lastnosti ali opraviti kalibracijo. Metoda temelji na podatkih o lastnostih materiala, ki se običajno uporabljajo v standardnih analizah simulacije brizganja plastike. Proces brizganja plastike je mogoče razdeliti na tri faze: fazo polnjenja, naknadno fazo in fazo hlajenja. Vsaka faza ima svoj režim obnašanja tlaka v staljeni sredici izdelka, ki skupaj s temperaturno spremembo predstavlja elastomehansko breme za plastični izdelek. Obnašanje tlaka v vsaki od faz je v tem delu pojasnjeno, novost je enostaven model obnašanja tlaka v naknadni fazi ter izračunavanja ravnovesnega tlaka v staljeni sredici v fazi hlajenja, ki je ključen za napovedovanje deformacije kosa in ne nazadnje tudi za ocenjevanje možnosti za nastanek lunkerjev. Natančnost termoelastične metode smo preverili tako, da smo rezultate primerjali z meritvami izdelkov, ki smo jih nabrizgali z različnimi procesnimi parametri. Prav tako smo delovanje metode preverili z rezultatom profilometrske meritve, objavljene v literaturi.

• Keywords: notranje napetosti, napake pri brizganju plastike, posedenost