TY - BOOK

T1 - Process optimization and simulation of film insert molding for electronic applications

AU - Hubmann, Martin

N1 - no embargo

PY - 2023

Y1 - 2023

N2 - The in-mold electronics (IME) process is a technique to produce lightweight three-dimensional objects with electronic functionalities. Here flexible films with printed circuits and surface-mounted devices (SMDs) such as resistors are merged with a thermoplastic through injection molding. As a result, the electronics become part of the load-bearing, protective, and geometrical-defining structures instead of being attached to them through cables. Those so-called structural electronics are novel approaches to the human-machine interface (HMI). But, unsurprisingly, integrating such films in injection-molded parts is challenging. During overmolding, high pressures, shear stresses, and temperatures prevail due to the injected viscous melt. After molding, insufficient adhesion between films and parts, distorted films, and, or detached components must be reliably omitted. This thesis aims to develop a more profound understanding of the driving forces behind those complications. To that end, experimental investigations based on case studies were made alongside injection molding simulations to find correlating relations. The impact of the molding parameters on the adhesion between O2 plasma-treated fluoropolymer (THV) films overmolded with polycarbonate (PC) were assessed through peel tests. The measured interface strengths were then correlated (R²=85%) with simulations through a derived temperature¿time integral. Here the temperatures at the PC-THV interfaces during molding were examined. Seemingly, the higher and the longer the temperatures remain above the glass transition temperature (Tg) of the amorphous PC, the stronger the interface becomes. This can be achieved through higher set mold and melt temperatures and faster injection speeds (viscous dissipation). Like the experiments, the simulations indicated (slightly) reduced adhesion with increasing packing pressures. It was denoted to a higher heat transfer coefficient (HTC) prevailing for longer when the cavity is pressurized. This results in faster cooling of the interface and, subsequently, weaker bonding. Next, distortion on injection molded laminated structural electronics was examined. To that end, films comprising PC sheets as outer and thermoplastic polyurethanes (TPU) sheets as middle glue layers incorporating flexible printed circuit boards (flexPCBs) were overmolded with PC. The observed distortion on the films was correlated with the simulations by deriving a shear distortion factor. Seemingly, distortion is triggered by the melting of the TPU layer. Hence molding settings that keep the TPU layer from melting and simultaneously restrain the occurring shear stresses during filling are desirable. Next, the detachment of SMD resistors on assembled foils when overmolded with PC was investigated. Here foils using a conductive adhesive for assembly, overmolded in the thinner mold at lower melt temperatures, exhibited the most detached components. Simulations calculated filling forces on the components ranging from 1 to 7 N. These were substantially lower than the experimentally obtained shear loads of at least 20 N (conductive adhesive). The discrepancy was denoted by the mechanical test being conducted at room temperature, while injection molding involves high temperatures (thus softening the assembling material). The results of this thesis show that optimizing the complex IME process using injection molding simulation software is possible. Additionally, through such virtual observation, a deeper physical understanding of the phenomena occurring during the parts manufacturing can be obtained.

AB - The in-mold electronics (IME) process is a technique to produce lightweight three-dimensional objects with electronic functionalities. Here flexible films with printed circuits and surface-mounted devices (SMDs) such as resistors are merged with a thermoplastic through injection molding. As a result, the electronics become part of the load-bearing, protective, and geometrical-defining structures instead of being attached to them through cables. Those so-called structural electronics are novel approaches to the human-machine interface (HMI). But, unsurprisingly, integrating such films in injection-molded parts is challenging. During overmolding, high pressures, shear stresses, and temperatures prevail due to the injected viscous melt. After molding, insufficient adhesion between films and parts, distorted films, and, or detached components must be reliably omitted. This thesis aims to develop a more profound understanding of the driving forces behind those complications. To that end, experimental investigations based on case studies were made alongside injection molding simulations to find correlating relations. The impact of the molding parameters on the adhesion between O2 plasma-treated fluoropolymer (THV) films overmolded with polycarbonate (PC) were assessed through peel tests. The measured interface strengths were then correlated (R²=85%) with simulations through a derived temperature¿time integral. Here the temperatures at the PC-THV interfaces during molding were examined. Seemingly, the higher and the longer the temperatures remain above the glass transition temperature (Tg) of the amorphous PC, the stronger the interface becomes. This can be achieved through higher set mold and melt temperatures and faster injection speeds (viscous dissipation). Like the experiments, the simulations indicated (slightly) reduced adhesion with increasing packing pressures. It was denoted to a higher heat transfer coefficient (HTC) prevailing for longer when the cavity is pressurized. This results in faster cooling of the interface and, subsequently, weaker bonding. Next, distortion on injection molded laminated structural electronics was examined. To that end, films comprising PC sheets as outer and thermoplastic polyurethanes (TPU) sheets as middle glue layers incorporating flexible printed circuit boards (flexPCBs) were overmolded with PC. The observed distortion on the films was correlated with the simulations by deriving a shear distortion factor. Seemingly, distortion is triggered by the melting of the TPU layer. Hence molding settings that keep the TPU layer from melting and simultaneously restrain the occurring shear stresses during filling are desirable. Next, the detachment of SMD resistors on assembled foils when overmolded with PC was investigated. Here foils using a conductive adhesive for assembly, overmolded in the thinner mold at lower melt temperatures, exhibited the most detached components. Simulations calculated filling forces on the components ranging from 1 to 7 N. These were substantially lower than the experimentally obtained shear loads of at least 20 N (conductive adhesive). The discrepancy was denoted by the mechanical test being conducted at room temperature, while injection molding involves high temperatures (thus softening the assembling material). The results of this thesis show that optimizing the complex IME process using injection molding simulation software is possible. Additionally, through such virtual observation, a deeper physical understanding of the phenomena occurring during the parts manufacturing can be obtained.

KW - injection molding

KW - overmolding

KW - in-mold electronics

KW - IME

KW - structural electronics

KW - flexible electronics

KW - simulation

KW - Spritzgießen

KW - Spritzguss

KW - Hinterspritzen

KW - In-Mold-Elektronik

KW - IME

KW - structural electronics

KW - Flexible Elektronik

KW - Simulation

M3 - Doctoral Thesis

ER -