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Materials Science in Additive Manufacturing Adhesion study for multi-material 3D printing

adhesion promoters that can bond to both PLA and will provide crucial insights into maximizing the interfacial
TPU. Process-based modifications, such as optimizing adhesion, thus enabling the reliable creation of composite
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print temperature, speed, and layer height, have also materials with integrated soft and rigid characteristics for
shown potential in improving adhesion. 30,31 Moreover, the advanced manufacturing applications.
integration of nanomaterials and the development of hybrid
filaments are emerging as promising frontiers in addressing 2. Methods
these adhesion challenges. Each approach carries its 2.1. Materials
own set of advantages and trade-offs, necessitating a
comprehensive study to identify the most effective method In this study, we utilized cPLA and TPU filaments, with a
for a given application. However, these works mainly standard diameter of 1.75 mm, to investigate the adhesion
focused on the interaction between the pure polymeric properties relevant for multi-material FFF. The selected
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materials. Nevertheless, the existing literature primarily cPLA filament, sourced from Protopasta , is a composite
emphasizes the interplay between unmodified polymeric material enhanced with carbon black nanofillers to infuse
materials. 32,33 There remains a substantial research gap the PLA with electrical conductivity, while maintaining its
in understanding the interlaminar properties when one inherent rigidity. The soft material chosen is a NinjaFlex
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introduces nanocomposites into the mix, particularly TPU filament, acquired from Ninjatek , notable for its
between cPLA enhanced with nanoparticles and unaltered flexibility and a Shore hardness rating of 85A, which
TPU. Furthermore, the mechanical interlocks approach falls into the medium-hard category for thermoplastic
often requires a large overlap/transition region to achieve elastomers. Comprehensive details regarding the
satisfactory interlaminar strength which may not be specifications, material properties, and manufacturer data
suitable for some applications. These areas of study are for both the cPLA and TPU filaments are systematically
crucial for advancing the application potential of multi- cataloged in Table 1. The table provides an overview of key
material 3D printing, as it addresses the interface between parameters, such as tensile strength, elongation at break,
technologically augmented and standard polymers. thermal properties, and electrical conductivity, offering
Recognizing the need for enhanced interfacial adhesion, a foundation for the ensuing analysis of interlaminar
this study delves into two specific enhancement approaches: adhesion. This selection of materials facilitates a focused
the engineering of low-profile mechanical interlocking assessment of the interfacial adhesion challenges between
features (<2 mm in thickness) and the tailoring of surface a conductive, rigid filament and a non-conductive, flexible
roughness at the interface. These approaches are assessed counterpart, contributing to the broader understanding of
against a benchmark part with no modifications, providing multi-material 3D printing dynamics.
a comprehensive comparative analysis. By exploring these
strategies, this research aims to unravel the complexities 2.2. Design and fabrication of test coupons
of the cPLA-TPU interface and to contribute to the The test coupons were designed using a 3D design CAD
optimization of multi-material FFF processes. The outcomes software, Solidworks 2022. In this work, a multi-material
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Table 1. Summary of materials used in this work for the fabrication of test coupons
Material type Brand Model Property
cPLA Protopasta ® Electrically conductive composite PLA • Appearance: black
• Additives: carbon black
• Volume resistivity of molded resin: 15 Ω-cm
• Volume resistivity of 3D‑printed parts along layers (x/y): 30 Ω-cm
• Volume resistivity of 3D‑printed parts against layers (z): 115 Ω-cm
• Melt temperature: 155°C
• Density: 1.24 g/cm 3
• Tensile modulus: 3.6 MPa
• Tensile yield strength: 60 MPa
TPU Ninjatek ® NinjaFlex • Appearance: glossy black
• Shore hardness: 85A
• Maximum elongation: 660%
• Melt temperature: 216°C
• Density: 1.19 g/cm 3
• Tensile modulus: 12 MPa
• Yield strength: 4 MPa
Abbreviations: cPLA: conductive polylactic acid; PLA: Polylactic acid; TPU: Thermoplastic polyurethane.

Volume 3 Issue 1 (2024) 3 https://doi.org/10.36922/msam.2672