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Materials Science in Additive Manufacturing 3D-printed LMPA-integrated soft robots

A B

Figure 4. Experiment setup and results (A) Servohydraulic test system for tensile tests; (B) tensile test results of pure TPU and LMPA-integrated TPU
robotic arms
Abbreviations: LMPA: Low-melting-point alloy; TPU: Thermoplastic polyurethane

The tensile tests were conducted to quantify the factors, including improper material flow, incomplete fusion
mechanical properties of the pure TPU and LMPA-integrated of layers, or trapped gases during the printing process.
TPU grippers, providing insights into their respective These variations were minimized through meticulous
strengths and durability. The outcomes of these tests are control of the injection process and temperature settings,
crucial for understanding the impact of LMPA integration but they highlight the need for further optimization in the
on the mechanical performance of the soft grippers. manufacturing process to ensure consistent quality.

3. Results and discussion The quality of 3D-printed parts is significantly
influenced by the nozzle size used during the AM process.
3.1. Fabrication process Nozzle size affects the resolution, surface finish, and
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The fabrication process of the soft grippers involved mechanical properties of the printed components. In our
distinct steps for the pure TPU and the LMPA-integrated integrated extrusion-based AM process, the influence of
TPU samples. Both types of grippers were initially nozzle size was primarily observed in the printing of TPU.
printed using the same TPU parameters, including a print Larger nozzle sizes tended to produce parts with lower
temperature of 205°C, a print speed of 20 mm/s, a layer resolution and rougher surface finishes, while smaller
height of 0.15 mm, an infill density of 30%, and a bed nozzles could achieve higher precision and finer details.
temperature of 45°C. This ensured that the foundational However, the trade-off includes potential issues with
structure of each gripper was consistent and comparable. clogging and slower printing speeds when using smaller
For the LMPA-integrated TPU grippers, additional nozzles. In contrast, the impact of nozzle size on LMPAs
steps were required post-printing. A customized 3D printer was minimal. The LMPAs were introduced into the TPU
equipped with a dual nozzle system was used, where one matrix through an injection-like process, which ensures
nozzle injected the LMPA at a controlled temperature uniform distribution and solidification. This method
of 100°C. This method ensured that the LMPA was mitigates the potential adverse effects of nozzle size
evenly distributed within the gripper’s internal structure. variations on the quality of LMPA components.
Maintaining precise control over the injection temperature Our manufacturing process for LMPAs, however,
and flow rate was critical to avoid air pockets and ensure presents a distinctive advantage in this regard. Unlike
uniform LMPA filling. layer-by-layer AM processes, our method involves the
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Despite these precautions, minor deviations in the continuous deposition and solidification of LMPAs as
manufactured samples were noted. The LMPA integration a single, cohesive unit. This continuous solidification
process occasionally led to slight variations in the internal reduces the thermal gradients and the associated residual
structure (shrinkage 50,51 and porosity ), which could affect stresses that typically arise in layer-by-layer approaches. The
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the uniformity and mechanical properties of the grippers. reduced residual stress in our LMPA parts can be attributed
Shrinkage defects occur due to the contraction of the to the following factors. First, the entire LMPA part solidifies
material as it cools and solidifies, leading to dimensional together as a whole, rather than in discrete layers. This
inaccuracies and potential weaknesses in the printed uniform solidification minimizes thermal gradients, which
components. The porosity in LMPAs can arise from several are the primary source of residual stress in layer-by-layer

Volume 3 Issue 3 (2024) 5 doi: 10.36922/msam.4144