I. Interpretation of the Core Summary

This study investigates the relationship between the viscosity of two-component RTV silicone rubber and its extrusion‑based 3D printing processability, proposing a practical, universally applicable rheological modification approach. Using the commercially available EcoFlex™ 00‑50 low‑hardness silicone, we systematically adjusted viscosity and thixotropy by incorporating THI‑VEX thixotropic agent in a gradient fashion, formulating six distinct mixtures with varying addition levels. Leveraging the Lynxter S300X industrial extrusion printer, we designed four categories of geometric benchmark specimens—thin walls, overhangs, slender columns, and gaps—and quantified print quality through multi‑rater blind scoring combined with reliability analysis. Experimental results demonstrate that a 4.0 wt% loading of the thixotropic agent yields the optimal formulation, balancing extrusion smoothness with high geometric fidelity. The entire study establishes an integrated design and operational protocol for soft silicone 3D printing materials and processes, demonstrating that rheological modification can adapt standard commercial silicones to industrial extrusion additive manufacturing systems, thereby overcoming the limitations imposed by proprietary consumables.

II. The Core Role of Silicone 3D Printing in This Study

The Lynxter S300X dual-extrusion silicone 3D printing system serves as the central platform for all experiments and validations in this study, fulfilling four key functions:

• A unique validation platform for the molding performance of modified materials. This equipment is equipped with independent dual material barrels—LIQ21 for printing RTV‑2 silicone and LIQ11 for printing water‑soluble supports—and can simultaneously perform automatic A/B adhesive ratio mixing, perfectly replicating industrial‑scale extrusion conditions. Modified silicones formulated with different thixotropic agents are all processed using identical printing parameters, enabling direct comparison of how viscosity and thixotropy affect defects such as collapse, oozing, and fracture, thereby facilitating precise optimization of the optimal formulation.
• Geometric‑limit quantification test platform: Leveraging the printer’s layer‑by‑layer deposition and co‑printing of supports, it enables batch printing of unsupported thin walls, 45°/60° overhangs, slender cylinders, and standard specimens with varying gap widths. This allows for the quantitative determination of the minimum wall thickness, minimum gap width, maximum overhang angle, and maximum slenderness ratio that can be stably fabricated using modified silicone elastomers, thereby establishing geometric design thresholds for soft‑silicone additive manufacturing.
• An extrusion parameter calibration test platform, using modified silicone rubber as the material, was employed to conduct a series of extrusion ratio gradient tests. This enabled the identification of an optimal extrusion range of 0.72–0.78, effectively addressing common defects in soft silicone rubbers—such as under‑extrusion, over‑extrusion, and interlayer grooves—and resulting in a standardized process‑parameter template applicable to all modified RTV‑2 systems.
• A practical, industrially validated material platform that overcomes the barriers posed by proprietary consumables, demonstrating that standard medical-grade soft silicone, after simple modification, can be stably processed on industrial‑grade silicone 3D printers. This provides a solid basis for deploying such equipment in applications like medical orthotics, soft robotics, and low‑volume, moldless fabrication of custom seals.

III. Four Major Core Innovations and Breakthroughs of the Paper

Breakthrough 1: A low-cost modification scheme for general-purpose RTV‑2 silicone rubber, breaking the monopoly on equipment and consumables.

Currently, industrial silicone‑gel printers can only use manufacturer‑specific high‑hardness silicones. In contrast, this study requires no custom‑made specialty resins; instead, it employs a commercially available THI‑VEX thixotropic agent to rheologically tailor the widely available EcoFlex 00‑50 ultra‑soft silicone. By simultaneously adding equal amounts of components A and B, mixing viscosity imbalances are prevented. The modification process is straightforward and cost‑effective, and this approach can be readily applied to various standard two‑component silicones, significantly expanding the range of printable silicone materials.

Breakthrough 2: A standardized, multi‑stage rheological screening method for rapidly predicting printability.

Innovative vertical‑flow qualitative testing: Different silicone‑gel formulations are coated onto a vertical panel and allowed to stand, with flow‑induced collapse observed. This approach enables rapid preliminary screening of compatible formulations without the need for high‑end rheometers, significantly reducing upfront material‑selection time. By first conducting qualitative preliminary screening followed by quantitative print‑based validation, a low‑cost material‑evaluation workflow is established, readily adaptable for small and medium‑sized enterprises.

Breakthrough 3: A Standardized, Multi-Dimensional Geometric Benchmark Evaluation Framework

A comprehensive suite of test specimens has been specifically designed to address the inherent collapse‑prone nature of soft silicone, covering four typical printing failure scenarios: unsupported thin walls, cantilevered structures, slender columns, and narrow‑wide gaps. The evaluation employs a multi‑rater independent blind assessment combined with inter‑rater reliability analysis, quantifying print quality objectively through numerical scores to mitigate the biases of single‑rater subjective judgments. This methodology can serve as a universal industry benchmark for 3D printing of soft materials.

Breakthrough 4: Comprehensive Hands-On Standards Integrating Materials, Processes, and Design

Determine the optimal formulation: add 4.0 wt% THI‑VEX to adhesive A; concurrently provide a complete set of fixed process parameters, including nozzle settings, layer height, and extrusion ratio, and define clear geometric design guidelines for soft silicone printing: minimum cross-sectional area for unsupported thin walls is 28 mm², overhang angles must be no less than 45°, the maximum slenderness ratio for cylindrical features is 0.58, and the minimum gap between straight lines is 0.21 mm—allowing designers to directly reference these criteria to prevent print failures. Additionally, specify the mechanical property changes after modification (a slight reduction in tensile strength, increased elongation at break, and unchanged hardness) to facilitate material selection based on application requirements.

IV. Application Scenarios and Future Development Directions

(1) Current Real-World Application Scenarios

• Rehabilitation and Medical Custom Devices

Personalized prosthetic liners, orthotic soft pads, and human soft-tissue simulation models; traditional casting and mold-making are extremely costly, whereas modified soft silicone 3D printing enables single‑piece customization, closely conforming to the body’s low‑hardness, high‑compliance requirements while meeting medical‑grade biocompatibility standards.

• Soft robotic flexible actuators

Low‑hardness biomimetic muscles, flexible grippers, and miniature pipe‑climbing robots; 4% thixotropic‑agent‑modified silicone exhibits stable printability and excellent deformation‑extension properties, making it well suited for safe soft‑robotic structures in human–robot interaction.

• Custom Sealing and Microfluidic Devices

Irregular thin-walled silicone sealing gaskets and microfluidic chip channels; leveraging high-precision gap‑forming capabilities, complex microchannels can be integrally printed without the need for secondary bonding.

• Cultural and Creative Special Effects and Wearable Flexible Interfaces

Soft silicone props for film and television special effects, wearable haptic sensor substrates—offering rapid prototyping in small batches and multiple styles, eliminating the need for mold development cycles.

(II) Directions for Future Research Development

• Multi‑category RTV silicone rubber—universal modification model that expands the range of base materials (high‑transparency, conductive, and magnetic silicones), establishes quantitative correlation formulas between thixotropic agent dosage and rheological and mechanical properties, and enables one‑click determination of modification ratios for any commercially available silicone.
• Quantitative rheology‑coupled printing simulation: By integrating a rotational rheometer to accurately measure shear‑thinning and thixotropic recovery behaviors, and coupling these data with a printing‑force simulation model, we can proactively identify potential forming defects for various formulations, thereby reducing the need for physical trial prints.
• Multi-material simultaneous printing expands capabilities by co‑printing modified soft silicone with rigid thermoplastic filaments, enabling monolithic fabrication of a “hard skeleton + soft joint” bioinspired composite structure, thereby enhancing the manufacturing potential for humanoid robots and biomimetic devices.
• A modified system for compensating mechanical losses, combined with synergistic reinforcing fillers, counteracts the reduction in tensile strength caused by thixotropic agents, restoring the material’s mechanical strength while maintaining excellent printability, thereby enabling its use in high‑load soft actuators.
• High-speed mass-production process optimization: By aligning with high-speed extrusion parameters and investigating temperature-controlled barrels paired with modified silicone systems, we reduce the printing time per part, meeting the batch‑customization production needs of small and medium-sized factories.

Image and Text Guide

Figure 1. Lynxter S300X (a); LIQ21 (b); LIQ11 (c)

Figure 2. Lynxter S300X Process Flow

Table 1. Performance parameters of SIL-001 and SIL-002 silicones; data source: Lynxter

Table 2. Recommended THI-VEX ratios for Ecoflex™ 00-50 silicone rubber, as specified by the manufacturer.

Figure 3. Photographs of the vertical plate test recorded at three time points (0 minutes (a), 30 minutes (b), and 60 minutes (c)). These images illustrate the relative displacement of the silicone samples over time, enabling a qualitative comparison of their flow behavior and an indirect assessment of viscosity values at different THI‑VEX concentrations.

Figure 4. A custom piston assembly designed for airtight filling of silicone syringes. The system comprises a cylindrical chamber capable of accommodating the volume required by a 55 mL syringe, along with a hollow plunger fitted with an O-ring to ensure material sealing and controlled transfer.

Figure 5. Presence of air in the syringe at 0 hours (a) and 48 hours (b).

Figure 6. Extrusion ratio test results decreased from 0.85 to 0.65; shown from left to right in sequence.

Figure 7. Supported (a) and unsupported (b) states of a thin-walled structure. Black arrows indicate the building orientation.

Figure 8. Notch. The black arrow indicates the building’s orientation.

Figure 9. (a) Side view of the cantilevered section; (b) Cantilevered structure. Black arrows indicate the building’s orientation.

Figure 10. Needle-like structure. The black arrow indicates the building’s orientation.

Figure 11. Images of experimental samples printed according to their THI‑VEX composition. The images were acquired perpendicular to the deposition direction, respectively.

Figure 12. Thin-walled structure (without support): failure (a), deformation (b), success (c); thin-walled structure (with support): failure (d), deformation (e), success (f); rectangular gap: failure (g), deformation (h), success (i); circular gap: failure (j), deformation (k), success (l).

Figure 13. Overhanging structure: (a) failure; (b) deformation (bottom surface bending); (c) success; Pin: (d) failure; (e) deformation; (f) success

Table 3. Summary of geometric printability assessment scores for six silicone formulations with varying THI‑VEX contents. Scores are based on visual classification by multiple raters and are calculated as a composite across all test geometries (thin walls, pins, overhangs, and gaps), with failure, deformation, and success receiving progressively higher scores.

Table 4. Inter-rater reliability analysis for Test 1 (2.00 wt% THI-VEX formulation), showing the individual classification results provided by three independent raters, as well as the pairwise agreement values used to calculate the concordance rate. Classification criteria: S = Success; D = Morphological abnormality; F = Failure.

Table 5. Summary of inter-rater agreement results for all tested silicone formulations. The table presents the mean scores for each rater and the overall percentage of agreement, thereby reflecting the level of consistency and reliability of the visual assessment system across different THI‑VEX concentrations.

Table 6. Success Rate Calculated Based on Assessments Conducted by All Evaluators

Table 7. Summary of the design and process guidelines for extrusion additive manufacturing of EcoFlex™ 00-50 silicone modified with THI-VEX, using the Lynxter S300X system. These guidelines apply to the printing parameters described in Section 2.4.

Table 8. Mechanical properties of EcoFlex 00-50 and EcoFlex 00-50+ with 4.00% THI-VEX. The data for EcoFlex 00-50 were provided by the manufacturer (SmoothOn).