Magnetic Particle Reinforced Elastomer Composites for Additive Manufacturing

Abstract

Magnetorheological elastomer (MRE) is a magnetic particle reinforced elastomer composite, which its mechanical properties can be controlled with the application of a magnetic field.1 The magnetorheological effect induces changes in stiffness and damping properties in the composite with the response of the magnetic particles to the external magnetic field.2 Smart sensor and actuator components can be fabricated for adaptive vibration control systems where mechanical properties can be controlled by the application of magnetic field. Typically, composites are prepared by mixing magnetic particles with the uncured rubber matrix and then the mixture is poured into a mold for curing. Recently low-cost 3D printers are widely available, utilizing various polymers. Manufacturability of MRE composites using additive manufacturing (AM) process has gained great attention so the geometrical design of the MRE composite fabrication can be achieved.3 In this research, development of filament of MRE for fused filament fabrication based on additive manufacturing is performed. The magnetorheological effect depends on the particle’s magnetic properties and volume fraction, and distribution, etc. Filament materials are developed by mixing thermoplastic polyurethane (TPU) pellets with the magnetic particles (iron oxide, carbonyl Fe particles), followed by extrusion process to produce rolls of filament. Due to the nature of the mixing process relying on gravity to feed material into the extruder, denser and much smaller magnetic particles (~10 μm) than the TPU pellets (~mm) tend to fall to the bottom of the hopper, leading non-uniform distribution of magnetic particle along the filament thru extrusion process. Therefore, the trickling systems with a screw feeder were developed to provide consistent trickling rate of each TPU and magnetic particles respectability. [Figure 1 (a)] The trickling rate of TPU was fixed at ~4.9 g/min and the trickling rate of iron oxide was varied from 0.0 to 2.5 g/min. Five sets of filaments were produced with different trickling rates of iron oxide.[Figure 1(b)]. Diameter ranges from 1.45 ± 0.01 to 1.55 ± 0.06 mm. The density of filament increases with the higher trickling rate of magnetic powder and the densities change from 1.33 ± 0.05 to 1.46 ± 0.05 g/cc, indicating that the volume fraction control of magnetic particles in the extrusion process was successful. The grayscale level plot [Figure 1 (c)] indicates the iron oxide concentration in each filament in the yellow box region. The grayscale plot profiles the images with each point represents the average pixel intensity. This data shows that the filaments produced with lower trickling rate (0.6 and 1.0 g/min) did not show significant difference but the lower gray scale values of filaments with 1.5 and 2.5 g/min indicate that there are more iron oxide particles. Micro-focus X-ray tomography scans were performed on the produced composite filament (Figure 2). The results show that magnetic particles are well dispersed in the matrix. The small grains are < ~30 mm and the larger grains are ~60 mm. The iron oxide volume fraction ranges from 9.31% to 12.95%. This demonstrates that MRE filament feedstock materials can be successfully manufactured for use in fused filament deposition processes. **