## 1 Introduction  
Absorbing material is a functional material that converts electromagnetic waves into thermal energy or other forms of energy and has small reflection and transmission [1]. Generally, absorbing material is composed of an absorbing agent and substrate; [2, 3]. According to the loss mechanism, the absorbing agent is divided into the following three types: First, the dielectric-type absorbing agent, such as FeN and Si\({}_{3}\)N\({}_{4}\), which achieves the absorption effect through dielectric polarization relaxation loss; second, the magnetic-type absorbing agent such as ferrites, which attenuates electromagnetic waves through magnetic loss; and third, the resistive-type absorbing agent that absorbs electromagnetic waves by the attenuation of the electronic polarization or interfacial polarization of the medium such as carbon materials. The substrate is divided into metal (Al and Cu), non-metal (ceramic and carbon substrate), and polymer (thermoplastic and thermosetting). [PERSON] et al. used graphene as the absorbing agent and polymethyl methacrylate as the matrix, and the graphene/polymethyl methacrylate absorber was produced by melt blending. The results showed that when the volume fraction of graphene was 1.8 vol%, the graphene formed a conductive network in the polymethyl methacrylate matrix, thus, the shielding efficiency of electromagnetic interference was 13 to 19 dB, in the frequency range 8-12 GHz [4]. [PERSON] et al. prepared a super paramagnetic ferroferric oxide (Fe\({}_{3}\)O\({}_{4}\)) nanocrystal using the in situ thermal decomposition method, and then mixed it with a paraffin matrix to prepare a Fe\({}_{3}\)O\({}_{4}\)/paraffin composite absorber (the mass fraction of Fe\({}_{3}\)O\({}_{4}\) nanocrystal was 50 wt%). The test results showed that, when the thickness was 1 to 5 mm, the minimum reflection loss value was \(-\)4.4 dB, in the range 2-18 GHz. As many atoms exist on the surface of Fe\({}_{3}\)O\({}_{4}\) nanocrystals, local symmetry is damaged. Thus, the polarization of dipoles was increased and Fe\({}_{3}\)O\({}_{4}\) nanocrystals had super paramagnetic property which was beneficial for absorbing the electromagnetic waves [5]. [PERSON] et al. prepared a graphene/Fe\({}_{3}\)O\({}_{4}\) composite absorber using a solvothermal synthesis method (the ratio of graphene to Fe\({}_{3}\)O\({}_{4}\) was 1:10). The test results showed that the reflection loss value of the graphene/Fe\({}_{3}\)O\({}_{4}\) composite absorber with a thickness of 2 mm was less than -10 dB, and the minimum reflection loss value was \(-\)15.38 dB, in the frequency range 10.4-13.2 GHz [6]. The aforementioned researches covered numerous efforts to improve wave absorbing performance through the existence of synergy between graphene and Fe\({}_{3}\)O\({}_{4}\). Currently, the manufacturing methods of the absorber mainly include an etching method and a machining method. The former has higher costs and has a limit on the size of the absorber; the latter is difficult to meet the requirements of the complex structure of the absorber, and the processing cycle is long, and sometimes special molds or tools are required. Three-dimensional (3D) printing technology is an advanced manufacturing technology which builds objects by modeling layered processing of stacked materials. This technology can realize the rapid manufacturing of complex products with any structure, as well as the combination of multiple materials and structures and is expected to provide a new process technology for efficient microwave absorbers. In this paper, graphene/nano-Fe\({}_{3}\)O\({}_{4}\)/polylactic acid (PLA), nano-Fe\({}_{3}\)O\({}_{4}\)/PLA, and graphene/PLA composite absorbers are made by fused deposition modeling (FDM) technology. Electromagnetic parameters of the FDM samples are measured by the waveguide method, and the absorbing property is calculated according to the transmission line theory.