## 1 Introduction
The relationship between the surface roughness and mechanical properties was close. [PERSON] [12] experimentally studied the effects of the gap between nozzle and substrate, filament feeding rate and printing speed on the ME parts' surface roughness. The width, height and cross-sectional shapes of the extrudate were examined. The results showed that these factors had significant effect on the built parts' surface roughness. [PERSON] [13] developed a methodology able to improve the surface roughness of ME parts by computer numerical control machining. The proposed method was validated by a case study, achieving a great reduction of average roughness and a reliable uniformity of finished surfaces. [PERSON] [14] set up a reliable algorithm that could predict the surface roughness of ME parts with an accurate definition of the centerline taken into account. The method was then validated experimentally. With an ensemble learning algorithm considered, [PERSON] [15] proposed a data-driven predictive model for predicting the surface roughness of ME parts. Based on the condition monitoring data measured by multiple sensors of different types from an ME device, the predictive model is validated, and it was able to accurately predict the surface roughness of ME parts. [PERSON] [16] combined atomic layer deposition (ALD) and ME technique to coat the aluminum oxide on the built small satellite propulsion parts. The benefits of the coating were studied in the context of in-space propulsion fluidics, with propellant flow properties taken into account. It was found that the parts' surface roughness was improved, leading to improved flow properties. [PERSON] [17] investigated the mechanisms controlling the bonding formation between the extruded filaments in ME process. The bonding quality was assessed in terms of measuring and analyzing the mesostructure and the degree of healing between the adjacent extrudate. The results showed that the build direction, liquefier temperature and convection coefficient had strong effects on the cooling temperature profile, as well as on the mesostructure and bonding strength of the built part. [PERSON] [18] provided an efficient method to evaluate the quality of the parts built by ME. It was found that the wall permeability of the part was dependent on its geometric shape. The part's quality was primarily influenced by the filament feeding rate, wall geometry and G-code-defined wall structure. Optimizing these parameters could improve the quality (decrease in the porosity) and sealing properties of ME parts. Based on short beam shear tests, [PERSON] [19] evaluated the effect of layer height and fiber content on the interlaminar bonding performance of the fiber reinforced nylon composites manufactured by ME technique. It was observed that the layer height of nylon influences the interlaminar shear performance slightly. Fiber reinforced samples showed higher interlaminar shear performance than unreinforced ones, but the level of increase was moderate with the increase of fiber content. [PERSON] [20] used the atmospheric pressure plasma to carry out the hydrophilic treatment to the ME printing layer. This method could significantly improve the rupture strength and surface free energy (up by 75%) of ME parts, and thereby enhance their inter-layer bonding strength. Besides, more and more attentions have been paid to ME parts' accuracy levels and tolerances [21] to increase their industrial applications. [PERSON] [22] studied the dimensional accuracy of the parts built by an ME device under the same manufacturing condition. The analysis showed that the test specimens varied significantly from the nominal dimensions. Careful selection of processing parameters was suggested to optimize the dimensional accuracy. [PERSON] [23] developed a design for manufacturing methodology able to improve the dimensional accuracy of ME parts. This approach, predicted the obtainable dimensional deviations in the model-design stage, provided compensations of the deviations generated in the fabrication stage.