# Process-Dependent Influences on Adhesion in Multi-Material Extrusion
## 2 Multi-Material Material Extrusion and Mechanisms of Adhesion  
Material extrusion (MEX, also referred to as Fused Layer Modeling or Fused Deposition Modeling) is an additive manufacturing process that uses line-by-line and layer-by-layer application of liquefied thermoplastic polymer in order to build complex parts. Inmulti-material MEX, the material interface can show almost any geometry and is not restricted to layer boundaries. By utilizing material-specific functions, this switch in material can help to increase the range of functions or integrate functions into fewer parts. In this way, mechanical [1; 2; 3; 4; 5; 6; 7; 8; 9], electrical or sensory [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23], and magnetic [24; 25] functions can been implemented, for example. Figure 1 schematically depicts the concept of a multi-material MEX machine as well as the fundamental adhesion principles at play in a multi-material print. In order to produce multi-material components, a MEX machine either needs a mechanism that switches between several extruder units, or a purging step is required while switching materials on a single extruder. While the former is prone to deviations in nozzle position from the mechanical switching process, the latter might lead to contamination at the interface zone due to material residue inside the extruder, which, in turn, might affect adhesion at the interface. As both options might negatively affect the interface strength of the produced component, the switching process requires special attention to ensure optimal mechanical properties. The final properties of the AM components only form during manufacturing. Hence, the process parameters are significant in producing high strength components, especially when considering multi-material parts. Since a multitude of process parameters might affect the interface strength of multi-material components, a complete overview of all influences is impossible in the scope of this publication. Thus, this paper focuses on extrusion temperature, build plate/chamber temperature, and over/under-extrusion as influences on composite strength. The aim is to determine the degree of variance that can be expected from using suboptimal process parameters in a simple multi-material part, in order to give suggestions for good component or printer design with regard to multi-material manufacturing. Regarding the possible adhesion mechanisms for polymer-polymer interfaces, only a handful of influences are apparent. On the one hand, temperature is the driving force for several mechanisms of adhesion. Inter-diffusion can only occur due to the movement of the polymer molecules at elevated temperatures. Hence, extrusion temperature and build plate/chamber temperature are expected to influence diffusion at the material interface. In  
Figure 1: Schematic representation of material extrusion (MEX) concept and material transition, including relevant coupling mechanisms. Based on [26]. (**a**) schematic setup of a manufacturing machine; (**b**) schematic mesoscopic view of the material interface, (**c**) schematic microscopic view of the material interface  
addition, the viscosity of most polymers decreases at higher temperatures, which can affect the filling of microscopic pores at the interface, thereby affecting mechanical adhesion. However, as extrusion temperatures approach the decomposition temperature of the used polymer, impurities at the interface increase, increasing the chance of failure as described by the weak boundary layer theory [27]. On the other hand, the proper amount of extrusion is crucial to ensure complete overlap of the two materials. Slight over-extrusion of the second material at the interface is expected to promote mechanical adhesion, as the molten polymer is more strongly pressed into the microscopic pores at the interface [27].