# Thermal Layer Design in Fused Filament Fabrication
## 1 Introduction  
Fused filament fabrication (FFF) is one of the most popular additive manufacturing (AM) processes, where the material is extruded to build 3D models layer-by-layer from digital information [1]. AM omits many of the constraints associated with conventional manufacturing processes, yet most AM technologies have several limitations, such as anisotropic material properties and the need for support and postprocessing [2]. Whether the technology is utilized as a tool in the design process or to manufacture end-use parts, it is necessary to consider both opportunistic and restrictive design for additive manufacturing (DfAM) aspects [2]. There are still key enabling technologies that must be further developed to achieve genuine industrial manufacturing via FFFF [3]. The behavior of the materials must be properly understood during both the manufacturing and the use of a component, while proper metrology methods are key in predicting the quality and properties of a component. One of the considerations regarding FFF is that the layer-by-layer process defines a part's anisotropic material properties, determined by the bonds between and within the layers [3]. The interface bonding state, affected by the heat transition within a structure, determines the layer adhesion quality. Process parameters, such as layer thickness, deposition velocity, infill rate, and environment temperature, affect the bonding [4, 5]. Thus, adjusting these parameters changes the mechanical properties of the part manufactured. The degree of molecular diffusion in a polymer material after deposition depends on the local temperatures and associated times [6]. Altering the deposition velocity alters the layer temperature and cooling time, thus also altering the quality of molecular diffusion and heat transfer between deposited filaments. For polylactide (PLA), it has been shown that the deposition of a new layer increases the temperature of the preceding layer above the glass transition temperature [7]. Low layer temperatures result in less diffusion, meaning strong bonds between adhesives and adherents do not occur [8]. Excessively high temperatures cause a material to flow, which may lead to dimensional inaccuracy. Maintaining a layer temperature within the lower and upper bounds is generally not a challenge for low-cost commodity materials such as PLA. However, for high-performance polymers this may be an issue. As more high-performance polymers and polymer composites become available as filaments [9], the design of AM components must be adapted to utilize the properties of these materials. By combining new engineering-grade polymers with fiber reinforcement, new mechanical properties can be achieved. While continuous fiber requires specialized equipment, short fiber composites for use in AM are being rapidly developed as they can be used in any traditional FFF 3D printer resulting in composite materials becoming commercially available at an increased rate. Currently, there is a lack of research on high-performance polymers, as most of the mechanical properties and manufacturing parameters that are tested are of commodity polymers that are not suitable for load-bearing applications [8; 10; 11]. This study explores how the temperature between the layers in the weak direction affects properties such as the ultimate tensile strength (UTS), surface roughness, and geometrical tolerances using an engineering-grade, high-strength, and carbon-fiber-reinforced polymer composite. Through tensile testing and measuring the layer temperature in different designs, differences in the material properties throughout a component can be assumed. To complement the existing DfAM guidelines, we introduce the concept of thermal layer design (TLD) and demonstrate the impact that design and geometrical changes have on the layer temperature of a component.