## 2 Basics of Injection Molding  
In order to accurately simulate mold filling processes and validate them experimentally, it is essential to have a fundamental understanding of the injection molding process. Figure 1 illustrates the basic structure of an injection molding machine, highlighting its core functional units. The plasticizing and injection unit play a crucial role in this process. Here, thermoplastic polymer granules are melted, homogenized, and metered to the required quantity. The polymer melt is then transported to the nozzle and accumulated between the nozzle and the screw. Subsequently, the material is injected under high pressure into the molding cavity within the clamping unit. In the injection phase, the cavity undergoes volumetric filling. This is succeeded by the holding pressure phase, during which the polymer is compressed, cooled, and its volumetric shrinkage is partially mitigated. Once sufficient rigidity is achieved to facilitate demolding, the mold opens, and the injection-molded part is ejected. This cyclic process is termed the injection molding cycle, often resulting in ready-to-use polymer components. The primary advantages of injection molding include rapid, automated, and cost-effective production of complex parts in large quantities, typically without the need for post-processing. However, a significant drawback is the high manufacturing cost of the injection molding tool, particularly for intricate geometries. Therefore, thorough numerical investigations during the initial stages of product and mold design are imperative to avoid costly modifications, cf. [10].  
The achievement of optimal injection molding results requires the careful coordination of a number of parameters, including those pertaining to the part properties, mold design, and material characteristics. This requires a process-oriented design approach, optimization of gating systems, and the establishment of appropriate speed profiles to ensure uniform filling of single and multiple cavities [11; 12]. The injection molding process is inherently susceptible to significant inhomogeneities and often presents transient thermal or rheological behavior. For example, variations in mold thickness result in heterogeneous flow resistances [13]. Furthermore, the solidification of the melt's boundary layers against the cooler mold wall induces local temporal changes in flow cross-sections, resulting in transient flow conditions [10].  
Inhomogeneous filling conditions, which are characterized by varying flow velocities and shear stresses within the cavity, have an impact on the properties of the part, including the molecular chain orientations and the distribution of fillers. Such conditions can lead to typical processing defects, including geometric deformations such as shrinkage and warpage. Other phenomena that have been analyzed through injection molding simulation include weld lines, air traps, sink marks, surface defects, shrinkage voids, and burning [12]. A significant challenge is to achieve a balanced filling process, especially for complex geometries and multiple cavities. Consequently, numerical filling studies are of paramount importance during the early stages of product development and mold design, as they serve to identify and rectify potential design issues [10; 14].  
Subsequent validation of the mold can be conducted through systematic filling studies, which precisely reproduce the behavior of the injection mold under real conditions. This enables a direct comparison of the results with previously performed simulations, facilitating the identification and correction of potential inaccuracies in production that may have been undetected in the absence of simulations [10].