What is Cavity filling?

Cavity filling is a critical phase in the plastic injection molding process. It involves injecting the molten material into the mold through the injection system until it fully occupies the geometry of the designed cavity or cavities. This stage largely determines the dimensional accuracy, structural integrity, and surface quality of the final part.

During filling, molten plastic flows at high pressure and speed from the machine’s nozzle, through the runner system and gates, and into the mold cavity. The way this flow behaves inside the mold directly affects key factors such as flash formation, flow marks, air traps, and the appearance of weld lines.

One of the main goals during cavity filling is to ensure that the material flows in a balanced, uniform, and controlled manner. This allows all areas of the part to fill simultaneously, preventing internal stress, warping, and variations in surface finish or mechanical properties.

Injection speed is a crucial parameter. If the cavity is filled too quickly, it can cause turbulence, gas entrapment, and visual defects. Conversely, if the filling is too slow, the material may begin to solidify prematurely, resulting in incomplete filling or weak weld lines. At Plásticos Lezo, we carefully adjust injection speed profiles based on the polymer type, part size, and mold complexity.

Material and mold temperature are also decisive. To ensure efficient cavity filling, the plastic must maintain optimal viscosity—avoiding both thermal degradation and premature cooling. A poorly temperature-controlled mold can lead to cold spots that interrupt the flow, causing defects such as flow lines, rough surfaces, or visible gate marks.

The design of the runner and gate system directly influences how the molten plastic is distributed in the cavity. A gate that is poorly located or improperly sized can result in uneven filling, with some areas overpacked and others underfilled. At Plásticos Lezo, we perform flow simulations and cavity balancing—especially for multi-cavity molds—to ensure simultaneous and balanced filling in all parts.

In multi-component or multi-cavity molds, controlling cavity filling requires even greater precision. Minimal differences in pressure, temperature, or surface finish within the runner system can create imbalances that impact product quality. To prevent this, we use sequential valve gates, real-time pressure monitoring, and thermal balancing via optimized cooling circuits.

Proper filling is also essential to avoid air entrapment—one of the most common issues in injection molding. Trapped air must escape through venting systems; otherwise, it may become compressed, oxidize the material, or cause burn marks on the part. That’s why our mold designs include strategic venting and precise cavity-core fitting.

In short, cavity filling is much more than a technical step—it’s a decisive moment in achieving a functional, aesthetically sound, and dimensionally stable part. At Plásticos Lezo, we apply technology, experience, and rigorous process control at this stage, ensuring that every molded part is the result of an optimized process from the very first second of injection.