What is Fill time?

Fill time is a critical parameter in the plastic injection molding process. It refers to the exact period during which the molten plastic fills the entire mold cavity, starting from the beginning of the injection phase. Precisely controlling this time is essential to ensure dimensional accuracy, surface appearance, and the structural integrity of the molded part.

During the injection cycle, fill time is influenced by multiple factors: injection speed, material viscosity, mold and melt temperatures, part geometry, and the design of gates and runner systems. A fill time that is too fast can cause turbulence, air entrapment, flow marks, or flash. Conversely, if the fill time is too slow, it may result in weak weld lines, poor material packing, and visible surface defects.

Properly balancing fill time is key to producing consistent, defect-free parts with excellent surface quality. That’s why mold designers and process engineers fine-tune machine parameters to achieve controlled, uniform, and efficient filling. Fill time is usually measured in seconds and may vary from fractions of a second to several seconds, depending on the size and complexity of the part.

One widely used tool for optimizing fill time is mold flow simulation. This technique predicts how the molten plastic will flow inside the mold and estimates the ideal fill time, helping identify potential issues before the mold is built. These simulations assist in making decisions about gate placement, runner design, and even material selection.

Beyond product quality, fill time also directly affects cycle time and production efficiency. A faster fill time may reduce the overall cycle per part, but if not well controlled, it can increase scrap rates and rework costs. On the other hand, a fill time that is too slow can limit machine productivity without delivering quality benefits.

At Plásticos Lezo, we carefully optimize fill time for every project. We conduct process studies, mold validations, and machine adjustments to find the ideal balance between speed, pressure, temperature, and quality. This optimization allows us to produce high-precision technical parts efficiently and consistently—even in long cycles or high-volume series.

We also record fill time as part of our traceability and quality control protocols, ensuring repeatability across all production batches. In complex parts or critical geometries, this data becomes a key factor in guaranteeing compliance with client technical and regulatory requirements.

In summary, fill time is a vital injection molding parameter that impacts both part quality and manufacturing performance. Accurate control enables the production of clean, dimensionally correct parts with uniform mechanical properties. At Plásticos Lezo, we consider fill time a strategic factor in our commitment to technical excellence and continuous improvement.