What is Injection speed?

Injection speed is a key parameter in the plastic injection molding process, as it determines how quickly the molten material is introduced into the mold cavity from the injection unit. This variable, usually expressed in millimeters per second (mm/s) or cubic centimeters per second (cm³/s), has a direct impact on the final quality of the part, as well as on the efficiency and stability of the production cycle.

Properly adjusting the injection speed allows the plastic to flow evenly and completely before it begins to solidify. If the injection is too slow, the material may cool down before completely filling the cavity, causing defects such as weld lines, voids, or even incomplete parts. On the other hand, excessive speed may result in flash, air entrapment, deformations, or internal pressure that could damage the mold.

Determining the optimal speed depends on several factors. The type of plastic material used is one of the most crucial aspects: each polymer has specific rheological behavior that influences its flow characteristics. Materials with higher fluidity allow for faster injection, while more viscous ones require careful control to prevent filling issues. The design of the part also plays a role, as complex geometries with thin walls or intricate details often require higher speeds to ensure the material reaches every area before cooling.

The mold design also affects injection speed. Elements such as the number and position of gates, the runner system, and cooling efficiency directly influence the material’s flow behavior. Additionally, processing temperatures — both of the material and the mold — must be properly set to ensure that the plastic remains fluid throughout the filling stage.

In practice, modern injection molding machines allow for speed profiles to be programmed across different phases of the cycle. This means a constant speed is not required; it is possible to start with a moderate speed to avoid pressure peaks, increase the speed during the main filling phase, and reduce it near the end to prevent overpacking or the formation of defects. This segmented control leads to greater precision in producing technical parts and improves process repeatability.

Improper adjustment of the injection speed can result in a wide range of problems: from visual defects such as flow marks, bubbles, or flash, to more critical functional issues like deformation, incomplete filling, or internal stresses in the part. It can also accelerate wear on mold components, shorten their service life, or cause instability in the production cycle.

For all these reasons, precise control of injection speed is essential in the manufacturing of plastic parts through injection molding. At Plásticos Lezo, we have a specialized technical team and state-of-the-art equipment that allows us to fine-tune this and other critical parameters, ensuring a high-quality final product tailored to each client’s specific needs.