Injection Speed Explained: Five Filling Stages and Adjustment Techniques

Injection speed refers to the rate at which the plastic melt enters the mold during the injection molding process. It is typically measured during the initial injection phase, from the moment the screw starts moving forward until the injection is completed.

Injection speed directly affects the filling behavior of the melt inside the cavity. If the speed is too high, defects such as bubbles and flow marks may occur. If the speed is too low, the part may have irregular edges or even incomplete cooling and solidification.

Based on melt flow behavior, injection speed is generally divided into five stages:

Stage 1 – Main and Branch Runners

In principle, high-speed filling is used at this stage to shorten the molding cycle. However, care must be taken to ensure that the high speed does not cause surface quality issues.

Stage 2 – Gate and Gate-Area of the Part

This stage requires adjustments depending on the material and gate type. Generally, a lower injection speed is used, especially for high-viscosity resins such as PC, PMMA, and ABS, as well as for side gates (e.g., direct-impact gates used in transparent parts, window unit panels, door covers, and vertical vanes). This helps prevent jetting marks and gate haze. However, when using pin gates or tab gates with low-viscosity resins such as PP, PA, or PBT — and when surface quality near the gate is not critical — high-speed injection may be acceptable.

Stage 3 – Main Body of the Molded Part

This stage begins after the gate area is filled and continues until approximately 70–80% of the part weight is achieved. High-speed injection is used here to shorten the cycle time, reduce viscosity changes of the melt against the metal cavity surface, improve surface gloss, minimize deformation caused by pressure drop during final filling, enhance weld line strength, and improve the appearance of weld lines.

Stage 4 – Near the End of Cavity Filling (approx. 85–90%)

Medium-speed injection is used at this stage as a transition to the final low-speed stage. It also helps prevent excessive thickness in certain areas and flash caused by unbalanced gates.

Stage 5 – Final Cavity Filling

Low-speed injection is used here to prevent flash and burn marks, improve dimensional and weight consistency of the part, and reduce the required clamping force.

Methods for Adjusting Injection Speed

a. Via the Injection Molding Machine Control Panel

Modern injection molding machines are equipped with user-friendly control panels. Operators can directly input the desired injection speed value, which is typically expressed as the screw forward speed in millimeters per second (mm/s).

b. Via the Hydraulic System

The hydraulic system of an injection molding machine plays a key role in controlling injection speed. By adjusting the flow rate of hydraulic oil, the screw advance speed — and thus the injection speed — can be changed. A flow control valve in the hydraulic system allows precise regulation of the oil flow.

c. Based on Mold Structure and Product Requirements

Complex molds or thin-walled products require higher injection speeds. For example, in molds with fine internal features — such as plastic gear molds — injection speeds of 80–100 mm/s may be needed to ensure the melt quickly fills tiny gaps. In contrast, for thick-walled products, the injection speed can be reduced to avoid internal defects such as bubbles or sink marks. A typical range for such products is 30–50 mm/s.