Why Must Injection Speed Be Controlled in Stages During the Injection Molding Process?

In the injection molding process, injection speed is one of the few parameters that can directly determine product appearance, internal quality, and production stability. Many people new to injection molding wonder: why can't the material be injected at one speed from start to finish? Set one speed value, and the machine runs from beginning to end—isn't that simpler?

The answer lies in this: the flow state of the melt inside the cavity keeps changing from the start of filling to the end. The gate, runner, cavity cross-section, venting locations, and the cooling degree of the melt front—these conditions are completely different at different stages of filling. Using one speed to deal with flow conditions that are constantly changing will inevitably be appropriate at some stages and problematic at others.

Staged injection speed control essentially lets the speed "adapt as it goes" along with the filling progress. Each speed stage corresponds to a specific interval in the filling process and solves the problem unique to that interval.

First Stage: From Injection Start to Near the Gate, Speed Should Be Slow

In the first stage of injection, the melt has just entered the mold from the nozzle, passing through the sprue, runner, and reaching near the gate. The core consideration for speed setting at this stage is avoiding jetting and reducing shear stress in the gate area.

If the first stage speed is too fast, the melt will jet when passing through the gate at high speed, shooting into the cavity like a water gun rather than advancing smoothly. Jetting leaves serpentine marks on the product surface and also creates high shear stress near the gate, causing localized material degradation or flow marks. For hot runner molds, the first stage speed also needs to consider the opening process of the needle valve gate. If the speed is too fast, the melt impacts the needle valve at the moment the gate opens, potentially causing needle valve wear or material degradation in the gate area. Therefore, the first stage is usually set to a slower speed, allowing the melt to pass smoothly through the feed system and establish a stable flow front.

Second Stage: From the Gate to the Middle of the Cavity, Speed Should Be Fast

After the melt smoothly enters the cavity, it enters the middle stage of filling. The main task at this stage is to rapidly fill the main body of the cavity and prevent the melt front from cooling prematurely.

Once the melt front cools, viscosity rises, flow resistance increases, higher pressure is required, and cold slugs or weld lines are likely to form at the end of filling. Therefore, the second stage is usually set to a higher speed, allowing the melt to advance rapidly while still at a relatively high temperature, shortening filling time and reducing temperature loss at the melt front. The speed setting for this stage needs to consider the material's flowability and the product's wall thickness. Thin-wall products dissipate heat quickly and require higher speeds to race against time; thick-wall products dissipate heat slowly, and speed can be appropriately reduced. For glass-fiber-filled materials, too low a speed causes uneven fiber orientation, while too high a speed may cause fiber breakage—a balance must be found in between.

Third Stage: Approaching the End of Filling, Speed Must Slow Down

When the melt front approaches the end of the cavity, the third stage begins. Speed must be reduced at this stage because of venting and flash prevention.

When the melt reaches the end of filling, the air in the cavity needs to escape through the vent grooves. If the speed is too fast, the melt front rushes toward the vent grooves at high speed, compressing air at the end before it can escape, causing burning (Diesel Effect) or poor filling. At the same time, high-speed melt impacts the parting surface at the end, easily forcing the parting surface open and causing localized flash. Reducing the end-stage speed allows the melt to smoothly fill the last section of the cavity, giving air time to escape while reducing impact on the parting surface and vent grooves. The speed setting for this stage needs to be considered in conjunction with the position and depth of the vent grooves and the rigidity of the mold. If the mold lacks rigidity, the end-stage speed should be even more conservative.

Fourth Stage: Packing Switchover, Speed Control Transitions to Pressure Control

Strictly speaking, the packing stage no longer falls within the scope of "injection speed"—it is a switch from speed control to pressure control. But the staged design of injection speed must consider the handoff to packing switchover.

The switchover point from injection to packing is usually triggered by position (screw position) or pressure (cavity pressure). If the switchover is too early, the cavity is not yet full, packing pressure cannot be applied, and the product will be short. If the switchover is too late, the cavity is already overfilled, and packing pressure instead pushes the product into flash, or causes over-packing and increased internal stress. The end-stage setting of the staged speed directly affects the position of the switchover point and the melt state at switchover. If the end-stage speed is too fast, the melt still has considerable kinetic energy at switchover, the pressure impact after switchover is large, and flash is likely. If the end-stage speed is too slow, the melt has already cooled before switchover, packing pressure cannot be transmitted, and the product sinks.

The Core Logic of Staged Control: Each Stage Solves One Main Contradiction

Looking at the four stages together, the logic of staged control becomes clear:

The first stage is slow to pass smoothly through the feed system, avoiding jetting and shear degradation.

The second stage is fast to fill the main body of the cavity before the melt front cools.

The third stage is slow to let air escape, avoiding burning and flash.

The fourth stage connects to packing so that the switchover point falls at the correct position and compensation is effective.

The speed setting for each stage is not isolated. If the first stage is too slow, the melt cools too much near the gate, and the second stage needs a higher speed to compensate; if the second stage is too fast, the end-stage melt temperature is too high, and the speed window for the third stage narrows; if the third stage is too slow, the melt has already cooled at switchover, and packing effectiveness is poor.

What Happens Without Staging?

If there is no staging and one speed is used from start to finish, what happens?

Using a high speed suitable for the middle stage, the first stage will jet, and the third stage will burn and flash. Using a low speed suitable for the first stage, middle-stage filling is too slow, the melt front cools, and the product is short or has severe weld lines. Using a low speed suitable for the third stage, the entire filling time is too long, the cycle is extended, and the material may degrade.

This is why injection molding machines provide multi-stage injection speed settings. It is not because the machine wants to complicate things, but because the flow conditions inside the cavity are themselves staged. Using one speed to deal with staged flow conditions will inevitably cause problems at some stages.

Staged Control Also Relates to Material, Mold, and Product Structure

There is no universal value for the specific settings of staged speed. It depends on several factors:

Material flowability. Materials with good flowability (such as PP and PE) can have speed appropriately reduced; materials with poor flowability (such as PC and PA) require higher speeds to avoid premature cooling.

Product wall thickness and flow length ratio. Thin-wall, long-flow products require higher speeds; thick-wall, short-flow products can have speed reduced.

Gate type and position. The gate area of a pin-point gate is shear-sensitive, so the first stage speed should be slow; the gate area of a large gate is not shear-sensitive, so the first stage speed can be appropriately increased.

Mold venting design. For molds with adequate vent groove design, the third stage speed can be appropriately increased; for molds with insufficient venting design, the third stage speed must be reduced.

Injection molding machine performance. The control precision of injection speed, response speed, and screw acceleration capability all affect the actual effect of staged settings.

Ultimately, Staged Control Is About Finding the Balance Between "Filling Up" and "Filling Well"

The goal of staged injection speed control is not to make any single stage speed "optimal," but to keep the entire filling process in a reasonable state at every stage. Filling the cavity is the basic requirement; filling it well—no jetting, no burning, no flash, no short shots, controllable weld lines, controllable internal stress—is what staged control truly aims to solve.