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How Should Injection Speed and Pressure Be Balanced During Precision Injection Molding?

2026/10/12 By le zhan

How Should Injection Speed and Pressure Be Balanced During Precision Injection Molding

In injection molding, the injection speed is primarily used to control the cavity filling process. In contrast, injection pressure must provide enough force to maintain the preset flow conditions without exceeding the limit. For precision injection molding operations, we typically recommend that customers first set a stable speed curve and set the pressure upper limit high enough to avoid restricting speed control; Subsequently, as the filling process nears completion, the control switches from speed control to holding pressure control. The purpose of this approach is not to achieve the highest speed or maximum pressure, but to obtain repeatable melt front flow rates and cavity pressure curves, thereby ensuring the part is filled without causing flash, excessive shear, scorch marks, or over-holding pressure.

What Roles Do Injection Speed and Pressure Play in Precision Injection Molding?

The first step in balancing injection speed and injection pressure is to understand that they serve different functions.

During the main filling phase, the injection molding machine typically controls the screw speed. Injection speed determines the melt front flow rate through the runner, gate, and cavity. This directly affects shear rate, cooling during filling, surface appearance, weld-line formation, and required pressure.

Pressure, on the other hand, is the force required to overcome resistance as the melt flows through the runner. In speed-controlled mode, the injection molding machine generates sufficient pressure to maintain the set screw speed until a preset pressure limit is reached. Generally, operators should not attempt to control the filling process by increasing pressure first.

Simply put:
Injection speed controls melt flow.
Injection pressure provides the force required to achieve that flow.

For this reason, even with the same injection speed, two different molds may require vastly different pressures. If the part has thin walls, a small gate, a long flow path, a low melt temperature, or high material viscosity, greater flow resistance will result, requiring higher pressure to maintain the same flow rate.

What Roles Do Injection Speed and Pressure Play in Precision Injection Molding

What Happens If the Speed and Pressure Are Too High?

Increasing both parameters simultaneously does not necessarily improve filling results.

Excessively high injection speed generates high shear stress and shear heat. If the machine injects material at high speed and maintains excessive pressure during the final stage of filling, cavity pressure can rise sharply. This can lead to the following consequences:

  • Flash (burrs) forming at the parting line;
  • Excessive internal stress;
  • Difficulty in demolding;
  • Burn marks near trapped air pockets;
  • Excessive holding pressure and dimensional deviations;
  • Unnecessary stress on the mold and clamping mechanism.

Therefore, the right approach is to maintain adequate speed while keeping pressure under control, rather than simply increasing set values.

In precision injection molding, should injection speed or pressure be adjusted first?

For most precision injection molding applications, set the injection speed profile first.

Use the processing parameter ranges provided by the resin supplier, along with the mold geometry, wall thickness, gate size, and expected fill time, to set these parameters. Adjust the screw speed until the cavity fills smoothly and repeatedly, with no stalling, jetting, excessive shear, or premature solidification. Then, monitor the actual injection pressure.

If the injection molding machine reaches the pressure limit before achieving the preset speed, the process is in a “pressure-limited” state. In this case, increasing the speed setting further won’t accelerate melt flow because the machine has already reached the maximum allowable pressure.

At this point, engineers should analyze the causes of excessive pressure requirements. Common causes include:

Too small a gate → Too thin wall thickness → Low melt temperature → Low mold temperature → Obstructed runner → High resin viscosity.

If the mold and material perform as expected, operators can increase the pressure limit as long as the injection molding machine, mould, and clamping system remain within their safe operating limits. However, if the pressure demand is already too high, simply raising the limit may mask defects in the process or mold design.

The recommended adjustment logic is as follows:

Set a stable speed curve → Verify actual pressure requirements → Confirm sufficient pressure margin → Optimize the V/P transition → Adjust the holding pressure.

In precision injection molding, should injection speed or pressure be adjusted first

Why Do Different Filling Stages Require Different Speed and Pressure Settings?

A single injection speed is often insufficient to meet the filling requirements of the entire cavity. Flow conditions at the beginning, middle, and end of the filling process often differ significantly.

During the initial injection phase, the melt must flow through the nozzle, runner, and gate. Excessively high initial acceleration can cause jet formation or flow instability; therefore, for certain molds, a controlled initial speed is preferable. During the main phase of cavity filling, you can typically increase the filling speed to maintain a stable melt-front advance, preventing viscosity increases caused by excessive cooling.

As the cavity nears full filling, you usually need to reduce speed or switch to pressure-controlled mode. Maintaining high injection speeds during the final stage of filling can cause a sudden pressure surge because very little cavity space remains for the melt.

This is precisely the purpose of the V/P switch: transitioning from speed-controlled filling to pressure-controlled holding.

This phased process strategy is particularly useful for molding precision parts with varying wall thicknesses, multiple gates, optical surfaces, fine features, or strict dimensional tolerance requirements.

Why Do Different Filling Stages Require Different Speed and Pressure Settings

Practical Methods for Balancing Injection Speed and Pressure in Precision Injection Molding

From a Topstar engineering perspective, the first step is to ensure stable material dryness, melt temperature, mold temperature, and shot volume. If the material or thermal conditions are unstable, adjusting speed and pressure alone will not provide reliable compensation.

Second, establish a filling curve. By observing short-shot conditions, part weight, screw position, filling time, cavity pressure (if applicable), and surface quality, confirm that the melt front advances consistently.

Next, evaluate the pressure requirements. The upper limit of injection pressure should be set to ensure the machine can operate according to the preset speed curve while maintaining a sufficient safety margin.

Then, optimize the speed/pressure (V/P) transition. Transitioning too early may result in short shots or require excessive holding pressure to complete filling; transitioning too late may cause flash and pressure spikes.

Finally, adjust the holding pressure and holding time separately. Holding pressure compensates for material shrinkage after filling, not unstable speed control during the filling phase.

This balance can be summarized as follows:

Injection speed determines the flow behavior of the melt front → Injection pressure provides the force required to maintain that speed → V/P switching prevents pressure spikes at the end of filling → Holding pressure stabilizes the final dimensions of the part.

We recommend this method for balancing injection speed and pressure in precision injection molding. The optimal process isn’t the setting with the highest numerical values on the controller; it’s the one that ensures stable speed during filling, keeps pressure within a controlled operating range, and maintains consistent switching between cycles.

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