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Injection Pressure vs. Injection Speed: Understanding the Balance Between Driving Force and Resistance
Views: 75415 Author: Site Editor Publish Time: 2026-11-09 Origin: Site
Actually, every machine movement is a motion that overcomes resistance. In other words, for the machine to move, it must first be provided with a driving force slightly greater than (or approximately equal to) the resistance. On an injection molding machine, this is called pressure. Only when this force is provided can movement occur. Movement also has a speed, which involves flow rate. Flow rate is the source of power. Once resistance is overcome and movement occurs, the larger the flow rate, the faster the movement; at the same time, the faster the movement, the greater the resistance. Therefore, it can be said that motion itself is a process of seeking a balance between driving force and resistance.
What most people are probably concerned about is the relationship between flow rate and pressure during injection and holding/packing.
Injection itself is the process of the hot melt stream filling the mold cavity. Theoretically, disregarding product surface defects and mold-related factors, the filling speed should be as high as possible.
However, resistance (cavity pressure) is inevitably generated as the hot melt flows. Because this internal cavity pressure resistance exists, the machine must provide a driving force (hydraulic pressure during injection) greater than or equal to this resistance in order to fill the cavity with the hot melt. In other words, the injection action is also an action of overcoming resistance.
Some may ask: Is this pressure constant from the beginning to the end of injection? Certainly not—even during constant-speed injection (also called single-stage speed injection). As more melt fills the cavity, the contact area continuously expands; that is, the pressure-receiving area continuously increases. Therefore, even during constant-speed injection, the injection pressure changes. During multi-stage speed injection, the injection pressure certainly also changes continuously, because in addition to the changing pressure-receiving area, the melt filling speed also changes.
Some people may be confused: How exactly should I set this injection pressure? In fact, setting the injection pressure means setting the cavity pressure (theoretically). How large should it be? Unknown! Do you know exactly how large the cavity pressure is? Presumably not. So what should be done?
Let us first assume a condition: on the machine screen, injection pressure cannot be set; only a pressure-reducing valve is available to adjust injection pressure, and injection speed is adjustable. In this case, we would certainly say: safety first—adjust the pressure-reducing valve to a level that will not cause the mold to be overpacked or damaged (the limit value can be found from the mold design). If it is set too low, there is a risk that the material will not completely fill the cavity.
In actual injection, when the cavity resistance (theoretically equal to the hydraulic injection force at this time) is lower than the pressure-reducing valve limit, the pressure-reducing valve does not operate. If the cavity resistance exceeds the pressure-reducing valve limit, the valve forcibly reduces its input value.
When the cavity resistance is lower than the pressure-reducing valve limit pressure, the melt fills the cavity completely according to the set speed. When the resistance is greater than the set pressure-reducing valve limit, the action of the pressure-reducing valve inevitably causes the melt filling speed to slow down naturally, in order to seek a new balance point between the cavity pressure resistance and the hydraulic injection pressure.
In fact, injection molding is exactly such a process. It is sufficient to have only a limit pressure setting (the pressure-reducing valve limit); it is not necessarily required to manually set the hydraulic pressure during injection. This is the case with many imported injection molding machines using variable closed-loop control. Such control systems can adjust the input/output load of the hydraulic pump and can advance the melt to fill the cavity according to the speed set by the operator. While maintaining the speed, they provide a hydraulic pressure approximately equal to the cavity resistance. When the cavity resistance tends to exceed the set limit force, the machine automatically reduces the injection speed to reach a new balance. In this injection mode, it can be said that injection speed dominates injection pressure.
Why, then, in actual production are there often three or even four settable injection pressure stages? Machines produced domestically generally use fixed-displacement or open-loop variable-displacement control systems, and are controlled by two independent proportional adjustments: a proportional flow valve and a proportional pressure valve. Their output load is always the maximum design load.
Therefore, such systems cannot confirm whether their output state is consistent with the set requirement. Moreover, advancing the melt itself requires hydraulic pressure as the power source, but the hydraulic pressure of such control systems cannot be automatically adjusted and supplied according to the cavity pressure. Therefore, the change in hydraulic pressure must be manually set to promote a change in flow rate, that is, a change in injection speed. At the same time, the manually set injection speed also affects the hydraulic pressure during injection.
Thus, in this mode, injection pressure and injection speed interact with and conflict with each other. In actual production, adjustment can only rely on experience. That is why people often encounter a situation where changing the injection speed from 30% to 50% produces no change in the product. In fact, it is not entirely a problem with the machine itself; it is simply that the injection pressure you have set is just approximately equal to the cavity resistance at an injection speed of 30% (of course, other pressure losses must also be added, such as those at the nozzle and the mold gate).
Colleagues who have experience with imported machines using variable closed-loop control will certainly say that the machines they have used can set three pressure stages.
Yes, and more and more brands are tending to set pressure in this way. The reason is that abnormal conditions will always occur during melt filling. For example, in a multi-cavity mold, if the gate of one cavity is blocked during production, and if only a limit pressure can be set as described above, the other cavity portions excluding this cavity will inevitably be subjected to localized high pressure. This pressure may cause damage to the cavity or mold opening/overpacking.
Therefore, if multi-stage pressure setting is available, the maximum hydraulic pressure—that is, the injection pressure—within a certain segment can be determined according to the structural characteristics of the mold and the melt-fill position, so as to avoid unexpected situations and achieve safe and smooth production.
Reliable pressure and speed control depends not only on the control system but also on the performance and consistency of the screw and barrel at the heart of the injection unit.
About SUPERIOR MACHINERY
Established in 1992, SUPERIOR MACHINERY has been engaged in the rubber and plastic molding machinery industry for more than 30 years. With an increasing market share, the company has established itself in the field of high-quality fully hardened screws and bimetallic barrels that are highly resistant to wear, corrosion, high pressure, and high speed.
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