The relationship between pressure and flow in injection molding process

2025-10-24

Is the machine action the result of pressure or flow? What is the relationship between them?

In fact, the movement of the machine itself is a movement that overcomes resistance. That is to say, in order for the machine to move, it must first provide a power slightly greater than (slightly equal to) the resistance, which is called pressure on the Injection Molding machine. Only when this force is provided can it move, and the movement is fast or slow, which involves flow, which is the source of power. After overcoming the resistance and moving, the greater the flow, the faster the movement, and at the same time, the faster the movement, the greater the resistance. Therefore, it can be said that the movement itself is looking for the balance point between power and resistance.

The relationship between flow rate and pressure during injection and pressure holding:

Plastic Injection is a process in which hot material flows into the mold cavity. Theoretically, if surface defects of the product and mold factors are not taken into consideration, the faster the filling speed, the better.

However, since the hot material flow will inevitably generate resistance (pressure inside the mold cavity) during the flow process, the reason for the existence of this internal pressure resistance of the mold cavity is that the machine must provide a power (liquid pressure during injection) greater than or equal to this resistance in order to fill the hot material flow into the mold cavity. In other words, the injection action is also an action of overcoming resistance.

Some people may ask, does this pressure remain constant from the beginning to the end of the injection?

Definitely not, even with a uniform injection speed (also known as a single-stage injection). As the material fills the cavity, the contact surface continuously expands, meaning the area receiving force continuously increases. Therefore, the injection pressure changes during uniform injection. In non-single-stage injection, the injection pressure also changes continuously, because in addition to the changing force area, the material filling speed also changes.

I believe some people may be confused. How should I set the injection pressure?

In fact, setting the injection pressure is actually setting the pressure inside the mold cavity (theoretically). How high should it be set? Do you know how high the pressure inside the mold cavity is? I believe you don't know either. What should you do? Let's assume that the injection pressure can’t be set on the machine screen. There is only one pressure reducing valve that can be used to adjust the injection pressure, and the injection speed is adjustable.

In this case, we would definitely say, safety first, and adjust the pressure reducing valve to a level that does not cause the mold to burst (the magnitude of this force is determined by the mold design limit). If it is adjusted too low, there is a risk of insufficient material filling. During actual injection, if the mold cavity resistance (theoretically, it is equal to the hydraulic injection force) is less than the pressure reducing valve limit, the pressure reducing valve will not operate. If it exceeds the pressure reducing valve limit, the pressure reducing valve will forcibly reduce the input value.

When the cavity resistance is lower than the limit pressure of the pressure reducing valve, the material flow fills the cavity completely at the set speed. When the resistance is greater than the set limit of the pressure reducing valve, the action of the pressure reducing valve will inevitably cause the material flow filling speed to slow down naturally in order to find a new balance point between the cavity internal pressure resistance and the injection liquid pressure.

In fact, the injection molding process itself is such a process. As long as the limit pressure can be set (the limit force of the pressure reducing valve), it is not necessary to manually set the hydraulic pressure during injection. Many variable closed-loop control injection molding machines from Europe are like this, because this control system can adjust the input and output loads of the oil pump and can push the material flow to fill the mold cavity according to the speed set by the operator.

While maintaining injection speed, a hydraulic pressure slightly equal to the cavity resistance is provided. When the cavity resistance approaches a set limit, the machine automatically reduces the injection speed to achieve a new balance. This injection method can be said to be dominated by injection speed and injection pressure.

But why are there often three or even four levels of settable injection pressure in actual production?

Domestically produced machines are generally quantitative or open-loop variable control systems, and are controlled by two independent proportional adjustment methods: a proportional flow valve and a proportional pressure valve. The output load is always the maximum design load, so this system can't confirm whether its output state is consistent with the set requirements.

The propulsion of the material flow itself requires hydraulic pressure as a power source, but the hydraulic pressure of this control system can't be automatically adjusted and supplied according to the pressure inside the mold cavity. Therefore, the change of hydraulic pressure must be artificially given to promote the change of flow rate, that is, the change of injection speed. At the same time, the artificially given injection speed will affect the hydraulic pressure during injection.

Therefore, this method involves a conflicting interaction between injection pressure and injection speed. In actual production, adjustments are made based on experience. This is why, in many cases, people experience no change in the product even when increasing the injection speed from 30% to 50%. This isn't a problem with the machine itself; it's simply that the injection pressure you're applying is roughly equal to the cavity resistance at a 30% injection speed (of course, this also accounts for other pressure losses, such as those at the nozzle and the mold inlet).

Some colleagues who have used imported machines with variable closed-loop control will undoubtedly say that their machines offer three pressure settings. Indeed, this is a trend increasingly seen among brands. The reason is that abnormalities can occur during filling.

For example, if the glue inlet of a multi-cavity mold is blocked during the production process, and if only the limit pressure can be set as described above, the other cavities except this cavity will inevitably be subjected to local high pressure, which may cause damage to the cavity or expansion of the mold.

Therefore, if there are multiple sections of settable pressure, the maximum liquid pressure in a certain section, i.e. the injection pressure, can be determined according to the structural characteristics of the mold and the position of the material flow filling to avoid some unexpected situations and ensure smooth production.