Injection molding machine pressure and flow of the machine

Injection molding machine pressure and flow of the machine itself functioned ? What is their relationship?

In fact, the action of the machine itself is a movement that overcomes the resistance, that is, to act, the machine 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 be active, and the action can be fast or slow, which involves the flow, which is the source of power, After overcoming the resistance, the greater the flow, the faster the action, and the faster the action, the greater the resistance. Therefore, it can be said that the movement itself is looking for the balance point between power and resistance.

Relationship between flow and pressure during injection and pressure holding

Injection itself is a process of filling the mold cavity with hot material flow. In theory, if we do not consider the product surface defects and mold factors, the faster the filling speed, the better. However, because the hot material flow must produce resistance (pressure in the mold cavity) in the process of flow, with the existence of the internal pressure resistance in the mold cavity, the machine must provide a power greater than or equal to this resistance (hydraulic pressure during injection) to fill the hot material flow into the mold cavity, that is, the injection action is also an action to overcome the resistance.

Some people will ask, is this pressure constant from the beginning to the end of injection?

Definitely not, even if it's a uniform injection (or a period of speed). As the material flow fills the mold cavity, its contact surface is expanding, that is, its stress area is expanding, so the injection pressure is changing during uniform injection. The injection pressure must also be changing when the injection is not at a single speed, because in addition to the force area, the filling speed of the material flow is also changing.

I believe some people will be confused. How can I set this injection pressure?

In fact, setting the injection pressure is to set the pressure in the mold cavity (theoretically). I don't know how big it is! Do you know how much pressure is in the mold cavity? I believe I don't know. What should I do? Let's start with a hypothetical condition. The injection pressure cannot be set in the machine screen. Only one pressure reducing valve can be used to adjust the injection pressure, and the injection speed is adjustable.

In this case, we will certainly say that safety first, adjust the pressure reducing valve so that the mold will not burst (the size of this force can be checked according to the mold design limit), because it is too low for fear of insufficient filling. When the actual injection cavity resistance (theoretically, it is equal to the hydraulic injection force at this time) is less than the limit of the pressure reducing valve, the pressure reducing valve does not work. If it exceeds the limit of the pressure reducing valve, it is forcibly reduced by the pressure reducing valve.

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

In fact, the injection molding product 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 artificially give the hydraulic pressure during injection. This is the case with many variable closed-loop control injection molding machines from Europe, because this control system can adjust the input and output load of the oil pump, It can push the material flow to fill the mold cavity according to the speed set by the operator;

When the speed is guaranteed, the hydraulic pressure slightly equal to the cavity resistance is provided. When the cavity resistance tends to be greater than the set limit force, the machine will automatically reduce the injection speed in order to achieve a new balance. This injection method can be said that the injection speed dominates the injection pressure

But why are there often three or even four sections of injection pressure that can be set in actual production?

Domestic machines are generally machines with quantitative or open-loop variable control system, which are controlled by two mutually independent proportional adjustment methods of proportional flow valve and proportional pressure valve, and the output load is always the maximum design load. Therefore, this system cannot confirm whether the output state is consistent with the setting requirements;

The propulsion of material flow itself requires hydraulic pressure as the power source, but the hydraulic pressure of this control system cannot be automatically adjusted and supplied according to the pressure in the mold cavity, so the change of hydraulic pressure must be artificially given to promote the change of flow, 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 way is the mutual influence and contradiction between injection pressure and injection speed. In the actual production process, it can only be adjusted by experience, so many times someone will encounter that there is no change in the injection speed from 30% to 50%. In fact, it is not entirely the problem of the machine itself, but at this time, the injection pressure you give is just about equal to the cavity resistance when the injection speed is 30% (of course, there are other pressure losses, such as on the nozzle, on the mold inlet, etc.)

Some colleagues who have been exposed to variable closed-loop control imported machines will say that the machines they contact can set the three-stage pressure. Yes, and more and more brands tend to set the pressure in this way. The reason is that abnormal conditions always occur in material flow filling.

For example, the first mock exam hole is blocked in the production process of multi-mode cavity mold. If the limit pressure can be set up at the same time as described above, it is inevitable that the other cavity parts of the die cavity will be subjected to partial high pressure, which may cause damage or expansion of the die cavity.

Therefore, if there are multiple sections where the pressure can be set, the maximum hydraulic pressure, i.e. injection pressure, in a certain section can be determined according to the structural characteristics of the die and the position of material flow filling, so as to avoid some unexpected situations, so as to ensure safe and smooth production.