Relationship between pressure and flow rate in the injection molding process

Is machine action a function of pressure or flow rate? What is the relationship between them?

The movement of the machine itself is to overcome the resistance of the movement, that is to say, the machine to move first must provide slightly greater than (slightly equal to) the resistance of the power, in the injection molding machine is also known as pressure, only to provide this force to move, and the action is fast and slow, which involves the flow, it is the source of power, after overcoming the resistance to action, the greater the flow of action will be faster, while the faster the action of resistance is also greater, so you can say that the movement itself is in search of the balance between power and resistance.

The relationship between flow and pressure during injection and pressure-holding.

The injection itself is a process of filling the mold cavity with hot material flow. Theoretically speaking, the faster the filling speed, the better, if surface defects and mold factors are not taken into account.

However, because the flow of hot material in the process of flow is bound to produce resistance (pressure in the mold cavity), the reason for the existence of the internal pressure resistance of the mold cavity, the machine must provide a power greater than or equal to this resistance (injection liquid pressure) to fill the hot material flow into the mold cavity, that is, the action of injection is also in the action of overcoming resistance.



One may ask, is this pressure constant from the beginning to the end of the injection?

Not, even if the injection is at a constant speed (or a period of speed). Because the contact surface is expanding as the material flow fills the mold cavity, which means that the force area is expanding, the injection pressure is changing when injecting at a constant speed. The injection pressure is also changing during non-single-stage speed injection because, in addition to the changing force area, the filling speed of the material flow is also changing.

I believe some people will be confused, how do I set the injection pressure?

Setting the injection pressure is to set the pressure inside the mold cavity (theoretically), in the end, how much to set, do not know! Do you know how much pressure inside the mold cavity, in the end, I believe also do not know, how to do it? First, a hypothetical condition, the machine screen can not set the injection pressure, only a pressure reducing valve available to adjust the injection pressure, the injection speed is adjustable.

In this case, we will certainly say that the safety of the first pressure reducing valve will not be adjusted to the mold will shoot the storm on the line (the size of the force of the mold design limit value can be found), because the adjustment is too low to fill the fear of material is not enough. The actual injection cavity resistance (theoretically, it is equal to the hydraulic injection force) is less than the limit of the pressure reducing valve when the pressure reducing valve does not work if beyond the limit of the pressure reducing valve is forced to reduce its input value.

When the resistance of the mold cavity is lower than the limit pressure of the pressure reducing valve, the material flow fills the mold cavity at the set speed, while when the resistance is higher than the set limit of the pressure reducing valve, the action of the pressure reducing valve inevitably leads to the natural slowdown of the material flow filling speed to find a new equilibrium point between the internal pressure resistance of the mold cavity and the injection fluid pressure.

The injection product itself is such a process, as long as the limit pressure can be set (the limit force of the pressure reducing valve) is sufficient, it does not necessarily need to be artificially given the liquid pressure at the time of injection, which is the case with many variable closed-loop control injection molding machines from Europe, because this control system can regulate the input and output load of the oil pump, and can advance the material flow to fill the cavity according to the speed set by the operator.

The machine automatically reduces the injection speed to reach a new equilibrium when the cavity resistance tends to be greater than the set limit force. This injection method can be said that the injection speed dominates the injection pressure.

However, in actual production, why are there often three or even four settable injection pressures?

Domestic machines are generally quantitative or open-loop variable control system machines and are controlled by two independent proportional adjustments of proportional flow and proportional pressure valves, and the output load is always the maximum design load, so this system cannot confirm whether the output state is consistent with the set requirements.

The propulsion of the material flow itself requires hydraulic force as the power source, but the hydraulic pressure of this control system cannot be automatically adjusted and replenished according to the internal pressure of the mold cavity, so the change of the liquid pressure must be artificially given to promote the change of the flow rate, i.e., the change of the injection speed, and the artificially given injection speed will affect the hydraulic pressure during injection.

So this way is the injection pressure and injection speed affect each other contradictorily. The actual production process can only be adjusted by experience, so many times someone will encounter the injection speed from 30% of the speed to 50% of the product is no change, is not entirely the machine itself has problems, but at this time you are given the injection pressure is just about equal to the injection speed of 30% of the cavity resistance (of course, some other pressure losses, such as the nozzle, mold The injection pressure is just equal to the cavity resistance at 30% of the injection speed (plus other pressure losses such as on the nozzle, the mold, etc.).

There must be some colleagues who have been in contact with variable closed-loop control imported machines who will say that the machine he is in contact with can be set in three stages. Yes, and this is the way more and more brands tend to set their pressures. The reason for this is that there are always irregularities in the material flow filling.

For example, if a cavity is blocked during the production of a multi-cavity mold, and only the limit pressure can be set as described above, the cavities other than the cavity will be subjected to localized high pressure, which may cause damage to the cavity or swelling of the mold.

Therefore, if there are multiple settable pressures, the maximum liquid pressure, i.e. injection pressure, can be determined according to the structural characteristics of the mold and the location of the material flow filling to avoid some unexpected situations, so that the production can be carried out smoothly.