The relationship between injection pressure and speed

In fact, the motion of the machine itself is a movement that overcomes resistance, which means that in order for the machine to move, it must first provide a force slightly greater than (slightly equal to) resistance, which is also known as pressure in injection molding machines. Only by providing this force can it move; And there is a distinction between speed and slow motion, which involves flow, which is the source of power. After overcoming resistance, the larger the flow, the faster the action. At the same time, the faster the action, the greater the resistance. Therefore, it can be said that motion itself is seeking a balance between power and resistance.
I think what everyone is concerned about may be the relationship between the flow rate and pressure during injection and pressure holding of the injection molding machine.
The injection itself is a process of filling the mold cavity with a hot material flow. In theory, if surface defects of the product are not considered and mold factors are not considered, the faster the filling speed, the better. However, due to the inevitable resistance (cavity pressure) generated by the hot material flow during the flow process, the existence of internal pressure resistance in this mold cavity requires the machine to provide a power greater than or equal to this resistance (liquid pressure during injection) in order to fill the hot material flow into the mold cavity, which means that the injection action is also an action to overcome the resistance.
Some people may ask, is this pressure constant from the beginning to the end of the injection? Definitely not, even if it is a uniform injection (or a certain speed). Because as the material flow fills the mold cavity, its contact surface continues to expand, that is to say, its stress area continues to expand, so the injection pressure changes during uniform injection. When injecting at a non single speed, the injection pressure is definitely constantly changing, because in addition to changing the force area, the filling speed of the material flow is also changing.
I believe some people will be confused. How can I set the injection pressure to be good? In fact, setting the injection pressure is just setting the pressure inside the mold cavity (theoretically). I don't know how much it should be set! Do you know how much pressure is inside the mold cavity? I believe I don't know either. What should I do?
Let's first assume that the injection pressure cannot be set in the machine screen, only a pressure reducing valve can be used to adjust the injection pressure, and the injection speed is adjustable. In this situation, we will definitely say that safety is the first priority. Adjust the pressure reducing valve to prevent the mold from exploding (the magnitude of this force can be checked by the mold design limit), because setting it too low may cause insufficient material filling.
When the actual injection resistance of the mold cavity (theoretically equal to the hydraulic injection force) 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, the input value is forcibly reduced by the pressure reducing valve. When the resistance of the mold cavity is lower than the limit pressure of the pressure reducing valve, the material flow completely fills the mold cavity at the set speed. However, when the resistance is greater than the limit of the set pressure reducing valve, the effect of the pressure reducing valve inevitably causes the material flow filling speed to slow down naturally in order to find a new equilibrium point between the pressure resistance of the mold cavity and the injection pressure.
In fact, the process of injection molding products itself is like this. As long as there is a limit pressure that can be set (the limit force of the pressure reducing valve), it is not necessary to manually set the liquid 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 load of the oil pump, and can advance the material flow to fill the mold cavity according to the speed set by the operator, while ensuring the speed, Provide a hydraulic pressure that is slightly equal to the mold cavity resistance. When the mold cavity resistance tends to exceed the set limit force, the machine will automatically reduce the injection speed to achieve a new balance. This injection method can be said to be dominated by the injection speed
But why are there often injection pressures that can be set in three or even four stages in actual production? Domestic production machines are generally machines with quantitative or open-loop variable control systems, and are controlled by two independent proportional adjustments: proportional flow valves and proportional pressure valves. The output load is always the maximum design load, so this system cannot confirm whether its output state is consistent with the set requirements, The propulsion of the material flow itself requires hydraulic pressure as the power source, but the hydraulic pressure of this control system cannot be automatically adjusted and replenished based on the pressure inside the mold cavity. Therefore, it is necessary to artificially set the change in hydraulic pressure to promote the change in flow rate, i.e. the change in injection speed. At the same time, the artificially set injection speed will also affect the hydraulic pressure during injection.
So this method is contradictory to the mutual influence between injection pressure and injection speed. In the actual production process, it can only be adjusted based on experience, so many people will encounter that changing the injection speed from 30% to 50% of the product does not change. In fact, it is not entirely due to the problem with the machine itself, At this point, the injection pressure you give is just about equal to the mold cavity resistance at an injection speed of 30% (plus other pressure losses, such as on the nozzle and the mold inlet).
There will definitely be colleagues who have been exposed to variable closed-loop control imported machines who will say that the machines they are exposed to can be set with three stages of pressure.
Yes, and more and more brands tend to set the pressure in this way. The reason is that there will always be abnormal situations when the material flow is filled. For example, when the rubber inlet of the first mock examination mold cavity is blocked in the production process of the multi mold cavity mold, if it is also the case that only the limit pressure can be set as described above, it will inevitably cause other mold cavities to suffer local high pressure, This pressure may cause damage to the mold cavity or expansion of the mold.
So if there are multiple sections that can be set with pressure, the maximum liquid pressure or injection pressure within a certain section can be determined based on the structural characteristics of the mold and the position of the material flow filling to avoid unexpected situations and ensure safe and smooth production.