This question came in last month. Although I could not get any specifics about the machine such as age or model, I believe the question still makes a good discussion…
Today I tried to adjust limit of injection pressure on Demag machine. I adjust maximum of machine 210 bar actual injection pressure was 160 bar, so I decided to decrease limit to 190 bar, but actual pressure decreased to 150 bar. I tried to decrease limit to 170 bar, but actual pressure decrease again to 130 bar.
I would say this is not a good thing. It seems like that this is built into the programming of the Demag machine. Might be automatically trying to create a pressure differential, but by doing it this way it moves the process back to pressure controlled rather than velocity controlled.
Although I cannot speak specifically about that particular machine, there are some general reasons for this behavior.
Most machines do have a buffer to ensure the machine does not overcompensate when approaching the maximum setting. Sometimes this buffer compensates for line pressure or pressure losses in the system.
Often machines set the limit near the pump and take peak pressure measurements at the cylinders… which does not compensate for losses in the system. Certain machine types even use a pressure differential to control velocity, which could cause this difference.
Another cause for this could be dynamic breaking… to compete with the accurate controls of electric molding machines, machines are applying a small amount of hydraulic pressure to the front of the injection cylinders to better control the velocity transitions during injection. The displayed injection pressure will most likely be calculated as a differential between the front and back of the cylinders.
In any of these cases, to test if you are actually hitting this peak limit, increase your max pressure and see if the peak pressure increases.
You may also want to perform a Dynamic Load Sensitivity Test on the machine to ensure the machine controls are functioning properly and there is not a problem with the hydraulics or controls.
As the molding machine technology increases, the specific functionality of molding machines is becoming less obvious. This is why it is very important to good relationship with the technical representatives at your machine supplier. If the machine is an older model, a machine re-builder can also be a great resource as they tend to have great knowledge of your machine’s shortcommings.