Definition: 1. the exertion of force upon a surface by an object, fluid, etc in contact with it. 2. Physics. Force per unit area. Symbol P 3. Pressure is created whenever the flow of a fluid is restricted.
Though yes there are many other definitions of pressure which I am sure we are all aware of each and every day, the three listed above are of our concern now. Especially in concern with injection molding given that we use pressure to squirt the material into a feed system and than into the mold cavity / form and further to pack it out to our given shape /size / dimensions.
Now understand that the following is related to a hydraulic machine and for those with electric's everything from the screw tip forward is applicable to our discussion. Electric machines use servos to drive the screw forward much in a similar manner that we drive it forward with a hydraulic actuator (piston/ pistons). Yes there is a delta P in / with electric machines too.
In a molding machine we can set how much pressure we wish to use in both filling of the part, packing the part and holding of part. This is via computer setting, pressure regulator valve, etc within the hydraulic / electrics of the press. This pressure for machine purposes is read in psi, bar, Newton, Plastic pressure, etc. This is either actual pressure read off a gauge / screen or computed pressure off a screen. The pressure we are measuring is the force against the injection screw tip which will have an area, basically the diameter of the screw.
So as we inject we will push fluid into our hydraulic cylinders which are mechanically coupled to our injection screw. The screw than pushes the plastic (now in a fluid state) into our mold to form our parts, and this pushing of the fluid (injecting our plastic) is what creates our resistance to flow. As the cavity fills up with plastic we create more resistance to flow (some due to cooling effects such as narrowing of flow channels) and higher pressures are recorded within our monitoring system.
This is why nozzle diameter, sprue diameter, runner diameter, gate diameter and wall thickness are all important to us in molding as they act as pressure restrictions in the injection of the plastic into our mold. Again pressure is only created when we have a restriction to flow. An example would be blowing up a car tire. After mounting the tire on the rim and sealing it, it takes a while for the air pressure to register as it fills the tire, but once the tire is full with our molecules of air we than pressurize it and can now register the increase in air pressure on the gauge. We basically filled the void and upon placing enough air into the tire to fill said void it will than pressurize it to give it strength so that we can drive on it. This is much the same as we do in molding in filling the cavity, but we also must fill the feed system prior to the cavity, and with plastics they have a viscosity or thickness and stickiness which restrict the flow unlike air. Thus as we fill our cavities to form our parts we have lost pressure along the way at various points in the feed system. It has been noted in some articles that it takes up to 2000 psi of Plastic pressure to just get the plastic to move out the nozzle of the machine.
Yes within our mechanical system we have friction, which causes resistance and within the fluid of hydraulics we may have land areas which cause resistance and loss of pressure but this can be predicted and calculated for. The terminology here is DELTA P (▲P), the difference in pressure from a supply side to the work side that allows for the flow of fluid to maintain its rate of flow.
Rate of Flow: within our hydraulic system we have a pump which is capable of creating pressure but the fluid is flowing at a rate, either Gallons per Minute (gpm) or Liters per Minute (lpm). Now understand that within the flow path from our pump to our cylinders we have a pressure relief value that is set at a pressure, so that for example if it is set at 100 psi we could apply 100 psi to our cylinder before the relief value would open, allowing fluid to flow back to our tank while still maintaining a 100 psi on the cylinder, and what ever residual flow to maintain that pressure. What is trying to be explained here is that the flow is constant from the pump; it is the settings on the relief valves that can be used to change the resulting forces / pressures. (Yes there are variable speed and pressure pumps but that is for a later discussion) Basically the pump is capable of delivering to a maximum psi pressure, but is always flowing at its gpm rate; the pressure is dependant of the forces exerted on the area of work. To be able to have flow we must have a pressure difference across the work area. Using the example above if the work being push was a load of 80 pounds we could push it but if it was 100 pounds we may or may not push it. (Our cylinder is only 1 square inch of area folks)
The rate of flow gpm /lpm is set by a flow control valve which regulates how much can flow through it, much like the faucet on a sink. This does not affect pressure but only flow rate.
The point
We can set our pressure available on the controls of the machine and the pressure we use is the result of the force it takes to make our part. Thus the filling and packing pressure are different in how we achieve our results.
Filling the part:
The pressure we actually develop and use is set by our plastics and design of part in the filling stage of our process. For the processor to be in control of this stage of the process the pressure available must always be more than that we use. To check if you have enough difference referred to as ▲P, check your fill time and see if it is constant to +/- 0.04 seconds repeatability. (See previous topic)
Packing the part:
The packing of the part is what gives us the finished part size, the cavity is full so we have maximum resistance to flow and this is a direct correlation to what we set on the press minus of course all the loses in the system. Note that in typical molding operations we just record and state what we have set it to or are recording it at and do not take into effect the pressure loses in the system as all are constant, runner size, gate size O of sprue etc. If in effect you change these areas than the pressure you use may change because the losses may be less or more.
Fill Pressure Graph;
Some whom have a pressure graph on filling of the part should see a gradual increase in pressure all the way to the point of transfer. If we see the pressure increase and than level off it may be pressure limited. What this means is that we have reached our pressure set maximum on the press. The question here is HAVE YOU????? Is the pressure on the graph the same as what you have set as maximum? If it is than you are in fact pressure limited, not necessary a good thing. If you have not reached the pressure set point but have this flat line graph than we have an issue with a restriction in our feed system. This can be caused by a gate being too small, or not, maybe our cavity is extremely thick in regards to our feed, but it is having an effect on the pressure we develop in the tool or are capable of recording. An example of this later issue may be foam molding where I have an extremely small gate to kick off foam and large cavity for the foam to fill. As I fill we would reach the restriction of the gate and this is where my greatest pressure would be so our resulting pressure graph to fill the part would now increase to the gate point and than be flat, and in this case with foam I would not want to see an increase since we want the reduce density in our cavity.
Fill Pressure gage:
If no graph is available use the gage and watch as the cavity fills. Is it a gradual increase in pressure and than a drop for pack / hold or is it wham up to maximum set and than down to our pack / hold pressure. The gradual increase and than drop to pack hold is showing that we are building pressure in a slope and not pressure limited. To see if we are limited in pressure, in a manual mode carefully bottom out the screw and hit the inject and record pressure, is it higher than what was recorded for our flat line we are not pressure limited, if is the same than you are pressure limited. Of course watching fill time will tell the same story, i.e. if pressure limited on certain machines you will notice a variation in fill time.
Electric Machines:
Understand that on the electric machines the readout and set point to the best of my knowledge are in Plastic Pressure. Also note that we still need more pressure available than what we are actually using so as to drive the screw forward in the time allotted and that observing the fill time clock will confirm that you have or have not supplied enough pressure. The same principle applies in that our registered use of pressure is a result of what is needed to do the work, if we do not have enough pressure available to do the work it will not get done in a repeatable manner.
*** On certain brands of machines they are programmed to mold via speed and will use more pressure than you set so as to maintain what ever speed setting was set.
