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Definition: Understanding: plural of understand, comprehension, knowing, Graph: a display of data in composite or visual display showing a range Injection Transfer: A point of transfer between filling and packing with the injection screw. Pack pressure: the pressure used to achieve final part dimensions.
On many presses of the hydraulic type the transfer from fill peak to pack occurs by the reduction of both pressure, and speed. It must be remembered the at the fill pressure is usually set higher than necessary so as to maintain fill time, and that when one transfers to pack the pressure is key in that this pressure is what gives the final dimensions and part weight. Yes there are parts which are filled and packed on fill due to their configurations and most importantly wall thickness. This discussion is on the ones that do use packing pressure to complete the filling.
The area of graph that is of a concern is that from the peak to the point of achieving the pack pressure. This is the downward slope, drop from transfer to pack.
An example is the travel on the curve of point B to point C.
Figure 1
Fig 1 shows the general curve of a fill and pack for part. With point A being the start, B the transfer point at fill C the start of the pack pressure and D the pack pressure.
While in the example one can notice that the slope of the line B to C is there, and on some machines this is programmable, it should be a vertical drop. That is straight, and has no slope. If it is straight that is an indication that no time has expired, while with a slope time has passed. The time it takes to stabilize pressure for pack is critical for repeatable part production.
Between these two points the screw may be going forward depending on type of machine, but really it has no pressure behind it to speak of as it is dropping pressure, thus in most cases the movement of the screw has slowed down. This is best understood when one thinks of the plastic which was being injected into the cavity, and flowing, shearing and stretching out it molecules to flow, and now because of the drop in pressure and at the same time speed the plastic molecules can spring back to their normal shape during a long period of time should it occur in this transition of speed and pressure that occurs between these two points. If the time is excessive (greater than ˝ second for example) issues with filling some plastics may occur.
Though our example shows a slope it also shows that packing pressure was reach immediately and is stable.
Figure 2
Fig 2 shows a hydraulic machine which is trying to find the pressure for its pack pressure.
In Fig 2 the example shows that the pressure for pack was not reach immediately and tended to over-shoot by going below set pressure and then overshoot again when going back up in pressure and then repeated this again but lower until it found the correct pressure at point C. While this is not unusual at times on a hydraulic press with electrical presses the author has not noticed to this point in time that this is happening. The factor here is how much time did it take for the press to find its' pressure and speed? If the time has been too long it is possible that the material has frozen either at the gate, or become so entangled in the cavity that it well not now flow at the speed and pressure setting used to pack the part. The author has observed this more than once, and the solution is to minimize this searching for pressure on the pack side. Further if this is observed it is an indication that the machine needs attention to take care of this problem.
Figure 3
Fig. 3 shows a vertical drop from transfer point to that of the packing pressure.
In figure 3 above the drop of pressure is vertical and fast as there is no slope and the pressure for pack is achieved immediately. This is a great example of how the curve should look for this area as no time (minimal time) is lost in the transfer by a slope and or seeking pressure for pack.
***please note that while the discussion was mainly in hydraulic press the slope can be placed into electric presses too. It is the comprehension of what happens during these times which is critical.
27052013.02
SL SILVEY
www.silveysplasticconsulting.com
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