|
Definition: Understanding: plural of understand, comprehension, knowing, Graph: a display of data in composite or visual display showing a range
One of the options many have with today's machines is the choice of what they shall graph. Besides a pressure curve which is important and has been described there is the relationship to the time function, the screw speed, and possibly position. Some or multiple of these can be plotted on the same screen along with the set position, speeds and pressures. (Review previous articles for graphs)
If one thinks about what is happening in the barrel than one need to understand when it is happening. What better way than time and or position. While not all machines are capable most should be. Thus while one looks at the pressure rising on fill and notices that there is a peak other than the one we wish, it can then be calculated out that it occurs at the beginning , within 5 mm of stroke and or 0.25 seconds of injection on a 2 second injection fill time. Is this pressure spike a cold slug in the nozzle, (which may be checked via raising the temperature resulting in a decrease in pressure at this point) or is it the fact that suck back was used and that this is the impact of check valve seating and or coming into contact with the mass of plastic in the barrel, or is it the fact that the nozzle is small and acts as a pressure regulator.
If watching the injection speed and comparing it to the set speed is the machine reaching the speed desired or set? Though one has fill time to judge the results of the setting, the question is if the injection screw is not reaching the set speed, is it possible to reduce this speed setting while maintaining the same fill time? This in the long term now would hopefully guarantee that even if lots change and or viscosity of the resin that the fill time would be maintained. While if left high though not achieving it would leave room for a possible faster injection (shorter fill time) if something changed in resistance to flow. In many cases it is that the shot stroke is too short to reach this speed, in car language trying to get the ¼ mile top speed but only in 1/8th of mile or a shorter distance, there just is not enough distance to achieve it. The distance needed to achieve the speed is not available or resistance to flow is too great. (**note that the concept of load compensation need be understood for above explanation)
In either case of not making speed one than can see what the actual speed is. This is where the machine is really working at. It is also checked or recorded as noted by fill time (which should be monitored) which equals the time it takes for the screw from start of injection to transfer point. So the question then becomes, if one lowers speed does the fill time remain the same?
The position curve which sometimes is tied into pressure or speed on some presses shows the relationship of what happens with the screw. To see if these are on your machine remove the pack and hold pressure and watch the graph to see if when the transfer occurs which line moves back and also what the screw within our press is doing. On a hydraulic press this can be noted but with an electric since a ball screw is mechanical there should not be much bounce back. Bounce back is what one would notice on a hydraulic press, as the material is compress and then pressure is lost the pressure in the mold thus greater than that of ram cause the screw to bounce back. In some cases while observing the position of the screw one can then use this to set up a closing of the valve gates at a point of full parts, due to screw movement.
Using of the graphics package on the machine can yield many answers and also present many questions.
One must understand what the machine is doing with the data capture, and also at what speed is it processing the data and or capturing the data. As machines become more capable, this collection is faster and more accurate. Unfortunately it can also be manipulated to look nice or what it is suppose to look like and not the actual data. Talking and communication with your machine manufacture of choice is a good starting point and graphics should be some of the tools that are purchased with the machine if an option.
Currently and in the past there have been many process monitors on the market which monitored the various functions of the machine. While many machine manufactures supported these units with sensors and or connections points built into the machine they at the same time increased and updated their own collection and data monitoring which can be done via the machine control panel itself. This along with faster computer chips has lead to advancement in the graphic packages on the machines. The next step may be or maybe it is currently available is to allow capture of these data graphs so as to print and or hold much like a process monitor to create a visual of what is happening. Yes the monitor or data collection in the press of today has a collection system that holds the last so many cycles and the data points that were chosen, but does it also hold the graph.
The issue that remains is that the processors need to be up to speed so to speak on the capabilities of their machines in their shops to take advantage of these capacities. They must also understand how to read them so as to understand them well enough to explain to those whom do not.
01072013.01
SL SILVEY
www.silveysplasticconsulting.com
| 
