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Definition: Linearity; a straight line, of equal parts equal to sum of segments, a screw thread.
The basis of injection speed is that it can be calculated fairly simple. This is done by dividing the distance traveled by the time it took which than equals a rate of mm/second or inches per second. Further it may be calculated to cubic mm or cubic inches per second. Though a simple calculation many do not do so are unaware their machine is not performing as they wish.
The question is what distance is traveled? This is taking the start point, screw position at start of injection and subtracting the point of switch over and coming to the distance traveled. The mistake here is that we must be careful not to take shot size, on the press but shot size plus the suck back being used. If one can read off a screen the actual start point, or off the screw position on the barrel great. The switch over is where we transfer from first stage (fill) to second (pack/hold). Further we need to use one speed on the injection speed. Note if using multiple speeds you could by means of measuring establish what the speed was at each segment.
So the distance has been calculated, and than by using the time of fill we can divide the distance by time with the results being distance /time. Now if we did this calculation from the fastest the machine can inject to the slowest the machine well inject, the results could be placed into a graph and compared to what was set on the machine. Further the results could be calculated as to what % of actual the machine achieved.
If you have a machine which is set by % than the math is such that you must know what maximum speed the machine can move is and than by multiplying that percent in decimal form against the maximum you may calculate actual mm or inches per second.
Some Machines may be in a volume which is great but now requires math to convert to linear measurement or just work in volume. Note if you stay in volume it is a universal setting and can be used on any machine as it is volume metric amount per second, much like the fill time is universal per tool.
Why do we care? By knowing the capability of our machine and knowing the speed rate of fill we have established knowledge for the filling of the mold, and knowledge of a machine that may or may not be capable, and or in need of calibration or repair.
For example let us take a machine which at 50% speed fills a part 95% in 2 seconds. At every speed setting up to 99% it stays at 95% full and the time is 2 seconds we have established that for that machine 50% speed is really the maximum speed of the press. Thus if on this press someone had multiple speed setting above the 50% setting there is no change in speed. The issue than is the press out of calibration or were we sold a bill of goods as to the capabilities of the press? The math has to be done so as to calculate where the speed is in comparison to the press manufactures specifications.
Another example would be that changing the speed by 1 or 2% is a major difference. The question than is it? If the measured fill time at the one setting is changed to the next setting did the fill time change? If so how much? This could lead to the establishment that the press is non linear in that changing from 45% to 40% is not a 5% change in speed but 25% or more. This than is a red flag to calibration of the machine and or establishing a chart to state what actual speeds are on the press. Again fill time is the result and should be monitored.
The why of this is we need to know our presses. Too many times everyone goes off a machine set point / start up condition and not the fill time or plastic condition set point. The other issue here is what is the minimum shot size on that press where you well drop off on speed? If we use 90% of barrel capacity do we achieve our speed setting, and if so do we also achieve it at 10% of barrel capacity? What does the machine manufacture say, what is it that you require for that mold/factory? When moving from machine A to machine B does the consistency and or issue with part production become an issue, even though they are supposedly identical machines?
SL Silvey
silveysplastics@hotmail.com | 
