It does not matter what injection velocity you set on any given machine-the critical parameter for the plastic is keeping the fill time the same. Our range in this case is a tight ☐.04 sec. That is, shot to shot, summer to winter, machine to machine. If the biggest single factor in the viscosity of most plastics is injection speed or fill time, what does that mean for processing? First, you must keep the fill time the same to have a stable process that can make the same part on any machine. Temperature and lot variations cannot come close. So in this case, just by changing injection rate you can change the viscosity 10-fold or an order of magnitude. If you double the viscosity it would be 4000, five times the viscosity would be 10,000, and 10 times stiffer would be 20,000. If you round up a bit, the viscosity at the fastest speed, a fill time of 0.39 sec, is about 2000 psi-sec. In fact, viscosity is influenced more by shear rate-injection speed-than temperature or even resin lot variations.Ĭheck out the numbers in the table accompanying the graph. That is the way it should be-plastics are non-Newtonian, meaning they all change viscosity with changes in injection rate or fill time. In fact, once you do a few of these you’ll recognize that if you have good data that spans a decent range of fill times, the curves all have the same shape. Starting with the basics: The graph shows a typical curve. Viscosity is influenced more by shear rate-injection speed-than temperature or even resin lot variations.īut how do you interpret the curve? The concept is that if you understand how plastic flows and fills a mold, you can use this information to make robust processes that produce consistent parts. That’s time well spent to get the data for determining fill time and the start of a production-capable process. Once you become adept at the procedure and log the data into a spreadsheet, the experiment should take about 30 minutes. This fill time is now set for the life of the project. You can find the equation on .)Īfter interpreting this data in relation to the machine, resin, and part, the processor is guided to a fill time for this particular mold and resin. (For those who want to know the science behind the terms in the chart, the parameters in this experiment are related by a fluid dynamics expression known as the Hagen-Poiseuille equation. From the table below, you can use the injection pressure at transfer and fill time to develop the viscosity curve. Take data as you gradually lower the injection velocity-don’t adjust anything but injection velocity.
Viscosity meaning full#
Lower the hold pressure to near zero then, starting with a high injection velocity, make a short shot, 99% full by volume. Start the experiment by taking hold pressure off. 3 Ways to Reduce Costs in Your Plastics Process Cooling Systems
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