Plastics Technology

JAN 2019

Plastics Technology - Dedicated to improving Plastics Processing.

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Last month, in the first part of this two-part installment, we focused on the reasons viscosity can change during injection molding. Because viscosity is a variable, keeping fill time constant minimizes the influence of these variations and provides a more stable process and more consistent parts. If driven to the same shear rate, viscosity variations can be minimized to provide a more con- sistent process. Run to run, shot to shot, summer to winter, and machine to machine—keep fill time the same and your process will be more consistent. Once you find a fill time that makes good parts, use that fill time for the life of the mold. So how do you maintain that fill time? There are differences of opinion in the industry. Some proces- sors feel it is their job to adjust for viscosity changes. However, is it really possible, plausible—or the right strategy—to expect an operator to stand at the machine adjusting for viscosity variations? I prefer the strategy where the machine automatically adjusts for changes in viscosity, something like a car on cruise control. If you set up a machine correctly, it will provide consistent fill time. For most processes and machines, I target less than ±0.04 sec of variation. This does not apply if you have a fill time of 0.06 sec or a long fill time, so use common sense in establishing this range. Dare I say it: Do a DOE! Any machine—open- or closed-loop, electric or hydraulic— will keep fill time constant, provided that it is set up with an appropriate Delta P. Delta P is the difference between the set first- stage pressure available and the peak pressure during injection or first stage. This, in combination with the required "load-compen- sation" circuit, will attenuate viscosity changes. To find the appropriate Delta P for each machine (yes, they do vary), you need to find how much higher the set or available pressure for first stage should be than the actual peak pressure The Importance of Consistent Fill Time during injection. The principle is the same for electric or hydraulic machines. The set or available pressure must be set higher than the peak (not transfer) pressure during injection. Peak pressure can be the same as the pressure at transfer but sometimes it is not, espe- cially if you profile injection. For this procedure you must be able to read peak pressure during fill or first stage. The question is how much higher the allowable or set pressure should be than the peak pressure to allow the machine to control fill time. THE DELTA P PROCEDURE A caveat before we get into the nitty-gritty: Do not try this without proper at-the-press training. There are safety concerns for per- sonnel and the possibility of damaging the mold or machine if one does not understand the procedure correctly. This procedure involves using high temperatures and pressures. If at any time, you are unsure of what will happen, stop and seek help. • Bring the machine to steady-state operating conditions while molding parts and following all appropriate safety practices associ- ated with the operation of the mold and machine selected. Ensure that you can read the peak hydraulic pressure during first stage. This may not be the pressure at transfer, especially if you are profiling injection speeds. Location of the pressure-measuring device, gauge or transducer, should be after the flow-control valve or directly on Run-to-run, shot-to- shot, summer-to- winter, machine-to- machine, keep fill time the same and your process will be more consistent. To make identical parts, you need to keep fill time constant. In part one we covered the why. Here's the how. Shot No. Set Pressure psi or bar Fill Time sec Peak Pressure psi or bar Delta P psi 1 1200 1.50 1156 44 Bad 2 1300 1.39 1229 71 Bad 3 1400 1.38 1244 156 Bad 4 1500 1.24 1355 145 Bad 5 1600 1.25 1376 224 Bad 6 1700 1.22 1453 247 Bad 7 1800 1.22 1467 333 Bingo 8 1930 1.22 1460 470 Too Much Delta P Data for an Hydraulic Injection Molding Machine 26 JANUARY 2019 Plastics Technology PART 2 K now How By John Bozzelli INJECTION MOLDING

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