Because plastic changes viscosity with injection rate, a processor must be in velocity control, not time or pressure limited. That means that you check the pressure at transfer (may or may not be peak pressure) during molding and you compare it to the pressure limit available on 1st stage (at all velocities). The pressure limit available must be higher than the peak pressure or pressure at transfer. Usually I like to see at least 200 to 400 psi hydraulic pressure over that seen at transfer. Not full system pressure unless needed. Transfer from velocity control to 2nd stage should be done by position or cavity pressure, not hydraulic pressure.
Ok, with pressure set to provide velocity control the questions become how many velocity profiles to use and when? (Assuming a two stage process, filling on first or boost and packing on hold) My opinion is: keep it simple. If you can get by with one, use only one. Why?
- Fill time is easier to reproduce.
- It is easier to set up the process.
- 95% of the time when I do have to profile velocities it is because of a TOOL PROBLEM, fix the tool!
- Some machines have 10 velocities…this is creeping featurism.
- 80 to 90% of the time, you can get by with one velocity.
- Some machines do this by percents or 10 equal segments. This is a nightmare to set up; profiles must be done by mm or inches better yet volume especially if you want to reproduce this setup.
- 95% of the machines that I check are out of velocity calibration so you need to duplicate fill times NOT machine setting.
- Due to momentum of the screw etc., inertia will override valve moment, the laws of physics and inertia will beat you every time. Imagine that you can move a throttle on a speedboat 10 times within 30 meters (~100 feet); will the boat respond?
Next if you do have to profile how many and when. What I like to do on ALL my process optimizations is make a series of 8-12 of short shots (90 to 99.9% full) starting at the machines highest injection rate and ending with the slowest injection rate, not adjusting position. (For those that know you can get viscosity data at the same time). So slow, it may take 10 or more seconds to fill the part (assuming reasonable nominal wall, for thin wall molding the longest fill time will not be near 10 seconds). Lay these parts in sequence and inspect for cosmetics, warp, blush, jetting, burns, flash etc. This sequence should show the influence of different velocities on the part. It will also show the influence of momentum or inertia on filling the part. As you go slower, the parts will be shorter due to inertia. Jetting, burns and flash are tool problems, again fix the tool, do not slow down the velocity. This is a pay me now or pay me later deal. I pick the velocity that does best for the part, with a keen eye on the viscosity curve. If there is a blush or something that I cannot find the cause for and fix then I will use that velocity that does not give me the problem. If early in filling the mold, I will speed up after the flow front is through that area. I also look at the amount of control at transfer. Too fast in particular molds or lots of inertia filling, then I will slow down but not get too close to the knee of the curve. You also have to worry about can other machines in your shop reproduce this fill time. The mold will not only be scheduled into the new fast machine.
That said, ~90% of the time one velocity will do, rarely more than three. Briefly stated: go as fast as far as possible UNDER CONTROL. Repeat under control 88 times.
Are there exceptions to the above, yes but rarely.
For thin walled parts, speed is critical. With speed you need abundant plastic pressure, 30,000 and higher plastic pressures may be needed. Speed will also give you the least residual stress, but lots of surface stress. Slow fills may give you a uni-directional strength that may be required for particular applications. For thick walled parts (a waste of plastic and they should be redesigned, thicker is not stronger) and lens applications fill can be agonizingly slow. You are going so slowly that you are densifying the flow front and in a sense packing the part as you go. This is what gas counter pressure (50 to 400 psi) can do for you
All the above assumes you have uniformly melted plastic, there are screws that can do this today, but they are NOT standard general-purpose screws. A stepped angle non-return valve (switching on position requires the non-return valve to work properly) and a machine that transfers crisply from 1st to 2nd stage (~0.1 sec or less) with a stable second stage are required.
About the Author
| John Bozzelli Injection Molding (IM) Solutions 1019 Balfour St. Midland, MI 48640-3227 Phone: 989-832-2424 Fax 989-832-8743 Email: john@scientificmolding.com www.ScientificMolding.com |
John Bozzelli is a graduate of Marietta College (BS) and Ohio University (MS). His studies were interrupted for a stint in Vietnam (US Army, Purple Heart; Silver Star). Twenty years in Dow Plastics provided extensive experience in polymer synthesis, development, production, and processing. John has been a seminar leader with RJG Associates, Injection Molding Magazine, University of Wisconsin Milwaukee, General Polymers and John Klees. Competent in resin characterization and analysis, his specialty is practical, hands-on injection molding training with both small and large machines. National recognition has come through ten patents, over 60 papers covering plastics, processing, machine specifications, and over 12 years on the national seminar circuit. Feature articles such as the “Productivity”; by Plastics World and ”Scientific Molding” by Injection Molding Magazine October, November and December 1997, have highlighted a couple of exemplar case histories. Check out the August 2001 issue for applications of The Universal Set Up Sheet.John is the initiator of Scientific Injection Molding and teaches the plastic’s point of view for design and processing with a passion you will remember. Take some of your valuable time to learn practical molding techniques that improve your profits tomorrow while eliminating the state of ”fire fighting“ currently found in many molding facilities. Let us keep plastic manufacturing strong in North America. |
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