Part of optimizing any molding process is determining the second-stage injection, or pack and hold, parameters. Most molders deal with two parameters—time and pressure. Let’s add a third: Should you “hold” until the gate freezes or not? There are other subtle issues, but let’s focus on figuring out the second-stage time.
Is this a critical plastic variable? No doubt about it. My experience dates back a couple of decades, beginning at Dow Plastics, where I was a technical representative charged with solving customer problems. Time and again, molders would tell me: Some parts were fine while others failed prematurely. They’d insist the process was the same, yet it produced both good and bad parts.
Often it had to do with how long it took the gate to seal. It was years before I caught on to the importance of gate seal or unseal relative to part performance. Bottom line: before determining second-stage time you must know which way the molecules in the part are happier—with the gate frozen or unfrozen/unsealed. It can make a vast difference in part performance.
Before we discuss experiments and testing, and because most of us were trained to run with the gate sealed, let me tell you a quick story. Last fall I needed to buy a new rake, so I went down to the home center and bought one. The first time I used it—on a relatively warm day—things went well. The next week the weather turned cold, near freezing (I live in Michigan), and after five minutes of raking my brand-new plastic rake had cracked.
And guess where the crack started? Yes, at the gate. Why? Consider the plastic’s point of view: First, how much pressure does it take to fill the solid area of the rake? Not that much, since it’s polypropylene and not a thin wall. Next, how much pressure does it take to fill the tines? A fair amount is needed here because the tines are harder fill. Rakes with shorts at the end of the tines will not sell, so second-stage pressure has to be set to make sure there are no shorts. Hold pressure might also be relatively high to ensure there is enough pressure to fill the tines completely.
Factor in the possibility that the molder also opted for a long second-stage hold time to ensure against shorts. As a result, molecules near the gate froze while packed tight, and this was a gate-sealed process. Pressure at the end of the tines was lower due to the pressure drop along the flow path, so the plastic molecules were less tightly packed. At warmer temperatures, the molecules are not happy, but they do their job. But at lower temps, a bit of shrinkage occurs and the molecules in the denser gate area are fighting one another with no place to go. That produces higher stresses in the gate area. When I added to the stress by raking, the molecules gave up and the part cracked; the combination of the internal stress plus external stress exceeded the strength of the plastic.
Sure, the cause of failure could have been due to other factors, like the wrong resin grade, too much regrind, etc., but my bet would be that the processor ran with the gate sealed. I’m also betting that this entire lot of rakes will break if used in cold weather.
There are a number of issues that influence the resin properties near the gate, and the only way to find out what is best for a particular part’s performance is to test the parts with and without gate seal. To do this, find what the part, resin, mold, and process demand. Run a gate-seal experiment and ascertain the approximate gate-seal time. It does not have to be exact, and it’s easy.
First, find a pressure the makes the part look okay. Then change the second-stage time and add or subtract the difference to/from the cooling or mold-closed time to keep the total cycle time the same. Then weigh parts to four significant figures. Do this in several steps. Make sure you get to a second-stage time long enough that the part weight stops increasing. Then plot the data. For cold runners it will look like the accompanying graph. For hot runners, the slope will change. For valve gates anything can happen.
Once you know how long it takes the gate to freeze, you have some testing to do. Make an appropriate number of parts with a hold time known to provide gate seal and a second set with a second-stage time known to provide parts with unsealed or unfrozen gates. Take these to Quality Control and put them through the performance tests, spanning the full range of use temperatures (do not worry about dimensions). You will be surprised at how many times you see all the parts fail on one side and pass on the other side of gate seal. And even if there is no difference in performance between parts with the gate sealed or unsealed, you will learn that you can save money on resin, as the parts made with the gate unsealed will be lighter than those with the gate sealed.
You can now objectively determine second-stage time. If a sealed gate gives best results, and you found the gate-seal time to be 5.5 sec, then set it a bit higher, say at 6.5 sec or more, to ensure gate-sealed parts. That is, providing you can get the screw back within the cooling time so as not to extend the cycle. Even if you have to extend cycle time and increase your costs, it is certainly less expensive than the cost of failed parts in the field.
If gate unsealed works best, start at a second-stage time about half of the gate-seal time. In this case, set 2.75 sec on the hold timer, and run parts at a range of pressures to see if you can minimize sinks and provide dimensions to specifications. Once the data is in, increase (but not close to the point of gate seal) or decrease the hold time and/or pressure to find the center of the process window that makes parts to specification.
Consider the influence of gate seal or unseal on other aspects of processing and mold design. How critical is consistent cycle time if the process is run with the gate sealed or unsealed? Does gate sealed or unsealed influence the decision to use a cold or hot runner, or a specific type of hot runner? What kind of part performance consistency will you have if you run at the exact gate-seal time? Do the testing and get the data before the part goes into production.
About the Author
John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. E-mail firstname.lastname@example.org or visit scientificmolding.com.