After you establish the fill time for first stage, you then have to figure out 2nd stage or hold. You have to determine three things for 2nd stage:
- Whether to run with or without gate seal.
- How much time to put on the hold or 2nd stage timer
- How much pressure to use.
To establish these three issues you first do a gate seal study to find out whether to run with or with out gate seal. The complete procedure to do a gate seal study correctly is in the last section of my seminar manual. Basically, set hold time to a much longer time than you think is necessary for gate seal. Then start with a low pressure and bring it up slowly until you have a pressure that makes the part Look like it might be OK. This is only a pressure to do the gate seal study, it is NOT the pressure you will run production at. Now weight this part. Take 1-2 seconds off of 2nd stage time and add it to the cooling or mold close timer to keep the cycle constant for this experiment. Weight this part. Repeat this procedure until there is only 0.5 seconds of hold time. Weigh all the parts and plot part weight vs. hold time (2nd stage time). If it is not flat for cold runner molds at the look hold times you did not use enough hold time. Hot runner tools will not flatten out.
Now take parts that you know have gate seal and parts you know were made without gate seal and test them for function. NOT Size. Run the part with whatever gives you the best strength, warp, or function. If running with gate seal always choose a time a bit longer than the actual gate seal time. This will provide a more robust cycle. If you have to run with no gate seal it will be CRITICAL to have constant cycle time. Cycle time is still important for running with gate seal but not nearly as critical for a process running without gate seal. Bottom line this establishes hold time.
Once hold time is established you now revisit the pressure question. Start all over at very low pressure maybe 200 psi on a normal intensification ratio machine. Increase pressure until you think you have parts the may be OK. Make as many as QC needs for testing. Now start raising the hold pressure until you get flash or parts begin to stick. Drop the hold pressure just a bit and run another set of parts for quality control at this high hold pressure. Now run a third set in between the low and high pressure limits you found. Send all to QC and find out where you need to run the pressure. There is no rule as to how much hold pressure you use. It has nothing to do with first stage.
About the Author
Injection Molding (IM) Solutions
1019 Balfour St.
Midland, MI 48640-3227
|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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