Plastics Technology

AUG 2018

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pressure vs. actual peak pressure during injection. I find this common rule of thumb to be inadequate. So now that we know the procedure for making a first stage part, how full should it be? No easy answer here. Somewhere between 90 to 99.9% full—based on volume, not weight. Factors that influence this are the type of part, number of cavities, balance of filling, a weld line at the end of fill, a living hinge, etc. Bottom line for 99% of parts being molded: The first- stage part(s) should be visibly short, not full with sink. But why by volume and not weight? The emails and blogs I've received and read covered a variety of rationales for volume or weight, dealing with melt density vs solid density, sinks, bounce- back, and other factors. For me ,the answer—volume—comes from studying cavity-pressure curves and witnessing the wear on parting lines along with the resulting development of flash on parts. Figure 2 provides the injection-pressure curve in first and second stage, along with an end-of-cavity pressure curve for a first stage developed by weight. Note the rapid, near-vertical instant rise FIG 2 First Stage by Weight FIG 3 First Stage by Volume In first stage, if you fill parts by weight you'll get a rapid, near- vertical instant rise on cavity pressure. This creates a significant force on the parting line of the mold and will likely prematurely wear it, leading to flash. Note the rounded end-of-fill cavity-pressure curve you get when filling parts by volume and not weight. It suggests less stress is being generated on the parting line, allowing it to withstand significantly more shots before wear and subsequent development of flash. of cavity pressure. This is a significant force on the mold's parting line and will likely prematurely wear it, leading to flash at some point. Figure 3 provides the injection-pressure curve for first and second stage, along with an end-of-cavity pressure curve for a first stage developed by volume. Note the rounded end-of-fill cavity- pressure curve. It suggests less stress is being generated on the parting line allowing it to withstand significantly more shots before wear and subsequent development of flash. Please understand there are a number of factors that influence parting line wear; this is only one of several. Also, the shape of an end-of-fill cavity- pressure curve is part-dependent. In short, develop first stage with time on the second-stage timer and based on volume. It should provide less wear on the parting line. Plastic Pressure, psi Plastic Pressure, psi Time, sec Time, sec 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 20,000 16,000 12,000 8000 4000 0 20,000 16,000 12,000 8000 4000 0 Cavity Pressure at the end of Fill Plastic Pressure in the Nozzle Cavity Pressure Near the Last Area to Fill PTonline.com 24 AUGUST 2018 Plastics Technology I N J E C T I O N M O L D I N G K now How Fill greater than 90% by volume, not weight. 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 special- ties. Contact john@scientificmolding.com; scientificmolding.com. Permanent Platen Thread Repair Watch demo video Self-tapping and Self-aligning Limited Lifetime Warranty Made in the USA Quick to Install They succeed in the heaviest, most demanding, highest torque industrial applications. They won't wear out, back out, or pull out. And they are by far the fastest and easiest to install of all thread-repair inserts.

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