Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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in./sec. We use two tests to help determine an optimal flow rate: the Fill Flow Study and the Dynamic Cavity-Imbalance Study. During the Fill Flow Study, you look for defects that appear on the parts when using a range of different speeds. With the Dynamic Cavity-Imbalance Study, you want to see the effect of different speeds on the balance of the cavities. With the results of these two tests, you can choose a flow rate that will yield quality parts. Pressure: Plastic pressure involves the pressurization stage of the process. This includes both second stage in a DII process (pack and hold) and second stage (pack) and third stage (hold) in a DIII process. Pack focuses on two things: 1) It finishes filling the part and applies pressure to imprint the cavity onto the plastic; and 2) plastic is non-hydrostatic, meaning that the melt does not transmit pressure equally. This is evident when you com- pare pressures at the screw tip vs. pressure just past the gate cavity (post gate) vs. pressure at the end of cavity. In most cases, you will see that pressure at the screw tip is greater than at post gate. Likewise, you will generally see the post-gate pressure is higher than at the end of cavity during the pack stage. The other half of plastic pressure is the hold stage. Up to this point in the process, you have filled and packed the part. The last thing you want is to let plastic backflow out of the cavity. To prevent this, hold time and hold pressure are applied. To determine how much time is needed for the gate to freeze—at which point we know the plastic won't backflow—perform a Gate-Seal Study. This involves increasing the hold-time setting on the machine until you see that the part weights stop increasing, or plateau. Cooling: The final—and often most overlooked—variable is plastic cooling. Cooling is critical and typically accounts for around 80% of cycle time. How is this possible? If you stop and think about when plastic really starts cooling, you'll realize that it occurs as soon as the material hits steel. This variable focuses on the means of how plastic cools and includes things like cooling lines and their contents and achieving turbulent flow. A classic yet common issue processors encounter is forgetting to turn the water (coolant) on. Often, this can lead to extended hold times and ultimately longer cycle times. You can do a couple of things to ensure you have water flow. One option would be to monitor Just because barrel temperatures are set at 500 F does not mean the actual plastic tempera- ture will be 500 F. QUESTIONS ABOUT INJECTION MOLDING? Visit the Injection Molding Zone. Traditional Molding Decoupled Molding II Decoupled Molding III Make Perfect Parts and Save Money! • Preload cores for perfect parts. • Save $$$$ with a smaller cylinder. • Simplify mold operation and design. • Smaller mold base = low cost parts. • Fully Preloads at only 1,500 psi. • Stays locked without hydraulics. • 400˚ F systems available. • Rush 0-2 day shipping available. pfa-inc.com N118 W18251 Bunsen Drive Germantown, WI 53022 (262) 250-4410 • Fax (262) 250-4409 KOR-LOK ® , Hydra-Latch ™ and Hydra-Jaws ™ are trademarks of PFA, Inc. ©2018 PFA, Inc. All Rights Reserved. Mold OPEN Core Out Mold CLOSED Core OUT Mold CLOSED Core IN Core PRE-LOADED Against FULL Injection Pressure CAN'T DO IT! CAN'T DO IT! KOR-LOK ® TRADITIONAL SIDE-ACTION VS. SIDE-ACTION SYSTEMS SYSTEMS YES! YES! YES! YES! YES! YES! SIDE-ACTION SYSTEMS Designed & Made in USA HYDRA-JAWS ™ Every Mold, Every Machine, Every Time! Adjusts to Fit Every Mold Quick and Secure Consistent Clamping Force NEW Service Center PFA-MEXICO/CNI-MEXICO Queretaro +52 442-198-0194 @plastechmag 49 Plastics Technology Sys tematic Molding

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