Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Late last year, a coworker and I were assisting a third party to process a mold that they hadn't run beforeā€”a disc mold (essen- tially two round discs). We did some initial calcu- lations and found a press where it would fit. As we were building our process, we noticed that there were burn marks at the end of fill. In many molding plants, when a processing issue like this arises, operators are typically seen frantically pushing lots of buttons to solve the problem based on their various opinions. However, burns can be traced to one of five areas: part design, material, mold, machine, or process. Regardless of the main cause of the burns, the process is also influenced by the other four areas. Therefore, if you want the process to be successful, all five pieces must fit together nicely. For example, if the part design is not engineered for the material, or the machine cannot produce enough clamp force to support the mold, then the chances of building a robust and repeatable process are unlikely. By making sure that all these pieces fit, you are following a proven methodology or system. Whether you are relatively new or have been molding for many years, you have probably heard the terms "scientific molding" and "systematic molding." These terms are often used interchange- ably, but there is a subtle difference. Scientific molding is centered around learning about key molding principles and theories. The strategic application of those principles and theories is what is known as systematic molding. Ask Apple's Siri, "What does 'systematic' mean?" and she'll respond, "The definition of 'systematic' is done or acting according to a fixed plan or system; methodical." In injection molding, if processors just "button-push" their way through processing, they'll eventually make a good part. However, molding from a systematic approach cuts the time it takes to build a robust and repeatable process. Using systematic molding techniques often results in decreased machine run time, cycle time, scrap rates, and so much more. The cumulative benefit of these cost reductions leads to increased profits. Taking a systematic approach involves separating the three stages of the molding process: fill (first stage), pack (second stage) and hold (third stage). Separating the stages is called Decoupled Molding and is the focus of systematic molding. There are two main types of decoupled processes: Decoupled Molding II (DII) and Decoupled Molding (DIII). In a DII process, fill is separated from pack and hold. To accomplish a DII process, during first stage the part is filled to a minimum of 95% visually full and to a maximum of 98% full by part weight before transferring to second stage. This is accomplished by adjusting the transfer position until the desired fill-only part is reached. For a DIII process, pack and hold are also separated. This is generally achieved by transferring based on cavity pressure. Building a DIII process requires extra training, equipment and software. Imagine decoupling to be like pulling into your garage after work. You've suspended a tennis ball from the ceiling to help you know exactly where to park. The tennis ball represents a full shot. Traditional molding would have you drive 45 mph from start to finish. With this method, how many times are you likely to stop exactly at that tennis ball without going out the back wall? Probably not very many. Using a DII process, you'll drive 45 mph to your driveway and then slow down to 15 mph until you reach the tennis ball. Your chances of stopping at that tennis ball just became a lot more likely, but you can do even better. Using a DIII process, you'll drive 45 mph to the driveway, 15 mph to the door of the garage, and then slow down to 5 mph until you reach the tennis ball. So, what is the benefit of processing with Decoupled Molding? When you decouple your processes, you minimize the impact of viscosity fluctuations. Change in material viscosity is a molder's greatest processing variable and can cause a process to go from producing short shots to parts with flash in the of blink of an eye. Like stopping the car right at the tennis ball in the garage consistently, the goal in Decoupled Molding is to stop the flow of plastic at the end of cavity consistently. Before you start generating a process, develop machine-perfor- mance baselines for each press that will potentially be running the mold. Some studies used to determine this baseline include the following: Injection-Speed Linearity, Check-Ring Repeatability, Load Sensitivity, and Pressure Response. These studies will tell you the current state of the machine to determine whether a particular press should be selected to run the mold. Each study should have an acceptable deviation limit to use as a type of grading scale. Keep in mind that if a machine's performance falls outside the desired deviation limit, it does not necessarily mean that the machine Molding from a systematic approach cuts the time it takes to build a robust and repeat- able process Like stopping a car at exactly the right spot in the garage time after time, the goal in Decoupled Molding is to stop the flow of plastic consistently at the end of cavity. By Logan Teut RJG Inc. @plastechmag 47 Plastics Technology Sys tematic Molding

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