Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 49 of 75

is bad. If you are still able to make good parts within a desired cycle time, then it is perfectly fine to continue processing on that machine. On the flip side, if the machine is producing bad parts, then that mold should not operate in that specific machine. This baseline machine data will tell you which of your presses is your "limiting" press. For example, if you have three machines where the mold will be used, and one of the presses has a much slower injection rate, build your process using the limitations of the least capable press. When you build your process for the "limiting" press, you accomplish three things: 1. It ensures that the proc ess makes quality parts in the "lim- iting" press; 2. You know you can transfer those machine values to the other more capable presses; 3. When you transfer the process values, you continue making quality parts. You can start building your process once you know the capabilities of your machines. Similar to machine baselines, you also create process baselines specific to each mold in order to process "from the plastic's point of view." To do this, you must look at the four plastic variables: temperature, flow, pres- sure and cooling. Temperature: It is critical to document plastic melt temperatures, not barrel temperatures. Just because barrel temperatures are set at 500 F does not mean the actual plastic temperature will be 500 F. Three machine settings will affect plastic temperature: Barrel temperatures (electrical), Screw rpm (mechanical), and backpressure. Generally, a large portion of melt energy comes from mechanical shear generated via screw rpm. In most cases, you can monitor the plastic temperature by taking a melt temperature at the nozzle. If you're happy with the results, maintain the settings. If not, then make adjustments. Flow: The impacts of plastic flow rate are the main focus of the plastic flow variable. Plastic flow is the first stage of a Decoupled Molding process. Plastic flow rate is important because it has the biggest impact on the material's viscosity. The faster the material is pushed, the thinner it becomes and the easier it flows. In contrast, the slower the material is pushed, the thicker it becomes and the harder it is to flow. This is known as Non-Newtonian behavior and explains why you might end up with flash when increasing the flow rate from 5 in./sec to 15 in./sec. When choosing a flow rate or fill speed, select it based on part quality, not machine capability. Just because you have a press that is capable of filling at 20 in./sec does not mean you should fill at 20 If you have three machines where the mold will be used and one of the presses has a much slower injection rate, build your process using the limitations of the least capable press. Macro Engineering and Technology Celebrating 40 years of providing innovative machinery to the plastic film and sheet industry. New MacroPack™ Blown Film Dies For Fast Purging +1 905-507-9000 | • Fastest purge time • Most versatile blown film die • Up to 13 layers for barrier film • Up to 30 layers for shrink film 40 1978-2018 48 OCTOBER 2018 Plastics Technology Note the burn mark on this part, made in one cavity of a two-cavity disc mold. Though there are many causes of part defects, decoupling a process makes it easier and faster to diagnose their root causes and identify solutions. In this case the problem was solved not by frantic button pushing, but by collecting and analyzing data. FE ATURE

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