Plastics Technology

SEP 2018

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shearing in the hot runner, which in turn can create problems in the molded parts. Conversely, the hot runner with an X-style melt- channel path creates a much more even shearing profile and resulting filling pattern. The overall message here is that the screw and hot runner act in conjunction with one another to manage and control melt creation and distribution in the injection molding process. If either the screw or the hot runner is poorly designed, the other element cannot function properly. Failure to optimize the interaction between the screw and hot runner can cause significant problems, resulting in reduced quality, yields and profits. As in many things, the chain is only as strong as its weakest link. The same can be said of the melt-distribution system. An optimized solution from the pellet to the gate has a material and meaningful benefit to the molder. The difference between average and optimal is material to profitability. Variations in dimen- sions, part weight, balance, gate quality, recovery, and color change can easily make a 1% difference in the system output— scrap, uptime, color change, cycle time (Fig. 8). In this experimental example, a molder could improve its OEE by 6% simply by optimizing the melt-distribution system. With a 48-cavity PP cap mold based on this experimental design (Table 3), running 0.4 sec faster than baseline could mean 9 million more parts/yr. Saving 0.05 g in part weight could save $17,000 in material cost alone. And only 1% greater uptime could yield 6.5 million more parts/yr. Clearly, retrofitting your existing melt-distribution system is worth considering, as the payback for the new components is in many cases only a matter of a few months, and is meaningful and lasting. ABOUT THE AUTHOR: Bruce Catoen has held the role of Chief Technology Officer for Milacron since 2014 and has worked in a variety of senior leadership roles at Mold-Masters and Husky since 1988. Bruce is the named inventor on 52 patents. He co-authored the book Selecting Injection Molds (2007) and has published numerous papers in trade journals and presented at many technical symposiums. 5. Hot Runner. The task of the hot-runner system is to convey the melt to each of the mold cavities in precisely the same state in which it left the barrel/nozzle. The hot-runner system is not able to adjust any melt inhomogeneity. If a hot-runner system is not correctly designed or manufactured, it may cause disruption to production and flaws in parts. A hot-runner system requires uniform tempera- ture distribution along the flow path, minimum possible pressure drop, no hang-up spots, and moderate shear rates. An unbalanced layout of the hot-runner melt channels can lead to many problems in the parts. As can be seen from the 3D particle-tracer images in Fig. 7, two seemingly balanced hot-runner channels can produce dif- ferent results. The single level H-style manifold creates preferential Summary of impacts of melt homogeneity on productivity, quality and economics. The first and foremost consideration in optimizing a melt-distri- bution system is to know the resin you are processing. FIG 8 TABLE 3 Average to Great: A few % Matters (Best Values Boldfaced) Poor gate quality due to variance in packing Overall Melt-Homogeneity Impact Part weight or dimensions vary due to variation in packing Flashing in some cavities, short shots in others Higher injection pressure required to make parts Unnecessarily long cycle times Long color changes, burning, splay, other defects 48-Cavity Mold; PP @ 50¢/Lb; 300-Ton Servo-Hydraulic Machine; Conversion Cost: $100/Hr; Run Hrs: 7000/Yr Baseline Faster Cycle Lighter Parts Greater Uptime Energy Efficient All Cycle, sec 6.5 6.1 6.5 6.5 6.5 6.1 Weight, g 1.75 1.75 1.70 1.75 1.75 1.70 Uptime, % 90 90 90 91 90 95 Yield, % 96 96 96 97 96 98 Energy Use, Relative 1.0 1.0 1.0 1.0 0.93 0.93 Annual Impact 9 MM Parts $17,000 6.4 MM Parts $2600 6% Higher OEE Impact of Poor Melt Homogeneity Benefits of Pellet-to- Gate Melt-Delivery Optimization: • Improved Precision • Reduced Reject Rates • Uniform Filling of Cavities • Less Part-Weight Variation and Lower Average Weight • Shorter Cycle Times • Improved Part Quality • Lower Energy Consumption • Lower Part Cost 60 SEPTEMBER 2018 Plastics Technology INJEC TION MOLDING

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