Plastics Technology

SEP 2018

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small runner sizes and a contoured channel layout. It is well established that color-change times can be enhanced with higher shear rates (smaller channel and flight depths), as this reduces the residence time of the plastic nearest the barrel and melt-channel walls. The contoured melt-channel path and the lack of a barrier section in the screw are also important for short color- change times, as the smooth paths reduce locations where the old color can hang up. Overall, any geometry that promotes "washing out" the previous color will reduce the time it takes to change colors. Energy Consumption: A properly designed melt system is also the most efficient from an energy standpoint. The experimental results (Fig. 6) show that a molder could save between $4800 and $7200 per year on a 48-cavity mold of this configuration by using screw design B and hot-runner design B. This repre- sents a 5% to 8% improvement in the required energy. OVERALL SUMMARY OF PERFORMANCE Table 2 shows the relative performance of each screw and hot-runner combination. Note that no single combination of screw and hot runner was optimal for each variable tested. For instance, the best performing combination for pressure drop was not the best solution for optimal color change or energy consumption. This is why a molder may settle for a "general" solution that has a balance of good results versus a best result for their particular case. However, the molder should carefully weigh the costs of optimization versus the net benefit it creates. Even a couple of percentage-points improvement in OEE can easily justify the cost of a new hot runner or screw and barrel. THE KEYS TO EFFECTIVE MELT MANAGEMENT 1. Know the pellets. The first and foremost consider- ation in optimizing a melt-distribution system is to know the resin you are processing. Each type and spe- cific grade of resin has different characteristics and the system design must take these into consideration for optimal performance. Designing the hardware to match the needs of the plastic is the first and foremost require- ment in optimization. 2. System thinking versus equipment thinking. The process of melting the plastic and conveying it to the gate of the mold is a complex interaction of the resin, dryers, loaders, color or additive mixing, injection molding machine, and hot runner. All these parts of the process interact and depend on one another for optimi- A properly designed melt system is also the most efficient from an energy standpoint. Experimental results showed that a molder could save between $4800 to $7200 per year on a 48-cavity mold of this configuration by using screw design B and hot-runner design B. This represents a 5% to 8% improvement in the required energy. FIG 6 Energy-Consumption Impact of Screw & Hot Runner Energy Consumption ($/Yr, $0.066/kWh, 7000 Hr/Yr 16 Cavities) Energy Consumption, $/Yr 20,500 20,000 19,500 19,000 18,500 18,000 17,500 17,000 Screw and Hot-Runner Combination Screw A + HR D Screw A +HR A Screw B + HR A Screw B + HR B Screw A + HR C Screw A + HR B Hot Runner Type A B C D Screw A, G-P Screw B, Barrier $4800 to $7200 per Yr Savings/48 Cavities (5-8%) TABLE 2 Overview of Performance (Non-Weighted) Values Indicate Relative Impact – Higher Values Indicate a More Positive Impact Hot Runner Type A B C D Screw A, G-P Energy 1 3 3 3 Color Change 1 3 2 5 Thermal Stability 2 3 3 3 Fill Balance 3 1.5 2.5 3 Pressure Drop 5 3 3 2 Process Window 5 2 3 4 Total 17 15.5 16.5 20 Screw B, Barrier Energy 1 5 Color Change 3 3 Thermal Stability 1 1 Fill Balance 2 1 Pressure Drop 5 4 Process Window 4 1 Total 16 15 (Amorphous PC: SABIC Lexan LS2) 58 SEPTEMBER 2018 Plastics Technology PTonline.com INJEC TION MOLDING

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