Plastics Technology

SEP 2018

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part. The balance of fill was also found to have a significant effect on the required injection pressure. An unbalanced hot-runner system required 10% to 15% extra pressure to fill out all the parts. Screw design additionally impacted injection pressures by up to 2000 psi (14 MPa) or 6% to 8% of the total fill-pressure requirement. Together, an optimized screw and hot-runner design could reduce required injection pressures by almost 50%. On an overall basis, the most optimized melt-distribution and control system reduced injection time by 0.25 sec. This equates to more than a 3% improve- ment in OEE and almost 6 million addi- tional parts a year. Fill Balance: One of the main objectives of the melt-delivery system is to deliver molten plastic in an identical rheological condition to each cav- ity in the mold. A well-balanced mold runs a faster cycle, higher uptimes and more uniform parts. The degree of balance is typi- cally determined by molding a statistically significant series of short shots (80% to 90% filled) and measuring the part-weight variation between the molded parts. The smaller the part-weight variation, the better the balance. In this experiment, two screw designs and four hot-runner designs were evaluated to find the best hardware configuration for fill balance. The best combination of screw and hot runner could improve fill balance by 17% over the worst combination (Fig. 3). Improved fill balance was also shown to reduce average part weight 0.05 g and reduce part-size variation by 0.1 mm. The barrier-type screw produced parts with higher part- weight variation and higher average part weights. Typically, the hot runner is the singular focus to resolve fill-balance issues, but it can be seen in this test that the screw has a meaningful impact on fill balance, regardless of the hot- runner configuration. In terms of the hot runner, the design with the largest channels and the smoothest rounded corners resulted in the best fill balance. This is because the flow of molten plastic in a runner channel is laminar. The material nearest the walls of the melt channels experiences more shear than the material that flows in the center of the channel. When the plastic rounds a sharp corner, even more shear and momentum loss occurs. A 3% to 5% improvement in OEE could be achieved with the most optimal combination of equipment configuration. Frequently, each of the elements in the flow path is optimized indepen- dently; as a result, they could be sub-optimized for the system's performance. The best combination of screw and hot runner improved fill balance by 17% over the worst combination. Improved fill balance reduced average part weight 0.05 g and reduced part-size variation by 0.1 mm. The hot runner with the largest channels and the smoothest rounded corners resulted in the best fill balance. Also, the barrier screw produced parts with higher part-weight variation and higher average part weights. Typically, the hot runner is the singular focus to resolve fill-balance issues, but this test shows that the screw itself has a meaningful impact on fill balance. As injection speed increased, part weight decreased and weight range also decreased. A hot-runner design that reduces pressure drop helps take optimum advantage of injection speed. FIG 3 FIG 4 Fill-Balance Impacts of Screw & Hot-Runner Design Avg. Std. Deviation Weight, g Part Weight Range, g Avg. Part Weight, g Weight, g Part Weight Distribution (150 RPM Screw) Weight Range vs. Injection Speed Avg. Weight vs. Injection Speed Part Size Distribution (150 RPM Screw) Shot Number Injection Speed, mm/sec Injection Speed, mm/sec 1.8 1.75 1.7 1.65 1.6 1.55 1.5 0.3 0.25 0.2 0.15 0.1 0.05 0 1.62 1.6 1.58 1.56 1.54 1.52 1.5 1.72 1.70 1.68 1.66 1.64 1.62 0 2 4 6 8 10 12 14 16 60 70 80 90 100 110 120 130 80 90 100 110 120 130 A/A B/A B/B A/B A/C A/D Screw/HR A/A B/A B/B A/B A/C A/D Screw/HR A/A B/A B/B A/B A/C A/D Screw/HR A/A B/A B/B A/B A/C A/D 0.050 0.040 0.030 0.020 0.010 0.000 Diam., mm Shot Number 16.05 26 25.95 25.9 25.85 25.8 25.75 25.7 0 2 4 6 8 10 12 14 16 Screw/HR A/A B/A B/B A/B A/C A/D Avg. Std. Deviation Diam., mm 26.00 25.95 25.90 25.85 25.80 A/A B/A B/B A/B A/C A/D 0.050 0.040 0.030 0.020 0.010 0.000 Fill-Balance Impact of Injection Speed Part Weight 1.7 g. Sample Size 36 Shots per Curve 56 SEPTEMBER 2018 Plastics Technology PTonline.com INJEC TION MOLDING

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