Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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ciency). In these cases, the importance of melt quality and balance of fill, for instance, become more and more critical. Not many molders consider the entire process from pellet to gate. Frequently, each of the elements in the flow path is optimized inde- pendently; and as a result, they could be sub-optimized for the system's performance. The difference between an average and a world-class molder is only a few percentage points (see Table 1). Could an optimized melt-distribu- tion system help an average molder become a world-class molder? EXPERIMENT IN OPTIMIZED MELT DISTRIBUTION A set of experiments was conducted to determine if an optimized melt-distribution system could create a more homogeneous melt and if that, in turn, would help to improve the overall perfor- mance of an injection molding workcell. A 200-ton Milacron- Fanuc Roboshot machine with two different screw designs; a 16-cavity 28-mm cap mold; four different hot-runner configura- tions (see Fig. 1); and Lexan PC resin were used in different com- binations in order to determine the optimized configuration and the difference between the worst and best configurations. Fill balance, pressure drop, part dimensions, color change, energy consumption, and process windows were evaluated to determine the optimal perfor- mance. Here's what was found: Injection Pressure vs. Injection Time: Ideally, a part should be injected with as little pressure as possible, as that reduces clamp tonnage, machinery stress, mold wear, energy consumption, molded-in stresses on the part, flash, ejection issues, and other plastic part issues. If the molding workcell has insufficient injection pressure or if the resistance to fill the part is too high to make a good part, a variety of defects such as sinks, warpage, short shots and dimensional variation can occur. Each of these defects could cause significant quality and uptime issues, which would reduce the overall OEE of the workcell. The pellet-to-gate system can have a major impact on the required injection pressure to make a good part (Fig. 2). The hot- runner layout impacted the required fill pressures by up to 6000 psi (41 MPa), or 30% to 40%. Channel sizes were the major factor deter- mining the required injection pressure to produce an acceptable OEE Factor Average Molder Top 10% Molder Availability, % 90 94 Performance, % 96 98 Quality, % 96 98 OEE, % 83% 90% TABLE 1 Average to Great: A Few % Matters An experimental test of the influence of various elements of the melt-distribution system on processing and part uniformity used four different hot-runner designs, including two each with larger and smaller melt channels. FIG 1 Four Hot-Runner Designs Hot-Runner Design A 1-level, brazed, large channels (88 cc) Hot-Runner Design C 2 -level, gun drilled, large channels (88 cc) Hot-Runner Design B 1-level, gun drilled, small channels (54 cc) Hot-Runner Design D 2 -level, brazed, small channels (54 cc) Could an optimized melt distribution system help an average molder become a world- class molder? Hot-runner layout impacted the required fill pressures by up to 6000 psi, or 30-40%. Channel sizes were the major factor. Also, an unbalanced hot-runner system required 10-15% extra pressure to fill out all the parts. Screw design additionally impacted injection pressures by up to 2000 psi or 6-8% of the total fill-pressure requirement. FIG 2 Injection Pressure vs. Injection Time 0.07 sec Screw B + HR A Screw A + HR A Screw B + HR B Screw A + HR B Screw A + HR C Screw A + HR D 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 Injection Time, sec Injection Pressure, psi 37,000 32,000 27,000 22,000 17,000 12,000 Screw 1.5 - 2 ksi Hot Runner 6 - 8 ksi 0.25 sec @plastechmag 55 Plastics Technology Melt Management

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