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INJEC TION MOLDING • Premature Measurements The first error occurs when at the end of a busy day there is not enough time to do measurements properly. If technicians decide to quickly take parts from 50 shots and place them on the scales, they often find out too late—namely, when the shot weights are available, or in an extreme case, never—that the process had not settled down (Fig. 3). Before the repeatability test, it should therefore always be determined whether the process is in a stable state (see Practical Tip 1 below). • Erroneous Measurements The next pitfall comes during weighing. You could think that the ability to use a set of kitchen or bathroom scales would be adequate for deter- mining the weight in the repeat- ability test. That's a mistake! Like any other measurement skill, weighing must be learned. The operator's instrumentation proficiency must be ascertained, and air drafts must be avoided. It is advisable to weigh not in the production envi- ronment, but in a lab, and the parts must not overhang the weighing pan. It should also be observed that different storage times after removal can falsify the result due to different moisture absorption; the same applies to electrostatic charging of the parts, attracting dust and stray particles. The last point is not widely known. It is therefore advisable to discharge the parts with ionized air. • Suboptimum Settings Anyone who has followed the tips so far will probably obtain a correct result—more precisely, a result that is correct under the given conditions. One of the key conditions is the operating point—i.e., the total of all the settings that the user has chosen. To make things a little more complicated, process repeatability is dependent on the settings. If the mold temperature is reduced somewhat, or the injection velocity increased, the repeatability can change—both in short-term tests and, probably even more, in the longer term. In practice, it will be difficult to optimize both the robust- ness and repeatability during process optimization. Usually it is enough to perform at least one optimization according to the conventional rules of injection molding. Two key indica- FIG 3 To test whether a process has settled down, it is necessary to monitor several parameters shot by shot. The values can be graphically displayed in the injection machine control system (such as Engel's CC300). 5 15 25 35 45 55 65 5 15 2 5 3 5 45 55 65 5 15 2 5 3 5 4 5 55 65 5 15 25 35 45 55 65 5 15 2 5 3 5 45 55 65 5 15 2 5 3 5 4 5 55 65 ΔT Circuit 2, C Shot Count Shot Count Shot Count Shot Count Shot Count Shot Count Injection Volume, cc Viscosity Change, % Plasticating Time, sec Clamping Force Peak Value, kN ΔT Circuit 1, C 55.5 55.3 55.1 54.9 54.7 54.5 26 24 22 20 18 1130 1120 1110 1100 6 5.8 5.6 5.4 5.2 5 3.5 2.5 1.5 0.5 -0.5 2 1 0 -1 3a 3c 3e 3b 3d 3f PRACTICAL TIP 2: Actual-Value Display in the Target-Value Graph One value per cycle is recorded in the process-data log and in the asso- ciated graphs. However, these records do not give any information about the variation of values within a cycle. But controls like the CC300 let the user display the actual-value curves for injection velocity and injection pressure together with the target-value profile of the injec- tion velocity (Fig. 5). This allows a simple check of whether and how the injection drive follows the set injection velocity. Moreover, the profile of the injection pressure can be observed and analyzed for irregularities (pressure spikes during injection, differences at the beginning of the pressure curve as a result of cold plugs, etc.). The so-called "actual-value display" in the target-value graph, which is read from right to left, is subdivided into three regions on the CC300 display screen (Fig. 5). Region a, on a dark-blue back- ground, marks the transition through the compression relief. Region b, on a light-blue background, models the velocity-controlled phase of the injection process. The changeover point takes place at the transition from b to c. That in turn means that region c is part of the pressure-controlled phase of the injection molding process. The curves illustrated here show that the injection drive in this example follows the preset profile and there are no abnormalities, such as pressure spikes, during the injection. 50 SEPTEMBER 2017 Plastics Technology PTonline.com