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 determining the weight in the repeatability test. That’s a mistake! Like any other measurement skill, weighing must be learned. The instrumentation proficiency must be ascertained, and air drafts must be avoided. It is advisable to weigh not in the production environment 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. The last point is not widely known. It is therefore advisable to discharge the parts with ionized air.
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: the repeatability is dependent on the settings. The mold temperature is reduced somewhat, the injection velocity increased and 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 robustness 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 indicators for the quality of a setting are mentioned here as examples.
- First: can the injection drive follow the selected injection velocity profile? That would not be the case if the injection pressure limit chosen for protection of the mold is reached.
- Second: is the injection pressure curve the same from shot to shot in its principle form? This would not be the case, for example, if cold slugs in the nozzle caused fluctuations at the beginning of the injection process.
Modern machine controls like the CC300 from ENGEL AUSTRIA help engineers to find an-swers to these questions, and thereby fulfill important prerequisites for the robustness of the injection molding process (see Practical Tip: Actual value in the target-value graph).
Repeatability Tests with Process Data
Measurement is difficult and expensive. Why not, to a first approximation, use process data – i.e. actual values from the machine – instead of measurements on the part to judge the repeat-ability? Do not a uniform melt cushion and a stable metering time already say a lot about the quality of the machine?
However convenient this procedure may seem, it is not possible to relate them to the part quality. It becomes particularly critical when a machine capability index is calculated from process data. As described above, this requires tolerance limits. For process data, however, they can-not be found in the part drawing. They must be freely invented, which takes us directly to the next point.
Relaxed Handling of Tolerance Limits
“The simplest way of increasing the process capability of a given process consists in relaxing the specification limits.” The greater the difference between the upper specification limit and lower specification limit, the more the standard deviations can be accommodated between them. This declaration by Wikipedia should not be used as a guide to self-deception . If the tolerance limits are freely invented, and the desired machine capability is not reached, it is tempting simply to reinvent the limits. Judging the advisability of this method is left to the reader.
Assistance Systems for Process Control
Anyone who is aware of the above-described pitfalls can sidestep them where possible and thereby obtain a value for the repeatability that is actually convincing – at least for the moment. Since, as mentioned above, it is not foreseeable how disturbances will change in future and how they will consequently influence the quality of the parts and the repeatability of the process.
A solution to this problem is offered by new intelligent assistance systems for process quality, such as “iQ weight control” from ENGEL. The module forms an automatic control loop that is superimposed on the familiar injection and holding pressure controllers. When the disturbances change, it adapts the injection, changeover and holding pressure parameters in real time, i.e. during the same cycle, so that the shot weight remains largely constant . The soft-ware package is part of the “inject 4.0” program, with which ENGEL is paving the way to the smart factory for its customers.
For processes with poor repeatability in short-term tests, the software can provide an immediate improvement. If, on the other hand, the repeatability in short-term tests is very good, the software will not appear to offer any further improvement at first sight. Nevertheless, iQ weight control continually monitors the injection volume and intervenes reliably when something changes. The system is thus an insurance that the process will continue to run reliably in future. The good thing about this type of insurance is that it does not wait for a damage signal to be activated. It is already active before the rejects are produced.