Representation, supervision possibilities of injection molding computer control systems.

The development of new computer controls has created possibilities of making the injection molding process more transparent and monitoring certain parts of the process. This is done through the acquisition, control or regulation, display and monitoring of selected process parameters, which in turn influence certain quality characteristics of parts of the elastomer molds.

This requires a machine equipped with enough measurement sensors to guarantee a reproducible process run.

The injection molding process is subdivided into partial processes and consideration of the total process for display or monitoring.

Partial processes

The partial processes include the plasticizing mold-filling and vulcanization steps.

Plasticizing

In the plasticizing process, by using an optional five-stage dosage profile (piston position), the mass temperature and the screw hydraulic pressure can be displayed and monitored (figure 1).

Figure 1 - metering profile

MCS II                                          20 Mar 91 14:19:18
4/ 4 10.0         23.6 112.1         0.0 0.0         3 209     62
Measurements when metering
Measurement profile               Supervision      (0/1)   1
Measure at step                      1      2      3       4     5
Piston position    (mm)           25.0   50.0   75.0   100.0   End
Set temperature    ([degrees]C)     81     81     81      81    81
Tolerance +/-      ([degrees]C)      3      3      3       3     3
Act. temperature   ([degrees]C)     81     81     81      81    81
Set. val. pressure (bar)            87     87     87      87    87
Tolerance +/-      (bar)             5      5      5       5     5
Act. val. pressure (bar)            89     86     69      48    39
Pressure tolerance metering step 5 exceeded

If there is uneven insertion of the liner strip or load deviations or mistakes (viscosity difference) that are outside the given tolerance range, an error message appears at the respective measuring position.

Tears in the strip on the front-feed pocket, depending on the dosage time, can be recognized after a few seconds (ref. 1) (figure 2).

[CHART OMITTED]

Filling the molds

The mold-filling process is composed of the injection and the dwell-pressure phases.

The injection process can take place at various speeds at 10 supporting points. The injection speed profile created therefrom is thus matched to the various mixture-tool combinations.

The given speed guidelines are shown with the actual runs and can be given a tolerance band and a monitoring range.

In the injection phase, the injection energy, which consists of measuring the hydraulic pressure and the injection distance during the injection phase, is surrounded by tolerance bands and thus monitored.

Changes in mixture viscosity, which can occur for various reasons, are thus recognized immediately (refs. 2-4).

The monitoring of the injection energy immediately finds unusual tolerance excesses (figure 5, Alarm 1), which can be caused, for example, by a runner system that is not completely removed (clogged injection point).

Figure 5 - cure time calculation

MCS II                                          20 Mar 91 13:52:39
0/ 0 00.0         23.6 0.3        0.0 0.0        3 215       60 32
Cure time
                                                       Set    Act.
Controlled cure time                          [sec]    120    0.00
Controlled/calculated cure time                 0/1      0
Parameters for cure time calculation
Low temperatures                       [[degrees]C]    175
Cure time for low temperature                 [sec]    188
High temperature                       [[degrees]C]    195
Cure time for high temperature                [sec]     93
Geometry factor                                       1.09
Temperature cure time calculation      [[degrees]C]    179     179
Calculated/remining cure time                 [sec]            149
Actual cure time                              [sec]              0

Moreover, a so-called "run" (figure 5, Alarm 2), which can occur due to lot change or processing of another mixture that is wrong, is indicated by permanent measuring value displacement.

The dwell-pressure process, which can be given by a 10-stage variation of dwell pressure height or length, can also, like the injection process, be given a monitoring range and a tolerance standard. The dwell-pressure guidelines (desired/actual comparison) can be displayed.

Using the pad tolerance data, the maximum setting of the injection piston in the fill phase can be recorded and monitored. Unusual tolerance excesses, which can occur if the mixture spends too much time in the injection cylinder, are immediately noted, since the dwell pressure, height and length set is not sufficient to push the scorched mixture completely to the normal pad position (figure 3, Alarm 1).

[CHART OMITTED]

A "trend" runout in steps from the tolerance range of the upholstery position, which can occur, for example, when there is a defect in the injection cylinder tempering device and a subsequent rise in the viscosity of the mixture, is recorded as quickly as possible (figure 3, Alarm 2).

The complete fill phase can be shown using desired/actual guidelines, the accompanying pressure sequences and the switch point from the speed control to the dwell pressure control.

Vulcanization

In the vulcanization process, the regulated heating platens and heating circuits and the measured mold temperatures are given a narrow and a wide tolerance band (figure 4).

[TABULAR DATA OMITTED]

Depending on the size of the deviation from the standard value, only an alarm message appears or the press goes to the end of the cycle and can then restart only if the measurement value is in the narrow range of the standard value.

Moreover, it is possible to eliminate mold temperature deviations that occur during the production process using the heat-time calculation (figure 5).

The crosslinking characteristic of the mixture for the mold part to be produced, which is based on a correlation between the mold temperature and the accompanying vulcanization time, serves as a base.

The longer the opening times of the press between the individual cycles and the more the tool is thereby cooled, the longer the necessary heating time will be and the more cycles there will then be with the extended heating times (ref. 1).

Total process

Consideration of the total process is divided into detection of the measuring data and an SPC program.

Measurement data detection

In measurement data detection, any parameter desired (pressure, temperature, speeds, etc.) can be measured over a set time period and will be shown as a curve. Actual values in online mode or stored measurement data from the hard disk can be seen in graphic or tabular form (figure 6).

[TABULAR DATA OMITTED]

SPC

The SPC program part consists of displaying the normal distribution on a graph with calculated capability factors and a control card, whereby the average or standard deviation of optional test characteristics can be recorded.

Thus all process parameters that are important to the product can be documented as part of the process on a control card. The SPC recognizes trends (for example, deviating temperatures) promptly and gives a corresponding "trend message," before a drift from the desired course leads to damage.

Conclusion

Modern injection molding machine controls are thus a helpful means of optimizing the production process, with the goal of reducting damage and simultaneously guaranteeing uniform, high quality for the article produced.

Literature

1. Schmid H.-M./Katzer M.: ProzeBanalyse und -uberwachung beim Spritzgie[beta]en von Gummiformteilen Kautschuk + Gummi Kunststoffe Heft 12/89, Seiten 1156-1161.

2. Janke W., Rechnergefuhrtes SpritzgieBen von Elastomeren Diss. 1985, IKV, Aachen.

3. Kautschuk + Gummi Kunststoffe Heft 8/88, Seite 801.

4. IKV-Kolloquiumsbericht 1984.