5. Mold viscosity test - Validation procedure for injection molds

>> Thursday, 24 June 2010

The fifth step in validating a injection mold with the overall process shown in figure injection mold validation flow chart is mold viscosity test. The steps before:

1. Mold certification
2. Dry cycle mold
3. Process stability test
4. Gage repeatability & reproducibility (R&R) test

The mold viscosity test was developed to identify the maximum and minimum fill rates permissible for the mold with a specific plastics material. The theory behind using high speed injection rates is to take advantage of the decreased viscosity which results when plastics are subjected to high shear rates. The mold viscosity test should be performed on the correct size press, for which the mold was designed. The injection rate should be recorded in volume per unit time so that the learnings and results of the mold viscosity test can be transferred to another press that may have a different size injection unit.

The mold viscosity test could be skipped if mold fill analysis has been performed on the mold. A fill time analysis should have been performed to determine the proper injection time for the mold. These results, when interpreted properly, can be used to identify the proper fill time.

All the steps during the procedure that involve intimate contact with the injection molding machine are to be done by a qualified injection molding machine operator.

Caution: Part geometry dependent. Part may stick in mold and need to be removed manually.

1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set mold temperature to resin manufacturer's recommended mid-range.
3. Measure and record the melt temperature.
4. Set hold pressure and time to zero.
5. If the machine is equipped, set pack time and pressure to zero.
5. Set cooling time long enough so that parts eject consistently without being distorted.
7. Set fill rate as fast as possible without flashing, burning or damaging the mold.
8. Transfer from injection to hold phase by screw position.
9. Adjust feed stroke so that the most filled part is approximately 90% filled by weight.
Note: Use full part weight established during process stability test to calculate 90%.
10. Have sufficient cushion to prevent the screw from bottoming out against the barrel during injection.
11. Add adequate hold time and pressure, as well as pack time and pressure, so no sink marks appear and parts eject.
Note: It is not important to be molding good parts to complete this study.
12. Record the fill time and machine peak hydraulic pressure for 3 shots.
13. Compute an average for all 3 shots.
14. Decrease fill rate.
Note: When you decrease the fill rate the parts will become less filled.
15. Set hold pressure and time to zero.
16. If the machine is equipped, set pack time and pressure to zero.
17. Adjust feed stroke to maintain approximately 90% part weight with lower fill rate. If you do not adjust the feed stroke, you risk having bogus results.
18. Add adequate hold time and pressure, so no sink marks appear and parts eject.
19. Record the fill time and machine peak hydraulic pressure for 3 shots.
20. Compute an average for all 3 shots.
21. Repeat steps 14-20 while incrementally decreasing the fill rate until the machine’s pressure limit is reached.
22. Use the following formulas to evaluate:
Shear rate = Fill time-1
Relative viscosity = Peak hydraulic pressure*Intensification factor*fill time
The intensification factor is the ratio of the area behind the hydraulic cylinder to the area of the barrel in the nozzle. Generally the intensification factor is supplied with the machine specifications and it does not need to be calculated.

intensification factor23. Complete a table similar to Mold Viscosity.
24. Graph the relative viscosity versus shear rate and determine optimum fill rate.

Relative Viscosity Graph

As can be seen from figure Relative Viscosity Graph, there is a region on the left side of the graph where small changes in fill rate result in large changes in viscosity. These large changes in viscosity result in molding variation with regards to actual cavity pressure seen by the molded part. It can also be seen on the graph, the right side levels out nearly horizontal indicating that the viscosity is more stable. The graph visually depicts that a faster fill rate would be more forgiving with regards to inherent molding variation. Corresponding the data from the mold viscosity graph with the mold viscosity worksheet, it is apparent that a fill time less than 0.6 seconds is ideal for this mold.
In some cases, the graph will not become flat. It is still possible to determine the proper fill rate from such a curve. Recall, the goal is to take advantage of the shear properties of plastics. While the curve may not be flat, the change in relative viscosity does decrease and become more stable. Tangency lines have been drawn in to illustrate how the curve changes. At the selected fill rate, the curve varies little from the tangency line. Conversely, the curve pulls away from the tangency a great deal at a slower fill rate.

If the mold you are qualifying is a production scale-up of a smaller cavitation pilot tool you should be able to achieve the same mold fill time, regardless of cavitation, as long as the press has been sized correctly for the mold (screw length, diameter etc.) This is not always the case, if the only way you can get the same fill time is to inject at maximum speed this is not a good idea since it will lead to instability when you process different resin lots. It is preferable to leave a margin of at least 30mm/sec from the maximum speed.

The further steps are required in validating a injection mold according to injection mold validation flow chart is dry cycle mold:

6. Balance of fill analysis
7. Gate freeze test
8. Commissioning (multi-cavity analysis)
9. Design of experiments
10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

2 komentarze:

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