IMM eShots, MPW eWeekly, IMM Moldmaking Monthly and more

>> Thursday, 7 October 2010

The coverage in this industry leading eNewsletter is brought to you by the editors of Injection Molding Magazine.

Request Free!
Each issue includes global breaking news from the plastics industry brought to you by the editors of Modern Plastics Worldwide.

Request Free!
The coverage in this industry leading eNewsletter is brought to you by the editors of Modern Plastics Worldwide.

Request Free!
Every day their experienced editorial team will present to you the top stories impacting the global plastics market.

Request Free!

Read more...

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

Purpose:
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.

Procedure:
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)

Read more...

4. Gage repeatability & reproducibility (R&R) test - Validation procedure for injection molds

>> Wednesday, 23 June 2010

The fourth step in validating a injection mold with the overall process shown in figure injection mold validation flow chart is gage repeatability & reproducibility (R&R) test. The steps before:

1. Mold certification
2. Dry cycle mold
3. Process stability test

Purpose:
The purpose of the gage R&R analysis is to quantify the magnitude of variation of a measurement system. To ensure the benefit derived from using measurement data is of value and cost effective, attention must be focused on the quality of the measurement data. One of the most common reasons for low-quality data is too much variation in the measurement data. For example, a measurement system with a large amount of variation may not be appropriate for use in analyzing an injection molding process because the measurement system’s variation may mask the variation in the injection molding process.

A gage is any device used to obtain measurements. This includes devices like coordinate measurement machines, vernier calipers, micrometers, tensile testers and go/no go devices. A measurement system is the collection of operations, procedures, gages and other equipment, software, and personnel (operators of the measurement device) used to assign a number to the specific characteristic being measured; the complete process used to obtain measurements. There are procedures used to assess the statistical properties of a measurement system: repeatability, reproducibility, bias, stability and linearity. Collectively, the procedures are often referred to as “Gage R&R” procedures because they are frequently used only to asses the statistical properties of reproducibility and repeatability. In general, the procedures are easy to use in a production environment and, although the procedures are statistical in nature, they are presented in a manner allowing them to be used by people who are not statisticians.

Table gage R&R excel epreadsheet shows an example of a typical gage R&R spreadsheet template that may be used to assess the reproducibility and repeatability of a selected measurement system. Measurement device operators are requested to record ‘blind’ measurement data on three separate groups or ‘trials’ of molded package components. The measurement data is then entered in to the gage R&R spreadsheet and the variation of the measurement system is interpreted. If the variation of the measurement system is relatively high then the source(s) of the variation must be identified and reduced or an alternate measurement system must be selected.

Procedure:
1. Prepare groups of molded parts (form the process stability run) for the operators to measure.
(in case of destructive testing more samples should be taken vs. non-destructive testing)
2. Prepare separate measurement data sheets (one sheet for each group or ‘trial’ of parts) so the operator cannot see the previous measurement data.
3. Request that operator perform the measurement operation and observe how the operator completes the measurement task. As each operator completes the measurement task note any significant differences in the procedure or manner in which the operator completed the measurement task.
4. Complete the gage R&R spreadsheet and record the ID information from the measurement device that was used to measure the parts.
5. Interpret the statistical results: repeatability and reproducibility for the measurement system. If the variation of the measurement system is relatively high, then the source(s) of the variation must be identified and reduced or an alternate measurement system must be selected and evaluated.

IF TOTAL "R&R EXCEEDS 20% OF THE TOLERANCE, THE SYSTEM OF MEASUREMENT (EQUIPMENT + SKILL) NEEDS TO BE IMPROVED. IF EQUIPMENT VARIATION IS VERY LOW, AND OPERATOR VARIATION IS HIGH, TRAINING AND PRACTICE MAY CORRECT THE PROBLEM. IF EQUIPMENT VARIATION IS HIGH, THE GAGE MAY NOT BE SUITABLE FOR THE TASK.

X-bar and Range Charts are also provided in figures X-Bar & R Chart for Gage R&R study & X-Bar Chart per Operator. A goal of less than 20% variation when calculated against the specified part tolerance was not achieved. In such a case, alternative methods for part measurement should be investigated. In some cases, these results may indicate lack of training for the individuals operating the equipment.

Gage R&R excel spreadsheet
Gage R&R excel spreadsheet

X-Bar & R chart for gage R&R study



X-Bar chart per operator
The further steps are required in validating a injection mold according to injection mold validation flow chart is dry cycle mold:

5. Mold viscosity test
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)

Read more...

3. Process stability test - Validation procedure for injection molds

The third step in validating an injection mold according to injection mold validation flow chart is dry cycle mold is process stability test . The steps before are:

1. Mold certification
2. Dry cycle mold

Purpose:
The purpose of the process stability test is to identify any gross mechanical design/construction flaws which will cause part variation over time. In addition, the process stability must be established with no short shots, flash, etc., prior to running designed experiments. The test is run at mid-range resin molding conditions for a period of time to allow thermal expansion equilibrium. In most instances, the mold builder may already have a process where the mold could run comfortably for several hours.

During the process stability test the mold must run with one ejection stroke unless it was designed to cycle with multiple strokes. A robust ejection system will be able to eject the part and runner with one ejection stroke. Multiple ejection strokes per molding cycle increases the wear on the ejection system and increases the molding cycle.

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.

Procedure:
1. Set melt temperature to resin manufacturer's recommended mid-range.
2. Set mold temperature to resin manufacturer's recommended mid-range.
3. Set hold pressure and time to zero, as well as pack time and pressure.
4. Set cooling time long enough so that parts eject consistently without distorting.
5. Adjust feed stroke and screw/ram position cutoff so that parts fill approximately 95% during injection.
6. Set adequate hold time and pressure, as well as pack time and pressure, so no sink marks are visible.
7. Record process settings of the machine, melt temperature and mold temperature.
8. Run the process long enough that you feel comfortable the mold and process has reached steady state, and there are no functional issues with the mold.
9. Evaluate visually for any gross part or mold variation, e.g., flash, short shots, scratches, blemishes, water (leakage) and voids.
10. Collect one full shot and weigh each part. Record individual part weight.
Note: This will be used as the weight of a full part when determining 90% part weight during the following tests.
11.Explore the processing window of the mold. Vary parameters such as injection rate, hold pressure, hold time, cooling time in order to begin to understand the mold processing window. For each range of process settings collect at least one set of parts. These parts will be necessary to perform the gage R&R analysis. Recall for the gage R&R one is trying to minimize the variation within a sample but maximize the variation between samples.

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

4. Gage repeatability & reproducibility (R&R) test
5. Mold viscosity test
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)

Read more...

RECENT NEWS

OTHER NEWS

BOOKS RECOMMENDATION

  © Blogger template Webnolia by Ourblogtemplates.com 2009

Back to TOP