Qualification (Process Capability Study) - Validation procedure for injection molds

>> Friday, 1 April 2011

The tenth step in validating a injection mold with the overall process shown in injection mold validation flow chart is Qualification (Process Capability Study). The steps before:
1. Mold certification
2. Dry cycle mold
3. Process stability test
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 (DOE or DOX)

Purpose:
The purpose of the qualification study is to determine if the process can meet the specified key part tolerance ranges. The first mold being manufactured to produce a molded part might be made “metal safe”. In this case, the qualification step will determine how much metal needs to be modified in the mold. Resin and colorant properties also need to be evaluated so that process capability may be determined.

Once a process has been selected from performing the Design of Experiments (DOE or DOX), the qualification study needs to be performed to determine the amount of variation of each key dimension (via control charts). This variation is compared with specified key part tolerances to estimate percent out of specification and product quality measures (Cr, Tz & Cpk). A solid understanding on creating and interpreting statistical control charts is necessary to perform the process capability study.

Material and Colorant Variation
Resin differences and the addition of colorants effect molded part performance. The rate of the shrinkage changes depending on the resin properties and type of colorant used. This will effect molded part dimensions and mechanical performance. In many cases, there is variation from one resin lot to the next, i.e., lot to lot variation. This lot to lot variation in resin properties is inherent. The resin variation must be evaluated to determine if the material specification range will drive the molded part to be out of the specification. In addition, the Qualification study must properly evaluate each colorant, while including the lot to lot variation in the base resin. Understanding the effects of different colorants is imperative since the same part must be produced in multiple colors from the same mold. The information obtained from the qualification study results can be used to properly modify the mold.

To quantify base resin, and resin to colorant blend properties, a reliable test method must be selected. Equipment such as the gel permeation chromatograph (GPC), differential scanning calorimeter (DSC) and capillary rheometer are reliable tools for quantifying the lot to lot range. However, it is sometimes difficult to obtain the data from these tools. Material suppliers in all regions generally provide data from the melt flow index (MFI) apparatus. The disadvantage of using the MFI unit is it only provides one data point on the shear rate versus viscosity curve. And, more importantly for qualification purposes, the accuracy of the test is poor. When data from the GPC, DSC or capillary rheometer is not available, regress to the MFI data as a means to quantify the upper, lower, and mean specification of the base resin.

While the mold is being designed, a lot to lot variation representing the maximum expected variation in the resin should be requested from the material supplier. Three lots of the base resin representing the upper, lower and mean specifications from the supplier will deliver an accurate indication of the capability ratio achievable in a production environment. Couple this together with an investigation of all colorants and the percent regrind to capture the remaining sources of inherent variation. Multiple options of the procedure for the qualification study were created to allow for a variety of different circumstances. A description of when to use each option is provided with the procedure. Review these descriptions to find an option which best meets the needs of your qualification study. It may be necessary to edit these options to identify the best qualification possible. Be sure to capture the intent of the qualification which is to examine all the sources of variation by means of an extended molding run. The qualification provides the confidence to make proper metal modifications to the mold and to provide the go ahead to move into the verification stage.

Option 1 Description:
This is the best option to study the inherent material and colorant variation. It requires representative virgin resin lots of the upper, lower and mean specifications. In addition, it requires adequate amounts of all colorants which will run on the mold.

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.

Option 1 Procedure:
1. Run a qualification study at the selected process conditions with the mean specification resin lot appropriately mixed with primary colorant for a period of 8 hours.
2. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
3. Switch to the resin lot representing the upper specification. This resin lot should be appropriately mixed with the primary colorant. Run for a period of 8 hours.
4. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
5. Switch to the resin lot representing the lower specification. This resin lot should be appropriately mixed with the primary colorant. Run for a period of 8 hours.
6. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
7. For each additional colorant, appropriately mix the colorant with the resin representative of the mean specification. Run each additional colorant blend for a period of 8 hours.
8. During each additional run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
9. After proper conditioning, measure and record all critical dimensions of all the parts.
10. Develop an and R Chart (n=4).
Note: in the course of a day, you may have several measurements from each cavity to repeat the multi-cavity analysis (within cavity/between cavity variation), if needed.
11. Both the and R charts should show control, otherwise investigate the sources of variation (root cause.)
12. Proceed to determine percent out-of-spec. and calculate the appropriate Product Quality Measures1 (Cr, Tz & Cpk).
13. Compare results to the results from the DOX process. Also, compare to the target and develop a histogram of sample population with and to assess normality of the parent.

Option 2 Description:
This option does not evaluate the inherent material variation. It does investigate the differences attributed to molding the same part in a number of colorants. It requires a representative virgin resin lot of the mean specification. In addition, it requires adequate amounts of all colorants which will run on the mold.

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.

Option 2 Procedure:
1. Run a qualification study at the selected process conditions with the mean specification resin lot appropriately mixed with primary colorant for a period of 24 hours.
2. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
3. For each additional colorant, appropriately mix the colorant with the resin representative of the mean specification. Run each additional colorant blend for a period of 8 hours..
4. During each additional run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
5. After proper conditioning, measure and record all critical dimensions of all the parts.
6. Develop an and R Chart (n=4).
Note: in the course of a day, you may have several measurements from each cavity to repeat the multi-cavity analysis (within cavity/between cavity variation), if needed.
7. Both the and R charts should show control, otherwise investigate the sources of variation (root cause.)
8. Proceed to determine percent out-of-spec and calculate the appropriate Product Quality Measures2 (Cr, Tz & Cpk).
9. Compare results to the results from the DOX process. Also, compare to the target and develop a histogram of sample population with and to assess normality of the parent population.

Option 3 Description:
This option evaluates the inherent material variation along with the complication of using regrind. This Qualification study investigates the variation attributed to molding the same part in a number of colorants. It requires a representative virgin resin lot of the upper, lower and mean specification. In addition, it requires adequate amounts of all colorants which will run on the mold and regrind.

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.

Option 3 Procedure:
1. Run a qualification study at the selected process conditions with the mean specification resin lot appropriately mixed with primary colorant and percent regrind for a period of 8 hours.
2. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
3. Switch to the resin lot representing the upper specification. This resin lot should be appropriately mixed with the primary colorant and percent regrind. Run for a period of 8 hours.
4. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
5. Switch to the resin lot representing the lower specification. This resin lot should be appropriately mixed with the primary colorant and percent regrind. Run for a period of 8 hours.
6. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
7. For each additional colorant, appropriately mix the colorant with the resin representative of the mean specification and percent regrind. Run each additional colorant blend for a period of 8 hours.
8. During each additional run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
9. After proper conditioning, measure and record all critical dimensions of all the parts.
10. Develop an and R Chart (n=4).
Note: in the course of a day, you may have several measurements from each cavity to repeat the multi-cavity analysis (within cavity/between cavity variation), if needed.
11. Both the and R charts should show control, otherwise investigate the sources of variation (root cause.)
12. Proceed to determine percent out-of-spec. and calculate the appropriate Product Quality Measures3 (Cr, Tz & Cpk).
13. Compare results to the results from the DOX process. Also, compare to the target and develop a histogram of sample population with and to assess normality of the parent population.

Option 4 Description:
This option does not evaluate the inherent material variation. It does investigate the differences attributed to molding the same part in a number of colorants along with the complication of using regrind. The Qualification study requires a representative virgin resin lot of the mean specification. In addition, it requires regrind and adequate amounts of all colorants which will run on the mold.

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.

Option 4 Procedure:
1. Run a qualification study at the selected process conditions with the mean specification resin lot appropriately mixed with primary colorant and percent regrind for a period of 24 hours.
2. During the run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
3. For each additional colorant, appropriately mix the colorant and percent regrind with the resin representative of the mean specification. Run for a period of 8 hours.
4. During each additional run, take a reading from a single cavity (selected at random) once every 15 minutes. More than one reading may be necessary for destructive test methods or to keep additional parts on hand.
5. After proper conditioning, measure and record all critical dimensions of all the parts.
6. Develop an and R Chart (n=4).
Note: in the course of a day, you may have several measurements from each cavity to repeat the multi-cavity analysis (within cavity/between cavity variation), if needed.
7. Both the and R charts should show control, otherwise investigate the sources of variation (root cause.)
8. Proceed to determine percent out-of-spec. and calculate the appropriate Product Quality Measures4 (Cr, Tz & Cpk).
9. Compare results to the results from the DOX process. Also, compare to the target and develop a histogram of sample population with and to assess normality of the parent population.

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

11. Mold metal Adjustments - centering process
12. Verification (30-day run)

2 komentarze:

Navin Patel 11 July 2015 at 02:56  


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Oracle Equipments 6 April 2016 at 23:12  

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