9. Design of Experiments (DOE or DOX) - Validation procedure for injection molds

>> Saturday, 12 March 2011

The ninth step in validating a injection mold with the overall process shown in injection mold validation flow chart is Design of Experiments. 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)

Purpose:
The purpose of the Design of Experiments (DOX or DOE) is to identify the optimum mold process and the mold process window. A solid statistical understanding of DOX is necessary. There are many different types of software that can be used to assist you in performing design of experiments properly. Use the software that you are most comfortable with.

The DOE can require a large amount of time to perform depending on the number of variables selected to test. In some cases a properly conducted DOX can require 2-3 days to perform as well as additional time to measure part attributes. It is best to perform an extensive DOE on the pre-production tooling (Pilot Tool) which replicates the production tooling. This will give a good representation as to which key variables will effect the critical attributes of the molded part from the production tool. It is also recommended the DOX be performed immediately after the commissioning test. Especially is you are confident with the skill and precision of your mold builder to produce a mold which has minimal variation between cavities.

It is recommended that a fractional factorial be performed to study the main effects. Variables that do not have a significant effect on critical part dimensions can be eliminated by performing the fractional factorial first. Significant variables for a full factorial experiment are determined from the fractional factorial, eliminating guesswork. The information from the full factorial as well as the fractional factorial analysis can then be used to perform Statistical Process Control (SPC).
Fractional Factorial Description:
This step should be performed when it is not clear which variables will effect key dimensions and attributes of the part. In some cases, it may be necessary to perform the fractional factorial DOE to determine the main effects for the aesthetics of the part. This will ensure that a full factorial DOX for the critical dimensions can be run while achieving aesthetically pleasing parts. The fractional factorial is to be used as a screening devise for identifying significant processing variables and not for process optimization.

Caution: The fractional factorial will not determine interaction effects.

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.

Fractional Factorial Procedure:

1. The multi-cavity analysis has been successfully completed, i.e., there is no difference between cavities on the critical part dimensions.
2. Set up fractional factorial experiment to include all significant molding variables.
3. Typical mold process variables to include in DOX are:
Melt Temperature, Hold Pressure, Hold Time, Cooling Time, Mold Coolant Temperature, Cavity Pressure at Injection Cutoff, Peak Cavity Pressure (Packing), Screw Speed, Back Pressure.
4. Test extreme conditions (corners) of the DOE to verify you are within the molds processing window.
5. If a test "Fractures" then stop the DOX and rerun the experiments with acceptable degrees of variation for each factor.
6. For each change in processing conditions, allow time for system to equilibrate for changes in machine set up. For example, changes in mold temperature may require the mold running for 1 hour before it reaches steady state conditions.
7. Collect 5 shots for each process conditions. Attach a process set-up sheet and test code to samples.
· For each destructive test method, collect one additional shot.
8. Condition parts for 24 hours at 23C (73.4F) and a relative humidity of 50% with a standard tolerance of 2.0C (3.6F) and 5% relative humidity, respectively, for 2 days.
9. Measure and record all key attributes for one cavity, representative of the mold (see multi cavity analysis).
10. Analyze the data using any statistical software with a DOX option. Use coded units for the analysis.
11. Identify significant processing variables to be evaluated in the full factorial DOX.

Full Factorial DOX Description:
The full factorial is to be used to determine the optimum processing conditions based on key part dimensions. When it is unclear which variables are significant, the fractional factorial should be performed first.

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.

Full Factorial Procedure:

1. The multi-cavity analysis has been successfully completed, i.e., there is no difference between cavities on the critical part dimensions.
2. Set up full factorial experiment using significant variables determined from the fractional factorial (if applicable).
3. Typical mold process variables to include in DOX are:
Melt Temperature, Hold Pressure, Hold time, Cooling Time, Mold Coolant Temperature, Material Lot (different lot of same resin), Colorant, Cavity Pressure at Injection Cutoff, Peak Cavity Pressure (Packing), Screw Speed.
4. Test extreme conditions of the DOE to verify you are within the molds processing window.
5. If a test "Fractures" then stop the DOX and rerun the experiments with acceptable degrees of variation for each factor.
6. For each change in processing conditions, allow time for system to equilibrate for changes in machine set up. For example, changes in mold temperature may require the mold running for 1 hour before it reaches steady state conditions.
7. Collect 5 shots for each process conditions. Attach a process set-up sheet and test code to samples.
· For each destructive test method, collect one additional shot.
8. Condition parts for 24 hours at 23C (73.4F) and a relative humidity of 50% with a standard tolerance of 2.0C (3.6F) and 5% relative humidity, respectively, for 2 days.
9. Measure and record all key attributes for one cavity, representative of the mold (see multi cavity analysis).
10. Analyze the data using any statistical software with a DOX option. Use coded units for the analysis.
11. Perform the optimization of the process based on the various critical dimensions and most efficient cycle time.
12. Identify upper and lower control settings for each processing variable evaluated (create a mold processing window).

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

10. Qualification (process capability study)
11. Mold metal Adjustments - centering process
12. Verification (30-day run)

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