3. Process stability test - Validation procedure for injection molds

>> Wednesday, 23 June 2010

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)

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