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)

2 komentarze:

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