6. Balance of fill analysis - Validation procedure for injection molds

The sixth step in validating a injection mold with the overall process shown in injection mold validation flow chart is balance of fill analysis. 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

Purpose:

The purpose of the balance of fill analysis is to evaluate the thermal and flow balance of the plastic distribution system in the mold. The plastic distribution system encompasses the hot or cold runner system, as well as the core and cavity. Only a naturally or symmetrically balanced manifold should be specified.

The cavity-to-cavity weight difference is an indicator of the quality of the hot runner system. It is critical to have the flows balanced to each cavity or the part-to-part variation may be large and process capability may not be achievable. Recall, the purpose of the mold validation procedure is to reduce variation throughout the injection mold. A typical mold is generally within +/- 10%. If the mold you are qualifying exceeds these values it should not cause you to cease the qualification process. Do investigate for possible causes such as, blocked gates, hot tip controller malfunction, hot tip failure, and non-uniform cooling. If the issue is not identified, document the data and continue. This data will be of use while analyzing the data collected during commissioning (multi-cavity analysis).

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: This test is part geometry dependent. The 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 manifold and probe tip temperatures to resin manufacturer's recommended mid-range.
3. Set mold temperature to resin manufacturer's recommended mid-range.
4. Set hold time and hold pressure to zero.
5. If the machine is equipped, set pack time and pressure to zero.
6. Set cooling time long enough so that resin has cooled and parts eject consistently.
7. Set fill rate using the results from the mold viscosity test.
8. Transfer from injection to hold phase by screw position.
9. Have sufficient cushion to prevent the screw from bottoming out against the barrel during injection.
10. Adjust feed stroke so that the heaviest part is approximately 90% filled by weight.
Note: To achieve accurate results it is imperative none of the parts be completely filled.
11. Add adequate hold time and pressure, as well as pack time and pressure (if the machine is equipped with pack time and pressure) so no sink marks appear and cycle for five shots.
12. Remove hold time and pressure, as well as pack time and pressure.
13. Collect one all cavity shot.
Note: With certain mold designs it is difficult to perform a manifold balance test because of ejection issues with short shots. For example, some parts are designed with a slight undercut to ensure the part will stay on the side of the mold with the ejection action. If this is the situation, the undercut might not be filled during a short shot and the part will stick on the wrong side of the mold. In this case, you should evaluate the number of samples to be collected. In addition, study the parts to verify the balance of fill is not a result of poor venting. Lack of venting can artificially make the mold appear in balance.
14. Repeat steps 11-13 until 3 full shots have been collected.
Note: It is necessary to cycle five shots before each collection of shots to ensure each collection is subjected to the same thermal conditions.
15. Weigh all the parts and average the data per cavity.
16. Chart the weight by cavity.
17. For each cavity, use the following formula to calculate % imbalance:
%Im balance = (Wf-Wn)*100/Wf, where Wf = Weight of heaviest cavity, Wn = Weight of cavity n, where n = cavity number
18. Graph and interpret results.
Figure Balance of Fill Analysis depicts typical results for a mold which is not properly balanced, i.e., cavities 9, 10, and 12 are not within 5% of the weight of the full cavity #7.

Plotting the data relative to a mean weight is also a good method of identifying problem cavities. The 0.00% level of imbalance is based on the mean weight of all cavities in the mold. Cavity to cavity imbalance calculated against the mean part weight. Showing the data in this manner helps to highlight cavities to both extremes - higher than normal, or lower than normal. The formula below shows the calculation for calculating the imbalance using this method. %Im balance = (Wa-Wn)*100/Wa, where Wa = Average Weight of all the cavities, Wn = Weight of cavity n, where n = cavity number
It is also recommended the data be plotted in other ways. Doing so will make it easier to trouble shoot molds when necessary. Figure Balance of Fill Analysis shows the cavities in “clusters” according to their position in the mold. This is a good method to identify problems in a runner system, cooling system, one face of a stack mold, or inadequate clamp tonnage across the entire face of the mold. The “cluster” method of plotting the data according to mold construction is probably the best method. This method is very useful when plotted using average part weights for calculated imbalance.
On a hot runner system if you plot the imbalance and find that the majority of the cavities are reasonably well balanced but you have one or two obvious deviations (i.e. on a 32 cavity mold 30 cavities are 4 % imbalanced and 2 are 20% imbalanced) it is evident there is an issue with those two cavities. Steps you could take are:
1.Ensure correct hot tip operation, read power usage and temperature setting/variation on display unit.
2.Check for a foreign particles which may be blocking the gates of the suspect cavities
3.Measure gate size/shape (i.e. circularity) in cavity. Measure hot tip height in relation to the gate.
If one or two cavities have substantially (over 10%) higher imbalance than others the above analysis should highlight the reasons.

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

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)