7. Gate Freeze Test - Validation procedure for injection molds

>> Saturday, 12 March 2011

The seventh step in validating a injection mold with the overall process shown in injection mold validation flow chart is Gate Freeze Test. 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


Purpose:

The purpose of the gate freeze test is to identify the hold conditions necessary to freeze the gate. The extent and duration of hold pressure has a large effect on the dimensional stability and outer appearance of the molded part. If the hold time is too short, the gate will not have had enough time to freeze off and sink marks could appear on the part. This is especially true of larger parts and when higher hold pressures are employed. After the mold gates "freeze", hold pressure has no effect and should be terminated at that point. It is typically better to be a little high on the hold pressure timer setting. This will cause a slight wear increase on the hydraulics of the injection press but the molder will be able to ensure higher dimensional accuracy.

Hold pressure is set at a pressure which allows no plastic melt to enter or leave the gate as the part solidifies. A high hold pressure setting could pack the part excessively, beyond dimensional tolerances. A low hold pressure setting will allow the melt to exit through the gate causing sink marks and voids in the part.

The gate freeze test is designed to achieve minimal dimensional variation by ensuring no plastic leaves the cavity before the gate is frozen. Hold times in the sharply rising part of the weight curve introduce additional variation. The gate freeze can be accurately determined via flow analysis programs which utilize the cooling circuitry layout, mold geometry, tool steel and hot/cold runner configuration. A predominant amount of the flow analysis programs assume isothermal conditions in the mold. This would produce best case results and typically the gate will never freeze off in this short of time. Properly interpreted, the flow analysis results serve as a good starting point.

Notes: 1) When using a valve gated system, no hold time is required. Once the valve is closed, no material will enter or leave the cavity. The valve is held open long enough to properly pack the part, and then closed. At this point, no pressure needs to be applied. 2) Having poor shot size control on your molding machine and an imbalanced mold will lead to less than desired results (lack of precision).

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 cooling time long enough so that parts eject without being distorted.
4. Set fill rate from results of mold viscosity test and if desired, profile the injection stroke to have velocity controlled pack. At this moment, record the dosing stroke and change over position. For the remainder of the validation process this will remain constant.
5. Set hold time based on the machine operators experience, take their estimate and multiply by 1.5. Use this as the starting point for hold time. If after running the test you do not identify a point at which the gate freezes off, increase the hold time incrementally. Caution: On some molds, high hold pressure and hold times can cause ejection issues. Pay heed to the advice of the mold builder and do not increase the hold pressure and time to the point the parts are difficult to eject off the cores. If you see this issue on your pilot mold, you may want to modify the mold or part design to make your part ejection more robust.
6. Set hold pressure so that there are no visual sink marks.
7. Collect and weigh 3 consecutive shots to 0.01 grams or better.
8. Subtract one second from hold time.
9. Add one second of time to cooling in order to maintain a consistent molding cycle.
10. Begin a table similar to Table: Gate Freeze Test.
11. Graph the shot weight versus the hold time Figure 1: Gate Freeze Test.
12. Repeat steps 7 - 9 until the part weight begins to decrease.
13. Repeat the test with a high mold temperature and high melt temperature to document the worst case scenario for hold time.

A typical "gate freeze test" curve is shown in Figure 1: Gate Freeze Test. There is a region on the left side of the graph where small changes in hold time result in large changes in part weight. These large changes in part weight may result in part quality variation with regards to dimensions or mechanical performance. The region on the right side levels out at 3.5 seconds indicating that the part weight is more stable and that the gate has frozen off, i.e., no polymer melt enters or leaves the cavity.

Table: Gate Freeze TestTable: Gate Freeze Test
Figure 1: Gate Freeze TestFigure 1: Gate Freeze Test

Figure displays a gate freeze test when the gate never froze. This is typical for hot tips directly gated onto the part. The thicker the part, the more likely this will occur. When such a curve is graphed, identify the region on the curve where a change in slope is evident. In figure 2, this area is at 1.0 seconds. A hold time of at least 1.5 seconds is recommended.
Figure 2:  Gate Freeze TestFigure 2: Gate Freeze Test

It is possible the part will become over-packed and either flash or stick in the core, causing ejection problems, before obtaining a level curve. The above test had to be stopped at 4.0 seconds due to the part sticking in the core at longer hold times. Therefore, the hold time could be set anywhere between 1.5 & 4.0 seconds. If cycle time is not limited by hold time, a design of experiments could be performed using hold time as a variable. This will help to select the optimum hold time.

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

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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