|
Definition: Mold Qualification. 1) To qualify for use 2) a set of procedures/steps which lead to the approval for production of a mold Gate freeze 1) a point at which material no longer flows through the gate due to solidification of material 2) may be referred or referenced back to a time factor such that it is stated the "gate freeze time is xx seconds"
The term gate freeze though self-explanatory and by the definition above relating to the time it takes to freeze off is one of those points of reference. Like the short shot, and or pressure drop study it brings forth a point in the cycle / process when something is complete. It is again a simple test on the machine and may be used as a reference point.
The test for the gate freeze study is in that it measures part weight versus pack time. The collection of this data is critical in how it is done though. In preforming this test the cycle time remains constant, along with material and mold temperatures. The packing pressure has been established, and the fill rate has been established and do not change with this test.
The tools required for this procedure / study are a scale capable of measuring to 3 significant numbers. Thus if a large part in weighed then not too much of an issue but if a small part than multiple shots may need to be combined to result in a weight large enough to gather 3 significant numbers. Since this is to measure gate freeze, it can be done for each cavity or for all cavities at once if one wishes an average for the shot. The runner is not weighed only the parts.
It is the cavity that is weighed, not the runner or any part of the gate, since the results are to be the gate freeze, or at what point did the gate freeze off. The trimming of the gate on the parts is critical that each should be done the same so as to minimize the effects of non-consistent trimming.
The second factor is that cycle time is always the same. Since the overall thermal cycle can have an effect on gate freeze time, the overall cycle time has to remain constant for this test.
The steps:
- Determine the existing part weight and record for each cavity or overall shot (average) and record it along with
- The pack time / injection forward time / time used to packed out parts
- Cooling time set on press
- Cycle time
- Packing pressure used
- Adjust the following:
- Add 2 seconds to the packing time
- Subtract 2 seconds to the cooling time
- Note that this (a &b) than maintains the overall cycle time and thermal cycle of the process.
- Record the part weights
- Understand that the adding or subtracting of time may be in whatever amount of units one wishes to use and detail one is to achieve.
- If the weight has increased than the gate was not frozen and one must continue in the direction of adding time to pack while taking it off cooling until a stable part weight is achieved.
- If the weight does not increase than the gate was frozen at the original settings
- One now should move in the opposite direction of testing by removing pack time and increasing the cooling time
- The testing should be done until it results in a drop in part weight.
- Once weight reduction is achieved than moving in the opposite direction establishes the gate freeze time.
- Once a point has been established where the gate has frozen, it than becomes a reference point and for safety purposes dependent on the operation an additional 1 to 2 seconds may be added to the gate freeze time to insure it is always frozen.
- Note that cycle time should be maintained by reducing cooling time the 1 to 2 seconds.
What has been established is that this is the point the gate freezes off. This can be critical in thick walled parts, along with thin wall parts, in that it the point when no more material is being injected into the cavity. Any additional time used beyond a safety point is only packing out the runner, sprue or other component of the feed system. In some cases one may need to add time so that the runner does pull, but than this fact should be noted, recorded and placed in a file to be reviewed and discussed.
An example of this testing showed that a 9 second pack time was not long enough for a part manufactured from polycarbonate. The issues at hand were rejects due to bubbles in the finished part. A gate freeze study showed that it took 11 seconds for the gate to freeze off. Adding the 2 seconds plus an additional second (total of 12 seconds) to the pack time resulted in a yield increase in this example of 30% to approximately 98% good part manufactured and zero bad parts for the effects of non-gate freeze (bubbles).
The other established point of conducting this test is that it may now determine if the gate needs to be increased when used in conjunction with a pressure drop test. (Please note that a viscosity study should be performed first). For example if one was to lose a great deal of pressure through the gate and it froze off too quick to pack out than it may be considered an option to open it up. But if it was to lose pressure but gate freeze was long than it may not be advisable to open it up. The 3rd situation is that the gate freeze is quick, pressure drop is not high, but the gate cannot by design be opened up, this than results in a case that pressure used may have to be increased to pack out a part if that is necessary.
There are times that gate freeze is not used in the manufacture of the part and this is when a high strain rate at the gate results in a poor performance of the part, such as cracking or stress. In these cases by determining gate freeze one than molds to a non-gate freeze point.
Once gate freeze time has been established the use of this time to optimize the tool and process can proceed. It is important in gate freeze to understand that a change in fill speed, mold temperature, melt temperature, and cooling time may than effect the gate freeze time and a possible check need be performed to see that gate freeze is still maintained. Since part weight was established at a gate freeze point it is a simple thing now to weigh a part to see that weight was maintained. If time allows than performing a gate freeze study may confirm that all is good or a new time is now established under these new molding conditions.
www.silveysplasticconsulting.com
25022013.01
SLSILVEY | 
