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There are many things in tooling but sometimes the simplest are those that are overlooked. The issue or results of such missteps is often downtime, process issues and time wasted.
Some of the most common are:
1- Venting is inadequate
2- Exhaust of venting is not adequate
3- Runner is not vented.
4- In and outs not properly marked on tool
When the issue of venting comes up, most tooling people (and others) seem to say wait until we test shoot the tool and then go from there. Unfortunately the molder by than has slowed down injection speed to take care of burning and the inadequate or nonexistent vents that were not present this due to the fact everyone wanted good parts right now and the OEM was standing on sidelines waiting. Currently though the author has not confirmed or read there is a program out of a company in Chicago area that seems to be able to predict venting and sizes. In truth the air has to be removed from the cavity and this is a volume, which when one calculates exhausts in the fill time. If one were to do the calculation one would find that in some tools the speed of the air leaving the cavity may be approaching the speed of sound. A rule taken from a paper years ago stated that typically venting on a cavity should be 30% of perimeter area and 20% if in fact we were to vent the runner itself.
The exhaust of the vent is as critical as the venting itself. In many cases the cavities are proud of the mold base parting line, and this is thought to be enough. But in fact after a period of time this exhaust gets thinner and thinner, or was only a few thousands proud to start. When talking exhaust we wish to eliminate all resistance to air flow, this means big and even polishing the exhaust channels. If one is wiping the mold and or the buildup on the perimeter of the tool it is an indication that the tool is not adequately exhausted to atmosphere.
As long as the vent is constructed properly the exhaust can and should be larger, and also polished so as to aid the flow of the air stream to atmosphere. Again if one notices that the buildup in the channel at a certain point it is that the polymer in the gas is condensing and things must be checked. Sometimes the tool temperature itself is the issue.
Venting of the runner on a typical 2 plate or 3 plate tool to atmosphere provides an exhaust for the air in the runner, and this vent can be larger than on the part as if we flash it should not be an issue with the runner. If we do not vent the runner than the air is than combined with that in the cavity and much more work must be accomplished with the cavity vents. Basically we are not adding air from the runner to the mold cavity and than trying to understand why our vents do not work.
The marking of the water lines must be done, this is both for in and outs and which lines are matched. An example is "in #1" and "out #1" thus the circuit is defined. The further color coding with a blue for in and possibly red for out may be better, as it is now easier to read. This color coding can be done with inserts, or just using die ink to fill in the stamped or engraved lettering. This saves time in setup and troubleshooting.
There are many other things that need looking at but the above was what the author has found to be most ignored.
SLSilvey
silveysplastics@hotmail.com | 
