Hot Runner Continued

In the past articles we have covered a majority of facets with the hot runner.  To somewhat wrapped up the subject, a review of some of the points and some additional thoughts.

1- Our system is designed for the material we are running. This would be the actual material or if generic one of the two families, semi-crystalline or amorphous.
2- We use a slow start on start up to dry out the heaters.
3- We always start the manifold first and after a bit of time we start the drops.
4- The drops are all set at the same temperature, usually the melt temperature.
5- The first step for balancing the mold is through the manifold and then the drops are minimally adjusted, if needed.
6- Knowing the system that you are using is important, in setting temperature. This is both because of the thermocouple placement, as well as the temperature setting, some are above what we think they should be too run.
7- The manifold and drops are in a sealed compartment in the mold base. We are not open for air flow where we are trying to heat the plastic material via our manifold.  This prevents the chimney effect and not being able to control the temperature.
8- We have placed an insulation sheet on the mold base for the A side to help maintain the temperature and not use the platen of the machine as a heat sink.
9- We have adequate cooling to the mold and around the drops. This is especially true for semi-crystalline material.
10-  We have a map, schematic of the manifold and drops for what zone controls what in our mold. This way we know when we raise or lower zone one that this is our sprue bushing or drop for the # 2 cavity.
 

Some common problems with hot runners:
1- Stringing, this is an interesting one as what is happening is the gate has not frozen off, or solidified enough to cause a breakage that we want. If the opening is too large we will get stringing, along with too high a temperature. Normally there is cooling in this area so that when the mold opening and the part is ejected we break the area of the gate. With amorphous materials we can drop gate temperature, with semi-crystalline we need to take away a bit of energy to cause the gate to freeze. This is where cooling comes into play.
2- Long gates, this is a little tit on the part. It could be by wrong placement of probe, too cold of a drop temperature, mold temperature too cold. If all else fails look at design of drop and interference with the mold base, do we have a gap around this area or are we drawing heat energy away, good for semi-crystalline bad for amorphous.
3- Imbalance in the fill.  Dependent on the system and number of drops. Remember that the first to start to fill is actually the last to fill. Knowing the manifold layout is critical in solving this issue.
 

Though the above is not totally complete it is a good starting point and review of the system.
Thanks for the time. As always if anyone has ideas for tips let me know.

Steven L Silvey
Silveys’ Plastic Consulting
360-882-3183
peipai@aol.com