HOT RUNNER THOUGHTS

Have you ever had to clean out the manifold section of a mold due to the fact that it filled up with plastic?  What about replacing heater bands and or assemblies because they were damaged due to flashing or in this case filling up the tool internally with plastic due to leakage.  The reasons and problems I hear are numerous and have been over the years. It is amazing how long something can leak before we do the maintenance etc. to take care of the system and or problem.

This is a discussion on the hot manifold system that is based on 2 parts, that of a manifold and the drops that use counter pressure to keep them sealed. This is not the system that screws together, but some of what I am talking about applies to all.

The first thing is safety.  Is the mold adequately marked with maximum injection pressure that may be applied to the system? There should be a tag or plaque on the side of the mold to describe this.  On that tag should be part numbers, product number, date, and manufacturer but most important are maximum pressure, temperature range, and plastic material.  There may even be a mention of mold temperature range.

The point to all this is this data tells what can be done with the machine and manifold. Understanding how the manifold is designed and works is very important.  In the case of the manifold the steel is heated and as it heats it expands and grows (thermal expansion). Dependent on the size of the system this could be a very small variation to possibly a large variation. Thus this piece of steel needs to grow for its proper location. When the manifold has expanded, than the drops are heated and they then line up with the manifold but by their growth / expansion they apply the pressure to the manifold to seal up the passage between the manifold and drops. Understanding of this concept of how your system works and is designed is very important. In the system that screws together the heat up cycle is important too, but the possibility of leakage is reduced greatly due to a sealed system.  There is still expansion and movement but we have everything mechanically locked together, thus minimizing the problem a bit.

So now we are ready to turn the heats on for a period of time. Slow (soft) start or fast start? Understanding, knowing the type of your heater system is important here. The reason for a slow start is that given the make up of a heater band we need to dry it out prior to applying the full voltage to it thus preventing a short in the system. Most units for power control today have this feature/option built into them, but they also allow the setting to be by passed. The difference between a slow start and fast may be 30 minutes in being able to start up. This time function is money, and some people will by pass. If we by pass and short out a heater what is the possible cost? One would be a new heater and the time to replace it, maybe due to the short in the heater which was not caught we blew our system because those cavities feed by the drop were not filled and the material went elsewhere which was not the other cavities but causing the manifold to separate and coating it with plastic.  Or worst yet we put all the plastic into 3 cavities instead of 4 and damaged the mold.  With some of the new heater types this slow start may be eliminated but check with the heater supplier and or manufacture of the system.
 
Thanks for the time, part two next time.

Steven L Silvey
Silveys’ Plastic Consulting
360-662-3183
ssilvey643@aol.com