Hot Runner Continued

Previously we had discussed the manifold and the fact that by increasing the heat we can decrease pressure lost in the system by creating a larger flow channel. We also discussed to a point that we must know our system and what temperature set point is necessary to maintain our flow.
Probe / tip balance to cavities. This is an area that is critical and very much dependent on the type of probe, tip and manufacturer. First we must agree that the height of the probe /tip is set the same for each cavity and that the opening and design is the same. Second is the fact that upon heating as discussed in the first part, we achieve a degree of thermal expansion. Thus the result is that the probe/tip grows. Thirdly we need to have the system design for the material classification we are running and hopefully for the particular material.  We are talking Plastics 101 here, amorphous or semi-crystalline. Materials that soften versus those that melt and have that latent heat of fusion.

Having stated all the above we now, in filling the mold we find that some parts fill faster than others. The key to a hot runner is that the first cavity to start to fill is actually the last to fill. Thus when we raise the heats (common mistake) the particular cavity still starts to fill first until the pressure gets large enough to open another drop and than this drop which started to fill first stops. So in an abstract way the cavities fill in a non uniform sequence, but generally they are close enough for what the part needs. It is usually when they are not that people tend to raise heats too high and we see 100 to 150 degree swings in probe setting. The adjustment of probe temperature is a last resort to fill parts, and should be about 25 to 50 degrees max variance if any. Understand that we did adjust the manifold temps first! Can I get a greater variance in temperature on my probes if I don’t first optimize the manifold, you bet.

The key is temperature, and understanding that the thermal expansion drives the probe tip into the opening and closes off our opening, creating a higher pressure drop, higher shear (which may help in certain cases) and may in fact effect what is thought of as gate freeze time. The cavity that has no flow, may be too cold and individually this may need to be determined, but with short shots, incomplete fill we may wish to drop our probe temperature and note results. Watch the pressure on your machine, does it vary or by going down on temperature does it actually drop too?

Manifold design plays an important role here. You should have a drawing of the manifold to look and see that we don’t have the effects of shear in the manifold causing us our problems. (Different topic)

In one case I dropped 100 degrees off the drop temperature and actually decreased the pressure used to fill the part by 8,000 plastic pounds per square inch. This is an exception due to design but it showed the molder what had happened.

Thanks for the time, more next time.

Steven L Silvey
Silveys’ Plastic Consulting
360-882-3183
ssilvey643@aol.com