.

Tech Tip March 99

Hot Runner Tool manifolds, part 2

Last month I talked a bit on the hot runner manifold and some key points of consideration. This month let us continue with some points to ponder. How are we to hold that manifold system in place? If we purchase or construct the manifold it is usually placed between the top clamp plate and the 'A' plate. Within this space we have these pieces of steel that are heated so as to allow the plastic to flow. The key here is heated and; the effect this heat has on the steel. As we heat steel up it will thermally expand in all directions. While it thermally expands the manifold will grow and the locations for the drops will align along with the thickness of the manifold to apply pressure to the drops. The drops themselves also thermally expand allowing for growth and the effect of sealing the joint between the manifold and the drop.

The manifold is located by a central pin below the sprue area and one other pin along either the x or y axis. But this pin is only located in one piece with the mating piece being slotted so that as the manifold expands we do not restrict the movement of the manifold. Please note that the larger the manifold system the more important this point is.

The manifold should not be in full contact with the mold base. If examined, the manifold is spaced within the area of the mold with an air gap around it. At areas of pressure such as below the nozzle contact, and the gate drops should be placed standoff to take the pressure and upon thermal expansion apply sealing force to the manifold. If you consider what is going on in the manifold you are able to limit the number of standoffs needed. The construction of these standoffs and the type of materials used are very important. Each manufacture has ones they like and provide; the key here is what is the thermal expansion and thermal conductivity of these pieces. These pieces can actually act as heat sinks within the manifold system thus causing some minor processing issues and temperature control issues also. The compressive strength of the material is very important and must be taken into consideration too.

The other issue I come across in the area of the manifold is that at times the ends of the area are open. This is usually the result of using two rails or a C-Frame similar to the ejector box of the tool. What in effect this can do is create a chimney effect in the manifold area and now we are transferring the heat from the manifold out to atmosphere due to the airflow across the manifold. Some suppliers have even recommended this in effect to take care of the overheating, lack of temperature control, or other inefficiencies
within the system. In some cases maybe it does work, but what are we trying to do? Are we not trying to get as uniform temperature as we can to this block of steel and control, the temperature of the plastic within?

Thanks for the time, to be continued next month.


Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Chemical