Definition: Hot runner: insulated -runner mold, where runners remain molten. Manifold: receiver of the outflow of the material from molding machines and distributes it to feed multiple out feeds.

In a hot runner system as previously described we have a heated sprue bushing, a manifold and the drops, (the heated sprue bushing has been described, Volume 4 no. 12, 13, 14).

In regards to the manifold there are two types of designs:

                External heating

                Internal heating

With external heating the heating is placed on the outside of the flow channel within or against the manifold block and thus heats or maintains the temperature of the melt stream on the inside of the manifold. The cross sectional flow of material inside this type of heating is free of obstructions and pressure drops are easy to calculate. A solid flow channel of material, think of a bar or rod of material which is solid. With internal heating the heater is place in the middle of the flow channel, thus in direct contact with the melt stream, this leads to higher pressure drops due to the obstruction of the heaters and decrease in the cross-sectional flow path of the materials. Think of a tube or thick wall pipe in which the material is now the wall of said pipe.

Externally heated:

In externally heating of the manifold block the typical or most practical today is the use of tubular heaters, which are placed into a groove or channel cut into the external surface of the manifold. These are placed on both sides of the manifold and the flexibility of this type of heater allows it to be bent and shaped so as to fit the proper path to heat the block and place heat where it is best suited. This type of heater also allows for a tight fit and or the use of heat conduction cement, this thus yields good conduction of heat to the manifold. This heater may be long and complex in its shape and contours, but it typically can heat the whole side of the mold and is only one heater, and controlled by a single thermocouple. The other side of the mold would have a similar configuration and control.

Another way to heat a manifold block externally is with the use of cartridge heaters placed inside the block itself. Thus a hole is drilled though and the cartridge is placed inside. There is a slight loss of heat due to non-contact to the metal, (air gap) and these can only be placed in straight lines, since they are placed inside drilled holes. With this type of heating element an individual thermocouple can be incorporated into each heater, which may allow for a variation in temperature settings to accommodate possible issues. Further there are wide variety of types, and some with tapers which can allow for better fit to the manifold. A down side is removal, dead spots at ends, and care has to be taken with placement.

Other methods of heating from an external manner are the use of heat pipes, cast in place heaters in a solid mountable plate and thick film technology.

In the use of heat pipes and or thermal pins, these are similar to the use of cartridge heater in that they are placed into the manifold block. The issue here is that they must be placed in the horizontal position or vertical position with the heater in the proper position to the tube. If in fact these are not maintained in their originally design position they may not be effective (this refers to how is mounted in press). The word effective is used, as due to their construction they tend to function minimally if installed wrong due to the copper and its heat transfer. These provide uniform heat along the entire path of the tube with the heater element at one end. These can be individually controlled.

Cast in place heaters inside a block, which is than mounted directly to the manifold, allow for a large thermal mass to control the manifold. Thus there is the chance of a more uniform temperature throughout the manifold block. There is concern of loss of heat with this system, thus in some cases the block may have a coating of a lower thermal conductive material on the sides not exposed or mounted to the manifold to prevent or reduce loss of heat.

Thick-film heater technology is similar to the plate heating in that it is mounted directly to the outside of the manifold. This technology can also allow for placement of heating in particular section thus localizing the heating control.

In the use of externally heated manifolds the flow path is free of obstructions and is a uniform path. The heating of the manifold can and should be suited to the material in use and designed in a way to apply a uniform heat where necessary. The placement of the heaters of which ever type and configuration should be matched to both sides of the flow path within the manifold.

Placement of thermocouples is critical and understanding of their placement /location should be understood by the personnel operating the equipment. Distance from tubular heater and or cartridge heaters that are not individually controlled can lead to hot spots and or cold spots within the manifold block, though with a tubular heater since it is one this is limited and may only lead to the fact that the block is not as hot as one would think, or too warm in area of heater. This is where bench testing of the uniformity of the manifold heating comes to play with installing the manifold. It should be noted that as the mold runs and the manifold processes the heat stabilization starts to occur, at what time this occurs can only be determined on an individual mold basis. Knowledge of this is important to proper processing.

Silveysplastics@hotmail.com

SL Silvey

07042012