Hot Runner: Manifold heating part 2
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 the previous part it was discuss the external heating of a hot runner manifold, meaning that the internal flow path for the plastic material is not restricted, or similar to a full round runner. The other method of heating a hot runner manifold is via the internal type. What this mean is the melt flow channel is cut with a larger diameter and the heating is place inside the channel so as to provide heat directly to the plastic material inside the flow channel.
From the design standpoint the channel is larger in diameter so as to hold the tube which in turn holds the heater cartridge to control the melt stream. This than allows for direct control of each heater cartridge, and does apply heat directly to the melt stream, and not the manifold block though the block itself does heat up.
The downside or issue with internally heated design is twofold. One is that there is a greater pressure drop in the use of the system; this is due to the cross sectional flow channel reduction. The other is there is now a larger frozen layer of material against the wall of the channel, which both decrease the flow path, and raises the pressure necessary to fill parts. The design of these systems also only allows the use of straight paths for the manifold design, as when the tubes are placed inside the manifold they in turn are that straight tubes, thus at junctions of flow there is now an offset in height to achieve the turn and also apply the heat.
As taken from a mold base suppliers catalog:
For 1.25" diameter bores the ID is 0.5" and the OD is 0.87" for the distribution tube.
Doing the math we than can calculate that the flow path, that area between the wall of the channel and tube is only 0.19" thick and thus if we subtract an additional 10 to 15% for cold material on the manifold side we now have a thickness of 0.179 to .1615".
As can be seen from the above the area of flow is thin. This is the reason for such a high pressure drop with these types of system.
In using these type of systems the areas of concern are the pressure drops, or pressure required to push material through them, and does the machine have adequate pressure enough to compensate for a the pressure drop in the manifold , and to fill and pack the part to the desired requirements.
The other issues are at the junctions of one channel to the next, as these are off set and now flow can be from one side of the tube versus the full tube design. Below shows this junction, from one direction of flow to another, it is important to note the offset, and decrease in path volume at these points.
As the above shows the use of internal heaters while giving control of temperature directly to the plastic does have the down side of pressure drop and possible stagnation of material especially at junctions due to the tangent design of flow in the path for the manifold. As stated previously knowing what one has within the mold is critical as to troubleshooting and processing the materials.
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29052012
