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first in, last in?
Definition: Fountain flow: as pertaining to fluid, flowing in the center, falling to the outside. Volume: an amount as measured in cubes. Thought: idea, notion
In the abstract within a molding process the material flows from the nozzle of the material into the mold, via pressure and speed as set on the machine. If the mold is equipped with a hot runner or heated sprue bushing, these additional components and flow channels only act as extensions to the machine and in particular the nozzle.
What is meant by this is that there is no mechanical mixing or extreme shearing being performed on the material, only the maintaining of the temperature so as to provide a flow path. Yes there is some shearing in a hot runner, nozzle etc. that occurs and does in fact affect the balance of flow within a hot runner.
The term fountain flow is used in the filling of the material into the cavities, meaning that the material is flowing from the center and then dispersing out to the side walls. What this means is the first material into the mold, runner system (cold runner) is that material which is in the nozzle. Therefore if one were to calculate out the volume of material in the nozzle of the machine and that material which is in the runner, and or part the abstract is that one now knows approximately where that material was in the barrel, nozzle arrangement.
Also the understanding that between shots, a percentage of the shot material sets in what may be called the heated chamber area, consisting of the area in front of the screw tip but with concern to all material that was in front of the screw at the point of pack forward of the cushion created. Typically the nozzle, but also if used a heated sprue bushing, and or hot runner system.
An example may be that the shot size is 10 cubic inches, and this is calculated total shot weight, and confirmed via the shot size / cushion calculation on the machine. Now let us look at the volume of the nozzle which calculates out to 1.77 cubic inches (.25 diameter by 6" long) and understand that this material just changes, meaning that with a cushion of 0.25" that the material in front of the screw (cushion Nozzle) acts as pressure transference in packing but is not yet into the mold. Thus on the next shot this material which just sat there is now injected into the tool. It has sat in the nozzle for 1 cycle, and either heated, cooled or if lucky maintained its temperature. Under pack the material in the nozzle is forced to the sides of the chamber thus increasing the thermal conductance to this steel.
Now let's say the part is sprue gated and the sprue has a volume of 0.75 cubic inches. The results are that the material in the nozzle of the machine shall flow to form the OD of the sprue and into the part at the sprue junction, while the newer material shall at some point flow through the sprue and center of the part thus filling the rest of the part. Again the material fountain flows, meaning through the center, and rolling out to fill the sides.
In a further abstract now consider the shot volume is 0.5 cubic inches, meaning 3 shots exist in the nozzle alone.
What is important in the above example is the volume of the nozzle in this example. This can be extended in cases where a hot runner and or heated sprue bushing are used in relation to shot volume. Within a hot runner this is referred to as turnover rate, meaning volume of the hot runner versus the shot weight/volume.
Since the material flows into the mold in reverse to what the shot on the machine is, than one may determine where within the shot volume an issue is occurring. In the above example if the part exhibited surface issue at the gate point of sprue to part than the nozzle would be the area to look at. But if in fact it was halfway through the part than back in the barrel of the machine may be the concern. This could be found on the machine by decreasing the shot volume to just fill to said location in the part (as long as shot shots can be produced with no damage). This type of technique would be used if the area of the problem was always in the same location, shot to shot.
Another trick with a hot runner or heated sprue bushing is to place a pellet of a contrasting color in the tip of the nozzle, and then after appropriate time inject material into the mold and look for where this contrasting color exhibits itself. This contrasting colors than shows visually the volume change between the hot runner/ heated sprue bushing and the nozzle.
The ultimate trick would be segment the shot itself with various colors and inject to view the fountain flow of material into the mold. While used for demonstration, and very time consuming, it is very educational especially when all other attempts to define the issue have failed.
The flow of material into the tool is such that the hottest material is always at the end of fill, again given fountain flow, thus if a weld line issue exists, than something other than melt temperature should be investigated, if in fact the melt temperature has been confirmed.
As the production of smaller shot volumes continues especially in custom shops the calculation of nozzle volume to shot volume may be of a greater concern, leading to better control of the nozzle temperature and type of nozzle employed.
SLSILVEY
10092014.01
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