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An example of a stepped runner
Definition: runner: a feed system for the flowing of plastic into individual cavities, which is ejected with the parts after it, solidifies.
An example of runner is shown below in the photo. While the runner itself is somewhat large, the gate is somewhat small given that it is a tunnel gate arrangement. The runner is for a family tool, and the sprue bushing may turn which allows for the closing off of branches of the runner from the sprue area. In this case it is only feeding 1 part.
In previous articles the cross sectional shape and the design for balance has been discussed. From a mathematical point this runner has a bit of an issue, as the cross sectional shape of the runner changes as the runner moves from one plane to another in this step runner design.
What is meant is that the sprue and runner is on one plane and then the runner shifts to another plane, approximately a 40mm shift. The runners design is that of a modified trap which has a radius in the bottom. Thus the overall height and width are shown in figure 1. The tunnel gate is 2.2mm in diameter approximately, and the O of the sprue is 7.3mm.
The part produced in this tool was acceptable from the molders standpoint, though they did have some minor issues with the cosmetics and packing of the part on occasion.
From a tooling standpoint the flow of the material through the runner is not constant. Thus we are shearing and possibly causing a pressure drop issue within the runner itself.
The above photo shows the flow of material from the sprue to the gate, which in linear distance is approximately 16 inches long. One can notice the 2 planes, one from the sprue and the other after the thin section; this is a stepped runner design.
Figure 1
The above figure shows the approximate cross section views of the various runner segments as depicted in the runner above it. This starts at the left hand side with the full round sprue O and then the runner on plane 1, and then the thin section, than back to runner in plane 2 and finally the gate cross section.
In simple terms one can just perform simple math. (For simplistic sake all calculation are based on a full round equivalent.)
Area of sprue O is 41.85 mm˛
Area of first level: 42mm˛
Thinnest area of drop section: 12.56mm˛
Area of runner 2nd level: 42mm˛
Area of gate: 4mm˛
Since the runner was working the molder tended to stay with what they had. But had they experienced more issues, than a simple pressure drop on the runner system, mold may have indicated if they needed to open up the thin section of runner, and or by performing shear calculation. Given the volume of material in the part the gate area was the high shear area, and there was a minor pressure drop through the thin area, but not enough to cause an issue with the material used.
SLSILVEY
02092014.01
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