Balance continued
In the last article we presented balance and how it is important and how to calculate from an unbalanced standpoint. At the same time we suggested the 5 Step Process® and how it looks at the balance from both a shear function and also steel function.
Again if you want the full story or data check out the website for Beaumont and down load the step by step for the 5 step or purchase the software.
Understanding that naturally balanced means that we have the same flow length and design to each gate is important. We also have to have the same gate at each part, and diameters to the parts. It means that the runner path is identical to each cavity in both length, and diameter.
Understanding this the importance is to understand how the material flows, in that we have a flow channel and different degrees of flow /stress within that flow path. What I mean is that the outside of the flow path is seeing high levels of stress/ shear as the layer between the wall of the runner path and those layer proceeding into the center of the runner path. The center does not see much stress, shear as it is flowing basically being insulated. Understand we are keeping this simple.
Now when we have high shear in an area the material flows easier, (think shear viscosity studies) thus when the material meets a junction such as a corner the material on the wall side flows easier than that in the center. Think of a donut as the higher stress area and the hole as low stress. We now come to a T, if we cut the donut in half and keep the material to the wall we now find that it is only in ½ of the runner. As we come to another junction if the turn is to the same side, now all that material is flowing into that gate. The other gate has material that started in the center and than had some shear applied as it flowed down the other wall. An experiment is to run short shots and see which cavities fill first or most, or if running clear material do not pack out the runner and see which side the vacuum bubbles form in, usually the lowest viscosity material or higher temperature material.
Now in the 5 step process we separate out cavities by groups, these groups are based on where they are located on the flow path of turns.
Example:
An 8 cavity H type runner system. We have 4 cavities on each side of the sprue to main runner, and 2 cavities on the each end of the secondary runner.
Thus we would have material flow flowing to the T and than taking a right and left turn splitting the flow in half. The next section is also a right and left turn splitting that flow in half. So the cavity with a right and right and left and left are basically filling the same and would be of the high shear stress material. The cavities that are right and left and left than right would be of less stress, or filling second. Think position of flow as to how close it is to the wall of the runner, a donut that is the flow is split form 12:00 to 6:00 thus the 12 to 6 section goes right and the 6 to 12 section goes left. At the next turn the 12 to 6 goes right and similarly the 6 to 12 section turns left. Thus the material that has seen the highest stress for the most time is filling similar cavities.
Now that we have segregated the cavities as to flow, we now run a short shot of the tool and than compare the 4 inside cavities by themselves and the 4 outside cavities by themselves (review the last article for the example).
The weight comparison of the 4 inside cavities to each other and or the 4 outside cavities, to themselves would indicate imbalance by steel, as the stress is comparable within these groups, and any weight difference is due to steel differences (part size).
If we average the weight of the 4 inside and compare to average weight of 4 outside this is a shear imbalance. This is due to shear/stress induced via our turns in the molds, related back to shear of materials sliding against itself, as it travels the flow path of our runner. If the cavitation is higher it would result in a greater number of groups / flow paths.
The data generated allows for what is going on within our tool and whether we have an imbalance due to steel, and also what is the imbalance due to shear. As a processor, OEM, or whom ever in the supply chain we wish all parts to be the same. If we have this imbalance it not only affects our part quality, but minimizes our processing window thus affecting the bottom line of the plant. The solution is to investigate the use of the Melt Flipper®, or to live with a minimal processing window and high cost of sorting parts.
Balance is important to even in a single cavity with multiple gates, such as well trays, or other parts especially when folks talk about warpage, flatness etc.
Melt Flipper is a registered trade mark of Beaumont Technologies, Inc.
5-Step Process is Trade Marked by Beaumont Technologies, Inc.
SL Silvey 360-882-3183
