Living a Balanced Work Life
Designers and engineers face a multitude of challenges whenever they approach an injection molding machine. One that has persisted for years is the difficulty of getting multi-cavity tools to produce identical parts. While the financial benefit of multi-cavity production is undeniable, rarely has the full financial benefit been realized. In many cases, the quality problems are so great that single-cavity production can be less expensive. So what is the problem with multi-cavity tools? Logic indicates that all one has to do is geometrically balance the runners. Moldmakers do a pretty good job of this, but the problem persists nonetheless. SOLIDWORKS Plastics can balance the runner system to fill the parts evenly.
A brief thought experiment or study in polymer flow may shed light on causes. As an example, follow a sequence of filling and packing in a two-cavity hot- or cold-runner mold. The runner is balanced with identical gate diameters and identical steel dimensions for each cavity. A short shot shows that the polymer fills the sprue and runner evenly. A next progressive short shot shows that even though the flow path is balanced, the polymer flow filling each cavity is not. There can be several reasons for this, such as a slight difference in gate land or venting. SOLIDWORKS Plastics will indicate what sizes the runners and gates need to be.
The point is that in most multi-cavity molds, all cavities do not fill evenly—this is true the world over. The consequence is that parts are not truly identical. Couple this with increasing part complexity and tighter tolerances required for assembly or function, and we have projects running with high reject rates or blocked cavities. Frustration builds, and the finger-pointing begins. Is the part design, resin, mold, or processing at fault? In our imaginary example, the mold-filling analysis does not show the source of the problem—an all too real possibility.
For a partial explanation, take a look at the material, particularly the shear sensitivity of all resins. In particular, look at typical curve plotting viscosity vs. shear rate on linear axes instead of the typical log-log scales. Fill times are included to provide information on injection rates. Short fill times mean high shear rates and long fill times mean low shear rates. Nearly all plastics show this relationship of changing viscosity in response to shear rate. The resin is significantly stiffer at long fill times with low shear rates and flows significantly easier at short fill times with high shear rates. The viscosity difference between the two is huge. What has this had to do with our non-uniform multi-cavity parts?
Now, what do we do? Many people start jacking up the clamp pressure or looking for a parting-line problem or other tool rework. However, all these are a waste of time; nothing helps. By now the importance of delivering uniform flow to both cavities during the entire filling stage becomes clear. What are the chances of processing around this situation? None! You are fighting Mother Nature, the inherent law of physics that pressure will take the path of least resistance.
If this discussion makes sense to you, apply the thought process to molds where you have one or more blocked cavities. Again you are trying to violate a law of plastic flow, and it is not nice to buck Mother Nature. The only answer is to find and fix the reasons for uneven filling in the first place. SOLIDWORKS Plastics can help solve this issue.
Jeff Osman
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