Whether it's a single cavity tool, multi-cavity tool or family tool, having a balanced tool is extremely important if part quality and performance are to be achieved.
How do imbalances occur?
In a single cavity tool, an imbalance can occur when one location of the cavity finishes filling while another has yet to be filled. In a multi cavity tool and family tool, the same imbalance may occur within each cavity but an imbalance may also occur from cavity to cavity. These imbalances may be due to gate location(s) on the part, part geometry, or a combination of both. It is extremely difficult to determine how balanced a part will fill before analyzing it in flow simulation software.
How can imbalances cause problems with part quality and performance? Understanding polymer flow will help one appreciate how an imbalance can potentially affect part quality and performance.
As the cavity fills, the flow front temperature must be maintained in order to keep the properties of the polymer consistent throughout the cavity. If there is flow hesitation in any part of the cavity, the flow front will cool down. A frozen layer will form more rapidly in the slower moving regions and form exceptionally fast in regions where flow has stopped.
Once the cavity develops enough pressure to continue filling these hesitating regions, the flow will begin to move once again but it is now much cooler and will create tremendous shear stress as it fills out the remainder of the cavity. IT IS THIS SHEAR STRESS THAT WILL CREATE POTENTIAL FOR PART DEFLECTION AND EVEN PART FAILURE.
If imbalances occur in a multi or family tool, then the entire cavity that experiences the hesitation is at risk for this increased shear stress. The imbalance also causes the hesitating region of the cavity to become non-uniformly packed which translates into non-uniform shrinkage; another precursor to part warpage.
How does shear stress cause quality and performance issues?When polymer experiences shear stress, the naturally 'coiled' molecules are stretched out and frozen into the 'stretched out' position within laminates of the part depending on when and how fast it cooled. Sometimes the stress is so great that it actually breaks the molecules because they are nearly frozen when they are being pushed to fill the cavity.
If the part contains frozen-in stresses and molecules are stretched out or even broken, then the part contains a frozen-in level of stress without having any forces applied to it. Over time, the stress will relieve and can even be accelerated due to heat cycling. The regions in the cavity that stress relieve (typically those regions that hesitated) cause the warpage or deflection of the part. The high stress regions are usually the first to fail when additional loads are applied in the field as the part functions.
Can you see shear stress?
Most of the time, the part will look just fine and there will be no signs of shear stress or any other problems. Shear stress is typically invisible unless the part is so severely stressed that it actually cracks at the time of molding.
How can shear stress be prevented?
The best way to prevent shear stress is to prevent any imbalance in the tool. Imbalance in a cavity or in a multi (or family) tool can be prevented by running a mold filling analysis beforehand. An analysis will provide more insight into the process and the final product so that neither time nor money is lost when creating the tool for any part.
In conclusion, imbalance in a cavity or in a tool can potentially cause part problems such as warpage/deflection or even part failure in the field. Filling imbalance causes shear stress within the part which cannot typically be seen by the naked eye. Imbalances can also cause non-uniform shrinkage. Both stress and shrinkage variation can lead to part warpage.
The best known method to identify and prevent the potential of filling imbalances is an up-front mold filling analysis.
Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs. We invite you to visit our website at www.BozillaCorporation.com.
