Part 2: Can Moldflow Validate the Quality of your Part?
In Part 2 we will continue our discussion of utilizing the Moldflow software to ensure the part does not fail in the field. (Part 1: Can MoldFlow Validate the Quality of Your Part?)
Is the gate in the best location?
In many cases, the customer provides a part with a suggested gating location. The gating location is sometimes determined during the part design process and is designed based on the assumed gate location. However, the pre-determined gate location may not always be the best location for several reasons:
- The gate location may not provide a balanced filling pattern which can cause severe velocity changes to the flow front or an undercut scenario where the flow direction changes after the frozen layer on the wall has been established which causes tremendous shear within the part laminae. Both of which can cause part deformation or even failure.
- If the gate location creates an imbalanced filling pattern it can create a pressure spike which will also create a clamp tonnage spike. Both of which lead to issues mentioned in aforementioned bullet.
- The gate location may not be in the best location with regards to the material freezing back to the gate which is essential for proper part packing. Some regions of the part may not be able to be packed due to a poorly selected gate location. This can cause excessive volumetric shrinkage in isolated regions of the part which can lead to sink marks or even voids. In the images below there is a wheel for a drawer carrier which is failing due to internal voids. There is a cut-away showing the voids which are revealed in the analysis where the high volumetric shrinkage shows the regions where the voids can potentially occur:
Has the tool design been verified?
This may seem like a redundant question but there are times when the tool may undergo some design changes which may affect the outcome of the part design such as:
- Cooling inserts added
- Rerouting of cooling lines
- Cavity orientation changes
- Feed system changes (perhaps due to cavity orientation changes)
- d possibly other tool design changes
Many businesses integrate a final tool review before or at the initial trial run, so if this is not standard procedure it is highly recommended. If anything HAS changed, the mold filling analyst will need to re-optimize the process based on those changes.
Has the material been qualified/verified? (Keep the material supplier involved)
This is another point that is typically not considered when trialing a tool but is absolutely critical. We have had several customers reach out to us with various conditions of part failure only to find that the batch of material that they were utilizing has changed and was compromised.
The material could have a specified glass content that may change from batch to batch. In some cases, the chemical binder used for binding the glass fiber to the polymer may be re-formulated and cause a situation where the polymer can bond to the tool resulting in a part that does not release properly during ejection.
We always recommend having the material supplier closely involved when both trialing a tool and during initial production start-ups.
Once an analysis has determined the optimum processing conditions for the part, the part will naturally contain inherent stress from the molding process. It is this stress that may be the ultimate cause of failure in the field and must be kept to a minimum.
Frozen-in stress coupled with functional stress imparted during usage may cause the part to fail and this must be headed off before the parts ever make it to the field.
This is when it’s vital to run a post process using FEA.
Perform post-molded CAE performance tests utilizing real-world stress loads to ensure the parts do not fail.
The Autodesk Moldflow software has the functionality to export the in-molded stresses from the injection molded process. This allows the user to import those stresses into a CAE software such as Ansys or Nastran which will allow them to run specific performance tests on the part while considering the in-molded stress from the injection molding process. The process of exporting the data and running the post-molded tests in a CAE software can be both time consuming and costly, therefore the risk/reward ratio should be considered.
Should you forego this testing? Perhaps, but what if the part fails in the field? Part failure could have easily been discovered and mitigated with this testing process. Again, if possible, we highly recommend performing the post-molding CAE performance tests in order to further increase the potential for part success in the field.
Because the frozen-in stress is accounted for in the analysis, the analysis may reveal that the part would fail when it normally wouldn’t have failed if the frozen-in stress was not accounted for. The inherent frozen-in stress due to the injection molding process is crucial data to consider in order to create a successful part.
Bear in mind that each one of the points discussed should be performed, at a minimum, on a first-generation tool. Engineers should consider conducting an analysis on a second-generation tool considering as many of these points as possible as an ‘insurance policy’ moving forward.
We have touched on a few of the key points of how to remedy the potential failure of parts in the field but there are many more to consider. We would like to emphasize the importance of reviewing these points in order to mitigate part failure. Once the tool is at the initial production run, it is very costly to make any changes that will influence the quality of performance of the part in the field.
Bozilla Corporation has performed this unique set of analyses and identified many potential causes of failure long before the parts were ever manufactured.
Bozilla Corporation’s Plastics Injection Molding Team has over 20 years of experience analytically and on the floor. We specialize in optimization, consulting, engineering, troubleshooting, mentoring and Autodesk Moldflow software training. Additionally, our plastics engineers have a full understanding of polymers and how they influence an injection molded part. Your success is our success. Our skilled Team is focused upon meeting the goals and timelines of our customers.
About the Author:
Chris Czeczuga is a Plastics Engineer, Injection molding expert, Military Veteran and the President of Bozilla Corporation. He has proven success with many Fortune 500 companies throughout the injection molding industry. A graduate from UMass Lowell, he is Expert Certified with Autodesk, has 20+ years of field experience, intimate knowledge of injection molding part, tool and feed system design. Bozilla Corporation’s success is built on providing the highest level of injection molding simulation and consulting advice to businesses who have short lead times, require an efficient, cost-effective molding process, and desire to produce a correct part the first time.