The Reality of Core Shift- Is this happening to You?

Core shift is not always obvious or suspected. Recently, Bozilla Corporation was called upon to investigate a part that was warping differently and more than expected. The customer had a flow simulation conducted by a third party and the warpage results did not match the actual part data. Sometimes part warpage does not match the flow simulation and in many cases, it is easily explained. However, after a quick investigation, the underlying cause of the excessive deflection was not easily understood. It was time for our Team to troubleshoot.

(The animations and images presented in this article do not represent the Customers actual part file and is just an example of how core deflection occurs)

Core deflection fill time graph

To begin the investigation, we compared the floor process to the simulation, which is standard operating procedure. They matched fairly well. They are never a perfect match but were very close. We then looked at the part data and tool design then compared it to the data utilized in the flow analysis. The data matched. This was good news because through process of elimination, we were nearing the target.

We then began taking a closer look at the part along with the flow simulation results. We noticed that there were long features extending from the core side of the tool that the polymer had to flow around and down. The features were thin so they did not have cooling in them therefore it was suspected that these long cores were heating up excessively causing the polymer to stress relieve and therefore warp. However, the simulation software accounted for this to some degree and we did not see a trend that suggested the hot core feature was contributing to additional deflection.

Having a long history with examining many polymers and how they behave in varying geometries caused us to take a closer look at the differential pressure within the cavity as it flowed around and along the long core features. We discovered a significant pressure differential that occurred on either side of the core. We also learned that the polymer did not freeze uniformly around that core during the 2nd stage pack process. Having differential pressure and non-uniform freezing threw up a few flags.

Core shift pressure graph

We had to investigate the impact of the differential pressure and non-uniform freezing on these features. We knew it was time for a core-deflection analysis. The customer was fairly confident that the P-20 tool steel was robust enough to resist any core deflection but over the years, we’ve learned that anything is possible when injection molding.

After running the analysis, we discovered that the long core feature actually did deflect! This was good news for us, not-so good news for the customer. We dug a little deeper into the simulation and found that even though the core deflected during the filling phase, it actually moved back to its nominal position. The most critical piece of information is that the majority of the deflection occurred during 2nd stage pack. The pressure distribution combined with the frozen layer variation across the core created a significant pressure increase on one side of the feature. Because the feature was deflecting during the pack phase, the polymer quickly finished freezing and held the core feature in the deflected position. Not good.

The situation of the deflected feature caused a change in part thickness around the feature. The change in thickness also changes the volumetric shrinkage and residual stress within the polymer. These factors are great contributors to deflection. The warpage results of the core-deflection analysis showed the additional deflection. We have now identified the root cause.

The customer was not very pleased with the findings and was a bit concerned about having to redesign the tool or modify the tool with high-strength inserts. We then offered that before making the large expenditure of modifying the tool he could first try to work with the process to reduce the core deflection. He was a bit surprised but eager to hear what we could suggest.

Since the examination determined that the majority of the final core deflection occurred during 2nd stage pack, we suggested reducing the pack pressure and rerun the gate freeze analysis on the floor.

The customer made his adjustments and found that the part was warping a little less and the measured core deflection was reduced by over 30%. While this was not a perfect fix, it allowed the part to meet specification in which case, the customer was delighted.

We didn’t want to burst the customer’s bubble but we had to give him a dose of reality. We had to explain to him that the core shift that is taking place is fatiguing the tool steel and it may break after a significant amount of cycles. We thought he knew this because he wasn’t surprised. He immediately made plans to insert the core feature with a high-strength steel so that he wouldn’t have a failure in the future. This was a great measure to take and we were happy to hear it.

The graph below is a great illustration of how steel and aluminum fatigue. It is obvious that aluminum is not as strong but the important things to point out are that the steel begins to fatigue but eventually reaches a limit and levels off. As compared to Aluminum, the aluminum begins to fatigue and continues on a downward trend and eventually fails. This is critical for anyone to understand cycle limits in tooling when considering both steel and aluminum.

steel and aluminum fatigue graph

The animation below shows the filling pattern of a basic test-tube shape with a long core. It is gated in a typical location but it is not ideal based on the flow as it travels up the length of the core.

Core shift fill time graph

The resulting deflection, shown in the animation below, illustrates how the core deflects. The deflection reaches a maximum during the filling phase but relaxes slightly once the part finishes filling and packing.

core shift displacement graph

As the core deflects, it experiences a tremendous amount of stress. The image below shows exactly how much stress is calculated during the deflection analysis.

Von Mises Stress graph

Many tools in the industry have cores that may have the potential to physically shift or deflect during the filling and packing process. Whether your tool steel is P20, aluminum or 414 stainless steel, core deflection can take place and happen to you and reveal itself in unexpected ways.

For more information, support with your tool design or assistance troubleshooting any of your injection molded issues, contact the Team at Bozilla Corporation.

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 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., 800-942-0742,

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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.