Kristen Kilroy
Dive into the fascinating world of Autodesk CFD and discover the power of Local Mean Age (LMA)! This innovative tool reveals the hidden dynamics of fluid flow, providing crucial insights into how fluids behave in your designs. By understanding LMA, you can optimize performance, identify stagnation zones, and enhance overall efficiency like never before. Get ready to transform your simulations and elevate your projects to new heights!
When looking for a new car, the vehicle’s appearance is one of the first things that attract buyers. Misaligned side panels, a wavy front hood, or depressions on the bumper negatively affect potential buyers’ interest. These issues portray a low-quality product and increase the chances of losing a sale.
This is one reason why upfront engineering is extremely helpful for many manufacturers within the automotive industry. With so many of the visible components of a vehicle now being created with plastic through injection molding, identifying shrinkage (side panel alignment), warpage (the wavy hood), sink marks (the depressions on the bumper) using Moldflow is handy to help manufacturers achieve that high-quality look they desire.
Part Shrinkage Prediction:
Analysis requirement:
Fill+Pack+Warp
What to look for within the simulation:
Shrinkage is a key influencer to how a molded part deflects, so the process for investigating and minimizing unnecessary shrinkage includes many similar approaches as addressing part warpage. Within Moldflow simulations, the warpage analysis includes analyzing the shrinkage, but enabling the feature, “Isolate Cause of Warpage”, prior to running the analysis will provide more detailed result plots including a set of results called, “Deflection, Differences in Shrinkage”. These plots display the amount of part deflection due to shrinkage variations. In addition to this plot, other Moldflow results including volumetric shrinkage, shrinkage compensation values, and even the fill result. How the material flows through the cavity (orientation of flow) can help identify where more shrinkage may occur.
How to approach minimizing material shrinkage:
Catching excess or unexpected shrinkage through simulation aids mold design. Traditionally, mold designers apply rule-of-thumb values for shrinkage based on general material properties. Moldflow simulation can help make additional steel-safe recommendations to tool designers or provide insights for reducing shrinkage if areas are out of tolerance. Some options for this may include:
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- Modify process settings: Experiment with ranges of process settings, such as hold time/pressure or melt/mold temperatures. Analyze these settings first to determine if processing can resolve issues, rather than making extensive mold changes. However, note that some process modifications may lead to other concerns, such as increased internal stresses. For example, freezing the material with a colder mold may stop shrinkage.
- Explore gating options: If the shrinkage is directly related to flow direction, relocating the gate (or adding additional gates) may reduce areas with excess shrinkage. Running Flow simulations can help identify if this is an option, given the part geometry and gating restrictions.
- Identify high-shear regions: While reviewing Moldflow shear results, they often coincide with high-shrink areas due to the increased orientation as a result of shearing then the relaxation of polymer chains. Adding rounds/fillets, flow directors, or other part features to minimize shearing in the high-shrink regions may be helpful.
- Modify part geometry for uniform thickness: Variations in part wall thickness can result in different shrinkage rates. Adjusting the wall thickness of these regions can be simulated to identify if that is necessary, or if there are processing changes (ie. increase hold time/pressure, adjust mold or melt temperatures). The Moldflow parametric analysis generates a range of specified thicknesses to analyze the best option while comparing to specific quality criteria.
Read more about shrinkage in injection molding: Technical Article: Dynamics Affecting the Shrinkage of Injection Molded Parts.
Part Warpage Prediction:
Analysis requirement:
Fill+Pack+Warp
What to look for within the simulation:
Although Moldflow’s default warpage result plots provide insights into part warping trends, additional plots reveal the full deflection story. Moldflow Insight solves for three main categories that cause part warpage. As mentioned in the shrinkage section, enable advanced warpage result plots before running a Warp analysis by selecting the “Isolate cause of warpage” option. After the simulation completes, review result plots for warpage caused by “Differential Shrinkage,” “Differential Cooling,” or “Orientation Effects.” These plots direct analysts to the main source of deflection. Default result plots like Volumetric Shrinkage and Part Temperature also provide insights into warpage causes.
How to address reducing part warpage:
When reviewing result plots for deflection in Moldflow, identify the main cause of deflection first to reduce warpage. Then, rerun the deflection analysis to see how those changes affect the other warpage plots.
Read more about warpage in injection molding: Technical Article: The Causes of Warpage.
Sink mark evaluation:
Analysis requirement:
Fill+Pack+Warp
What to look for within the simulation:
Moldflow’s sink mark result identifies potential part design or processing concerns by predicting material behavior opposite features like ribs. Review built-in sink mark results (Sink Mark, Estimate; Sink Mark Index; Sink Mark, Depth) to find the probability of sink marks and their predicted depth.
For areas not captured by built-in sink mark results (thick regions and non-rib related areas), use other Moldflow results for insight. One example is the Volumetric Shrinkage result, as observing shrinkage differences can hint at voids or sink marks.
How to approach resolving sink marks:
Optimizing packing profiles may reduce or eliminate sink marks caused by insufficient packing. A virtual design of experiments (DOE) study in Moldflow Insight can identify if packing pressure resolves sink marks. Select sink marks as a quality criteria factor during set-up. Including melt and mold temperatures in the DOE is also helpful, as some temperatures may affect surface strength for sink marks.
If optimizing packing pressure does not suppress sink mark defects, costly and time-consuming tooling modifications may be necessary for an acceptable appearance. In this case, simulating the following options in Moldflow Insight is ideal. This approach allows you to avoid jumping directly into a tool modification that may fix the issue but could also result in additional problems that need to be addressed.
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- Optimize runner system and design: If the gate is unable to stay open long enough to pack the part (the gate is freezing early), increasing the size of the gate or runners may help. This is typically a quick tool fix if it is confirmed to show improvements when reviewing it within the simulation.
- Relocate gate: Position gates from thick to thin. If the area showing sink marks is thicker than the surrounding regions, consider relocating the gate there (if possible). This modification requires more invasive tool changes, potentially delaying the production timeline by several days (if not weeks). Therefore, simulating first is highly recommended to understand how gate relocation will affect other aspects of part quality as well as sink marks.
- Material change: Switching to another material may require additional tool work, but it could offer wider processing windows and different flow characteristics. Simulating a material change helps determine if existing tool geometry (steel safe areas) works for the new material. It also identifies if changes are needed for the cavities, runners, and gates. Every material performs differently, including different grades of the same material.
- Part modification: Reducing thick regions of the part can allow those problematic regions to pack better. Using a parametric study or Design of Experiments (DOE) in Moldflow Insight allows you to quickly set up multiple test simulations. You can then run these simulations in a batch to compare the results. The analyses provide visual comparisons showing the maximum thickness a rib can have to produce an in-tolerance surface appearance (sink mark). Additionally, these results incorporate process settings to show the ranges the mold must operate within to achieve those outcomes.Read more about sink marks in injection molding: Technical eBook: Missing the Mark.