Injection Molding Plastics Design Optimization

Ahh, the allure and challenge of plastics!

Plastics part designers are facing a variety of issues in today's highly competitive environment. Project timelines may be compressed; requests to scale to high-volume production may happen quickly and unexpectedly; parts may have ever-increasing performance requirements; and mold design, mold making and manufacturing often occur halfway around the world from where a part was designed.

When faced with these pressures, an engineer's natural response may be to pass poorly understood issues along in the product development chain, making their problem Someone Else's Problem. In part one of this series, we characterized Someone Else's Problem Syndrome (SEPS) and described its downstream consequences. We offered cues to diagnose affected organizations and presented tools for its cure. Part two provides details on how to use these tools to prevent and mitigate the effects of SEPS.

We believe that it's important to provide each member of the team with tools that help to manage the design-to-manufacture process.

Simulation and Analysis of Injection-Molded Parts

Moldflow Part Adviser provides a means to quickly check the manufacturability of plastics part designs early in the design process. Once the filling analysis is complete, Part Adviser displays Moldflow's unique "Confidence of Fill" plot. The Confidence of Fill plot filters the analysis data and summarizes the findings in an intuitive "traffic-light" plot. The results are overlaid on the part geometry to indicate the ease or difficulty of filling the mold where green is good and red is bad.

If the Confidence of Fill plot shows areas of concern, the part designer can query it using Moldflow's Dynamic Adviser. Based on the underlying simulation results, the Dynamic Adviser will reveal the explicit cause of problems as well as suggest possible resolutions. Other results available in Part Adviser include: injection time, injection pressure, and visualization of weld lines and/or air traps. Using this process, a part designer can quickly optimize a design for manufacturability, minimizing the possibility of downstream design-related problems. By resolving basic filling problems and injection location issues during the design stage, the part designer gives the mold designer and process engineer greater leeway in specifying final processing conditions.

In a non-SEPS world the mold designer should be able to begin with the knowledge that a part has already been optimized for injection molding. This eliminates unnecessary iterations between part and mold design. Moldflow's tool for the mold designer is Mold Adviser. With this tool, mold designers can quickly create single-cavity, multi-cavity or family-mold layouts. When a mold designer runs a filling analysis on a full system, Mold Adviser's Confidence of Fill plot indicates the manufacturing feasibility of the entire layout including sprue, runners, gates and part cavities.

Other analyses in Mold Adviser help the mold designer determine optimum sizes for runners and gates as well as balance the flow to all cavities in a multi-cavity or family-mold tool. With data on shot size, injection pressure and clamping force, mold designers can provide manufacturing engineers with the information required to accurately specify an injection molding machine for a given mold and part.

The Mold Advisers' design and analysis features enable mold designers to save tool maker and machine operator time and effort, as well as sidestep manufacturing issues that cannot be addressed through other process optimization techniques.

Advanced Analyses

With Cooling Circuit Adviser, mold designers can place cooling circuits, specify their characteristics and analyze circuit performance. This enables optimization of processes for uniform part cooling and minimal production cycle times. Mold designers can import cooling channel data from a CAD system, place channels using Moldflow's Cooling System Wizard, or model channels using tools to place and size individual circuit segments. Target cooling systems can include circular or semi-circular channels, hoses, baffles and bubblers.

After the user places cooling circuits, identifies the inlets to each circuit and specifies a coolant and coolant temperature, they can launch a cooling analysis. Mold designers can use analysis results such as pressure drop through the system, Reynolds number, and flow rate and coolant temperature to help identify inefficient circuits. They may also review part surface temperatures to identify hot-spots during processing, a frequent cause of part warpage, or inspect cooling time plots to identify regions where slow cooling drives long cycle times. With Performance Adviser, users can simulate the packing phase of the injection molding process. The resulting analysis estimates part shrinkage and provides a warpage plot indicating whether a part is likely to deform beyond acceptable levels.

Proper use of this tool enables molding professionals to improve a part's conformance to its original geometric specifications.

Part warpage has a number of different possible causes. Temperature differences from one side of the mold to the other can result in various layers within the part freezing and shrinking at different times and generating internal stresses. Physical property variations induced during processing, inconsistent packing and varying mold and melt temperatures can cause non-uniform shrinkage. Orientation of polymer chains and fillers introduced during processing also affect the part's final geometry. Tools that simulate the packing phase of the injection molding process enable the mold designer to predict whether or not the part is likely to warp or deform beyond acceptable levels.

Moldflow's Performance Adviser allows the mold designer to evaluate various packing profiles to determine optimum packing pressure and time.

With Performance Adviser's Warpage Indicator plot, which is analogous to the Confidence of Fill plot, manufacturers can examine areas where warpage exceeds a user-specified level relative to a chosen reference plane, and scale the deflected shape for better visualization of part deformation.

Issues with process cycle time, part shrinkage or part warpage can be difficult, if not impossible, to resolve at the injection molding machine. With proper simulation and analysis, the mold designer or process engineer can prevent these problems before the cores, cavities and cooling channels are cut into the mold plates.

Conclusion

By incorporating simulation and analysis tools early in the plastics part design and manufacturing process, designers of plastic parts and injection molds can avoid forcing manufacturing engineers to compensate for inherent design flaws.

Given that injection molds can cost anywhere from thousands to hundreds of thousands of dollars (and up!), validating a potential layout before cutting steel is an absolute requirement to avoid unnecessary rework and lost time-to-market. To prevent unnecessary cost increases and dangerous product delays, all members of the design-to-manufacture team must strive to eliminate Someone Else's Problem Syndrome.