Editor's note: Laura Carrabine is a consultant for Moldflow Corp.
It is well understood that proper design of injection molds is critical to producing functional plastic parts. What is less well understood is how to use simulations to optimize part quality and an operation’s overall profitability.
Designers of high-cost molds have generally embraced injection molding simulation to optimize their designs. However, the same cannot be said of designers of medium-cost molds.
At far right, a simulation identifies circuit flow rate. The near right image shows Reynolds numbers, a measure of laminar, transition, or turbulent flow.
The far left image shows deflection from all effects. At near left is a “stop light” representation (red, yellow, green), identifying locations of warpage in the part.
Using the average mold cost as the criteria, injection molds can be classified into three groups. Molds with an average cost exceeding $75,000 can be considered high-cost molds and comprise less than 10% of all molds manufactured. About 50% of molds cost between $25,000 and $75,000. The remaining 40% of the molds manufactured are considered low-cost at less than $25,000.
The Moldflow Plastics Advisers (MPA) 7.0 software addresses the needs of both medium- and high-cost molds.
Optimizing Molds
Simulation technology enables designers of both high- and medium-cost molds to reduce their reliance on time-consuming and often problem-wrought past project experience.
With the software, users can predict and solve problems in the earliest stages of product development rather than relying on “rule-of-thumb” engineering. Manufacturing constraints can be considered at the same time as form, fit, and function.
The software allows users to create and simulate plastic flow through single-cavity, multicavity, and family molds. Users can optimize gate type, size, and location, as well as runner layout, size, and cross-sectional shape. Analysis results include cycle time, clamp tonnage, and shot size, which help the design team choose the clamping force and the platen size of the injection molding machine and minimize cycle times. One add-on module allows users to simulate multiple phases of the injection molding process and evaluate anticipated molded-part performance. Another evaluates cooling-circuit design.
Packing and Cooling
One module simulates the packing phase of the injection molding process to predict and minimize undesirable part shrinkage, and also provides an indicator that shows if a part is likely to warp or deform beyond acceptable levels.
Packing, the second stage of the injection molding process, holds the key to achieving the right balance between part quality, part cost, and cycle time. Mold designers can set up and evaluate packing profiles to determine the optimal packing pressure and duration of packing.
Using the packing analysis results, mold designers can identify areas of high, nonuniform volumetric shrinkage that could contribute to part warpage and view the distribution of cooling time to identify areas that dictate cycle time.
The warpage indicator analysis shows the deflected shape of a part—a valuable tool in visualizing the part shrinkage and warpage. Mold designers can also scale the deflected shape for better visualization of part deformation. Using this tool to view the net shape of the part, mold designers can evaluate specific areas of the part that need to be within specified warpage levels. The warpage indicator result is a traffic-light (red, yellow, green) plot that highlights the areas where part warpage exceeds a user-specified, acceptable warpage level relative to a user-specified reference plane.
Using this tool, mold designers can evaluate whether changes made to the part or mold design, or to the material or process conditions, will bring the part warpage to within acceptable levels. The cooling module simulates the cooling phase of the injection molding process so that users can optimize mold designs for uniform cooling and minimum cycle times.
Mold designers can leverage several options to design their cooling circuits, including importing from a CAD system, using an automatic wizard, or using modeling tools that are integral to the cooling module.
The cooling circuits can incorporate circular and semicircular channels, hoses, baffles, and bubblers. After the cooling circuits are laid out and cooling entrances to each circuit specified, mold designers can launch a cooling analysis. Indicators such as pressure drop, Reynolds number, flow rate, and coolant temperature can be used to help identify inefficient circuits.
Part-surface temperature is useful in spotting nonuniform cooling patterns that can potentially induce warpage in the part. In addition, the software features a tool that helps users estimate the total job cost by considering expenses including resin costs, mold manufacturing costs, molding machine operating costs, and the cost of post-molding operations.
The software automatically generates Internet-ready reports to facilitate communication among all members of the design-through-production team, including those at remote locations. Using these reports allows early review and feedback from all parties involved in the part and mold design.
Contact information
Moldflow Corp., Wayland, MA
Laura Carrabine
(508) 358-5848; www.moldflow.com(end)
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