Moldmakers: The First Line of Defense

Over the past two to three years, substantial advancements have been made in injection molding CAE software. What started out as a tool to give designers a general idea of how a simple plastic part will fill, can now accurately analyze packing, cooling, warpage, fiber orientation in any complex part geometry and conditions in the mold.

 

Identifying Problems in the Mold

The goals of flow analysis can range from basic knit-line location prediction to measuring the exact displacement due to anisotropic conditions on a low-tolerance part. When performed early in the design process, users of the technology save thousands of dollars in startup costs, and thousands more by improving part quality, eliminating downtime and reducing cycle times and scrap rates. Flow simulation helps to perfect the part design by reducing or eliminating conditions that may lead to gas traps, burns, sink marks, voids or excessive warp. This is done by optimizing factors such as gate size and location, runner balancing in multicavity tools, mold design including inserts and cooling line circuitry, material selection and process conditions. The technology identifies problems in the mold before they become problems in the part, and the mold designer and moldmaker are the first line of defense in eliminating these costly tool and part problems.

The simulation serves as the perfect medium for trial-and-error techniques that are very expensive and time-consuming to perform on the mold, and must be used early in the design process to gain the most ROI. The reputations of tool designers and builders depend on how well a tool performs when it is placed in the press for the first time, and profits are gained by not having to go back and recut the tool multiple times. The competition in today's market requires vendors to reduce costs while improving quality. Flow analysis allows moldmakers to reduce the mold building cost by 10 to 30 percent, shave weeks off the delivery time and reduce piece-part cost; this is all while improving the quality of the end product for their customer. This competitive edge represents the difference between profit and loss since the days of building in the extra startup costs of time and tool recutting are over. Modern plastic products have extreme performance standards with very strict tolerances, often involving hybrid blends of materials with many additives and stabilizers that make it impossible to know exactly what the final molded part will look like. Without understanding the characteristics of these new materials upfront, the design criteria may be outside the physical scope of cut and try tooling, which in some cases requires a complete rebuild.

 

Midplane Analysis

The most significant breakthrough in flow simulation technology was the advent of true 3-D solid element analysis in 1999. Prior to this, the only way to perform a plastics flow analysis was using midplane technology based on the Hele-Shaw approximation. In a midplane analysis, often referred to as 2.5-D, the part model is represented by a shell of 2-D triangular mesh elements, which are then assigned an appropriate thickness. Similarly, runner systems and cooling lines are modeled with 1-D beam elements. Since each element represents conditions through its entire thickness, many assumptions are made within the predictive software code, which may or may not skew the final results. Extracting a midplane mesh is a time-consuming, arduous and ambiguous process that can take several days, in some cases accounting for up to 80 percent of the man-hours that go into a given flow analysis project. While this approach works well for simple part geometries with uniformly thin walls, it does not capture the true phenomena occurring in the runner system and mold base. Significant accuracy can be lost on parts with a moderate to high level of detail, variable wall thickness and/or thick and bulky areas.

Bulky parts with varying wall thickness cannot be accurately represented with midplane technology and require a more advanced solution. In a true 3-D simulation, solid mesh elements, predominantly tetrahedral, fill the entire volume of the part geometry, without the modifications and assumptions associated with a midplane. This results in a much better representation of the original part file, and therefore much more accurate simulation results. The 3-D meshing process is highly automated allowing users to create a solid element model in a fraction of the time spent generating a midplane. However, 3-D models have far more elements than midplane models and require longer solve-times and more computer hardware. This is a small price to pay, considering that 3-D analyses can run on an unmanned PC overnight and offer significantly better results.

 

The Fountain Effect

Users can observe the true volumetric fountain effect of the melt front, including jetting and gravitational effects. With several element layers per thickness, gradients of results such as temperature, pressure, volumetric shrinkage and fiber orientation can be seen across all the thicknesses. When modeling the true mold base geometry with solid elements, moldmakers can incorporate multiple insert materials, such as beryllium copper, into the analyses. By reviewing outputs such as temperature distribution, cycle-average heat flux and part warpage, users can evaluate the effectiveness of these inserts prior to building the tool.

 

Saving Costs

In addition to identifying and eliminating potential problems, flow analysis helps to optimize overall part and mold design while preventing overengineering. This occurs when too much effort is put into part/mold design. The design often ends up being more complicated, using more plastic than necessary. Flow analysis can prevent unnecessary overengineering by proving that a simpler part/mold design is sufficient. Today's technology allows the moldmaker to evaluate different water layout designs to reduce flow rates and optimize temperature distribution throughout the part and mold, ultimately minimizing differential shrinkage, warpage and cycle time. Different water layout designs refer to the cooling line circuitry in a mold. Without running a cooling analysis first, moldmakers often have to redesign and modify the cooling lines after they prove to be ineffective in the machine.

What effects will changing the tool steel have on the part? Does it need a hot runner system or will a three-plate mold be sufficient? By running a flow analysis upfront, these types of questions can be answered, saving thousands of dollars in unnecessary tooling costs.