The following example demonstrates the power of simulation in optimizing a generic mold, A 3-D geometry of the mold was initially obtained from an after-market U.S. plastics molder in the STEP format. This was then read into a FEA pre-processor for preparation of a computer-based (or virtual) model of the mold in operation.

• Simplification of Mold Geometry for FEA
To reduce the size of model and run time on the computer, the mold shown here was reduced to half by virtue of symmetry. Also, computer analysis ought to start with a component at a time, such as the cavity plate in this example. More parts could then be added to an analysis as components get optimized in a virtual environment (prior to ordering steel or machining parts for trials).

• Material Model for Mold FEA
For design purposes in FEA, only two material constants on tooling material are necessary: (1) the elasticity or Young’s modulus which is the slope of the linear reaction to pulling a coupon in unit of stress; and, (2) Poisson’s ratio which relates to the contraction of coupon when stretching. In this example 29 ksi and 0.3 were respectively used off a materials database for modeling.

• Boundary Conditions for Mold FEA
Here, a “mirror” condition was assigned to the plane cutting the mold in two, to account for the missing half by symmetry. Other boundary conditions include compression only at the base half of the mold besides restraints at the slots. Analyzing a full or portion of the mold with the right boundary conditions is to be exactly the same (except that the reduced-sized model would “run” a lot faster on a computer compared to the full-size model).

• Loading of Mold in FEA
“Loading” a mold consists of press tonnage or clamp force and injection pressure in part cavity, Pressure by the molten plastics could come from a “mold fill” analysis or assumed to be at maximum deliverance of the press, as was the case here. Nonetheless, the analysis being linear elastic, deformations and stresses in the mold will remain proportional to the applied loads.

Results Analysis
While stresses are of prime importance in assessing a mold, when post-processing FEA results, displacements should be looked at first to see if a tooling analysis (static structural in this case) was well set and completed properly.

Color contours of displacements in space showed zero values at the mounting slots and opening of the cavity plate under internal pressure—the highest blue contour corresponds to 0.049 inch in the lateral direction. Transitions from least to maximum displacement contours are gradual and make sense in distribution and magnitude.

Color contours of von Mises equivalent stress (combining all tensile and shear components of stresses in all three directions and plans) indicate that most of the mold—except for the bottom of the part cavity and mounting slots—is below the yield stress of structural steel used in this study. Color contours of deformations, strains and/or stresses can interestingly be plotted on non-deformed or deformed geometries, or in fact, on deformed geometries with non-deformed ones still drawn for reference (to help a mold designer or analyst assess FEA outputted results quickly and efficiently).

Mold Optimization
After examining the results of the model-building stage, the geometry of the original half plate of the mold shown was returned to the FEA pre-processor to easily modify its overall size. The goal was to slim down the block of steel required for the job at a toolshop, speeding its delivery, reducing its machining, easing the finding of a press at the customer for molding the parts, while quickening the heating and cooling of plastics and part in production.

Now assuming the cost of P20 or 4140 heat-treated steel to be at $2.50/lb and a density of 0.29 lb/inch3, the cost savings in this example application is $12,317.00 in steel alone on a volume of 4,247 inch3 per quarter core and cavity plates—an amount that could in actuality be crucial in the quoting stages leading to the selection of the mold shop for the project (locally or overseas).

Still, while the initial investment in a numerical or FEA model development of a mold may take several weeks (that would definitely qualify for government R&D dollars), optimizing a mold (such as the example) on a routine basis should not take more than a working week.