By Masahiro Matsuno, Keeper Company Ltd., Kanagawa, Japan

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Keeper is a manufacturer of rubber products for a wide range of industrial and domestic applications. Its oil seals are made using a compression molding process. The molds are filled with a heated polymer, and one or more walls are moved to compress and shape the final product. Non-uniformities that develop during this process need to be avoided, since they can have a negative impact on the shape and integrity of the product. At Keeper, early attempts to understand the filling process using a structural analysis code failed, so last year, FIDAP was introduced to the company, and simulations using CFD were initiated instead.

Using silicone rubber as the working material, a typical oil seal mold was modeled in a 2D axisymmetric simulation using the volume of fluid (VOF) model. At the start of the transient simulation, a rectangular slab of material was positioned in the corner of the mold. As time progressed, the left mold wall was gradually moved to the right, squeezing the rubber and forcing it to seep out and fill the remainder of the complex mold space. After about 5 seconds, when a preset position was reached, the motion of the mold wall was stopped.

Comparison of the FIDAP predictions for volume fraction of rubber (right) with two sets of experimental images as the mold closes (left and center)

The geometry of the axisymmetric model showing the initial position of the rubber

FIDAP predictions for the shape of the rubber as a function of time were in good agreement with experimental results, with the FIDAP predictions for the free surface location lagging the actual free surface measurements by about 0.5 seconds. Another discrepancy observed was that the order in which certain corners were filled in the experiment was not always the same as the order in which they were filled in the simulation. This difference was attributed to behavior observed in the experiments, but not included in the model, such as the apparent expansion or swelling at the surface of the rubber as it rounds sharp corners and seeps into small crevices in the mold. Because it is not fully plasticized, this swelling occurs during the release of the stress that acts on the rubber at the entrance of the cavity. This phenomenon requires further investigation.

In future work, this and other effects will be incorporated into 3D models with more of the actual geometric features of the compression mold, in hopes of improving the ability of CFD to capture more of the flow details during processing. A transparent die is also being developed so that the entire process can be monitored visually.

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