Modeling of Foam Flow From Adjacent Pour Regions in Reactive Polyurethane Foaming

Molding of flexible polyurethane foam starting from a reacting mixture of polyol, water and isocyanate is a fast process where the structure is formed in a matter of minutes.  The blowing reaction usually proceeds much faster than the crosslinking reaction: the reacting foam attains gel consistency well before the gel conversion of the crosslinking reaction – see Mora et al. (1991).   There is considerable literature on numerical simulations of the expanding and reacting flow – see e.g. Bikard et al. (2007). However, predictions of unfilled regions near the end of fill were found to be inconsistent with observations in molding of seat cushion foam.

The object of this paper is to track the approach and merging of foam flow fronts from adjacent and separate pour regions in a rectangular mold cavity (see example in Figure). Numerical simulations were carried out by using the VOF method and the finite element software FLUENT with the help of user defined functions for the time varying density and viscosity of the foam.  As a first step, foam flow patterns were obtained for the rectangular mold filling case reported by Mitani and Hamada (2003) with their specifications on density and viscosity and our results were found to be consistent with their results. Next, the flow field near the merging fronts was investigated for varying distance between pour regions. This was done at first with time varying but Newtonian viscosity for the expanding foam and then with a different viscosity function near the approaching fronts.  In particular, conditions that locate an air pocket in the merging region were identified.

Bikard, J., Bruchon, J., Coupez, T., Silva, L., “Numerical simulation of 3D polyurethane expansion during manufacturing process,” Coll. Surf. A 309, 49-63 (2007)

Mitani, T. and Hamada, H., “Prediction of flow patterns in the polyurethane foaming process by numerical simulation considering foam expansion,” Polym. Eng. Sci. 43 (9), 1603-1612 (2003)

Mora, E., Artavia, L.D., Macosko, C.W., “Modulus development during reactive polyurethane foaming,” J. Rheol., 35 (5), 921-940 (1991)

Adjacent pour regions in a rectangular mold cavity at the start of expansion