Modelling the Uniaxial Rotational Moulding of Reactive Liquid Polymers
Brad Crawford, Niamh Corrigan, John Watterson, Eileen Harkin-Jones
Aeronautical Engineering, Queen's University Belfast
N. Ireland
Keywords: CFD, Rotomoulding, RLP
Historically, the rotational moulding process has been laboured with long cycle times, due to the use of powders or granules that require long melting and cooling cycles. Recently, however, attention has turned to the use of reactive liquid polymers (RLPs), which offer the prospect of significantly reduced cycle times due to the absence of the melting and cooling phases. Amongst the many technological challenges involved in the rotational moulding of RLPs is the need for a better understanding of the fluid dynamics involved in the process.
To this end, the commercial CFD package Flow3D is used to simulate the flow inside a partially filled cylinder that is rotating about its horizontal axis. Flow3D was chosen because it specialises in the Volume of Fluid (VOF) method, which enables us to track the position of the free surface accurately. The CFD results are compared with experiments performed on a uniaxial rig. Initially we model a Newtonian fluid, and then, by customising Flow3D to account for the time-dependent viscosity function obtained from rheological tests, we model the RLP.
As the cylinder rotates, the liquid is subjected to inertial, viscous, gravitational and surface tension forces, resulting in several interesting flow regimes. At the lowest rotation rates, gravitational forces dominate, and the liquid remains in a pool at the bottom of the cylinder. As the rotation rate increases, the liquid is drawn up from the pool, and coats the entire inner surface of the cylinder. At intermediate rotation rates falling waves and hygrocysts (fluid fingers) may be observed. At high rotation rates, centrifugal forces dominate over gravity, and rimming flow is observed, where a smooth film of liquid coats the cylinder almost uniformly. Theoretically, an infinite rotation rate sees the fluid enter a solid body rotation mode, where it rotates rigidly with the cylinder walls.