Stiffening of PαMS Mandrels During Curing

Poly(α-methylstyrene) (PαMS) mandrel precursors (a fluorobenzene solution of PαMS surrounding a water core) must be agitated to center the core. The initially fluid PαMS solution stiffens as the organic solvent is extracted, and it eventually becomes brittle; a collision with a stirring propeller will either dent or crack the surface. Dried mandrels often exhibit dents on the surface or cracks in the wall that may result from such collisions. We have studied the deformation relaxation rate of mandrel precursors as a function of curing time. We have found a point in the cure at which the core centering process has stopped but permanent denting has not started, and have found a simple test for this condition. We can use this information to produce highly spherical plastic mandrels by controlling propeller speed as a function of time.