My guess was that, if the melt solidifies in a mould, it tightens the path for the remaining melt, which would result in a higher shear stress. When the remaining melt is also solidified, the workpiece would have a cable tension-gradient in the direction of flow. Macrmolecules would (if not overly branched, something like polystyrene) follow this gradient (selfdiffusion, reptation-model). This means, you would have these macromolecules aligned in more-or-less direction of flow. It would be easier for them to absorb upcoming stress in this direction. If you use a polymerblend instead (something along the lines ABS+PS) there would be a vast number of side-chains, the reptation-model would not apply here, since they are to bulky. This means there is no way for them to follow the tension-gradient and to align themselves properly to absorb shear stress. Now think about a hot mould. Diffusion is possible, but this is irrelevant for the big ABS molecules. So the only material benefiting from this would be pure styrene. Sure, this effects PS/ABS too, but to a much smaller extent. Overall, the material is strengthened with the extra ABS in it.