Stress Cracks can happen in the plastic, over a period of time, if there is uneven cooling of the plastic during forming. Above is a photo of the shelf of a tub surround ( manufacturer unknown ). The hair line cracks (fractures) are caused when the area of the cracks has cooled more slowly than the rest of the plastic part. Plastic shrinks when it cools. In this situation the plastic part cooled first and hardened. As the area of the cracks cooled, it was put under a tensile stress. It was only a matter of time before cracks would form to relieve these tensile stresses. As it happens, these cracks are in a nasty place because the water on the shelf can drip through the cracks to the wall behind.