Flow profile in injection moulding cavity can also be evaluated from the moulded product. When molten polymer is forced into the mould cavity, the polymer molecules will experienced stresses and deformation from the processing history and/or setup, and this can be investigated using optical birefringence technique. This technique is conducted using polarized light and known to be used for transparent polymers. Examples of these polymers are polystyrene (PS), polymethyl methacrylate (PMMA) and polycarbonate (PC). An example of the use of birefringence technique to evaluate flow profile in the vicinity of a weld line is shown below.
The following is another example where polarized light (via optical microscopy) was used to follow flow behaviour of molten polymer.