A modified rotational moulding process is discussed and analysed for polymer liner production in composite overwrapped pressure vessel (COPV) designs. An integrally heated mould tool contained within the rotating axes was used to produce breadboard polymer liner components. The integrally heated tooling provided improved control of heating parameters which in turn increased the dimensional accuracy of the part while also reducing energy consumption. A square-shaped mould with flat sides was constructed to provide samples for wall cross sectional analysis. Processing parameters such as temperature profile and cooling rates on the liner formation have been analysed with respect to typical issues encountered with moulded parts. Trials have been conducted with polyethylene in similar processing environments with different temperature cycles, hold times and cooling rates to analyse the formation of voids and void content percentages. Microscopy has been used in conjunction with image processing software to characterise void contents in liner samples. Results have shown that increased hold temperatures have reduced void content percentages and simultaneously increased void size. High percentage air void contents in polymer materials have been shown to increase permeability as they act as leakage paths for permeation; hence, reducing air void contents for liner applications is of paramount importance. Finite element modelling of diffusion processes in sample liners has also been undertaken with varying levels of void content densities present. By varying the void content density the mass flow rate through the liner can be assessed and values for mass flow rates through prospective liners were found.