Pellets of lubricated and fully biodegradable thermoplastic wheat starch formulations were produced by extrusion. The lubricants used were a vegetable oil or an internal biodegradable release agent such as magnesium stearate. After being conditioned for one week at 20 °C at 65% of relative humidity, (HR), the pellets were injected to produce ISO/R 527 standardised samples for tensile tests. These samples were conditioned in the same way as the pellets, before determining the mechanical properties and the moulding shrinkage of these fully biodegradable materials. The automatic injection mode for production of samples was only possible with pellets lubricated with magnesium stearate. This formulation is considered hereafter as the reference injection grade. The rheological study in this paper emphasises, all other factors being equal, that the injection parameters depend on the relative humidity of the conditioning atmosphere of the pellets.