The other week, I received a multiple part question regarding many aspects of materials, this is one part…
Although the 1st stage injection time is the same, the virgin PC/ABS (<0.06% moisture) has a 1RMPa higher peak injection pressure than virgin PC/ABS (0.08% moisture). Is this change in pressure common?
Moisture in the polymer causes hydrolysis to occur. As the water molecule is heated, one of the hydrogen atoms breaks away and both the H+ and OH- ions look for pairs along the polymer chain. In hygroscopic polymers such as PC/ABS these ions break up the long polymer chains resulting in a lower average molecular weight. This drop in molecular weight results in a drop in overall material viscosity.
Because water has a molecular weight of only 18, it alone acts as a plasticizer which reduces the overall viscosity of the material. This is one reason we often recommend molders of non-hygroscopic materials ensure they remove surface moisture from their materials.