The Affect of Moisture on Injection Pressure…

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?
My Response
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.
Additional Thoughts
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.

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