As plastic flows through the
different sections of the machine and the mold, because of drag and frictional
effects there is a loss of the applied pressure at the flow front of the
plastic. Additionally, as the plastic hits the walls of the mold, it begins to
cool increasing the viscosity of the plastic requiring additional pressure to
push the plastic. The skin of plastic that is formed at the walls decreases the
cross sectional area of the plastic flow that also results in the pressure
drop. The molding machine has a limited maximum amount of pressure available to
push the screw at the set injection speed. The required pressure to push the
screw at the set injection speed should never be more than the maximum
available pressure. In this case the process becomes pressure limited. During
process development, knowing the pressure loss in every section helps in
determining the overall pressure loss and the sections where the pressure drops
are high. The mold can then be modified to reduce this pressure drop and
achieve a better consistent flow.
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