D. Delaunay et al., Nature of contact between polymer and mold in injection molding. Part II: Influence of mold deflection on pressure history and shrinkage, POLYM ENG S, 40(7), 2000, pp. 1692-1700
In injection molding of thermoplastic parts, high hold pressures are set du
ring the packing phase to generate a post-filling, which compensates the sh
rinkage of polymer due to its cooling. The polymer pressure in mold cavity
leads to a cavity deformation due to mold and machine compliance. Then, the
Increase in cavity thickness can modify the post-filling and consequently
the pressure history, the Volumetric shrinkage and the part mass. The first
goal of this paper is to present a simple method to locally determine mold
rigidities: over-packed slabs are injected and local deflections are deter
mined from measurements of the local residual pressure, the local in-plane
shrinkages and the plate thickness. In the studied plate mold, which can be
considered as stiff compared to some industrial molds, a rigidity of more
than 1 mu m/MPa has been measured close to the center of the plate. The sec
ond goal of this paper is to show the influence of mold deflection on dimen
sional properties. If the cavity thickness is small as for our 1-mm-thick p
late mold, considering an infinitely rigid mold cannot do realistic predict
ions of polymer pressure history, volumetric shrinkages and part mass. Neve
rtheless, in-plane shrinkage seems to be less affected by mold deflection.
It means that the additional polymer mass due to mold deflection is mainly
distributed in the part thickness.