Fundamentals of Mold Cavity Pressure Measurement

The Cavity Pressure Curve
At the onset of the filling phase (1) the melt enters the cavity. As soon as the flow front reaches the sensor (2), pressure is registered. The pressure should rise in a near-linear gradient parallel to the duration of the filling time. At volumetric filling of the cavity, the filling phase has been completed (3) and the melt is compacted during the compression phase to ensure the forming of the contours of the mold cavity. The holding pressure phase follows after the maximum cavity pressure has been reached (4). The holding pressure phase compensates for the high thermal contraction of the polymer material – i.e., the reduction of its volume following the cooling down process – by introducing more material. During this phase, up to 10 % of the part volume is being pushed into the cavity. The fact that the molded part starts to cool down and solidify near the cavity wall inhibits the pressure transfer. The melt flow from the screw antechamber to the cavity is slowed down as the viscosity of the material increases and the flow channel becomes more constricted in the process. The progressive solidification of the melt (5) and the progressive thermal contraction causes the pressure within the cavity to drop to ambient levels (6).

The different characteristics of the molten amorphous and semi-crystalline plastics make no difference during the filling phase as long as their viscosities are identical and their compressibility factors are different. With semi-crystalline materials, more melt must be added to the mold in order to increase the pressure at the start of the holding pressure phase than is required with amorphous materials. While the plastic is cooling down under holding pressure, additional melt should be injected into the mold in order to compensate for the volume contraction and to prevent sink marks on the finished part.