Solidification under Pressure
During injection molding, thermoplastics pass through several physical states. Upon ejection from the mold, the material cools and makes a transition from the molten state through the elastic state, finally ending up in the solid state. These transitions are accompanied by local temperature and pressure differences that vary with time and can lead to inhomogeneous, anisotropic shrinkage and internal stresses.
The Plastics Engineering Department (LKT) in Erlangen has developed a novel process in which the plastics resin solidifies at a constant temperature solely through use of pressure. The transition from the molten state or elastic state to the solid state (glass transition point) thus occurs at the same time and uniformly throughout the entire part. As a consequence, the parts satisfy the most demanding requirements regarding dimensional accuracy and optical characteristics.
The temperature of the mold cavity is initially above the melting point of the resin. The cavity is filled without applying holding pressure, so that the melt does not solidify anywhere. This means that the orientation that occurs during injection of the melt into the cavity can relax completely. Next, a high pressure is applied to the melt, causing it to solidify upon passing through the solidus line (glass transition point). The molded part cools under uniform pressure and subsequently relaxes at ambient (room) temperature. This technique precludes the simultaneous existence of the liquid and solid states.
Good prerequisites for pressure-induced solidification are exhibited by PC, cycloolefin polymers (COP) and microcrystalline PA, since the glass transition point in these thermoplastics has a very strong dependence on pressure. Because of the relatively slight pressure dependence of the glass transition point in PMMA, it is less suited for this process.
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