2. Residual strains and stresses

2.1 Introduction

The deformation of parts leaving the mold is one of the most difficult problems for tool designers to solve. These deformations are linked to residual stresses. They also affect the mechanical and optical properties of molded parts. Residual stresses can have two origins:

• stresses due to flow during filling and compaction. They do not relax completely before solidification, due to the viscoelastic nature of the material (they would cancel out at the end of flow for purely viscous behavior). Pressure does not cancel out at the end of flow, due to the confinement of the polymer in the cavity. These are called flow-induced fixed stresses;

• thermal shrinkage associated with heterogeneous cooling....