Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 21

Earlier reported studies on RHCM focused mainly on controlling the mold temperature distribution and temperature history (22, 25) as well as the relationship between process parameters and macroscopic properties of parts (9, 22).

Although it is well known that the morphology determines the macroscopic properties of parts, there are only a few studies on morphologies in the RHCM part.

Isotactic poly(propylene) (iPP) is a typical crystalline polymer with polymorphism that is commonly used to study the relationship between the molding process, morphology, and part quality in injection molding (26). A multilayered structure was prepared that was divided into the skin layer, shear layer, and core layer depending on the morphology. In particular, the crystal was highly oriented in the skin layer, there were a large number of fibrous shish-kebab crystal structures in the shear layer, and there were spherulite crystal structures in the core layer.

The thickness difference of the multilayered structures, crystallization and lamellae orientation at different sampling sites or molding and between RHCM and conventional injection molding (CIM) processes also reflected the variations of the temperature and shear gradient during the filling process.

The main difference between these two processes was the different temperature histories in the mold, which introduced various thermal and shear histories to the polymer. Accordingly, the multilayered structures differed from each other, which could be observed from SEM images (26).

In the CIM part, the degree of lamellae orientation was also roughly centro-symmetric, which increased and then decreased from the skin layer to the core layer. The degree of lamellae orientation reached a maximum at the shear layer and a minimum at the core layer. For the RHCM part, the degree of lamellae orientation of the different layers varied slightly, and the degree of lamellae orientation of all layers was less than those in the CIM part. The maximum degree of lamellae orientation was at the skin layer next to the moving half, and it decreased slightly to a minimum at the core layer. Then it increased slowly until reaching the skin layer next to the heated stationary half, at which point the degree of lamellae orientation was less than that of the opposite skin layer. The variation of lamellae orientation was attributed to the different flow and temperature histories of the RHCM and CIM molding processes (26).