Effect of Rapid Mold Heating on the Structure and Performance of Injection-Molded Polypropylene

Abstract

The tailoring by the process of the properties developed in the plastic objects is the more effective way to improve the sustainability of the plastic objects. The possibility to tailor to the final use the properties developed within the molded object requires further understanding of the relationship between the properties of the plastic objects and the process conduction. One of the main process parameters that allow adjusting the properties of molded objects is the mold temperature. In this work, a thin electrical heater was located below the cavity surface in order to obtain rapid and localized surface heating/cooling cycles during the injection molding process. An isotactic polypropylene was adopted for the molding tests, during which surface temperature was modulated in terms of values and heating times. The modulation of the cavity temperature was found able to control the distribution of relevant morphological characteristics, thus, properties along the sample thickness. In particular, lamellar thickness, crystallinity distribution, and orientation were analyzed by synchrotron X-ray experiments, and the morphology and elastic modulus were characterized by atomic force microscopy acquisitions carried out with a tool for the simultaneous nanomechanical characterization. The crystalline degree slightly increased with the cavity temperature, and this induced an increase in the elastic modulus when high temperatures were adopted for the cavity surface. The cavity temperature strongly influenced the orientation distribution that, on its turn, determined the highest values of the elastic modulus found in the shear layer. Furthermore, although the sample core, not experiencing a strong flow field, was not characterized by high levels of orientation, it might show high values of the elastic modulus if temperature and time during crystallization were sufficient. In particular, if the macromolecules spent adequate time at temperatures close to the crystallization temperature, they could achieve high levels of structuring and, thus, high values of elastic modulus.

Morphology-Mechanical Performance Relationship at the Micrometrical Level within Molded Polypropylene Obtained with Non-Symmetric Mold Temperature Conditioning

“…A small increase in the modulus values in the shear layer, with respect to the CIM sample, has been observed in the sample obtained with 140 °C temperature on the hot cavity side, held for 18 s. However, this effect is even more pronounced in sample 160-18. This behavior can be ascribed to a more efficient structuring achieved by the polymer chains when high temperature of the conditioned side is maintained for long time [ 45 ].…”

“…If the cooling rate is so fast as to freeze the orientation of the flow, fibrils can be found in the solid. If the cooling rate is slow, the molecules can relax the stress induced by the flow and fibrils cannot be found in the solid [ 45 , 46 ]. During the filling stage of the CIM test, the cooling rate and the shear rate are high close to the cavity wall, thus, the fibrils form in these regions.…”

Fused Filament Deposition of PLA: The Role of Interlayer Adhesion in the Mechanical Performances

“…The melting behavior of the materials was investigated in the range 25–200 °C, with different heating/cooling rates. The latent heat of melting of PLA was 93 J/g [ 14 , 15 ].…”

Application of Magnetic Concentrator for Improvement in Rapid Temperature Cycling Technology

“…This can be also used to minimize the reduction in tensile strength in parts, where weld lines are present [ 21 , 22 , 23 ]. Besides, the high cavity surface temperature is commonly used to improve surface quality (removal of weld lines’ presence, obtaining high-gloss surface, better replication of microstructure of the cavity surface) [ 6 , 11 , 23 , 24 , 25 , 26 , 27 , 28 ].…”