Injection-molded Polypropylene Parts Research Articles

Tribological and Micro-Mechanical Properties of Injected Polypropylene Modified by Electron Radiation

Today, more and more importance is given to the improvement of polymer materials’ wear resistance, i.e., their micro-mechanical and tribological properties, which could widen their application in practice. The properties of materials can be modified by several methods, among them exposure to electron radiation. This study focuses on the effect of varying radiation intensity (15 kGy to 99 kGy), depth of penetration, and subsequent structure modification of injection-molded polypropylene on tribological and micro-mechanical properties. Electron radiation influences the structure of individual layers, thus improving or degrading their properties. Hence, the depth of penetration can be examined by instrumented hardness tests and scratch tests. Due to irradiation, surface properties and wear resistance increased by up to 105% (from 38 MPa to 78 MPa). As the results show, an increase in mechanical properties was recorded in the direction towards the center of the sample (from 72 MPa to 82 MPa). Micro-mechanical tests were also confirmed by the observation of cross-linking changes (gel test) as well as crystallinity increases (wide-angle X-ray diffraction and microtome cuts). This finding could have a significant effect on the manufacturing and subsequent modification of injection-molded polypropylene parts, which opens new possibilities in practice for this material. The increased surface wear resistance enables the use of parts for which the durability and abrasion resistance of the surface are demanded, especially in applications facing exposure to long-term cyclical loads (e.g., gears).

An Approach to the Impact Simulation on Foamed Injection Molded Polypropylene Parts: An Example of Application in the Automotive Industry.

An approach to the simulation of foamed injection molded Polypropylene parts subjected to impact loading is presented in this paper. The proposed method, which considers strain-rate-dependent material properties and the possible occurrence of fracture, is, in particular, suitable for parts manufactured with core-back technology. The method was developed to be used within the functionality of a commercial Finite Element solver using a shell-type element mesh. The material model is based on a three-layer structure, with two compact skin layers and a foamed core layer made of expanded material. The properties of the foamed material are assumed as those of the compact grade scaled by a suitable factor, which is identified via inverse engineering on a set of bending tests executed on specimens having different foam densities. The fracture of the material is then predicted using a damage model which considers the effects of triaxiality. The approach is then validated on industrial parts from the automotive sector, subjected to impact in a component test. Despite the simplicity of the presented approach, which makes this method suitable for industrial applications and especially for early-stage design, the validation shows a sufficiently accurate simulation of part behavior under the impact, with a reasonable prediction of damage and fracture.

Investigation on surface roughness of injection molded polypropylene parts with 3D optical metrology

Investigation on surface roughness of injection molded polypropylene parts with 3D optical metrology

Fiber Orientation and Concentration in an Injection-Molded Ethylene-Propylene Copolymer Reinforced by Hemp.

This paper characterizes and analyzes the microstructures of injection-molded polypropylene parts reinforced with 20 wt% of hemp fibers in order to understand the process induced variations in thermomechanical properties. In-thickness fiber orientation and fiber content were determined by X-ray tomography along the flow. The fiber content along the flow path was also determined by direct fiber content measurements after matrix dissolution, showing an increase of 2%/100 mm for a 2.2 mm-thick plate due to fiber migration during the filling stage. A typical shell/core structure for fiber orientation in injection molding was observed, but with a very clear transition between the layer solidified under high shear rates and the core in which the fiber content was reduced by more than 50%. The orientation of hemp fibers is lower than the one of glass fibers, especially in thickness direction. However, the overall fiber orientation in the injection direction induces significant anisotropic thermomechanical properties, which cannot be explained by simple micromechanical models that consider isotropic mechanical properties for hemp fibers. These phenomena must be taken into account in process simulation codes for injection molding to better predict thermomechanical properties as well as part shrinkage and warpage to design molds.

(Invited) Modeling Morphology Distribution in Injection Molded Polypropylene Parts

The morphology distribution in the final molded object is determined by temperature, pressure and flow, experienced during the process, and their effects on relaxation times, nucleation density, spherulitic growth rate, and the interrelation among these quantities with the deformation rate. The combination of the models developed to describe all these effects becomes a model for predicting the morphology evolution during the injection molding process. This model was implemented in the UNISA code. In this work, samples obtained with a fast evolution of cavity surface temperatures were analyzed and the results were compared with the UNISA code simulations. The fibrillar layer morphology was related to the achievement of critical values of both the molecular stretch and the mechanical work, the latter being performed after the achievement of the critical molecular stretch. The dependence of the morphological layers thicknesses upon the heating conditions was satisfactorily described by the model.

(Invited) Modeling Morphology Distribution in Injection Molded Polypropylene Parts

The UNISA code, a software for the simulation of injection molding developed at the University of Salerno, considers a series of specific models to consider several aspects of the mutual effect of flow and crystallinity: the effects of temperature, pressure and flow on relaxation times, the effect of crystallinity on viscosity, the effect of flow on nucleation density and spherulitic growth rate. Each of the effects mentioned above was experimentally analyzed and described by a model for the iPP grade considered in this work. The combination of all these specific models becomes a model for the morphology evolution during polymer processing. In order to verify the accuracy of the predictions, a series of injection molding tests were obtained applying a system which allows to control the cavity surface temperatures. This technique allows to keep, for assigned time intervals, the cavity surface temperature at intermediate values between injection and cooling channels temperatures. A modulation of the level of the cavity surface temperature and of the time, it was kept active, allowed to control the final sample morphology all the way from the complexity of a standard injection molded part down to a completely (skin and shear layers free) spherulitic structure. The fibrillar layer morphology was related to the achievement of critical values of both the molecular stretch and the mechanical work, the latter being performed after the achievement of the critical molecular stretch. The dependence of the morphological layers (skin, shear, spherulitic layers, the latter including the transition from the shear zone) thicknesses upon the heating conditions was satisfactorily described by the models adopted.

Effect of fused layer modeling (FLM) processing parameters on impact strength of cellular polypropylene

Fused layer modeling (FLM) or fused deposition modeling (FDM) can be used to produce polymer components with a cellular structure. The existence of cells (voids) in FLM parts degrades mechanical properties. This study was done to understand the influence of two printing parameters, layer height (0.1 mm and 0.3 mm) and extrusion temperature (200 °C and 250 °C), on the Izod impact strength of polypropylene (PP). Morphological analysis showed that smaller layer height and higher extrusion temperature generally resulted in smaller cell size but larger cell density. Printed PP components were lighter than injection molded PP parts. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses confirmed that both α and β type crystals existed in printed PP. PP printed at 250 °C had lower impact strength while components printed at 200 °C had similar impact strength to injection molded PP.

A method to improve dimensional accuracy and mechanical properties of injection molded polypropylene parts

Abstract Gas counter pressure (GCP) technology can impose a reverse pressure to melt and thereby effectively increase the pressure acting on the melt at flow front. Theoretically, it has a potential to solve some defects often occurring in conventional injection molding (CIM) process. This paper designed and manufactured a GCP injection mold. GCP injection molding experiments were conducted. Effects of GCP process on melt flow and density, dimensional accuracy, and mechanical properties of molded samples were investigated. The results showed that GCP process can effectively inhibit the “fountain effect” in melt filling process, decrease the dimensional shrinkage of molded samples, increase dimensional accuracy of samples, and effectively improve impact property of samples. For the samples without weld line, tensile strength and flexural strength of GCP injection molded samples are slightly increased in comparison with those of CIM samples, but for the samples with weld line, GCP process can greatly improve the tensile strength and flexural strength of molded samples. When GCP is 9 MPa and GCP holding time is 10 s, the dimensional accuracy of molded samples without weld line, the tensile strength and flexural strength of the molded samples with weld line all increase up to maximum values. In comparison with CIM samples, the dimensional shrinkage of samples without weld line decreases by 17.2%, the tensile strength and flexural strength of samples with weld line increase by 30.51% and 23.69%, respectively. The impact value of the samples molded by process parameter combination of GCP 9 MPa and GCP holding time 20 s is the highest, and the impact value increases by 18.65%.

Anisotropic Wetting Behavior on Injection Molded Polypropylene Parts Inspired by Surface Structure of Moss

During the last years the carmakers interest is focused on polymer manufacturers to develop breakthrough solutions addressing the key drivers of reducing weight and cost. The use of polypropylene (PP) and other thermoplastic olefins is growing because of their low cost and good properties. Unfortunately, some of the same properties that make these plastics attractive to designers, also make them difficult to bond with adhesives or varnish. Non-polar character of PP results in poor surface properties and low wettability. Several methods of surface pretreatments have been used to improve its wettability, but all involves cost extending additional processing. Based on lessons from nature, one of the ways how to increase wettability is surface structuring. Injection mold insert designing and manufacturing process inspired by hydrophilic structure of the moss is presented. Due to cost reducing demands were used conventional processing methods.

Relating microhardness to injection molding induced morphology: A study of microbeam synchrotron WAXD

AbstractThe relationship between microhardness and morphology of polypropylene near the surface of injection molded plaques was investigated. Crystal structures and molecular orientation from the surface to shear oriented layer were characterized by microbeam synchrotron wide angle X‐ray diffraction, polarizing optical microscope and Fourier transformed infrared spectroscopy. From precise measurements, using an X‐ray beam with width of 0.35 μm, the frozen layer was divided into two layers. Molecular and crystal orientations changed greatly in the surface layer but remained constant in the inner layer. These oriented morphology did not affect microhardness along the depth direction on the cross‐cut surface. Microhardness was proportional to crystallinity and showed a local maximum at shear oriented layer and a minimum at the surface. Then, positional dependence of microhardness was compared at the gate and the center of the plaque. Although crystallinity near the gate was lower than that at the center position, microhardness was higher. Therefore, crystallinity mainly affects microhardness but superstructure like cross‐hatching lamellae which is induced in biaxially extended melt flow is also not neglected in injection molded polypropylene parts. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Temperature-dependent influence of molecular orientation and internal stresses on the deformation of injection-molded polypropylene parts

Temperature-dependent influence of molecular orientation and internal stresses on the deformation of injection-molded polypropylene parts