**1.2 Injection molding common defects**

*Polypropylene - Polymerization and Characterization of Mechanical and Thermal Properties*

lated by two or more of these components have been proposed.

technologic knowledge and scientist knowledge.

**1.1 Injection molding process**

machine can be observed in **Figure 1**.

toughness, dimensional stability, and even esthetics of PP parts [1]. To achieve this purpose, several particles have been used, such as glass fibers (GF), nanoclay (NC), carbon nanotubes (CNT), and rubber. In a further step, hybrid materials formu-

From an engineering point of view, mixing a polymer matrix with a particle is an effective low-cost way to achieve the required properties when parts are produced by injection molding. However, it is important to understand the way in which particles and processing affect the structure and properties of processed parts. It is important to keep in mind that PP composites' performance depends not only on their intrinsic properties but also on processing conditions. PP is also strongly sensitive to defects produced during manufacturing processes such as injection molding, which deteriorate and decrease a lifetime of composite parts [2]. These defects are even more pronounced in the case of composites. In recent years, a number of texts regarding properties of injection-molded reinforced polypropylenes have been published [3–5]. However, because of the continuing developments of PP composites, the achievable property values are continuing to improve. In structural and semi-structural applications, particularly, in addition to high stiffness and mechanical strength, adequate fracture toughness is often required. In order to optimize these properties, the knowledge of the relationship between morphology and deformation behavior seems to be essential. The understanding of the fracture, micro-deformation, and mechanics of failure of composites is therefore crucial for engineers. This chapter will summarize the relationship between processing and performance of several PP composite micro, nano, and hybrid—injected parts aiming to generate a bridge between

Injection molding of thermoplastic polymers is a repetitive process in which a molten polymer is forced to go through a mold cavity where it is held under low pressure until it solidifies and it is finally ejected. A scheme of injection molding

First stage of injection cycle begins with the molten polymer filling the cavity mold which is closed (filling stage). During filling stage, the screw doesn't rotate but acts as a dashpot which drives the molten material into the mold. At the end of the filling stage, a lower pressure is held by the feeding system allowing a small amount of additional material to enter the mold cavity to compensate the volumetric contraction of the injected part (holding stage). Holding pressure eventually decreases to zero; this defines the beginning of the third stage known as "cooling stage." In this stage, the molten material solidifies inside the cavity, the

**58**

**Figure 1.**

*Injection molding machine scheme.*

Although the main advantage of injection molding process is to manufacture complex parts in a single, fast, and automatic operation, there still are some processing inherent defects such as flow and weld lines which may deteriorate the mechanical performance and appearance of final injected parts. Weld lines are the result of the convergence of several flow fronts during filling stage. The origin of these flow fronts may be due to several reasons: inserts inside the mold cavity, thickness differences in the piece, or the presence of two or more injection points [7]. Weld lines are usually V-shaped as it can be seen in **Figure 2**.

Cross section of the welding plane shows two different zones within the weld line with some particular characteristics: a V-shaped zone where there is almost no molecular diffusion and unfavorable orientation and a central zone with a better molecular diffusion (see **Figure 2**). Weld line is a weak zone from a mechanical point of view and uses to present visual defects too [8, 9]. Weld line performance is determined by material nature, part complexity, and processing variables.

Another common and important injection molding defect is warping. Warping is a macro-geometric deformation of injected pieces which remains after cooling. The main causes of warping are differential contraction between different parts of pieces and released residual stresses formed during cooling stage. These deformations are mainly due to confinement of pieces in the mold cavity, orientation, crystallization, or cooling differential.

#### **1.3 Injected polypropylene**

PP performance—mechanical, thermal, and electrical—depends mainly on its morphology and crystallinity [10]; and processing affects both morphology and crystallinity of polymers. In the case of injection molding, the molten polymer is subjected to thermomechanical complex conditions characterized by high cooling speeds and stress fields. These conditions change along the flow path and mold thickness, i.e., polymer pieces present an intrinsic heterogeneous microstructure, characterized by a gradual and hierarchical variation of morphology, which evolves through the spatial domain of the piece. Injected PP particularly develops a "skin-core"

**Figure 2.** *Cross section of a weld line zone.*

#### **Figure 3.**

*Skin-core microstructure seen by polarized optical microscopy.*

microstructure, which can be seen by polarized optical microscopy (PLM), as in **Figure 3**.

The number of observed "layers" in the microstructure depends on the resolution of the experimental technique used. A simple analysis considers a three-layer model (two external skins and an inner core) [11–13], but other layers may be also observed (two external skins, two sub-skin regions, two shear zones, and an inner core). The intrinsic molecular nature of the polymer together with this layer morphology determines the mechanical performance of injected PP parts.

Besides, adding a second component—particles or additives—into a PP matrix may also change its crystalline structure, i.e., may produce changes in injected piece performance.

Through this chapter, the relationship between processing and performance is reviewed for injected PP composites. The combined effect of the molding process and the fillers on the properties of the polymer composites is reviewed. Also, the effects of the occurrence of inhomogeneities, such as weld lines or flow lines in microstructure and therefore in performance, are summarized.
