Abstract
Clear relationships between the semi-crystalline microstructure of injection molding polymers and their mechanical behavior are not yet totally established for all polymers. Part of this relative lack of understanding is because an unambiguous scientific approach is difficult to build up. The processing of samples promotes a microstructure which is not uniform and can be described in various ways on different scales. This introduces uncertainty in the correlations. Most completed studies were conducted on polyolefin, which exhibits an evolution of microstructure that is quite easy to observe and to correlate to mechanical properties. This paper intends to illustrate a more diffuse case. To achieve this, combined characterizations along the flow path and throughout the thickness of a plaque as well as characterizations of the local microstructure and tensile behavior of polyamide 66 are described. The microstructure was explored in terms of skin-core structure, spherulites sizes, crystallinity ratio and lamellae organization. Mechanical properties were addressed with non-monotonic tests with the use of DIC (Digital Image Correlation) to assess true behavior. The effect of humidity is also accounted for. It is demonstrated that small changes in lamellae or interlamellar amorphous phase are likely to be responsible for non-uniform mechanical properties, whereas more macroscopic levels (skin core structure, spherulites level of crystallinity ratio) appeared to be irrelevant levels of description. Consequently, the usual simplified analyses based on optical microscopy and differential scanning calorimetry (DSC) can be inefficient in improving knowledge in that field.
Highlights
Injection molding is a common process for manufacturing thermoplastic parts or samples.it induces a heterogeneous microstructure that responds to heterogeneous thermo mechanical histories during the processing itself.From a technological point of view, injection molding involves three main steps: filling, packing-holding and cooling
A polyamide 66 is considered as an example
As the targeted application is injection molding, we focus on injection-molded plaques
Summary
Injection molding is a common process for manufacturing thermoplastic parts or samples. The specific profiles of the pressure, temperature, strain rate and stress can be estimated by computational simulations and, in some cases, assessed by adequate mold instrumentation This has led to numerous studies in the past (for example [1]) but general features of the relationships between processing conditions and properties are still an open question [2]. Moisture may need to be considered [16,17,18] The goal of this general study is to contribute to the understanding and the modeling of the relationships between the processing and the mechanical behavior of injection-molded polymers. In this framework, a polyamide 66 is considered as an example. The most relevant level of microstructure description is proposed for further and broader study
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