Flow Induced Crystallization of Poly(ether‐block‐amide) from the Microinjection Molding Process and its Effect on Mechanical Properties

Abstract

Abstract:Crystallization of Poly(ether-block-amide) during microinjection molding is investigated using polarized light microscopy, atomic force microscopy, small/wide angle X-ray scattering and differential scanning calorimetry. Special morphologies of micro dumbbell parts, such as spherulitic or spherulite-free core structures and unique spherulite structures generated beneath the skin layers, are examined and explained relative to processing conditions. Nanoindentation tests confirm that both the modulus and hardness of the skin layer are higher than the core layer, regardless of the core structure. Uniaxial tensile testing indicates that Young's modulus, strain at break and yield stress all increase with an increase of skin ratio. The relationship between process, morphology and mechanical properties are systematically studied for micro products. Additionally, by using in-line process monitoring, flow induced crystallization of polymer filling of a dumbbell part is characterized by shear stress and apparent specific work. By comparison with a "short-term shear protocol", shear stress is validated to be a good candidate to characterize the formation of highly oriented structures under an actual microinjection molding process. This may provide a method for in-line control of morphology development and then final properties by controlling the flow conditions.Additional Information:Question ResponsePlease submit a plain text version of your cover letter here.Please note, if you are submitting a revision of your manuscript, there is an opportunity for you to provide your responses to the reviewers later; please do not add them to the cover letter.Dear Editor, Please find attached extensively revised manuscript that addresses all of the reviewers criticisms and queries, along with Support Material.I trust that you will consider this suitable for full review and ultimately for publication in the journal and look forward to hearing from you in due course. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems CorporationYours faithfully, Email: michael.gilchrist@ucd.ie Michael Gilchrist Powered by Editorial Manager® and ProduXion Manager® from Aries SystemsABSTRACT. Crystallization of Poly(ether-block-amide) during microinjection molding is investigated using polarized light microscopy, atomic force microscopy, small/wide angle Xray scattering and differential scanning calorimetry. Special morphologies of micro dumbbell parts, such as spherulitic or spherulite-free core structures and unique spherulite structures generated beneath the skin layers, are examined and explained relative to processing conditions. Nanoindentation tests confirm that both the modulus and hardness of the skin layer are higher than the core layer, regardless of the core structure. Uniaxial tensile testing indicates that Young's modulus, strain at break and yield stress all increase with an increase of skin ratio. The relationship between process, morphology and mechanical properties are systematically studied for micro produc...

Effects of gate design and cavity thickness on filling, morphology and mechanical properties of microinjection mouldings

“…When cavity thickness is more than 300µm, the skin area reduces to less than 20% of the entire cross-section, similar to cases where process induced crystallization was studied [21]. Yield stress broadly reduces with an increase of cavity thickness, since the skin ratio decreases, as shown in Figure 19 (b).…”

“…Pebax 7233 indicated that the Young's modulus is approximately half that of a standard tensile specimen, strain at break is 30% less than expected from material supplier data sheets, and yield stress is 1.6-2 times higher than standard samples [21]. Such comparison suggests that process induced crystallization and size induced crystallization have a similar level of influence on mechanical properties for microinjection moldings.…”

“…Such morphology is made up of four distinct layers: skin, fine-grained, oblate and core layers. Details on the evolution of such morphology layers can be found in our previous work [21]. The distribution of morphology along the flow direction is influenced by the flow regime and heat transfer, both of which are closely related to the thermomechanical history that polymer melt experiences during its journey to its final position in a molded part.…”

“…The skin layer accounted for ~16%-26% of part thickness. Our recent work [21] systematically studied the effect of process on morphology evolution and mechanical properties of Poly(ether-block amide) (Pebax 7233) based on a 400μm thick miniaturized tensile specimen. It was found that both the thickness of the highly oriented skin layer decreased with an increase of injection velocity and mold temperature, while transcrystalline layers, such as fine-grained and oblate layers, increased with mold temperature; this trend was similar to what occurs in conventional injection molding [22].…”

“…Therefore, experimental gate design and its influence on filling and morphology can give more insight into flow behavior though a micro gate and can assist in gate optimization. Following our previous study on process, morphology and mechanical properties for Pebax 7233 micro parts [21], the present work endeavors to study the morphology distribution in a micro part with special attention being given to 100 µm and sub-100µm features. The effect of the openings and thickness of micro gate on filling behavior is revealed by measuring part flow length and by evaluating the morphology distribution of the part.…”