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Polymers02:34

Polymers

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The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the...
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Plastic deformation represents a fundamental concept in materials science, which explains the irreversible change in the shape of a material when it experiences stress beyond its elastic capability. This phenomenon is important in structural engineering, especially in designing and analyzing cantilever beams—structures that are securely fixed at one end and bear loads at the opposite end. When these beams are subjected to loads within their elastic range, they will return to their...
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Plastic Deformations01:14

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It is essential to understand how structural members behave under plastic deformation when the bending stress exceeds the material's yield strength. This state of deformation permanently alters the shape of the member, in contrast to the linear elastic behavior observed before yielding. The strain at any point in the member is expressed in terms of maximum strain. Notably, the neutral axis, which coincides with the centroid during elastic bending, shifts away from the centroid under plastic...
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When materials are subjected to forces that surpass their yield strength, they undergo a process known as plastic deformation. This results in a permanent alteration or strain in their structure. This concept can be specifically applied to circular shafts, where the deformation leads to a change in its shape. The precise evaluation of this plastic deformation requires understanding the stress distribution within the circular shaft, which is achieved by calculating the maximum shearing stress in...
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Polymers: Molecular Weight Distribution01:10

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For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.
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A Soft Tooling Process Chain for Injection Molding of a 3D Component with Micro Pillars
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Process Induced Skin-Core Morphology in Injection Molded Polyamide 66.

Yvonne Spoerer1, René Androsch2, Dieter Jehnichen3

  • 1Institute of Polymer Materials, Department of Processing, Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Str. 6, 01069 Dresden, Germany.

Polymers
|April 16, 2020
PubMed
Summary

This study examined how the structure of injection-molded polyamide 66 changes from the surface to the core. Using advanced imaging techniques, the researchers found that the outer layer, or 'skin,' has less perfect crystals compared to the inner part, or 'core.' This gradient in crystal structure could be controlled by adjusting the molding process. The results may help improve injection molding techniques to produce materials with desired properties.

Keywords:
X-ray scatteringcrystallizationinjection moldingpolarized-light optical microscopytransmission electron microscopypolymer skin-core structureX-ray scattering analysisinjection molding materialspolyamide 66 crystal structure

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Area of Science:

  • Polymer processing and morphology analysis
  • Materials science within injection molding
  • Crystallography in polymer science

Background:

The structure of injection-molded polymers often includes a gradient between surface and interior layers. Prior research has shown that skin-core structures form due to cooling rates and flow dynamics. However, the exact crystalline morphology and perfection in such structures remains unclear. This gap motivated a closer examination of semi-crystalline morphologies in polyamide 66. Existing studies have focused on macroscopic properties rather than microscopic crystal arrangements. No prior work had resolved the variation in crystal perfection across skin and core regions. This uncertainty limits the ability to tailor material properties through processing. Understanding these gradients could improve product performance in injection molding applications.

Purpose Of The Study:

The aim of this work was to investigate the skin-core morphology in injection-molded polyamide 66. The specific problem addressed is the lack of detailed understanding about crystal perfection and structure gradients. The motivation comes from the need to control material properties through processing parameters. The researchers propose that injection molding conditions influence crystal perfection. They suggest that surface layers differ from core regions in crystallinity and structure. This study was performed to provide a reliable method for mapping semi-crystalline morphologies. The goal is to enable tailored material structures for improved performance. The findings may help optimize injection molding processes for desired mechanical properties.

Main Methods:

The researchers used injection molding to produce polyamide 66 samples with skin-core structures. They analyzed semi-crystalline morphology using polarized-light optical microscopy (POM). Thin sections with thickness around 50 µm were prepared for further analysis. Wide-angle X-ray scattering (WAXS) was employed to detect crystal perfection. Small-angle X-ray scattering (SAXS) was used to examine structural features. The samples were cut at varying distances from the surface to study gradients. The study focused on α-crystal perfection and structure variation. The methods allowed detection of crystal differences in skin and core regions.

Main Results:

The skin layer of injection-molded polyamide 66 was found to be transparent and non-spherulitic. This layer contains α-crystals with lower perfection compared to the core. Crystal perfection increases continuously with distance from the surface. The core region showed higher α-crystal perfection than the skin. These findings suggest a gradient in crystal structure from surface to interior. The study confirmed that skin-core morphology is process-dependent. The results indicate that injection molding conditions affect crystal perfection. The researchers observed a clear transition in crystal structure across the sample.

Conclusions:

The study provides a reliable route to map semi-crystalline morphologies in injection-molded polyamide 66. The authors propose that skin-core structures can be tailored through molding conditions. They suggest that crystal perfection varies with distance from the surface. The findings may help in optimizing injection molding for desired material properties. The study confirms that α-crystal perfection increases in the core region. The researchers propose that process parameters influence crystal structure gradients. This work may guide future efforts in controlling polymer morphology. The conclusions align with the observed crystal structure variations in the samples.

The main outcome is the mapping of skin-core morphology and crystal perfection gradients in injection-molded polyamide 66.

The researchers used polarized-light optical microscopy (POM), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS) to analyze the structure.

The skin layer is important because it contains α-crystals with lower perfection compared to the core, indicating a structural gradient.

WAXS was used to detect crystal perfection in thin sections of injection-molded polyamide 66 samples.

Crystal perfection increases continuously with distance from the surface, from the skin layer to the core.

The findings suggest that injection molding conditions can be tailored to control skin-core morphology and crystal perfection.