Influence of scale effect and injection speed on morphology and structure of the microinjection molded isotactic polypropylene parts

Abstract

The morphology and microstructure as well as their forming mechanism of the parts in microinjection molding process are critical. In this work, the coupling effect of scale factor and injection speed on the morphology of the microparts was systematically investigated. Neat isotactic polypropylene parts with thicknesses of 1 mm, 200 μm, and 100 μm were molded at different injection speeds. Polarized light microscope and wide-angle X-ray diffraction were used to inspect the microstructures along the sample thickness. In this way, three kinds of typical morphology were observed in the parts, including typical skin-core structure for the parts with the thickness of 1 mm, noncore shear layer structure for the parts with the thickness of 200 μm, and special skin-core structure with large fraction of columnar crystal for the parts with the thickness of 100 μm. Most interestingly, it was intuitively and straightforward found that the wall slip occurs when the injection speed exceeds a certain value. Specifically, opposite morphological change trend can be obtained when the parts were molded at different levels of injection speeds. Based on these experimental observations, the formation mechanism was proposed to interpret the morphological evolution. Our work provides a new insight for better understanding the morphology evolution mechanism for microinjection molding parts. K E Y W O R D Scolumnar crystal structure, microinjection molding, morphology and structure, noncore structure, wall slip 1 | INTRODUCTION Microinjection molding (MIM) is currently one of the most promising processing methods for fabricating polymeric microparts, such as biochips, microgears, microheat exchangers, and so on. 1-3 MIM process not only reduces the actual dimensions of the molded products, but also involves extreme and complicated thermomechanical history, including high-temperature gradient, extremely high shear rate, high injection pressure, and so on. 4-6 These special phenomena can affect the crystallization behavior of the semicrystalline polymers during the MIM manufacturing process. It is for this reason that the morphologies of the obtained products were different from traditional injection molding. 7-10 As is known to all, morphologies and physical properties (mechanical, optical, electrical, transport, and chemical) 11-14 are closely correlated for polymeric parts. Hence, the first step toward highperformance MIM parts is to thoroughly and systematically investigate the morphology evolution mechanism.At present, researches of MIM techniques have mainly focused on specific injection machines, 13,15 numerical simulation, 16-20 and replicating capability analysis. [21][22][23][24] Investigations of the morphology

Experimental investigation and numerical simulation of the microinjection molding process through an expanding flow configuration

“…The effect of the microinjection molding parameters on the induced morphology and the electro‐mechanical properties represents one of the greatest challenges of thermoplastics forming. Considerable attention was paid over the years to the material structure development under severe processing conditions of the microinjection molding process, 4 and its consequences on the final product features. Jiang and al 5 investigated the effect of the injection velocity on both local mechanical micro properties and the morphology of blends made by polycarbonate/poly(ethylene terephthalate) through the microinjection molding process.…”

Effects of Cavity Thickness and Mold Surface Roughness on the Polymer Flow during Micro Injection Molding

“…However, there are many intractability problems to be solved in μIM [ 13 ], especially the polymer flow during molding, which mainly includes two aspects: insufficient filling and incomplete microstructure replication [ 14 , 15 ]. Both of the aspects are mainly affected by process parameters [ 16 , 17 ], cavity thickness [ 18 ] and surface roughness [ 19 ]. Among these factors, the cavity thickness and surface roughness are not only more difficult to adjust, but are often constrained by product requirements.…”

“…The effect of the cavity size on the micro injection molding process was different, and the effect law was different from the CIM case [ 29 ]. Wang et al [ 16 ] investigated the effect of cavity size on the product microstructure. Three different morphologies were observed in the microstructure of products with thicknesses of 1 mm, 200 μm and 100 μm.…”

Microinjection molding of polyoxymethylene stepped‐parts: Morphology, crystallinity, shrinkage, and thermal characterization

“…6 Moreover, the highly oriented "Shish-kebab" structures dominate the relative thickness, 7 favorable for the enhancement of the mechanical properties of the part. 8 The coupling effect of μIM machine settings 9 and part scale factors 10 on the induced morphology attracts widespread interest. It is commonly known that macropart and micropart exhibit similar skin-core layers.…”

“…The coupling effect of μIM machine settings 9 and part scale factors 10 on the induced morphology attracts widespread interest. It is commonly known that macropart and micropart exhibit similar skin‐core layers.…”