Morphology and mechanical properties of polypropylene micro-arrays by micro-injection molding

ORIGINAL ARTICLE

Morphology and mechanical properties of polypropylene micro-arrays by micro-injection molding

Zhen Lu&K.F.Zhang

Received:2October2007/Accepted:19December2007/Published online:28February2008

#Springer-Verlag London Limited2008

Abstract To study the effects of particular process conditions and micro size factor on microstructures manufactured by micro-injection molding,four types of polypropylene micro column arrays are fabricated.The filling performance,morphology,and mechanical proper-ties of these column arrays are investigated by polarized light microscopic,SEM,X-ray microbeam diffraction, nanoindentation,and microindentation.The process pa-rameters are optimized by analyzing the filling perfor-mance of these micro columns under different process conditions.These micro columns represent“skin-core”structure and spherulite size diminishes gradually with the decrease of diameter of micro columns.Micro columns contain bothαandβphase.The hardness and modulus of the same micro column increase from core zone to skin layer.There is no obvious difference of hardness among the micro columns with different diameters.

Keywords Micro-injection molding.Skin-core structure. Spherulites.Micro size effect.Nanoindentation.

X-ray microbeam diffraction

1Introduction

Micro-injection molding(MIM)is one of the most promising technologies for microstructures because of its low production cost,mass production capability,applica-bility for many materials,good tolerance,etc.[1–3].Many micro or micro-structured parts fabricated by MIM have been applied in the fields of information and communica-tion technology,medical and biotechnology,micro-sensor technology,and daily life[4,5].Up to now,the inves-tigation of MIM mainly focused on the mold technology, special machine,production process,filling performance analysis,and numerical simulation[6–8].However,re-search on the morphology and mechanical properties of microstructures made by MIM is deficient.

The unique thermal,mechanical and electrical charac-teristics of polymer micro or micro-structured parts by MIM allow them to be used in many applications[9–12]. The filling performance and morphology of injection molded parts are affected by process parameters and sizes of parts[13,14].Furthermore,the morphology has a pronounced effect on the characteristics(mechanical, optical,electrical,transport,and chemical)of polymer parts.Most polymer parts made by conventional injection molding represent“skin-core”structure[15].MIM has many features differing from conventional injection mold-ing,such as higher injection pressure and speed,higher mold temperature,cavity vacuum,etc.In addition,the sizes of microstructures reduce to micron or sub-micron.How-ever,some polymer crystal sizes are in the range from several microns to several tens of microns,for example,the polypropylene spherulites.The special process conditions of MIM and micro size factor may induce microstructures to have particular morphology and mechanical properties. The micro-array as one kind of micro-structured part is widely used in optics,chemistry,medical fields,etc.In this paper,the filling performance,morphology,and mechanical properties of polypropylene micro column arrays by MIM were analyzed.

Int J Adv Manuf Technol(2009)40:490–496

DOI10.1007/s00170-007-1364-6

Z.Lu

:K.F.Zhang(*)

National Key Laboratory for Precision Hot Processing of Metal, Harbin Institute of Technology,

Harbin150001,China

e-mail:kfzhang@