Mechanical properties and crystallinity of polypropylene injection molded in polyjet and aluminum tooling

Abstract

Purpose

Rapid tooling has numerous advantages when prototyping injection molded components, but the effects of the tooling on the resulting part properties are often overlooked. The purpose of this paper is to consider the effect of tooling on the final part properties and morphology.

Design/methodology/approach

Digital polyacrylonitrile-butadiene-styrene (ABS) tooling and aluminum tooling were used to mold test specimens from isotatic polypropylene (iPP). Tensile behavior, impact strength, shrinkage, surface roughness and porosity were evaluated for both sets of samples. Additionally, differential scanning calorimeter (DSC) and wide-angle X-ray scattering (WAXS) were used to assess the crystallinity of the samples.

Findings

Characterization of the molded parts showed that slower cooling rates in the Digital ABS inserts promoted the formation of ß-PP, while this crystal structure was not found in the parts molded using aluminum tooling. Additionally, parts molded on the digital ABS inserts exhibited higher mold shrinkage and SEM images identified microscopic shrinkage voids within the material. The change in morphology and the presence of voids significantly affected the tensile behavior with the parts molded in Digital ABS, which broke with little cold drawing and exhibited higher tensile moduli and higher yield strengths.

Practical implications

The results show that the choice of rapid tooling technique plays an important role on determining the properties of the final parts.

Originality/value

Previous studies have not characterized the effect of rapid tooling on the morphology of the molded articles fully or over a variety of processing conditions. This study builds on prior work by using both WAXS and DSC to characterize morphological changes over a wide range of processing conditions and comparing results to mechanical property and shrinkage data.

Citation

Mendible, G.A., Saleh, N., Barry, C. and Johnston, S.P. (2022), "Mechanical properties and crystallinity of polypropylene injection molded in polyjet and aluminum tooling", Rapid Prototyping Journal, Vol. 28 No. 4, pp. 686-694. https://doi.org/10.1108/RPJ-09-2020-0221