Weld strength of injection molded short fiber reinforced polypropylene


Fiber-reinforced thermoplastic, such as fiber-reinforced polypropylene (FRP), is a type of easily moldable thermoplastic used to create a variety of components used primarily in the automotive industry. FRTPs are one of the fastest growing categories in thermoplastic technologies. Basically, their structural properties and low cost per part have enabled FRTPs to replace metal parts in the automotive industry. They are lightweight and possess a higher specific strength and modulus compared to other materials. In industrial setups where mass production is a necessity, injection molding is often used. In parts produced using injection molding, some resin joints must be welded at locations called welds. At welds, the resin is usually oriented perpendicularly to the flow direction. FRTP strength depends strongly on the fiber orientation direction. As such, the weld region strength/weld strength tends to be much lower than it is in other regions; a consequence of the fiber orientation being parallel to the resin.

Consequently, prediction of the FRTP weld strength using finite element method considering the fiber length and fiber orientation has been studied, however, a gap separates the experimental value and the predicted value. Therefore, the factors that determine weld strength cannot truly be identified. In recent publications, certain equations have been seen vital for unravelling the shortfalls encountered in existing theories, nonetheless, some elements of these equations still demand further investigations. Bearing this in mind, Professor Tetsuo Takayama at the Yamagata University in Japan investigated the fiber content dependence on the weld strength of injection molded fiber-reinforced polypropylene (FRP). His work is currently published in the research journal, Mechanics of Materials.

In this view, Professor Tetsuo Takayama prepared FRTPs with polypropylene as matrix, FRPs, by changing the fiber content and fiber type. Next, he evaluated the weld strengths of the injection molded products. Finally, from the comparison of the experimental results with the calculated results obtained using the conventional equation, he assessed the validity of the current theory on the fiber content dependence and modified the conventional equation.

The author observed that the FRP weld strength tended to decrease as the fiber content increased. This tendency was confirmed irrespective of the fiber type. Moreover, he identified that the conventional theoretical formulas were insufficient to predict this trend. Therefore, the conventional equation was corrected to an equation that incorporates the fiber content dependence in the thermal expansion strain term.

In summary, the FRP weld strength dependence on fiber content was investigated. The results obtained suggested a negative correlation between the fiber content and weld strength. In fact, in an interview with Advances in Engineering, Professor Takayama highlighted that the experimentally obtained results clarified that the FRP weld strength could be determined by the residual strain derived from thermal deformation during molding processing and the interaction force between the fiber and the matrix. Remarkably, by adding the term of interface interaction, the researcher was able to reproduced the FRP weld strength dependence on fiber contents.

Weld strength of injection molded short fiber reinforced polypropylene - Advances in Engineering
Fig.1 Specimen overviews of dumbbell shaped specimen. Thickness is 1mm.
Weld strength of injection molded short fiber reinforced polypropylene - Advances in Engineering
Fig.2 Stress-strain curves of injection molded fiber reinforced PP composites with weld.
Weld strength of injection molded short fiber reinforced polypropylene - Advances in Engineering
Fig.3 Comparison of theoretical values obtained by equation and experimental results on weld strength.

About the author

Tetsuo Takayama is a assistant professor in Organic Materials Science of Yamagata University, Japan. He received his Doctor Degree in Mechanical Engineering from Kyushu University, Japan.

He is interested in the mechanical properties of injection molded polymer products. He is trying the modelling including processing temperature dependences about mechanical properties of injection molded polymer products and the designing the properties of polymer by polymer blending and filler compounding techniques.


T. Takayama. Weld strength of injection molded short fiber reinforced polypropylene. Mechanics of Materials, volume 136 (2019) page 103064.

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