Polymer/short fiber composites fabricated by direct fiber feeding injection molding

23 April 2018
Xiaofei Yan, Hua Shen, Guangbiao Xu, and Shengbin Cao
Direct fiber feeding injection molding is an effective way of improving the mechanical properties of polymer/short fiber composites by enhancing the fiber length.

Short-fiber-reinforced polymers (SFRPs) enjoy widespread use in the aerospace, automotive, and construction industries, among others, because they are affordable and easier to produce than continuous-fiber-reinforced composites.1, 2 Injection molding is a popular means of manufacturing SFRPs, but high-performance SFRPs require a large fraction of high-aspect-ratio (i.e., longer) fibers.3 The high fiber content in turn leads to fiber attrition (from fiber–fiber interaction, fiber contact with equipment surfaces, or fiber interaction with the polymer matrix) during injection molding, which reduces the reinforcing efficiency of the fibers and, consequently, the mechanical properties of the composites.4

Ways around the problems posed by these high-fiber-content SFRPs include creating pellets by cutting fibers to desired lengths and then injection-molding them to make relatively ‘long’-fiber-reinforced composites with improved material properties. However, the process is complex and time-consuming.5 A faster and easier method of increasing fiber length (i.e., fiber aspect ratio) to improve the performance of SFRPs is direct fiber feeding injection molding (DFFIM).6

In DFFIM, continuous fiber bundles are introduced into an injection-molding machine through the vent hole, which is typically used to release volatile gases emitted by hydroscopic (i.e., water-absorbing) materials (see Figure 1). This process obviates having to pre-compound short or long fiber pellets, which reduces costs.7 DFFIM is an extension of earlier work aimed at improving the quality of parts made by direct incorporation of continuous fibers.8 Where others have focused on characterizing the tensile modulus and tensile strength of SFRPs, we have extended the application of DFFIM from 2D thin plates to 3D materials and have analyzed the structure of the resultant DFFIM products. Here, we describe the structure, tensile properties, and fiber length of different types of composites, i.e., carbon fiber/polypropylene (CF/PP) and glass fiber/carbon fiber/polypropylene (GF/CF/PP) composites made using the DFFIM technique, as well as CF/PP and GF/PP composites made using conventional injection molding.

Schematic drawing of the direct fiber feeding injection-molding (DFFIM) process.

We used a 30-ton injection-molding machine (TI-30F6, TOYO Machinery & Metal) with a vented barrel to make the specimens. All specimens were injection-molded into dumbbell-shaped tensile bars 3mm thick and 10mm wide. To make the GF/CF/PP hybrid composite by DFFIM, pre-compounded GF/PP pellets with 25wt% loading content (GWH42, Sumitomo Chemical) were fed through a controllable feeding hopper with constant speed (50rpm). CFs (Grade TR50S12L, 1200 Tex, diameter 7μm) from Mitsubishi Rayon, used as the hybrid fiber, were guided into the vent of the devolatilizer unit of the barrel and fed into the melt by the shearing action of the screw during plasticization. The DFFIM CF/PP composite was made by inserting CFs through the vent hole and feeding PP pellets (Y101S, Sumitomo Chemical) through the hopper. The conventionally injection-molded GF/PP and CF/PP composites, which served as controls, were fabricated exclusively by feeding pre-compounded GF/PP and CF/PP pellets, respectively, through the hopper. All the composites were made under the same machine processing conditions.

We measured the length of the fibers by burning off the PP, casting the fibers onto glass slides, and then taking optical photographs of the samples and processing the images to mark the fiber length. We determined the tensile properties of the specimens according to ASTM D638. The tests were performed in a universal testing machine (55R 4206, Instron) at a constant crosshead speed of 1mm/min, with five samples for each measurement. An extensometer (Instron) with a gauge length of 50mm was used for the strain measurements.

Figure 2 shows scanning electron micrographs of the CF/PP composites fabricated by DFFIM and conventional injection molding. We observed fiber agglomeration in the DFFIM composites (top and middle rows), especially in the core layer. Conventional injection molding (bottom row) does not exhibit this phenomenon. Where agglomerations occur, almost no matrix can be found between the fibers. In the hybrid composite, feeding all the fibers through the vent hole would result in severe fiber agglomeration. This could weaken mechanical performance, with consequences for the potential application of the materials. That is why we fed the high-performance fibers (specifically, CFs) through the vent hole, and added GF/PP pellets through the feeding hopper. (The advantage in feeding the CFs through the vent hole is that much longer fibers result than would be the case with the hopper.)

Scanning electron micrographs of carbon fiber/polypropylene (CF/PP) composites fabricated using DFFIM (top and middle rows) and conventional injection molding (bottom row).

Table 1 shows the lengths of the different fibers in the composites. The GFs in the DFFIM hybrid composite are slightly shorter than those in the GF/PP composite made by conventional injection molding. The CFs fed through the vent hole in both the CF/PP and hybrid composites are much longer than the GFs. Both the GFs and CFs in the hybrid composite are shorter than in the individual composites owing to fiber attrition.

Fiber length in the different composites: a glass fiber/polypropylene (GF/PP) composite made using conventional injection molding, and CF/PP and GF/CF/PP composites made using DFFIM.

CompositesFiberMaximum fiberMinimum fiberAverage fiberStandard
length (mm)length (mm)length (mm)deviation (mm)
GF/PP compositeGF2.5900.0690.3620.245
CF/PP compositeCF12.2120.1991.6141.094
GF/CF/PP composite {GF1.3660.0530.3580.196

We measured the tensile modulus and strength of the GF/PP composite made by conventional injection molding, and the CF/PP and GF/CF/PP composites fabricated by DFFIM. The GF mass fraction was 10wt% and the CF mass fraction was 12wt% in each composite. The tensile strength and modulus of these composites are shown in Table 2. The GF/PP composite shows higher tensile strength but a lower tensile modulus than the CF/PP composite. The GF/CF/PP hybrid composite shows better tensile properties than the other two. Manufacturing hybrid composites by DFFIM could therefore significantly improve the tensile properties of the materials.

Tensile modulus and strength of the GF/PP composite made by conventional injection molding and the DFFIM composites (CF/PP and GF/CF/PP). CV: Coefficient of variation.

MaterialsModulus (GPa)CV (%)Strength (MPa)CV (%)
GF/PP composite3.7030.57455.4262.171
CF/PP composite5.2070.59344.8623.049
GF/CF/PP composite7.7480.64074.5473.817

In summary, DFFIM is an effective way of increasing fiber length and consequently improving the mechanical properties of composites. However, fiber agglomeration could weaken the mechanical properties of composites fabricated by DFFIM. To further improve the mechanical properties of DFFIM composites, fiber agglomeration should be avoided by distributing the fibers uniformly in the final product. This can be achieved in two ways: first, by feeding the fibers in a dispersed manner instead of in clumps; and second, by improving the interfacial properties of the fibers and matrix by adding a coupling agent to the composite system. As a next step, we plan to work on solving the fiber agglomeration issue in hybrid composites and improving interfacial bonding between the fibers and matrix to enhance the mechanical properties of DFFIM products.


Xiaofei Yan
Kyoto Institute of Technology

Xiaofei Yan graduated with a bachelor's degree from Xi'an Polytechnic University, China, in 2012, and obtained his master's degree from Donghua University, China, in 2015. He is currently a PhD candidate in the Department of Bio-based Materials Science at Kyoto Institute of Technology. His main research interests are focused on composite and textile materials.

Hua Shen
Key Laboratory of Textile Science and Technology Ministry of Education, College of Textiles Donghua University

Hua Shen is currently a lecturer at Donghua University. He obtained a PhD from Kyoto Institute of Technology in 2017. His primary research area is the thermal properties of textiles.

Guangbiao Xu
Key Laboratory of Textile Science and Technology Ministry of Education, College of Textiles Donghua University

Guangbiao Xu is deputy dean of the College of Textiles and deputy director of the Textile Testing Center at Donghua University. His main focus of interest is fiber and textile science.

Shengbin Cao
Institute of Materials Science, Shanghai Dianji University

Shengbin Cao is an instructor. His focus of interest is textile materials.


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DOI:  10.2417/spepro.006928