Predicting the fiber-breakage history in injection molding

3 April 2018
Chao-Tsai Huang
Simulations of the injection-molding process for long-fiber-reinforced thermoplastics enable the prediction of complex fiber behavior and can therefore be used to investigate the effect of screw type.

In recent years, there has been a growing demand for green, lightweight material technologies, particularly for automotive applications.1, 2 Short and long fiber-reinforced thermoplastics (FRTs), for example, have replaced metal in some components because they are significantly lighter. Furthermore, FRTs can achieve greatly improved mechanical properties compared with other injection-molded products because of the variables in their microstructure (e.g., orientation, length, and concentration).3–6

In particular, long fiber-reinforced thermoplastics (LFRTs) have become one of the most popular materials for lightweight applications. In an LFRT, the fiber microstructure plays a crucial role in enhancing the mechanical properties of the composite.7, 8 However, it is difficult to gain full control over the fiber performance (i.e., to enhance the material's strength) because of the complexity of the fiber microstructure within the matrix. Moreover, when an LFRT is used as a fabrication material in the injection-molding process, the structure and operation of the injection-molding screw significantly affect the residual fiber length (which is inversely proportional to the process-induced fiber-length reduction).

The mechanism behind the influence of the screw on fiber breakage in LFRTs is not yet clear, however. We have therefore investigated the use of different screw designs in the injection-molding process and have considered the fiber-length variation in thermoplastic/fiber composites along the screw during plastication. Specifically, we have used simulations to consider two kinds of screw design (regular and barrier) and two types of fiber (glass and carbon).

For a conventional regular screw—see Figure 1(a) and (b)—there are three zones: the feeding zone, the compression zone, and the metering zone. The feeding zone is the point at which the feedstock enters the screw. The material is then forced into the smaller compression zone, where it undergoes external heating and is subject to increasing shearing forces (as a result of the compression). Finally, the melt is homogenized in the metering zone before injection. In terms of the injection-molding process, we used the following settings in our simulation: filling for 0.74s, packing for 3s, and cooling for 10.6s. The time taken for opening and closing the mold was 5s. The overall cycle time was therefore 19.3s. For the injection-molding material, we simulated the use of a resin from SABIC (ABS Cycolac CGF20). The injection-molding barrel consisted of three zones, with temperatures of 225°C, 235°C, and 245°C, respectively. The screw was operated at 100RPM with a stroke of 34.91mm. The original length of the fibers was 13mm, and we assumed that the material was dropped into the resin from the hopper in the screw. The barrier screw geometry design—see Figure 2—is similar to that of the regular screw. The main difference between the two screws is the structure of the compression zone. In general, barrier screws offer better plastication and mixture efficiency.


Schematics showing screw designs, for use in injection molding. (a) Diagram illustrating the different sections of the regular screw: the feeding zone (I), the compression zone (II), and the metering zone (III). d1: Channel depth in the feeding zone. d2: Channel depth in the metering zone. (b) Schematic diagram of the regular screw design. 11D, 6.5D, and 4.5D represent the length-to-diameter ratios of the screw sections (i.e., the length of the section is 11, 6.5, and 4.5 times its diameter at the feeding, compression, and metering zones, respectively). D: 36mm. The channel depths, which are uniform across the feeding and metering zones but vary across the compression zone, are also shown.


Schematic diagram of a barrier screw with a compression ratio of 7.2/3.0. D: 36mm.

To study how these screw designs affect fiber length as the material travels through the screw, we performed numerical simulations based on a number of theoretical models (using Moldex3D). In particular, we considered the plastication unit in the injection-molding simulation, using the Phelps–Tucker fiber-breakage model. After the fiber residual length was thus predicted, we further integrated details of the fiber microstructures (including orientation, length, and density) into Moldex3D to determine the influence of the microstructures on the characteristics of the final injected parts.9

Our findings regarding the effect of shear rate on the fiber length along the operating direction of the regular screw (filled with glass fibers) are shown in Figure 3. These simulation results show that when the plastic flows from the feeding section to the compression section, the shear rate is significantly increased (from 20s−1 to a maximum of 160s−1 at the end of the compression section). The high shear rate is then retained until the end of the metering section because the channel depth is the same as at the end of the compression section. For the regular screw, we find that 26% of the fiber length is broken in the metering section, 19.2% in the compression section, and 15% in the feeding section. Our results therefore illustrate that the screw design is an important factor to consider, due to its influence on fiber length in injection molding.


Simulation results showing the shear-rate variations and changes to the fiber length along the direction of operation for the regular screw (Screw 1). LW: Weight-average fiber length. R: Shear rate.


Simulation results for the shear-rate variation along the screw length for regular and barrier screws. SR: Shear rate.

We also investigated the shear-rate variation along the screw operating direction for the regular and barrier screws: see Figure 4. Generally, the shear-rate variation of the barrier screw exhibits a similar pattern to the regular screw. In the barrier screw, the highest shear rate occurs in a similar region, but with a much higher shear force. Figure 5(a) and (b) show the average fiber lengths along the screw operating direction for different screw designs. Our results show that the fibers break earlier and more severely in the barrier screw (from the feeding zone to the metering zone), thus reducing fiber length.


(a) Simulation results for the number-average fiber length (LN) along the operating direction of the screw for different screw designs. (b) An enlarged version of the red-dashed box shown in (a).

To further study the fiber breakage behavior, we validated our simulation with the results of Patcharaphun and Opaskornkul,10 and found them to be in good agreement. We then simulated the use of two different fiber types (glass and carbon) in the screw for plastication. The fiber length distribution (FLD) at the end of the regular screw is shown in Figure 6. Although the average fiber length of the glass fiber is greater than that of the carbon fiber, the carbon fiber exhibits a higher peak of FLD. This finding is consistent with the observations of Chen et al.11


Simulation results for fiber-length distribution at the end of Screw 1 for (a) glass fiber and (b) carbon fiber.

In summary, we have investigated fiber behavior in two different screw types using simulations. For a conventional three-section screw used with injection molding, we found that the fiber degradation occurs in every section, but a more serious degradation occurs in the compression and metering sections. Using a barrier screw design is an effective way to enhance the melting process, but the stronger compression force (expressed as a higher shear rate) will damage the residual length of the fiber. Indeed, we found the fiber length of LFRTs that underwent injection molding with a barrier screw system to be 10% shorter than those processed with a regular screw system. We also investigated the fiber breakage phenomena for two types of fibers. Our discovery is consistent with experimental results reported in other recent studies.10, 11 In our future work, we intend to investigate the relationship between the LFRT microstructure and warpage of the injection-molded parts.


Author

Chao-Tsai Huang
Tamkang University

Chao-Tsai Huang is an assistant professor, and received his PhD in chemical engineering from Washington University in St. Louis, Missouri. In recent years, he has focused on special injection-molding technologies—including hot runner, conformal cooling, and microcellular foam injection molding, and multi-component molding (e.g., over-molding and co-injection)—as well as FRTs, and variotherm technology.


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DOI:  10.2417/spepro.006989