Molding Compounds: Characterization of Flake Reinforced Thermoplastic Composites
Start / End:
April 2012 to April 2016
P.O. Box 770
7500 AT Enschede
P : +31 88 8773877
W : www.tprc.nl
University of Twente
Faculty of Engineering Technology
Chair of Production Technology
P.O. Box 217
7500 AE Enschede
P : +31 (0)53 489 4346
E : m.i.abdulrasheed[a]utwente.nl
This particular short term project is driven by Cato Composites and the University of Twente.
The inherent property of thermoplastics (TP) of being able to be recycled and the increasing production rate of TP composite products; generating a considerable quantum of process scrap, motivates to recycle the process scrap into a product with potential applications in industrial and consumer products. In order to conserve the parent materials’ properties (reinforcing effect of fibers), the recycling is limited to grinding the scrap in to flakes of smaller dimensions. The ground flakes can be compression molded or injection molded into products based on the product complexity and the desired product properties.The end products’ properties can be tailored by the statistical size distribution and orientation distribution of the flakes and is essentially a function of various other parameters like overlap between flakes, agglomeration and interface properties. However prescribing the distributions and arriving at the desired properties is a non trivial task and needs extensive investigation. The method of delivery of the molding compound (mold filling), flow parameters inside the mold, process parameters and the properties of the flakes itself are all interdependent on each other, which creates a need for building a primary knowledge-base on the molding compounds and eventually proceed to a more usable predictive semi-analytical model of the product incorporating the various parameters discussed above.
The properties of the flake reinforced composite (FRC) is primarily determined by the combined properties of the discontinuous dispersed phase and the polymer matrix, whereas the processing parameters influence the arrangement pattern and the orientation pattern of the flakes within the product. The focus of the current research is on the following :
- Strength of FRC - Every application demands a designed strength or atleast a range of strength taking into account of the various uncertainties involved in the material processing. The strength (tensile, impact, bending) of the FRC depends on the uncertainties in the flake distribution and its orientation. This research focuses on the influence of the above uncertainties considered probabilistically on the strength of the composite using fundamental theoretical relations. The results are validated experimentally to develop a predictive strength model.
- Process parameters - Once the transfer function between the desired composite property and the input parameters are arrived, it would become a starting point for investigating the interrelations between the process parameters and the desired output. The optimization of the process parameters with respect to the product properties and the input material properties is of primal importance for practical implementation.
In addition to the generation of a knowledge-base based on various experimental and theoretical analysis of the FRC, the knowledge has to be brought out in a useful medium. The following deliverables are considered as a good starting point for a practical implementation of the attained knowledge in the research process.
- A formulation of guidelines for an effective molding compound design (size distributions, pre-treatements,…) and process design taking in to account the properties of the molding compound and the end product properties.
- Formulation of a dedicated failure model (facilitating statistical inputs for molding compounds’ properties) for finite element analysis.
Recycled FRC:(Scrap to product)
A generalized version of the production process of the recycled FRC is illustrated in Figure 1. The process scrap is ground in a grinding device which chops the parent composite into flakes of a broad size distribution. The flakes are then sieved into different classes of sizes and mixed in the designed proportions for specific strength properties. The mold is filled with the flakes in a molten state or dry state depending on the production process. The mold is closed and the process cycle with the designed parameters (viz. pressure, temperature, cooling rate,..) is executed, finally the formed component is ejected out of the mold.During the process, depending on the existence of a melt flow or a quasi static melt flow the flakes are positioned and oriented (measured and approximated on a statistical basis), the inter-phase properties between the fiber and matrix gets established and a composite is formed. A cooling rate is chosen to avoid the thermal stress induced in the component and to get optimal thermo-mechanical properties of the thermoplastic matrix [1,2].
Fig. 1 A bird’s eye view of the production process of FRC
The mechanical properties of the FRC are determined by the size distribution and alignment of the flakes within the component [3,4]. Experimental analyses of the mechanical properties are carried out with respect to various parameters. Such an experiment to determine the tensile strength is carried out in this case. Figure 2 shows a fractured FRC specimen during a standard tensile test. Observation of the fracture path on the tensile specimens gives valuable information on the damage propagation. Figure 3 shows a scanning electron microscope image of a part of the fracture surface of the specimen in figure 2. Fractography analysis reveals the qualitative interface properties of the fiber and matrix which can be utilized to further narrow down the approach in the modeling process.The results of a series of strategically designed tensile tests can be used to validate the predictive model and to analyse the strength of the FRC using statistical principles.
Fig 2. Fractured tensile specimen
Fig 3. SEM Fractography of fracture surface
Currently work on experimental analysis of the FRC specimens are being carried out with integrated methods to capture the strain field on the surface of the specimen while testing and at the same time an attempt to capture the load at which the cracks within the material get initiated (by using acoustic emission techniques) is also done. These experimental analyses focuses on the development of a predictive model to predict the strength of the FRC as a function of various parameters shown in figure 4. The figure shows the measured parameters and observed phenomenons, which are integrated in the fundamental relations, taking into consideration of the various intermittent models (overlap model, interface model, agglomeration effects,..), put together as a predictive model for FRC. By validating the relations in the model with experimental results, a generalized model for predicting the properties of the FRC can be achieved.
Fig 4. Predictive model
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