Using Non-Isothermal Resin Transfer Molding Simulations to Control Rheological Properties

Vacuum resin transfer molding, or VARTM, is a common and low-cost process used for developing wind turbine blades. In this process, vacuum is applied to allow the flow of resin into a fiber mat preform in a single-sided, closed mold induced by the variation between in-mold and external atmospheric pressure. The VARTM is a highly efficient process because it can produce composites that have a fiber volume fraction of 60% or more. Therefore, this process can be used for manufacturing high-quality products.

Since the molding cycle is longer, slow-reactive and low-viscosity resins are typically selected to guarantee a proper filling without early curing at the time of the filling stage, wherein the resin’s viscosity will be influenced by the shear stress rate, temperature, and curing rate. Moreover, as the curing rate continues to increase with time to the point when the viscosity is sufficiently high for the part to be ejected, the resin can be perceived to be fully cured.

Challenges Faced with VARTM and Solution

However, issues like unanticipated product deformation can occur if the resin is ejected before it is fully cured. In order to overcome this problem, engineers should perform careful material characterization testing to fully understand the resin’s curing reaction and the rheological properties, and subsequently use Moldex3D’s non-isothermal, true 3D simulation technology to estimate how the VARTM process is affected by various thermosetting resins. Discussed below is a case of the exterior cover of a carbon fiber wind turbine blade. Moldex3D’s non-isothermal, true 3D simulation technology will be applied to study how different resin materials affect the curing and filling behaviors.

First, the accurate rheological properties and curing behaviors of Resin A and B are obtained by performing a material characterization testing. This is followed by studying the changes of the curing and the viscosity behaviors of Resin A and B at 25 °C and 75 °C within a timeframe of 4 hours.

Results

A comparison is made between the results attained from material characterization testing and those obtained from Moldex3D’s Material Wizard, and based on the observations, it was seen that both these results display similar trends. In other words, the viscosity and curing rate of Resin B are both lower when compared to those of Resin A under the same reaction time (see Figures 1 and 2).

The curing rates of Resin A and B at 25 °C and 75 °C.

Figure 1. The curing rates of Resin A and B at 25 °C and 75 °C.

The viscosity of Resin A and B at 25 °C and 75 °C.

Figure 2. The viscosity of Resin A and B at 25 °C and 75 °C.

In this case, a 1-kW wind turbine blade of a one-sided mold is used as the model. Figures 3 and 4 show the fiber layer and the design layout, respectively. The curing process starts when the filling reaches 99.8%. At this stage, to speed up the curing process, the temperature of the mold is increased from 25 °C to 75 °C (Figure 5), allowing the resin to cure faster and the part to be ejected sooner.

The geometry of the part and the resin injection design.

Figure 3. The geometry of the part and the resin injection design.

The distribution of the sensor nodes on the outer cover of a wind turbine blade.

Figure 4. The distribution of the sensor nodes on the outer cover of a wind turbine blade.

The mold temperature settings.

Figure 5. The mold temperature settings.

The viscosity, curing rate, and curing time of Resin A and B at different stages are shown in Table 1. As demonstrated, Resin A has a longer filling time and higher viscosity, but has a shorter curing time. On the other hand, Resin B easily fills the mold but has a longer curing time. The total processing time of Resin A is 2.5 hours, and that of Resin B is 2 hours.

Although there is just half an hour difference in the total processing time, their filling and curing times demonstrate a substantial difference between the two resins. In simple terms, Resin B has more processing benefits when compared to Resin A because of its lower viscosity and easy-to-fill properties. Therefore, it can be concluded that before you start to design, the material properties should be properly understood to eliminate unwanted trial and errors.

Table 1. The curing rate, viscosity, and filling time of Resin A and B at different stages

Resin A Resin B
Before Filling
Curing Rate 0% 0%
Viscosity 710 cps 154 cps
Filling Stage
Time 1 hr 27 min 17 min
Curing Rate ~11.5% 0%
Viscosity 1300 cps 154 cps
Curing Stage
Time 1 hr 1 hr 44 min
Curing Rate 75.00% 81.40%
Viscosity Completely Cured
 (1.3×109 cps)
Completely Cured
 (2.5×108 cps)
Total Processing Time (Filling + Curing)
Time 2 hr 27 min 2 hr 1 min

Conclusion

Generally speaking, accurate control of the curing time in a real production setting is not easy. Particularly when the temperature of the mold increases, the reaction will vary differently from material to material; therefore, evaluating the material property changes simply based on the engineers’ experience is very difficult. To achieve a better control of the VARTM process, the material data obtained from Moldex3D’s Material Wizard can be used to gain further insights into the material properties of various resins.

Moreover, with the aid of Moldex3D’s mold-filling analysis, the flow behavior and curing reaction of the resin can be predicted in a better way to determine the most suitable mold temperature and processing parameters to increase the advantages of using the VARTM process.

This information has been sourced, reviewed and adapted from materials provided by Moldex3D.

For more information on this source, please visit Moldex3D.

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