RTM Process Flow Analysis

 

The RTM process includes resin filling flow, heat transfer and curing reaction. Among them, heat transfer and curing reaction are common to other composite materials processes, and the focus of process control is the filling flow problem.

 

During the RTM filling process, the mold cavity is filled with fiber preforms, which can be called fiber beds, which contain solid phases – fibers and mobile phases – air. The resin filling process is to ensure that the resin flows through these irregular pores and displaces the air so that the resin fills the pores. The flow of resin in these irregular pores is very complicated, and there are two types of flow at the same time, one is the macroscopic flow between fiber bundles, and the other is the microscopic flow within the fiber bundles. These two flows compete with each other during the filling process, which may lead to poor wetting or bubble encapsulation.

 

When the injection pressure is low, the capillary force in the fiber bundle plays a major role. The flow front of the fluid is shown in Figure

(a). The flow front in the fiber bundle is ahead of the flow front between the fiber bundles. When the leading fluid front converges along the lateral flow, the air that has not been discharged in the fiber bundle is wrapped, forming large bubbles between the fiber bundles. On the contrary, when the injection pressure is high, the capillary pressure has a smaller effect than the dynamic pressure, so the flow front of the fluid in the fiber bundle lags behind the flow front between the fiber bundles,as shown in Figure

 

(b). When the flow front converges lateral flow, small bubbles are formed inside the fiber bundle. Bubble content is one of the important indicators of composite material quality. The presence of bubbles makes the fiber impregnation degree low and the bonding poor, resulting in inconsistent mechanical strength and poor surface quality of composite material parts; at the same time, it is easy to cause stress concentration, trigger cracks, and reduce the durability, fatigue resistance and weather resistance of composite materials.

 

 

Formation of Bubbles During Flow

 

Bubbles form microscopic defects in products, mainly between fiber bundles or between single fibers in fiber bundles, while dry spots are macroscopic defects in products. When processing fiber preforms, problems such as preform bending, loose fabric edges, and changes in local preform permeability often occur. When cutting preforms, it is generally difficult to achieve precise dimensional accuracy, and the looseness of the fiber bundles will reduce the fiber volume content at the edges. In addition, when the mold design is unreasonable or the mold is not properly closed, it is easy to cause gaps between the preform and the cavity wall and at the corners, and this gap forms a preferential flow channel for the resin. The flow of the resin in this channel is ahead of the flow in the preform, destroying the normal flow front pattern. This effect is called “edge effect” or “flow channel effect”. This type of change in the performance of the reinforcing material caused by the shear, compression or compaction effect of the preform, especially the edge effect, has an important impact on the filling process, and is prone to cause problems such as insufficient fiber impregnation and dry spots. The general flow channel effect area is at the millimeter level. In some cases, a static flow area of ​​1 to 2 mm can have a considerable impact on the filling process, causing large dry spots on the product.

 

2. Fiber Permeability

 

The analysis and research on the RTM process filling process is based on the theory of fluid flow through porous media, using Darcy’s law as the momentum control method. Darcy’s law was proposed by Henry Darcy in the mid-twentieth century based on a series of experimental results and is widely used in soil science. Darcy’s law describes that the speed of fluid flow through porous media is proportional to the applied pressure gradient and inversely proportional to the viscosity of the fluid. Darcy’s law is a special form of the momentum balance equation, which can be directly substituted into the continuity equation to obtain the control equation for pressure.

 

For horizontal flow, ignoring gravity, Darcy’s law can be expressed as follows:

 

 

 

For a special orthotropic preform, when the local coordinates coincide with the principal directions of the fiber preform, the permeability tensor can be described as:

 

The RTM process is mainly used to manufacture shell components. The resin flows mainly in the surface of the preform, and the flow along the thickness direction is very weak and can be ignored. Therefore, the permeability along the thickness direction can be ignored. Therefore, in this case, the permeability can be simplified to:

 

 

The permeability of fiber preforms in RTM applications is usually obtained by three approaches: experimental measurement, analytical solution and numerical estimation.

 

3. Flow Simulation

The flow simulation of RTM process can qualitatively predict the resin flow process. If the input parameters are reasonable and reliable, the injection pressure, flow rate, flow state, etc. can be accurately predicted through simulation. The core issue of establishing analysis technology is how to obtain important input parameter data, such as permeability, resin chemical rheological properties, thermal diffusion, and edge flow. These parameters are closely related to the resin system, reinforcement material system, mold, and process of composite materials.

 

The finite element simulation theory of liquid flow process is the difficulty and hot spot of today’s simulation technology. At present, the finite element simulation software for resin flow has become mature, such as RTM-Worx developed by Polyworx in the Netherlands, PAM-RTM developed by ESI in France, and MOLDFLOW developed by MoldFlow in the United States. Applying these flow simulation systems, referring to the actual product’s use requirements, performance requirements, and molding process parameters, and establishing the actual product finite element model can effectively simulate the product’s production process and guide the production of actual products.

 

Through the study of the application of flow simulation technology in the actual product production process, the general process of engineering application of flow simulation technology can be obtained:

(1) Computer geometric modeling of the workpiece, and finite element segmentation of the model with the help of finite element method;

(2) Select several feasible resin injection methods according to the geometric shape of the workpiece;

(3) Select and measure the various parameters required for resin flow simulation, including injection pressure, permeability and fiber volume content of the preform, and resin viscosity;

(4) Simulate the resin flow process of various injection methods;

(5) Comprehensively consider various factors such as resin flow time and final filling position, as well as the difficulty of process operation, and select the optimal resin injection method to guide actual production.

 

The following is an example of simulating the RTM process filling process of the antenna cover using RTM-Worx software. The geometric shape and size of the workpiece are shown in the figure below:

 

 

Part Geometry and Dimensions (unit: mm)
During the simulation, the injection pressure is set to 0.5 MPa. It is assumed that the reinforcement preform is isotropic, the permeability is 1×10-10m2 and the distribution is uniform, the fiber volume content is 40%, the thickness of the spherical cap and the working area is set to 14 mm, the thickness of the annulus is set to 20 mm, and the resin viscosity is set to 0.4 Pa•s. The simulation results are shown in the figure below:

 

Simulate Resin Flow Front Diagram and Pressure Distribution