Optimizing RTM process with pressure sensors

1 Paddle made from fiber-reinforced plastics, produced with RTM

Monitoring of the mould cavity pressure improves the control of transfer moulding resin processes. The goal? Make the technology useful to large-scale industrial production

Due to energy-related requirements, accelerated systems contain an increasing number of lightweight components and structures. Thanks to their high weight-specific strength and rigidity, fiber-reinforced plastics have become the lightweight material of choice for these applications. Processing methods for composites such as resin transfer moulding (RTM) have so far failed to become established in large-scale industrial production.

Suitable sensor technology however, can optimize the control of RTM processes. This is prerequisite for the wider use of composites in areas such as lightweight automotive engineering which is considered the key to future mobility. Typical examples of fiber-reinforced plastic products are components for aerospace, automotive and machine engineering as well as parts for sports and leisure applications (Figure 1).

RTM technology

The following processing methods are suitable for producing long fiber-reinforced plastic structures:

• manual lamination, for small batches with minimum quality requirements;

• resin infusion moulding for supporting high-quality structural components, for medium-size batches (e.g. resin transfer moulding, RTM);

• preform autoclave processing, for highly structural components and special applications (particularly for aerospace engineering).

The complex processing requirements of preforms and the high investment and energy costs involved in autoclave processing are arguments for the use of the simpler method of resin transfer moulding as it delivers the same quality as preform processing. RTM is the method of choice, particularly for high-tech applications which are produced on a large scale of between 1,000 and 10,000 units/year (Figure 2).

2 Schematic diagram of an resin transfer moulding (RTM) process

As the process stages show, the infusion process itself is preceded by a preforming process which produces threedimensional fiber structures. Figure 2 shows how dry fiber reinforcements are subjected to a thermoforming process, which produces preforms. Alternatively, these structures can be produced by means of braiding, knitting or sewing. After insertion of the reinforcing fibers into a two-part mould, a reactive thermoset resin (typically epoxy systems) is injected into the mould. The low-viscosity resin penetrates and wets the fibers under pressure. Following the injection, the part cures under an exothermic reaction and can then be removed.

Efficient process control requires suitable mould and sensor technology. Both technologies are not available as comprehensive products. This is one of the main reasons why RTM has not become established in large-scale industrial production.

Process simulation

Part design and product development involve the selection of materials such as fiber preforms and resins as well as the conduction of feasibility studies concerning the production process.

One essential part of these tests is the simulation of the mould filling operations, which is carried out to verify whether the filling is “robust”, i.e. whether it occurs within a certain time and with the required reliability. The filling simulation of a paddle, for example, shows that the cavity can be filled successfully (Figure 3). With the selected injection strategy, voids are not expected.

3 Flow front profile during resin injection

Process parameters

As the RTM process occurs in a closed mould, sensor technology is of utmost importance. The process can be divided into three phases:

• insertion/heating of the dry preform,

• impregnation of the fibers during form filling,

• curing of the resin by means of chemical crosslinking.

The decisive process parameters are temperature, pressure, viscosity and degree of resin cure. The following process information can be derived from these parameters:

• the temperature profile provides information on the heating of the dry preform, the passage of the resin flow front and the exothermic chemical crosslinking reaction;

• the pressure profile provides information on the passage of the resin flow across the measuring point and the pressure distribution in the mould;

• the viscosity provides information on the flowability of the resin;

• the degree of cure provides information on the progress of the chemical crosslinking reaction.

In principle, the process parameters listed above change over time, as shown in Figure 4.

4 Process parameters: temperature (T), pressure (p), viscosity (η) and degree of cure (α) over process time

The degree of cure is inferred from the temperature and viscosity (Figure 5).

5 Correlation between degree of cure (α), temperature (T) and viscosity (η)

The process parameters affect the injection process as well as the resin curing (Figure 6).

6 Interaction between process parameters in RTM processes

The heat exchange between mould, fiber preform and resin determines the temperature profile. Both the temperature and the degree of cure determine the viscosity and the passage of resin through the fiber preform. The temperature and the degree of resin cure also determine the reaction rate.

Sensor technology for process control

As shown above, process data can be used to monitor the mould filling process. The information is useful for assuring the quality of production processes. The sensor signals provide information on the position of the flow front, i.e. this information can be used to determine whether the resin flow front has already reached the checkpoint sensor. The passage of the resin flow front provides information on the “robustness” of the process control. Process data is useful for assuring the quality of large-scale production processes and allows detailed tracking of all process steps. Moreover, process data is useful for validating simulation results.

Combining simulation results with data from real processes will become increasingly important. The combination of numeric simulations with real-time process information allows intervention in the process to improve the result.

Kistler’s cavity pressure sensor Type 6161AA… (Figure 7) is optimized for the special requirements of RTM processes.

7 Kistler’s cavity pressure ultra-sensitive sensor Type 6161AA…

As these sensors are highly sensitive, they can detect minute pressure changes and reliably reveal irregularities in the pressure profile during the mould filling. The recorded pressure profile is therefore useful for process documentation. Moreover, adjusting the injection pressure can ensure that the part is filled correctly. In bulky moulds with several gates sensor signals can facilitate the strategic control of injection nozzles.

The optimized sensor is distinguished by its solid, compact design which is ideal for heavy-duty requirements of the industrial production of fiber-reinforced components. Equipped with an o-ring seal, the sensor can also be used after evacuation of the cavity and for vacuum measuring. Applying vacuum to the cavity can support the injection of the resin-curing agent composite and can help eliminate voids in the component. Sealing also prevents resin from penetrating the mounting holes and facilitates sensor assembly and disassembly.