Vacuum-assisted resin transfer molding process for large-size composite structures

Vacuum-assisted resin transfer molding process for large-size composite structures

Vacuum-assisted resin transfer molding process for large-size composite structures
Core Tips: Vacuum-assisted resin transfer molding process for large-size composite structures bookmark0 is used to manufacture large-size composite parts. In this study, we developed an advanced VARTM called sequential multi-gate automated resin transfer molding (SMARTMoling).

The vacuum-assisted resin transfer molding process for large-size composite structures, bookmark0, is used to manufacture large-size composite parts. In this study, an advanced VARTM system called sequential multi-gate automated resin transfer molding (SMARTMoling) was developed to fully automate the impregnation process. During resin injection and on-line control, the system introduced a sequential injection of VARTM process. Control the flow of the operating mechanism. Detecting the flow characteristics of the sensing element opens the operating mechanism. Off is optimized. In), the preform is impregnated with a liquid resin under a negative pressure (eg, vacuum), followed by curing and demoulding. Advantages of the VARTM process include the use of single-sided molds that can reduce tooling costs and other major input costs. In addition, large parts are rapidly impregnated with a highly permeable distribution medium suitable for the surface layer of the preform. When the injection sprue is open, the resin preferentially penetrates the surface and simultaneously penetrates through the thickness of the preform. For the flow front, the surface flow front is ahead of the mold flow front. Lead time is important for thick preforms. In large structures, many injection gates and vacuum exhaust ports are required, and the flow front becomes very complex, depending largely on the sequence of opening the gates.

Sequential injection is often applied to impregnated large-size composite parts. At this point, several injection gates are set for the distribution medium, and the injection pipes are opened in sequence to minimize the cycle time. The resin impregnation time increases exponentially with the duration of the injection during VARTM processing. In order to optimise the injection of large parts, the requirements are as follows: The number of injection gates is required to be the minimum (the lower limit of the number of gates poured), in order to ensure that the resin filling time is less than the gel time to ensure full resin filling.

Adding the number of injection gates will reduce the cycle time, but add costs (additional device labor costs) and hardware requirements, resin waste, and so on. The minimum spacing of injected catheters and the upper limit of the number of injected catheters are related to the leading time of the flow front. Analytical studies have shown that the lead time is obviously related to the distribution medium, the permeability of the preform, and the thickness of the preform.

The best time to open the injection gate in sequence is the injection gate, when no dry spot can expand. The injection port is opened when the surface of the mold below the gate is wet, to ensure that the total wetting and cycle time costs are minimized, the optimum opening time is before wetting the mold surface below the gate, and the cost is minimal.

The shape of the flow front must be considered to determine the best sequential injection process. The area below each gate must be completely filled to ensure wetting of the composite part. The problem is to open the injection gate at the correct time when the lower part is completely filled with the full moon. Tests have shown that when the material properties are kept constant, the unsaturated area does not change its shape and extent during the immersion time. The analytical model can predict the shape and extent of the unsaturated zone, as shown. The unsaturated area increases with the thickness of the preform. For 10 layers of 6804 gE fiberglass preform and 50% of Roxford cover material, the length of the unwetted zone is about 32 cm; for 20 layers, about 50 cm; for 30 layers, about 68 clotting time or the desired injection time is used for Calculate the upper limit of the number of injection gates. (a) shows 100 cm and 500. Development of graphical user interface (GUI) input parameters including performance of preforms and distribution media, application of vacuum level, resin viscosity, preform length, and number of injection gates. Calculate the arrival time during resin injection, the flow rate and the unsaturated flow shape. Digital optimization is used to find the minimum number of injection gates with the desired injection time or setting time, which is an important tool in the injection stage of large-size composite parts. It can be used to select the correct distribution down to obtain the desired pre-flow shape and injection time, and to observe the effect of material properties on the number of injection gates needed.

These results indicate the potential benefits of sequential injection. Currently, the problem with the sequential injection is that there may be large dry spots at depths of penetration of a few centimeters that need to be repaired or must be scraped off. To improve the wettability, the large parts are delayed by several minutes to several hours, which adds cycle time. An automated system with sensor element feedback (summarized in the next section) can improve part and part quality, minimize cycle times, and eliminate process monitoring.

2 The multi-gate automated resin transfer molding processing unit component is formed in sequence and consists of a valve system that automatically opens or closes the injection gate and a supervisory control computer that optimizes the opening of the gate. In addition, real-time arrival times can be calculated during the test using a computer-controlled CCD camera equipped with processing software to capture the flow characteristics on the surface. The data is input to the precision balance of the monitoring computer, and the injected resin quality is measured. At the same time, the system is able to measure the injected resin mass and flow rate, the time it takes the resin to reach the skin, the thickness flow characteristics of the resin to penetrate the part, the viscosity and the cure characteristics.

Injection gates can be used to calculate the number of injected catheters and the distance between injection catheters, as described in the previous section. In general, the injection pipes that are parallel to one another are embedded, keeping the distance constant and the resin arrival time during the injection. If the formed part is complex, or the preform thickness, permeability, etc. are not uniform, other configurations may be considered.

In order to successfully control and automate the VARTM process, various sensor elements can be added. Especially for large-thickness composite parts, during the resin impregnation and curing process, due to cost, complexity, and reliability of readout technology, resin impregnation is also referred to as SMART weaving for impregnation process studies that penetrate part thickness. Regional resin transfer technology monitoring. The SMART weaving system measures the conductivity between individual sensing and interfering nodes.

When the resin reaches the junction, the injection resistance is continuously controlled and the drop is monitored. Low-cost process monitoring automation systems have the ability to monitor the flow front and viscosity, cure, and degree of condensation in real time. The system can be shaped to handle 4096 node sensors in complex multi-planar components, enabling three-dimensional monitoring of the impregnation process.

During the sequential injection control, the flow characteristics of the permeation thickness must be taken into account. In traditional resin transfer models (RTM) or VARTM machining thin parts, the arrival time between the top and bottom of the preform is almost the same (for example, the permeation thickness is uniform).

The actual flow characteristics during thick section VARTM machining are shown. The resin was injected into the woven roving fabric 30 layers, 1(1) cm long, and 680 4g in mass from the back surface of the preform having holes on the opposite side, and the SMART consisting of 40 flat patterns was formed in 7 (permeate thickness) positions. The sensor element monitors the resin flow. The spacing of the planar sensor elements was fixed at 5 cm and penetrated in different layers.

After the initial unstable region, the flow front follows a single permeation thickness angle (approximately 2* spread, independent of position and thickness, controlled only by the preform configuration (fabric thickness and permeability and distribution medium).

Based on the resin reaching the upper surface, opening of the resin inlet will allow the resin to quickly wet the lower layer below the injection conduit and actually separate the two resin flow fronts, resulting in a large dry spot spread. Conventional VARTM machining using sequential injection is based on the visual inspection of the above surface flow. For thicker parts, this method has a higher probability of dry spot formation during processing. Buried or die-mounted flow sensing elements can reduce the risk.

In order to prove the principle of SMARTM moulding, embedded woven sensing elements can be replaced by mould-mounted sensing elements, such as the flow characteristics of a 30-ply woven roving fabric with a pore number permeation thickness of SW nodes. This allows for repeated manufacturing without the additional expense and time associated with burying the inlet of the sensor element. The working principle of the mold installation sensing element is the same as that of the DC resistance measurement principle and the SMART braided sensing element. Placing the sensing element underneath the injection conduit on the preform mold interface (eg, bottom) allows measurement of fully wetted preforms at these points. Multiple sensing elements are installed while the injection catheter is monitored to ensure wetting at each location. The sensor element spacing can be chosen to ensure that any potential dry spot is less than the critical size. When all the sensing elements on the mold surface measure the flowing resin, the next conduit is opened and a sequenced automated injection process is obtained by the clamping valve, which controls the opening and closing of the injection conduit. Initially, the resin stream opening the first injection conduit is flowing above the distribution medium toward the hole. The resin slowly infiltrates the thickness impregnation, after which it reaches the second gate on the surface, delaying the opening of the second gate until all the sensing elements indicate this thickness is wet. Using multiple sensing elements will ensure that all dry spots disappear before the next valve opens. In addition, the initial injection conduit is closed in order to ensure the compaction pressure in this area and to prevent lateral flow. Continue this process until you inject the entire part.

6804g woven parts are injected sequentially. Using a computer control system, 20 layers of thick composite parts were injected with SC 15 resin. As outlined in the flow chart in the previous section, the injection conduit was placed and separated by an exhaust passage located on the left side of the part. The pinch valve is visible at the top of the flow chart and opens in sequence during the process. The resin in the distribution medium reached the position of the injection catheter at 0s10s46s150s and 201s, respectively.

Analyze the recorded image and visually measure the arrival time along the length of the part as shown. It can be seen that, on the surface, the arrival time increases exponentially until about 150 s. At this point, the resin arrives under the second conduit and opens the gate. The slope of the curve decreases, which corresponds to the faster resin. . However, the cycle time only from the complete wetting of the entire surface layer does not indicate an improvement in the control method.

For such a single port injection, the surface layer is completely filled after about 200 s and 300 s.

Shows the loss of resin in the container in sequential injection and conventional injection. At approximately 146 s 254 s and 420 s, the pinch valve is opened at these points and the flow rate is increased to the level during the initial injection. The final part was completely filled with resin during the 580s. The difference between the theoretical predictions (850s) and the results obtained from the test can be explained by the fact that the analytical model considers stable unsaturated regions throughout the impregnation period.

As experimentally verified (as shown), conventional injections of the same size parts (with one injection catheter) require 3 times the injection time (~180s). For large-size, large-width composite parts, due to the injection of Alg by the % injection, the injection distance of the injection port/cm, the injection time of the order of time/s, the arrival time of the resin permeation thickness on the length of the single raft injection arrival time/s M length. With an additional conduit, the amount of injected resin can be ignored. 1 shows the injection from a sequential gate and a single gate injection, at a distance of 25 cm from the injection gate, 50 3 Conclusion VARTM analysis simulations have been used to predict the optimal interval between sequential injection gates. The interval is based on the required injection time and the shape of the unsaturated flow during the injection. The graphical user interface has been researched to automate the best conditions and results. The GUI has been used to show the benefits of an in-order injection process: the cycle reduction is up to 90% for large-size composite parts. In order to support the sequential injection process, VARTM's automated control system is introduced, which enables online adjustment of a single injection gate Open and close. The injection of the system in sequence is accomplished by a computer-controlled pinch valve that is capable of feeding sensor system feedback from the mold. The cycle time is reduced by 66%. The system eliminates process monitoring and guarantees the shortest cycle time for complete soaking. Next-generation control systems will increase the automation of the VARTM injection process and include automated resin mixing, cure monitoring, and control.

Cao Yunhong Yang Hongchang

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