Vacuum Assisted Resin Infusion Molding Process and Application

 

Vacuum Assisted Resin Infusion Molding (VARIM) is a high-performance, low-cost composite molding process developed on the basis of RTM (Resin Transfer Molding).

 

Since its development in the late 1980s, VARIM, as a new type of liquid composite molding technology (LCM), has been widely recognized in aerospace, defense engineering, shipbuilding, energy industry, infrastructure engineering and other application fields, and has been studied and applied as a key low-cost manufacturing technology by the Composite Affordability Initiative (CAI) implemented by the United States.

 

 

As shown in Figure 1, the basic principle of the VARIM process is to use the flow and penetration of resin to impregnate the fiber fabric reinforcement material in the closed mold cavity under vacuum negative pressure conditions, and then solidify and mold.

 

The Basic Process of the VARIM Process Includes:

 

Preparation Stage.

It includes the design and processing of the single-sided rigid mold, the cleaning and coating of the mold surface with a release agent, the preparation of reinforcement materials (fiber fabrics, preforms, core materials, etc.) and vacuum-assisted media (demolding media, high-permeability flow-guiding media, gas-guiding media, etc.).

 

Laying Stage.

Reinforcement materials, demoulding cloth, peeling layer media, high-permeability flow-guiding media, resin infusion pipes, vacuum gas-guiding pipes, etc. are laid on the single-sided rigid mold in sequence.

 

Sealing Stage.

The reinforcement materials and vacuum-assisted media are sealed in the elastic vacuum bag film with sealing tape, and vacuum is drawn to ensure that the closed mold cavity reaches a predetermined vacuum degree.

 

Infusion Stage.

Under vacuum negative pressure, the resin glue is introduced into the closed mold cavity through the resin infusion pipe and fully impregnated with the reinforcing material.

 

Curing Stage.

Continue to maintain a high vacuum degree, and the liquid resin undergoes a curing and cross-linking reaction at room temperature or heating conditions to obtain a product preform.

 

Post-Processing Stage.

Including cleaning the vacuum bag film, guide medium, peeling layer medium, demolding cloth and other vacuum auxiliary media and demolding and trimming, and finally obtaining the product.

 

Compared with the traditional open mold molding process and RTM process, the VARIM process has the following advantages:

 

Low Mold Cost.

Compared with the RTM process that requires a double-sided rigid mold, the VARIM process only requires a single-sided rigid mold; compared with the molding mold that needs to withstand high temperature and high pressure in the compression molding process, the mold manufacturing cost is low, which is suitable for designing and developing large molds with different complex structures and shapes.

 

The Product Shape is Controllable and the Size is Precise.

The VARIM process has fewer restrictions on the size and shape of the product. It can be used for the molding of large-thickness and large-size structural parts in the fields of aerospace, defense engineering, shipbuilding industry, energy industry, infrastructure engineering, etc., such as rocket shells, wind turbine blades, and automobile shells.

 

The Product has Good Mechanical Properties and High Repeatability.

Compared with hand-layup components, the mechanical properties of VARIM process molded products can be improved by more than 1.5 times, and the products have high fiber content, low porosity, few structural defects, uniform and smooth surface, and high consistency between components. Therefore, the quality of VARIM process molded products is stable and has good repeatability.

 

Good Environmental Protection.

Compared with open mold molding, the volatilization of volatile organic compounds (VOCs) such as styrene and acetone is as high as 35~45%. As a closed mold molding technology, the VARIM process confines volatile substances and toxic air pollutants in the vacuum bag film during resin infusion and curing. Therefore, it almost does not cause pollution to the environment, which is one of the most prominent advantages of the VARIM process.

 

High Production Efficiency.

The resin under vacuum negative pressure can be quickly introduced into the closed mold cavity along the resin infusion pipeline, and fully and quickly penetrate and impregnate the reinforcing material before gelation, so that large and complex geometric sandwich and reinforced structural parts can be integrally formed. Compared with the open mold process, the VARIM process can save more than 50% of labor.

 

Main Raw Materials of VARIM Process

 

Resin

Resins suitable for VARIM process include low-viscosity resins such as epoxy resin, vinyl resin, unsaturated polyester resin, phenolic resin, etc. The VARIM process generally has the following requirements for resin:

 

Low Viscosity of the Resin System.

Generally, the viscosity of the resin system is required to be between 100 and 800 mPa•s, and the optimal viscosity range is 100 to 300 mPa•s, so that the resin can completely impregnate the reinforcing material under the action of vacuum negative pressure. If the resin viscosity is too high, the filling flow speed is slow, and the impregnation effect on the fiber fabric is not ideal; if the resin viscosity is too low, the resin flow speed is too fast, and it is easy to form defects such as dry spots.

 

Appropriate Gel Time.

Different processes have different requirements for gel time. Therefore, gel time should be variable and easy to control, with a suitable operation cycle, which is an important indicator of the VARIM process-specific resin system. Generally, for large-scale parts molding, the low viscosity platform time of the resin system (i.e., the process operation window) is required to be no less than 30 minutes to avoid violent gel reaction and curing cross-linking reaction of the resin during the infusion process.

 

The Curing Exothermic Peak is Moderate.

High exothermic peaks will reduce the service life of the mold and may affect the core material, reinforcement and other components in the product. At the same time, high exothermic peaks may cause cracks in the components and affect the performance of the product.

 

Other Physical and Chemical Properties.

Including good mechanical properties to meet the high requirements of engineering applications, resistance to thermal oxidation aging, chemical corrosion resistance, flame retardancy, non-toxicity, low cost, etc.

 

Reinforcement Materials

Reinforcement materials generally include E glass fiber, carbon fiber, Kevlar fiber, Spectra fiber, and a mixture of E glass fiber and several other fibers. The reinforcement material can be chopped fiber or fiber fabric, but usually fabric is used, such as untwisted roving fabric, twisted fabric, bidirectional stitched fabric, etc. Among them, new knitted materials and plain unidirectional fibers are ideal choices.

 

Vacuum Bag Film

High-temperature resistant nylon film and polypropylene film are the most commonly used vacuum bag films, mainly using their ductility, flexibility and puncture resistance; at the same time, the material is required to have a high heat resistance temperature (specific resin properties need to be considered) and excellent barrier air tightness.

 

Sealing Adhesive Tape

Sealing adhesive tape is a vacuum bag film sealant with butyl rubber as the base rubber and additives such as heat-resistant reinforcing agents and tackifiers. The material is required to have high elasticity, surface adhesion and temperature resistance, etc., to ensure excellent sealing performance during the product molding cycle.

 

High Permeability Medium

The function of the high permeability medium is to ensure that the resin can quickly penetrate and flow during the vacuum infusion process, greatly improving the filling flow rate. Nylon mesh and woven fibers can usually be used.

 

Stripping Layer Medium

The function of the stripping layer medium is to separate the product from the high permeability medium or vacuum bag film to prevent the vacuum-assisted medium from adhering to the product. Generally, low-porosity and low-permeability film materials are selected as stripping layer media, such as PE, PP porous films, etc.

 

Lightweight Core Material

Generally, core materials are within the optional range, such as lightweight wood, PVC, PEI, PMI, SAN, PS foam and other linear microporous closed plastics. For open-cell core materials (such as honeycomb), the resin will fill its cavities, increasing the weight and cost of the product, so this type of core material should not be selected.

 

Common Defects and Causes of VARIM Process

 

Bubbles and White Spots

As shown in Figure 2, in the VARIM process, the permeation flow of resin in the fiber fabric can be divided into macroscopic flow and microscopic flow, among which the flow of resin between the gaps in the fiber bundle is called macroscopic flow, and the flow of resin between the fiber filaments inside the fiber bundle is called microscopic flow. If the flow speeds of the macro flow and the micro flow are different, that is, there is an inconsistency between the flow fronts of the two, the resin will laterally penetrate into the fiber fabric layer, resulting in a local “air encapsulation” phenomenon, in which bubbles are generated on the surface layer of the part and white spots are generated in the internal layer of the part.

 

 

Figure 2 Schematic Diagram of the Macroscopic and Microscopic Flows of the Resin in the Vacuum Assisted Molding (VARIM) Process[6]

 

The local “gas encapsulation” phenomenon occurs because the macroscopic and microscopic flows of the resin are inconsistent. The flow velocity of the macroscopic flow front is related to the injection pressure gradient. The larger the injection pressure gradient, the faster the macroscopic flow; while the flow velocity of the microscopic flow front is related to the capillary force between the fiber filaments. The larger the capillary force, the faster the microscopic flow.

 

Therefore, as shown in Figure 3(a), when the injection pressure gradient is less than the capillary force, the flow velocity of the resin microscopic flow front will be greater than the flow velocity of the macroscopic flow front. At this time, the resin inside the fiber bundle will penetrate laterally and wrap the residual gas between the fiber bundle gaps to form large bubbles.

 

On the contrary, as shown in Figure 3(b), when the injection pressure gradient is greater than the capillary force, the flow velocity of the resin macroscopic flow front will be greater than the flow velocity of the microscopic flow front.

 

At this time, the resin in the gaps between the fiber bundles will penetrate laterally into the fiber bundle and form small bubbles inside the fiber bundle.

 

In order to reduce and avoid the occurrence of local “air inclusion” phenomenon, it is usually necessary to pre-evacuate and maintain the set vacuum degree for a certain period of time, so as to remove the air in the closed mold cavity as much as possible.

 

At the same time, it is appropriate to design the resin infusion flow channel so that the resin flows along the direction perpendicular to the fiber fabric (90°), rather than the resin flowing along the direction parallel to the fiber fabric (0°) as shown in Figures 3 and 4.

 

Figure 3 Schematic diagram of the formation of bubbles and white spots in the vacuum assisted molding (VARIM) process [6]

 

Dry Spots and Dry Areas

 

In the VARIM process, the flow rate of the resin between the fiber bundles is inconsistent. If the resin infusion channel or fiber fabric layer design is unreasonable, it will lead to the occurrence of “channel effect” or “short circuit effect”. The flow rate of the resin in the low resistance area will be significantly greater than the flow rate in the high resistance area, up to 10 to 100 times. As a result, the resin will mainly flow and penetrate in the low resistance area, making the fiber fabric in the high resistance area unable to be fully impregnated or even completely unimpregnated. The part will show dry spots and dry areas on a macro scale. Poor wettability matching between fiber fabric and resin, loose or too tight or twisted local structure of fiber fabric, and excessive gap between sandwich core material and fiber fabric may all cause dry spots and dry areas in the part.

 

Wrinkles and Warping

During the layering stage, if the fiber fabric is not laid tightly and flat, the resin may squeeze or even disperse the fiber bundles during the infusion process, resulting in wrinkles and warping of the cured parts. In addition, when the resin undergoes gel reaction and curing cross-linking reaction, it will have a certain volume shrinkage rate and release a large amount of reaction heat, which will cause the loose fiber fabric to twist and deform under large internal stress or thermal stress, and then cause the parts to warp. In order to eliminate the occurrence of wrinkles and warping, the fiber fabric and preforms must be laid flat, and a resin system with small volume shrinkage and small heat release should be selected, and a reasonable curing system and heat dissipation circulation system should be adopted.

 

Over-Extraction and Lack of Glue

In the VARIM process, in order to maintain a high vacuum degree during the resin infusion process and ensure the vacuum pressure gradient required for infusion and the quality of the product, it is necessary to continuously evacuate the residual gas in the closed mold cavity and the gaps between the fiber bundles. If the vacuum channel is not set up properly, or the resin infusion pipeline is not set up properly, a large amount of low-viscosity resin will be easily extracted while the air is being pumped out, resulting in a large area of ​​glue shortage in the product and the undesirable phenomenon of over-pumping.

 

Spots and Glue Enrichment

 

During the layering stage, if lumps of objects are mixed in the fiber fabric layer, the fiber fabric in the local area will be deformed, resulting in local enrichment of the resin glue, and uneven spots will appear on the cured product.

 

Similar to the glue shortage phenomenon, the glue enrichment phenomenon is mainly caused by the unreasonable laying of the vacuum channel and the resin infusion pipeline. This is because the pressure of the resin at the infusion inlet is atmospheric pressure, while the pressure at the flow front is almost zero. In this way, the farther away from the vacuum pipe mouth (i.e., the resin infusion inlet), the higher the resin content and the lower the corresponding fiber content; and the closer to the vacuum pipe mouth (i.e., the resin flow front), the lower the resin content and the higher the corresponding fiber content. Therefore, if the vacuum channel and resin infusion pipeline are not laid properly, or the resin inlet and vacuum system are closed immediately when the resin reaches the outlet, the resin infusion inlet area will be rich in glue, and large-sized and thick parts will also have uneven thickness.

 

In order to weaken the above-mentioned rich glue phenomenon, it is necessary to reasonably set the vacuum channel and resin infusion pipeline, and close the resin infusion inlet after the resin reaches the outlet, and continue to maintain vacuum for a period of time without over-pumping, so that the resin pressure can be steadily reduced, and the resin content in each area of ​​the part can be made uniform as much as possible. In addition, the thicker core material and the boundary of the reinforcement rib will also have the phenomenon of glue enrichment, so it is necessary to lay some triangular or trapezoidal materials as a transition to avoid the occurrence of rich glue.

 

Application of VARIM Process

As a new composite material molding process, VARIM process began in the late 1980s. At the beginning, the process was not highly valued by people and failed to realize its potential huge commercial value. It was not until 1996 that the VARIM process was recognized and valued by people at the SPI Composite Material Annual Conference due to its successful application in ships. Since the VARIM process has many advantages such as low cost, high product quality, and suitability for manufacturing large and complex integral structural parts, after more than ten years of research and application, the VARIM process is no longer limited to the shipbuilding industry, but has been widely used in the construction of many military and civilian facilities, such as military ships, missile bays, radar covers, wind turbine blades, bridges, car shells, refrigerators, etc.

 

Large Aviation Parts

 

Fighter cockpit: The F-35 fighter developed by Lockheed Martin Corporation of the United States used the VARIM process to manufacture the cockpit for the first time, and the cost was reduced by 38% compared with the autoclave process.

 

Large aircraft wings: In the “Boeing Preform” program funded by NASA (National Aeronautics and Space Administration), V System Composites used the VARIM process to study the forming of wing structure composite materials and integral composite sandwich structures with stiffeners, while Boeing studied the integral forming of large aircraft wing skins.

 

Large Ships and Superstructures

 

In the shipbuilding industry, the British Vosper Thornycroft (VT) company has used the VARIM process to manufacture more than 270 composite minesweepers for the British Royal Navy, and has also manufactured transport ships, work boats, lifeboat hulls and marine port engineering structures. North End Company used the VARIM process to manufacture a 27.5 m long hull, and after inspection, the void ratio of the hull laminate was almost zero, and the mechanical properties were equivalent to those of the parts formed by low-temperature curing in an autoclave, but the manufacturing cost was greatly reduced. The British Sandown-class minesweeper is made of non-magnetic materials. The superstructure and some internal structural parts of the entire ship are formed by the VARIM process, which can withstand strong impacts. The US Navy DD21 Zumwalt-class stealth destroyer and the Swedish Navy YS2000 Visby-class stealth anti-submarine light cruiser both use the foam sandwich structure formed by the VARIM process as the ship hull. Foshan Baoda Marine Engineering Co., Ltd. uses the VARIM process to composite the hybrid reinforcement material containing aramid fiber and vinyl resin to manufacture a 13.6 m customs super-high-speed motorboat.

 

Large Composite Wind Turbine Blades

 

In recent years, the VARIM process has been widely used in the integral molding of large composite wind turbine blades. Compared with the hand lay-up molding process, the VARIM process has greatly improved the production efficiency of wind turbine blades, significantly improved the operating environment, reduced the amount of resin used by 30%, and achieved stable product quality and good repeatability. Denmark’s LM Fiberglass Products Co., Ltd. has developed a 60 m wind turbine blade using the VARIM process.

 

The manufacturing of blades using the VARIM process can be divided into the following steps:

 

Mold Preparation:

Clean the mold and apply a release agent.

 

Lay the Reinforcement Material:

Lay the fiber fabric according to the design requirements. In this process, in addition to the fabric model, position and overlap size must meet the design requirements, the draping must also be flat and clean.

 

Figure 4 Laying Glass Cloth Reinforcement Material on the Blade Mold

 

Laying Out Vacuum Pipelines:

According to the process requirements, lay out vacuum pipelines and cover them with vacuum. This step is a critical step in the VARIM process. Usually, it is necessary to combine theoretical simulation and repeated experiments before formal production; during production, the vacuum degree of the entire system needs to be guaranteed.

 

 

Figure 5 Vacuum Pipeline Layout and Vacuum Inspection

 

Resin Infusion and Curing:

Under vacuum conditions, the mixed resin is infused into the compacted reinforced material preform. After the resin fills the entire mold cavity, the resin flow channel is closed and cured according to the specified conditions.

 

 

Figure 6 Vacuum Infusion

 

Skin Bonding and Post-Curing:

After the skin is cured and formed, the upper and lower skins and shear webs are bonded together and cured according to the specified process.

 

 

Figure 7 Skin Bonding

 

Post-Processing:

After the product is demoulded, the blades are trimmed, reinforced, polished and painted.

 

Figure 8 Coating treatment on the external surface of the blade.