Vacuum assisted resin transfer molding

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Vacuum Assisted Resin Transfer Molding (VARTM) or Vacuum Injected Molding (VIM) is a closed mold, out of autoclave (OOA) [1] composite manufacturing process. VARTM is a variation of Resin Transfer Molding (RTM) with its distinguishing characteristic being the replacement of the top portion of a mold tool with a vacuum bag and the use of a vacuum to assist in resin flow. [2] The process involves the use of a vacuum to facilitate resin flow into a fiber layup contained within a mold tool covered by a vacuum bag. After the impregnation occurs the composite part is allowed to cure at room temperature with an optional post cure sometimes carried out.

Contents

Process

Typically, this process uses a low viscosity (100 to 1000 cP) polyester or vinyl ester resin along with fiberglass fibers to create a composite. [3] Normally the process is capable of producing composites with a fiber volume fraction between 40 and 50%. [3] The resin to fiber ratio is important for determining the overall strength and performance of the final part, with mechanical strength being most influenced by the type of fiber reinforcement. The type of resin used will primarily determine the corrosion resistance, heat distortion temperature, and surface finish. [4] Resins used in this process must have low viscosities due to the limited pressure differential provided by the vacuum pump. High performance fibers, such as carbon fiber, can also be used. However, their usage is less common and is mainly for the fabrication of high end parts.

Air Leakages

For VARTM to create high quality composite parts it is crucial that air leakages are avoided. Air leakages can cause resin to improperly flow through the mold and also lead to the formation of air bubbles. Defects in the form of voids occur when the composite cures with air bubbles inside of it. Air leakage can be caused by a defect in the vacuum bag, an improper application of the sealant tape, or an improper seal at the points where the hose meets the vacuum bag.

Air leakages can be detected using various methods. In some situations air bubbles, and consequentially air leakages can be detected simply through a visual inspection of the composite. The most simple ‘Leak isolation’ method involves monitoring the vacuum pressure level to determine if there are air leaks. If the vacuum pressure level does not decrease after vacuuming all of the air out of the mold, then it can be determined that there is no air leakages. [5] However, if there was a drop in the vacuum pressure level it would be an indication that there was an air leakage. Unfortunately, this method for identifying the presence of an air leak does not determine the air leak's location.

Sound magnification is also utilized to locate leaks. Since air leaks make noise, this method utilizes a microphone to amplify sound to a set of speakers or headphones to help in the identification of leaks. [5] This allows a user to detect an air leak and use the microphone to help them find the location of the leak. Unfortunately, this method is ineffective in noisy environments.

Heated air can also be utilized to detect leaks. In this method heated air is forced through the mold prior to the use of a vacuum pump. If there are any air leaks in the process's set up the hot air will be expelled through the leak. An infrared detector, can then be used to determine if there are any heat releases on the surface of vacuum bag, which would be an indication of the presence of an air leak. [5]

VARTM vs RTM

Both VARTM and RTM are closed mold processes where pressure is used to inject resin into the mold. There are few differences in the materials used in VARTM vs RTM, with the resin and fiber basically being the same for both processes. Therefore, if factors such as fiber to resin ratio and cross-sectional fiber distribution were held constant for each process, the molded part performances would be similar. [4]

RTM has a fiber preform placed between mold halves, while VARTM uses the bottom part of the mold tool and a vacuum bag with resin flow caused by the use of a vacuum. RTM results in small-medium-sized parts that can also be complex in shape, while VARTM can also create very large parts. VARTM also advantageously has lower equipment costs than RTM. The single sided nature of the VARTM mold has the drawback of only allowing for one side of the composite to have an A-class finish. However, parts can be manufactured with an A-class finish on both sides with RTM due it having both a top and bottom mold.

Advantages and Applications

This process offers the benefit of not requiring an expensive autoclave while also being capable of producing large, complex aerospace-grade parts. [1] Products produced using this method vary widely in their application with parts being used in transportation, wind energy, marine, infrastructure, and aerospace applications. The process's ability to create large and complex parts has allowed it to effectively reduce manufacturing costs when utilized to produce parts that are traditionally constructed of numerous small components. For instance, LOCKHEED Martin Space Systems (LMSS) experienced a manufacturing cost saving of up to 75% when it began to produce the quarter section of the equipment bay for the Trident II D5 missile using VARTM. [6]

Related Research Articles

Fiberglass or fibreglass is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinyl ester resin—or a thermoplastic.

Thermosetting polymer Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system.

Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or "film" when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that permits it to be stretched into or onto a mold and cooled to a finished shape. Its simplified version is vacuum forming.

Compression molding Method of molding

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured; this process is known as compression molding method and in case of rubber it is also known as 'Vulcanisation'. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.

A helium mass spectrometer is an instrument commonly used to detect and locate small leaks. It was initially developed in the Manhattan Project during World War II to find extremely small leaks in the gas diffusion process of uranium enrichment plants. It typically uses a vacuum chamber in which a sealed container filled with helium is placed. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer.

Filament winding is a fabrication technique mainly used for manufacturing open (cylinders) or closed end structures. This process involves winding filaments under tension over a rotating mandrel. The mandrel rotates around the spindle while a delivery eye on a carriage traverses horizontally in line with the axis of the rotating mandrel, laying down fibers in the desired pattern or angle to the rotational axis. The most common filaments are glass or carbon and are impregnated with resin by passing through a bath as they are wound onto the mandrel. Once the mandrel is completely covered to the desired thickness, the resin is cured. Depending on the resin system and its cure characteristics, often the mandrel is autoclaved or heated in an oven or rotated under radiant heaters until the part is cured. Once the resin has cured, the mandrel is removed or extracted, leaving the hollow final product. For some products such as gas bottles, the 'mandrel' is a permanent part of the finished product forming a liner to prevent gas leakage or as a barrier to protect the composite from the fluid to be stored.

Electronic packaging is the design and production of enclosures for electronic devices ranging from individual semiconductor devices up to complete systems such as a mainframe computer. Packaging of an electronic system must consider protection from mechanical damage, cooling, radio frequency noise emission and electrostatic discharge. Product safety standards may dictate particular features of a consumer product, for example, external case temperature or grounding of exposed metal parts. Prototypes and industrial equipment made in small quantities may use standardized commercially available enclosures such as card cages or prefabricated boxes. Mass-market consumer devices may have highly specialized packaging to increase consumer appeal. Electronic packaging is a major discipline within the field of mechanical engineering.

Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small-scale production like industrial prototypes and dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the production of collectible toys, models and figures, as well as small-scale jewellery production.

Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

Vacuum bag moulding is the primary composite manufacturing process for producing laminated structures. It is common in the aerospace industry.

Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave (industrial) curing process commonly used by the aerospace manufacturers for manufacturing composite material. Out of autoclave (OOA) is a process that achieves the same quality as an autoclave but through a different process. OOA curing achieves the desired fiber content and elimination of voids by placing the layup within a closed mold and applying vacuum, pressure, and heat by means other than an autoclave. An RTM press is the typical method of applying heat and pressure to the closed mold. There are several out of autoclave technologies in current use including resin transfer molding (RTM), Same Qualified Resin Transfer Molding (SQRTM), vacuum-assisted resin transfer molding (VARTM), and balanced pressure fluid molding. The most advanced of these processes can produce high-tech net shape aircraft components.

Tailored fiber placement

Tailored fiber placement (TFP) is a textile manufacturing technique based on the principle of sewing for a continuous placement of fibrous material for composite components. The fibrous material is fixed with an upper and lower stitching thread on a base material. Compared to other textile manufacturing processes fiber material can be placed near net-shape in curvilinear patterns upon a base material in order to create stress adapted composite parts.

A void is a pore that remains unfilled with polymer and fibers in a composite material. Voids are typically the result of poor manufacturing of the material and are generally deemed undesirable. Voids can affect the mechanical properties and lifespan of the composite. They degrade mainly the matrix-dominated properties such as interlaminar shear strength, longitudinal compressive strength, and transverse tensile strength. Voids can act as crack initiation sites as well as allow moisture to penetrate the composite and contribute to the anisotropy of the composite. For aerospace applications, a void content of approximately 1% is still acceptable, while for less sensitive applications, the allowance limit is 3-5%. Although a small increase in void content may not seem to cause significant issues, a 1-3% increase in void content of carbon fiber reinforced composite can reduce the mechanical properties by up to 20% Void content in composites is represented as a ratio, also called void ratio, where the volume of voids, solid material, and bulk volume are taken into account. Void ratio can be calculated by the formula below where e is the void ratio of the composite, Vv is the volume of the voids, and Vt is the volume of the bulk material.

Transfer molding is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. Furthermore, unlike injection molding the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding process.

A Lay-Up process is a moulding process for composite materials, in which the final product is obtained by overlapping a specific number of different layers, usually made of continuous polymeric or ceramic fibres and a thermoset polymeric liquid matrix. It can be divided into Dry Lay-up and Wet Lay-Up, depending on whether the layers are pre-impregnated or not. Dry Lay-up is a common process in the aerospace industry, due to the possibility of obtaining complex shapes with good mechanical properties, characteristics required in this field. On the contrary, as Wet Lay-Up does not allow uni-directional fabrics, which have better mechanical properties, it is mainly adopted for all other areas, which in general have lower requirements in terms of performance.

The main stages of the Lay-Up process are cutting, lamination and polymerization. Even though some of the production steps can be automated, this process is mainly manual, leading to laminates with high production costs and low production rates with respect to other techniques. Hence, nowadays, it is mainly suitable for small series production runs of 10 to 1000 parts.

Pressure bag moulding is a process for moulding reinforced plastics. This process is related to vacuum bag molding.

Resin transfer moulding is a process for producing high performance composite components.

Light resin transfer moulding is a process by which products of Composite materials are manufactured using a closed mold system.

References

  1. 1 2 Autoclave Quality Outside The Autoclave? [Online]. Available: http://www.compositesworld.com/articles/autoclave-quality-outside-the-autoclave.
  2. ↑ X. Song, “Vacuum Assisted Resin Transfer Molding (VARTM): Model Development and Verification,” Blacksburg, VA, 2003.
  3. 1 2 J. S. Tate, A.T. Akinola, and D. Kabakov. Bio-based Nanocomposites: An Alternative to Traditional Composites. The Journal of Technology Studies. 35(1). 2009. DOI: http://scholar.lib.vt.edu/ejournals/JOTS/v35/v35n1/tate.html#tate2004.
  4. 1 2 Vacuum Assisted Resin Transfer Molding (VARTM) - What it is, What it is Not, What it Can, and What it Cannot Do [Online]. Available: https://www.rtmcomposites.com/process/vacuum-assisted-resin-transfer-molding-vartm.
  5. 1 2 3 S.G. Advani, F.Zhou, J.B. Alms, and C.C. Corlay, “System and method of detecting air leakage in a vartm process,” U.S. Patent 11 742 243, Nov 5, 2009.
  6. ↑ T. Steve. VARTM cuts costs. Reinforced Plastics. 45(5), pp. 22. 2001.
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