The Resin Transfer Molding Process has been in use for decades. It is one of the best methods for making a composite mold, and is uniquely capable of satisfying the low-cost/high-volume 500-50,000 parts per year of the automotive industry as well as the higher performance/lower volume 50-5,000 parts per year of the aerospace industry. If that sentance didn't make sense, here's the short version: Resin Transfer Molding is excellent for mass production of composite parts.

The automotive industry has used resin transfer molding (RTM) for decades. The resin transfer molding process is fairly simple: A two-part, matched-metal mold (or tool) is made. A preform is placed into the mold, and the mold is closed. The resin is then pumped under low pressure through injection ports into the mold and follows predesigned paths through the preform. Both the mold and resin can be heated as needed for the application.

The above is a simplified model of the RTM production machine. As I am sure we have all been looking forward to this part, without further adu let us discuss the steps of this process.

The first step in the Resin Transfer Molding technique is creating the preform. The preform is the matrix, already in the shape of the finished product, that the resin will be injected into. There are a few steps in making a preform (you didn't think it would be that simple did you?) which are:

• 1. Selecting the type of fiber. There are several different types of fiber available for use with composites.
• Random Mats: This fiber type consists of continuous or chopped fibers randomly laid and loosely bonded by an adhesive. While random mats have high permeability and flexibility, these advantages are offset by relatively poor stiffness and strength, and lack of control of fiber orientation.
• Two Dimensional Woven Fabrics: These fiber types are fabricated on a loom, and woven in a manner similar to the clothing that you are wearing now. (I hope you are wearing clothing.) This fabric has a few advantages in a composite. Its properties , on the plane of the fabric, are quite balanced. It has good impact resistance, and conforms well to different shapes. The disadvantages of this fiber type are that it does not conform well to unusual geometric shapes, and due to the method of weaving the threads are weakened somewhat.
• Unidirectional Fabric: Another fiber type, unidirectional fabric consists of parallel filaments held loosely by stitches on a plane. This has high stiffness and strength in the filament direction, but not in the direction of the stitches. Filaments may 'wash' away as well. Unidirectional fabric for use as a matrix is a trade off, but it is still good for many applications.
• Other fiber forms: There are many, many different fiber forms that are not listed here. Knit fibers, other 2 dimensional weaves and 3 dimensional weaves are also viable manufacturing alternatives, though 3 dimensional weaves are for more specialized applications.
• 2. After selecting the fiber type, the fiber must be preformed into the shape of the finished product. There are several methods of preforming (pretend to be surprized at that!). Some of these are listed below.
  • Cut and Place Preforming: In this method, nearly any 2 dimensional woven fabric and some 3 dimensional ones may be used. Individual layers of the preform are seperately cut, placed into the female mold cavity by hand, and then contact preformed within the tool. (You know, the one with the conceptual picture above and all that good stuff.)
  • Directed Fiber Preforming: This method uses a 'sprayer' to fire chopped fibers at a perforated screen shaped like the finished product. A low vacuum is created on the opposite side of the screen to prevent the fibers from falling off of it. The fibers are then cured in an oven. Afterwards, they are removed from the screen, which may be re-used. Advantages to this method include the ease with which it can be used to make large structures, low cost, and easy adaptability to a manufacturing process. It has several disadvantages though. The things molded using this method will not always have the same density, and chopping the fibers up before application takes away a considerable amount of their strength.

  There are several benefits to using the resin transfer molding process over the alternative processes available. One of the most prominent benefits is the seperation of the actual molding process from the design of the fiber shape. Having the fiber preform stage occur at a different time than the injection and curing stages allows the manufacturer a much greater amount of flexibility and precision when designing a peice.