Resin transfer moulding (RTM)

Resin Transfer Molding (RTM) is a low pressure, closed molding process which offers high-quality dimensional and surface finish composite moldings using liquid thermoset polymers reinforced with various forms of fiber reinforcements. Typically polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with typically fiberglass reinforcement. Other reinforcements, are offered for more demanding applications such as Arimid, Carbon and Synthetic fibers either individually or in combination with each other.

A process using a two-sided mold set that forms both surfaces of the panel. The lower side is a rigid mold. The upper side can be a rigid or flexible mold. Flexible molds can be made from composite materials, silicone or extruded polymer films such as nylon. The two sides fit together to produce a mold cavity. The distinguishing feature of resin transfer molding is that the reinforcement materials are placed into this cavity and the mold set is closed prior to the introduction of matrix material. Resin transfer molding includes numerous varieties which differ in the mechanics of how the resin is introduced to the reinforcement in the mold cavity. These variations include everything from vacuum infusion (see also resin infusion) to vacuum assisted resin transfer molding. This process can be performed at either ambient or elevated temperature.

The matrix selection of polymer and reinforcement, dictates both molding material cost, as well as molding mechanical and surface finish performance. Along with the polymer and reinforcement the addition of mineral fillers may be added to enhance fire retardantcy, flex modulus and surface finish.

Reinforcements are presented in their dry form to the mold in either binder-bound chopped mat, random-continuous strand mat or woven cloth format. The fiber has been either "preformed" to the exact shape of the molding tool in a previous operation or is hand-tailored during the loading process in the molding tool. After the fiber is installed into the mold, a premixed catalyst and resin is injected into the closed mold cavity encapsulating the fiber within. The primary surface of the molding may be gel-coated, a process of spraying the mold surface before installing the fiber. If a gel coat is not required, the exterior finish would be the same from the front to back of the molded part. RTM having the inherent advantage of low-pressure injection usually does not exceed 100 psi of resin injection pressure during the mold-fill process. the most common application of RTM composites is molded with a cross-section thickness of 4mm with 30% fiber content by weight of the total laminate.

Resin Transfer Molding (RTM) is a low pressure, closed molding process, where a mixed resin and catalyst are injected into a closed mold containing a fiber pack or preform. After the resin has cured, the mold can be opened and the finished component removed. A wide range of resin systems can be used including: polyester, vinylester, epoxy, phenolic and methyl methacylates, combined with pigments and fillers including aluminum trihydrates and calcium carbonates if required. The fiber pack can be glass, carbon, arimid, or a combination of these.

EDO has placed considerable emphasis on the development of in-house resin transfer molding for the fabrication of high performance structural composites. Resin transfer molding produces parts to net shape with good surface finish, low void content and required levels of fiber volume. Raw material costs for the resin transfer molding process are significantly lower than those associated with pre-impregnated materials. Being a "closed mold" process capable of automation, resin transfer molding does not pose the health and environmental risks associated with traditional methods of resin impregnation.

Resin transfer molding is a logical choice for the impregnation of dry fiber preforms. Braided preforms can be held in inventory indefinitely without concern for resin out-times and cure schedules. As separate processes, braiding and RTM schedules can be designed for optimum production rates. The RTM production area can be configured to operate a variety of molds simultaneously to accommodate the diversity of preforms being produced by the braiding operation.

The Benefits of using RTM

· Closed Molding process is cleaner and healthier which attracts higher skilled employees.

· Closed Molding operator turn-over is dramatically reduced through improved working conditions.

· Closed Molding area has organized process flow to maximize throughput efficiency.

· Moldings can be manufactured to close dimensional tolerances.

· Components will have good surface finish on both sides.

· Selective reinforcement and accurate fiber management is achievable.

· Ability to build-in fiber volume fraction loadings up to 65%.

· Uniformity of thickness and fiber loading, resulting in uniform shrinkage.

· Inserts may be incorporated into moldings.

· Tooling costs comparatively low compared to other manufacturing processes.

· Uses only low pressure injection.

· Low volatile emission during processing.

· Ability to produce near net shape moldings, reducing material wastage.

· Process can be automated, resulting in higher production rates with less scrap.

· Ability to mold complex structural and hollow shapes.

· Low resultant voidage in molded components.

· Ability to achieve from 0.1mm to 90mm laminate thickness.

Main Disadvantages:

  • Matched tooling is expensive, and heavy in order to withstand pressures.
  • Generally limited to smaller components.
  • Unimpregnated areas can occur resulting in very expensive scrap parts.