RTM METHOD (Eng. Resin Transfer Molding), also called embossed shaping, consists in pressing the liquid mixture and hardener into the space between the tightly closed two rigid parts of the mold in which the reinforcement is located, usually in the form of a mat or fabric, as shown in Figure 2.27 a. The pressed mixture infiltrates the reinforcement laid. The applied pressures are not large (from 0.2 to 1 MPa), which makes it possible to use relatively inexpensive forms made of polymer composites. Sometimes the upper part of the rigid mold is replaced by a molded silicone diaphragm (Figure 2.27 b). Then this method is referred to LRTM (Light Resin Transfer Molding), it is colloquially called light RTM.

The station for manufacturing products by RTM also consists of the dispensing and pumping device with a mixing head. The dosing and mixing process is fully automated. The curing reaction of the resin occurs after the mold is completely filled.

The most commonly used methods are polyester or epoxy resins with low viscosity and low curing temperature. The low viscosity of the reactive mixture allows for good impregnation of the reinforcement laid in the mold and allows free filling of even large molds with complex geometries. On average, the product cycle of a large-size RTM product is as follows: 20% of the time it takes a thin layer of gel coat, 7% positioning of the reinforcement in the mold, 13% operation related to closing the mold, 8% injection of resin and hardener mixture, 31% resin curing, 9% removal of the product from the mold, 12% inter-operative break. The filling time of the mold may, however, vary depending on the size and shape of the product, the form and type of reinforcement, degree of filling, viscosity of the reactive mixture and temperature. This method uses unheated as well as heated forms (up to 80 ° C) – which allows shortening the time of making products by shortening the resin curing time. Products with a degree of reinforcement filling up to 60% are obtained.

The method has found application mainly in the automotive industry (eg car body elements of  passenger cars and lorries, interior fittings, leaf springs and spring – loaded) and in the aviation industry (eg emergency door and ribs stiffening the wings). It is used for the production of medium series from 35,000 to 100,000 products a year. The advantages of products manufactured by this method are homogeneous and repeatable mechanical properties, high surface smoothness, low tendency to create defects, high fill factor and full repeatability of quality.

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