What are the Methods for Preforming RTM Reinforced Materials?

 

In RTM Molding, the fiber-reinforced material must first be made into a shape similar to the product to be processed before molding. The process of making a preform of the reinforced fiber and its fabric is called preforming, and the preform of the reinforced fiber and its fabric processed by preforming is also called filler. There are currently three methods for processing preforms: stitching, spraying and stamping.

 

Stitching Method

 

The processing cost of the preform mainly depends on the application field of the product to be processed. Taking aerospace and aviation as an example, RTM molded products used in this field are usually processed by cutting and stitching. In this processing process, the fiber weaving or textile fabric is usually divided into several simple-shaped areas according to the shape of the pre-processed product, cut from the overall fiber-reinforced fabric, and then stitched into a preform similar to the shape of the product to be processed by stitching.

 

At this time, RTM molding can be used. This preforming process is inefficient and labor-intensive, and is developed on the basis of traditional textile and stitching technology. This method can be used to process two-dimensional or three-dimensional fabrics. The three-dimensional geometric preforms prepared have high interlayer strength and toughness, excellent overall performance, and can realize automated production of preform processing. Therefore, this process is mostly used in RTM molding of aerospace products, and it is also applicable to the molding of other RTM products. But it is limited to the production of small products.

 

Spraying Method

 

The preparation of larger volume preforms requires faster processes, such as chopped fiber bundle spraying technology. In the spraying technology, a perforated metal mesh is manufactured according to the shape of the preform, and a vacuum is applied to the back of the perforated mesh. The reinforcing fibers are cut to the required length by a shredder and sprayed with resin. Then, air is blown in by cutting the air inlet pipe to disperse and randomize the fibers and directly enter the perforated mesh. The vacuum system on the back of the perforated mesh will firmly position the fibers in the mesh and make the fibers contact each other.

 

Once the required thickness of the reinforcement is reached, the shredder power is turned off and the preform is sent to the oven together with the perforated mesh. In the oven, the resin can be cured to shape the preform. Spraying technology can produce large preforms very quickly, that is, one preform per minute. However, the mechanical properties of composite products will be affected by cutting the fiber bundle. To solve this problem, a new spraying technology that can spray continuous annular fiber bundles or continuous oriented fiber bundles has been developed.

 

Stamping Method

 

In the stamping process, the reinforcement material used is a continuous random glass fiber bundle mat with a centrifugal configuration. It can be coated with a thermosetting resin and fixed together after stamping.

 

To make the hot stamped fiber mat into the required shape, the following steps should be followed:

 

① Prepare the blank;

② Prepare the skeleton and clamp it;

③ Heating;

④ Stamping;

⑤ Compacting;

⑥ Cooling;

⑦ Demolding and trimming.

 

The blank is a fiber mat that is initially cut into the geometric shape of the image product. The skeleton should provide sliding action for the fiber mat and can be fixed and controlled around the blank to reduce the formation of wrinkles. The ideal skeleton should be tilted and warped, have a good covering effect on the female mold, and be as close to the product shape as possible. The clamping action of the blank causes the fiber mat to stretch and then thin. The sliding action of the blank shrinks the fibers and slows down the thinning of the fiber mat. The fibers cannot be over-shrinked in products with curved flanges. In the curved area, the velocity compression of the inner plane will deform the fiber mat, which will cause wrinkling of the product after compaction. The design of the blank skeleton should also include the setting of the blank holding force, the design of the blank holder geometry, and the adoption of measures to reduce fiber mat waste.

 

Preform Process Design

 

According to the preform process design, the second and third steps can be changed. The heating or melting of the fiber mat depends mainly on the properties of the resin used. The temperature setting of the oven (such as a high-efficiency infrared heater) should take into account the glass transition temperature (Tg) of the resin and the cooling rate of the fiber mat after it is removed from the oven and placed in the stamping station.

 

In addition, the friction between the fiber mat and the tool surface will also seriously affect the strain and the quality of the preform. Reducing the friction at the fiber/resin interface will make the fiber distribution more uniform, which can prevent the phenomenon of localized thinning and fiber breakage. Conversely, increasing friction will reduce the bendability of the fiber. After the mold is closed, the resin can be used to bond at the fiber-to-fiber contact points to give the preform dimensional stability, and then compact it. The cooling effect is to turn the resin into a solid state, which helps to keep the preform with a similar geometry to the mold.

 

Because RTM mostly uses glass fiber reinforced epoxy composite molds, this mold has poor thermal conductivity compared to metal molds, so the cooling rate is slow. If a perforated metal mold is used and vents are set, the cooling rate will be faster. After demoulding, the preform can be trimmed. Although stamping technology is fast and simple to preform large structures, its performance is not on the same level as textile stitching preforms due to random fiber reinforcement, and its comprehensive performance is relatively low.

 

Preforming of Blanks

There are two preforming methods for the blanks of the bag-pressed skylight hood, as described below.

1. Hand lay-up method, using negative molding.
2. Mechanized molding.

 

The blanks are manufactured using a preforming machine, as shown in the figure below.

 

This method is suitable for manufacturing products with complex shapes and large dimensions. When producing preforms, the glass fiber roving is first cut into fixed-length glass fibers of about 50 mm in length by cutters 2 and 3. In the molding chamber 6, the cut strands are scattered by air turbine disperser 5 to make them settle evenly. The exhaust fan 9 creates negative pressure in the molding chamber to adsorb the glass fibers to the mesh mold 8.

 

In order to ensure that the blank has sufficient strength and will not fall apart during transportation, a certain amount of water-soluble resin adhesive must be sprayed during the molding process. The commonly used amount of adhesive is 5% of the resin matrix. When producing light-transmitting composite material skylights, adhesives that do not affect the light transmittance of the product must be selected.