SMC (Sheet molding compounds)

SMC or Sheet Molding Compounds consists of the molding of a thermostable resin generally reinforced with fiberglass in the form of cut or continuous threads, obtaining the final piece through polymerization of the resin by applying pressure and temperature.

This manufacturing method has a preliminary stage in which the prepreg is created that will later be used to make the desired final piece. This pre-process is based on using fiberglass rovings in continuous cords that are cut into pieces (about 5cm) on a layer of resin filling transported on a polyethylene film. Subsequently, once the lower layer has all the fibers distributed, another layer of resin filling is placed on top of the first, forming a sandwich. This sandwich is compacted and rolled on calibrated packaging rollers.

The prepreg rolls will be stored to let the fiber settle with the thermoset matrix.

Glass fibers can be presented not only as cut fibers, but there are also cases in which continuous threads are used or as a mixture of both (continuous threads and cut fibers), in the latter case the best mechanical properties would be obtained.

Finally, the rolls of composite material are taken to a press, where they are cut with the appropriate configuration for the part to be obtained and placed in a tight, heated metal mold. Subsequently, the hydraulic press is closed and the SMC flows uniformly under pressure throughout the mold, forming the final product.

The main features are:

• - Good dimensional stability.

• - Excellent surface finish on both sides.

• - Good mechanical resistance properties.

• - Possibility of obtaining complex geometries.

• - High automation capacity.

It is a process that, due to its high automation capacity, is mainly used in large series production, with the automobile being its main sector (hoods, front panels...). This phenomenon makes SMC currently the most used method in the manufacture of composite materials with thermoset resins.

Injection

Infiltration of composite materials by injection uses the same technology as traditional plastic injection molding. But unlike introducing only a polymer (either thermoplastic or thermoset) into the mold, the polymer plus a reinforcement is introduced.

The most used fibers are glass, carbon and aramid. These reinforcements significantly affect the mechanical properties of the final material. But on the other hand they present some drawbacks such as:

• - Difficulty in controlling the positioning of the fibers in the piece.

• - The fibers can significantly reduce their length due to breaking with the endless screw that feeds the process.

Pultrusion

It is a very versatile automatic process through which constant section profiles are obtained.

A fiber embedded in a thermostable resin is used, which reacts when heat is applied. The material is pulled (pull) to avoid breaks and misalignment of fibers.

Through this process any complex section can be produced as long as its thickness is constant. Furthermore, due to the precision of the mold surface, high quality finishes are obtained.

However, the process speed is relatively low (3 m/min) compared to the extrusion speed (30 m/min). Furthermore, it is very difficult to orient the fibers at optimal angles.

Description of the process:.

The pultrusion process consists of several stages:

• - The reinforcing fibers are presented in the form of coils or rolls to promote the continuous flow of the material.

• - They are placed on reels on axles with special bearings to maintain constant thread tension when being wound.

• - When the fibers are directed towards the impregnation system, they are passed through alignment plates to avoid twists, knots or damage to the reinforcements.

• - The key to the entire pultrusion process is the mold. It must align the fibers and compress them to the desired volume fraction, ensuring the curing of the material in a relatively short time.

• - Heated plates are placed around the mold to heat the part and promote the polymerization reaction of the resin. The most important control parameter is the interior temperature of the mold, which ranges between 100 °C and 150 °C.

• - In the traction mechanism there must be a distance of at least 3 meters between it and the outlet to ensure correct cooling of the profile through natural or forced convection (by water or air jet).

RTM (Resin Transfer Molding)

It is a closed mold manufacturing process at low pressure. It can be summarized in five stages:

1. • The dry fiber fabric is placed in the mold and it is closed.

2. • The resin is injected into the mold using a pump.

3. • The resin inlet and air outlet are sealed.

4. • Heat is applied (curing).

5. • After a cooling period, the mold is opened and the piece is removed.

Due to the possibility of molding the fibers dry before injecting the resin, a better orientation of the fibers is achieved, increasing the mechanical properties of the material.

• - Manufacturing process.

• - Composite materials.

• - Machining of composite materials.

SMC (Sheet molding compounds)

SMC or Sheet Molding Compounds consists of the molding of a thermostable resin generally reinforced with fiberglass in the form of cut or continuous threads, obtaining the final piece through polymerization of the resin by applying pressure and temperature.

This manufacturing method has a preliminary stage in which the prepreg is created that will later be used to make the desired final piece. This pre-process is based on using fiberglass rovings in continuous cords that are cut into pieces (about 5cm) on a layer of resin filling transported on a polyethylene film. Subsequently, once the lower layer has all the fibers distributed, another layer of resin filling is placed on top of the first, forming a sandwich. This sandwich is compacted and rolled on calibrated packaging rollers.

The prepreg rolls will be stored to let the fiber settle with the thermoset matrix.

Glass fibers can be presented not only as cut fibers, but there are also cases in which continuous threads are used or as a mixture of both (continuous threads and cut fibers), in the latter case the best mechanical properties would be obtained.

Finally, the rolls of composite material are taken to a press, where they are cut with the appropriate configuration for the part to be obtained and placed in a tight, heated metal mold. Subsequently, the hydraulic press is closed and the SMC flows uniformly under pressure throughout the mold, forming the final product.

The main features are:

• - Good dimensional stability.

• - Excellent surface finish on both sides.

• - Good mechanical resistance properties.

• - Possibility of obtaining complex geometries.

• - High automation capacity.

It is a process that, due to its high automation capacity, is mainly used in large series production, with the automobile being its main sector (hoods, front panels...). This phenomenon makes SMC currently the most used method in the manufacture of composite materials with thermoset resins.

Injection

Infiltration of composite materials by injection uses the same technology as traditional plastic injection molding. But unlike introducing only a polymer (either thermoplastic or thermoset) into the mold, the polymer plus a reinforcement is introduced.

The most used fibers are glass, carbon and aramid. These reinforcements significantly affect the mechanical properties of the final material. But on the other hand they present some drawbacks such as:

• - Difficulty in controlling the positioning of the fibers in the piece.

• - The fibers can significantly reduce their length due to breaking with the endless screw that feeds the process.

Pultrusion

It is a very versatile automatic process through which constant section profiles are obtained.

A fiber embedded in a thermostable resin is used, which reacts when heat is applied. The material is pulled (pull) to avoid breaks and misalignment of fibers.

Through this process any complex section can be produced as long as its thickness is constant. Furthermore, due to the precision of the mold surface, high quality finishes are obtained.

However, the process speed is relatively low (3 m/min) compared to the extrusion speed (30 m/min). Furthermore, it is very difficult to orient the fibers at optimal angles.

Description of the process:.

The pultrusion process consists of several stages:

• - The reinforcing fibers are presented in the form of coils or rolls to promote the continuous flow of the material.

• - They are placed on reels on axles with special bearings to maintain constant thread tension when being wound.

• - When the fibers are directed towards the impregnation system, they are passed through alignment plates to avoid twists, knots or damage to the reinforcements.

• - The key to the entire pultrusion process is the mold. It must align the fibers and compress them to the desired volume fraction, ensuring the curing of the material in a relatively short time.

• - Heated plates are placed around the mold to heat the part and promote the polymerization reaction of the resin. The most important control parameter is the interior temperature of the mold, which ranges between 100 °C and 150 °C.

• - In the traction mechanism there must be a distance of at least 3 meters between it and the outlet to ensure correct cooling of the profile through natural or forced convection (by water or air jet).

RTM (Resin Transfer Molding)

It is a closed mold manufacturing process at low pressure. It can be summarized in five stages:

1. • The dry fiber fabric is placed in the mold and it is closed.

2. • The resin is injected into the mold using a pump.

3. • The resin inlet and air outlet are sealed.

4. • Heat is applied (curing).

5. • After a cooling period, the mold is opened and the piece is removed.

Due to the possibility of molding the fibers dry before injecting the resin, a better orientation of the fibers is achieved, increasing the mechanical properties of the material.

• - Manufacturing process.

• - Composite materials.

• - Machining of composite materials.