Variety of industries using fiber reinforced composite materials rapidly increases for a few decades now. Composites are preferred by several industries because of their high strength and lightweight properties. Compared to metals, manufacturing with composite materials is very different. The main reason for that material properties of virgin metal and the final part are not fundamentally changed when manufacturing a metal part. Vice versa, manufacturing process has key role for composite materials. During selected composite process type, the desired shape of the part also its specific properties are affected. Thereby, it is important to select proper manufacturing method according to your project. This comprehensive article about composite material manufacturing methods is prepared to clear all the question marks in your mind.
Composite Materials
Before we explain the manufacturing methods for composites in detail, let us first explain what the composites are made of. The usage of fiber-reinforced plastics (FRP) has grown exponentially since the 1960s. As a result, these fiber-reinforced plastics are used in many applications, from sport equipment to aerospace structures. In general, composites consist of two or more different materials used together to achieve a new set of material properties that high-quality materials cannot achieve on their own.
In polymer composites, individual constituents are typically reinforcement material (as glass, carbon, aramid, etc.) and polymer resin (polyester, epoxy, vinyl ester, phenolics, etc.). The main roles of polymer matrix are to bind the fibers together and distribute force applied to part for overall durability. The fibers are used to carry the structural load and to provide strength and stiffness. This new generation material technology, which has developed rapidly, is expected to reach a global market volume of $130 billion by 2024.
Common Composite Manufacturing Processes
The selection of proper manufacturing process for composites depends on part size and geometry, number of units, type of reinforcement material and polymer matrix, and the cost. In addition to the different types of classification criteria, composites processing can be divided into two main groups as open (wet hand lay-up, spray lay-up, automatic tape placement and filament winding) and closed mold (rein transfer molding, vacuum infusion, and autoclave processing) processes.
1. Open Mold Processes
Wet Hand Lay-Up
Wet hand lay-up is the earliest manufacturing method for composite parts. In this process, a fabric layer is placed on top of the one-sided mold and followed by pouring thermoset resin over the fabric layer. A hand-held roller is used to apply pressure for compacting fabric layer and impregnate it with thermoset resin. Once the layer of the fabric layer is completely impregnated, another layer is placed, and this procedure is repeated until the desired lamination is completed.
Although this process is the earliest manufacturing method for composites, it is still widely used in marine industry because of its simplicity and low cost. However, the major drawbacks of this process can be listed as follows:
· High labor cost
· High emissions of styrene to the environment
· Low surface quality and dimensional tolerances
· Low mechanical properties because of low fiber volume fraction
Spray Lay-Up
Spray lay-up is another open mold process for composite manufacturing which can be identified as the derivative of hand lay-up process. Compared to hand lay-up, the resin is not applied directly by hand, but with spray gun. In general, continuous fiber is chopped with chopper gun and sprayed with polymeric resin simultaneously or sequentially to the one-sided mold.
It should be noted that the mechanical properties of final laminate manufactured with spray lay-up process is low since the resin ratio of the laminate is high (approximately 60%). In addition, another major drawback of this process is the part thickness variation thus low dimensional tolerances.
It is ideal to use this process for the parts which are simple shaped and used for low performance required applications such as shower stalls, vehicle trims and machine coverages.
Automated Tape Placement (ATP)
Automated Tape Placement (ATP) is an open mold composite process which uses robotic arm, fiber/polymer resin prepreg tape, heat, and pressure. In ATP, robotic arm is actively controlled for the placement of prepreg tape over the mold also pressure and heat are applied for consolidation of fibers and curing of resin, respectively. This procedure is repeated until the lamination is completed.
The main advantage of ATP is the manufacturing of highly customized parts since each tape can be placed at different angles according to the required load directions. Other advantages of can be listed as:
· Low labor cost
· Low material waste
· High part quality
However, the disadvantages of ATP can be listed as:
· High equipment cost
· Limited part geometry and size
· Long process time
Filament Winding
Filament winding is a composite manufacturing process which is mainly used for manufacturing circular, cylindrical and hollow shaped parts such as pipes, poles, tubes, vessels, and tanks. This process requires winder machine to rotate the mandrel and wind the fiber tows (or tow pregs) over the mandrel according to the required orientations. Impregnation of the fiber tows with resin occurs either during the winding operation or previously impregnated towpregs can be used as well.
The main advantages of filament winding can be listed as:
· Automated
· Low labor cost
· Optimized fiber orientation
· Short process time
· Most suitable process for cylindrical or circular shaped structures
The major drawbacks of this process are:
· High equipment costs (winder machine and mandrel)
· Limited geometry (not capable of manufacture concave shaped products)
· Low surface quality since the outer surface is not molded.
2. Closed Mold Processes
Vacuum Infusion (VI)
In Vacuum Infusion, dry or semi-impregnated fabric layers are cut in desired dimensions, stacked, and placed on the rigid lower mold. As an upper mold, a vacuum bag is used to cover the fabric preform. A vacuum is applied for two reasons:
· compacting the fabric preform by drawing air inside the mold, and
· applying resin driving pressure to provide resin flow from inlet to exit.
Once resin is infused into the mold by a vacuum pump and it impregnates the fabric preform completely, composite part is ready to demold as it reaches its green strength.
Although, using a flexible bag as an upper mold has advantages as:
· manufacturing complex shaped and large-scale composite parts,
· having low molding cost
it leads to major drawbacks as:
· long mold filling time since resin driving pressure is limited to 101 kPa, and
· potential to form dry-spots or macro/micro voids because vacuum bag cannot resist against the change in resin pressure during flow propagation.
Resin Transfer Molding (RTM)
In Resin Transfer Molding (RTM), fabric layers are cut in desired dimensions, then placed inside the lower half of the rigid mold. Compared to VI process, rigid mold is used as an upper mold in RTM. The fabric preform is compacted until reaching desired thickness by enclosing it with a rigid upper mold. A resin is injected into the mold with positive injection pressure to impregnate the fabric preform. After the resin arrives at the vent and it is ensured that the fabric preform is completely impregnated with resin, the injection is discontinued by closing the inlet valve. After the composite part reaches its green strength, it is ready to be demolded.
Even though using rigid upper and lower molds increases the investment cost and limits to produce large scale parts compared to VI process, this process has advantage to manufacture composite parts with high mechanical properties and excellent surface quality.
For further details you can check our article about Resin Transfer Molding.
Autoclave Molding
An autoclave is identified as a large pressure vessel with a heating capability. Autoclave molding is one of the widely used method to manufacture advanced composites in aerospace industry. In the first stage of autoclave processing, prepreg fabrics (impregnated fabrics with uncured resin) are cut in desired dimensions, stacked, and placed over the lower mold halve. In general, this stage is practiced by hand lay-up, but it will have great advantage if ATP is used. After the fabric placement stage is completed, peel ply, breather and vacuum bag are placed on the fabric preform, sequentially. In the next stage, vacuum is applied inside the mold. After securing any air leakage inside the mold, the mold is placed inside the autoclave. In autoclave, temperature is applied for initiating the curing cycle and completing the solidification of resin and external pressure is applied for consolidation of fabric layers. The main advantages of autoclave molding are listed as:
· Capability to manufacture parts with high mechanical properties
· Low potential to form dry-spots in the part compared to VI and RTM
However, the main drawbacks of this process are:
· High equipment cost because of the initial investment of an autoclave
· Part size is limited to the size of the autoclave