Last Updated on: 14 6 月, 2025

 

Compare and Contrast Composite Liquid Molding, Resin Molding and Liquid Resin Infusion Molding Processes in One Article

 

1. Overview of Liquid Composite Molding (LCM) Process

 

There are many manufacturing technologies for high-performance composites today, the most widely used of which is prepreg technology. However, some parts can also be made directly from low-viscosity resins and dry reinforcements using liquid composite molding (LCM). The process selected for manufacturing a specific part depends on the required geometry, performance, cost and production efficiency. The composite liquid molding process (LCM) is significantly different from prepreg-based manufacturing technology. In the LCM process, resin and reinforcement fibers are combined and cured in the same molding operation, providing structural manufacturers with different processing routes.

 

 

Figure 1 Common Continuous Fiber Reinforced Composite Molding Technology

 

In recent years, many LCM processes have been developed (Figure 1), among which resin transfer molding (RTM), liquid resin infusion (LRI) and resin film infusion (RFI) are the main and most common processes. Figure 2 shows the main processing flow of the LCM process.

 

 

Figure 2 LCM Main Process Flow

 

In most LCM processes, a preformed reinforcement material called a preform is placed in a mold and impregnated with resin. The impregnation path can be along a plane, thickness, or a combination of plane/thickness, as shown in Figure 3 below.

 

 

Figure 3 Resin Infusion Direction in LCM Process

 

Although the resin infusion path helps determine the type of LCM process used, it is not the most important factor. The main difference between injection and infusion is the complexity of the mold and the pressure applied. For RTM, matched metal tools are usually used, which can provide good dimensional tolerances and surface finish for each surface of the part, and the injection pressure is usually greater than 1 bar. For infusion, one side of the mold will be a flexible vacuum bag, and the maximum pressure that can be applied to the part is atmospheric pressure. The main advantages of the LCM process are:

 

 Potentially higher degree of part integration

 

 Reduced investment in autoclaves and cold storage

 

 Reduced transportation costs

 

 Reduced bagging, packaging, trimming and sorting waste

 

 Shortened processing cycle

 

 Suitable for higher production rates

 

 Better thickness accuracy and surface finish compared to prepreg

 

2. Overview of Resin Transfer Molding RTM Process

 

Resin Transfer Molding (RTM) is a composite manufacturing process that pumps a low-viscosity, reactive resin into a matching mold containing a preform or reinforcement material. The resin fills the mold and wets the fibers before curing. RTM is used to manufacture high-quality parts in many applications. Its main features are relatively large-scale automated production and net shape and high dimensional tolerances. The RTM process is considered a cost-effective alternative to manual composite molding processes and has traditionally been used by the aerospace industry to manufacture primary and secondary structural parts

 

 

Figure 4 Airbus A380 aileron spar Manufactured by Airbus France, using HexFlow® RTM Resin and Injectex® Woven Carbon Fiber Reinforcement

 

2.1 Advantages of the RTM Process

 

1. a) Resin and Reinforcement Fiber Separation

 

▪ Dry fiber structures are easier to handle and drape

 

▪ Thicker and complex shapes can be achieved

 

▪ Adhesives facilitate the handling of dry reinforcement materials and layups

 

1. b) High-performance precision molded parts can be obtained

 

▪ Tight dimensional tolerances

 

▪ Good surface quality

 

▪ Mechanical properties comparable to autoclave molded parts

 

1. c) Void content <1%d) Reduce steps and reduce costs

 

▪ No bagging required

 

▪ Minimum resin handling can be achieved

 

▪ Reduced trimming and finishing operations

 

▪ Short curing cycle

 

▪ Low pressure operation (typically less than 7 bar)

 

1. e) The process can be automated

 

▪ High productivity

 

▪ Low scrap

 

2.2 Disadvantages of the Process

 

o Higher cost when used for large parts

o Only low viscosity resins (0.1 to 1 Pa.s) can be used

o Not cost-effective for low production rates

o Improper tool sealing can lead to resin leakage

o Lower toughness compared to the latest generation of prepregs

 

2.3 Processing Process Most RTM Processes include the Following Processes (Schematic Diagram is Shown Below):

 

a), Preforming (Prefabrication):

Dry reinforcement material is processed into a preform with the final geometry. The purpose is to facilitate handling and prevent the fabric from slipping during injection molding.

 

b), Molding:

First, the mold containing the preform is fully vacuumed to remove air; then the resin is injected into the mold. During the injection process, the resin fills the mold cavity and completely impregnates the preform.

 

c), Curing:

The mold is heated to achieve polymerization/curing of the resin.

 

d), Demolding:

Curing is completed and the part has sufficient strength for demolding when it cools.

 

 

Figure 5 RTM Process Flow Diagram

 

3. Overview of Liquid Resin Infusion (LRI) process

 

Liquid Resin Infusion (LRI) is a variant of the traditional RTM process, which uses a single-sided mold and combines it with a specific vacuum bag device.

 

The vacuum bag device can provide resin distribution and curing. Since LRI is carried out under a vacuum film, it does not require a special mold, so the manufacturing cost is greatly reduced, and large and thick complex parts can be made with this process.

 

 

Figure 6 Schematic Diagram of a Typical LRI Process

 

3.1 LRI process Flow

 

1. a) Laying up the fiber preform:

The reinforcement fibers are placed on the rigid tool surface and once the reinforcement layer is complete, they are covered with a resin distribution medium and finally with a formable vacuum bag.

 

1. b) Resin preparation:

In some cases, the resin can be preheated to give it a lower viscosity. If the resin has been treated, degassing is recommended.

 

1. c) Impregnation of the preform with resin:

 

Depending on the part size and shape, the resin is introduced through a single or multiple inlets. A network of distribution channels for the resin is integrated into the vacuum bag device to control the flow of resin into complex parts.

 

Before impregnation, air is exhausted from the preform through the vacuum port. A resin tank is connected to the inlet. The tank can be maintained at atmospheric pressure.

 

The pressure difference between the vacuum port and the vacuum tank forces the resin into a highly porous flow medium, which distributes the resin on the surface of the part before being injected through the thickness of the part.

 

The vacuum bag provides consolidation pressure. Sometimes a second vacuum bag can be used to ensure vacuum and consolidation pressure.

Figure 7 Injection Bag Sample

 

1. d) Curing the Impregnated Preform:

The infusion process is usually carried out at an intermediate temperature, when the resin viscosity is low enough for good infusion, but not enough to start the curing process. Once the infusion is completed, the temperature will be increased to the final curing temperature. Based on experience and the use of appropriate reinforcement materials, high-quality components with 60% fiber volume required for high-quality composite parts can be obtained.

 

3.2 Advantages of LRI Process Compared to RTM

 

Compared with the RTM process, the main advantages of LRI are:

 

▪ Larger parts can be processed, especially parts with large surfaces, such as thick covers and fuselage panels;

▪ Compared with the relatively expensive RTM paired molds, LRI uses a simple single-sided mold;

▪ Reduced the cost of basic equipment and tools.