RTM Process Overview, Advantages and Processing Flow

Abstract

Today, there are many manufacturing technologies for high-performance composites, among which prepreg technology is the most widely used. 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.

In order to enable readers to have a clearer understanding of the characteristics and advantages of the LCM process, this official account will launch a series of articles on the LCM process of composite materials. In recent years, many LCM processes have been developed, among which Resin Transfer Molding (RTM) and Liquid Resin Infusion (LRI) are the main and most common processes. In this article, the characteristics of the RTM process will be mainly introduced.

Attachment: This official account’s composite material LCM technology guide article:

“LCM process overview and main advantages analysis”

Overview of 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.

The door hinge arms for the Airbus A380 are manufactured by Airbus Helicopters using HexFlow® RTM6 resin and HexForce® G0926 carbon fiber fabric. There are 16 door hinge arms on each A380. In addition to reducing weight, they also provide better insulation in the cabin, reducing heat loss

RTM is used in many applications to manufacture high-quality parts. Its main features are: automated production in relatively large quantities 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.

Airbus A380 aileron spar manufactured by Airbus France using HexFlow® RTM6 resin and Injectex® woven carbon fiber reinforcements

Embraer Legacy 500 BizJet horizontal tail fairing using HexFlow® RTM6-2 resin and HexForce® G0926 carbon fiber fabric

2.1 Process Advantages

a) Separation of resin and reinforcing fibers

Dry fiber structures are easier to handle and drape

Thicker and more complex shapes can be achieved

Adhesives facilitate the handling of dry reinforcement materials and layups

b) High-performance precision molded parts can be obtained

Strict dimensional tolerances

Good Surface Quality

Mechanical properties comparable to autoclave molded parts

c) Void ratio <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 (usually less than 7 bar)

e) The process can be automated

High Productivity

Low Scrap

2.1 Process Disadvantages

o Higher cost for large parts processing

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

o Not cost-effective for low productivity

o Improper tool sealing can cause resin leakage

o Compared with the latest generation of prepreg, the toughness is lower

RTM Process and Equipment

3.1 Processing Flow

Most RTM processes include the following processes (schematic diagram is shown below):

a) Preforming (prefabrication): Processing the dry reinforcement material 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: Heating the mold to achieve polymerization/curing of the resin.
d) Demolding: Curing is completed and the part has sufficient strength for demolding when cooled.