Send Your Inquiry Today
Quick Quote

Resin Transfer Molding: Definition, Characteristics, Types, and Applications

Welcome to our comprehensive introduction to Resin Transfer Molding (RTM) technology. This article will explore various aspects of RTM, including its definition, working principles, application scenarios, and types of materials used. Whether you are an engineer, designer, or a materials science enthusiast, you are sure to find valuable information and inspiration in this article.
Continue reading to learn more.

What Is Resin Transfer Molding?

Resin Transfer Molding (RTM) is a manufacturing process used to produce high-performance composite material parts, particularly suited for medium volume production. It involves a closed mold system where both surfaces of the part are formed by rigid double-sided molds, typically made of materials like aluminum and steel, and occasionally composites. RTM is preferred for producing lightweight yet high-strength composite material parts and is a favorite for many high-performance applications.

How Does Resin Transfer Molding Work?

The principle of RTM is based on a closed mold process. Pre-shaped preforms or fiber-reinforced materials shaped according to the finished product’s required shape are placed inside the mold cavity. The mold is then closed and sealed. Under pressure, catalyzed low-viscosity resin is pumped into the heated mold, expelling air through exhaust vents until the mold is filled. This process ensures the preform is completely impregnated with resin. After the injection phase, the curing cycle begins, during which the resin polymerizes into a hard plastic. The curing time depends on the mold temperature and the chemical composition of the resin used.

RTM is primarily used for molding parts with large surface areas, complex shapes, and smooth surfaces, making it highly suitable for large-scale production of composite material parts. The process allows precise control of part thickness and provides high-quality, durable surface effects on both sides of the part.

What Is the Origin of Resin Transfer Molding?

The origin of RTM can be traced back to the development in the shipbuilding sector in the 1950s, post-World War II. However, it was not until the 1980s that this process began to be applied in aerospace composites. Although the basic concept and initial application of RTM started in the shipbuilding industry, significant advancements and broader applications, particularly in high-performance sectors like aerospace, appeared decades later.

What Materials Are Used in Resin Transfer Molding?

Reinforcing Fibers

Various types of fibers can be used as reinforcement materials in RTM, including:

  • Glass fibers: E-glass, R-glass, and S-glass are common types of glass fibers used in RTM processes.
  • Carbon fibers: Known for their high strength and high modulus, offering lightweight yet strong characteristics.
  • Aramid fibers: Noted for their impact resistance and toughness.
  • Quartz: Sometimes used in RTM processes.
  • Other fibers: These include natural fibers or other synthetic fibers customized for specific applications.
    The forms of reinforcement materials are diverse, such as random mats, two-dimensional woven rovings, or pre-shaped fiber-reinforced materials tailored to the desired part shape.

Resin Systems

The resin systems used in RTM are typically thermosetting polymers, including:

  • Epoxy resins: Renowned for their excellent mechanical properties and good thermal resistance.
  • Polyesters: Commonly used due to their cost-effectiveness and good mechanical properties.
  • Vinyl esters: Well-balanced in performance and cost between polyesters and epoxy resins.
  • Phenolics: Have flame-retardant properties but may encounter difficulties when used with certain fiber types.
    The choice of resin system depends on the performance requirements of the final product, such as mechanical strength, thermal resistance, and environmental durability. The resin mixes with a catalyst or hardener and begins to cure once injected into the mold.

What Is the Process of Resin Transfer Molding?

The RTM process includes several key steps:

  1. Creating the preform: The first step is determining the type of fiber to use and creating a preform or glass fiber reinforcement, which is pre-shaped to the final product’s shape.
  2. Preform stacking: The preform is then placed inside the mold cavity, which has been shaped according to the required part. This allows precise control of part thickness and smoothness on both sides.
  3. Mold closing: Once the preform is in place, the mold is closed and clamped, preparing for resin injection.
  4. Injection stage: Under pressure, catalyzed low-viscosity resin is injected into the heated mold. The resin expels air through exhaust vents until the mold is filled, ensuring the preform is fully impregnated. The injection stage is critical as insufficient impregnation can lead to dry spots and weak areas.
  5. Curing: After the mold is filled with resin, the curing cycle starts. The resin polymerizes into hard plastic. The curing time depends on the mold temperature and the resin’s chemical composition.
  6. Demolding: After the resin has cured, the mold is opened, and the finished part is removed.
    RTM may also include variations, such as applying a gel coat for high-quality surface smoothness and using Vacuum Assisted Resin Transfer Molding (VARTM) to enhance fiber saturation.

What Equipment Is Used in Resin Transfer Molding?

The equipment used in RTM includes various specialized tools and machines designed to facilitate efficient production of composite material parts. Here are some key components:

  1. Molds: RTM requires rigid molds capable of withstanding the pressures and temperatures of the resin injection and curing processes. These molds are usually made from materials like steel or aluminum.
  2. Resin injection equipment: This includes pumps and systems for mixing and injecting the resin into the mold. The resin is mixed with a catalyst or hardener and then injected at a controlled low pressure (about 5 psi) to ensure thorough impregnation of the fibers.
  3. Static mixers: Used to regulate the proportion of resin and catalyst, ensuring a consistent and uniform mix before injection into the mold.
  4. Gel coat applicators: Used to apply a gel coat on the mold surface before placing the fibers, which helps achieve high-quality molded part surface smoothness.
  5. Directional fiber preforming machines: These machines help to precisely arrange the fibers in the mold before resin injection, which is crucial for achieving optimal mechanical properties and part geometries.
  6. RTM presses and mold handling equipment: These are used to close and seal the mold and handle the mold during the production process.
  7. Secondary finishing equipment: Includes tools and machines for trimming, drilling, and finishing the molded parts, such as CNC milling machines for precise cutting and shaping.
  8. Quality assurance laboratories: Equipment for testing and ensuring the quality of manufactured parts is also essential.

How Strong Is Resin Transfer Molding?

RTM is known for producing robust, durable parts with a high strength-to-weight ratio. Parts produced via RTM typically contain 25-50% fibers, making them lightweight and durable. The process enhances the compression rate of laminates, the glass-to-resin ratio, and an excellent strength-to-weight ratio. This makes RTM parts suitable for applications requiring robust performance, such as in the automotive, aerospace, and maritime industries. The strength of RTM parts is also due to the process’s ability to produce complex geometric shapes with two processed surfaces, which are structurally more effective than simpler shapes.

What Are the Key Features of Resin Transfer Molding?

The main features of Resin Transfer Molding include:

  • Production of large, robust parts: RTM can produce both sturdy, durable, and aesthetically pleasing large parts.
  • High strength-to-weight ratio: Parts produced through RTM have a high strength-to-weight ratio, which is particularly beneficial for applications requiring lightweight yet strong parts.
  • Complex geometric shapes: RTM can manufacture complex geometric shapes, regardless of their size. The use of molds that can generate detailed features on both the interior and exterior surfaces of parts enhances this capability.
  • High glass-to-resin ratio: The process achieves an excellent glass-to-resin ratio, which helps improve the final product’s mechanical strength and quality.
  • Multicolor capability and aesthetic quality: RTM can incorporate multiple colors and provide high-quality surface effects. The process can also add gel coats inside the mold during processing to further enhance the effects.
  • Cost-effectiveness: The materials used in RTM generally cost less than those used in other molding processes. Additionally, molds can be made from materials like aluminum, and because the temperatures used in the molding process are lower, the costs are reduced.
  • Low material wastage: RTM is known for minimal material waste, making it more environmentally friendly compared to other molding techniques.
  • Flexibility in mold making and material selection: RTM offers great flexibility in mold making and material selection, which can be customized according to different production scales and requirements.

What Is the Surface Treatment of Resin Transfer Molding?

The surface treatment of RTM can employ various techniques to enhance the smoothness and quality of molded parts:

  1. Gel coats: Gel coats can be applied inside the mold to provide high-quality, durable surface effects. Gel coats can be used to improve surface effects or provide a paintable surface.
  2. In-mold coatings: In-mold coatings can be used to improve the appearance of parts and minimize the printing penetration of reinforcing fibers. These coatings can match almost any color, including the color of the gel coat itself.
  3. Glass fiber veils: A thin layer of glass fiber veil can be applied to the preform to minimize visible glass in the final product, which helps reduce perforation phenomena.
  4. Surface treatment agents: A specialized demolding system consisting of cleaners, primers, sealers, and topcoats (release agents) has been developed to ensure thorough protection of the mold and provide multiple demolding cycles. These release agents prevent defects like pinholes and fisheyes, helping maintain the quality of the surface treatment.
  5. Flame surface treatment: Flame surface treatment can be used to alter the surface characteristics of molded parts, particularly when using thermoplastic resins in RTM. This treatment can affect the mechanical properties of carbon fiber-reinforced polyamide composites.

What Is the Color of Resin Transfer Molding?

The color of RTM parts can be matched to almost any desired shade according to customer requirements. This includes matching the color of the gel coat or in-mold coating itself. The resin systems used in RTM can be formulated with specific pigments to achieve the required color of the final product.

What Are the Applications of Resin Transfer Molding?

Resin Transfer Molding products

RTM is capable of producing high-quality, complex, and large parts, making it widely used across various industries. Here are some of the main applications:

  1. Aerospace Industry: RTM is used to manufacture high-performance aerospace parts. This includes fuselage parts and other structural components that require precision and strength.
  2. Automotive Industry: In the automotive industry, RTM is used to produce lightweight, high-strength parts such as body panels, frames, and other structural components. This helps reduce the overall weight of vehicles, which is crucial for improving fuel efficiency and performance.
  3. Maritime Industry: RTM is used to produce various marine parts, including hulls and other structural components of ships. The process can manufacture large, complex shapes and has waterproof and resistance capabilities against harsh marine environments, making it highly esteemed.
  4. Wind Energy: Specifically, RTM has been used to manufacture the lower structures of wind turbine blades. This application benefits from RTM’s ability to produce large, durable parts capable of withstanding the mechanical stresses and environmental conditions associated with wind turbines.
  5. Infrastructure and Construction: RTM is also used in the construction industry to produce items like beams and panels that require high strength and durability. The process can incorporate various types of reinforcing materials to meet specific structural requirements.
  6. Sports and Recreation: For sports equipment such as golf carts and other recreational vehicles, RTM maintains lightweight characteristics while also providing the necessary strength and durability.

What Are the Limitations of Resin Transfer Molding?

The limitations of RTM include:

  • High mold and equipment costs: The process involves high costs for mold design and maintenance, as molds can be complex and made from materials like steel or aluminum.
  • Lower production speed: Compared to other molding processes like injection molding, RTM has a slower production speed because the resin needs time to cure within the mold.
  • Material wastage: Material wastage can occur during the preparation and transfer stages, particularly due to the use of sprues and overflow channels, leading to excess material being discarded.
  • Air entrapment: To avoid defects in the final product, air must be properly expelled. Air can become trapped in the mold, leading to voids or incomplete impregnation of the reinforcing material.
  • Limited to thermosetting polymers: While RTM is widely used, it is primarily suited for thermosetting polymers and not applicable for thermoplastic plastics, limiting the range of materials that can be used.
  • Smaller production volume: RTM equipment typically cannot accommodate as many mold cavities as injection molding machines, meaning fewer units are produced per cycle.
  • Maintenance requirements: The molds used in RTM, especially those with inserts, may require more extensive maintenance to maintain quality and functionality.


Resin Transfer Molding offers a robust solution for producing high-quality, complex, and durable parts across a wide range of applications. Despite its limitations, the benefits of RTM, such as high strength-to-weight ratio, complex geometric capabilities, and cost-effectiveness, make it a valuable process in fields that demand high-performance components.

Looking Forward
Your Next Project!

Update cookies preferences
Scroll to Top