The different closed mold processes
25 April 2024

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When it comes to precision part production, you have many choices. However, not all options are created equal. Closed mold processes such as Light Resin Transfer Molding (LRTM), Reusable Bag Molding (RSBM) and Vacuum Infusion Process (VIP) are all popular ways to create precision parts. These processes often offer a lower cost and can be used in a variety of applications. Let’s explore these, so you can determine if it’s right for your business and how Magnum Venus Products (MVP) can help you today.

In the process of closed molding, the raw materials of fibers and resin cure inside a two-sided mold or within a vacuum bag that is devoid of air. Closed molding is an automated approach that requires special equipment that you will most likely find in large plants that are creating a high volume of parts.

What is it?

Closed molding is a consistent and repeatable manufacturing method enabling higher quality parts, faster production, reduced emissions, and with less waste compared to open molding. With this method, fiber reinforcements are laid into the base mold, the mold is closed and sealed, and the resin is injected into the closed mold cavity. Once the resin has cured, the mold is opened and the part is removed.


Closed Molding Processes

To determine what type of closed molding is best for your production, you’ll need to consider each process.

Vacuum Bag Molding

The use of this process improves the mechanical properties of laminate, which consists of two or more layers of fiber reinforcement bonded with a resin. The vacuum forces out trapped air as well as excess resin, resulting in a compacted product. The high fiber concentration allows for better adhesion between layers of sandwich construction.

Vacuum Infusion Process (VIP)

This process is best for manufacturing large structures. It produces strong, lightweight laminates. It also produces less emissions, when compared with open molding. VIP uses the same low-cost tooling as open molding, requiring minimal equipment.

Resin Transfer Molding (RTM)

Classic RTM is used to produce a high volume of parts per day. The process uses a composite mold reinforced by metal which is then injected under high pressure. This closed molding method uses reinforcement material, and the mold is clamped. Injection portals pump resin into the mold under pressure. This process is best for parts that are highly complex. It finishes the mold smooth on all exposed surfaces. This process has the ability to be automated. Because the material is laying up reinforcement, the material dries inside the mold. You can use a variety of different materials and orientations, including 3-D reinforcements.

Some key benefits of RTM over other processes include:

  • Good surface quality
  • Wide range of reinforcements
  • Large, complex shapes
  • Dimensional tolerances
  • Low capital investment
  • Less material waste
  • Tooling flexibility
  • Low environmental impact
  • Labor savings
  • Add inserts and reinforcements at a point of infusion for greater strength
  • Zero air entrapment

Light Resin Transfer Molding (LRTM)

LRTM, one of the most common closed mold methods, uses a rigid mold and semi-rigid counter mold which is sealed by vacuum. Resin is then injected with an injection machine.

Flex Molding Process

Sometimes referred to as reusable bag molding, Flex Molding was developed by MVP and utilizes one rigid mold half and one flexible mold half, either a plastic bag or a silicone membrane.

Fast Flow LRTM

The latest closed mold process from MVP, Fast Flow LRTM is a blend of the Flex Molding Process and LRTM utilizing collapsible resin channels and a unique channel layout for faster injection times. The process enables the production of larger parts with even higher resin viscosity, and eliminates the need for trimming once parts are demolded.


Resin Infusion Process

Resin Infusion Process is utilized for production of large parts and where higher glass content is required. MVP created a single injection port solution for simple operation and developed a unique silicone resin channel network and plastic bag closing system to speed production times for this process even more.


For creating rods or bars, pultrusion is the process used most frequently. Continuous strands of reinforcement pull through a resin bath to saturate them. Then, the process pulls the heated steel molds to sculpt the composites. It’s a continuous process that can be automated. The finished product is strong and requires little labor interaction.

Reaction Reinforced Injection Molding (RRIM)

RRIM is a process for external and internal automotive parts. Two or more resins heat separately then combine with milled glass fibers. Molds receive the mixture via injection under high pressure. The resin cures quickly. RRIM composites have many advantages, including a fast cycle time, low labor, low mold-clamping pressure and low scrap rate. The RRIM process requires special resins and reinforcements.

Centrifugal Casting

In centrifugal casting, a deposit of reinforcements and resin goes inside the surface of a rotating mold. The centrifugal force holds them in place as the material cures. This process produces hollow parts, such as pipes, including extremely large diameter ones for oil and chemical applications.

Continuous Lamination

Continuous lamination is the process to create flat or corrugated sheets and panels, which you’ll find in truck and RV sidewalls, road signs, building panels, and other materials. Fibers and resin combine, squeezed between two plastic carrier films, and then go through a conveyor process. Forming rollers shape the sheets, and the resin cures to form the composite panel.