​Light Resin Transfer Molding

Light Resin Transfer Molding

Light Resin Transfer Molding, or Light RTM, is a process by which composite products are manufactured using a closed mold system. The closed mold consists of an “A” side mold (base mold) and a semi-rigid “B” side mold (counter mold) that is sealed to the “A” side mold using vacuum pressure. Resin is drawn into the resulting cavity under vacuum.

The resin infusion may be assisted by a resin injection pump, which will accelerate the infusion process. Once an “A” side mold is cured, the “B” side mold is removed and the part is demolded from the “A” side mold.


Light RTM is a very versatile manufacturing process with the following characteristics:

  • High productivity
  • Mold cycles as fast as 20 minutes per part using a single “B” side mold and three “A” side molds
  • Reduced labor costs
  • Cosmetic surfaces on both “A” and “B” side of part
  • Enhanced dimensional stability
  • Improved process control
  • Consistent material usage
  • Long mold life
  • No VOC emissions
  • Greatly improved shop working conditions

While LRTM can be an improvement over open molding for most products, it requires that the product you wish to manufacture is designed with this process in mind and that the molds you use are designed for the process. A common mistake is to simply try to produce a part that was originally designed for open molding with this process. A better approach is to consider the possibilities and limitations of the LRTM process and design or redesign your products to take advantage of them. This can result in substantial savings as well as improvements in productivity. A good rule to live by is “Design for the Process,” regardless of which process you use.

LRTM mold sets consist of a rigid “A” mold that is solidly reinforced and a semi-rigid “B” mold. The “A” mold is made with high temperature and low shrink resin. The “B” mold is semi-rigid and retains a small amount of flexibility. “B” molds are made with vinyl ester resin. This flexibility enables the “B” mold to fit perfectly with the “A” mold, which is essential to achieving the necessary vacuum pressures and precision parts.

LRTM production is more efficient than most other processes. You can use a single “B” mold in conjunction with multiple base molds to achieve production rates of up to three molded parts per hour. In this case there are three “A” molds, which move from the mold prep area through the production cycle, while the “B” mold remains in a fixed location.

What should you consider when switching to Light RTM?

There are always considerations when it comes to closed molding, but some of the major ones to think about when thinking about Light RTM are as follows:

  • Moderate start-up costs, higher than VIP or Silicone Bag Molding
  • Longer initial set-up time than VIP or Silicone Bag Molding
  • Not suitable for parts with negative draft
  • Part thickness is fixed at the time of mold construction
  • Glass / resin ratios less than 35%

See more in the Closed Mold Road Map.