Resin Transfer Molding

RTM processes use closed (2-sided) molds and low pressure to force external resin to enter the mold and flow through the layup. A vacuum can be used to assist RTM and in specialized processes like Vacuum Assisted Resin Transfer Molding (VARTM). External vacuum can enhance resin flow through the layup to help speed up the process and remove trapped air. Resin Transfer Molding or RTM as it is commonly referred as a “Closed Mold Process” in which reinforcement material is placed between two matching mold surfaces — one being male and another being female. The matching mold set is then closed and clamped and a low-viscosity thermoset resin is injected under moderate pressures (50 – 100 PSI typically) into the mold cavity through a port or series of ports within the mold. The resin is injected to fill all voids within the mold set and thus penetrates and wets out all surfaces of the reinforcing materials. The reinforcements may include a variety of fiber types, in various forms such as continuous fibers, mat, preforms, or woven type construction as well as a hybrid of more than one fiber type. Vacuum is sometimes used to enhance the resin flow and reduce void formation. The part is typically cured with heat. In some applications, the exothermic reaction of the resin may be sufficient for proper cure.

Resin Transfer Molding (RTM)

is used when higher volumes are needed, finished inside and/or part thickness is critical. A benefit of this process is both surfaces have a “Class A” finish. Tooling cost is more for this process because of the heavy structured tooling needed to withstand the hydraulic pressure when the male & female tools are closed. But, the part piece price goes down because its less labor intensive and automated.
Process – Fiberglass mat pre-cut patterns are placed in the female tool. The male tool is hydraulically lowered into the female tool. With hydraulic  pressure, the resin flows through the part. When the resin cures the male tool is removed and the part is taken out. The part can have a gel coat finish or top coat paint.
If you need to manufacture large parts with a high strength to weight ratio, resin transfer molding (RTM) might just be the process you need. Resin transfer molding is a form of composite liquid molding in which resin is mixed with a catalyst or hardener before being injected into a closed mold filled with reinforcing fibers. RTM’s strength, durability and high glass-to-resin ratio make it perfect for creating large, strong parts. In addition, RTM is also a viable option for molding complex shapes, up to and including compound curves.
CREATING THE PREFORM
PREFORM LAYUP
CLOSING MOLD
INJECTION PHASE
CURING PHASE
The preform, or contoured fiberglass reinforcement, is the matrix, already in the shape of the finished product, into which the resin will be injected. First, the type of fiber must be defined. There are several different fiber types available for use in RTM composites, from random mats to two-dimensional woven rovings.
Once the preform or fiberglass reinforcement is created, it is packed into a mold cavity that has the shape of the desired part.
The mold cavity is then closed and clamped. The mold cavity allows for precise control over part thickness and allows for a smooth finish on both the A and B sides of the part. Gel coats may be used inside the mold to provide a high-quality, durable finish.
Catalyzed, the low-viscosity resin is then pumped into the heated mold under pressure, displacing the air through vents , until the mold is filled. The injection phase must guarantee the complete impregnation of the preform. Bad impregnation of the fibers results in dry spot areas with missing adhesion between the layers.
After the injection phase, the curing cycle starts, and the resin polymerizes to become rigid plastic. Curing time varies and is dependent on the mold temperature and chemistry of the resin being used. A variation on standard resin transfer molding also exists. It is known as vacuum-assisted resin transfer molding (VARTM). In the VARTM process, a partial vacuum is used to pull the resin into the mold. This achieves a complete saturation of the strengthening fibers within the mold. Because VARTM uses lower plunging forces when compared with its standard counterpart, it is extremely economic, especially in regards to the creation of larger parts or parts which utilize thicker fibers.

BENEFITS OF RTM

  • Good surface quality
  • Wide range of reinforcements
  • Large, complex shapes
  • Dimensional tolerances
  • Low capital investment
  • Less material wastage
  • Tooling flexibility
  • Low environmental impact
  • Labor savings
  • Ability to add inserts and reinforcements at a point of infusion for greater strength
  • Zero air entrapment within the product.
Resin transfer molding continues to be an increasingly popular method of fabrication. Contact Prazastinav Compositech Private Limited today for further information on how we can assist in your next molding project. To achieve repeatability and consistency in your fully automated RTM or VARTM production process, Prazastinav has custom-designed its equipment to instantaneously monitor and control all of your critical process variables. These variables, which you digitally enter at the operator- interface touchscreen, include ratio, independent material temperatures to 500F, vacuum levels in the heating and degassing tanks and inside the mold, plus inside-the-mold mixed resin pressures and mixed resin flowrates. With this level of process control and automation, our customers fill void-free vacuum-assisted preforms on a production basis. They also easily select saved part recipe presets to consistently dispense air-bubble-free resins into RTM or VARTM molds of many different sizes and shapes.
Where your budget allows, we can provide you with state-of-the art resin injection systems with digital PLC and PC control that operate in real-time with digital pressure sensors and mass flowmeters — to track and verify mixed material penetration into preforms. Our systems operate at very low PSIA infusion pressures for Vacuum-Assisted Resin Transfer Molding (VARTM) and at elevated injection pressures from 100 to 500-PSI for closed-mold Resin Transfer Molding (RTM) applications. If you are at the experimental stage or on a temporary tight budget, we can offer alternative lower-cost RTM and VARTM systems that will help you get your project under way.