Overview
Vacuum-assisted resin transfer molding (VARTM) is likely the most common acronym of all used in the discussion of low-pressure closed mold reinforced composite molding. The often-used VARTM acronym is most accurately applied to the process of vacuum infusion, which is where the composite is molded using a rigid mold to provide part geometry and a thin flexible membrane over the fiber, with outer atmospheric pressure compressing the fiber tight against the rigid mold surface.
It should be noted however that VARTM simply means to add vacuum at the exit vent of the molding tool, thus any form of resin transfer molding in which vacuum is applied to the vent would qualify for the VARTM association. Often, the aerospace industry will have very high fiber volume composites molded within a rigid matched mold set in which the fixed cavity mold is clamped closed using a press and the resin injection pressures typically range less than 100 psi, yet could be as high as 600psi as used to fill the mold encapsulating the dry fiber within.
VARTM Process
The vacuum-assisted resin transfer molding (VARTM) process has been developed to use effectively during the past two decades. This closed-mold technique can produce high-performance and various types of composites, especially fiber-reinforced polymer structures, at a low cost. The method involves primarily putting the fibers or cloth fabrics in a performance in the desired configuration.
Often, these fabrics are held together by a binder and pre-pressed to the mold shape. A top (second) matching mold tool is clamped over the first and vacuum-sealed used as a deformable vacuum bag. Then, the pressurized resin is injected into the cavity with the aid of a vacuum.
Afterward, the laminate is healed, and both injection and cure can occur at ambient or elevated temperatures. In this process, it is possible to use any fibers, and the stitched materials work well since the gaps allow for the rapid transport of resin. This is a flawless process using low-cost composite materials without prepregs and autoclaves compared with the conventional composite fabrication process used in the aeronautical field. The resin cup is typically open to the atmosphere, creating a pressure differential between the inlet and exit, causing the resin to be drawn into the layup. The figure above shows a typical VARTM process setup.
There are different names to describe this process, viz. vacuum-assisted resin infusion molding (VARIM), vacuum-assisted resin infusion (VARI), and vacuum bag resin transfer molding (VBRTM). Further, many other developments have been in the VARTM process, such as Seeman composites resin infusion molding process (SCRIMP), vacuum-assisted process (VAP), and controlled atmospheric pressure resin infusion (CAPRI).
Advantages of Vacuum-assisted Resin Transfer Molding (VARTM)
The VARTM procedure was formerly produced to fabricate superior and huge composite parts, such as infrastructure and transport structures. This process is cheap and established for mass production, and the basic principle is to develop the pressure difference between the vacuum and environment pressure to get the desired necessities. The points of advantages in the VARTM process are summarized below in the Figure below.
Figure 2. Advantages of VARTM
Defects in VARTM Processed Composites
Despite the VARTM process’s advantages, there are different types of defects, like fiber misalignment and voids, whereby defect-free manufacturing is impossible. In mass production, voids formation is the primary defect. Many factors like resin-flow pressure variation or temperature changes initiate voids formation in this process. The figure below shows void formation factors in the VARTM manufacturing process.
Figure 3. Voids Formation Factors
The figure below illustrates actual images of voids in different VARTM processes for woven (a) and non-crimp (b) reinforcement from experiments. This figure indicates that the voids’ conditions are higher in the mass production process (VARTM). This is why it is necessary to consider the manufacturing details (voids effect) in failure design conditions.
Summary
The method involves primarily putting the fibers or cloth fabrics in a preform in the desired configuration. A vacuum bag is applied over the preform, then, the pressurized resin is injected into the cavity with the aid of a vacuum. VARTM is beneficial for large-size parts and provides a lot of flexibility in material selection. The process is not under control once the vacuum is engaged and shifting of fiber, voids are very common defects.
About Addcomposites
Addcomposites is the provider of the Automated Fiber Placement (AFP) ecosystem - including the Fiber Placement System (AFP-XS), 3D Simulation and Programming Software (AddPath), and Robotic Cells (AddCell). With the leasing program for the AFP system (AFPnext), composites manufacturers can work with thermosets, thermoplastics, dry fiber placement, or in combination with 3D Printers on a monthly basis.
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