Vacuum Assisted Resin Transfer Molding

Since VARTM processes use only vacuum pressure for both injection and cure, the single biggest advantage of VARTM is that the tooling cost is much less, and simpler to design, than for conventional RTM. In addition, since an autoclave is not required for curing, the potential exists to make very large structures using the VARTM process. Also, since much lower pressures are used in VARTM processes, lightweight foam cores can easily be incorporated into the lay-ups. VARTM type processes have been used for many years to build fiberglass boat hulls, but have only recently attracted the attention of the aerospace industry.

Line for Vacuum Degassing Resin Prior to Infusion line to Vacuum

Sealant Tape

Line for Vacuum Degassing Resin Prior to Infusion line to Vacuum

Sealant Tape

Resi

Resin Trap

Tool

Fig. 7.48. Typical VARTM Process Setup1

Resi

Resin Trap

Tool

Fig. 7.48. Typical VARTM Process Setup1

A typical VARTM process, shown in Fig. 7.48, consists of single-sided tooling with a vacuum bag. VARTM processes normally use some type of porous media on top of the preform to facilitate resin distribution. The porous distribution media should be a highly permeable material that allows resin to flow through the material with ease. When a porous distribution media is used, the resin typically flows through the distribution media and then migrates down into the preform. Typical distribution media include nylon screens and knitted polypropylene. Since resin infiltration is in the through-the-thickness direction, race tracking and resin leakage around the preform are largely eliminated.28

Since the VARTM process uses only vacuum pressure for both injection and cure, autoclaves are not required and very large part sizes can be made. Ovens and integrally heated tools are normally used, and since the pressures are low (i.e., <14.7psia), low-cost lightweight tools can be used. Some manufacturers use double vacuum bags to minimize variations in compaction pressure, and guard against potential vacuum leaks in the primary vacuum bag. A layer of breather between the two bags increases the ability to remove any air from leak locations. Reusable vacuum bags can also be used to reduce the cost of bagging complex shapes.

The resins used for VARTM processing should have even a lower viscosity than those used for traditional RTM. Resin viscosities less than 100 cP are desirable to give the flow needed to impregnate the preform at vacuum pressure. Vacuum degassing prior to infusion is normally used to help remove entrained air from the mixing operation. Some resins may be infused at room temperature, while others require heating. It is desirable to keep the resin source and vacuum trap away from the heated tool. This makes it easier to control the temperature of the resin at the source and minimizes the chance of an exotherm at the trap.

For large part sizes, multiple injection and venting ports are utilized. As a rule of thumb, resin feed lines and vacuum sources should be placed about 18 in. apart. As the resin moves away from its source, its velocity decreases in accordance with Darcy's law, and the final thickness and the resin content decreases. It is also more difficult to obtain high fiber volume contents in thick preforms. Since perfect fiber bundle nesting does not occur, there is an increase in free volume with every additional layer, which results in lower fiber volume contents in thick parts.

Since the pressure is much lower than that normally used in the conventional RTM or autoclave processes, it is more difficult to obtain as high a fiber volume percent as with the higher pressure processes; however, this process disadvantage is being overcome with near net preforms. In addition, the VARTM processes cannot hold as tight dimensional tolerances as conventional RTM, and the bag side surface finish will not be as good as a hard tooled surface. Thickness control is generally a function of the perform lay-up, the number of plies, the fiber volume percent, and the amount of vacuum applied during the process.

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