In order to understand the physics behind the vacuum infusion process (VIP), net compaction, fiber volume, resin content and the effects it has on the quality of the finished composites product, we need to take a closer look into what happens during the infusion process. The driving force behind VIP is a pressure differential (atmospheric pressure outside the system minus the absolute pressure inside the bagged system) across the fiber preform – which pushes the resin through the layup.

pressure prior to evacuation

Prior to evacuation the mold, dry preform, flow media and vacuum bag are all in a relaxed state. At this point, the pressure within the bagged system
is the same as that outside of the bagged system, namely atmospheric pressure. Thus, the net pressure compacting the dry preform is zero.

Vacuum Resin Infusion - Net compaction pressure after evacuation

After evacuation and prior to resin flow, vacuum has been applied so that absolute pressure Pabs in the system is uniformly low (vacuum is uniformly high)
and the mold, preform, flow media and vacuum bag are in a compacted state. With 10 mbar inside the bagged system and 1000 mbar outside the bagged system the pressure differential is 990 mbar, which is the net compaction pressure upon the dry preform and the force that drives the resin through the laminate.

Once the resin inlet is opened, the resin begins to flow and the pressure of the filled volume approaches atmospheric pressure since it is connected via the
resin inlet line to the resin reservoir which is open to atmoshere. The rise in pressure inside the bagged system acts against the atmospheric
pressure outside the bagged system. The pressure differential is the remaining net compaction upon the preform in the mold and it drops during the infusion process, as shown in the picture below.

Vacuum Resin Infusion - Net compaction pressure during the infusion process

This pressure differential will vary depending on a number of factors including the permeability of the dry preform and flow media and the timing sequence of clamping the resin inlet(s) and vacuum lines. With less compaction, more resin can flow into the preform. In traditional resin infusion, this net compaction can approach zero, resulting in laminate relaxation which leads to increased thickness and reduced fiber volume fraction. Thus, achieving a high fiber volume fraction requires compaction of the layup.

The membrane assisted vacuum resin infusion process utilizing the MTI® hose (Membrane Tube Infusion) allows to adjust or control the net compaction pressure during and after the infusion only through the use of hydrostatic affected by the height difference between the mold cavity and resin source. This affects the fiber to volume ratio as well as the thickness of the laminate.