Vacuum  or Resin Infusion

Vacuum or Resin Infusion fits into the broad fabrication category of closed moulding processes where composite manufacturing is conducted within an enclosed cavity. In vacuum infusion the mould cavity pressure is lower than atmospheric pressure, in other words the resin flow is driven by vacuum.


  1. Apply mould release to the mould surface carefully.
  2. If a gelcoat surface is required, apply directly to the prepared mould and allow to cure. 
  3. Locate and stack all dry reinforcements into the mould in pre-determined locations that optimise the laminate performance.  This may include surface media1, glass reinforcement in the form of fabrics and continuous filament mat and core2. A peel ply is often positioned over the dry stack to ease the separation of the cured laminate from the vacuum bag3.
  4. A flexible film ('vacuum bag') is laid over the reinforcements and sealed airtight around the edge of the mould, beyond the perimeter of the required laminate.
  5. A minimum of two connections are put into the vacuum bag.  One is for resin entry, the other for air removal to achieve vacuum.
  6. With the resin entry point closed, the area under the bag is placed under vacuum by use of a vacuum pump. This compacts the reinforcement stack and removes all air from the system.  The vacuum integrity of the system can be tested using a leak detector around all seals.  All leaks need to be corrected before continuing.
  7. A drop test which isolates the part from the vacuum pump via the catch pot must be carried out to ensure the part can maintain full vacuum integrity for at least 15 minutes.  The gauge on the catch pot must not show a drop in vacuum of more than 1” of vacuum over 15 minutes.  Any drop in vacuum indicates air leaks in the system.  These need to be located and sealed before the drop test is repeated and infusion begins.
  8. Once the system is deemed leak proof, the vacuum point is re-opened.
  9. The container for resin supply is filled with the required amount of catalysed resin and the resin supply connection is opened so the vacuum pulls the resin through the tubes to enter into the reinforcement filled cavity between the vacuum bag and mould.
  10. Once the part is fully infused with resin, it is allowed to cure.  Timing depends on the resin system and conditions, which may be hours or overnight.
  11. The bag and other consumables are removed from the part and disposed of.  The remaining laminate will be one homogenous structure.  Some minor finishing will be needed to grind off the flange edges and some sharpened areas where resin has cured into the creases in the vacuum bag.


1A finer reinforcement such as tissue, print blocker or light weight chopped strand mat used in the reinforcement layer against the gelcoat will minimise fibre print through, providing a smoother, high quality surface finish.
2Reinforcements such as chopped strand mat are not recommended for reinforcement as they will wash and relocate when the resin breaks down the binder system and moves the glass strands in the direction of resin flow.
3To facilitate resin flow, flow media need to be utilised.  Interlaminar flow can be achieved via the use of speciality infusion products which remain as part of the finished laminate, or disposable surface flow media.

Hints & Comments

  1. Open moulds similar to those used in hand lay-up are used for infusion, however more care must be taken to ensure the entire mould surface is air tight. Larger flanges are also required to allow for the necessary vacuum lines.
  2. Accurate reinforcement placement provides localised strength and compression without unnecessary additional weight. This includes the inclusion of stiffeners and/or the addition of high density inclusions (e.g. metal, high density core) in areas where localised screwing may be required. Resin flow software is available to optimise the process.
  3. The end laminate has one finished (moulded) side only.
  4. Vacuum infusion can be used with epoxy, polyester and vinyl ester resins, most conventional fabrics and cores with the exception of honeycombs. 
  5. Workshop conditions are much improved compared to open mould processes due to the reduction in volatile emissions.
  6. Infusion process is suitable for limited production runs, and parts requiring higher glass content than can be achieved with open mould processes such as spray up or hand-lay up.  It is also suitable for extremely large and or detailed parts.
  7. Compared to hand lay-up, this process gives better fibre wet-out, lower void content, higher reinforcement content and better adhesion between layers. Laminates made by vacuum infusion therefore have higher mechanical properties than those made by hand lay-up.
  8. Skilled fabricators are required for vacuum infusion.  
  9. Thicker parts can be achieved by the inclusion of light weight core materials to the laminate, which contribute greatly to stiffness but not to overall part weight.