Vacuum Bagging

The Vacuum Bagging process is a relatively low cost method to consolidate a wet lay-up (see Hand Lay-up) during the curing using atmospheric pressure. Often used for complex shapes and large components where other methods of pressurisation are not achievable or cost effective.

Prepregs may also be consolidated using the vacuum bagging process, however they must be cured within an enclosed oven, which adds to the process cost.

The atmospheric pressure applied during cure compacts the laminate, facilitates removal of excess resin, improves fibre wet-out and lowers void content. The vacuum bag reduces the amount of volatiles emitted during cure thus improving health and safety in the workshop.  Vacuum Bagging results in good quality parts which have a higher reinforcement content and better adhesion between layers when compared with parts prepared using the hand lay-up process.

Considerations must be made for the relatively low cost equipment, cost and disposal of the consumables and the additional time involved in bagging and consolidating the laminate. Fabricators must also have a higher level of skill than for hand lay-up.  Re-usable bags may be utilised.

allnex products commonly used:

  • Resins (UPE, vinyl ester, epoxy)
  • Catalyst
  • Gelcoats
  • Reinforcements (chopped strand mat, fabrics, tissues, speciality fabrics)
  • Accessories (brushes, rollers, bags, peel ply, breather cloth, flow mediums, tacky tape)


  1. The wet lay-up is prepared as normal in the mould.
  2. Peel ply and breather materials are placed over the laminate, followed by a flexible plastic film ('bag') which is placed over the top and sealed to airtight around the edge of the mould1.
  3. The air under the bag is then extracted with a vacuum pump, achieving up 1 atmosphere (14psi) pressure evenly across the laminate in the mould.  Resin flow and cure can be further assisted by oven heating the assembly, as required for prepregs.  
  4. Once the resin is cured the peel ply, breather materials and bag are removed from the part.


1Any vacuum process requires a near perfect seal between the bag and the part to be an effective process and achieve a good finished result.

Hints & Comments

  1. Wet bagging is well suited when epoxy resins are being used.  
  2. When using polyesters and vinyl esters use <1 atmosphere pressure.  Doing so will reduce the amount of styrene loss, which would otherwise have a detrimental effect on cure.