"Vacuum Bagging" is used when high strength is needed. Tooling cost is similar to "Hand Lay-Up" because only one side tool is needed. By vacuuming all the air out before injecting resin, you will get the highest glass to resin ratio. The higher the glass content the stronger the part.
Process - The fiberglass mat is placed into the tool. A fabric cloth is placed over to help with air flow. Then a bag is placed over and sealed down around the perimeter with gummy tape. Vacuum is applied to remove all air and then the resin is vacuumed through. When part cures the vacuum is removed and the bag & cloth are thrown away.