Fabrication of Composites using Closed Mold Process
Closed Mold Processes
1. Compression Molding 2. Vacuum Bag Molding 3. Autoclave Molding 4. Injection Molding
5. Resin Transfer Molding (RTM)
6. Vacuum Assisted Resin Transfer Molding (VARTM) 7. Resin Film Intrusion (RFI)
1. Compression Molding:
Fully formed parts are molded in matched metal compression molds that give the final part shape using pressure and temperature.
The moving platen is heated either by steam or electricity to promote curing.
After placing the laminate to be cured called the charge in the core of the mold, the cavity is then closed at the rate of usually 4-12 mm/sec.
The molding pressure based on the projected area ranges from 0.7 to 9 MPa (100 to 1200 psi).
The curing time is usually between 25 sec to 3 minutes.
Advantages of Compression Molding:
Very less variations in Dimension of the parts.
A wide variety of shapes, sizes and complexity can be produced.
Disadvantages of Compression Molding:
High tooling cost.
Heating requirement is high.
Not practical for volume production. Cure time is critical.
If the resin cure exotherm is not properly controlled, cracking, blistering or warping may occur.
2. Vacuum Bag Molding:
Vacuum Bagging techniques have been developed for fabricating variety of aerospace composites and structures.
The required number of piles are cut to size and positioned in a mold.
When individual piles of a prepreg material are formed to the lay-up tool, certain amount of voids exits between layers.
The lay-up is covered with a flexible membrane or vacuum bag, which is sealed around the edges of the mold by a sealant.
An edge bleeder placed so as to absorb excess resin. The lay-up is allowed to cool at room temperature. To reduce the curing time oven is preferred.
Requirements of Vacuum Bag:
Should be impervious to air.
Should uniformly apply the cure pressure. Should not leak under over pressure.
Good and high capacity vacuum path to be provided to evacuate air from between the bag and the tool.
Types of Vacuum Bag Molding:
i. Expendable Vacuum Bagging ii. Reusable Vacuum Bagging
(i)
Expendable Vacuum Bagging:
The surface of the mold is prepared with the release agent.
The composite piles are applied and rubbed to remove the entrapped air.
A perforated film is applied over the composite laminate and extended approximately 3.2 mm beyond all edges.
A predetermined number of bleeder plies are applied over the release film and extended to the perimeter of the lay-up.
A perforated release film is applied over the lay-up and extended over the release film. Sealant tape is applied around the perimeter of the bleeder.
The vacuum bag is positioned and sealed.
The contents are evacuated and the bag is checked and sealed against leaks.
Advantages of Expendable Vacuum Bagging:
This allows for a better control of the resin content of the composite structures.
Disadvantages of Expendable Vacuum Bagging:
Vacuum Bag Bridging causes resin rich and excessive voids in the corners of composite laminates.
Material and recurring cost is high.
(ii)
Reusable Vacuum Bagging:
To reduce the material and recurring cost of the expendable vacuum bags, Silicone rubber reusable bags are used.
Advantages of Vacuum Bag Molding:
High Fiber content.
Improved Mechanical Properties because of low void contents. The Vacuum bag reduces the amount of volatiles emitted during cure.
Disadvantages of Vacuum Bag Molding:
Limited pressure can be applied
The extra process adds cost both in labor and in disposable bagging materials. A higher level of skill is required by the operators.
Mixing and control of resin content largely still determined by operator skill.
3. Injection Molding:
It refers to the process that generally involves forcing or injecting a fluid plastic material into a closed mold.
The molding compound is fed into injection chamber through the feed hopper. It is then heated there wherein it changes into a liquid form.
Advantages of Injection Molding:
More readily automated process. Permits finer part detail.
High Volume and Low cost Manufacturing process.
Disadvantages of Injection Molding:
Not all the plastics are molded except thermoplastic and thermosets. Limited to materials with very short fiber lengths.
Material non-uniformities exist because of large amount of flow.
4. Resin Transfer Molding (RTM):
It is closed mold low pressure process that allows the fabrication of composites.
The dry reinforcement and the resin are combined within the mold to form the composite component.
The displaced air is allowed to escape through the vents to avoid dry spots. The part cure is normally done by heating.
Advantages of RTM:
Production cycles are much faster than wet lay-up. Large and complex shapes can be made efficiently.
Volatile emissions are low because RTM is a closed process.
Disadvantages of RTM:
The mold design is critical and so it requires greater skill. Reinforcement may move during the injection of resin. Resin uniformity is difficult.
Uncontrolled flow pattern of resin and so resin rich edges are possible.
5. Vacuum Assisted Resin Transfer Molding (VARTM):
The fabrication parts are injected through a single open mold.
VARTM employs the injection of resin along with vacuum and captured under a bag thoroughly impregnate the fiber reinforcements.
Fabric are laid up as dry stack and covered with peel ply and a knitted type non-structural fabric.
The whole dry stack is vacuum bagged and leaks if any are eliminated and the resin is allowed to flow through the laminates.
The resin distribution is aided by flowing resin easily through the non-structural fabric and wetting the fabric.
Advantages of VARTM:
Much lower cost due to the usage of one half of the tool being the vacuum bag. Large components can be fabricated.
Cored Structures can be produced in one operation.
Disadvantages of VARTM:
Relatively complex process to perform well. Resins must be very low in viscosity.
6. Resin Film Infusion (RFI):
Dry fabrics are laid up interleaved with layers of semi-solid resin film supplied on a release paper.
The lay-up is vacuum bagged to remove air through the dry fabrics.
It is then heated to allow the resin melt and flow into the air-free fabrics and a certain time to cure.
Advantages of RFI:
High fiber volumes is achieved with low voids. Good health, safety and a clean lay-up.
High resin mechanical properties due to solid state of initial polymer materials and elevated temperature cure.
Disadvantages of RFI:
Limited within Aerospace industry.
