Composite manufacturing processes.

1. Compression Molding
Technique

2. What is compression molding?
• Compression molding is the one among the oldest materials processing techniques.
• It was followed in the industries for plastics and is also known as matched die molding.
• This process is used mainly for thermoset composites (polyester, epoxy, polyurethane).
• Compression molding is a high-volume, high-pressure method suitable for molding
complex, high strength fiber glass reinforcements.
• This technique is typically used to make flat or moderately curved parts.
• The process cycle time ranges from 1 to 3 minutes depending on the part thickness.
• During the process, the material temperature is in the range of 130 degree Celsius to 160
degree Celsius under low molding pressure of 10MPa.
• The force developed by the press can reach up to several hundred tons depending on the
part size which is useful to obtain good part uniformity and to collapse the voids that
may be generated during the molding process.

3. Pictorial representation of compression molding
process

4. Compression molding process
• Preparation and placement
The molding material known as sheet molding composite sheets (ready to mold fiber reinforced
polymer) is placed in the hot mold. The material dimensions are selected to cover about 50% of the mold
surface area. The position of the material in the mold is a key factor that affects the part quality since it
influences the fibre orientation and void content.
• Mold closure
Once the material is placed in the mold, the upper mold moves down quickly to touch the top surface
of the material. Then the upper mold continues to move down slowly usually at 5-10mm/s to compress the
material. As the molding pressure builds up with the mold closure, the material flows to fill the mold cavity
and causes the air to escape through the air vents of the mold. The mold closing speed and mold temperature
are key factors that affects the process performance and product quality.
• Curing
After the mold cavity is completely filled with the material, the mold is kept closed while the molding
pressure is maintained for a period of time. During this period the resin is cured and the part is consolidated.
The curing time depends upon the resin mixture formulation, part thickness and mold temperature.
• Part release
Once the resin is cured and the part is solidified, the part is removed from the mold with the aid of ejector
pins. Then the part is cooled down outside the mold while the mold surfaces are cleaned and an external mold
release agent is applied to the mold surfaces for the next molding cycle.

5. Compression molding process - Advantages
• Excellent part reproducibility is obtained.
• The fiber content and type can be easily controlled to yield a variety of mechanical and physical
properties.
• Excellent electrical performance and flame resistance can be obtained depending on the resin
formulation and the filler type.
• Complex shapes can be fabricated.
• High production rates can be obtained.
• Low labor costs.
• Scrap material is minimal.
• Minimum trimming of parts.

6. Compression molding process - Disadvantages
• More equipment is needed.
• Compression molds and tooling are more expensive.
• There can be surface imperfections.

7. Compression molding techniques - Application
• Electrical fixtures.
• Control boxes.
• Machine guards.
• Grille opening panels and hoods.
• Exterior body panels.
• Home appliances.
• Furniture.
• Automotive industries.
Front end module made by glass fiber
reinforced thermoplastics.

8. Vacuum Assisted Resin Transfer Molding (VARTM)
• The vacuum assisted resin transfer molding (VARTM) process is now a widely used process for
manufacturing fiber reinforced polymer (FRP) composite laminates.
• The VARTM process, which is a closed-mold process with reduced volatile organic compounds (VOC)
emission, combines the benefits of high quality, repeatability and clean handling of the resin transfer
molding (RTM) process with the advantages of flexibility and scalability of open-mold hand layup
processing.
• The VARTM process plays many important roles in promoting the quality, affordability and part
complexity of large closed-mold FRP composite structures.
• VARTM processes are widely used in the marine, energy, infrastructure building, aerospace and defense
industries. Many variations of VARTM have also been developed recently for manufacturing more
complex composite parts with improved quality and lower cost.

9. • Clean the mold and apply mold release (wax) on the mold surface.
• Lay up the dry fiber preform, which could also be layers of dry fabrics, on the mold surface.
• Apply the peel ply to cover the fiber preform.
• Apply the flow distribution medium layer on the top of the peel ply as necessary. The fl ow distribution medium layer can help to enhance
the resin infusion speed and is commonly used for manufacturing large composite laminate parts. Note that the fl ow distribution medium
layer will later be connected to the resin injection port; and the fl ow distribution medium layer must not directly contact with the vent
port.
• Place the resin injection port on one end of the flow distribution medium layer (or on the preform if the flow distribution medium layer is
not used). A helical open tube or an omega-shaped tube can also be used as a resin injection line source, which serves the purpose of
promoting fast resin supply in the helical or omega tube and simultaneously infusing the resin into the fl ow distribution medium layer (or
the preform if the flow distribution medium layer is not used).
• Place the vent port on the top of the peel ply above the preform; note that the vent must be placed slightly away from the fl ow distribution
medium layer to avoid direct competition against the dry fi ber preform for the resin supplied from the distribution medium layer.
• Apply the sealing tape, which is a double-sided tacky tape for adhering to the mold surface and the vacuum bag together, surrounding the
preform assembly.
• Carefully lay up the vacuum bag on the assembly and secure it against the sealing tape on the mold.
• Connect the vent tube and the injection tube to the vent port and the injection port, respectively. Connect the vent tube and the injection
tube to a vacuum source and a resin reservoir, respectively. Do not fi ll the resin into the reservoir at this time.
• Close the injection port and open the vacuum port to apply the vacuum inside the bagged preform assembly. Carefully check for and fi x any air leakage.
• Apply debulking process (optional) by cyclically compressing and relaxing the preform to better compact the fi ber preform.
• Fill the resin into the resin reservoir. Keep the vacuum port on. Open the resin injection port to allow the resin to be drawn into the vacuum bagged fi ber preform assembly.

10. • It should be noted that the resin flows quickly through the fl ow distribution medium layer and
gradually infuses into the fiber preform in the thickness direction. As the length of a VARTM part
is usually much larger than its thickness, the fl ow distribution medium layer will greatly
accelerate the resin infusion process.
• Once the resin reaches the vent, allow some extra resin to be bled out for a few more minutes to
remove the tiny air bubbles in the resin fl ow front.
• Close the injection port and keep the vacuum port open until the resin cures into the solid phase.
The vacuum will keep the preform assembly tightly pressed against the mold and will also
maintain the uniform compressive pressure on the preform to create a composite part with a
uniform thickness (i.e., a uniform compression ratio or a uniform fi ber volume fraction).
• Once the resin fully cures into the solid phase, one can turn off the vacuum and demold the
composite part from the mold.

11. VARTM - ADVANTAGES
• Flexible mold tooling design and selection of mold materials
• Able to manufacture large and complex composite parts with good quality.
• A VARTM mold, which is similar to the open mold of a hand layup process, can be easily modifi ed for
manufacturing different part geometries.
• The resin and the catalyst can be stored separately and mixed just before the resin infusion.
• With a transparent plastic vacuum bag, a visible dry spot occurring during the resin infusion process can
be removed by inserting a vacuum needle at the dry spot and drawing the air out.

12. VARTM - DISADVANTAGES
• Vacuum bag, fl ow distribution medium, peel ply, sealing tape and resin tubing may not be reusable.
These consumables will need to be prepared for each individual VARTM process every time.
• Chance of air leakage is high and this strongly depends on the worker’s skill, experience and the consumable
(sealing tape, vacuum bag, etc.) quality of each VARTM process. The air leakage can cause dry spot and incomplete
resin infusion. A careful and frequent inspection for the air leakage is necessary before the resin infusion, during
the infusion resin and during the curing cycle as a leakage can be initiated at any time during these three processing
stages and ruin the composite part.
• The resin injection pressure is limited between the environmental pressure (e.g., the atmospheric
pressure) and the vacuum.
• The compressive pressure on the preform is limited between the environmental pressure (e.g., 1
atmospheric pressure) and the vacuum. A lower compressive pressure on the fi ber preform can limit
the fi ber volume fraction of the composite part. Typical fi ber volume fraction achieved by VATRM is
within the low 40% to high 50% range and mainly depends on the fi ber preform used.

13. RESIN TRANSFER MOLDING (RTM)
• Resin transfer molding (RTM) is a low-pressure molding process.
• Resin transfer molding makes it possible to make large complex shapes with a very smooth surface on both
sides.
• The technique takes place in a closed mold.
• A mixed resin and catalyst are injected into a closed mould containing a dry fiber reinforcement which is
packed into a mold cavity that has the shape of the desired part. When the resin has cured, the mould can be
opened and the finished component can be removed.
• The uniformity of resin flow can be enhanced by using a catalyst as an accelerator.
• The resin is transferred over the already placed reinforcement.
• Molding unit has two halves, upper half mold and lower half mold.
• A wide range of resin systems can be used including polyester, vinyl ester, epoxy, phenolic and methyl
methacrylate etc. combined with pigments and fillers including aluminum trihydrates and calcium carbonates
if required.
• Natural fibers can also be used as reinforcements.
• The fibre pack can be either, glass, carbon, aramid, or a combination of these.
• RTM is more cost-efficient than other processes, manufacturing PMC parts using this method may often
induce defects.

14. RTM PROCESS
• First, a dry fibrous preform composed of multiple layers is placed into a mold cavity (surface of lower half
mold) matching the desired geometry.
• A release gel is applied on the mold surface for easy removal of the composite.
• The mold (upper and lower halves) is closed and clamped together.
• A polymeric resin is mixed with a curing agent, and then injected (pumped) into the mold cavity until a
predetermined volume through single or multiple inlets.
• During injection, the resin displaces air out of the mold cavity and mixes the preform before the
polymerization begins.
• After the curing cycle, the mold is opened and composite part is removed from the mold.

15. RTM - ADVANTAGES
• As a closed mold process, emissions are lower than open mold processes
such as spray up or hand lay up.
• The mold surface can produce a high quality finish.
• This process can produce parts faster as much as 5-20 times faster than open
molding techniques.
• Resin transfer molding produces tighter dimensional tolerances
• Complex mold shapes can be achieved. Cabling and other fittings can be
incorporated into the mold designs.

16. RTM - DISADVANTAGES
• Reinforcement materials are limited due to the flow and resin saturation of
the fibers.
• Size of the part is limited by the mold.

17. RTM APPLICATIONS
• Complex structures can be produced.
• Automotive body parts, big containers, bath tubs, helmets etc.
• Vehicle panels.
• Wind turbine blades.
• Aerospace parts.

18. REFERENCES
• Park, Chung Hae and W. I. Lee. “Compression molding in polymer matrix composites.” (2012).
• Tatara, Robert. (2011). Compression Molding. 10.1016/B978-1-4377-3514-7.10017-0.
• Hsiao, K.-T., & Heider, D. (2012). Vacuum assisted resin transfer molding (VARTM) in polymer matrix composites.
Manufacturing Techniques for Polymer Matrix Composites (PMCs), 310–347. doi:10.1533/9780857096258.3.310.
• Hamidi, Y. K., & Altan, C. M. (2018). 2.5 Process-Induced Defects in Resin Transfer Molded Composites.
Comprehensive Composite Materials II, 95–106. doi:10.1016/b978-0-12-803581-8.09902-1.
• Park, S.-J., & Seo, M.-K. (2011). Element and Processing. Interface Science and Technology, 431–499.
doi:10.1016/b978-0-12-375049-5.00006-2.