What is Closed Mold Processing?
Closed mold processing is a manufacturing method where components are formed inside a mold cavity made of two mold halves.This technique uses high pressure and temperature to shape and consolidate materials within the mold, ensuring that the final product has a precise shape and finish.This technique has a lot of applications and is really common in composite material production, where it's used to create parts with complex geometries and high strength-to-weight ratios.
Examples of Closed Mold Processes
There are a few different closed mold processes used to make composite materials, like:
- SMC (Sheet Molding Compound) and BMC (Bulk Molding Compound): These processes involve pre-impregnated sheets or bulk materials that are pressed into shape within a mold.
- RTM (Resin Transfer Molding): In this process, reinforcing fibers and core materials are placed in a mold cavity, and then resin is injected under pressure to impregnate the fibers and form the part.
- VARTM, or Vacuum-Assisted Resin Transfer Molding, is a type of RTM that uses vacuum to help resin get into the fibers.
- Vacuum Infusion with matched metal molds is another process where vacuum is used to pull resin into a mold filled with reinforcing fibers and core materials.
These processes are considered more technically advanced than open mold techniques because they require more planning and often expensive equipment. But the precision and quality of the final product make it worth the investment.
The Plus Sides of Closed Mold Processes
Compared to open mold methods, closed mold processes have a lot going for them:
- Greater production efficiency: Closed mold processes can produce parts faster and with less waste.
- Fewer labor requirements: Automation and mechanization are more feasible in closed mold processes, reducing the need for manual labor.
- Smooth, finished surfaces on both sides: Parts produced using closed mold processes have a smooth, finished surface on both sides.
- Greater design capability: They can make parts with complex geometries and intricate details.
- Lower emissions of Hazardous Air Pollutants (HAPs): Closed mold processes generally produce fewer emissions compared to open mold techniques.
These benefits have led to more closed mold techniques being used in composite manufacturing. Right now, unsaturated polyester resin (UPR) applications account for about 10% of the market, but this percentage is expected to grow as manufacturers continue to adopt closed mold processes.
Environmental Considerations
There's been more environmental awareness and regulatory pressure, so composite manufacturers are focusing on two key aspects:
- Reducing HAP emission rates: They're using low-emission materials and processes to cut down on hazardous air pollutants (HAPs).
- Controlling absolute HAP emissions: Along with cutting down on emission rates, manufacturers are also working to control the absolute amount of HAPs emitted. That depends on both emission rates and production volumes.
By focusing on these two aspects, composite manufacturers can reduce their environmental impact and comply with regulatory requirements.
Types of Closed Mold Processes
There are two main types of closed mold processes: Resin Transfer Molding (RTM) and Compression Molding.
Resin Transfer Molding (RTM)
RTM involves placing reinforcing fibers, core materials, and inserts into a mold cavity before injecting resin. The resin impregnates the fibers and forms the part under pressure. Variants of RTM include:
- Conventional Resin Transfer Molding (CRTM): This is the basic RTM process without any extra help.
- Vacuum-Assisted Resin Transfer Molding (VARTM): This uses vacuum to help the fibers soak up the resin, using less resin and making the final product better.
- Vacuum Infusion (VI): This is like VARTM, but it's used for bigger parts that need more resin.
Compression Molding
Compression molding is when you squeeze composite preforms into a mold using clamping pressure. The preform is heated and pressed into shape within the mold cavity. This process can be used for things like:
- Sheet Molding Compound (SMC): This is a pre-impregnated sheet material that is pressed into shape within a mold.
- Bulk Molding Compound (BMC): It's a bulk material that is pressed into shape within a mold, similar to SMC but with a thicker cross-section.
- Wet-laid composite materials: These are materials that are laid up wet and then pressed into shape within a mold.
Conclusion
Closed mold processes, like RTM and compression molding, have a lot of advantages over open mold techniques. They reduce emissions, increase production efficiency, and produce high-quality parts with complex geometries and intricate details. As a result, they're becoming more popular in composite material manufacturing. With continued advancements in technology and environmental regulations, closed mold processes are expected to play an increasingly important role in the future of composite manufacturing.