Vacuum Impregnation Process
The vacuum impregnation process involves five key stages:
- Part preparation
- Cold wash
- Hot cure
Before the vacuum impregnation process begins, porosity within the component must be completely clean and dry. While this is achievable through a separate pre-process, Ultraseal offers modules for aqueous washing and vacuum drying that can be integrated into the system.
Ultraseal International offers two types of autoclave with top-load equipment:
Vertical Transfer System (VTS)
Here, the workload is suspended in a basket during the vacuum impregnation phase before being lowered and immersed in the sealant. The impregnation vacuum is then released, forcing the sealant deep into the porosity. VTS-based impregnation machines are simple in design and have a relatively small footprint.
Sealant Transfer System (STS)
Here, the workload is loaded into the autoclave with the sealant in an adjacent tank. After the impregnation vacuum has been drawn on the autoclave, a valve is opened and the sealant transfers into the autoclave, immersing the components. The vacuum is then released, forcing the sealant into the porosity, after which unused sealant returns to its original tank under vacuum. STS machines are simpler to automate and benefit from well-conditioned sealant due to the length of time the sealant is held under vacuum. For critical applications, a pressure autoclave can be supplied, enabling a pressure cycle to be incorporated following the vacuum phase.
The drain cycle involves removing excess sealant from the component after the vacuum impregnation process. Good draining is essential to reduce 'carry-over' to the cold wash tank and limit the amount of sealant lost as effluent. While a static drain is suitable in some applications, sealant may become trapped in pockets or features within more complex components. A rotational drain system physically rotates the components in their basket to maximise sealant removal. This is then collected and automatically returned to the autoclave.
After draining, the components are transferred to the cold wash tank for removal of surface sealant. For more complex components, a rotational system is recommended. The wash solution is pumped from the lower tank into the chamber, passing over and through the rotating workload, before returning to the tank. The workload is not immersed, as if the components were to remain in the water during washing, contamination in the wash solution would adhere to the components and cause potential cleanliness problems during curing. The weight and volume of washing media passing through the components affect a high level of cleanliness and the subsequent rotation drain minimises carry-over to the hot cure module.
This takes place in a separate tank with water controlled at a minimum of 90°C (195°F). For maximum productivity, a rotational system is recommended. Rotational Technology, which was patented by Ultraseal, is where the hot cure solution is pumped from the tank to the manifold at the top of the chamber. The solution fills the chamber to a point where it automatically weirs back to the lower sump while the chamber is continually filled with filtered, reheated solution from the lower storage tank. This method not only cures the sealant within the porosity but provides a very effective final washing action.