Vacuum Impregnation: A Brief History
Vacuum impregnation was developed in the 1950s to seal internal, connected porosity that forms during the casting or molding of metal parts. The technique seals the porosity without changing the casting's dimensional or functional characteristics. The process allows the use of parts that would otherwise be scrapped. The process was adopted quickly in various industries, particularly in the automotive and aerospace sectors. It became the preferred method to prevent leakage of liquids or gases under pressure.
The traditional vacuum impregnation method involves the use of a batch system, in which workers load multiple parts into large baskets for processing (Image 1). This approach typically has a cycle time of 30-40 minutes. To increase productivity, operators can increase the process equipment's size, but this is often accompanied by a reduction in finished product quality and process safety.
How Vacuum Impregnation Works
To seal casting porosity, vacuum impregnation process parts through four stations:
1. Impregnation chamber. The operator seals the chamber and draws a vacuum. This removes air in the porosity and leak path in the casting wall. Then, the parts are covered with sealant, and positive pressure is applied. More energy is required to penetrate the porosity with sealant than to evacuate the air. The operator then releases the pressure and drain the chamber.
2. Excess sealant recovery. The operator removes excess sealant through gravity, rotation, or centrifugal force.
3. Wash/rinse station. Next, the operator washes residual sealant from the part's internal passages, taps, pockets, and features.
4. Cure station. Finally, the operator polymerizes the impregnated sealant in the leak path.
Safety and Quality Concerns
Over the years, vacuum impregnation became standardized, as other manufacturing operations (e.g., machining, pressure testing, and assembly) were modernized. These other operations became more cellular, more automated, more ergonomically sound, safer for operators, and more efficient. Vacuum impregnation, however, remained a manual process with significant safety concerns
Among the safety concerns:
- Open tanks emit hot vapor with elevated VOC levels, which can cause health problems.
- System components like overhead hoist chains, actuating tank lids, locking rings, and chain drives can cause injuries.
- Part baskets are bulky and heavy, and moving them can create stress on the operator's body or cause injury if mishandled.
- Open modules can jeopardize operator safety. For example, an operator could be splashed with sealant or fall into an open, 800-gallon container of 195 °F water.
Quality Issues
Batch impregnation systems are prone to quality issues, too. These include:
- Complex castings are difficult to impregnate. Large batches cannot be washed and rinsed adequately, increasing sealant contamination, which renders many parts unusable or jeopardize their use in assembly.
- There is a high probability of human error. The operator might pack the basket incorrectly or skip processing steps, potentially damaging parts.
Until the mid 1980s, most automotive OEMs handled the vacuum impregnation process in house, using this method. However, with the poor safety and quality record of batch systems, many OEMs outsourced the process to third-party providers. This allowed OEMs to alleviate risk and focus on their core competencies.
Re-imaging Vacuum Impregnation
In the early 2000s, many OEMs sought to bring vacuum impregnation in-house, intending to meet the volume demand for lighter, aluminum parts that increased in volume following the increase in fuel efficiency regulations and subsequent pressure to produce more fuel-efficient vehicles.
Vacuum impregnation systems were thus modernized to meet the demands of the new manufacturing environment. Rather than large, top-loading batch systems, new equipment is designed to be front-loading. It also can process just single pieces or a small number of castings.
Robotic handling allows parts to move continuously between each station. The robotics reduce cycle times and improve overall cycle time and production volumes.
Automated impregnation technology has further led to the development of compact, manually operated systems. This allows OEMs to bring vacuum impregnation in-house at a fraction of the cost. These new systems are smaller than batch systems, and the modular design enables them to integrate with other production operations.
The operator of a modern vacuum impregnation system is safer than ever before, as self-contained modules protected them from contact with sealant and hot fluids (Image 4). Mist eliminators collect water vapor in the exhaust and returned it through a drain line for re-use. Better ergonomics allow the operator to easily slide a lightweight fixture onto the platform for each module, eliminating the injury risk.
Improved Production and Quality
The redesign of vacuum impregnation systems also has improved recovery rates and cycle times. The old economies of scale have given way to smaller, more efficient systems, which has yielded greater productivity and quality.
New equipment has been designed to function with an automated and repeatable process. With robotics, parts can be impregnated automatically, which reduces the possibility of human error. Stations can contain operator demands of the new manufacturing environment. Rather than large, top-loading batch systems, new equipment is designed to be front-loading and process just single pieces or a small number of castings.
Robotic handling is incorporated, and robotic arms allow parts to move continuously between each station. Robotics reduce cycle times and improve overall cycle time and production volumes.
The evolution of automated impregnation technology includes the development of compact, manually operated HMI and cycle status lights presenting real-time process data and fault diagnostics.
In Summary
As the 21st century proceeds, companies will continue to wrestle with challenging design standards, fewer resources, and shorter cycle times. Those that thrive will do so by increasing productivity, quality, throughput, and cost reduction.
Batch vacuum impregnation systems are still in use in some corners of industry, but they’re no longer competitive in high volume and modern environments. Today’s vacuum impregnation systems will continue to offer safety to the operators, with increasing production volumes and the continuing effectiveness at sealing casting porosity.
