Comparing APG Technology and Vacuum Casting for Epoxy Resin Bushings
Epoxy resin bushings are critical insulation components in medium-voltage switchgear, transformers, and power distribution equipment. Their electrical reliability and mechanical integrity depend not only on material selection but also on the manufacturing process.
Two production methods dominate the industry today: APG (Automatic Pressure Gelation) and vacuum casting. Each has its own advantages, limitations, and suitable application scenarios. Understanding the differences between these technologies helps engineers and buyers make informed decisions when selecting epoxy resin bushings.
Overview of Epoxy Resin Bushing Manufacturing
Epoxy resin bushings must meet demanding requirements, including:
High dielectric strength
Stable partial discharge performance
Good mechanical strength
Long-term aging resistance
Manufacturing defects such as voids, cracks, or uneven curing can directly lead to insulation failure. Therefore, the production method plays a decisive role in final product quality.
What Is Vacuum Casting Technology?
Vacuum casting is a traditional and widely used process for epoxy insulation components.
Process Description
Liquid epoxy resin is mixed with hardener and fillers
The mixture is degassed under vacuum
Resin is poured into molds under vacuum or low pressure
Curing occurs over a relatively long cycle
Key Characteristics
Low injection pressure
Flexible mold design
Suitable for complex shapes and prototypes
Vacuum casting has been used for decades and remains common for small batches and customized designs.
What Is APG (Automatic Pressure Gelation) Technology?
APG is a more advanced and highly controlled molding process developed specifically for epoxy insulation components.
Process Description
Resin is injected into a closed mold under controlled pressure
Injection continues during the gelation phase
Pressure compensates for shrinkage during curing
Temperature and curing parameters are precisely controlled
Key Characteristics
Stable pressure during curing
Shorter production cycles
High repeatability
APG is now widely adopted for medium- and high-volume production of epoxy bushings and solid insulation parts.
Comparison of Electrical Performance
Void and Porosity Control
Vacuum Casting: Good degassing is possible, but voids may still form due to resin shrinkage during curing.
APG: Continuous pressure during gelation significantly reduces void formation and internal porosity.
Lower void content directly improves:
Partial discharge performance
Dielectric strength
Long-term insulation reliability
Partial Discharge Behavior
APG-produced bushings generally show lower and more consistent partial discharge levels, especially at higher voltage stress, making them better suited for critical medium-voltage applications.
Mechanical Strength and Dimensional Stability
Vacuum Casting
Acceptable mechanical strength
Greater risk of internal stress if curing is uneven
Dimensional variation between batches
APG Technology
More uniform curing
Better fiber-resin bonding (when inserts are used)
Higher dimensional accuracy and repeatability
APG bushings tend to perform better under mechanical load, vibration, and thermal cycling.
Production Efficiency and Consistency
AspectVacuum CastingAPG TechnologyProduction speedSlowerFasterBatch consistencyModerateHighAutomation levelLow–mediumHighLabor dependenceHigherLower
For large-scale switchgear manufacturing, APG offers clear advantages in consistency and efficiency.
Design Flexibility and Customization
Vacuum casting remains attractive for:
Small production volumes
Customized designs
Prototyping and development projects
APG molds require higher initial investment, making them more suitable for standardized products with stable demand.
Cost Considerations
Vacuum Casting
Lower tooling cost
Higher labor and unit cost
Longer production cycle
APG Technology
Higher initial mold and equipment investment
Lower unit cost for mass production
Reduced rejection rate
In long-term, high-volume projects, APG often results in lower total manufacturing cost.
Typical Application Scenarios
Vacuum Casting Is Commonly Used For
Low-volume orders
Custom epoxy bushings
Special dimensions or non-standard designs
APG Is Preferred For
Medium- and high-voltage switchgear bushings
Solid insulated switchgear components
High reliability and high consistency requirements
Quality Control and Reliability
APG technology allows better integration of:
Process monitoring
Temperature and pressure control
Statistical quality control
This results in more predictable long-term performance, which is especially important for utilities and industrial users demanding long service life.
Conclusion
Both vacuum casting and APG technology are mature manufacturing methods for epoxy resin bushings, but they serve different needs.
Vacuum casting offers flexibility and lower upfront cost, making it suitable for custom or low-volume applications. APG technology, on the other hand, provides superior consistency, lower defect rates, better electrical performance, and higher production efficiency—making it the preferred choice for modern medium-voltage switchgear production.
Selecting the right process should be based on voltage level, production volume, reliability requirements, and long-term cost considerations rather than initial tooling cost alone.
