Comparing APG Technology and Vacuum Casting for Epoxy Resin Bushings

Introduction

Epoxy resin bushings are critical insulation components in medium-voltage switchgear, transformers, and gas-insulated equipment. Their long-term electrical and mechanical performance depends not only on material formulation, but also on the manufacturing process used to mold the epoxy insulation.

The two most widely used processes are:

• Vacuum casting

• APG (Automatic Pressure Gelation)

While both processes produce solid epoxy bushings, they differ significantly in process control, production efficiency, void control, and suitability for different voltage levels and applications. Understanding these differences is essential when selecting bushings for reliability-critical equipment.

Overview of Vacuum Casting Technology

Process Principle

Vacuum casting is a batch-type manufacturing process where:

• Epoxy resin and hardener are mixed under vacuum

• The mold is evacuated to remove air

• Liquid resin is poured into the mold under vacuum

• The casting cures slowly, often with post-curing

The primary objective is to eliminate air bubbles and achieve homogeneous insulation.

Key Characteristics of Vacuum Casting

• Low-pressure or gravity-driven filling

• Long gelation and curing time

• Strong dependence on operator skill

• Flexible for complex shapes and prototypes

Vacuum casting has been used for decades and remains common for low-volume or specialized epoxy parts.

Overview of APG Technology

Process Principle

APG (Automatic Pressure Gelation) is a highly controlled, closed-mold process where:

• Pre-heated mold is clamped and sealed

• Epoxy resin is injected under controlled pressure

• Resin gels quickly inside the heated mold

• Short demolding time with high dimensional accuracy

APG combines pressure filling and thermal control to achieve consistent results.

Key Characteristics of APG

• Pressure-assisted injection

• Short gelation time

• High repeatability

• Automated process control

APG is widely adopted for medium- and high-volume production of MV insulation components.

Comparison of Process Control

Vacuum Casting

• Relies heavily on vacuum level and pouring technique

• Higher sensitivity to resin viscosity changes

• Greater risk of operator-related variation

• Less consistent from batch to batch

Process stability can vary, especially in large or thick bushings.

APG Technology

• Injection pressure ensures complete mold filling

• Mold temperature tightly controlled

• Automated dosing and timing

• Excellent process repeatability

APG offers superior control, especially for mass production.

Void and Partial Discharge Performance

Vacuum Casting

• Effective at removing macro air bubbles

• Micro-voids may still form in thick sections

• Higher risk of localized PD if process is not optimized

PD performance depends strongly on resin formulation and curing discipline.

APG Technology

• Pressure filling minimizes void formation

• Better resin flow around inserts and conductors

• Consistently low partial discharge levels

For MV bushings, APG typically achieves more stable PD performance.

Mechanical Strength and Dimensional Accuracy

Vacuum Casting

• Longer curing can cause shrinkage variation

• Dimensional tolerances may vary

• Post-machining often required

Adequate for simple designs, but less ideal for tight mechanical interfaces.

APG Technology

• Controlled shrinkage due to fast gelation

• High dimensional accuracy

• Excellent integration with metal inserts and flanges

APG is well suited for wall bushings and embedded-conductor designs.

Production Efficiency and Cost

Vacuum Casting

• Long cycle time

• Lower tooling cost

• Suitable for small batches or custom designs

• Higher labor cost per unit

Often preferred for prototype or low-volume production.

APG Technology

• Short cycle time

• Higher initial tooling investment

• Lower cost per unit at scale

• Ideal for standardized products

APG becomes more economical as production volume increases.

202 double-pass bushing

Suitability for Different Voltage Levels

Vacuum Casting Applications

• Low- to mid-voltage bushings

• Non-standard or custom geometries

• Repair or retrofit components

• R&D and pilot production

It remains viable for limited production runs.

APG Applications

• 12 kV, 24 kV, and 36 kV epoxy bushings

• GIS wall bushings

• Embedded pole systems

• High-current MV components

APG is the preferred choice for modern MV switchgear insulation.

Integration with Conductors and Inserts

Vacuum Casting

• Insert positioning relies on fixtures

• Risk of resin movement during long cure

• Greater chance of interface defects

Careful mold design is required.

APG Technology

• Pressure injection locks inserts in position

• Excellent wetting of conductor surfaces

• Strong resin-metal bonding

This improves thermal and mechanical reliability.

Quality Consistency and Testing Results

Vacuum Casting

• Wider variation in PD and dielectric test results

• Strong dependence on batch quality control

• More inspection required

Quality consistency can be challenging in large-scale production.

APG Technology

• Narrow tolerance band for electrical performance

• Stable dielectric and PD results

• Easier compliance with IEC test requirements

This is a key reason utilities prefer APG-produced bushings.

Environmental and Process Safety Considerations

Vacuum Casting

• Open resin handling

• Longer exposure to chemicals

• Higher operator dependency

Requires strict procedural discipline.

APG Technology

• Closed system operation

• Reduced resin exposure

• Better working environment

APG supports modern manufacturing safety standards.

Summary Comparison Table

Which Technology Should Be Chosen?

The choice depends on application and production strategy:

• Choose vacuum casting for:

  • Low-volume production

  • Custom or experimental designs

  • Cost-sensitive tooling requirements

• Choose APG technology for:

  • Standardized 12–36 kV epoxy bushings

  • GIS and switchgear mass production

  • High reliability and consistency requirements

For most modern MV epoxy resin bushings, APG has become the industry standard.

Conclusion

Both vacuum casting and APG technology can produce functional epoxy resin bushings, but they serve different manufacturing and performance needs. Vacuum casting offers flexibility and lower tooling cost, while APG delivers superior process control, consistency, and electrical performance, especially for medium-voltage applications.

As switchgear systems demand higher reliability, lower partial discharge, and tighter dimensional tolerances, APG technology is increasingly preferred for epoxy resin bushings used in modern MV power equipment.