In high-voltage switch insulation pull rods, basin-type insulators, and instrument transformer cast bodies, trapped bubbles will gradually erode insulation life through partial discharge. Bubbles have a far lower dielectric constant than epoxy resin, withstand higher field strength under electric fields, and become the starting point of breakdown. The core value of vacuum casting equipment lies not in tank size or pump capacity, but in whether it can reduce bubbles in the mixed compound to a sufficiently low level and prevent new defects throughout casting and curing.
I. Bubble Control: From "Pulling a Vacuum" to "Molecular-Level Degassing"
A single vacuum pull only removes large bubbles; micro-bubbles of 10–100 μm require far more.
The effective path is staged vacuum with dynamic adjustment: first pull to a lower vacuum level to let large bubbles escape smoothly, then gradually deepen; briefly release and re-pull under ultimate vacuum, using pressure changes to break surface tension and force micro-bubbles to coalesce and escape. Resin temperature must be precisely controlled near the lowest-viscosity point (typically 40–45°C) to minimize bubble escape resistance. Thin-film degassing technology forms a thin layer of material through rotational stirring, achieving better degassing efficiency and uniformity than static vacuum.
Modern equipment uses PLC recipe management to lock in validated degassing curves, reproducing the same vacuum, temperature, and time sequences batch to batch and reducing human variation.
II. Casting: Flow Rate and Temperature Are Equally Critical
No matter how well degassing is done, entraining new air during casting undoes everything.
Molds are typically preheated slightly above resin temperature (e.g., 50–60°C) to avoid cold-mold viscosity drop and surface defects. Casting gates at the mold bottom with stable, slow flow minimize splashing and air entrapment. High-voltage switch insulators have complex inserts and precision cavities; temperature control accuracy throughout casting directly affects whether resin fully wets metal inserts and fillers—poor interfacial bonding is also a partial discharge hazard.
III. Curing: Internal Stress Is the Invisible Crack Source
Epoxy resin shrinkage during curing is unavoidable. Excessive heating rate or uneven heat dissipation leaves micro-cracks inside the cast body, which become field distortion points under electric fields.
Gradient curing is the effective approach: the initial curing stage slowly raises from casting temperature to curing onset temperature, allowing gentle gelation; the post-curing stage gradually rises to final curing temperature, completing the crosslink network while releasing stress. For high-voltage switch insulators, primary curing temperature typically ranges from 110–135°C, with the curve adjusted to resin formulation and workpiece thickness. Some processes apply 0.2–0.3 MPa pressure during gelation to compensate shrinkage while the material can still flow.
IV. Three Parameters More Important Than "Volume" in Selection
1. Ultimate vacuum and leak rate. Casting tank working vacuum typically needs to reach below 1 mbar, but leak rate is more critical—when tank sealing degrades, vacuum may appear adequate while degassing effectiveness has already declined. Confirm leak rate specifications, not just pump speed.
2. Metering accuracy and filler adaptability. High-voltage switch insulator formulations contain high proportions of silica filler, testing both wear resistance and precision of metering pumps. Hydraulically driven high-pressure metering pumps avoid pump body error through linear motion, suiting high-filler-ratio conditions. Metering deviation directly causes hardener ratio imbalance; excess unreacted anhydride volatilizes during vacuum, introducing new bubble sources.
3. Zoned temperature control accuracy. From resin preheating, mixing, and pipeline delivery to the mold, each stage requires independent temperature control. Static mixers, filters, and delivery lines use medium circulation heating with temperature variation within ±1°C, preventing viscosity rise or crystallization from cooling during transport.
V. Continuous vs. Batch Selection
For high-voltage switch insulators—multi-variety, medium-batch production—batch vacuum casting systems remain mainstream: each batch independently completes mixing, degassing, casting, and curing, with flexible recipe switching, suiting alternating production of basin insulators, pull rods, and instrument transformers.
When single-variety capacity demand is larger, automatic pressure gelation (APG) offers another path: casting is completed under pressure, molds are fixed on pressure molding machines, and gel time is shortened through ultra-fast segmented heating, increasing single-machine capacity by approximately 50%. APG suits high-volume components such as insulators and switchgear accessories, but requires higher mold investment and product dedication. Choosing batch or continuous depends on the trade-off between product mix breadth and single-variety volume.
The reliability of high-voltage switch insulators is determined the moment resin enters the mold. Bubble content, interfacial wetting quality, and curing internal stress level are not visible on finished product appearance, but they emerge in partial discharge data from type tests and in breakdown incidents after years of operation.
We welcome your workpiece type (insulation pull rod, basin-type insulator, or instrument transformer cast body), resin system and filler ratio, single-piece weight, and daily capacity requirements. We will provide targeted recommendations on degassing solutions, metering configuration, and curing curve control. Good casting equipment ensures insulators withstand electric field testing from the inside out.