Bubbles & Rough Surface in Instrument Transformer Casting Optimized English Article

Bubbles & Rough Surface in Instrument Transformer Casting

Surface bubbles and rough surfaces are common defects in instrument transformer epoxy resin casting. Possible causes include:

1. Improper mold design — Vent groove position is wrong, too shallow, or the parting line is too tightly sealed. Air trapped in the cavity forms bubbles on the product surface.

2. Insufficient vacuum in the resin mixture — Bubbles introduced during mixing are not fully removed, leaving micro-bubbles inside the resin.

3. Uneven release agent — Thick areas of release agent form an isolation film, preventing resin from closely contacting the mold cavity. After curing, the surface becomes uneven and rough.

4. Abnormal raw materials — Epoxy resin or curing agent is damp, causing water vapor bubbles during high-temperature curing. Expired materials or incorrect component ratios cause abnormal reaction speed and fast curing, so bubbles cannot escape. Excessive moisture in filler powder causes agglomeration and rough, pitted surfaces.

5. Curing heating rate too fast — Rapid initial heating increases resin viscosity quickly. Bubbles are sealed before they can rise to the surface, forming sub-surface bubbles and pinholes.

6. Wet inserts or copper bars — Metal parts absorb moisture. If not dried properly before molding, they release gas during heating, creating bubbles around the inserts.

To prevent these defects, manufacturers should optimize mold venting, increase vacuum degassing, apply release agent evenly, control raw material moisture, use stepped curing profiles, and pre-dry inserts and copper bars.

CNYIHU applies vacuum degassing, gelation and curing time control, and partial discharge testing in production. For related details, see our Voltage Transformer Manufacturing Process and Current Transformer Manufacturing Process articles.