Why hot-end coating matters
A microscopic tin-oxide layer applied to ware at the hot end serves two functions: it improves scratch resistance during conveyor handling, and it primes the surface so that cold-end coating (polyethylene wax or oleic acid) bonds correctly. Without it, scuffing and scratching at the conveyor cause cold-end rejection rates to climb measurably. With incorrect dose, the same — too little fails to protect, too much creates haze.
SnCl₄ chemistry and operation
Tin tetrachloride is the lower-cost, more widely-used hot-end coating precursor. It is highly reactive — vapour reacts with hot glass to form SnO₂. Dose is controlled via vaporiser temperature and carrier gas flow. Drawback: HCl emissions require scrubbing.
MBTC chemistry and operation
Monobutyltin trichloride is the modern alternative — organotin precursor, lower emissions, less downstream corrosion. Higher cost per kg, but often pays back via reduced equipment maintenance and emissions compliance.
Dose, distribution and the daily check
The 30-second daily check that saves 2 OEE points: measure tin-oxide layer thickness with a contact-style gauge at three points across the lehr loader. Variance between points typically reveals coating-hood distribution issues that are otherwise invisible until cold-end rejects climb.
Equipment and maintenance
- Coating hood — alignment, condensate drainage, hood seals
- Vaporiser — temperature stability, refill cadence
- Carrier gas — flow steady within ±5%
- Exhaust scrubbing — periodic media replacement
Regulatory and emissions
HCl emissions from SnCl₄ are increasingly regulated under EU IED, US Clean Air Act, and equivalent regional frameworks. Plants converting from SnCl₄ to MBTC frequently cite emissions compliance as the primary driver.