SHEBOYGAN, Wis.(WHBL) - The American Orthodontics plant on North 18^th Street was evacuated yesterday afternoon due to an ammonia leak.  Spokesman Matt Klitzke said it happened shortly after one o’clock when a regulator valve on an ammonia tank failed, probably due to weather-related rapid temperature changes.  Klitzke said the levels of ammonia were non-threatening, but neighbors were told to stay inside as a precaution.  He said the leak was repaired by three o’clock and work inside the plant resumed.