Sigma DTK6 6" C153 Ductile Iron Mechanical Joint Fitting Tapped Cap - Sigma Corporation offers a wide range of water and wastewater infrastructure products, original equipment manufacturing products and services (OEM), precision castings, and HDPE products. Sigma quality assurance starts by thoroughly inspecting each factory before approving it as a SIGMA manufacturer. They examine every facet of the operation from the procurement of raw materials to the maintenance of equipment through the fabrication process.