Comparison of Gap and Hybrid Forming Rebuild Options for a Fine Paper Grade, 1997 Engineering Conference Proceedings

Traditional former rebuild selections for Fourdrinier machines producing fine paper are to install top or hybrid style formers for slow speeds (<800 mpm)="" and="" gap="" formers="" for="" high="" speeds="" (="">1200 mpm). At intermediate speeds, the question arises which would be more appropriate? One critical factor in this decision is the difference in paper properties that can be achieved with the different forming configurations. Both will offer improved formation and drainage over a Fourdrinier, but due to significant differences in the drainage process, it can be expected that there will be operational and paper property differences between the two twin wire styles.

A series of pilot machine trials are reported on where drainage and paper property differences are compared for a 75 gsm fine paper grade produced on both a gap and hybrid former at 1000 mpm for similar furnish conditions. The two former con-figurations examined are a horizontal roll blade style gap former and a high capacity hybrid former located at 2.5 meters from the breast roll. After the initial drainage sections (forming roll or table plus lead-in shoe), both formers on the pilot machine had identical elements. The most significant difference observed between the formers, was retention behavior. Higher first pass total and ash retentions were observed with the hybrid former, despite lower retention aid injection rates. While slightly better formation was observed with the gap former, lower minimum tensile ratios, less dense, stiffer, more closed up, more evensided sheets were found with the hybrid former.