Tow to Top Conversion in Textile Industry

Author: Dev Santosh Kshirsagar (VJTI, Mumbai)

"Tow to Top" conversion is a process in the textile and composite industries used to transform synthetic fibers from an intermediate state (known as "tow") into a more refined form called "top." It is an important post-spinning operation. The conversion process improves the quality of the fibers by aligning them, removing impurities, and preparing them for further processes like spinning or blending.

Tow - Tow refers to a bundle or rope of continuous filaments or fibers, usually in a raw or unprocessed state. These fibers are loosely packed. In synthetic fiber production, tow is the output after fibers are extruded through spinnerets in a continuous stream but before they are fully processed. A tow is a collection of approximately 300,000 continuous man-made fiber filaments kept in a parallel, untwisted form.

Top - Top refers to a processed and aligned sliver of fiber ready for spinning or further processing.

Tow to Top Conversion - To convert tows into a sliver, the individual filaments must be cut or broken collectively into staple fibers of a specified length. Conventionally, the tow was chopped to the required staple length and baled, ready for further processes like opening, blending, gilling, etc. The resultant output used to be a sliver after this long process. A much shorter process route, called tow-to-top, is to cut or break the filaments while retaining them in their straight, parallel state and producing a sliver. In converting the tow, the ends of the resulting staple fibers must not coincide to maintain the inter-fiber cohesion required to form a sliver.

To convert tow to top, two types of machines are generally used, namely: a) cutting converters and b) stretch-breaking converters. Both are used to cut the tow into the required staple lengths and form a sliver from it. The basic difference lies in the cutting mechanism used. A cutting converter uses a cutting cylinder that cuts the tow, while a stretch-breaking converter uses stretch to cut the tow.

Cutting Converters

A cutting converter basically comprises a feed creel, a cutting unit, a sliver-forming section, a crimping unit, and a sliver can delivery. In the feed creel (feeding unit), tows are mounted and tensioned over a series of bars, due to which the filaments are straightened and evenly spread across the width. This creates an even feed for the cutter. The cutter is a helical blade roller.

The tow is pressed between the cutting blade (on top) and a smooth-surfaced stainless steel roller (at the bottom). The helical shape provides overlapping of the cut lengths of fibers for cohesion, and the pitch of the helix on the roller determines the staple length. The gaps between the cutting edges of the helical blade have a rubber-covered surface that prevents filament misalignment. The cut lengths are then consolidated and gilled to form the sliver. For improved cohesion, crimp is imparted, and the sliver is passed through a stuffer box before coiling into the sliver can. The slivers from converters usually undergo two further gilling passages.

Stretch-Breaking Converters

Stretch-breaking converters work on the principle of extending to the breaking strain. This is generally preferred when bulky yarn is required as the end use. This process is divided into four zones: a) initial tensioning zone, b) heating zone, c) cooling zone, and d) filament breaking zone.

Filament tows are fed to stretch breakers via a creel, spread out. The tows are initially tensioned between rollers. Then they are heated to a temperature ranging between 120–170°C. After heating, they are cooled. After complete cooling, they are fed to the stretching zone. Here they are given a draft of 1.4-1.8 between the tensioning and stretching rollers. Cooling is done by air and water rollers. In the stretching zone, the final stretch is given to the breaking strain of the filaments, and the rollers used for the final stretch are spaced as per the mean fiber length required.

At a set draw ratio, the filament tenacity increases, and its potential shrinkage decreases with temperature. Within a suitable operating range of draw ratios, when the heater temperature is constant, both tenacity and potential shrinkage increase with an increased draw ratio.