Spinning PP Filaments

The textile industry has been making fabrics from polymers for more than fifty years; as a result, the processes and equipment are highly evolved.

A fiber of continuous length is typically referred to as a “filament.” With modern equipment it is possible to convert a thousand pounds of polymers into filaments without a single break occurring. In the textile industry, molten polypropylene (PP) passes through an adapter into a die that has a pattern of holes in it. The liquid polymer being forced through these holes solidifies in a short distance, typically about 100 centimeters or 40 inches (one meter), and it can be further stretched or treated before winding onto spindles.

In the manufacture of synthetic underlayment, the threads used to manufacture scrim are made by slitting an extruded polypropylene sheet into many strips, which are further processed into threads or filaments.

The primary raw material employed in the manufacture of PP filaments or fibers is polypropylene granules, which can also be referred to as “pellets.” Depending on the color requirement of the final product, color pigment is incorporated into the master batch. Furthermore, depending on the end use and expected outdoor exposure of the finished product, ultraviolet stabilizers (UV additives) are included in the master batch. The various ingredients are blended at precise formulations and then conveyed through a suction pipe to the hopper, which then follows to the extruder hopper.

The extruder hopper allows the raw materials to enter the feed section of extruder in a continuous fashion, by gravity. The feed section is connected with the pipes to the water supply system for flow and return of chilled water. The cooling system eliminates slippage of the raw materials as they are pumped in the direction of a series of drums towards the die.

Raw materials are continuously fed at a rate proportional to the speed of the extruder screw. The temperatures at each of the heating zones are maintained to assure proper mixing, melting and conveying of material. The extruder screw plasticizes the granules into melt form and conveys it forward. At the exit end of the extruder screw, a screen pack consisting of varying mesh sizes screens out the un-melted particles. The polymer melt must be filtered to ensure there are no bubbles or contaminants. Ideally, the polymer melt will have a consistent chemical composition and, hence, also predictable physical properties. The lengths of molecules within the polymer melt will not vary greatly such that the viscosity, melt flow characteristics, and melting temperatures do not vary over the length of the filaments (or “strips” or “threads” or “tapes”).

The polymer emerging from the slot die is chilled to a solid form at a water tank and the chilled film passes through two pairs of scrapers on the way to the squeeze-off rollers, where the remaining drops of water are removed. Then the film enters a slitter where a spreader roller straightens any wrinkles in the film, slitting blades cut the film into fibers or filaments. The width of individual fibers is adjusted by using spacers of suitable size in between the slitting blades.

A system of godets is used to draw the filaments. (In the textile industry, a “godet” is a roller for guiding synthetic filaments during drawing. It is derived from a French word for a “cup” or a Dutch word for a “cylindrical piece of wood.”) Filaments from a first godet station pass through a hot air oven and enter a second godet station, which is maintained at ambient temperature. The speed of this second godet is four- to-five times higher than the speed of the first godet. (The speed corresponds to draw ratio, which is chosen according to property of raw material and the expected quality of the fibers.) The filaments then enter a third godet station, which is maintained cold using chilled water circulating system, thereby freezing the molecules and aligning them in an oriented state.

Such oriented filaments possess high tensile strength and other desirable physical characteristics. Various properties of the filaments can be inspected to maintain quality control before the spools are sent to the next processing step.

Ultimately, these threads are gathered into spindles, which will in turn feed the weaving machines.

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