Extrusion of Polymer Tubing
Tubing has a constant cross sectional shape, which allows it to be efficiently manufactured by melting polymer compounds and forcing them through the necessary tooling in a continuous process called extrusion. In this process, tubing can be made in lengths limited only by the capacity of the downstream handling equipment available.
In the extrusion of plastics, a raw polymer blend in the form of small beads or pellets is vacuum loaded into a hopper located above a gravity-fed barrel. The barrel, a cylindrical metal chamber that is electrically heated on its outside surface to temperatures of several hundred degrees F, contains a close tolerance rotating screw auger. The barrel and tooling temperatures, as well as the rotational speed of the screw, are precisely controlled.
The screw serves to mechanically shear the polymer pellets, adding frictional heating and mixing the polymer as it becomes molten, and to push the molten polymer forward to the end of the barrel. The tooling located at the end of the barrel is comprised of a pin and a hollow die. The pin is centered within the hollow die, and the tubular shape is formed as the molten polymer travels over the surface of the pin and inside the surrounding hollow die. The polymer tubing is solidified but has minimal mechanical strength as it exits the tooling and immediately enters a vacuum water tank. The vacuum serves to prevent the tubing from collapsing while the water cools the tubing, allowing it to harden and develop full mechanical strength.
By carefully controlling process parameters such as the speed of screw rotation, the vacuum exerted on the tubing in the cooling tank, and the amount that the tubing is drawn down by pulling it out of the extruder faster than it is pushed out by the screw, the diameter and wall thickness can be accurately controlled. Lasers are often used to monitor the geometry of the tubing as it is extruded.
Downstream of the extrusion process, the tubing is either cut to length inline or placed on reels. Tubing that is cut inline will be straighter and rounder than tubing that is first placed onto a reel and then taken off the reel for cutting.
In the extrusion of plastics, a raw polymer blend in the form of small beads or pellets is vacuum loaded into a hopper located above a gravity-fed barrel. The barrel, a cylindrical metal chamber that is electrically heated on its outside surface to temperatures of several hundred degrees F, contains a close tolerance rotating screw auger. The barrel and tooling temperatures, as well as the rotational speed of the screw, are precisely controlled.
The screw serves to mechanically shear the polymer pellets, adding frictional heating and mixing the polymer as it becomes molten, and to push the molten polymer forward to the end of the barrel. The tooling located at the end of the barrel is comprised of a pin and a hollow die. The pin is centered within the hollow die, and the tubular shape is formed as the molten polymer travels over the surface of the pin and inside the surrounding hollow die. The polymer tubing is solidified but has minimal mechanical strength as it exits the tooling and immediately enters a vacuum water tank. The vacuum serves to prevent the tubing from collapsing while the water cools the tubing, allowing it to harden and develop full mechanical strength.
By carefully controlling process parameters such as the speed of screw rotation, the vacuum exerted on the tubing in the cooling tank, and the amount that the tubing is drawn down by pulling it out of the extruder faster than it is pushed out by the screw, the diameter and wall thickness can be accurately controlled. Lasers are often used to monitor the geometry of the tubing as it is extruded.
Downstream of the extrusion process, the tubing is either cut to length inline or placed on reels. Tubing that is cut inline will be straighter and rounder than tubing that is first placed onto a reel and then taken off the reel for cutting.