Plastic extrusion of tube, hose and pipe is a steady-state process for converting a thermoplastic raw material to a finished or near-finished annular product. The raw material is usually in the form of plastic pellets or powder. The conversion takes place by forming a homogeneous molten mass in the extruder and forcing it under pressure through an extrusion die orifice that defines the shape of the product's cross section. The formed material, or extrudate, is cooled and drawn away from the die exit at a controlled rate. The extrudate can then be wound on a spool, cut to a specified length, or directed into another in-line process.
By contrast with injection molding or blow molding, which are cyclic processes, extrusion is a steady-state process. This steady-state characteristic produces some unique benefits and challenges as a manufacturing process. Extruded products are very long and continuous, and have a cross section that is usually constant with respect to the axis or direction of production. Injection-molded products are discrete items with varying cross sections in each axis. The fact that the extrusion of tube, hose and pipe is a steady-state process will be discussed in many areas of this article.The terminology used for describing tube, hose and pipe varies from industry to industry.
The major components of a tube, hose, or pipe extrusion system are often divided into components that are upstream of the extruder, the extruder itself, and those that are downstream of the extruder. Upstream components include resin handling equipment for conveying the raw materials to the extruder, and resin conditioning equipment that will pre-heat the resin, or control the moisture content of the resin before it enters the extruder feed throat. The downstream components include a molten resin filter or screen changer, the extrusion die assembly, the quench or vacuum sizing tank, any product measurement instruments, the extrusion line speed controller in the form of a belt puller or nip roll set, and finally a product cut-off device or winding equipment. There are additional extrusion process methods that will take the extruded product directly into another in-line process for the addition of a reinforcement braiding or for the addition of other coatings or resin layers.
Extrusion techniques can be used to process most thermoplastics and some thermoset plastics. The resins most commonly extruded for tube, hose and pipe products include high and low density polyethylene, polypropylene, polyurethane, polystyrene, fluoropolymers, PTFE, polyester, and flexible and rigid PVC. A characteristic that often differentiates extruded resins from injection-molded resins is the melt viscosity of the plastic at normal processing temperatures. Extruded plastics often have a higher melt viscosity, which allows the extrudate to retain the shape imparted to it by the die while the extrudate is in the quenching stages. Combinations of various resins can be used to gain special physical, biological, or chemical properties. Many additives can be used during the extrusion process to enhance processing characteristics of the polymer or to alter product properties. Such additives include lubricants, thermal stabilizers, antioxidants, radiopacifying agents, and colorants.
The parameters important to extrusion processing are similar to those of injection molding processes. Resin temperature, resin pressure, resin moisture content, screw speed, and screw motor amperage are usually controlled or monitored to provide a homogeneous melt at a controlled volumetric rate. Quenching temperature and the rate at which the extrudate is drawn are controlled or monitored to provide a controlled product size. Dimension measurements, using a variety of gauging methods, can be taken of the extrudate as it is produced. In contrast to injection molding, extrusion can vary the size of the final product without changing the die tooling. Common extrusion production tolerances or process consistency are held to within 1% of the nominal measured value. As these manufacturing processes involve steady state conditions, any action that can stabilize any parameter or condition is beneficial to the process.