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Extrusion is a conversion process which transforms plastic raw material into finished or semi-finished usable product. The thermoplastic or thermoset polymer, usually in the form of granules or powder, is fed from a hopper on to the rotating screw. It is then conveyed along the barrel where the plastic gets heated up by conduction from the barrel heaters and shear due to its movement along the screw. The depth of the screw channel is reduced along the length of the screw so as to compact the material. At the end of the extruder the melt passes through a die possessing the required cross section to produce an extrudate of the desired shape. It is cooled with a spray of water and drawn away from the die exit at a controlled rate. The extrudate can then be wound on a spool, cut to a preferred length by a plastic extrusion machine operator, or directed into another machine for further processing.
Extrusion vs Injection Moulding
Extrusion is a steady state process in contrast to injection molding or blow molding, which are cyclic processes. This steady-state characteristic yields some unique benefits and challenges as a manufacturing process. The injection molding process is almost similar to the extrusion method except that in injection molding, the molten material is injected into a mold rather than forced through a die. Hence, higher back pressures may be generated in single screw extrusion machines compared to injection molding machines. The primary function of the screw extruder in injection molding is simply to obtain a melt whereas in a plastic extrusion machine, the screw must mix, homogenize and melt the material. Also to achieve better mixing of the polymers used, the screws used in extrusion process are longer than the ones in injection molding machines. Extruded products are very long and continuous, and have a cross section that is usually constant with respect to the axis of production. Injection-molded products are discrete items with varying cross sections in each axis. Melt viscosity of the plastic is one characteristic which often differentiates extruded resins from injection-molded resins at normal processing temperatures. Extruded plastics often have a higher melt viscosity.
Extrusion Machine for Sale
The major extrusion machine parts can be divided into components that are upstream and downstream to the extruder and the extruder itself. Upstream components include resin handling equipment which conveys the raw materials to the extruder and resin conditioning equipment that will pre-heat the resin before entering the extruder system, thereby controlling its moisture content. The downstream components usually include a molten resin filter, the extrusion die assembly, the quenching tank for cooling and finally a product cut-off device. The two main components of a plastic extruder machine for sale are barrel and screw. The screw which is chiefly responsible for the melting of the plastic can be further subdivided into three zones namely,
Feed zone which maintains the optimum supply of pre-heated resin to the subsequent zones
Compression zone where the material is squeezed and compressed to liquid
Metering zone where the melt is homogenized and supplied at uniform temperature and pressure to the die.
Extrusion and its Types
Extrusion is commonly categorized based on the following three parameters:
Direction of Flow – Forward and Backward
Operating Temperature – Cold and Hot
Extrusion Equipment – Horizontal and Vertical
In forward extrusion, the metal is forced to flow in the same direction as the punch whereas in backward extrusion the metal flows in the direction opposite to the punch movement. Cold extrusion is the process done at room temperature or slightly elevated temperatures. Hot extrusion is done at fairly high temperatures, approximately 50 to 75 % of the melting point of the metal and at very high pressures ranging from 35 – 700 Mpa. Due to high temperatures and pressure it has a detrimental effect on the die life as well as on the other working components. Based on the direction of travel of the ram, the extrusion presses are classified as horizontal and vertical. Vertical presses have a higher production rate and occupy less factory space than their horizontal counterparts. Also deformation is non-uniform due to varying temperatures between top and bottom parts of the billet in horizontal presses.
Applications of Plastic Extrusion Machinery
The extrusion machine process has a few exclusive advantages when compared to other plastic processing techniques like low tool costs, ability to process complex profile shapes, and speedy processing even though it is limited to parts of uniform cross section. Extrusion is capable of forming anything with a constant cross section like fibres, tubing, pipes, sheets, films, cable sheathing, profiles, coated wires, filaments, rods etc. Extrusion is highly suitable for producing seamless pipes and tubing, especially with metals which have poor manufacturability. Lead, tin, aluminium alloys, copper, titanium, molybdenum, vanadium, steel are some of the metals that can be extruded. Aluminium extrusions are used for window and door frame systems, prefabricated building structures, curtain walling, etc. Typical parts produced by extrusion machinery are trim parts used in automotive and construction applications, window frame members, railings and aircraft structural parts.
Extrusion Machine Manufacturers
DAVIS STANDARD, REIFENHÄUSER, DOLCI, QINGDAO SHANSU, EAGLE, SAREM, USEON (Nanjing), TOSHIBA, KRAUSS-MAFFEI BERSTOFF, COPERION, BRÜCKNER and MAILLEFEE are few of the plastic extrusion machines manufacturers in the world. Find great used extruders and the best extrusion machine price on TradeMachines.