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You are here: Home » News » Extrusion principle of plastic extruder

Extrusion principle of plastic extruder

Views:171     Author:Site Editor     Publish Time: 2017-12-11      Origin:Site

Extrusion principle of plastic extruder

Basic principle of plastic extruder screw feeding and discharging principle the function of extruder is to use heating, pressurizing and shearing to transform solid-state plastics into homogeneous melts and to send the melt to the next process. The production of melt involves the process of mixing masterbatch such as additive, blending resin and crushing. The melt of the finished product must be homogeneous at the concentration and temperature. The pressure must be large enough to squeeze the viscous polymer. The extruder completes all of the above processes through a barrel with a screw and a helical path. The plastic pellet enters the barrel through the hopper at one end of the barrel, and then passes through the screw rod to the other end of the barrel. In order to have enough pressure, the depth of thread on the screw decreases as the distance to the hopper increases. External heating and internal heat generated by friction between plastic and screw rods make the plastic soft and melted. is a simplified extruder. Different polymers and different applications, the design requirements for extruders are often different. Many options relate to discharge, a plurality of feeder outlets, a special mixing device along the screw, cooling and heating of the melt, or no external heat source (adiabatic extruder), the relative size of the gap between the screw and the barrel, and the number of screw rods. For example, a twin-screw extruder can be more fully blended than a single screw extruder. In-line extrusion is a melt that is extruded with the first extruder, which serves as a second extruder, usually used to produce extruded polyethylene foams.

China's plastics extrusion technology market is showing three notable features: first, the industry's attention to efficient, energy-efficient technology is increasing, energy-efficient, high-yielding extruder has become an important technical innovation direction; second, the demand for high-performance flexible packaging films, such as food, beverages, and new energy industries, continues to soar, Push the plastic film multi-layer coextrusion technology to innovate unceasingly; Thirdly, China's extrusion technology began to increase research and development in a variety of ancillary components and ancillary equipment, the pace of new technology applications and development, and promote the rapid upgrading of the overall technical level.

Although affected by the international financial crisis and the external demand shortfall caused by the European debt crisis, since last year, China's plastic machine exports have generally declined, but due to the end of the year before the receipt of a good situation, this part of the first half of last year's volume, making the company last year export ratio increased, accounting for about 20% to 30%. Throughout 2013 China's foreign trade market can be seen, the key import market occupies the absolute share of China's plastic machine imports, while Indonesia, Thailand and Vietnam and other countries are leading the export market.

In the last two years, China's plastic extruder market has a big advance and breakthrough, in China's plastic machinery market, extruder as an important processing equipment, the development prospects are very optimistic, China's plastic machinery enterprises in recent years to expand the extrusion machine emerging markets, brand awareness further expanded, market competitiveness further improved, The share of exports to emerging countries will also rise significantly.

1. Extrusion system

Extrusion system including screw, barrel, hopper, head, and mold, plastic through the extrusion system and molded into a homogeneous melt, and in this process under pressure, the screw continuous extrusion head.

(1) Screw: Is the most important part of the extruder, it is directly related to the application scope and productivity of the extruder, made of high strength corrosion-resistant alloy steel.

(2) Barrel: is a metal cylinder, generally with heat-resistant, high compressive strength, rugged wear-resisting, corrosion-resistant alloy steel or lined alloy steel composite steel pipe made. Barrel and screw, to achieve the plastic crushing, softening, melting, plasticizing, exhaust and compaction, and to the molding system continuous uniform delivery of plastic material. 

(3) Hopper: The bottom of the hopper is equipped with a truncation device to adjust and cut off the material flow, and the side of the hopper is fitted with a hole and a calibration metering device.

(4) Head and mould: The head is made of alloy steel inner sleeve and carbon steel coat, and the die is equipped with mould.

The function of the head is to convert the rotational motion of the plastic melt into a parallel linear motion, evenly and smoothly into the mold sleeve, and give the plastic to the necessary molding pressure. Plastic presses in the barrel, through the porous filter plate along a certain flow path through the head of the head of the neck into the mold, mold core mold sleeve appropriate to form a continuous reduction of annular gap, so that the plastic melt in the periphery of the core line to form a continuous dense tube-like coating layer. In order to ensure that the plastic flow path reasonable, eliminate the accumulation of plastic dead corners, often placed with a shunt sleeve, in order to eliminate the pressure fluctuations in plastic extrusion, there are also set the pressure ring. The die calibration and adjustment device is also arranged on the head, which is easy to adjust and correct the concentricity of the die core and the die sleeve.

According to the angle between the head material flow direction and the screw centerline, the extruder is divided into a bevel head (angle 120º ) and a right-angle head. The casing of the head is fixed on the fuselage with a bolt, the mould inside the head is seated, and the nut is fixed on the head inlet port, the front of the die core seat is provided with a die core, the center of the die core and the die core seat has a hole, for passing through the core line, the front part of the nose is fitted with a pressure ring for equalizing pressure; The extrusion forming part is composed of a die sleeve and a die sleeve. The position of the die sleeve can be adjusted by the bolt support to adjust the relative position of the die sleeve to the die core, so as to adjust the uniformity of the thickness of the extrusion layer. The exterior of the head is equipped with a heating device and a temperature measuring device.

2. Transmission System

The function of the driving system is to drive the screw and supply the torque and rotational speed required by the screw in the extrusion process, which is usually composed of motor, reducer and bearing.

3. Heating and Cooling device

Heating and cooling are necessary for the plastic extrusion process to be carried out.

(1) Now the extrusion machine is usually used electric heating, divided into resistance heating and induction heating, heating sheet installed in the fuselage, machine neck, the parts of the nose. The heating device is heated by the plastic in the external heating tube to achieve the desired temperature of the process operation.

(2) The cooling device is designed to ensure that the plastic is in the temperature range required by the process. Specifically, in order to eliminate the screw rotation of the shearing friction generated by the excess heat, in order to avoid excessive temperature so that plastic decomposition, burning or stereotypes difficult. The cooling of the barrel is divided into two kinds: water cooling and air-cooled. General medium and small extrusion machine using air-cooled relatively suitable, large-scale use of water-cooled or two forms of combined cooling; screw cooling mainly using central water cooling, the purpose is to increase the material solid transport rate, stabilize the amount of glue, while improving product quality; but cooling at the hopper, First, in order to strengthen the transport of solid materials, to prevent the temperature of the plastic particles sticky plugging material mouth, the second is to ensure that the transmission part of the normal work.

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