Introduction to process of plastic movement in an extruder (calculation of extrusion volume, discussion of extrusion theory)

Introduction to process of plastic movement in an extruder (calculation of extrusion volume, discussion of extrusion theory)

The working principle of extruder: use a certain-shaped screw to rotate in a heated barrel to extrude plastic sent from hopper to front, so that plastic is evenly plasticized (that is, melted) and passed through machine head and molds of various shapes, so that plastic is extruded into continuous plastic layers of various shapes and extruded onto core and cable.

Introduction to process of plastic movement in an extruder (calculation of extrusion volume, discussion of extrusion theory)

Wire and cable extrusion

One, plastic extrusion process

The plastic insulation and sheathing of wires and cables is made by continuous extrusion, and extrusion equipment is usually a single screw extruder. Before extruding plastic, you need to check if plastic is wet or there are other sundries, and then pre-heat plastic and put it in hopper. In extrusion process, plastic in hopper is fed into drum by gravity or by a feeding screw, and is continuously pushed forward by action of rotating screw, and gradually moves from preheating section to homogenizing section; time, plastic is mixed and extruded by screw and converted into a viscous state by external heat of barrel and heat of shear friction between plastic and equipment, forming a continuous and uniform flow of material into thread groove. At temperature set by process, plastic turns from a solid state into a plastic object in a molten state, and then pushed or stirred by a screw, fully plasticized plastic is pushed into machine head, and flow of material reaches machine head passes through mold core Annular gap between mold and mold sleeve extruded from neck of mold sleeve, and core or cable core is extruded to form a continuous and dense insulating layer or sheath layer, and then cooled and solidified to make wire and cable products.

Second, three-step extrusion process

Plastic extrusion is mainly based on plastic state of plastic. The plastic molding process of plastic in an extruder is a complex physical process: including mixing, grinding, melting, plasticizing, venting, compacting and final molding, this process is carried out continuously. However, usually people tend to artificially divide extrusion process into different steps according to different reactions of plastics; ① plastification stage (mixing, melting and homogenization of plastics); ② molding stage (plastic extrusion molding); ③ molding stage (cooling and solidification of plastic layer).

2.1, plastification stage.

Also called compression phase. It terminates in extruder barrel and rotation of screw turns plastic from a granular solid to a plastic viscous liquid. There are two heat sources for plastics in plasticizing stage: one is electrical heating outside barrel, other is frictional heat generated by rotation of screw. The initial heat is generated by electrical heating on outside of barrel; after normal operation, heat is generated by friction between rotating screw of material and inner wall of cylinder during compression, shearing and mixing, as well as internal friction between material molecules.

2.2. Formation stage.

This is carried out in machine head. Through rotation of screw and pressure, viscous liquid is pushed into machine head, and viscous liquid is converted into extruded materials of various sizes and shapes through a mold in machine. head, and is wound on wire core or conductor.

2.3, finalization phase.

It is carried out in a cooling water tank. After extruded plastic lining is cooled, it changes from an amorphous-plastic state to a molded solid state.

Third, change in plastic flow at stage of plasticization

At plasticizing stage, when plastic moves along axis of screw and is pushed by screw to machine head, it experiences changes in temperature, pressure, viscosity and even chemical composition. These changes are different in different parts of screw. The plasticizing stage can be divided into three stages according to process of continuous change of physical state as plastic flows: ① Feeding section (also known as crushing section); ② Melting section (also called plasticizing section); ③ Homogenizing section, also known as leveling section). Each section has a different effect on plastic extrusion, and in each section, plastic has different shapes, indicating plastic extrusion characteristics.

3.1, feeding section

Firstly, softening temperature of granular hard plastic must be maintained, and secondly, shear stress that occurs between rotation of screw and stationary cylinder acts on plastic particles, crushing softened plastic. The most important thing is to create enough continuous and stable thrust and reverse friction when screw rotates, so as to form a continuous and stable extrusion pressure, and then realize mixing and uniform mixing of broken plastics, and initially realize heat exchange. providing basis for continuous and stable extrusion. Whether thrust generated in this step is continuous, uniform and stable, whether shear rate is high or low, and whether crushing and mixing are uniform, all these directly affect quality and productivity of extrusion.

3.2, section "Melting",

The hard plastic, which has been crushed, softened and initially mixed and mixed, moves along screw groove to machine head due to pushing action of screw and enters melting section from feeding section. In this area, plastic collides with heat of a higher temperature. The heat source at this time, in addition to electrical heating outside barrel, also plays role of frictional heat of rotation of screw. The thrust from feed section and reaction force from homogenizing section cause plastic to form a reverse flow as it advances. The reverse flow occurs in screw groove and inthe gap between screw and barrel. The reverse flow not only makes material more evenly mixed, but also increases heat exchange effect of plastic, and achieves heat balance of surface. Since exposure temperature at this stage exceeded rheological temperature of plastic, and exposure time is longer, plastic has undergone a physical transformation, and material in contact with heating barrel begins to melt, forming a polymerization layer on inner surface of barrel. When thickness of melted film exceeds gap between top part of filament and stem, it will be scraped off by rotating filament and collected in front of advancing filament, forming a molten pool. Due to relative movement of stem and base of thread, material circulation is created in melt bath

Circular flow. Behind edge of auger is a solid layer (hard plastic). When material moves forward along auger channel, hard layer is constantly

Extrusion on inner wall of machine frame accelerates process of heat transfer from barrel to solid bed, and at same time, rotation of screw has a shearing effect on melt film on inner wall of body. barrel, thereby melting material at interface between melt film and solid layer. The width of solid layer gradually decreases until it completely disappears, that is, transition from a solid state to a viscous-liquid state (plastic state). At this time, molecular structure of plastic has undergone a fundamental change, and intermolecular tension has been extremely relaxed. If it is a crystalline polymer, its crystalline area begins to decrease, and amorphous form increases. Except for super-large molecules, main body has completed plasticization. This is so-called "initial plasticization", and under pressure, gas contained in solid material is expelled to achieve initial compaction.

3.3 alignment section

There are several outstanding technological characteristics: thread depth of this section of auger is smallest, that is, volume of auger groove is smallest, so this is working section where pressure between auger and barrel is largest, in addition, draft from auger and sieve plate, etc. .d. process, so radial pressure and axial pressure on plastic at this stage are largest. This high pressure effect is enough for all gas in plastic to be exhausted and melt to condense and become dense. This section is called equal pressure section. Due to exposure to high temperature, polymers that have not been plasticized in melting section are plasticized in this section, so that "particles" are finally removed and plasticization of plastic is completely uniform, and then completely plasticized and melted plastic is quantitatively and under constant pressure. Extruded evenly from machine head.

Fourthly, flow state of plastic during extrusion

Duringextrusion, due to rotation of screw, plastic is pushed but barrel is not moving, which creates a relative motion between barrel and screw, and this relative motion causes friction against plastic, causing plastic to be compressed. In addition, due to resistance of mold in machine head, porous sieve plate and mesh filter, plastic will create a reactive force during advancement, which makes it difficult for plastic to flow in screw and barrel. The fluidity of plastics is usually thought of as consisting of following four forms of fluidity:

4.1 positive flow—

This refers to flow of plastic along groove of screw towards head of machine. It is created by pushing force of rotation of screw and is most important of four forms of flow. The positive flow size directly determines extrusion volume.

4.2, Reverse Flow—

Also known as countercurrent, its direction is directly opposite to that of positive current. This is caused by pressure generated in area of ​​the head (the reaction force of plastic advancing) due to fact that shape, sieve plate and filter mesh in machine head prevent plastic from advancing. "Pressure backflow" is also called "pressure backflow" from machine head to loading port. This may result in loss of performance.

4.3 crossflow -

Along direction of axis, that is, in direction of plastic perpendicular to thread groove. material flow. It is also formed by pushing as propeller rotates. Its flow is counteracted by side wall of screw groove. Due to mutual resistance of threads on both sides and rotation of screw, plastic flips in screw groove to form a circular flow, so cross flow is essentially a circular flow. Circulation has a great influence on mixing, plasticization and heat transfer of plastic in barrel, so plastic can be mixed and plasticized to a molten state in screw, which is inseparable from circulation effect. The circulation makes material mix and mix in barrel and promotes heat exchange between barrel and material, is of great importance to improve quality of extrusion, but has little effect on flow rate of extrusion.

4.4, Leak—

This is also produced by resistance of mold, sieve plate and screen in machine head. However, this is not a flow in screw groove, but a reverse flow formed in gap between screw and barrel. It can also lead to loss of productivity. Since gap between screw and barrel is usually very small, under normal conditions leakage flow rate is much less than forward flow and return flow. In extrusion process, leakage flow will affect extrusion volume, and leakage flow will increase and extrusion volume will decrease.

The four states of plastic flow cannot appear in a separate form. With regard to a certain plastic particles, there will be neither true countercurrent nor closed circulation. The actual flow of molten plastic in thread groove is a combination of above four flow states, moving forward in a helical path.

Five, extrusion volume

Extrusion volume is an important characteristic parameter of an extruder and one of important research in field of extrusion theory. As mentioned above, flow of plastic during extrusion process is an artificial division of screw into three parts according to a certain working characteristic. In fact, screw itself is a single whole, a change in physical state of plastic along entire length of screw. The screw arises and develops gradually and continuously. And you're done, there is no two-phase interface. For this reason, there are two assumptions about amount of extrusion: it is assumed that entire process of transition of plastic from a solid state to a viscous-liquid state occurs and completes so-called "junction point" in changing section, while it is assumed that plastic is densified, and perform so-called "filling point" in change section, thereby artificially dividing all materials into two parts, namely solid part before "glue point" and liquid part after "connection point". For extrusion mechanism with a reasonable design, due to continuous nature of extrusion, extrusion volume in solid state and extrusion volume in viscous state must be absolutely equal (released gas is not taken into account), so extrusion volume can be obtained from one of two parts. As a rule, it is calculated by fluid mechanics method at a later stage. The formula for calculating extrusion volume of a screw with same distance and different depth:

Introduction to process of plastic movement in an extruder (calculation of extrusion volume, discussion of extrusion theory)


Q—extrusion volume (cm3/min); V—Auger speed in advancing direction (cm/min);

b—width of threaded slot (cm); h1—thread depth of filling point (cm);

h2—— End Thread Depth (cm);g——Free Fall Acceleration (cm/min2);

p—extrusion pressure (kg/cm2);η—plastic viscosity (kg/cm min);

L——length from point of filling to end of thread (cm).

Based on formula for calculating extrusion volume, main factors affecting extrusion volume are:

1) The higher extrusion pressure, smaller extrusion volume. The extrusion pressure is formed by thrust and its reaction force. The greater extrusion pressure, greater reaction force, and reaction force is source of counterflow (backflow and leakage). Therefore, greater extrusion pressure, greater influence of displacement on positive flow, larger it is, there will be less extrusion.

2) The smaller screw groove, more stable extrusion volume. During extrusion process, small changes in temperature and screw speed will cause extrusion pressure to change. From second point of formula for calculating extrusion amount, we can know that when depth of screw groove is large, value (h1 square * h2 square) will be very large, even if extrusion pressure changes a little, it will cause a large number of fluctuations in second term, which affects large volume fluctuations extrusion.

3) The larger screw groove width and larger screw groove volume, larger extrusion volume. But you can't blindly increase width of screw slot to increase extrusion volume, because widening width of screw slot will reduce thickness of thread or shorten plasticizing path. The former reduces wear resistance of filament, and latter makes plastic less transformable.

4) The thread depth should be appropriate, if it is too shallow, extrusion volume will be small; If it is too deep, extrusion volume will be unstable and affect plasticization uniformity. .

5) Temperature, as screw speed, extrusion pressure, external heating conditions and temperature of environment around extruder change, temperature of material in barrel will also change accordingly. Proper temperature control is very important to ensure that plastic can always be heated and plasticized in extruder as needed and evenly delivered to die.

Low temperature extrusion has many advantages, it is easy to maintain shape of extrudate because polymer contains less thermal energy, cooling cycle or cooling time is shortened, and polymer degradation is reduced, especially for heat-sensitive plastics. But when temperature is too low, melt breakdown may occur, poor molding may result in poor looking or completely useless products,and may also reduce extrusion performance.

The relationship between temperature of extrudate and physical properties of product and other aspects is very complex. According to physical properties of polymers, equipment and product performance requirements, use of an appropriate temperature is one of important conditions for obtaining quality products.

Therefore, a good extrusion process requires extrudate to have a stable flow rate, constant pressure, constant temperature, and uniform extrudate composition. However, these conditions are not independent. For example, pressure fluctuations cause flow fluctuations, temperature fluctuations cause viscosity fluctuations, and viscosity fluctuations cause pressure fluctuations and flow fluctuations.

Extruder, whether single screw or twin screw, when talking about extrusion volume, two concepts need to be explained:

①Theoretical extrusion limit

②Actual extrusion volumeCalculation of theoretical limiting extrusion volume: Calculated from screw geometry (cross-sectional area of ​​the barrel interior - screw cross-sectional area = flow channel cross-sectional area) and then multiplied by feed is multiplied by assembly speed, all in international standard units, and resulting m³/h equals theoretical limit of extrusion at that speed. The actual extrusion volume does not need to be calculated. Generally speaking, this is a metered feed, and extrusion volume is feed volume you set. In starvation feeding (rarely in practice, because amount of feed is not stable enough) it is necessary to combine characteristics of material, for example powder or pellets, fluidity of its solids and structure of feed. feeder The size is related, but usually actual extrusion of solid fast food does not exceed 80% of theoretical limit.When designing dosing feed, it is usually 60% of theoretical limit. There is no need to adjust amount of feeding within a certain speed range of host.

Total calculated result:

The throughput of 51/105 mm extruder is about 150 kg/h

The output of 65/132 mm extruder is about 160-280 kg/h

The throughput of 80/156 mm extruder is about 350-450 kg/h

The throughput of 92/188 mm extruder is about 450-600 kg/h

Six, extrusion quality

Extrusion quality mainly refers to good plasticization of plastic and homogeneity of geometric dimensions. That is, whether radial thickness is constant and whether axial outer diameter is uniform. Apart from plastic itself, plasticization state is mainly determined by factors such as temperature, shear strain rate, and exposure time. Excessive paceThe extrusion time not only causes extrusion pressure to fluctuate, but also causes plastic to decompose, and may even cause equipment breakdowns, so extrusion temperature should be controlled according to process temperature. Reducing depth of screw groove and increasing screw length to diameter ratio favorably affects heat transfer of plastic and increases heating time to ensure uniform plasticization, but this will affect extrusion volume and cause difficulties in manufacturing and assembling screw. Thus, an important consideration to ensure plasticization should be to increase shear strain rate of plastic obtained by rotating screw in order to achieve uniform mechanical mixing and balanced extrusion heat transfer and thus guarantee uniform plasticization. The magnitude of this strain rate is determined by shear strain force between screw and cylinder, and value of shear strain rate is:

Δ= Πd*N/h

Among them: Δ — shear strain rate (l/min); D is screw diameter (cm);

N — screw speed (rpm); H - screw groove depth (cm);

It can be seen that if extrusion volume is required, depth of screw groove can be increased by increasing rotation speed. In addition, gap between screw and barrel also affects extrusion quality. When gap is too large, reverse flow and plastic leakage increase, which not only causes extrusion pressure fluctuations and affects extrusion volume; Excessive heat can cause plastic to burn or make casting difficult.

Seventh, study of extrusion theory

The study of theory of plastic extrusion is based on three processes of plastic in extruder, that is, from a solid state in feeding zone to a solid state in transition zone (melting zone) - a viscous liquid state to a viscous state in homogenization zone These three physical processes of liquid state are studied . The extruder extrusion theory is mainly divided into three functional areas for research, namely so-called solid body transfer theory in feeding zone, melting theory in melting zone, and viscous liquid transfer theory.

The theory reveals influence of material properties, machine design parameters, and process conditions on melting process and feed rate to varying degrees, and provides a basis for improving extruder design, formulating reasonable process conditions, and selecting materials. .

7.1, Solid State Transport Theory

During extrusion, hard plastic added to screw is pushed forward by force of rotating screw, and material is continuously compacted by resistance of machine head. Plasticized and non-plasticized materials are continuously and neatly arranged to form an elastic "solid plug" that fills auger groove of entire feed section. According to this phenomenonm, based on equation of static balance of friction of solids on solids, theory of transfer of solids is established.

7.2, Melting theory

The theory of melting is a theory based on thermodynamics and rheology. At end of feeding section, material in contact with heating cylinder begins to melt, and a layer of polymer melt film is formed on inner surface of cylinder. When thickness of melt film exceeds gap between screw top and barrel, it will rotate. scraped off, gathered in front of thread, forming a

Molten pool. Due to constant increase in temperature and shearing action of screw, melt pool continues to expand. The main factors influencing length of melting section are material properties, flow rate, screw speed, barrel temperature and initial temperature of material.

7.3, Melt transfer theory

The melt delivery theory is also known as theory of hydrodynamics, which studies how homogenizing section of screw ensures complete plasticization and extrusion of plastic at constant pressure, quantity and temperature to obtain stable quality and output.

Source: Internet, thanks to author of original, very interesting!

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