thUnder extrusion production method to understand the processing of polymeric materials continuously pushing through of the melt through a die, the geometric shape of the output channel which determines the profile of the resulting semi-finished products IPT.

The basic equipment is an extrusion process or a worm press extruder equipped with a forming head. In the extruder, the polymeric material is melted, plasticized and is then injected into the die. In most cases, various modifications of single- and twin-screw extruder, also called worm presses. Sometimes used disc and reciprocating kneaders.

There are two main methods of film extrusion: Extrusion blow extrusion sleeve and ploskoschelevaya. The first method provides a film tube which can be folded or cut, and the second method is obtained by a flat film. The unit for the extrusion of film includes an extruder equipped with a corresponding head (spinneret), cooling the molten film unit by pulling mechanism and a winding device.

Different extrusion methods use different design of extrusion dies, and film cooling types.

Haul-offs and winding units also differ.

Construction and operation of the extruder head and the same for both methods, and briefly considered here to consider the various film production methods. The extrusion process is a continuous transformation of the thermoplastic material in the form of granules in a product such as a film. The sequence of process steps below:

1) Softening materials in powder or granular form;
2) metering the kneaded melt through a die which gives it the desired shape (e.g., a sleeve or a flat film);
3) cooling and fixation desired shape;
4) wound into rolls.
Steps 1 and 2 occur in the extruder, steps 3 and 4 are complementary.
A typical extruder has a main working body – Archimedean screw (auger), which rotates inside a heated cylinder. Polymer granules enter through a feed hopper mounted on one end of the cylinder, and are moved forward by a screw, to the head along the cylinder. When moving forward the pellets are melted by contact tf hot cylinder walls, and due to heat generated by friction. Warming due to friction (exothermic heat) is very noticeable in the modern high-speed machines, and can provide all the heat required for steady flow, external heating is needed only to avoid stopping the machine at the start, when the material is cold. The auger then pushes the molten polymer through a die, which defines the final shape.

Typically screw design is selected according to the type of polymer being processed. Augers characterized ratio of length to diameter L / D and compression ratio. Compression ratio – the ratio of the coil volume of the screw at the feed opening to the volume from the head coil. As a rule, single-screw extruders use screws with a ratio L / D of 15: 1 to 30: 1 and a compression ratio of 2: 1 to 4: 1.

extruder screw typically consists of three zones: loading, compression and metering. Download Zone transporting polymer of the hole under the hopper to the hotter sections of the cylinder. The compression zone – a zone where the depth of cut decreases, and hence the volume of the coil, which leads to compression of melting pellets. The main compression effect – increasing shear to the molten polymer resulting from the mutual movement of the auger relative to the cylinder wall surface. This improves the mixing, heating increases friction and leads to a more uniform heat distribution in the melt. Assigning the last zone of the screw – further melt homogenization, uniform dosing it through a die, smoothing the ripple at the output.

Before you head grille is supporting packet networks with large and small holes. These mesh filter removes impurities contained in the raw material. This is particularly important in the case of extrusion of thin films, where even the smallest dirt particles may form holes and even destroy the film. Package grids also increases backpressure in an extruder, which improves mixing and homogenizing the melt.

The screw is usually a channel for heating it with steam or cooling water. When you need maximum mixing auger cooled. This improves the quality of the extrudate, but slightly lower performance.

Selecting the proper die design – an essential condition of the material flow without “dead zones” where the material can stagnate and decompose due to overheating. This is particularly important in the case of PVC, in which the decomposition point of the material is close to the temperature required for the normal flow.

One of the latest innovations in the design of extruders – degassing zone, which allows to remove from the melt volatile components before the release of the head. This is accomplished by melt-release state of compression in which it is located, whereby the water and other volatiles are evaporated, and the melt is foamed. In fact, the system is used as if the two screws separated by degassing zone. First has three sections – loading, compression and metering, the latter has a generally shallow cuts and filled. A second screw degassing zone filled by the first screw melt, followed by dosing again zone. degassing zone is deeper cuts than the last round of the first screw; thus, the polymer suddenly falls into a larger volume, whereby the pressure falls.

Formed in the allocation of volatile foam again compressed and fed to the head. Volatile vapors or removed through the vents in the cylinder, or the cavity and the screw hole in one of the ridges of the screw in the degassing zone. Sometimes a vacuum is used to facilitate removal of gases. Vacuum can be used to funnel suhosmeshannyh powder to reduce porosity, when the screw must be sealed to prevent spillage of powder.

Producing and multi-screw extruders, often – dvuhshneko-tions that have their advantages and disadvantages. In general, multi-screw machines expensive due to the complex structure and are less stable in operation, but they have better mixing properties and transport. Due to a higher pressure effect multi-screw extruders produce less shear heat which makes them very suitable for materials sensitive to overheating, having a low friction coefficient, and those that have come out of the head at low temperatures.

Extrusion blow sleeve

The molten polymer from the extruder enters the head from the side, but can be fed and bottom. Once in the head, the molten polymer flows around the mouthpiece and exits through annular die opening to form the pipe. The tube is inflated to the desired diameter of the air pressure supplied through the center of the die. Blowing sleeve accompanied by a corresponding decrease in the film thickness. Extrusion is usually carried sleeves up and down sometimes even horizontally. Pressure clamping sleeve support rollers on the one hand and the head – the other. It is important that the air pressure is kept constant to ensure uniform film thickness and width. Other factors which affect the thickness of the film: the extruder performance and the drawing speed and temperature of the cylinder head. They must be strictly controlled.

As with any process of extrusion, film blowing becomes more economical by increasing the process speed. The limiting factor is the speed of the cooling sleeve. Cooling is generally produced outside the blowing ring of the sleeve. Assuming a constant air flow increase the extrusion rate results in a higher crystallization line (line, wherein solidification of the extrudate begins), leading to instability of the sleeve. Increased air flow results in more rapid cooling of the sleeve and reduce the crystallization curve, but this method is limited in its application, as too high air flow rate causes the deformation of the sleeve. Various forms of cooling air for the rings have been developed. It consists of a conical ring shape provided with three slits for air, while the air streams are directed and adjusted so that the distance between the sleeve and the ring is gradually reduced to the top ring. This leads to an improved cooling by increasing the airflow. This design also creates a low pressure zone in the upper part of the ring, which increases the stability of the sleeve.

Extrusion blown film is extremely difficult, there are many problems associated with the production of high-quality film. Among the many possible defects can be called unevenness, surface defects such as “orange peel”, “applesauce”, “fish eyes”, low strength and impact resistance, haze, and adhesion of pleats. Wrinkles are a constant problem, they lead to Rejection of the film and can occur due to many reasons, even in well-regulated industries. If the film is, for example, reaches the cold suction rolls, it becomes inelastic, and may be bent to form folds and rolls. One of the methods of increasing the temperature of the film at the nip rolls – Increase the melt temperature, which can lead to other problems, such as sticking. In fact, it is just an illustration of the method of film blowing, which often requires trade-offs to achieve the best combination of properties. Wrinkles are often caused by unregulated gap in the die. As a result, there is a difference in the film thickness and uneven exhaust hood in rolls. Folds may arise due to failures in the extruder or due to air flows in the drawing zone. Both of these factors can lead to wobble sleeve and thus to the folds at the reel. the bubble can be stabilized while maintaining its stationary horizontal guide ( “jaws”) to protect all or by extruder flow fluctuations air curtain film. Other causes of defects can be: non-parallelism of the guide shaft and exhaust shafts, uneven pressure along the slit rolls.

Among the surface defects, as mentioned earlier, the defect “fish-eye” occurs due to poor mixing in the extruder and pollution. Both of these factors are controlled by a strainer that not only separates the dust, but also improves the homogeneity of the melt by increasing the pressure in the extruder. Defects of “orange peel” and “apple sauce” are also surface defects that occur due to the heterogeneity of the polymer melt.

Since the vast majority of films made of low density polyethylene, it is useful to consider the influence of various parameters of the polymer such as melt flow index and molecular weight, the properties of the films. Impact strength, for example, increases with increasing molecular weight (m. E. A decrease in melting point) and with decreasing density. Bags that must withstand large loads, typically made of polyethylene density of 0,916-0,922 g / cm3 and MFR = 0,2+ 0,5 g / 10 min. For manufacturing a thin film technology, which are used in the construction of reservoirs or impermeable coatings to be used polymers with a higher MFR because of the difficulty of drawing of thin films viscous melts. The melt flow rate from 1 to 2.5 are typical in these cases, and therefore the toughness is less than that of films for bags. But transparency is improved. When the required balance of properties such as transparency and moderate moderate toughness, the use several higher density (0,920-0,925 g / cm3) and a MFR in the range 0,75-2,5. When high transparency is required, the brand used with high density and MFR, as they increase leads to an increase of visual transparency, reduced haze, higher gloss. Highly transparent films have a relatively low toughness due to high MFR, and can not be used for packaging of heavy objects.

Extrusion cast film

When flat film extrusion, molten polymer is forced through a slot die, and then it gets into the cold water bath or a cooling drum. In any case, the meaning of the process is the rapid cooling of the extruded film, and therefore cooling is started at a very short distance from the die lips (usually 25-65 \ mm). This distance is also dictated by the need to reduce the narrowing of the film web. When watering the cooling drum using chrome-plated hollow shaft with water cooling. Rapid cooling results in the formation of small crystallites which gives a transparent film. When using a water quench bath temperature was necessary to maintain it constant. At the same temperature, lower than the extrusion temperature of the quench bath, the film glides better and less sticks together, and at high temperatures it is easier to wind the film, creases are not formed, the physical properties of such a film is better. Slotted dies for forming a flat film has a large width compared with the diameter of the die, and this means that the path which passes the flow to the edges of the die is longer than the path to its midpoint. Compensation flow is usually achieved at the expense of the die to the collector. It comprises a transverse channel (or collector) of such diameter that the flow resistance therein is less than the resistance imposed by the die lips. Purpose collector – to compensate for the melt flow – can be realized, if the melt viscosity is low enough so for the flat film extrusion requires a higher temperature. This limits the use of dies with manifold processing of materials with good thermal stability. Another consequence of the extrusion at a higher temperature is the need to use denser filters, in order to maintain a satisfactory blood pressure. The inner side of the slot die must be carefully handled, since even a small surface flaw can lead to the appearance of roughness on film or polythickness.

Comparison of methods for producing the bubble blown and watering

Some of the advantages of the process of obtaining a tubular film:

1) the mechanical properties of the films thus obtained, typically better than those of films obtained watering;

2) the width of the folded tubular film is easier to control, there is no loss on the cutting edge; cutting edges is necessary in the manufacture of flat film due to the thickening of edges of film that occurs due to narrowing of the web;

3) folded tubular film is easier to turn in a bag, for this you only need to make one end of the hose segment;

4) broader manufacturing cost blown film is much less than the cost of a wide cast film production, as the cost of cooling drums increases sharply with increasing length of the complexity of handling long shafts;

5) Holtsevye die film more compact and inexpensive compared to the slot die for the production of flat film width comparable;

6) The manufacturing process of the tubular film is more flexible and easier.
These benefits must be weighed against the advantages of the process, which uses a slit die:

1) ploskoschelevye extrusion systems provide better performance;

2) the film obtained using a slit die, typically have excellent optical properties, but it should be noted that the special rapid cooling processes were developed for blown film, particularly polypropylene

3) the variation in thickness of the film is usually obtained using a slit spinneret less.

Coextrusion films

Coextrusion technology is used to extend the operational capabilities of molded products by combining them in polymeric materials with different individual characteristics.

Most of these products has a layered structure in which each layer of material forms a specific operational or technological quality. For example, to successfully compete with conventional food packaging materials (foil, glass, cellophane, etc.) Modern film should have high stiffness and toughness, to ensure long-storage products, be gastight, and with all this to allow the high-speed processing processes. The use of plastics, not previously used together, for example, HDPE and PA; PVC and PP; PA, PC, PP and PVDC allowed to receive containers for storage of industrial and agricultural chemicals, fuels and lubricants, etc. With a capacity of hundreds of liters. Multi-layer coextruded sheets with the number of layers from 2 to 9, are widely used in the automotive and construction industries for the subsequent thermoforming products with a long useful life (on the assurances of manufacturing companies up to 20 years). Of particular interest is the production of multilayer coextruded technology pipes, hoses, tubes, capillaries, including biomedical applications. Coextrusion of PP and glass fiber reinforced PA receive pressure pipes and tubes for transporting gas and pipe used in the devices of hot and cold water. In some cases, the use of second polymeric raw material allowed in multi-layer constructions. Stratification produces a film with a very thin individual layers (2-5 microns), which is unattainable using other methods (lamination or Kashirova-set). In combination with the substrates of the solid polymer (PP, PA, PC, PET) can be formed from layers of low-strength polymers such small thicknesses (5-15 microns) which is completely impossible in the preparation of monolayer films of the same plastic. For example, the minimum thickness of the EVA monolayer film of 20 microns, and as components of multilayered products, it can be reduced to 5 microns. Thus, it is possible to use structures of thin layers of expensive, but operationally important polymers in combination with cheap constituting the main part of the construction, plastics.
Considerable importance is the economy of the method in which the hybrid or multi-layer product structure is achieved by one-stage process, precluding the operation of laying the individual layers or elements, and the bonding primer.

Coextrusion performed separately plasticizing polymer components in a single screw extruder melt streams followed by coupling of various polymers in the forming head. Thus, defining part of coextrusion technology are processes taking place in the forming head. All operations the extrudate after it exits from the forming head (blowing, orientation, etc.). Made by conventional and specific enough tehnologiyam.Estestvenno that since the materials are used in coextrusion different melting points and differ in rheological and thermal characteristics, they are kneaded in their optimal conditions, and supplied to the head, which temperature is set by the material to the highest temperature processing. It is obvious that, firstly, maintaining the melt in forming the desired layered organization head will be determined by layering different melt viscosity and, secondly, all polymers used should be heat stable at the selected processing temperature. Depending on the device heads are coextruded bag house, a flat sheet and tube. The number of layers in the product can reach 9, the minimum thickness of layers 2 mm, maximum – 3000 mkm.K coextrusion heads imposes very stringent requirements. The most important of them – to ensure flawless the layered structure of the product. Meant saving equal thickness layers in the longitudinal and transverse (radial) direction when monolithic coextruded products. In addition, the head should be (this corresponds to the logic of their actions) by referring to generic recyclable materials, and design head channels, respectively, to provide minimum resistance to the current melts. This movable coupling head parts are performed at a high accuracy class, and the whole design process should allow the assembly-disassembly. Applicable in the heads of construction materials require operation at elevated temperatures and pressures, the thermochemical processes. The cost of co-extrusion head to be economically feasible, that is the minimum.
The combination set forth the terms and conditions makes the task of designing and manufacturing co-extrusion heads very complex. Currently, three-layer coextrusion head design are most waste. four- and five-layer heads let you receive the product ABCD structure; ABCD, ABCBD. The ability to use one product of several varieties of polymers dramatically expands the potential of the materials produced. For example, the structure ABCD film thickness of 40-140 microns and a width of 1000 mm, comprising two barrier layers of EVA and PVDC and can be used for food and medical packaging.

Безымянный

Fig. 7.2. Driving head for coextrusion design proizvodstvadvuh- and five layer products: and – a head for a two-layer film structure AB, where A and B – polyolefins or polyamides; b – a head for a two-layer film structure ABC, wherein A and B – LDPE and PA C – adhesive c – a head for three-layer ABA or ABC structure film; g – a head for a four ABCD structure of the film; d ‘head to produce a five-layer film structure ABCD (three components) or ABCBD (four components)

 

orientation films

The orientation films (stretching them in a heated state) is widely used for films such as PP, PS, PA, PET, to increase the transparency, impact resistance and particularly (in the case of PP) barrier properties.

The base film PS in unoriented state is very fragile and has a very limited use as a dielectric in capacitors. Biaxially oriented, ie. E. Oriented in two mutually perpendicular directions, the film no longer be brittle and can be molded into transparent boxes, trays and in larger products, such as lids for cakes. The most widely used process orientation obtained in the production of PP films, and the various processes will be illustrated by the example of these films. There are two main ways: a flat and bag. The circuit may be flat orientation illustrated in Fig. 8.9. Watering on a cold drum, as described above, producing a thick cast film thickness of 500-600 microns. Its system is fed to the rolls, rotating at different speeds, for example rolls rotating with increasing from roller to roller speed. The rolls are heated to heat the film up to the desired temperature (below the polymer melt temperature). Under these conditions the film is stretched in the longitudinal direction with stretching ratio of from 4: 1 to 10: 1. After leaving the roller machine the film is fed into tenternuyu frame which consists of two independent endless belts or chains fitted with the die stock. Film die stock held, so that when moving forward die stock film is stretched in the perpendicular direction at approximately the same draw ratio as in the longitudinal direction. The space inside the frame tenternoy heated with precise temperature maintenance. After the film frame tenternoy cooled on the cooling roll and wound. There is a scheme to reverse the process sequence, ie. E., First vdet tenternaya frame, then roll the car.

Both stages can be carried out simultaneously. The film captures the edges at the exit from the drum irrigated, and it moves forward with increasing speed, simultaneously stretched in the perpendicular direction by the diverging chains. Mechanically, this type of orientation can be carried out using tenternoy frame in which the worm move die stock with an increasing pace. The degree of transverse stretching is defined as usual divergence angle chains and the degree of longitudinal stretching will depend on the pitch of the worm.
As shown, the orientation can be achieved in the course of a bag. The molten polymer is extruded from the ring die and cooled to form a sleeve. The thickness of the sleeve wall defined quantity • partially ring die gap and partly acceptance ratio and extrusion speeds. Hose passes through the slowly rotating the pinch rolls and then heated to a uniform temperature. Transverse drawing dostigachhg by increasing the air pressure in the sleeve, hood adjustable rate adjusting volume fed into the air. Air is held by nip rolls at the end of the blowing hose; These rolls are rotated faster than the first pair, stretching the film in the longitudinal direction. Bag a process – it is also a way to simultaneously longitudinal and transverse orientation.

Both methods allow to obtain various films – biaxially oriented by isotropic properties, that is, with equal extents in both directions through the two-osnoorientirovannye film with anisotropic properties to completely pultruded films (or in the longitudinal or in the transverse direction)… Pultruded split film under tension in a direction perpendicular to the direction of orientation, which reduces their utility as packaging films. This phenomenon is used, however, in the production of film fiber.

If oriented PP film heated to about 100 ° C immediately after stretching, it will shrink. The shrinkage can be prevented by heat-setting. The film was heated, under certain conditions, keeping taut. After cooling, this film will not shrink at temperatures below the curing temperature. Physical and optical properties of the film remain unchanged.

The foamed film

Reducing the density of the polymer due to formation of the cellular structure allows to obtain products with greater bending stiffness for a given weight of polymer. The method can be used for extruded films and molded products, and it is widely used for the SS, and more recently for polyolefins. Greater specific rigidity of the material is achieved due to the fact that the stiffness of the beam is proportional to the cube of the thickness. Therefore, for a unit surface area of ​​double thickness film may be made of material x grammes, if the density is reduced by half. Doubling the amount of stiffening means 23, ie. E. 8-fold for a given module but the module is changed linearly with a change in density so that the density reduction module halves doubled overall stiffness increase is obtained by 4 times. This can be very important economic advantage.

The films of foamed polystyrene

There are two main methods of producing foamed films. The first starts from polystyrene granules which are impregnated under pressure liquefiable gas, typically pentane. Blown film is extruded using a twin-screw extruder, adding a nucleating agent such as a mixture of citric acid and sodium bicarbonate. The nucleating agent is useful for tonkoyacheistoy structure. When the molten mass exits the die, the pressure falls and the material starts to foam. Sleeves blowing method is used to prevent morscheniya observed during the foaming process. The sleeve typically swell in the horizontal direction to facilitate the start-up period, as the material is hard and rigid as opposed to PE film. When the sleeve is folded, it is cut off on both sides to produce two flat sheets, wound separately. This is useful because the sleeve edges folded – a weak spot because of the rigidity of the SS.

Another method uses conventional PS pellets, and the gas is introduced into the extruder. The film is inflated in the same manner. For this method requires a specially designed extruder, economical only for large capacity – about 400 tons / year. Extrusion foaming granules cheaper with less productivity and more flexible, so as extruders can also be used for other purposes. The degree of orientation of the sheet is controlled in both directions, as usual, by adjusting the degree of acceptance rate and blown. The degree of orientation must be the same in both directions, since any difference in strength in the two directions may result in splitting of the sheet during thermoforming.

The foamed film
Reducing the density of the polymer due to formation of the cellular structure allows to obtain products with greater bending stiffness for a given weight of polymer. The method can be used for extruded films and molded products, and it is widely used for the SS, and more recently for polyolefins. Greater specific rigidity of the material is achieved due to the fact that the stiffness of the beam is proportional to the cube of the thickness. Therefore, for a unit surface area of ​​double thickness film may be made of material x grammes, if the density is reduced by half. Doubling the amount of stiffening means 23, ie. E. 8-fold for a given module but the module is changed linearly with a change in density so that the density reduction module halves doubled overall stiffness increase is obtained by 4 times. This can be very important economic advantage.
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