Development of Plastic Extrusion Technology

Mixer

In a wide range of production practices, the theory and technology of extrusion molding have been continuously deepened and expanded; the types of polymers, product structures and product forms that can be processed are increasing; the extrusion process has been continuously developed; the extrusion molding The equipment has been continuously improved and innovated, and the equipment has become more and more large, efficient, precise, intelligent and specialized; the application of computer technology in extrusion molding processing has become more and more extensive and in-depth.

New extrusion molding process:

Due to the continuous development of the petrochemical industry and the polymer processing industry, in order to expand the range of moldable materials and increase the types of extruded products, the traditional extrusion molding technology has been continuously developed to form some new technologies, which mainly include reactions Extrusion process, solid state extrusion process and co-extrusion process, etc.

1. Reactive extrusion process

Reactive extrusion process is a new technology that only emerged after the 1960s. It is generally valued for its ability to diversify polymer properties, functionalize, continuous production, simple process operation, and economical and practical. It is a method of continuously polymerizing monomers and modifying existing polymers. The biggest feature of this process is to combine the separate operations of polymer modification, synthesis and polymer processing.

The reaction extruder generally has a larger length-to-diameter ratio, multiple feeding ports and a special screw structure. The reactants are fed into the extruder from different feeding positions, the solid materials are fed from the hopper, and the viscous fluid or gaseous reactants are fed through the injection port at various points along the barrel according to the reaction sequence. The material is transported forward by the rotation of the screw. At a certain reaction temperature, the material fully reacts during the mixing process, and the volatiles generated during the reaction process are removed at an appropriate position. The fully reacted polymer is extruded through the die and cooled. , Curing, pelletizing or direct extrusion molding into products.

Reactive extrusion has long attracted widespread attention from people in the world of chemistry and polymer science and engineering, and it has developed rapidly in industry. The attention and rapid development is due to the unique function of the extruder to handle pure high-viscosity polymers. The extruder can melt extrude, compound polymer and vent and devolatilize polymer. This function is exactly what the chemical reactor needs.

The characteristics of the reactive extruder are: ①Easy pretreatment of molten feed; ②Excellent mixing dispersion and distribution; ③Stability of temperature control; ④Control of the distribution of the entire residence time; ⑤Continuous processing; ⑥Segmentation; ⑦ Unreacted monomers and by-products are easy to remove; ⑧ has the ability to limit the post-reaction; ⑨ can be used for viscous melt transportation; ⑩ can continuously manufacture special-shaped products.

Reactive extrusion processing is mainly used in polymer degradation, synthesis, grafting, compatibilization, etc. For example, in the process of reactive extrusion, by adding an appropriate amount of peroxide to PP, the P main chain is broken, branching is terminated, and the macromolecular free radicals generated by the break are produced with low melt viscosity that is difficult to obtain by general methods. , Narrow molecular weight distribution, small molecular weight can be used to meet the requirements of film extrusion, thin-walled injection products of PP. The special twin-screw extruder is used as the reactor, and the principle of rapid caprolactam anion polymerization is used to directly react and form, which can realize the integration of monomer-polymer products. The strength of PA6 prepared by this process is 120% of that of ordinary PA6, and the It is more than 3 times that of ordinary PA6. Connecting the twin-screw extruder to the molding device can directly produce PA6 products, which greatly shortens the molding cycle and reduces the cost. Grafting polar functional groups on the molecular chains of polar polymers can give products some special properties. This kind of grafts has a wide range of applications in plastic modification and composite material preparation, such as silane cross-linking or radiation cross-linking. The XPE pipe produced by joint reactive extrusion has the advantages of high temperature resistance, high pressure resistance, good flexibility, excellent chemical resistance, good creep performance, and good stress crack resistance. In terms of polymer compatibilization, in the PE/CaCO3 blend with the compatibilizer PE-g-MAH, the CaCO3 particle size is consistent and evenly distributed, so that the tensile strength of the blend is significantly improved, and the elongation at break changes. Smaller, MFR drops slightly.

Reactive extrusion molding technology is a new technology developed in recent years aimed at achieving high added value and low cost. It has attracted widespread attention from the world’s chemical and polymer materials science and engineering circles, and has developed rapidly in industry. Compared with the original extrusion molding technology, its obvious advantages are: ① save energy consumption in processing; ② avoid repeated heating; ③ reduce the cost of raw materials; ④ in the reaction extrusion stage, it can be adjusted in time on the production line The physical properties of monomers and raw materials ensure the quality of the final product.

2. Solid state extrusion process

Solid state extrusion is the process of extruding the polymer from a die below its melting point. Solid state extrusion generally uses a single plunger extruder, and the plunger extruder is a batch type operation. The movement of the plunger produces positive displacement and very high pressure. The polymer in the die undergoes great deformation during extrusion, which makes the molecules severely oriented, and its effect is far greater than that of melt processing, which greatly improves the mechanical properties of the product.

There are two methods of solid extrusion: one is direct solid extrusion, and the other is hydrostatic extrusion. In direct solid extrusion, the pre-formed solid round rod-shaped material is put into the barrel, and the plunger directly contacts the solid material, pushing the material to be extruded from the die. In hydrostatic extrusion, the pressure required for extrusion is transferred from the plunger to the ingot through the lubricating fluid, and the shape of the ingot is matched with the die to prevent the loss of lubricating fluid. Hydrostatic oil reduces friction, thereby reducing extrusion pressure.

The mechanical properties of solid-state extrusion HDPE are compared with some metal materials and ordinary melt-extrusion HDPE, see Table 1-1.

It can be seen from Table 1-1 that because the cross-sectional area of the barrel of the single plunger extruder is much larger than the cross-sectional area of the die, the extruded ingot needs sufficient pressure when passing through the die, and it is solid under the action of force. The material produces deformation and molecular orientation. In solid-state extrusion, the ratio of the area of the barrel to the cross-sectional area of the die is defined as the extrusion ratio. In the case of high extrusion ratio, the mold release expansion in melt extrusion is basically not generated, so the size of the extrudate is equal to the size of the die. The mechanical properties of solid-state extrusion HDPE are better than those of melt-extruded HDPE, and its tensile strength is about the same as that of carbon steel. The basic process of solid-state extrusion is discontinuous and cannot be molded with ordinary polymer processing equipment. It requires very high pressure to achieve extrusion.

3. Co-extrusion process

The application of coextrusion technology in plastic products can make products diversified or multi-functional, thereby improving the grade of products. The co-extrusion process is completed by two or more extruders, which can increase the cross-sectional area of the extruded product, form a composite product with a special structure, different colors, and different materials, so that the product can obtain the best performance.

Co-extrusion technology can be divided into soft and hard co-extrusion, core foam co-extrusion, waste co-extrusion, two-color co-extrusion, etc. according to the characteristics of co-extrusion materials. The production line of PVC foamed pipe made by co-extrusion of three extruders is more accurate than the way of co-extrusion of two extruders to control the extrusion process conditions, and the thickness of the inner and outer layers and the core foam layer is more accurate. More excellent pipe performance can be obtained.

In order to meet the needs of functional development of agricultural film and packaging film, co-extrusion blown film tends to develop with multiple extruders and multilayers. At present, the multi-layer co-extrusion blown film can reach 9 layers. Multi-layer co-extrusion has very high requirements on the rheological properties of various polymers, phase adhesion properties, and mutual matching between the extruders, and the design and manufacture of the die runner is more critical.

With the high efficiency of polymer processing and the continuous expansion and extension of application fields, types of extrusion molding products continue to appear, and new extrusion molding processes emerge in endlessly.

In order to meet the higher requirements of industrial and construction pipes in terms of pressure resistance, temperature resistance, and crack resistance, people have developed composite pipes with special fibers as the framework, composite pipes with porous metal pipes as the framework, aluminum-plastic composite pipes, and XPE. Pipes, RPP pipes and UHMWPE pipes, etc. The molding process of these pipes is different from the extrusion molding of ordinary pipes.

The production process of low-haze and high-gloss PP sheet is different from ordinary PP sheet. After the sheet is extruded by the die, it enters between the two steel belts that move synchronously with it. The steel belt is cooled by water and extruded with PP. After the objects are in contact, they are quickly cooled to form a transparent sheet, and then tempered to obtain a PP sheet with a haze of 2% to 5% and a gloss of 120% to 145% (thickness of 0.2 to 0.3 mm).

Double film bubble extrusion blow molding single-layer and multi-layer biaxially stretched heat shrinkable film is a newly developed film blowing process in recent years. The extruded tube film is first cooled by water, and then heated to a temperature suitable for inflation. Compress air and adjust the speed ratio between the two pairs of traction rollers to form a single-layer or multi-layer biaxially stretched composite shrink film.

In order to improve production efficiency, a new cooling process is used in the extrusion process. Germany’s Battenfeld company uses a liquid nitrogen cooling system to increase the production of blown film by 41%. In the blown biaxially stretched heat shrinkable film, there is also a process of changing the water bath cooling of the tube film to the water mist cooling in the sizing sleeve, and a larger stretch ratio can be obtained during biaxial stretching.

The development trend of extrusion molding process:

Large-scale products, high-efficiency production, new structures and new processes of products are the development trend of extrusion molding on the basis of traditional extrusion production.

1. Large-scale extrusion products

(1) Pipes With the continuous expansion of the application fields of plastic pipes, the production of pipes is moving in the direction of high performance and large diameter. The high performance of pipes depends on new polymer varieties, polymer modification and improvement of pipe forming equipment. The UPVC pipe diameter that can be produced for water supply and drainage pipes and municipal engineering sewage pipes has reached 630mm, while it can reach 1000mm in foreign countries. The maximum diameter of HDPE gas pipe can reach 100mm, and the diameter of water delivery pipe can reach 2500mm

(2) Film In order to reduce the workload in laying, people hope that agricultural shed film and mulching film can be widened significantly. The maximum width of the front extrusion blow molding shed film is 14m, and some can reach 25m. BOPP film can be processed into a thickness of 10~18pm, a film width of 9m, and part of the three-layer co-extruded film has a width of 6.1m. The wide film is more widely adapted to the needs of various packaging. Due to the increase in the width, the ratio between the cornea formed by the trimming of the flat film and the finished film can be reduced, the yield rate is increased, and the cost is reduced. Therefore, both cast film and biaxially stretched film develop in the direction of wide width.

(3) Plates and sheets are widely used as industrial liners, insulating building materials and advertising production plates, as well as food packaging, pharmaceuticals, and industrial packaging sheets. They are also developing to special performance and wide format according to market needs. At present, the width of the panels produced in China can reach 2500mm and the thickness is 55mm; the width of the co-extruded PVC foam board is 2000m, and the thickness of the foam layer is 20mm.

2. High efficiency of extrusion molding

Since the extrusion molding method is used in production, the pursuit of high quality and high efficiency has never stopped. People continue to improve the technology and equipment, taking high quality as the prerequisite to increase productivity.

The most advanced extrusion granulation equipment in the world includes: a single-screw extruder with a diameter of 700mm, with an output of 36t/h; a twin-screw extruder with a diameter of 600mm, with an output of 45t/h. China’s current pelletizing production line is moving towards large-scale and high-speed production.

The extrusion rate of several products whose output accounts for a large share of extruded products continues to increase. The drawing speed of blown film is increased from the current 100m/min to the high-speed drawing of 300~350m/min. Extrusion composite multilayer heat shrinkable film, width 2300mm, extrusion line speed up to 90m/min, extrusion output 200kg/h cast film In North America and Europe, the typical production line speed was 210~270m/ min, now it has reached 450~610m/min; three-layer co-extruded film with a width of 6.1m, with an output of 450kg/h; a composite cast stretch film with a width of 3.2m, the production line speed is up to 550m/min, and the production capacity is 9100t/ a. BOPP film, 10 years ago, the extrusion line speed was 200m/min, 5 years ago, the extrusion line speed was 250m/min, and currently it can reach 350~400m/min

Extrusion coating of woven cloth with a width of 4.2m, the extrusion line speed can reach 60m/min, the output is 550kg/h when coated with PP resin, and the output is 600kg/h when coated with PE resin

The output of the extruded structure foamed PVC sheet can reach up to 600kgh. The screw diameters of the two-stage single-screw extruder for extruding the foamed sheet are 114mm and 150mm respectively, and the length-to-diameter ratio is 30 to produce expanded polystyrene sheets. The output can reach 545kg/h per hour; the output can reach 455kg/h when producing foamed polyethylene sheets. The screw diameters of the large two-stage foamed sheet extrusion line are 120mm and 160mm, and the output can reach 815kg/h.

The extrusion molding of PVC profiles for building doors and window frames is generally at an extrusion line speed of 0.8~1.2m/min and an output of 80~120kg/h; through the improvement of the extruder and the shaping cooling device, the extrusion line speed can reach 6m/min, output up to 500kg/h

The development of extrusion molding equipment:

In recent years, high-speed extrusion has brought about a substantial increase in production efficiency. High speed, high efficiency and intelligence have always been the development direction of extrusion molding equipment. The total torque of the parallel twin-screw extruder is greatly improved, and it is widely used in high filling mixing, polymer blending, reactive extrusion and other processes. The screw speed tends to be high. The foreign screw speed can reach 600~1200r/min to increase the length-to-diameter ratio is another characteristic of the development of twin-screw extruders. The previous twin-screw length-to-diameter ratio is about 30. In the new and efficient mixing and reactive extrusion, there will be a process of addition polymerization, graft copolymerization, and control of the relative molecular mass and relative molecular mass distribution of the polymer. When multiple processes including polymer modification, polymer alloy preparation and molding are concentrated in the extruder, the length-to-diameter ratio of the screw has been developed to be more than 40. The twin screw extruder of Welding Engineers in the United States The ratio of length to diameter is 48~72. An important factor affecting the economic performance of an extrusion line is output. Taking the production of PVC profiles as an example, in the 1970s and 1980s, the linear speed of the production of PVC profiles with parallel twin-screw extruders was 12m/min; in the mid-1990s, the output increased rapidly due to the development of die heads and shaping technology. Development, the linear speed increased from 2m/min to 4m/min, the screw diameter of the extruder was about 11mm, the length-to-diameter ratio was 20-23, and the output was 150-350kg/h. In 1995, the screw diameter of the extruder was developed to 130mm. The output is 450~500kg/h. In order to continue to meet the needs of the market, Krauss Maffei has developed a new generation of parallel twin-screw extruders with a screw diameter of 160mm and a length-to-diameter ratio of 26. The output has been increased to 1000kg/h.

The development of extrusion molding technology is concentrated on the increase in extrusion rate. At present, the extrusion rate of UPVC profiles and pipes has reached more than 6m/min in single cavity and 4m/min in dual cavity; the extrusion rate of polyolefin pipes has reached 20m/min. In terms of biaxial stretching technology, biaxial stretching film machinery The production speed has reached 450m/min or more, the film width has reached 10m, and multi-layer co-extrusion is possible.

According to the requirements of the feeding order of the various components of the co-mixing reaction extruded material, the extruder is equipped with multiple feeding ports according to the shape of the added material except the main hopper, and the chemical reaction is sequentially added along the screw axis. Required auxiliary materials and various components that are conducive to melt mixing and uniform dispersion. Corresponding to multi-point feeding, multiple exhaust ports are set on the barrel to meet the needs of exhaust, dehydration, and devolatilization in different positions and different states.

With the improvement of the dimensional accuracy of extruded products, more and more extrusion production equipment uses melt pumps. Melt pumps have been used in the processing of nylon, polyolefin, fluororesin, polyurethane elastomer and other polymers, films, sheets, foam products, monofilaments, cables, medical hoses, composite extrusion molding, etc., greatly expanding In the past, it was only used in a few extrusion production occasions such as baking melt spinning and biaxially oriented film, making it possible for the extruded product to achieve high viscosity.

In terms of equipment innovation, in addition to increasing the screw diameter and length-to-diameter ratio, in order to increase the output, measures have been taken to increase the installed torque, increase the screw speed, and increase the specific drive energy. At the same time, in order to improve the quality of products and the stability of production, each extruder manufacturer is also making technological innovations in the automatic control of the extrusion line, on-line inspection and precise synchronization of the drive system. The development of extrusion equipment is also reflected in the improvement of energy saving and automatic control level.

The future development direction of extrusion molding technology is: ①Reduce labor and material consumption, which is mainly reflected in shortening the time of replacing products as much as possible, and replacing and automatically replacing during the production process as much as possible; ②By increasing the plasticizing capacity of the equipment, increasing the gear Torque, to further increase the length-to-diameter ratio of the screw to improve production efficiency; ③Break through the cooling limit. In extrusion molding, cooling has been limiting the increase in productivity. Multi-cavity and multi-line extrusion have become an important method to increase production; ④In The continuous application of sensing technology, control technology and artificial intelligence technology in the extrusion line control system has further improved the quality of products and the stability of production.

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