Plastic extrusion moulding, such as UPVC (Rigid Polyvinyl Chloride) profile or pipe product moulding, is mainly formed through the compounding of PVC resin and related additives, extrusion moulding, shaping, traction, and cutting. Among them, raw materials, formula equipment and operating processes are the main factors of plastic extrusion moulding, directly affecting the quality and output of extrusion moulding. Therefore, this article focuses on the impact of extrusion equipment and raw materials on extrusion.
UPVC raw material composition
1. PVC resin
Extruded PVC rigid products generally use suspension method loose resin, S-PVC polymerization degree, particle size, and degree of porosity should be suitable. It is not possible to use polishes with significant particle size differences or loose resins with dense particles.
Because PVC resin is a heat-sensitive resin, when the temperature reaches about 90 to 130 ℃, it will begin to degrade thermally, and the unstable HCL will be released, and the resin will turn yellow.
As the temperature rises, the colour of the resin becomes darker, and the physical and chemical properties of the product decrease. To solve the degradation problem, in addition to improving the production process of resin raw materials, stabilizers can also be added to PVC resin to absorb and neutralize HCL gas and eliminate its catalytic degradation effect.
Commonly used stabilizers are: lead salts, organic tins, metal soaps and rare earth stabilizers.
Lubricants are additives to improve lubricity and reduce interfacial adhesion. Their functions are divided into external lubricants, internal lubricants, and internal and external lubricants.
The external lubricant can reduce the friction between the material and the metal surface and prevent the UPVC material from adhering to the barrel and screw after plasticization.
The internal lubricant can reduce the friction between the particles inside the material, weaken the cohesion between the molecules and reduce the melt viscosity.
The use of lubricants has apparent effects on reducing screw load, reducing shear heat, and increasing extrusion output.
4. Filling material
To improve the hardness and rigidity of the product, reduce the deformation of the product, and reduce the cost of raw materials, more fillers such as CaCO3 are added in the production of UPVC products.
5. Processing modifier (ACR)
Processing modifiers are mainly used to improve the processing performance of materials, accelerate the plasticization of PVC resin, and improve the fluidity, thermal deformation and surface gloss of the products.
6. Impact modifier
Impact modifiers are mainly used to improve the impact resistance of products, increase the toughness of products, and improve the plasticizing effect. Commonly used modifiers for UPVC are CPE (chlorinated polyethene) and acrylic impact modifiers.
7. Coloring agent: titanium dioxide, carbon black, etc.
Plasticization mechanism of plastic extrusion equipment and the influence of formula on moulding
There is much equipment for plastic extrusion moulding. The main ones used for UPVC complex product extrusion are vented single screw extruder and counter-rotating twin screw extruder.
The following mainly discusses the plasticizing mechanism of commonly used extruders for extruding UPVC products.
1. Exhaust type single screw extruder:
1.1 Plasticizing mechanism:
The vented single-screw extruder can be used for powder-feeding moulding, extrusion and pelletizing of UPVC.
The screw comprises two ordinary single screws in series with a large length-to-diameter ratio (L/D=25～30). The single front screw is mainly used for heat absorption, compression, melting and homogenization of the material to make the material initially melt. The rear-stage single screw is primarily used for venting, melting and homogenization, and building up the extrusion pressure.
From the exhaust port, the material should be in a semi-melted state. The exhaust port is set in the conveying section of the rear screw, where the material can be exhausted after decompression.
In the conveying section, the dry powder material is gradually compacted to form a “solid bed”. Because the material temperature has not yet risen, only the air between and inside the powder particles is discharged.
In the compression section, the temperature of the material is about 160～170℃. As the volume of the screw groove decreases, the pressure builds upon the surface of the material and the barrel to force the material to pass through the gap between the screw and the barrel, and the tension between the material and the barrel surface increases.
The heat absorption effect of the material is enhanced, and the fabric close to the surface of the barrel forms a melt film due to shear, pressure, and heat.
Due to the relative movement of the screw and the barrel in the front of the screw groove, the area gathers and gradually increases, the material particles in this section are sheared and melted. Because the material in the screw groove receives less shearing force, the material in the screw groove is plasticized. The consistency is lacking.
In the homogenization section, the bottom diameter of the screw is reduced so that the material in the middle of the spiral groove is close to the barrel to promote shearing and heating to melt and further complete the melting of the material homogenize it.
The bottom diameter of the conveying section of the last screw (close to the machine head) becomes more prominent, and its displacement is much larger than that of the homogenizing section of the last screw. The volatile components are released, and the exhaust port will discharge them through the vacuum pump.
The material reaches the homogenization section through the second compression section, and the extrusion pressure is built up under the action of the die, screw and barrel to form a dense and uniform flow of extrusion moulding from the die, where the displacement of the homogenization section is larger than the previous one. One-stage removal prevents material offending.
From the above analysis, it can be seen that the single-screw melting is mainly caused by the rotation of the screw and the barrel static, and the relative displacement of the material in the different parts of the groove is sheared. The material is heated and compressed, and the heat conduction between the barrel and the screw forms a molten film pool—migration between liquid phases, etc.
1.2 Problems that should be paid attention to in formula design:
When designing the single-screw material formula, it should consider the long melting time of the material in the single-screw extruder, the apparent influence of the secular state in the solid conveying section on productivity, and the non-mandatory conveying of materials.
Due to the large aspect ratio of the vented single screw extruder (usually L/D=28～32), the material is heated for a long time and is not forced to convey. It is beneficial to increase the amount of stabilizer to prevent overheating decomposition—more prolonged and more giant load. An appropriate increase of lubricant can reduce screw torque.
Of course, too much lubricant harms the conveying of materials and the impact performance of products. The phenomenon of “screw holding” may occur in the extrusion when the fat is too much. Consider adding an impact modifier. The increase in the amount of impact modifier will increase the torque of the screw.
Adding a certain amount of filler CaCO3 can increase the strength of the melt, reduce the fluidity of the material, and affect the plasticization speed of the material. The effects of CaCO3 with different particle sizes are also very other. Therefore, the amount of CaCO3 added to products for various purposes varies greatly.
In addition, the characteristics of the mould structure are related to the size of the extrusion pressure and have a particular impact on the formulation.
2. Counter-rotating twin screw extruder
Although the melting mechanism of the twin-screw extruder is based on a single screw, the conveying principle is very different from that of a single screw due to the existence of the meshing zone.
2.1 Classification of twin-screw extruders
According to the direction of screw operation, it can be divided into:
- Counter-rotating twin-screw extruder: the rotation direction of the two screws is opposite.
- Co-rotating twin-screw extruder: the rotation direction of the two screws is the same.
According to the law of rotation, the counter-rotating twin-screw extruder can be divided into counter-rotating outward twin-screw extruder and counter-rotating inward twin-screw extruder.
The counter-rotating inward twin-screw extruder was eliminated because of its poor feeding ability and the large radial force generated by the material on the screw in the calendering area of the two screws, resulting in severe wear between the barrel and the screw.
Generally speaking, the counter-rotating twin-screw extruder refers to the counter-rotating outward twin-screw extruder (the same below).
Extrusion of UPVC profiles generally uses counter-rotating conical twin-screw extruders and counter-rotating parallel twin-screw extruders.
2.1.1 Counter-rotating conical twin-screw extruder:
The axis of the two screws and the axis of the barrel are symmetrically distributed at the included angle α (the α value is generally between 1° and 2°). However, the screw direction is different, the diameter of the two ends of the working section is different.
The screw with the same depth of the large and small screw grooves in the ordinary conical twin-screw extruder screw and the screw with the considerable screw groove depth are more significant than the minor screw groove’s depth super-cone (double-cone) conical twin-screw extruder screw.
The features of the counter-rotating conical twin-screw extruder: large screw head diameter, the large heat capacity of the screw, deep groove (ultra-cone) material, the large contact area with screw and barrel, the long residence time of material is beneficial to the material The heat transfer between the barrel and screw and the material. Based on this point, the screw length and aspect ratio (usually 13-17) under the same output is much smaller than those of other types of extruders.
The diameter of the small screw head is relatively small, the residence time of the material in the extrusion section is short, the linear speed of the screw operation is low, and the low shear rate is beneficial to reduce the friction heat between the material and between the material and the screw and barrel.
When the extrusion rate of the profile is within 400Kg/h, and the extrusion rate of the pipe sheet is within 800Kg/h, the use of a conical twin-screw extruder should be given priority. The conical twin-screw extruder for extruding UPVC profiles and pipes is the most widely used.
Plasticizing capacity: The plasticizing capacity of extruders results from the comprehensive effect of the extruder’s extrusion system, formula and operating process parameters.
The plasticizing ability of conical twin-screw and parallel twin-screw extruders cannot be said which is better or worse. It can only be determined based on the analysis of the specific structure of the screw, the composition of the formula, the operating process parameters, and the mould.
2.1.2 Counter-rotating parallel twin-screw extruder:
The axes of the two screws are parallel and symmetrically distributed with the axis of the barrel. The inner and outer diameters of the two ends of the screw working section are the same, and there are segmented screws and continuously variable lead screws.
Segmented screw refers to the screw with undercut grooves due to the different number of screw heads and different pitches between different functional sections of the screw.
The continuous variable lead flat twin-screw means that there is no undercut groove between the different functional sections of the screw. Therefore, the number of screw heads in the various application areas of the screw is the same. Because the generatrix of the screw and barrel is straight, the processability is better.
The screw of the counter-rotating flat-twin extruder can be taken out from the discharge end of the extruder, which is convenient for equipment maintenance. The screw can be designed into a full-course variable-lead structure. According to relevant information, the processing stress on the material when extruding the profile is small to obtain a good extrusion quality.
A parallel twin-screw extruder is relatively more used when the extrusion volume is large. It should be noted that the output of the profile production line is greatly affected by the mould. The moulding of the extruder head at high speed and the excellent shape of the product in the setting mould often become the bottleneck of the limited output.
2.2 The plasticizing mechanism of the conical twin-screw extruder:
Generally, the extrusion system of the conical twin-screw extruder includes: screw, barrel, heating and cooling device, and vacuum exhaust device.
2.2.1 Material plasticization mechanism of conical twin-screw extruder:
⑴ Conveying section:
The material enters the conveying section from the discharge port and is conveyed forward under the force of the screw. Every time the screw rotates, the material in the C-shaped chamber moves forward by one lead.
Due to the structure, the volume of the C-shaped chamber is getting smaller and smaller, and the material is gradually compressed. As the contact pressure between the material and the barrel screw increases, the heat absorption increases, and the temperature of the material slowly rises, ready for the next step of melting.
As the surface area of the barrel and screw of the cone twin extruder is enlarged in the conveying section, the heat conduction efficiency between the material and the barrel and screw is improved.
⑵ Pre-plasticizing section:
After the materials are heated and compressed in the conveying section, most of the air between and inside the powder particles is discharged, and the density of the materials increases.
As the material in the C-shaped chamber continues to move forward, the material in contact with the barrel and screw will be kept at the same speed as the barrel or screw due to adhesion. Under the drive of the screw, the shearing effect is stronger than the material in the middle of the screw groove. After being heated for a long time, it begins to melt, and the material in the C-shaped chamber is dissolved in a circulating flow from the outside to the inside.
As the volume of the C-shaped chamber changes, the exchange of internal and external materials is enhanced. Some extruder manufacturers set up a mixing tank in the pre-plasticizing section according to the characteristics of the extruder of their own company. The purpose is to communicate the materials in the front and rear C-shaped chambers, enhance the shearing effect and facilitate the exchange of materials between the C-shaped indoor and outer layers. Improve melting effect.
After the material passes through the pre-plasticizing section, the ample powder and granular materials are broken, and the materials are in a semi-molten state.
⑶ Plasticizing section:
Also known as compression section. The volume of the C-shaped chamber in this section is sharply reduced (the displacement of this section is only between 0.25 and 0.4 of the conveying area), and the materials are subjected to solid squeezing, shearing, and exchange when they pass through. Thus, most of the materials are basically in an initial plasticized state.
⑷ Exhaust section:
The UPVC mixture enters the exhaust section after the conveying, pre-plasticizing and compression section because the volume of the C-shaped chamber in the exhaust section is much larger than that of the compression section (generally, the displacement is more than three times that of the compression section).
When the material reaches this section, the pressure is reduced, and the material becomes segmented or large, and the gas and low-molecular volatile components in the material are released. This section is equipped with an exhaust port, and the gas is discharged through the exhaust port under the action of a vacuum pump.
The exhaust function of UPVC extrusion moulding is essential. Otherwise, the air bubbles in the product will seriously affect the mechanical properties.
⑸ Measurement section:
Due to the continuous change of the volume of the C-shaped chamber through the conveying section, pre-plasticizing section, plasticizing section, and metering section, and the different number of screw heads in each area of the screw, the material in the C-shaped chamber continuously changes its position and then enters the metering section. As a result, further plasticize, homogenize, and build up the extrusion pressure under the action of the die.
Because the volume of the C-shaped chamber in the metering section is reduced, the material is compressed again after homogenization to form a dense and uniform fluid that is extruded through the connecting body (transition body), perforated plate, and die.
2.3 The plasticizing mechanism of the counter-rotating parallel twin-screw extruder:
The plasticizing mechanism of the counter-rotating parallel twin-screw extruder is the same as that of the conical twin-screw extruder. The difference is that the diameter of the screw and barrel is the same everywhere. The material in the feeding section has a small heat absorption area, which is relatively compared to the screw metering section. Therefore, the diameter is larger than the cone double, and the screw speed cannot be too large.
Therefore, to improve the plasticizing effect, the length-to-diameter ratio of the counter-rotating parallel twin-screw extruder screw is more significant than that of the conical twin-screw extruder (generally L/D=25-30).
The volume of the C-shaped chamber in each section of the counter-rotating parallel twin-screw extruder is changed repeatedly as the cone and double, and the change mode is the same.
The transformation process of UPVC materials in the processing process is not only related to the composition of the mixed ingredients but also has a lot to do with the external processing conditions, such as the temperature of the mixed ingredients, the order of feeding during the mixing, the extrusion process temperature, the screw speed, and the feeding The amount of material and the strength of the material subjected to shearing.
This article makes a fundamental analysis of the melting mechanism and formula adjustment of the material in the extruder. Due to the high practicality of polymer processing and moulding technology, the product raw material formula, processing equipment, and process conditions are pretty different. Therefore, the actual production should be combined with the structure of the production equipment, especially the extrusion system, the composition of the ingredients, the product performance requirements, and the production volume. In addition, a comprehensive analysis of the influence of the ingredients in the additives on the melting of the materials should be combined to determine a reasonable production process.