Plastic extrusion molding, such as UPVC (rigid PVC) profiles or pipe products, is mainly formed by mixing PVC resin and related additives, extrusion molding, shaping, traction, cutting and other links. The factors that affect the performance of products cover every link of the production process. Each link interacts and influences each other through products as media. Problems in one link can be remedied through other links within a certain range, so each link becomes an organism. Among them, raw materials, formulation equipment and operation technology are the main factors of plastic extrusion, which directly affect the quality and output of extrusion. This paper focuses on the influence of extrusion equipment and raw materials on extrusion. Due to the influence of many factors, limited to my own level and experience, the one-sided views and errors in this paper are inevitable, and we sincerely invite readers to criticize and correct. 2、 Composition of UPVC raw material:
Generally speaking, UPVC raw materials are mainly composed of the following components:
1. PVC resin:
In general, the loose resin of suspension method is used for Extruded PVC hard products, and the polymerization degree, particle size and loose degree of S-PVC should be appropriate. Resin with large particle size difference and loose resin with dense particles can not be selected.
2. Stabilizer:
Because PVC resin is a thermosensitive resin, when the temperature reaches 90-130 ℃, it begins to degrade, and the unstable HCl is removed, and the resin turns yellow. With the increase of temperature, the color of resin becomes darker, and the physical and chemical properties of products decrease. In addition to improving the production process of resin raw materials, stabilizers are mainly added to PVC resin to absorb and neutralize HCl gas and eliminate its catalytic degradation. Common stabilizers are lead salts, organotin, metal soap and rare earth stabilizers.
3. Lubricant:
It is an assistant for improving lubricity and reducing interface adhesion. According to function, there are external lubricant, internal lubricant and internal and external lubricant. The external lubricant can reduce the friction between the material and the metal surface, avoid the adhesion of UPVC material to the barrel and screw after plasticization, and the internal lubricant can reduce the friction between the particles in the material, weaken the cohesion between the molecules and reduce the viscosity of the melt. The use of lubricant has obvious effect on reducing screw load, reducing shear heat and increasing extrusion yield. The design of lubricants in the formula is very important.
4. Filler:
In order to improve the hardness and rigidity of the products, reduce the deformation of the products, and reduce the cost of raw materials, more filling materials such as CaCO3 are added in the production of UPVC products.
5. Processing modifier (ACR):
The main purpose is to improve the processing performance of materials, accelerate the plasticization of PVC resin, improve the fluidity, thermal deformation of materials and the surface gloss of products.
6. Impact modifier:
The main purpose is to improve the impact resistance, toughness and plasticizing effect of products. The commonly used modifiers for UPVC are CPE (chlorinated polyethylene) and propionic acid lipids.
7. Colorant:
Titanium dioxide, carbon black, etc.
3、 The plasticizing mechanism of plastic extrusion equipment and the influence of formula composition on it:
There are a lot of equipment for plastic extrusion, the main ones for UPVC hard products are: exhaust single screw extruder, opposite twin screw extruder. The following mainly discusses the plasticizing mechanism of extruders commonly used for extruding UPVC products.
1. Exhaust single screw extruder:
1.1 plasticizing mechanism:
Figure 1 shows the exhaust type single screw based on the common single screw. It can be used for UPVC powder feeding, extrusion and granulation. The screw is composed of two ordinary single screws in series, with a large length diameter ratio (L / D = 25-30). The former single screw mainly makes the material melt through heat absorption, compression and melting homogenization, while the latter single screw mainly makes exhaust, further melting homogenization and establishes extrusion pressure. From the exhaust port, it can be seen that the material should be in the semi molten state, and the exhaust port is set at the conveying section of the screw in the rear section where the material can be discharged only after the pressure is reduced.
The former screw is mainly a process of material transportation, compression and melting.
In the conveying section, the dry powder 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 material temperature is about 160-170 ℃. With the reduction of the screw groove volume, the pressure between the material and the barrel surface is established to force the material to pass through the gap between the screw and the barrel. The pressure between the material and the barrel surface increases to enhance the heat absorption effect of the material. The material near the barrel surface forms a melting film due to shearing, pressure and heat, because the relative movement of the screw and the barrel is in front of the screw groove Because of the small shear force on the materials in the spiral groove, the consistency of the materials in the spiral groove is poor.
In the homogenization section, the screw bottom diameter is reduced, so that the material in the middle of the screw groove is close to the barrel to promote its shear heating melting, further completing the melting of the material and making it homogenized.
The bottom diameter of the conveying section of the latter screw (close to the head) becomes larger, and its displacement is much larger than that of the homogenization section of the former screw. The materials here are not compressed into sections or blocks, and the gas and volatile components in the materials are released. The exhaust port is set here to discharge them through the vacuum pump. The material passes through the second compression section to the homogenization section. Under the action of the head, screw and barrel, the extrusion pressure is established to form a dense and uniform material flow for extrusion from the head. The discharge capacity of the homogenization section here is larger than that of the previous section to prevent the material from escaping.
From the above analysis, it can be seen that the melting of single screw is mainly through the static rotating barrel of the screw, the shearing of the relative displacement of the materials in different parts of the spiral groove, the formation of melting film molten pool between the materials by thermal compression and the heat conduction between the barrel and the screw, and the transfer between solid and liquid, etc.
1.2 problems in formula design:
When designing the material formula of single screw, we should take into account the characteristics of long melting time of material in single screw extruder, obvious influence of material state in solid conveying section on productivity and non forced conveying of material. Due to the large aspect ratio of exhaust single screw extruder (generally L / D = 28-32), the material is heated for a long time and is not forced to be transported. It is beneficial to increase the amount of stabilizer properly to prevent overheating decomposition. Due to the long screw and large load, the torque of screw can be reduced by properly increasing the lubricant. Of course, too much lubricant has a negative impact on the transportation of materials and the impact performance of products. When it is too much, the phenomenon of "holding screw" may appear in the extrusion. It can be considered to add impact modifier, the increase of impact modifier will lead to the increase of screw torque. Adding a certain amount of filling material CaCO3 can improve the melt strength, reduce the fluidity of the material, affect the plasticization speed of the material, and the effect of different particle size CaCO3 is also very different. The amount of CaCO3 added to the products of different uses varies greatly. In addition, the die structure is related to the size of extrusion pressure, and also has a certain impact on the formula. Therefore, the design of the formula should be considered.
2. Opposite twin screw extruder:
Although the melting mechanism of twin-screw extruder is based on single screw, due to the existence of meshing area, the conveying principle is quite different from that of single screw. In order to analyze the melting mechanism, it is necessary to give a brief introduction to twin-screw extruder.
2.1 classification of twin screw extruder:
According to the screw running direction, it can be divided into:
(1) opposite direction twin-screw extruder: the rotation direction of the two screws is opposite.
(2) co rotating twin screw extruder: the two screws have the same rotation direction
According to the direction of rotation, the different direction twin-screw extruder can be divided into the different direction outward twin-screw extruder and the different direction inward twin-screw extruder. Due to the poor feeding performance and the large radial force exerted by the material in the calendering area of the two screws, the counter rotating internal twin-screw extruder was eliminated because of the serious wear between the barrel and the screw. Generally speaking, the counter rotating twin-screw extruder refers to the counter rotating twin-screw extruder (the same below).
Generally, the different direction conical twin-screw extruder and the different direction parallel twin-screw extruder are used to extrude UPVC profiles.
2.1.1 counter conical twin screw extruder:
The axis of the two screws and the axis of the barrel are symmetrically distributed at an included angle α (α value is generally 1 ° - 2 °). The common conical twin-screw extruder has different screw rotation direction, different diameter at both ends of the working section, and the same depth of the large and small head grooves. The super cone (double cone) conical twin-screw extruder has more depth of the large head grooves than the small head grooves.
The characteristics of conical twin-screw extruder: large diameter of screw head, large heat capacity of screw, large contact area between material and screw and material barrel with deep screw groove (super cone type), long residence time of material are all conducive to heat transfer between material barrel and screw. Based on this point, the length and length diameter ratio (generally 13-17) of conical twin-screw extruder under the same output is higher than other types The length to diameter ratio of the extruder is much smaller. The diameter of the small screw head is relatively small, the residence time of the material in the extrusion section is relatively short, the linear speed of the screw running is low, and the shear rate is low, which is beneficial to reduce the friction heat between the material and between the material and the screw and the barrel.
The conical twin-screw extruder should be preferred when the extrusion amount of profile is within 400kg / h and that of tube and plate is within 800kg / h. The conical twin-screw extruder for extruding UPVC profiles and pipes is widely used.
Plasticizing capacity: the plasticizing capacity of an extruder is the result of the comprehensive effect of extrusion system, formula and operation process parameters of the extruder. The plasticizing capacity of conical twin-screw extruder and parallel twin-screw extruder can not be simply said which is better or which is worse. It can only be determined by the analysis of screw specific structure, formula composition, operation process parameters, mold and other conditions.
2.1.2 opposite parallel twin screw extruder:
The axis of the two screws is parallel and symmetrical to the axis of the barrel. The internal and external diameters of the two ends of the working section of the screw are the same, and there are two kinds of screws: segmented screw and continuous variable lead screw. Segmented screw refers to the screw with machining undercut between different functional sections due to different screw heads and screw pitches. Continuous variable lead flat twin screw means that there is no undercut between different functional sections of the screw, and the number of screw heads in different functional sections of the screw is the same. Because the generatrix of screw and barrel is straight, the machinability is better. Due to the structural characteristics, the screw of the opposite flat twin extruder can be taken out from the discharge end of the extruder, which is convenient for maintenance of the equipment. The screw can be designed as a whole process variable lead structure. According to the relevant data, the processing stress of the material is small when extruding the profile, so as to obtain good extrusion quality.
Parallel twin-screw extruders are used more when the extrusion volume is large. It should be noted that the output of the profile production line is greatly affected by the mold. At high speed, the extruder head molding and whether the products are well shaped in the setting mold often become the bottleneck of the limited output.
2.2 plasticizing mechanism of conical twin screw extruder:
2.2.1 extrusion system structure and related concepts of conical twin-screw extruder:
Fig. 2 is a schematic diagram of the extrusion system of a foreign company's conical twin-screw extruder:
The general extrusion system of conical twin-screw extruder includes: screw, barrel, heating and cooling device, vacuum exhaust device.
It can be seen from Figure 2 that the outer diameter and bottom diameter of the screw from the screw conveying section to the metering section are decreasing, and the number of screw heads is also changing. There is a transition section with or without thread between each section, and the pre molding section is provided with groove (or without groove). It should be noted that the screw structure of extruders from different manufacturers is not identical