Six basic principles of plastic extrusion processing (feeding, conveying, compressing, melting, mixing, exhausting)
After the plastic is added to the extruder barrel by the hopper, the thread is forced to push the direction of the head with the rotation of the screw. Due to the resistance of the filter, the manifold and the die at the machine, and the gradual reduction of the volume between the threads, the forward material is subjected to a lot of pressure, and at the same time it is heated by the heat source of the barrel; in addition, the plastic is moving. When it is subjected to the action of compression, shearing, stirring, etc., the friction between the barrel and the screw and the friction between the plastic molecules generate a large amount of heat. As a result, the temperature of the plastic in the barrel is continuously increased, and the physical state gradually changes from the glass state to the high elastic state, and finally becomes a viscous flow state, achieving complete plasticization. Since the screw has been rotating at a constant rate, the plasticized material is pressed and equalized from the die mouth to form a plastic product having a certain shape. After cooling and setting, the extrusion molding work is completed. The core component for achieving the above process is a screw, and the extrusion process occurring on the screw includes the following stages.
After the plastic is added to the hopper, it can enter the space of the screw groove by its own weight or under the action of the forced feeder, and is pushed forward under the pushing of the screw edge. However, if the coefficient of friction between the material and the metal hopper is too large, or the internal friction coefficient between the materials is too large, or the cone angle of the hopper is too small, the bridging and hollow tube phenomenon will gradually form in the hopper, and the material will not be smooth. When the ground enters the groove, the extrusion will be forced to stop or extremely unstable. Therefore, if the extrusion productivity is abnormally lowered or not discharged, it is necessary to check the feeding condition and even change the design of the hopper.
In theory, after the plastic enters the screw groove, every time the screw rotates, all the plastic will carry a lead forward. At this time, we call the transfer rate 1. However, for each screw, the amount of forward transport depends in fact on the friction factor fb of the plastic on the barrel and the friction factor fs of the plastic on the screw. The larger the fb or the smaller the fs, the amount of solid plastic that will be transported forward. The more. A large number of experiments have shown that the friction coefficient of resin to metal mainly depends on the temperature of the system and the surface roughness of the metal or the structure and shape of the system, and is also related to system pressure and material movement speed.
It is absolutely necessary that the plastic is compressed during the extrusion process. First of all, plastic is a poor conductor of heat. If there is a gap between the particles, it will directly affect its heat transfer, thus affecting the melting rate. Secondly, it will only be between the granules under the pressure gradually increasing along the length of the screw. The gas is discharged from the hopper. Otherwise, the product will become defective or waste due to the bubble inside. In the end, the higher system pressure also ensures that the product is denser.
There are three reasons for the pressure on the screw:
1. In the structure, the screw groove depth gradually becomes shallower, and the material is gradually compressed;
2. The resistance element such as the splitter plate, the filter net and the head is installed in front of the screw head;
3. It is the pressure established along the entire length of the screw caused by the friction of the material against the metal. The smaller the cross-sectional area of the die, the greater the pressure peak will be, and the highest pressure will move toward the nose. Generally speaking, the pressure peak is at the front of the metering section or the back of the compression section.
At the same time of pressure increase, the moving solid plastic continuously contacts and rubs against the heated barrel wall, and the temperature of the plastic material near the barrel wall continuously increases, reaching a melting point and forming a thin layer on the inner wall of the barrel. Melt film, after that, the heat source of solid plastic melting has two aspects, one is the conduction heat of the external heater of the barrel, and the other is the shear (internal friction) in the melt film due to the different speed of the melt movement of each layer. ) Heat, that is, the viscous heat dissipation in rheology.
As the melting progresses, when the thickness of the melt film is larger than the gap between the screw and the barrel, the moving spiral edge scrapes the melt film to form a molten pool in front of the advancement of the spiral edge. During the melting process, the molten pool became wider and wider, and the remaining solid width became narrower and narrower until it finally disappeared. This is the epoch-making famous melting theory that Tadmor published in 1967.
During the extrusion process, under high pressure, the solid material is generally compacted into a dense solid plug. Because there is no relative movement between the granules in the solid plug, the mixing can only be carried out between the layers of the melt with relative motion. get on.
In general, the following mixing phenomena occur in the melt, especially in the melt transport section: first, the components in the material system are uniformly dispersed and mixed, and these components refer to the resin and various additives. The second is the mixing of heat, because in the extrusion process, the temperature of the first molten material is the highest, the temperature of the material after the melting is the lowest, and the temperature of the interface between the solid and the melt is exactly the melting point of the plastic. If the melt is extruded too early from the machine head, it will inevitably cause unevenness throughout the extrudate, and light chromatic aberration and deformation will occur, and the possibility of cracking of the product may occur. In addition, considering that the plastic itself has a certain relative molecular mass distribution, the mixing allows the relatively high molecular weight portion to be uniformly dispersed in the melt, and at the same time, under the action of shear force, the relative molecular mass Higher parts are likely to be reduced by chain scission, which reduces the likelihood of crystallites and lumps in the article.
Obviously, in order to ensure a uniformly mixed product, it must be ensured that the melt delivery section (i.e., the last section) of the screw has a sufficient length. Therefore, the melt transport section of the screw is also referred to as a homogenization section. At the same time, when calculating the output of the extruder, the screw groove volume of the last stage of the screw is used as the calculation basis, and the melt conveying section of the screw is also referred to as the metering section.
In the extrusion process, there are three kinds of gases to be discharged, one is the air interposed between the particles, as long as the screw speed is not too high, in general, this part of the gas can be gradually increased pressure from the hopper Discharged. However, when the rotation speed is too high, the material moves forward too fast, and the gas may not be able to be completely discharged, thereby forming bubbles in the product. The second gas is the moisture adsorbed by the material from the air, which becomes water vapor when heated. For those plastics with little moisture absorption, such as PVC, PS, PE, PP, etc., generally no problem occurs. These small amounts of water vapor can also be discharged from the hopper at the same time; but for some engineering plastics such as PA, PSF, ABS, PCs, etc., because their moisture absorption is too large, too much water vapor, so it is too late to be discharged from the hopper, which forms bubbles in the product, and the third is some materials inside the plastic particles, such as low molecular volatiles, Low-melting plasticizers, etc., which gradually vaporize under the action of heat generated during the extrusion process. Only when the plastic melts, these gases can escape the surface tension of the melt, but at this time, since they are far away from the hopper, Cannot be discharged through the hopper. In this case, a venting extruder has to be used.
It is thus possible that any of the six screws must perform the six basic functions of feeding, conveying, compressing, melting, mixing and exhausting. Obviously, the addition and delivery affect the output of the extruder, while compression, melting, mixing and venting directly affect the quality of the extruded product. The term "quality" here refers not only to whether the melting is complete, but also whether the product is compacted, whether the mixing is uniform, and that there are no bubbles in the product. This is the quality of plasticization.