Six basic principles of plastic extrusion

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with nearly 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

When the melt enters the transition section and the die, shear heating diminishes significantly, because the melt has started to transition from a spiraling, variable-speed flow to a linear, uniform-speed flow when it reaches the transition section. When the melt reaches the mold along the flow path defined by the transition section, it also consumes some heat. In order to ensure that the melt moves evenly along the dovetail groove of the mold, it is necessary to add appropriate heat. Therefore, the temperature of the mold is set slightly higher, so it is called the "Temperature Maintenance Zone".

After the plastic is fed into the extruder barrel from the hopper, it is forced to the die head by the screw flights with the rotation of the screw. Due to the resistance of the filter screen, splitter plate and die at the die head, and the gradual reduction of the volume (channel depth) between the screw flights, the advancing material is under great pressure, and at the same time, it is heated by the heat source of the barrel; In addition, when the plastic is subjected to compression, shearing, stirring and other forces in motion, the friction between the plastic and the barrel, screw and the friction between plastic molecules will generate a lot of heat. As a result, the temperature of the plastic in the barrel keeps rising, and its physical state gradually changes from the glassy state to the high-elasticity state, and finally becomes the viscous-flow state, reaching full plasticization. As the screw has been rotating steadily, the plasticized material is extruded from the die mouth of the die head at a constant pressure and rate, and becomes a plastic product with a certain shape. After cooling and shaping, the extrusion molding is completed. The core component to realize the above process is the screw, and the extrusion process along the screw can be divided into the following functional zones:

First: Feeding

After the feeding plastic is added into the hopper, it enters the screw channel (the space between flights) by relying on its own weight or under the action of the forced feeder, and extrudes forward conveyed forward by the rotating screw flights. However, if the friction coefficient 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 phenomenon of bridging and hollow pipe will gradually form in the hopper, the material will not enter the screw groove smoothly, and the extrusion will be forced to stop or extremely unstable. Therefore, if the extrusion productivity is abnormally reduced or does not discharge, it is necessary to check the feeding situation, or even change the design of the hopper.

Second: Conveying

In theory, after the plastic enters the screw groove, every time the screw rotates, all the plastic will be transported forward for one lead. At this time, we call the conveying efficiency 1. However, for each screw, the forward conveying volume actually depends on the friction factor fb of plastic to barrel and the friction factor fs of plastic to screw. The larger fb or the smaller fs, the more solid plastic will be conveyed forward. A large number of experiments show that the friction coefficient between resin and metal mainly depends on the temperature of the system, the surface roughness of metal or the structure and shape of the system, as well as the pressure of the system and the speed of material movement.

Third: compression

In the extrusion process, it is absolutely necessary for plastics to be compressed. First of all, plastic is a poor conductor of heat. If there are gaps between particles, their heat transfer will be directly affected, thus affecting the melting rate; Secondly, the gas between particles will be discharged from the hopper only when the pressure gradually increases along the length of the screw, otherwise, the products will become defective or waste products due to the bubbles generated inside; Finally, the high system pressure also ensures that the products are relatively dense.

There are three the pressure build-up along the screw is caused by:

1.The the decreasing channel depth (from hopper to tip) in structure, and the material is gradually compressed;

2.Resistance elements such as splitter plate, filter screen and head are installed in front of the screw head;

3.It is the pressure built up along the full length of the screw caused by the friction between materials and metal. The smaller the die section area of the head, the greater the pressure peak value will be, and the highest pressure point will move toward the head. Generally speaking, the pressure peak value is in the front of the metering section or the rear of the compression section.

Fourth: Melting

When the pressure rises, the moving solid plastic constantly contacts and rubs with the heated barrel wall. The plastic material temperature near the barrel wall continuously increases. After reaching the melting point, a thin melt film is formed on the inner wall of the barrel. After that, the heat source of solid plastic melting comes from two aspects: one is the heat conduction of the external heater of the barrel, The other is the shear heat (due to viscous dissipation) generated due to the different movement speed of each layer of the melt in the melt film, namely the viscous heat dissipation in rheology.

With the progress of melting, when the thickness of the melt film is greater than the gap between the screw and the barrel, the moving screw will scrape the melt film off and form a melt pool before the advance of the screw. In the melting process, the melt pool gets wider and wider, and the width of the remaining solid gets narrower and narrower, until it finally disappears completely. This is the epoch-making famous Tadmor's melting theory published by Tadmor in 1967.

Fifth: Mixing

In the process of mixed extrusion, solid materials are generally compacted into dense solid plugs under high pressure. Since there is no relative movement between particles in the solid plugs, mixing can only be carried out between layers of melt with relative movement.

Generally speaking, the following mixing phenomena occur in the melt, especially in the melt conveying section: First, each component in the material system is uniformly dispersed and distributed, which refers to resin and various additives. The second is the thermal homogenization. This is because in the extrusion process, the material that melts first has the highest temperature, and the material that melts later has the lowest temperature. The temperature of the interface between the solid and the melt is just the melting point of the plastic. If the molten material is extruded from the die prematurely, it will inevitably cause uneven extrusion everywhere, which may cause color difference and deformation, or even cause product cracking. In addition, considering that the plastic itself has a certain molecular weight distribution (MWD), mixing can make the part with higher relative molecular weight uniformly disperse in the melt. At the same time, under the action of shear force, the part with higher relative molecular weight may be reduced due to chain scission, which reduces the possibility of unmelted particles (gels) and inhomogeneities in products. Obviously, in order to ensure uniform mixing of products, it is necessary to ensure that the melt conveying section (the last section) of the screw has sufficient length. Therefore, the melt conveying section of the screw is also called homogenizing section. At the same time, when calculating the output of the extruder, the volume of the screw groove at the last constant depth section of the screw is taken as the basis for calculation, and the melt conveying section of the screw is also called the metering section.

Sixth: Venting

During the extrusion process, there are three kinds of gases to be discharged. One is the air mixed between the polymer pellets or powder. As long as the screw speed is not too high, generally speaking, this part of gas can be discharged from the hopper under gradually increasing pressure. But when the rotating speed is too high, the material moves forward too fast, and the gas may not be discharged completely in time, thus forming bubbles in the product. The second gas is the water absorbed by the material from the air, which becomes steam when heated. For those plastics with little moisture absorption, such as PVC, PS, PE, PP, etc., there is generally no problem. These small amounts of water vapor can also be discharged from the hopper at the same time; However, for some engineering plastics such as PA, PSU, ABS, PC, etc., due to their large moisture absorption and too much water vapor, it is too late to discharge them from the hopper, which forms bubbles in the products. The third is some materials inside the plastic particles, such as low molecular weight volatiles (LMWV), low melting point plasticizers, etc., which are gradually vaporized under the heat generated during the extrusion process. Only when the plastic is melted, Only by overcoming the surface tension of the melt can these gases escape, but at this time they are far away from the hopper, so they cannot be discharged through the hopper. In this case, an vented extruder has to be used.

Therefore, any screw must complete the above six basic functions of feeding, conveying, compression, melting, mixing and exhaust. Obviously, feeding and conveying affect the output of the extruder, while compression, melting, mixing and exhaust directly affect the quality of the extruded products. The so-called quality here refers not only to whether the melting is complete, but also to whether the products are compressed compactly, whether the mixing is uniform, and whether there are no bubbles in the products. This is the plasticating quality.

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