1. Mechanism of extrusion and plasticization of PVC materials
According to three-stage plastification process of PVC material in barrel of extruder, screw is divided into three zones: a solid conveying zone, a melting zone, and a melt conveying (extrusion) zone. The discussion of plastification mechanism in PVC material extrusion is a discussion of mechanism of the three-stage screw-in-barrel plastification process.
Scheme of three-section auger zone
1.1. Plasticizing mechanism in bulk material handling area
In drum of machine, zone of flow, preheating and compaction of solid polymer (PVC) and its additives is defined as zone of transport of solid. Firstly, flow of solid polymer particles from hopper to barrel is achieved by gravity. As screw rotates, particles in hopper move continuously while particles advance towards machine head. In solid particle conveying zone (cylinder zone C1), particles such as polymers and small molecules in PVC material are gradually heated, and at same time, shearing action of screw and friction between particles also increase particle heat, so that particles are completely in a dense state. Contact, Diffusion, Penetration.
In this area, due to changes in screw pitch, winding width, etc., PVC material particles are compacted tightly, forming a solid bed or solid plug that slides along screw groove. The movement of hard plug depends on friction between surface of barrel and hard plug, and friction between screw and hard plug prevents hard plug from moving. Therefore, in barrel, PVC material particles do not move in one direction, but from time to time they roll and slip, rotate with a screw, “bridge”, accumulate behind “bridge”, break through “bridge”, and squeeze out of PVC material. Flow from and PVC material into bunker is repeated.
In this area, extrusion and plasticization quality of PVC materials is better. From point of view of state, PVC changes from a glassy state to a highly elastic state; in terms of total structure of 50% to 60% PVC resin. particles break up, turn into primary particles, and surface of various additive particles fully contacts and diffuses with primary particles.
It is worth noting that for stable operation, height of solid material in hopper must always be above a certain critical value. Above this critical value, changing level height will not affect performance of extruder, however, if level height is below critical value, it becomes a strong destabilizing factor. Changing height of solid material level will cause a change in lower pressure, which will change extruder mode and lead to a deterioration in quality of extrusion and plasticization of PVC materials.
1.2.Mechanism of plasticization in melting zone
In a barrel, region of coexistence of a solid polymer and a melt is defined as a melting zone or a phase transition zone. This is heating zone C2, C3. The melting zone is main part of extruder, especially temperature setting (C2 barrel zone, C3 zone, screw core) and screw speed setting, screw-to-screw gap, screw-to-barrel gap, etc., The extrusion quality of PVC materials is important meaning. When PVC material reaches melting zoneDue to changes in screw pitch and flight width, PVC material particles already generate significant compaction pressure, and this pressure is combined with softening effect of surrounding heat transfer fluid. At present, hard layer is a mixed state, consisting of a part of highly elastic state of PVC, part of glassy state of PVC, and a small amount of viscous liquid state of PVC. The continuous layer has shape of a helical channel and slides inside channel. Due to this relative motion, a distribution of velocities is created in melt film between solid layer and barrel surface. As a result, melt in melt film begins to flow towards screw, and when it collides with screw flight, screw flight "scrapes" melt from barrel and collects it in back of screw groove in front of screw shaft. advancement of screw thread in molten bath. As layer of solids moves through channel, more and more molten material enters molten pool, so size of pool increases and size of solid layer decreases. The solid layer is gradually destroyed, passes into a viscous-liquid state and is transported forward.
In this area, quality of extrusion and plasticization of PVC materials is better. From point of view of state of PVC, it changes from a highly elastic state to a viscous-liquid state, from point of view of aggregate structure, it changes from 60 to 70% of primary PVC particles. This is a primary particle, and various additive molecules contact with primary PVC particles, forming a physical and chemical combination.
In melting zone, factors that improve quality of extrusion and plasticization of PVC materials are:
(1) Increase auger speed
(2) Increase set barrel temperature in melting zone
(3) Appropriate clearance between screw and cylinder
For a specific recipe for production of PVC profiles, there must be a set of optimal barrel temperatures in melting zone.
Strength Question: Can increasing screw speed definitely improve plasticization? For a single screw, an increase in speed means an increase in extrusion capacity, which usually reduces plasticization.
1.3. Plasticizing mechanism in melt supply zone
In barrel, solid polymer is completely melted, and part where melt is forcibly transported to die is defined as melt supply zone (barrel heating zone C4). In this area, molten macromolecules are further reacted with various additives under shear, and crystals are composed of superior particles and a small amount of primary particles, which can improve strength and toughness of final material. When material containing this crystal is extruded and cooled under action of an external force, primary particles can prevent movement of primary particles to achieve an increase in strength.and, and due to large surface area of the primary particles, they can absorb some of impact energy upon impact, improve toughness.
2. How to improve degree of plasticization of PVC products
2.1. The influence of degree of plasticization on product characteristics:
The effectiveness of PVC products is closely related to degree of plasticization. If degree of plasticization is low, product will be brittle and its mechanical properties will not meet requirements, if plasticization is too high, yellow lines will appear on product. , and mechanical properties are not qualified. This is very important when processing PVC products.
①When degree of plasticization is 60%, tensile strength is highest;
②When degree of plasticization is 65%, toughness is highest;
③With a degree of plasticization of 70%, maximum elongation at break;
For production of water pipes, degree of plasticization of material is 60-65%. Because in this range, it can reflect two properties of tensile strength and toughness.
2.2. Influence of temperature on degree of plasticization:
The resin material cannot be melted below 80°C, and is in state of glass. The material in state of glass is hard and brittle, and material cannot be processed in state of glass, as temperature rises to 160°C, material is in highly elastic condition, but material still cannot flow in this area, it can only make material soft and improve viscoelasticity; if it can really achieve PVC melt processing and has fluidity, temperature should be within 160-200°C, but for any stabilizer, when temperature is over 200°C, material will decompose after heating for a long time, so when controlling degree of plasticization, temperature can be adjusted only within 160-200°C. In temperature range of 40°C, when PVC temperature is set within 170-180°C, plasticization is better.
2.3. Ways to increase degree of plasticization:
①Increasing temperature of fuselage and propeller.
②When screw speed is normal, increase feeding speed of feeder to increase degree of plasticization. (Can this be improved by increasing amount of feeding?)
③In case of rated extruder speed and feed requirement, increase extruder speed.
④Give dry powder a good curing period (12-48 hours)
Question from Brother Tenacity: Is aging just mixing ingredients and leaving them for a while?
The role of ripening period:
a. Eliminate static electricity and reduce environmental pollution.
b. Increase Apparent Density
in. Increase degree of plasticization
d. Low molecular weight polymerization is uniformly dispersed to prevent instabilityduring extrusion.
e. Increase partial plasticization by lowering drain core temperature.
2.4 How to evaluate degree of plasticization:
①The degree of plasticization is judged by current of main motor. As an example of 65/132 production line, current of main motor is 46-52A. Because our company produces low calcium product, corresponding current is 45-50A. Premise: screw speed is 16 ~ 22 rpm, feed is full and matches screw speed, temperature setting is consistent with screw speed and host current);
② Observe degree of plasticization of material through vacuum outlet of main motor (that is, material is more than 60% filled in middle of scraper groove, powder in scraper groove is in state of bean curd residue, and material at bottom of groove is pressed);< /p>
④Determine degree of plasticization by melt pressure and melt temperature of confluence core (the disadvantage is that if instrument or sensor of confluence core fails, coke and other factors will affect accuracy of detection result)< /p>
Third, direction of winter PVC formula adjustment
PVC plastic itself has poor frost resistance and impact resistance at low temperatures. The lower limit of use temperature is usually -15°C, and outside temperature in summer is over 30°C, which drops to about -10°C. °C in winter. The temperature difference is large. If formula of PVC products is not corrected in time, productivity and quality of products will fluctuate and decline. For example, products will be more brittle than in summer, and processing will be relatively difficult. Therefore, as weather gets colder, formulation of PVC products must be adjusted accordingly.
The temperature is low in winter, and movement of PVC molecules is relatively difficult than in summer. Externally, strength of PVC decreases, and fragility increases. The production of PVC formula in winter should minimize amount of external lubricant. and moderately increase internal lubrication under condition of no sticking to mold.
For hard PVC products in my country, most of impact additives used are CPE, and amount used in winter should be increased accordingly. Fillers such as calcium powder and talc increase brittleness of PVC, which can be reduced accordingly in winter.
For products with high quality requirements and low calcium powder, shock-resistant ACR or MBS can be used to prevent products from being too brittle in winter and too soft in summer.
At same time, we also need to consider effect of processing temperature, cooling temperature, stretching speed, mold design and other aspects on frost resistance of PVC, combined with good formulation design and itsTimely make appropriate adjustments when temperature drops sharply.
Source: Internet, thanks to original author!
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