Extrusion channel polymer melting

We consider a co-extrusion die consisting of an outer circular distribution channel of rectangular cross-section, connected to an extrusion slot, which is a slowly tapering narrow passage between two flat, non-parallel plates. The polymer melt is fed through an inlet into the distribution channel and flows into... 

Several works (96-98) describe analytically swelling of polymer melts during extrusion through short channels. The obtained relationships have been chiefly tested on rubber blends and they do not take into consideration specificity of a structure formation in semicrystalline thermoplastics. 

Forcing of Cl into the head die is a multifunctional operation leading to reduced friction of the melt against the die walls, in addition to improved orientation stretching and reinforcement of the film. The distinguishing feature of the employed extrusion head [100] (Fig. 2.24) is the presence of an annular chamber 6 in mandrel 4. The chamber communicates with Cl channel 8 and is shut off by a porous wall 5 from the side of channel 7 over which the polymer melt is fed from the extruder into the head. The outside surface (a) of the porous wall 5 and inner surface (b) of matrix 1 are made in the form of a cone whose apex faces the side opposite to the inlet to channel 7. 

The increasing volume of polymer production in the chemical industry and the wide range of items being manufactured require an increase in the capability for the production and quality of the products produced. It has been established that an imderlying problem is the distortions introduced as the melted polymer flows through the extrusion channels. Analysis of the extrusion heads, has allowed identification of certain parameters which can positively be controlled to improved performance reduction of the pressure required leading to a decrease in cost, increase of output of the flow, increase in the hardness of extrudate and decrease the post-extrusion swelling. 

Mathematical description of the process of polymer melting in the extrusion channel is complex when ultrasound is used. The description requires firstly, consideration of the mass flow of the polymer, knowledge of the flow characteristics of the melt, the temperature and pressure of extrusion, sizes of the channel and frequency of ultrasonic oscillations. Secondly, coefficient of swelling of the extrudate, effective viscosity of polymer, pressure of melt, and frequency of oscillations. 

In order to establish the way in which these various factors influence the performance of the extrusion head, studies were conducted on the flow characteristics of the polymer melts. The effects of the extrusion temperature and pressure were also studied together with the influence of the frequency of the ultrasound and construction of the channel the effectiveness of producing a high elasticity of the spray in the melt and the final properties of the solid produced. 

Common defects encountered with extrusion include effects associated with the viscoelastic nature of plastic melts. As the melt is extruded from the die for example, it may exhibit sharkskin melt fracture and extrudate (die) swell. Diagrams of these defects are shown in Fig. 1.16. Sharkskin melt fracture occurs when the stresses being applied to the plastic melt exceed its tensile strength. Extrudate swell occurs due to the elastic component of the polymer melt s response to stress and is the result of the elastic rebound of the polymer as it leaves the constraints of the die channel prior to cooling. 

Melt viscosity, or flow, is typically measured using extrusion plas-tometers (or melt indexers), capillary rheometers, and parallel plate rheometers. The extrusion plastometer measures the flow of a polymer melt under conditions specified by ASTM standard D 1238. This test yields a single, low-shear-rate value which is typically used to specify resins. Capillary rheometers determine viscosity over a range of shear rates in channel flow. While they are subject to error, these rheometers are still the only means of measuring viscosity at high shear rates (typically -y > 1000 s i). Parallel-plate rheometers also measure viscosity over a range of shear rates, but the maximum allowable shear rate is about 100 s i. 

Analysis of the flow in extrusion dies is very difficult because of the nature of the polymer melt. Die design, therefore, is largely still an empirical science. Flow behavior in the flow channels will be discussed in Section 7.5 and die design will be discussed in detail in Chapter 9. 

From Eq. 7.115, it can be seen that an extreme condition is reached when the term i/(AzVs2PsVWi) becomes unity. In this case, the solid bed width becomes independent of down-channel distance, and the shortest possible melting length is obtained Zj = Hf/A. However, this condition cannot be achieved in practice because there is no room for the polymer melt. Thus, in practical extrusion operations, the term Q,/ (A Vs PsVWi) has to be larger than unity to ensure a continuous reduction in solid bed width with distance and to avoid plugging. 

Even if the polymer melt is assumed to be purely viscous, the analysis will generally be quite complicated because many dies have flow channels of complex shape. As a result, accurate analysis of flow In extrusion dies generally requires three-dimensional flow analysis. This presents quite a challenge for simple Newtonian fluids. Three-dimensional flow analysis of viscoelastic non-Newtonian fluids is beyond the capabilities of most (if not all) die designers. Not surprisingly, die designers often take an empirical approach to die design. 

Streamlining the flow channel geometry has been found to reduce the tendency for melt fracture in branched polymers. Increased temperatures, particularly at the wall of the die land, enable higher extrusion rates before melt fracture appears. The critical wall shear stress appears to be relatively independent of the die length, radius, and temperature. The critical stress seems to vary inversely with molecular weight, but seems to be independent of MWD. Certain polymers exhibit a superextrusion region, above the melt fracture range, where the extrudate is not distorted... 

Fig. 9.13(a) shows the T-die. This flow channel geometry is simple and easy to machine. However, the distribution of the polymer melt is not very uniform and the flow channel geometry is not well streamlined. Thus, this die is not suitable for high-viscosity polymers with limited thermal stability. The T-die is used in extrusion coating applications. Analyses of the flow in a T-die have been made by Weeks [3,4], Ito [5], and Pearson [6]. 

High stock temperatures are likely to be a problem in extrusion operations where the extruder is run at high screw speed and where the polymer melt viscosity is high. The main screw design variable that affects viscous heating is the channel depth. Increasing the channel depth will reduce the shear rate and thus the viscous heating. There are limits to how deep the screw can be cut. One limit is the physical... 

Figure 4. Schematic illustration of extrusion die for in situ x-ray scattering from polymer melts in channel flows. Also shown is represaiiative spcexr used for 1 4 slit-expansion flow.

There are basically three types of multiple layer extrusion techniques (1) melt streams flow separately (2) melt streams flow separately and then together (3) melt streams that flow together. Examples of type 1 are shown in Figure 7.47. In this process polymers A and B are extruded through separate flow channels and then joined together outside the die. 

In the study, the mathematical model of the polymer melt flows in the extrusion process of plastic profile with metal insert was developed and the complex melt rheological behavior was simulated based on the finite element method. The melt flow characteristic in the flow channel was analyzed. The variation of the melt pressure, velocity, viscosity and stress versus different metal insert moving rate was investigated. Some suggestions on its practical manufacturing control were concluded based on the simulation results. 

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