EXTRUSION RATE

The extrusion rate of a polymer is dependent on the rotational speed of the extruder which determines the residence time and shear rate of the polymer. The extrusion rate and the shear forces on the polymer have been found to affect its subsequent degradation profile [3, 4, 10]. One study found that a weaker, less drawn, and less crystalline fiber resorbed more quickly in PBS solution [10]. The extrusion speed determines the residence time at an elevated temperature, because it controls the time the material remains in the barrel of the extruder. Longer residence times (from 50 to 300 s) and higher shear rates result in higher rates of polymer degradation even at lower temperatures. 

Thus in general, hydrolytically sensitive bioresorbable polymers need to be processed under the mildest conditions possible with rigorous exclusion of moisture from the polymer as well as the surrounding atmosphere. This is contradictory to the optimum melt-processing requirements. Many times, high temperatures are needed to reduce the melt viscosity, or higher pressures are needed to enable the polymer to be forced through the fine spinneret holes. 

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In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Example 4.4 A blow moulding die has an outside diameter of 30 mm and an inside diameter of 27 mm. The parison is inflated with a pressure of 0.4 MN/m to produce a plastic bottle of diameter 50 mm. If the extrusion rate used causes a thickness swelling ratio of 2, estimate the wall thickness of the bottle. Comment on the suitability of the production conditions if melt fracture occurs at a stress of 6 MN/m. ... 

For filled thermoplastics (30-40% by mass of chalk, ash or asbestos), complex shear may, as reported in [235], provide an increase of apparatus productivity by 40-80%, or if the flow rate is to be constant, the pressure in the molding instrument may be reduced by at least 20-30%. It is to be noted that while some extra power is required to create the complex shear conditions, the total power consumption of the apparatus as a whole may be reduced, on the power per unit of product basis, due to the high extrusion rates [233]. 

Wu [353] who studied extruded fiber-filled composite samples established that the distribution of fibers along the radius of the specimen depended on the extrusion rate at low rates the fiber distribution is uniform, at medium rates the minimum of filler concentration occurs at 0.63 R (R is the extrudate radius) (in case of spherical particles this point corresponds to the maximum filler concentration) at high extrusion rates most of the fibers are concentrated about the flow axis and there are almost none on the extruded sample surfaces. 

The Ml test equipment is easy to operate, provides repeatable results, and low cost to operate. It is widely used for quality control and for distinguishing between members of a single family of plastics. Specifically, this MI makes a single-point test that provides information on resistance to flow at only a single shear rate. Because variations in branching or MWD can alter the shape of the viscosity curve, the MI may give a false ranking of plastics in terms of their shear rate resistance to flow. To overcome this problem, extrusion rates are sometimes measured for two loads, or other modifications are made. 

Addition of scrap rubber in the form of either ground waste vulcanizates or reclaim in rubber compounds gives economic as well as processing advantages. In addition to lowering the cost of rubber compounds, the use of cross-linked rubber particles has beneficial effects such as faster extrusion rate, reduced die swell, and better molding characteristics. 

Processability Extrusion rate, pilot scale, U.S. Patent 5 683 973 [8]... 

A 40 mm diameter laboratory extrusion line was used to make demonstration parts using a specialty PE resin. The desire was to build a larger line with considerably higher extrusion rates. The shear viscosity of the resin is shown in Fig. 9.1. For the temperature range of the process, the heat capacity and thermal conduc-... 

Figure 10.13 Extrusion rate and motor current for an HDPE resin at a screw speed of 50 rpm. The highest rate was achieved at a Zone 1 temperature of about 230 °C...

Figure 10.14 Extrusion rate and discharge temperature for an HOPE resin at a screw speed of 50 rpm. Discharge temperature increased when the Zone 1 temperature was increased...

Pf pressure at the tip of the screw for an injection-molding plasticator (Eq. 10.22) Q extrusion rate in kg/h O specific rate in kg/(h-rpm) for Eq. 10.21 R compression rate in the helical direction Pjs belt sheave reduction Rg speed reduction for the gearbox S support distance from the fixed end of the barrel shear strength of the material used for the screw t plasticating time in seconds (Eq. 10.21)... 

At the startup of the line, the extruder was operated at 91 rpm to produce the required rate of 148 kg/h for a specific rate of 1.63 kg/(h-rpm). The temperature of the extrudate was measured through the transfer line wall at 232 °C. Due to process safety constraints the extrudate temperature could not be measured using a handheld temperature sensor. The extrusion rate was required in order to maintain the downstream take-away equipment at its maximum rate. At first the extruder appeared to be operating well except that the specific rate was lower than predicted. That is, the screw was rotated at an rpm that was higher than expected to produce the 148 kg/h. At 91 rpm, the rotational flow rate was calculated at 228 kg/h the specific rotational flow rate was calculated at 2.51 kg/(h-rpm). Thus, the line was operating at only 65% of the rotational flow rate. A barrier design... 

Table 11.5 Extrusion Rates and Performance Before and After Modifications to the Entrance of the Barrier Section...

Seven commerciai case studies are presented in the next sections that describe rate-iimited processes and the strategies and soiutions to increase extrusion rates. Additionai case studies that had iimited rates and additional defects were presented in Chapters 11 and 12. 

Hyun, K.S. and Spalding, M.A., Troubleshooting TPU Resin Extrusion Rate Limitations Due to Solids in the Discharge, SPE ANTEC Tech. Papers, 58, (2012)... 

Blending of the lowest price commodity polymers from synthetic and carbohydrate polymer families [e.g., poly(ethylene) and starch] would appear to follow these laws. Although each polymer class is produced in large volume (first law), the production rate for com starch/synthetic polymer blends is much lower than that for the synthetic polymer this slower extrusion rate directly affects the final cost. Ignoring this limitation, the film properties of the blend are significantly poorer than those of the synthetic polymer film. Both deficiencies are related to the poor thermoplastic properties of water-soluble polymers such as cora-starch. 

The principal benefit of using reclaim is its lower cost compared to virgin mbbers. Reclaim typically sells for 20—30% of the cost of its none-reclaimed counterparts. Reclaimed mbber also imparts some desirable improvements in processing it has much lower nerve than virgin polymers. As a result, compounds containing reclaim have much lower die swell and extrusion rates. It also increases calender rates and improves flow and mold filling. 

An extrusion is comprised of a basket (cylinder) to contain the mix and a hydraulically actuated piston to press the mass through multiple extrusion dies. The baskets are jacketed for temperature control and contain internal screens to retain large agglomerates. Since the casting powder strands are unperforated, the dies are simple cylindrical barrels with conical lead-in sections. Extrusion rates are normally controlled by orifices in the hydraulic supply line however, constant-rate positive displacement devices are also used. As they extrude, the strands are collected separately in suitable containers. 

Extrusion parameters, such as basket temperature, extrusion rate, and die design, can be adjusted to suit a particular formulation and granule size. This is an empirical process based on experience. Extrusion characteristics represent an intimate interplay between press parameters and dough properties, the latter predominating. What is wanted is maximum production rate of a smooth, dense, homogeneous strand of these qualities, smoothness is perhaps most sensitive to extrusion conditions. 

Meskat (M8) has presented a mathematical analysis of the effect of fluctuations in pressure and other variables on the comparative fluctuations in extrusion rates of Newtonian and non-Newtonian fluids. This work indicates the possibility of amplification of such fluctuations under certain circumstances with non-Newtonians rather than the uniform damping predicted for Newtonian behavior. If the validity of this analysis can be proved, it would warrant major attention being given to the problem of unsteady flow of non-Newtonian materials. 

Rotation of the core at Q = 70 min-1 shifted flow rate/pressure characteristics of extrusion curves upwards and to the left, the result of which was a notable drop of (AP)70 compared to (AP)0 (indexes used here correspond to the speed of core s rotation). Increase in (AP)70 up to values of (AP)0 due to higher speed of the screw allowed as increase is the extruder s capacity by 40-80 % (depending on the nature and content of the filler). Results very much similar to that were obtained later in experiments with filled PVC-based compositions 36). Pressure in the head can be reduced by not less than 20-30 % at a constant extrusion rate, experiments with filled PVC-based compositions 36). Pressure in the head can be reduced by not less than 20-30% at a constant extrusion rate. 

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