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The influence of screw wear on blowmolding processablity.

The Influence of Screw Wear on Blowmolding Processability

Screw design, and the compatibility of the flight tip coating with the barrel lining, affect the wear rates of screw and barrel, and ultimately, cycle time and production rate.

Processability is a major concern for fabricators of HDPE bottles. The most common blowmolding technique is extrusion blowmolding, in which a parison is formed in free-surface flow and expanded into a bottle. The screw for melting and extruding the parison can either be continuous or reciprocating. Both methods require careful control over parison formation. The control of parison swell is the first criterion for a successful product in extrusion blowmolding.

A second criterion is the melt temperature in the parison. Melt temperature controls the cooling portion of the cycle, increasing or decreasing the overall cycle time for a given bottle mold, and thus increasing or decreasing the overall production rate (bottles/hr). The cooling portion is the longest segment in the overall cycle, so high melt temperatures, and the consequent slow heat removal, can become a disadvantage. High melt temperatures can result from inadequate screw design, or if the screw is satisfactory when new, performance can deteriorate because of normal wear on the screw and barrel. Such wear increases the clearance between screw and barrel, which reduces screw efficiency over a period of time.

Very few experimental results are available on the influence of wear on the behavior of the screw. One set of experiments on a 2.5-in extruder with a Union Carbide Corp. Maddock mixing screw showed: * Wear decreases output rate at constant

screw speed. * Melt temperature is not increased at

constant screw speed, but does increase

with increased screw speed. * Severe instability in output was

observed, which in blowmolding on shuttle

machinery would be manifest by

variation of bottle weight and length of

tails. On wheel machines this would be

observed as a variation in bottle weights

with time. The effect on reciprocating

machinery would be nonuniform shot


Theoretical studies on the effect of clearance on output and experimental results on the influence of clearance on output have been published, but no information is available on the behavior of melt temperature.

This article is the result of an effort to understand the influence of melt temperature on blowmolding machinery production, and to quantify the rise in melt temperature as a consequence of wear on the screw and barrel over an extended period.


Machinery. Two laboratory blowmolding machines were used--a single-head Impco reciprocating molder and a single-head shuttle Bekum molder.

Because the equipment has no trimming devices to simulate production facilities, a method was devised to measure the solid temperature of the parison after bottle ejection from the mold. A surface pyrometer was used to measure the surface temperature of the tail of the finished part as quickly as feasible. Significant variations in temperature were observed over the tail, so a single spot was chosen for repeatability. At two melt temperatures the blow time was varied and the tail temperature measured as a function of cooling time.

Material. An HDPE copolymer (DMDA-6300) with a 40 flow rate (ASTM D-1238 Condition F), 0.954 g/cc, and excellent stress crack resistance was chosen as the model resin for the experiments. The rheology, thermal properties, and specific gravity of this resin were used in the extrusion simulations as representative of the general characteristics of most blowmolding resins.

Results. Data from the melt temperature experiments on tail temperatures are plotted in Figs. 1 and 2. In these examples, the tail temperature is illustrated as a function of cooling time for two melt temperatures. Figure 1 shows the results for the Impco reciprocating molder. At a constant tail temperature of 57 [degrees] C, an increase in melt temperature of 28 [degrees] C results in a 3.0-sec increase in cooling time for the Impco system.

In the Bekum experiments, the changes observed are more indicative of a production environment in that the cooling times were shorter and the tail temperatures higher. At a constant tail temperature of 74 [degrees] C, the loss of time in the cooling cycle is 4 to 5 sec for a 28 [degrees] C melt temperature rise. This Bekum cooling change is used in the wear calculations to determine the loss in production rate from wear. The average penalty in cooling time from a melt temperature increase is 0.16 sec/ [degrees] C.

Effect of Screw Design on Melt Temperature

To demonstrate that screw design can have a major effect on melt temperature, three different screw geometries, shown in Fig. 3, were optimized to minimize melt temperature. The screws were fitted with UCC barrier mixing heads at either end. The simulation results--plotted in Fig. 4 as melt temperature versus output rates--demonstrate a temperature advantage for the double metering screw. This screw was used to determine the influence of extruder size on melt temperature. The results of output versus melt temperature for extruder diameters of 80, 90, and 100 mm are plotted in Fig. 5 (water cooling, 163 [degrees] C barrel temp., 13.8 MPa head pressure). The largest extruder gives the lowest temperature. Thus, for 136 kg/hr output rate, the lowest melt temperature is obtained with the 100-mm double metering screw.

If both screw design and extruder size are optimized, a 11.1 [degrees] C reduction in temperature can be realized with a potential 1 to 2 sec reduction in cycle time.

Effect of Wear on Melt Temperature

Screw clearance has an adverse effect on output as determined experimentally. Clearance is defined here as the difference in radius between the screw and the barrel; thus a clearance of 0.10 mm on a 100-mm-diameter extruder is a screw radius of 49.9 mm (or 99.8 mm diameter). The data from the simulations predict a decrease in output. Other data also show no increase in temperature. The extrusion simulations show the same behavior for a similar screw at 100-mm diameter. The constant melt temperature at constant screw speed with decreasing output is attributed to additional solids reaching the barrier mixing head, which acts as a non-shear melting device, thereby moderating the effects of reduced output on melt temperature. When the unmelted material in the barrier mixing head becomes excessive, melt quality can be affected.

In a production environment, any decrease in output from the extruder is compensated for by increasing the screw speed to achieve a constant output. This increase in screw speed increases the melt temperature, as illustrated in Fig. 8, where the increase in melt temperature is shown as a function of screw speed for four clearances. The average increase in melt temperature at 176 [degrees] C barrel temperature is 0.45 [degrees] C/rpm. At 149 [degrees] C, the average increase of temperature with screw speed is 0.61 [degrees] C/rpm.

Also important in estimating the loss in production is the relation between screw speed and output as the clearance between screw and barrel increases.

To estimate the increase in cooling time from wear, Fig. 9 is used to determine the increase in screw speed at constant output. For 136 kg/hr, as the screw wears, the screw speed increases from 44 to 62 rpm (that is, by 18 rpm) when the screw clearance is 0.81 mm. For 113 kg/hr, the increase in screw speed is only 15 rpm. Thus, operating the extruder at lower screw speed can have an influence on the temperature increase as function of wear.

To determine the increase in cooling time, the increase in screw speed is multiplied by the average increase in temperature with screw speed from Fig. 6 to obtain the estimated temperature increase, which is then multiplied by the increase in cooling time, 0.16 sec/ [degrees] C. For throughput of 136 kg/hr the increase in cooling time is 1.4 sec when the clearance is 0.81 mm. For a 15-sec total cycle time, this represents an 8.5% increase.

Table 1 summarizes the results of the simulation experiments on a water-cooled continuous extrusion molder. The production rate in bottles/hr is based on 4 bottles/cycle. For a cooling time of 10 sec, the cycle time is assumed to be 15 sec, so the production rate from the molder is 960 bottles/hr. For a cooling time of 11 sec, the cycle time is 16 sec, for a production rate of 900 bottles/hr. Thus the production rate decreases as the screw speed increases and the melt temperature rises.

Because many extruders operate with air cooling by blowers or natural convection, additional simulations were done to determine the influence of limited cooling on the extruder barrel. The heat transfer rate was limited to 569 kcal/hr/[mm.sup.2] (water cooling is 6 times greater). Table 2 lists the production rates for limited cooling. It is readily apparent from Tables 1 and 2 that water cooling reduces the influence of wear on melt temperature and production rates.

Extruder output is sensitive to the tooling pressure exerted on the screw and will have an effect on the behavior of the melt temperature as a function of wear. Table 3 illustrates the difference between 28 MPa and 3.4 MPa head pressure. The increase in screw speed at constant output is lower for 3.4 MPa and the reduction in production rate is less. The extrusion simulations are easily applied to a continuous extrusion blowmolder, such as wheel or shuttle machines. And the same principle applies to reciprocating screws, with the added possibility that reduced output will eventually result in a situation where the screw will control the cycle time because of inadequate plasticizing capability. When the screw is worn enough to control the cycle time, it must be replaced immediately.

Factors Affecting Screw Wear Rates

The rate of rise of melt temperature depends upon the rate at which the screw clearance increases or wear progresses. The rate of increase in screw clearance depends upon two major factors: the abrasion resistance of the screw and barrel (metallurgy), and the screw design. One of the better combinations for minimizing wear rates is Colmonoy 56 on the screw tips with a bimetallic barrel lining such as Xaloy 101. However, new materials such as tungsten carbide flight coatings and tungsten carbide-lined barrels should be considered even if more expensive.

Observations of wear problems in production environments suggest the following screw design considerations: * Screw flexibility, the result of the natural cantilever

deflection of the screw. The effect is minimized by reducing the

weight of the screw. The depth and length of the feed section

is a major factor in reducing flexibility. Feed section depth

should be minimized to reduce wear rates. Another

technique to reduce screw flexibility is to double flight the screw,

especially in the feed section. * Length of the transition section, believed to be a major factor

in screw design. A short transition forces the solid bed to

generate pressure, which pushes the screw against the barrel,

accelerating wear. * Amount of flight area, also a major consideration in

determining wear rates. Larger flight area reduces the pressure

loading of the screw flights against the barrel and decreases

the wear rate.

Estimates of Wear Rates

Wear rates depend upon flight tip coating and screw design. For the type of materials used in blowmolding the rates are 0.25-0.50 mm/yr. For the low wear rate, 0.25 mm/yr, three years would be acceptable wear life for a water-cooled extruder. At 0.50 mm/yr, two years would be the expected life of the screw. To determine the extent of screw wear a plot of screw speed versus time is recommended. When the increase in screw speed reaches 20 rpm, the dimensions of the screw and barrel should be measured. The data from this article can then be used to approximate the temperature rise and to determine the loss in production rate from wear.

For an air-cooled extruder, replacement or screw rebuilding should be more frequent than for water cooling, even with similar wear rates.

Conclusion The most important finding of this work is the development of quantitative data demonstrating the influence of melt temperature on the ejection temperature of the blowmolded bottle. Higher melt temperatures increase the cooling time perhaps as much as 5 sec when a continuous effort is made by the producer to limit cooling times in order to minimize cycle time and gain productivity. The quantitative data on the effect of cooling time permit calculation of the gains or losses from the decrease or increase in melt temperature via the experimentally determined number 0.16 sec/[degrees] C. In all blowmolding processes there is a limit to decreasing the melt temperature because of poor bottle appearance from melt fracture or pinch-off welding.

Screw design can influence melt temperature from either the basic screw design or from extruder size. Larger extruders will operate at lower screw speeds, which will provide lower temperature initially and will have lower wear rates in general. Wear slowly increases the melt temperatures as the output rate decreases with increasing clearance. The important factors in melt temperature increase have been demonstrated by extrusion simulations as tooling pressure, heat transfer limitations, and screw speed. Screw design and compatibility of flight tip coating with the barrel lining have been observed to be major influences on wear rates of screw and barrel. [Figure 1-9, omitted] [Table 1-3, omitted]
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Author:Miller, John C.
Publication:Plastics Engineering
Date:Oct 1, 1989
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