Time uniformity of extrudate melt temperature.

Time uniformity of extrudate melt temperature

Good temperature control is a prerequisite for high quality extrusion of elastomeric compounds. Two types of temperature fluctuations have to be controlled. One is along the length of the extruded piece. The other is across the extruded shape. The first is affected by parameters such as variation in feed temperature to the extruder, barrel heating and cooling. It can be measured with standard surface or probe type pyrometers and control usually is the responsibility of the extruder operator. The other is an extrudate temperature fluctuation across the die or screw channel cross section which requires special equipment to measure. This variation will not disturb product uniformity so long as flow is streamlined. However, if the crossflow temperature variation is great, flow in the die will not be streamlined and irregular fluctuations can occur. Crossflow temperature variation is affected by extruder and screw design.

The profile for temperature fluctuations across an extruder adapter was measured by Kessler[1] and is shown in figure 1. This figure shows that the maximum temperature fluctuation occurs about halfway between the wall and the center of the adapter. Using devices with a response time of less than one second, placed at mid-radius of the die, temperature fluctuations during extrusion of low density polyethylene were measured at various extrusion rates. These are presented in figure 2. As expected, temperature fluctuations increased as output increased and with a corresponding increase in die pressure fluctuation.

Squires and Wolf[2] used an unsteady-state heat transfer model to study temperature fluctuations in cross channel flow. They concluded that the approach to temperature equilibrium would be expected to correlate with the dimensionless group. (1) [Alpha] [Theta]/[r.sup.2] where [Alpha] is thermal diffusivity, [Theta] is the contact time and r is the distance over which heat is transferred. Substitution was made for the contact time [Theta] with the average residence time in the extruder, i.e., the channel volume of the screw divided by the throughput rate, or (2) [Theta] = [Pi]DhL/Q where D is the barrel diameter, h is the channel depth of the screw section, L is the length of screw section and Q is the throughput rate, all in consistent units. Also (3) r = [Beta]h where [Beta] is a proportionality constant (dimensionless). Substituting equations (2) and (3) into (1) yields (4) [Alpha] [Theta]/[r.sup.2] = ([Pi]/[Beta.sup.2]) ([Alpha]DL/Qh) Squires assumed [Beta] to be constant and have a value of 0.54 for the extruders and plastics under consideration.

The sum of these terms was named the "Quality Index," Nq, and defined (5) Nq = ([Alpha]D/Q) [Sigma] (L/h) where Nq is the dimensionless quality index and the length-to-depth ratio, L/h, is summed for the various zones along the extruder screw.

Good correlation between quality index and melt temperature variation in low density polyethylene was found using extruders ranging in diameter from 50 to 200 mm and larger. The theoretical and actual correlations are presented in figure 3.

While quality index has been useful in designing extruders and extrusion systems for quality performance, the term is ambiguous since limits on the numerical values of "Nq" representing good and poor extrusion quality will change with different types of materials. A simpler procedure, especially for elastomeric compounds, is to calculate temperature variation directly using (6) [T.sub.v] = ([T.sub.m] - [T.sub.f]) exp [([Pi]/[Beta.sup.2]) ([Alpha]DL/Qh)] where [T.sub.v] is the temperature variation, [T.sub.m] is the melt or extrudate temperature, [T.sub.f] is the feed temperature and k is the correlation constant or slope of the Gurney-Lurie dimensionless time-temperature plot[3].

It is the objective of this article to describe application of this model to the extrusion of several elastomeric compounds using a variety of screw designs and barrel length to diameter ratios. Additionally, the significance of the findings on equipment design and operation will be discussed.

Experimental

Four compounds of EPDM and two of CR elastomers were used in this study. Their formulations are presented in table 1. The formulations are typical extrusion type compounds for these elastomers. The non-productive batches were mixed in a No. 27 Banbury from Farrel Corp. using standard procedures.

Frensdorff[4] measured a value of 0.127 [mm.sup.2]/s for the thermal diffusivity of carbon black filled, EPDM compound. This value was used in all calculations. It agrees closely with a value of 0.120 [mm.sup.2]/s measured by this author on clay filled compounds using less precise techniques.

Theoretically, the value of [Beta] should be 1.0 for polymer melts. Elastomeric compounds show little or no crystallinity and are considered polymer melts at both room and processing temperatures so a value of 1.0 was used in all calculations.

Extrusion tests were conducted in a 90 mm diameter, Davis-Standard Model DSR 35-IN-35 extruder. The basic extruder has an electrically heated barrel with a length to diameter ratio of 10/1. Two additional barrel segments, each 5/1, could be added to provide 15/1 and 20/1 barrels. The interior of the head tapered to a throat of 50 mm diameter. The throat flared to a slit die of 100 mm width and 6.4 mm depth. A temperature probe was mounted at a depth of 12.5 mm in the throat.

The six extruder screws used in these tests are described in table 2. There are single stage, constant transition screws with a compression ratio of 2/1 for each of the three barrel lengths. For the 20/1 barrel there are three additional screws. Two of them are two stage compression screws. The other has no compression but a changing flight angle to provide pumping. It is sometimes referred to as a "rubber screw" since this is a design frequently used for extruding elastomeric compounds. One of the two stage screws has constant transition in both stages. The other has combinations of feed, transition and metering zones. The L/h values for these screws range from 68 to 195 which is nearly a four fold range for testing the effect of this parameter on temperature variation.

Extrusion tests were conducted using room temperature or cold feed of compound to the extruder. Barrel temperatures were set to provide an extrudate with bulk temperatures on the order of 95 to 115 [degrees] C. A minimum of 600 s operation after any screw speed adjustment was used to achieve quasi-equilibrium before recording temperature variations in the extruder throat using a Hewlet-Packard multi-channel recorder.

Results and discussion

Experimental results of the temperature variation measured while extruding the six compounds with the six different screws are presented in table 3 along with the predicted values for temperature variation under the test conditions. These data also are summarized in figure 4 for the CR compounds and figure 5 for the EPDM compounds as plots of actual versus predicted temperature variation. The designation of individual compounds was omitted from these plots since the composition of the compounds did not seem to be a significant factor in the results.

Also shown in figure 4 and figure 5 are lines representing the theoretical (perfect) correlation between actual and predicted values and the actual correlation determined by linear regression analysis of the data. It will be noticed that the actual correlation is high at low temperature variation and low at high temperature variation. This may represent the effect of more measurements in the lower temperature range but it should be noted that a similar divergence is seen in the correlations published by Squires for polyethylene as shown in figure 3. However, with a correlation coefficient of 0.92, there is a high degree of correlation between predicted values and those actually measured.

Predicted and actual values of temperature variation also are presented in figure 6 as a function of the Quality Index, Nq. It should be noted that the correlation for the elastomeric compounds is shifted to much lower values of Nq than those for polyethylene. This is due to use of a value of 1.0 for [Beta] instead of 0.54 as Squires used for polyethylene. Also, temperature variation in elastomeric compounds is more sensitive to changes in Nq than in polyethylene. This is another advantage for direct calculation of temperature variation rather than using the Quality Index.

Although the experiments reported herein are limited to a few compounds of CR and EPDM, one extruder and six different screws, it is believed that the model tested should be applicable to a wide range of elastomeric materials and extrusion equipment. Thermal diffusivity is the only material parameter needed. While it is not easy to measure, it can be calculated from thermal conductivity, specific heat and relative density but with poorer reliability. The other parameters involve screw dimensions which can be measured.

One way for using this model to evaluate extruder systems, extruder and screw designs is to compare theoretical output for a given temperature variation. To do this, equation (6) is rearranged in the following manner (7) q = (k [Alpha] [Pi] DL [Delta]) (L/h)/1n [[T.sub.v]/([T.sub.m] - [T.sub.f])] where q is the extruder output and [Delta] is the relative density, all in consistent units. For a given screw diameter D and temperature variation [T.sub.v], the term [T.sub.m] - [T.sub.f] describes the extruder system (hot or cold feed) and the term L/h describes the extruder and screw design. In the following applications of this model, screw diameter is 90 mm, relative density is 1.3, feed temperature is 25 [degrees] C and extrudate or melt temperature is 95 [degrees] C unless stated otherwise. Also, the term quality is restricted by definition to temperature variation across an extruded shape.

Extrudate quality performance for three of the screws studied is presented in figure 7 as a plot of melt temperature variation versus extruder output. The three screws represent the lowest, intermediate and highest L/h ratios studied. For each screw, temperature variation is small in extrudates produced at low output but increases rapidly as output increases. Finally, the rate of temperature variation tends to level off as it approaches the difference between extrudate and feed temperatures ([T.sub.m] - [T.sub.f]), the driving force for heat transfer. This pattern of temperature variation is precisely the same as reported by Squires[2] for extrusion of polyethylene at rates up to 40 kg/hr. It can be seen that the amplitude of temperature variation varies inversely with the L/h ratio of the screw and that high quality extrudates can only be achieved at low extrusion rates.

A plot of predicted output for the six screws used, based on a 5 [degrees] C temperature variation, is presented in figure 8. This shows a linear effect for the length/depth ratio on output and predicted outputs ranging from 10 to 31 kg/hr. This is very small compared to a maximum extrusion rate of 500 kg/hr for a 90 mm, cold feed extruder listed by Johnson in a recent review of rubber extrusion[5]. For the best screw used in this study (L/h = 195) this rate should produce an extrudate having a 60 [degrees] C temperature variation. This could not be described as a high quality extrudate.

Hot feed extruders usually have a relatively low L/h ratio. Their length/diameter ratios range from 3:1 to 8:1 and the depth of the screw is large so they will have low L/h values. They are usually fed with prewarmed strips from a mill. The effect of both feed and extrudate temperatures on temperature quality performance for such an extruder is presented in figure 9. It can be seen that prewarming increases output at equal quality. However, decreasing the extrudate temperature yields a higher output for the same quality level. This agrees with practical experience that the warm-up mill should be as close to extrusion temperature as possible.

A summary comparison of quality performance output is presented in figure 10. This shows that a hot feed, short L/D extruder cannot match the output of a 20:1 L/D, cold feed extruder at equal quality, even if the longer extruder has a poorly designed screw. It confirms that the current trend in the rubber industry to purchase cold feed rather than hot feed extruders[6] should be continued and that increased emphasis should be placed on screw design and mixing devices. Johnson describes several screw designs and mixing devices in his review[5]. An evaluation of their effect on temperature variation across an extrudate shape requires modification of the simple unsteady-state heat transfer model described. Some of these screw designs and mixing devices will be the subject of future studies.

Conclusion

An unsteady-state heat transfer model is a satisfactory and useful model for predicting temperature variation across the cross section of an elastomeric extrudate. [Figure 1 to 10 Omitted] [Tabular Data 1 to 3 Omitted]

References

[1]H.B. Kessler, R.M. Bonner, P.H. Squires and C.F.W. Wolf, Soc. Plastics Engrs. J. 16, 267 (1960). [2]P.H. Squires and C.F.W. Wolf, Soc. Plastics Engr. J. 27, 68 (1971). [3]W.H. McAdams, "Heat transmission", McGraw-Hill, New York, 2nd. ed., p. 27-44 (1933). [4]H.K. Frensdorff, Rubber Chem. Technol. 47, 849 (1974). [5]P.S. Johnson, Rubber Chem. Technol. 56, 575 (1983). [6]R.L. Christy, Rubber World 180, 100 (1979).