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Stratification of re-cycle and pellets is likely to occur in the delivery system of an extruder. Therefore, a sample datum for 50% ratio of re-cycle mass/total mass is shown in Figure 3. The loose bulk density is shown to be nearly the same as for the 50% mixed resin. A slightly lower bulk density might be expected as the void space in the pellet portion is only filled with air for the stratified condition. However, this is shown to not be significant for the 50% stratified mixture of pellets and re-cycle when compared to the 50% sample that is mixed.

The 25% and 50% mixture of re-cycle/total show that lateral stress ratio is less than that for 100% pellets over the entire range of primary stress, whereas 100% recycle has a higher lateral stress than 100% pellets. Therefore, the solids conveying rate with the addition of re-cycle will be affected.

Bulk densities of the mixtures are also lower at low stress, but are greater at higher stress. Therefore, the combined effect of bulk density and lateral stress on solids conveying is subject to the range of stress of the operation.

As with the bulk density regression functions, above primary stress values of 40 MPa, error is likely. Again, a linear extrapolation for primary stresses moderately above 40 MPa could be used to extend the useful range if need be.

Figure 7 shows the lateral stress ratio functions so determined from Equations 7-12 with the mathematical limit at zero primary stress (y intercept). The limits of lateral stress ratio values at zero primary stress are unique for the different ratios of pellets and re-cycle. It is highest for the 100% Pellets (~0.22) and stratified 50% re-cycle (~0.32). The mixtures of 25% re-cycle and 100% re-cycle have a limit of lateral stress ratio of about 0.1. Pellets with 50% and 75% re-cycle show a limit near to zero.

The results of Figure 7 clearly indicate that mixing virgin pellets and re-cycle produces non-linear results. Also, the result is dependent on the level of compressive stress. For example, resin with 25 % re-cycle has lateral stress ratio less than that for 100% pellets at primary stresses up to about 5 MPa. At 5 to 10 MPa stress the lateral stress ratio for the mixture is about the same as that for the 100% pellet resin. Above primary stress of 10 MPa, the lateral stress ratio of the 25% mixture is greater than the pure pellets. The non-linearity and the reversing of the effect can make processing difficult to manage and that day to day operation would not be consistent since mixture ratios likely could vary depending on the supply of re-cycle or lack of mixing.

Figure 7 shows that the lateral stress ratio for 50% and 75% re-cycle to always be lower than all other samples, including pure pellets. Also, at zero primary stress each of these two samples had zero lateral stress ratio, whereas all of the other samples had finite values of lateral stress at low primary stress.

Figure 7 also shows that the lateral stress ratio for 50% stratified pellets and re-cycle to have the greatest variability of all the samples. Stratification of the recycle/pellet mixture is inevitable, random, and difficult to detect in the extruder. Stratification will, thereby, make the lateral stress ratio inconsistent, and solids conveying less predictable. Therefore, stratification should be considered when flow instability occurs in the extrusion process of mixtures of PET pellets and re-cycle.

2. The lateral stress of mixtures of PET pellets and PET re-cycle flakes was measured to be a function of the mixture ratio and primary stress.

3. Lateral stress for pure re-cycle flakes was measured to be greater than for pure pellets. However, lateral stress for mixtures with 50% and 75% re-cycle was less than that for 100% pellets.

4. Lateral stress for a 50% stratified combination of pellets and re-cycle flakes resulted in the greatest values of lateral stress and the greatest variability of lateral stress ratio.

5. Loose bulk density was measured to be greatest for the pellets at about 0.75 g/cc. The addition of recycle diminished the loose bulk density in proportion to the amount of re-cycle. Pure re-cycle has a loose bulk density of about 0.5 g/cc.

6. The bulk density of mixtures of PET pellets and PET re-cycle flakes was measured in compression as a function of mixture ratio and primary compressive stress. The bulk density varied between about 0.5 g/cc and 1.2 g/cc.

2. S. J. Derezinski 2010. “Measurements of Biaxial Stresses During the Compression of Bulk Resin Feed,” ANTEC 2010 - Proceedings of the 68th Annual Conference & Exhibition, Orlando, Fl, May 16-20, 2010, Society of Plastic Engineers, pp. 617-622.

3. Chung, Chan I., Extrusion of Polymers Theory and Practice, Hanser Publications, Munich, 2000, pp. 161-162, 199-200.

4. M. A. Spalding, K. S. Hyun, and K. R. Hughes, 1996. “Stress Distribution in Solid Polymer Compacts,” ANTEC 1996 - Proceedings of the 54th Annual Conference & Exhibition, Indianapolis, IN, May 5-10, 1996, Society of Plastic Engineers, pp. 191-198.

5. K. S. Hyun. and M. A. Spalding, 1997, “A New Model for Solids Conveying in Plasticating Extruders”, ANTEC 1997 - Proceedings of the 55th Annual Conference & Exhibition, 43, 1997.

Figure 1. PET Resins. Bottom is re-cycle scrap and the top is virgin PET pellet. Different mixture ratios of these two components were tested. Pellet dimensions are about 3.2 x 4.2 mm.

Figure 2. The Test Cell for Measuring the Primary and Lateral Forces During Compression of Bulk Resin Feed as a function of bulk density [1,2]. Diameter of the cylinder is 12.7 mm. The linear displacement transducer records the height of the sample to establish the evolving bulk density value and lateral area of force.

Figure 3. Bulk Density of PET Feed versus percent of Re-cycle Flakes. Values are those measured with the test cell before load is applied.

Figure 4. Data for Bulk Density of the Pellet/Re-cycle Mixtures vs. Primary Stress

Figure 5. Raw Data for Lateral Stress for PET Pellets with Re-cycle vs. Primary Stress

Figure 6. Regression Functions for Lateral Stress versus Primary Stress A 6th order polynomial is used for each set of data of Figure 5. Regression coefficients are greater than 0.999.

Figure 7. Lateral Stress Ratio Functions for PET/Re-cycle Mixtures

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