Measuring the Compaction Characteristics of a Resin

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

At the start of the experiment, 11.5 g of feedstock resin is placed into the cylinder and the piston is inserted. The resin is then allowed to warm up to the preset cylinder temperature. In general, 10 minutes is adequate for thermal equilibration as measured by density fluctuations. The position of the piston is then recorded for each pressure setting, starting at the lowest and increasing to the maximum pressure. The piston position is recorded only after movement ceases. 

Amorphous HIPS resin pellets compact very little in the temperature range of 25 °C to about 75 °C, a temperature that is about 25 °C below Tg, as shown in Fig. 4.5. The HIPS resin compacts to a much higher degree for temperatures of 25 °C below Tg up to Tg. Like the LDPE resin, the bulk density at 25 °C and zero pressure was measured using the cell shown in Fig. 4.2 at 0.62 g/cmL At temperatures 25 °C below the Tg (100 °C), the bulk modulus of the resin is relatively high, and thus the pellets do not deform easily under pressure. At higher temperatures, how 

The original solids-conveying model developed by Darnel and Mol [7] assumed that the pressure (or stress) in the solid bed is isotropic. This assumption was made to simplify the mathematics and because of the lack of stress data for solid bed compacts. Previous research, however, showed that stresses in solid compacts are not isotropic [8]. Anisotropic stresses can be represented by the lateral stress ratio. It is defined as the ratio of the compressive stress in the secondary direction to the compressive stress in the primary direction, as shown in Fig. 4.7 and Eq. 4.1. 

The lateral stress ratio depends on the resin type and shape, surface treatments such as additives, temperature, and pressure. The ratio is measured using a compaction cell [2], as shown in Fig. 4.8. This cell is very similar to one shown in Fig. 4.3 except the piston for the lateral stress ratio cell is octagonal in cross section and a pressure sensor is mounted in the cylinder wall. The stress ratio is calculated by dividing the pressure measured at the side of the cylinder by the calculated pressure in the axial direction at the height of the sensor. The calculation method can be found elsewhere [2j. The lateral stress ratio for select resins at 25°C and 2.5 MPa are provided in Table 4.1. 

---