Polyblend melting in intermeshing counter-rotating twin screw extruders
Extrusion is the process where heated plastic is forced by a rotating metal screw through a shaping orifice (i.e., a die) in one continuous, formed shape to produce films, sheets, or pipes. The extruders are multi-component machines that require precise control over each step to ensure products are made accurately and with minimal waste. For example, polymer melting is one of the main functions of a plasticating extruder machine. The machine must melt the polymer rapidly to provide enough space to mix the material. To control this process necessitates good knowledge of the melting behavior of the polymer. Often, mathematical models are used to study this process and aid extrusion design. However, a number of poorly understood factors in plastic extrusion remain, including the transport of solid material, melting behavior of the plastics, and other phenomena related to the viscoelastic nature of polymeric materials. In contrast, the flow characteristics of molten polymers are relatively well defined in single screw extruders. The corresponding velocity profiles in twin screw extruders are complex and more difficult to describe because the mechanism of material transport is different to the simple, single screw system. Here, we study melting behavior in a twin screw, intermeshing extruder.
We studied polystyrene (PS), low-density polyethylene (LDPE), and polypropylene (PP). All these materials are commonly used plastics with favorable physical properties for their various applications. Despite this, there are no reports on the melting behavior of polystyrene or any polyblend in an intermeshing counter-rotating twin screw extruder. In such a system, the flights of one screw are inserted into the channel of the second screw, resulting in meshed screws. There has also been relatively little research on the melting process in such counter-rotating extruders.1–4 Recently, the first composite model of metering, melting, and solid conveying for these machines was developed,5 based on known melting and melt-flow descriptions.2, 3,6,7 This model was used to simulate the extrusion of LDPE, HDPE, and PP.
We used a modular intermeshing counter-rotating twin screw extruder in the study.8 We arranged various screw elements on shafts to produce a particular screw configuration (see Figure 1). We used an LDPE/PS (85:15 ratio) polyblend and compared the results to the extrusion of LDPE, PP, and PS. To study polymer behavior along the screw axes, we relied on the screw ‘pull-out’ technique. That is, after the machine had reached a steady state, screw rotation was stopped, and the barrel was quickly cooled to ambient temperature. Then, after slightly increasing the barrel temperature to the polymer melting point, the screws were pulled out from the barrels. We then investigated the melting mechanism by stripping polymer samples from screws (see Figures 2 and 3).
We found that the LDPE/PS polyblend pellets formed a pellet bed of sorts, which decreased in length along the screw axis. This indicated that our polyblend melts according to the mechanism determined by White and Wilczynski for polyolefins.2, 3 That is, the pellets are dragged into the calendering gap where they are melted by calendering action between the flight of the first screw and root of the second. The molten polymer is then expelled from the gap and pushed against the pellet bed, which is continuously dragged into the gap. This results in decreased lengths of the pellet bed.
We also found that in our LDPE/PS polyblend, the PS was dispersed in the matrix. The polyethylene has a lower melting temperature and melts first, encapsulating the PS pellets. This delays the softening and fusing of the PS granules in the polyblend further along the screw channels, which may result in poor product quality.
With these observations in hand, it is possible for us to model the extrusion of polyblend materials, which usually includes descriptions of melting and melt flow. Additionally, extrusion modeling of polyblends requires an evaluation of the morphology development. According to our observations, our LDPE/PS polyblend melted in the non-fully filled region of the screws. The molten material then flowed into the fully filled region. The melting region of the PS dispersion in the LDPE matrix and that of fully molten LDPE/PS polyblend might be discussed separately.
In summary, we studied the melting behavior of an LDPE/PS polyblend in a closely intermeshing counter-rotating twin screw extruder. In future, we will use our observations related to the polymer melting behavior and filling of the screw channel to develop a computer model for the counter-rotating twin screw extrusion to predict process behavior.
- Twin Screw Extrusion, Elsevier Science Ltd., 1978.
- K. Wilczynski and J. L. White, Experimental study of melting in an intermeshing counter-rotating twin screw extruder, Int'l Polym. Proc. 16, pp. 257-262, 2001.
- K. Wilczynski and J. L. White, Melting model for intermeshing counter-rotating twin-screw extruders, Polym. Eng. Sci. 43, pp. 1715-1726, 2003.
- D. Wang and K. Min, In-line monitoring and analysis of polymer melting behavior in an intermeshing counter-rotating twin-screw extruder by ultrasound waves, Polym. Eng. Sci. 45, pp. 998-1010, 2005.
- K. Wilczynski, Q. Jiang and J. L. White, A composite model for melting, pressure, and fill factor profiles in a metered fed closely intermeshing counter-rotating twin screw extruder, Int'l Polym. Proc. 22, pp. 198-203, 2007.
- M.-H. Hong and J. L. White, Fluid mechanics of intermeshing counter-rotating twin screw extruders, Int'l Polym. Proc. 13, pp. 342-346, 1998.
- M. H. Hong and J. L. White, Simulation of flow in an intermeshing modular counter-rotating twin screw extruder: non-Newtonian and non-isothermal behavior, Int'l Polym. Proc. 14, pp. 136-143, 1999.
- K. Wilczynski, A. Lewandowski and K. J. Wilczynski, Experimental study of melting of LDPE/PS polyblend in an intermeshing counter-rotating twin screw extruder, Polym. Eng. Sci.. in press