Test methodCharacterization of thermo-rheological behavior of polymer melts during the micro injection moulding process
Introduction
Rheological characterization of polymer melts is widely used in process monitoring, quality control, process design and simulation, and troubleshooting applications [1]. Generally, instrumentation based on rotational, capillary or slit flow is used to obtain accurate measurements at a series of set strain rates and temperatures [2]. Some work has been done by using slit/capillary dies embedded into either nozzles or moulds to test the material rheology with an injection moulding machine or an extruder [1], [2], [3], [4], [5], [6]. This method can provide rheological data associated with the same thermo-mechanical history that is experienced by materials during real process conditions, such as plastication and viscous heating.
Based on conventional injection moulding technology, micro injection moulding has been demonstrated to be the most efficient way to fabricate polymer micro components with complex shapes and products with high quality surface features [7]. However, both micro parts and micro/nano features have very high surface to volume ratios, making heat diffusion effects very significant. As a result, the polymer melt solidifies particularly rapidly. Elevated temperature and higher pressure are mainly used to fill the polymer melt into micro cavities as quickly as possible. Consequently, the polymer melt will experience very high shear rates and very high thermal gradients in the micro injection moulding process. Melt rheology under such extreme conditions controls the polymer flow behavior at the micro/nanometre scale. Based on numerical simulations, Yao et al. [8] studied the effect of size related viscosity, wall slip and surface tension on the filling of micro channels and found that micro viscosity and wall slip must be considered when channel size is smaller than 10 μm. They also found that surface tension is not important for micro injection moulding, and that viscous heating is not significant when channel size is smaller than 100 μm. Kelly et al. [2] used an embedded capillary die instead of a nozzle to obtain viscosity at extremely high shear rates. They found that when the shear rate approached or exceeded 10−6 1/s viscosity reached a rate independent plateau, and in some cases shear thickening occurred with further increases in shear rate. This was attributed to molecular structure. Chien et al. [9] developed a series of micro channels in a die that was embedded in a mould in order to study the rheological behavior of polymer melts under isothermal conditions using an injection moulding machine. They found that the viscosity of polymer melts in the micro-channels is significantly lower than that measured when using a conventional capillary rheometer. They also found that wall slip velocity increases with decreasing micro-channel size and that there is a large reduction in apparent viscosity when the size of a micro-channel decreases. Similar work by Vasco et al. [10] on the thermo-rheological behavior of polymer melts in micro channels, when mould temperature is lower than melt temperature, indicated that heat transfer analysis of conventional injection moulding is not applicable to a very thin micro injection moulding with a very large surface to volume ratio. Although these studies give some insight into polymer rheology behavior at the micrometre scale, they either focused on viscosity measurement under assumed isothermal conditions or they did not separate the effects of plastication, wall slip and non-isothermal conditions when analysing rheological behavior.
In the present work, the rheological behavior of polymer melts was measured with several adjustable dumbbell slit dies with thicknesses ranging from 600 μm to 200 μm using a commercial micro injection moulding machine and a process monitoring system. By recording the pressure drop and cross-section of the gauge length of the dumbbell parts, viscosity was calculated according to the slit flow model. By varying the machine process parameters, the dependence of viscosity on slit thickness was quantified along with the effects of plastication on viscosity. Based on a dimensionless analysis and the power-law slip model, it was possible to establish the influence of non-isothermal processes, slit thickness and wall slip on viscosity.
Section snippets
Slit flow model
A slit is defined as a straight, rectangular channel having width, w, which is much greater than its thickness, h. The effect of edges on pressure drop is ignored. In the measurement, the polymer melt is forced by a piston to flow though a slit die, as shown in Fig. 1. With the assumptions of a fully developed steady state laminar flow with no-slip on the wall, viscosity can be calculated by monitoring the amount of polymer exiting the slit die per unit time (Q) for a given pressure drop (ΔP)
Rheology monitoring system
The viscosity of polymer melts was measured with an insert mould utilising a reciprocating micro injection moulding machine, Fanuc Roboshot S-2000i 15B, which was equipped with a 14 mm diameter injection screw. We designed a series of adjustable dumbbell mould cavity inserts to form the mould cavities with depths of 600, 500, 400, 300 and 200 μm, as shown in Fig. 2(a) and (b). Steel and bulk metallic glass (BMG) were used as mould insert materials, as shown in Fig. 2(c). Pebax 7233 (Arkema
Conventional rheological behavior of polymer melts
The rheological data for Pebax 7233 at temperatures of 200 °C, 230 °C and 260 °C, as provided by the material supplier, is shown in Fig. 8 [23]. Pebax is a thermo-rheologically simple polymer and its viscosity-temperature relationship can be described by the Arrhenius equation,where A is a resin dependent constant, T is melt temperature, R is the gas constant and E is the flow activation energy. Based on data fitting at high shear rates (>1000 1/s), the flow activation energy E
Conclusions
In the present work, a measurement system to quantitatively evaluate the rheological behavior of polymer melts under a typical micro injection moulding process was established. The flow curves of a slit dumbbell cavity with thicknesses ranging from 600 μm to 200 μm were monitored. Three dimensionless numbers, Pe, Gz and Br, were used to evaluate the non-isothermal process that the material experienced during the actual moulding process. Four important observations can be made about the
Acknowledgments
The authors gratefully acknowledge financial support from Enterprise Ireland (Grant No. CFTD/07/314), the Chinese Scholarship Council, and University College Dublin. We also acknowledge Dr. Jingsong Chu (now President of Micromoulding Solutions Inc., Canada) for designing the mould and the inserts.
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