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CHARACTERIZATION OF THERMAL CONTACT IN INJECTION MOLDING VIA THE COMBINATION OF AN INFRARED HOLLOW WAVEGUIDE SYSTEM AND A TWO-THERMOCOUPLE PROBE A. Bendada, A. Derdouri, M. Lamontagne, and Y. Simard National Research Council, Industrial Materials Institute, Boucherville, Quebec, Canada Abstract: Flow of a molten polymer within the confining walls of a mold during injection molding is a complex phenomenon. The complexity stems from various sources: transient process, material nature, thermal effect, phase change, shrinkage, among others. Numerical simulation of this type of flow is very helpful in optimizing the mold design and processing conditions, but generally lacks appropriate knowledge of the thermal contact resistance (TCR) that is characteristic of the heat transfer across the interface between the polymer and mold-cavity. In this work, we aim to analyze the nature of thermal contact between polymer and mold through the different phases of a typical injection-molding cycle. The key idea is the combination of two measurement techniques: an infrared hollow waveguide device is employed for the first time to monitor the temperature at the surface of the polymer stream within the cavity, while a two-thermocouple probe is used to determine via an inverse heat conduction processing the heat flux crossing the polymer-mold interface and the temperature at the cavity surface. In a second part of the work, the data obtained with the infrared and the two-thermocouple instruments are used to determine TCR at the polymer-mold interface. The results show that TCR between polymer and mold is not negligible and not constant with time. The effect of packing pressure, melt and mold temperatures are investigated and discussed. Introduction: Injection molding of thermoplastics objects is a cyclic fabrication method that involves complex fluid flow and heat transfer coupled with phase change. Although, the same steps of the process are repeated regularly, they are inherently transient. First, an accurately sized shot of a pre-heated molten polymer is rapidly injected through a small orifice or gate in a cold mold cavity having a more or less intricate geometry. As thermoplastics, and more importantly semi-crystalline thermoplastics, shrink while cooling, a packing/holding pressure is applied to compensate for shrinkage by adding more molten material until the gate is frozen. When solidification of the injected polymer is deemed sufficient, the mold is opened and the part is ejected. Finally, the mold is closed and is ready for the next cycle to start. Excellent reviews of the subject can be found in the scientific literature [1-3]. While a great deal of attention has been given to the rheological and flow aspects during the filling stage, the solidification stage has been largely less considered. Modeling and simulation of the cooling phase of a molten polymer confined in the cavity of an injection mold rely on the use of realistic boundary conditions that in turn depends on the knowledge of the mechanisms by which heat is transferred between the polymer and the metal wall of the cavity. The heat transfer across the interface is described by what is called the thermal contact resistance (TCR). In most cases, this parameter is not known with precision but is rather empirical. During the cooling phase, after the gate has frozen, the cavity relative pressure can locally drop to zero resulting in the formation of an air gap between polymer and mold due to shrinkage. This gives rise to a significant change in the heat transfer mechanism at the interface. The purpose of this article is to investigate the thermal contact between polymer and mold during a typical injection molding cycle and its evolution with the molding conditions. TCR is characterized by a sharp drop in temperature at the interface and may be defined per surface unit as TCR = (Tps - Tms) / ϕ, where Tps is the polymer surface temperature, Tms is the mold surface temperature, and ϕ is the heat flux density crossing the interface. TCR may vary with pressure and temperature, with the type of metal used to make the mold, the surface roughness and the polymer in contact with the mold. Few researchers have investigated the problem of TCR in injection molding and most have found that it changes with time during the injection cycle [5-8]. However, in some of these studies the experimental determination of TCR was done in steady state conditions or the polymer surface temperature was not measured directly. To overcome this problem, we suggest in this work a new methodology to get accurate and reliable temperature monitoring at the surface of the polymer at the interface. The key feature of the proposed method is the use of a hollow optical waveguide that is incorporated into the injection mold to transmit the thermal radiation from the target to a photon detector [9]. The other physical parameters needed for the determination of TCR, namely the mold surface temperature and the heat flux density, are indirectly obtained with the use of a specially designed two-thermocouple probe similar to the one developed by Delaunay et al. [8, 10]. The two-thermocouple probe provides temperature histories at two different locations within the mold, close to the mold surface, situated on a line normal to the cavity surface. Then, these in-mold temperatures are introduced into an inverse algorithm to determine the mold surface temperature and the heat flux density that crosses the interface. From the data provided by the infrared pyrometer and the two- thermocouple probe, it is possible to estimate the TCR evolution for various process conditions. The determination of TCR and how it is affected by process parameters such as injection pressure, injection temperature, and mold temperature are addressed. Results: Experiments were carried out on a 400-ton Husky injection-molding machine. The experimental part has the shape of a box with the main average dimensions are 330x200x180 mm and a thickness of 2.3 mm. With such dimensions, one-dimensional heat conduction through the part thickness into the mold in locations far from the part features (such as ribs and bosses) is a valid assumption. To keep the mold temperature as uniform as possible a cavity temperature controller with heated circulating oil was used. The material used in the experiments was polypropylene. It was selected because it was opaque at the spectral bandwidth of the hollow waveguide pyrometer [9] and highly sensitive to temperature and inside cavity pressure changes. Shrinkage and warpage phenomena are in general quite significant for polypropylene during injection molding [8]. Figure 1 is a close up of the relative locations of three probes that were flush mounted with the cavity surface to monitor different process parameters. The infrared waveguide probe was incorporated at a central position. On its right-hand-side at a distance of 18 mm, a D.M.E SS-405C pressure transducer was also incorporated to monitor the inside cavity pressure. The two-thermocouple probe was set at the left-hand-side of the waveguide probe at a distance of 20 mm. Data acquisition for both infrared, pressure and temperature sensors were performed at a frequency rate of 500 Hz so that rapid and sudden signal changes could be observed. Eight series of experiments were undertaken for different combinations of holding pressure, mold temperature, and melt injection temperature. The plastication conditions were set so that an injection temperature of 220 oC or 275 oC was achieved; the mold temperature was regulated at 25 oC or 50 oC, while the hydraulic pressure during holding was set to 2.5 MPa or 16 MPa. The injection rate was 11 cm/s, the packing time was 1.75 s, and the holding time was 3.5 s. The cooling time in the cycle was set relatively long, 70 s, in order to be able to investigate the thermal contact in low-temperature ranges before the point of ejection. During the experiments, the operating parameters such as injection temperature, hydraulic pressure, mold temperature, and extrusion screw movement were also carefully monitored. All the recorded tests were done after the mold reached a thermally stable condition.
Fig. 1. Images of the mobile mold side and the polymer part showing the locations of the infrared probe, the pressure transducer, and the two-thermocouple probe. Figure 1 is a close up of the relative locations of three probes that were flush mounted with the cavity surface to monitor different process parameters. The infrared waveguide probe was incorporated at a central position. On its right-hand-side at a distance of 18 mm, a D.M.E SS-405C pressure transducer was also incorporated to monitor the inside cavity pressure. The two-thermocouple probe was set at the left-hand-side of the waveguide probe at a distance of 20 mm. Data acquisition for both infrared, pressure and temperature sensors were performed at a frequency rate of 500 Hz so that rapid and sudden signal changes could be observed. Eight series of experiments were undertaken for different combinations of holding pressure, mold temperature, and melt injection temperature. The plastication conditions were set so that an injection temperature of 220 oC or 275 oC was achieved; the mold temperature was regulated at 25 oC or 50 oC, while the hydraulic pressure during holding was set to 2.5 MPa or 16 MPa. The injection rate was 11 cm/s, the packing time was 1.75 s, and the holding time was 3.5 s. The cooling time in the cycle was set relatively long, 70 s, in order to be able to investigate the thermal contact in low-temperature ranges before the point of ejection. During the experiments, the operating parameters such as injection temperature, hydraulic pressure, mold temperature, and extrusion screw movement were also carefully monitored. All the recorded tests were done after the mold reached a thermally stable condition. The temperature at the surface of the polymer stream is a required boundary condition to determine the TCR value. As previously mentioned, it was non-intrusively monitored with a hollow waveguide pyrometer that was developed for injection molding operations. Bendada et al. [9] have already described the waveguide pyrometer elsewhere and only a brief description is given here. The main component of the pyrometer is the optical hollow waveguide which was devised for our needs by Miyagi's research group from Tohoku University in Japan [11, 12]. The waveguide gathers the thermal radiation emitted from the hot polymer through a sapphire window and transmits this energy to an infrared detector. The detector converts the radiation into an electric signal and in turn transmits it via long electric cables to a signal-processing unit. The waveguide consists of silver and fluoro-carbon- polymer (FCP) films deposited on the inside of a smooth glass supporting tube. The main characteristic of this type of waveguides is their low transmission loss of the thermal energy in the mid- and far-infrared. This allows the measurement of quite low temperatures, as low as room temperature. Furthermore, by the insertion of appropriate narrow-band-pass filters in the optical path of the waveguide pyrometer, it is possible to accurately measure the polymer surface temperature. Conventional optical fiber thermometers can neither measure such low temperature ranges nor measure the polymer surface temperature. We should mention here that like most radiometric thermometers, it was necessary to find out the emissivity of the polymer under investigation. Indeed, the infrared energy radiated by an object does not depend only on its absolute temperature, but also on its emissivity. To retrieve the true absolute temperature, emissivity must be known. Since the reflectance is typically 3 % for most plastics throughout the infrared spectral region [13], according to Kirchhoff's law and conservation of radiant energy considerations [14], emissivity was considered to be around 97 % in the aforementioned measurement procedure. Besides the polymer surface temperature, the other two physical parameters required to determine the TCR value are the temperature at the surface of the cavity and the heat flux crossing the polymer-mold interface. They were both obtained via the use of the two-thermocouple probe [10]. The latter was composed of two steel half- cylinders joined side by side. These were obtained by cutting longitudinally a cylinder that was 8 mm in diameter and 130 mm long. The shape and size of the cylinder tip in contact with the polymer stream were designed to fit commonly employed probe-housing cavities in injection molds. Two E-type fine-wire thermocouples 75 µm in diameter were spot-welded inside and along the axis of the cylindrical probe at two different locations (1 and 2 mm) from the probe tip. At the interface between the two half-cylinders, a narrow slot was longitudinally machined in one half-cylinder to contain the thermocouples wires. Two thermocouples rather than a single one were utilized because the additional information could aid in more accurately estimating the surface conditions via the sequential procedure detailed thereafter. To perform perfectly non-intrusive measurements, the two- thermocouple probe was manufactured with the same steel (P20 steel grade) as the mold material and the same roughness as the cavity surface. Two 3B47-conditioning modules from Analog Devices Company amplified the thermocouple signals to a data acquisition system and control unit. The latter devices were piloted with a computer via a GPIB card. The 3B47 module allowed the automatic conversion of the monitored voltage to temperature. A signal processing software, Labtec, was used for the visualization and the exploitation of the experimental results. Close to the polymer-mold interface, heat transfer in the mold can be assumed to be one-dimensional. The temperature field can thus be described by the variable T(x, t). The mold is considered as a semi-infinite body (defined by ) initially at temperature T 0 x ≥ o; for times t > 0 there is heat generation at the mold surface (x = 0) at a rate of q(t) per unit time, per unit surface. The mathematical formulation of this problem is given as: t ∂ -λ at 0 t 0 x > , = , (2) o ∂ 2 α in x ) t , x ( T 2 ∂ = ) t , x ( T 1 ∂ ∂ x 0 0 t |
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