effect of mold temperature on melt front …effect of mold temperature on melt front temperature of...

Journal for Technology of Plasticity, Vol. 39 (2014), Number 2

*Corresponding author’s email: [email protected]

EFFECT OF MOLD TEMPERATURE ON MELT FRONT TEMPERATURE OF THERMOPLASTIC RESIN AT

INJECTION MOLDING

Saša Nikolić1, Saša Ranđelović1*, Mladomir Milutinović2, 1)University of Nis, Faculty of Mechanical Engineering, Serbia

2)University of Novi Sad, Faculty of Technical Science, Serbia

ABSTRACT Design of tool and process parameters of injection molding is a challenging engineering task that combines different knowledge, skills and experience of the designer. Spread of melt front within the mold cavity during injection molding is one of the parameters that have a decisive role for the accuracy and quality of the finished part. However, calculation of this parameter is very demanding particularly in case of parts with complex geometry, multi-cavity molds and asymmetric layout of different forms in the mold, i.e. when non-uniform filling occurs. FEM simulation of injection molding process and an analysis of temperature field of molten material in dependence on the temperature of the mold provide a good base for the process optimization including elimination of potential defects on the final part. Key words: Injection molding, FEM model, temperature field, tool temperature 1. INTRODUCTION Injection molding is a method for mass processing of thermoplastics, in which complex spatial forms of variable wall thickness are obtained. Technologically speaking, this procedure consists of heating the selected polymeric material to the liquid state, followed by the injection into the mold. Part is then cooled to stabilize the desired shape of the projected dimension. It is a very complex both thermodynamic and mechanical system. Therefore, successful processing of the resin and production of the part of desired performance means determination of great number of parameters, their management, monitoring and control in the specified limits. According to [1] optimization and improvement of injection molding process requires detailed knowledge of the parameters during the entire process within and around the mold cavity. The new market demands and daily occurrence of different types of polymers on the market make the design of injection molding process design more and more difficult. Application of modern

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Finite Element Method (FEM) software for the simulation of injection molding could help engineers to reduce process uncertainness and eliminate “trial and error“ procedure in the process development. In this way significant savings in money and time may be attained. Also, by evaluating different design alternatives on virtual process models one can optimize the injection molding process to produce parts at a faster rate with a higher degree of accuracy. In this paper, FEM software package Moldex3D was employed for simulation of injection molding process in which pipe fittings Ø75/45o and Ø75/90o are produced using the mold with two asymmetric cavities. The goal of the investigation was to estimate the effects of the mold temperature on the melt front temperature and the parts quality. The temperature of the mold is measured experimentally using thermal imaging camera. The melt front temperature is the temperature of melted plastic when it reached a given point in the mold cavity. This value indicates the amount of heat that is transmitted and dissipated during the injection molding process. Analyzing the melt front temperature one can identify potential problems during injection molding process and defects such as: weld lines, flow marks, hesitations and material degradation due to high temperature. For example, if the melt front temperature near the possible locations of the weld lines appearance (places where different melt fronts joining together) is lower, the weld lines will be more noticeable. Low temperature of the molted material is one of the main reasons for occurrence of the flow marks (surface defect in shape of circular ripples near the gate). Therefore, in order to check if there could be any flow marks, one should look for low melt front temperature in the runner and near the gate. The melt front temperature in combination with the melt front time (the position of melt front with respect to time during the filling stage) can be also employed to identify the hesitation problem (surface defect that results from the stagnation of polymer melt flow along a defined path) and to check whether the cause is low temperature of molted plastic. If the melt temperature exceeds the temperature range that the material can retain the sharp decline in strength as well as degradation of the polymer structure may occurs [2]. 2. DESIGN OF THE MOLD Taking into account the economic criteria for injection molding of pipe fittings Ø75/45o and Ø75/90o the mold with two (different) cavities was designed (Fig.1 and Fig.2). The parts are made from Polypropylene (PP), which is widely used because of its good mechanical and chemical properties [3, 4]. The specificity of these parts are the inner grooves for sealing mandrel. Due to asymmetric cavities, it was necessary to design different running system for cavities that distribute melted resin from the conical sprue to the gates [3]. In addition, the flow within the both cavities divides into two substreams. Result of these are different thermodynamic conditions within the mold and cavities, which can lead to filling problems and failure of parts. Moldex3D Designer module [5] was used for design of the runner and cooling systems in mold plates. The number and locations of the gates for both cavities are defined using the software adviser and the corresponding map (disposition of the best location), as the layout and dimensions of runners are created manually. In the next step, overall dimensions of both mold plates are manually adopted first, after that cooling channels are designed. The cooling system consists of several components. Regular cooling channels are usually set-up inside of mold base to maintain the temperature of the mold. In this case, a baffle type of cooling channel is adopted. This channel was drilled perpendicular to a main cooling line with a blade that separates one cooling passage into two semi-circular channels. A manifold is used to connect many cooling channels into one. It was located between mold base and temperature controller.

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Fig. 1 - 3D model of the molds

Fig. 2 – Layout of mold cavities

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3. FEM MODELING OF INJECTION MOLDING PROCESS FEM analysis of the investigated injection molding process was performed using Moldex3D Project module. Mathematical models and assumptions in the field of fluid mechanics and applied in the FEM analysis depend on the type of finite elements mesh. Moldex3D Project module for FEM analysis of injection molding process use three models of finite element mesh for the plastic continuum: solid model, shell model and e-design model [6]. The solid model is based on assumption that melted plastic is poorly compressible viscous liquid. During cavity filling stage of the processing circle of conventional injection molding process, resin acts just as a poorly compressible fluid and due to low-pressure process characteristic may be considered as incompressible Newton's fluid. By another words, it is treated as Generalized Newtonian Fluid (GNF) in the FEM analysis. Additionally, the surface tension of the melt front is neglected in this model. Therefore, non-isothermal 3D flow of the molten plastic can be mathematically described by the governing equations based on general Hele-Shaw model as follows [7]: Continuity Equation: 0u∇ ⋅ = (1) Momentum Equation: ( )2du p g

dtρ ηγ ρ= −∇ + ∇ +& (2)

Energy Equation: ( ) 2 :pTC u T k Tt

ρ ηγ γ∂ + ⋅∇ = ∇ ⋅ ∇ + ∂ & & (3)

where is: u – velocity vector [mm/s] ρ – density [kg m–3] p – pressure [Pa] η - non-Newton viscosity [Pa s] γ& - strain rate tensor g – body force vector

pC – specific heat T- temperature [K] k – thermal conductivity [W·K−1·m−2]

The viscosity of the molten polymer depends on its chemical structure, composition and processing conditions, especially temperature [8, 9]. There are many different kinds of mathematical models to describe the viscosity of thermoplastics. One of them is the Cross modified model. This model is widely employed in the standard Moldex3D database. Since the model has an exponential dependence on temperature (Arrhenius temperature model), it is also known as the Cross-Exp model [2, 10]:

( ) ( )

( )0

140

,1

nT

Tη

η γη γ τ

−=

+&

& (4)

where is:

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( )0 exp bTT B

Tη = ⋅

(5)

where: n - Power law index, 0η - the zero shear viscosity, τ - parameter of the transition region between the zero shear rate and power law shear rate in the diagram of viscosity. Small portion of the volume ( f ) is introduced to monitor the progress of the melt front as a function of time. Here, f = 0 defines the air phase, f = 1 is the phase of molten polymer, as melt front is in the range 0 <f <1. The kinematics of melt front path can be modeled by the following improved transport equation:

( ) 0f uft

∂+ ∇ ⋅ =

∂ (6)

During filling stage, flow rate, pressure, resin and mold temperature are input parameters. Boundary conditions also should be set. For the transport equation (6) of hyperbolic particle of the volume, it is necessary to define only initial boundary conditions, i.e. f = 1 for filling and f = 0 for packing stage [8]. Cooling analysis is done in software module Moldex3D Project. This module provides an accurate analysis of the entire cooling process including the efficiency estimation of cooling channel layout and the required cooling time in the design phase. For proper cooling results, many parameters mast be taken into account, such as material of the mold, types of runners (cold or hot), flow rate of the coolant, the type of coolant etc. In Moldex, the cooling analysis can be performed in two ways. The standard analysis of the cooling process means to set manually the cooling time (estimated), and based on the input value the temperature distribution is calculated (the cooling time refers to the time required for plastic part inside the mold to cool to a temperature that is sufficient to eject it from the mold [9]). Advanced analysis is based on the approach where the minimum cooling time is predicted automatically by the software. In general, the next equation (Ballman and Shusman method) may be applied for calculation of the cooling time [11]:

( )( )

2

24

lnp m wi

e w

C H T Tt

T Tk

ρππ

⋅ −= −⋅

(7)

where are: ρ- density of cooling fluid, Cp – specific heat, k - thermal conductivity, H – part thickness , Tm- temperature of the resin, Tw - temperature of the mold, Te – ejection temperature t1 – cooling time of part core to be below Te

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4. RESULTS OF THE FEM ANALYSIS The melt front temperature results obtained in the first iteration of the analyzed molding process are shown in Fig.3. As it can be seen from Fig.3, for the pipe fitting Ø75/45o the temperature distribution at the end of filling stage is very inhomogeneous over the part volume. The most significant differences are noticed in the vicinity of the parting line what means that two melt fronts meet just along the parting line and weld lines can occur here. Since the meeting angle is very small, the weld line(s) might be highly visible after the part is ejected and consequently the part strength reduced substantially. On the other hand, in case of the pipe fitting Ø75/90o the problem related to the weld lines is less present, but there is a risk of short shot because the zone of the lowest temperature coincidences with the zone of the maximum pressure drop. The melt front time results indicate that melted resin reaches this part of the cavity at the very end of the filling stage. However, being the lowest predicted temperature of the melt front is 230°C, at which the polypropylene is still in a liquid state, the short shot should not occur. Incomplete filling of the mold cavity may happen eventually as a result of too low pressure in the cavity The highest temperature of the molten front are located around the groove for sealing ring in case of Ø75/90o, and with the maximum of 241.6°C. This temperature is far below the upper values for polypropylene (270°C) recommended by the manufacturer of the material. Thus, there is no serious risk of material degradation due to increased temperature of molten polypropylene. This finding is confirmed also by shear stress analysis.

Fig. 3 - Melt front temperature at first iteration

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Second iteration was performed after changes related to the gates locations for both cavities. It resulted in more uniform distribution of the melt front temperature (Fig.4) so that differences in the temperature between different flows in both cavities are negligible even in case of “problematic” fitting Ø75/45o. Hence, the possibility of noticeable weld lines occurrence and local weakness in the molded part is reduced to minimum. In addition, for the fitting Ø75/90o there is an increase in the temperature of the workpiece area that is the most distant from the gate (part of the cavity that is filled up at the end). Consequence of this is lower viscosity of the resin, which helps better filling of the cavity. At the same time, there is no a risk of the material degradation since the temperatures ranges from 229.9°C to 247.4°C, which is far away from allowed temperature maximum (270°C).

Fig.4 - Melt front temperature at second iteration

The solution that does not require any modification of the mold geometry (dimensions of the plates, running or cooling system, etc.) may be found in adding heat insulating coat over the movable plate in order to prevent fast cooling of the plate. Another similar solution is to increase the flow rate of the cooling fluid through stationary plates, or to connect the mold to a special device and which could temper the plates separately.

5. EXPERIMENTAL RESULTS The temperature of the mold during the injection molding cycle is variable and depends on a number of influential parameters:

• temperature and types of molten thermoplastic into the mold cavity, • dimension of the mold cavity (area, volume and thickness of the walls of the mold cavity)

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• cooling time of the mold, • temperature and flow of coolant in the mold, • ambient temperature, • contact surface of the mold with a working desk of machines, • material and dimensions of the mold

Measuring the temperature of the mold and finished parts was carried out using thermal imaging camera "FLIR". Measurements were made at the very end of the injection process (circle), i.e., immediately after the mold opening and just before the ejection of the molded part. The results of the temperature measurements indicate that there is a strong interaction between the temperature of mold and the parts (Fig.5 and Fig.6). The average temperature of the resin along running system toward the mold cavity for the fitting Ø75/45o is around 88.4°C, as the warmest point has the temperature of 103°C (Fig.5). At the same time, the temperature of the molted part is less than 50°C. Therefore, the temperature of the mold plate close to the running system are higher than the temperatures of the cavity surface. For comparison, Fig.6 shows temperature distribution of the plate near running system just before the mold closing (start of the cycle).

Fig. 5 – Temperature distribution over the plate and the part (Ø75/45o) immediately after mold

opening (end of cycle)

Fig. 6 – Mold temperature near running system

(start of the cycle)

The temperatures of the outer surfaces of the movable and stationary plates recorded during the injection process differ significantly (Fig.7). The temperature of the stationary plate is higher (around 30°C) compared to the temperature of the movable plate (around 17°C). It is quite expected because the heat from the nozzle heater is transferred directly to the stationary plate. Measured temperature along the parting line is around 25°C (24.6°C – see Fig.7). The measured temperature difference may explain the results of FEM analysis (the first iteration) that predicted considerable differences in the temperature of the melt front near to the parting line and occurrence of noticeable weld lines. As indicated by FEM analysis, the difference of temperature between two melt fronts are 4-5°C for fitting Ø75/45o. The photo of the fitting Ø75/45o obtained before the modifications in the running system is given in Fig.8. As it can bee seen from Fig.8, the location of the welding line (white line under the sealing groove) coincide with the results of FEM analysis. In addition, the defect known as “flesh” or “fins” is present on the real part. It refers to the excess material that penetrates into the mold gap. .

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Fig.7 – Temperature of the outer surfaces of the mold during injection

Fig. 8 - Fitting Ø75/45o with

defects 5. CONCLUSION In this paper, FEM and experimental approach were combined in order to investigate injection molding process for fabrication of pipe fittings Ø75/45o and Ø75/90o respectively, and made from polypropylene. In the process the mold with two (asymmetric) cavities and movable inserts were used. The results of FEM analysis (MOLDEX3D software) have shown that for initial design of the running system there are significant differences in the melt front temperature between movable and stationary plates for the fitting Ø75/45o. Obtained results deviate from expected since running system were balanced and the resin flows under the same conditions (the length of the flow was the same as well as the thickness of the plates). This finding indicates an increased risk of weld lines occurrence and local weakness of the part strength around the parting line. Especially the weld lines formed within the groove area are potentially harmful. This part of workpiece is exposed to elastic deformation when removing the side inserts and thus any weakness of the part structure can cause its permanent damage. The accuracy of FEM results including weld line predicted location were confirmed experimentally. By measuring the temperature of the closed mold during injection molding, differences in the temperature were observed between the movable and stationary plates. It is seen as a main cause of the melt front temperature differences. With modification in the gate locations of the both cavities temperature field becomes more uniform and weld line less noticeable. In addition, there was a slight increase of the melt front temperature due to which the viscosity of the material decreased providing in that way better filling conditions.

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REFERENCES [1] Lei X, Longjiang S., Bingyan J.: Modelling and Simulation for Micro Injection Molding

Process, Computational Fluid Dynamics Technologies and Applications, Prof. Igor Minin (Ed.), 2011, pp.317-332

[2] http://help.plastics-u.com/online-help/molding-knowledge/standard-injection-molding. Accessed on 15.12.2014

[3] Perosević B.: Molds for injection molding of (in Serbian), Naučna knjiga, 1988. [4] Pejak M.: Polypropylene – properties and processing (in Serbian), Hipol korporacija Odžaci,

1994. [5] Moldex3D Designer® R12SP2, Core Tech System Co. Ltd. Manuel References [6] Moldex3D Project® R12SP2, Core Tech System Co. Ltd. User Manuel [7] Shen Y.K., Yeh S.L., Chen S.H.: Three-dimensional non-Newtonian Computations of Micro-

Injection Molding with the Finite Element Method. Int. Comm. Heat Mass Transfer, 2002, Vol 29, No.5, pp. 643-652, ISSN 0735-1933

[8] Kemmanno O, Weber L, Jeggy C., Magotte O.: Simulation of the Micro Injection Molding Process. Proceedings of the Annual Technical Conference (ANTEC 2000), Orlando, FL, USA, pp.576-580, ISBN 1566768551,.

[9] Liou A. C., Chen R. H.: Injection molding of polymer micro- and sub-micron structures with high aspect ratios, Int. J. Adv. Manuf. Technol., 2006Vol 28, pp.1097–1103, ISSN 0268-3768

[10] Zaikov G.E., Jimenez A.: New Developments in Polymer Analysis, Stabilization and Degradation. Nova Science Publishers, Inc.,2006.

[11] Liang J.Z., Ness J.N.: The calculation of cooling time in injection moulding. Journal of Materials Processing Technology, Vol. 57 (1996), pp. 62-64

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UTICAJ TEMPERATURE ALATA NA TEMPERATURU RASTOPOLJENOG FRONTA TERMOPLASTA PRI

INJEKCIONOM BRIZGANJU

Saša Nikolić1, Saša Ranđelović1*, Mladomir Milutinović2, 1)Univerzitet u Nišu, Mašinski fakultet, Srbija

2)Univerzitet u Novom Sadu, Fakultet tehničkih nauka, Srbija

REZIME Projektovanje alata i optimalan izbor parametara procesa injekcionog brizganja predstavlja složeni inženjerski zadatak koji zahteva kombinovanje različitih znanja, veština i iskustva. Prostiranje fronta rastopljenog materijalča unutar kalupne šupljine je jedan od parametara koji imaju odlučujuću ulogu za tačnost i kvalitet gotovog dela. Međutim preciznu sliku o širenju temperaturnog fronta i vrednosti temperature veomaje teško dobiti posebno u slučajevima delova složene geometrije, alata sa više kalupnih šupljina i nesimetričnog rasporeda tih šupljina, tj. kada postoji neujednačeni tok i ispuna kalupa. FEM simulacije procesa injekcionog brizganja i analiza temperaturnog polja rastopljenog materijala u zavisnosti od temperature kalupa omogućavaju optimizaciju procesa i eliminaciju eventualnih grešaka na finalnom delu. Ključne reči: brizganje plastike, FEM model, temperaturno polje, temperatura alata

effect of mold temperature on melt front …effect of mold temperature on melt front temperature of...

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