- Email: [email protected]
Design and optimisation of conformal cooling channels in injection moulding tools
Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Design and optimisation of conformal cooling channels in injection moulding tools D.E. Dimla a, ∗ , M. Camilotto b , F. Miani b a
School of Design, Engineering and Computing, Bournemouth University, 12 Christchurch Road, Bournemouth, Dorset BH13NA, UK b DIEGM, Universit` a Degli Studi di Udine, via delle Scienze 208, 33100 Udine, Italy
Abstract With increasingly short life span on consumer electronic products such as mobile phones becoming more fashionable, injection moulding remains the most popular method for producing the associated plastic parts. The process requires a molten polymer being injected into a cavity inside a mould, which is cooed and the part ejected. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. The main objective of this study was to determine an optimum and efficient design for conformal cooling/heating channels in the configuration of an injection moulding tool using FEA and thermal heat transfer analysis. An optimum shape of a 3D CAD model of a typical component suitable for injection moulding was designed and the core and cavity tooling required to mould the part then generated. These halves were used in the FEA and thermal analyses, first determining the best location for the gate and later the cooling channels. These two factors contribute the most in the cycle time and if there is to be a significant reduction in the cycle time, then these factors have to be optimised and minimised. Analysis of virtual models showed that those with conformal cooling channels predicted a significantly reduced cycle time as well as marked improvement in the general quality of the surface finish when compared to a conventionally cooled mould. © 2005 Elsevier B.V. All rights reserved. Keywords: Tool design optimisation; Injection moulding
1. Introduction Injection moulding is one of the most exploited industrial processes in the production of plastic parts. Its success relies on the high capability to produce 3D shapes at higher rates than, for example, blow moulding. The basic principle of injection moulding is that a solid polymer is molten and injected into a cavity inside a mould; which is then cooled and the part ejected from the machine. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. As in most modern industries, time and costs are strongly ∗
Corresponding author. E-mail address: [email protected] (D.E. Dimla).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.162
linked. The longer is the time to produce parts the more are the costs. A reduction in the time spent on cooling the part before its is ejected would drastically increase the production rate, hence reduce costs. It is therefore important to understand and thereby optimise the heat transfer processes within a typical moulding process efficiently. Historically, this has been achieved by creating several straight holes inside the mould (core and cavity) and forcing a cooler liquid to circulate and conduct the excess heat away so the part can be easily ejected. The methods used for producing these holes rely on the conventional machining process such as drilling. However this simple technology can only create straight holes and so the main problem is the incapability of producing complicated contour-like channels or anything vaguely in 3D space. An alternative method that provides a cooling system that ‘conforms’ to the shape of the part in the core, cavity or both has been proposed. This method utilises a contour-like channel, constructed as close as possible to the surface of the
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
mould to increase the heat absorption away from the molten plastic. This ensures that the part is cooled uniformly as well as more efficiently. The first part of this investigation concentrates on reviewing and evaluating the injection moulding process, to set the knowledge and background on the subject. Then a study of proposed methods for developing and applying conformal channels is conducted, identify the most viable method. Specific software was used to optimise the design and construction of the mould, with attention on refining the tool design through application of finite element and thermal flow analyses. Successively, a study on the effectiveness of the conformal cooling channel based on virtual models was performed using I-DEASTM software for prototyping and simulation. The study is on going and hopefully would culminate in the suggestion of the level of proficiency required using virtual models in deciding moulding specifications for production parts.
2. Brief overview of the injection moulding process The injection moulding industry, like all industries, at present needs to reduce costs to remain competitive. This need has been addressed using various technologies ranging from design software to computer numerical control machinery. After these technologies are in place and moulding begins the cost is usually based on cycle time. Adjustments can be made to the moulding machine to help reduce the time to mould but in the final analysis the time is dictated by the ability of the mould to carry the heat away from the molten polymer. Liquid is passed through cooling channels in the mould at the required temperature. This must allow the molten polymer to flow into all sections of the cavity while at the same time remove the heat as quickly as possible. Up to now these channels have been produced by drilling which can only produce straight lines. If the channels carrying the water could be conformed to the shape of the part and their crosssection changed to increase the heat conducting area then a more efficient means of heat removal could be realised. This may also help to reduce warpage when the part is ejected, as the plastic would be cooled more uniformly.
Fig. 1. Temperature history during injection moulding [2].
2.2. Pressure control Both the injection unit and the clamping system require pressure with the latter developed to resist the former (Fig. 2). Three different pressures can be distinguished in the injection unit: initial, hold and back. All these are obtained by the action of a screw. In the clamping unit the oil pump of the hydraulic system controls the pressure needed to move the mould. Holding pressure is required to finish the filling operation and maintained during solidification to supply the shrinkage. 2.3. Time control Time is the most significant parameter in the entire operation. Cost and machine efficiency can be estimated from the cycle time. The principle temporal aspects to be controlled include: gate-to-gate time, injection time and cooling time. A simple schematic illustration of a typical cycle time is shown in Fig. 3. 2.4. Thermal proprieties Despite their large diffusion, for all plastic materials temperature range is a limit to their purpose. Both high and low temperature can create damage to plastic components. It is important to study thermal proprieties to understand and predict this behaviour. Therefore cooling times in moulding ma-
2.1. Temperature control Temperatures such as those for the molten polymer, the mould, the surround temperature and the clamping system temperature need to be controlled (Fig. 1). When molten plastic is injected in the mould it must be solidified to form the object. The mould temperature is regulated by circulation of a liquid cooler, usually water or oil that flows inside channels inside the mould parts. When the part is sufficiently cooled it can be ejected. Most (95%) of the shrinkage happens in the mould and it is compensated by the incoming material; the remainder of the shrinkage takes place sometime following the production of the part [1].
1295
Fig. 2. Pressure history during injection moulding [2].
1296
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 5. Same mould as Fig. 4 with conformal channels [3].
Fig. 3. Cycle time in injection moulding [2].
chines must be set carefully to permit, first, plasticization of the thickness and secondly dissipation of melting heat. Unlike metals, the thermal capacity of plastics is high with crystalline plastics having a higher capacity than non-crystalline. Plastics have a large coefficient of thermal expansion if compared, for example, with metals. A way to modify these values is to use mineral fillers such as fibre glass. 2.5. Cooling channels As with most manufacturing fields, production time and costs (lead and lag) are strongly correlated. The longer it takes to produce parts the more are the costs, and with injection moulding production industries cooling time is often taken as the indicator of cycle time. Improving cooling systems will reduce production costs. A simple way to control temperature and heat interchange is to create several channels inside the mould where a cooler liquid is forced to circulate. Conventional machining like CNC drilling can be used to make straight channels. Herein, the main problem is the impossibility of producing complicated channels in three-dimension, especially close to the wall of the mould. This produces an inefficient cooling system because the heat cannot be taken away uniformly from the mould and the different shrinkage causes warpage and cooling time increase (Fig. 4). On the other hand, if the cooling channels can be made to conform to the shape of the part as much as possible (Fig. 5), then the cooling system the cycle time can be significantly reduced with cooling taking place uniformly in all zones. Further-
Fig. 4. Cavity (A) and core (B) of a drilled channels mould [3].
more, if the part is ejected with the same temperature in every point the subsequent shrinkage outside the mould is also uniform and this avoids post-injection warpage of parts. Another advantage is that a mould equipped with conformal channels reaches the operation temperature quicker than a normal one equipped with standard (or drilled) cooling channels [3,4]. In this way one can reduce the time required when the moulding machine is started. When the polymer is injected, it solidifies immediately touching the wall of the mould. If the volume of the part is sufficiently big and its thickness is too small, polymer solidified can obstruct the flow and hinder a complete filling of the cavity. In this case the mould must be heated to a particular temperature in order to permit the polymer to flow. Despite all these advantages it may be noticed that new technologies involved in the production of moulding tools with conformal channels can increase initial costs for the additional complexity of the construction process.
3. Conformal channels—an overview Results from an investigation of the effectiveness of conformal channels by Ring et al. [5] through the construction of three different moulds with and without conformal cooling, showed that the latter technique led to significant improvements and a general reduction of the cycle time while ameliorating heat transfer. A contribution to understanding the importance of conformal channels and the employment of new high-conductivity materials is given by Jacobs [6]. This research showed that using nickel/copper moulds with conformal channels (copper layered) led to productivity improvements of about 70% when compared to a similar mould made with conventional steel with drilled cooling channels. A comparison between conformal channels and drilled cooling channels has also been conducted by Sachs et al. [3]. They based their investigation on modelling the core and cavity coupled with software using both techniques and proceeded to construct the moulds to compare theory and experimental data. Subsequent analysis shows that the conformal channel mould reaches operational temperature faster than the conventional one, attaining a more uniform temperature distribution with efficient heat transfer capacity. A method of controlling the moulding temperature suggested by Bayer [7] presents a means of finding the right posi-
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
tion for the cooling channels conformal to the mould surface. He also analysed the heat interchange with surroundings, essential to calculating a correct cooling system. Park and Know [8] conducted a thermal analysis of the moulding process based on modified boundary element method, joined to a design sensitivity analysis, which was shown to achieve an optimised design process in moulding. Through thermal analysis the cooling time and temperature gradient on the surface could be predicted illustrating that design sensitive analysis is a natural way to obtain an optimum mould design, especially on sizing and positioning of cooling channels. With the same instrument, optimal processing conditions in the cooling operation was found, minimising functions linked to process quality and productivity. The problem of cooling channels disposition in not only to find a way to construct them, but also to look for a method to apply this technology in all kinds of cavity shapes. A solution to overcome this issue is proposed by Xu et al. [9]. The initial object is divided in small zones easy to be analysed and for each of these a cooling channel system is constructed. Then, all the information is used to build the final tool. A similar approach for solving cooling problem is proposed by Li [10], who suggests a feature-based method where complex moulds are divided into simple shapes through a recognition algorithm. Then for each shape, a specific cooling system is constructed and at the end all of these are assembled. The algorithm is based on the “superquadrics”, a family of parametrical shapes capable of modelling features, such as those used in computer graphic. The main problem in this method is selecting the best superquadric in order to approximate the whole part. Once this is done, the cooling system becomes easy to be modelled. This approach is very useful when there are complex parts to create.
4. Mould part adviser (MPA) analysis The basic idea was to construct a virtual model using Model Master in I-DEASTM and then use its Moldflow analysis option to find the best position for the runner. Then a cooling system was designed for the part. Successively the model was ready for further analysis such as finite element analysis to refine the design, etc. MPA is a tool used exclusively on the virtual solid model of the object, to help the designer to determine the manufacturability of the parts of the mould. The only requirement of the software is the choice of the material from which the object is intended to be made from.
Fig. 6. 3D solid view of the model.
with a draft angle of 8◦ . A rib was joined to the part in the internal cavity to increase the mechanical resistance and avoid the possibility of deformation. 4.2. Gate location The best position of the gate is found by trial and error and different positions for the gate are suggested and visualise via inspection of the model part in terms of quality, weld line presence, air traps and sink (Fig. 7 shows a typical scenario with weld lines). With the gate placed in the centre of the bottom surface of either of these two solutions are possible: internal position and external position. From flow analysis it emerges that the two times for the total cycle are of the same order, but with the gate positioned on the external surface the weld lines are lower than in the internal position, especially on the external surface. The criteria that can be adopted for the choice of the position can opt for the quality surface or the production time. The gate in the external position involves more time spent in cutting the eventual track of polymer, which forms on the gate part area when this is ejected, and this can be unacceptable for production purposes. Placing the runner inside the cavity can lead to problems in creating the cooling system, as there is not much space so it can be difficult to place complex shape channels and runners. An increase in the number of gates not only does not lead to an improvement in both an improvement in cooling time and quality, but obliges the creation of a complex shape runner. So the solution of one gate runner is preferred.
4.1. The model The geometry of the model used in this exercise was chosen according to specifications and characteristics required in the object such as the inclusion of a draft angle to permit the part to be ejected easily once cooled. To create this model (Fig. 6) a rectangular surface was created and than extruded,
1297
Fig. 7. Weld lines on the model surface.
1298
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 8. Reduced weld lines on the external surface.
Fig. 11. Surface temperature.
Fig. 9. Single gate position: quality prediction. Fig. 12. Freezing time.
The following observations were made from the optimised solution of the part through computer prediction: • Significantly reduced weld lines on both the external and internal surfaces (Fig. 8) compared to Fig. 7. • Improved quality prediction (Fig. 9). • Further check on quality, cooling and sink marks was conducted. Results indicated that the position of the gate was optimum as no visible marks were evidenced and the injection time reasonable at about 1 s (Fig. 10).
• Similarly, good prediction rates were achieved for both the surface temperature and freezing time (Figs. 11 and 12). Fig. 10 essentially shows that the condition chosen from the optimisation lead to a better product as the process falls within the zone that indicates a good possibility of creating the object without problems (green area). In the same form the injection time is estimated to be 1.18 s.
5. Cooling channel positioning 5.1. General considerations
Fig. 10. Results form from Moldflow window analysis.
The final aspect of the object and the precision of its shape are determined not only by the process condition, but also by the temperature of the wall of the cavity [11]. An accurate positioning of the cooling channel system is thus needed to satisfy quality standards of the production, as the temperature of the mould must be kept high in order to permit the crystallisation of the material. The problem on positioning channels is to assure a uniform and equal temperature in both core and cavity. If there is a strong gradient in the cavity be-
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 13. Effect of the temperature gradient of the wall surface on the nonreinforced plastics [11].
tween the two halves the part may warp and distort its shape (Fig. 13). So the targets that a correct cooling system has to follow are the uniformity of the wall temperature and a gradual reduction of the polymer temperature, in order to find a compromise between the necessity of reducing cycle time and allowing for the crystallisation. 5.2. Temperature behaviour During the production cycle the temperature of the mould follows a periodic fluctuation (Fig. 14), due to several factors such as the properties of the material of the mould and the polymer. The cooling system is not able to control the amplitude of these fluctuation, but what is important is the maximum pick of temperature, reached when the flow of hot polymer arrives and touches the inside of the cavity [11]. To keep the temperature uniform some physical effects must be
Fig. 14. Fluctuation of temperature of the wall inside the cavity during the moulding cycle [11].
1299
monitored. Convex areas need high cooling because in these parts there is a concentration of heat. On the contrary concave areas need less cooling because the presence of more material helps the diffusion of heat in the mould. Thus, attention must be paid to designing corners and the cooling system in these areas. On the issue of dimensional criteria in designing cooling channels, three dimensions have to be considered: the diameter of the cross-section (or the cross-section area if not circular), the distance between channels and the distance between channel and wall of the mould. The main problems that arise when choosing these dimensions concerns the pressure losses derived from the choice of the diameter and the design of the channel. A heating/cooling relationship reported in Zollner [11] gives a guideline on the channels positioning. This states that the value resulting from the solution of the relationship should stay between 2.5 and 5% for semi-crystalline thermoplastics and between 5 and 10% for amorphous thermoplastics. 5.3. Cavity channels positioning Different solutions for the core and the cavity cooling system were suggested for this analysis, consisting of a conformal cooling system (Fig. 15) and a straight drilled cooling system (Fig. 16) for comparison. Because these parts had to be analysed after with a FEM package, only a quarter of each insert was analysed. A system of four channels was created following the surface of the object, with three channels placed to cool the lateral surface and one to cool the bottom one. 5.4. Core channels positioning The conformal channels system for the core consisted of two channels that followed the geometry of the shape (Fig. 17) with one channel cooling the upper and the short side surfaces and the second one cooling the big side surface. All corners of the channels were filleted to decrease fluiddynamic losses of the liquid cooler. In the straight channel solution one cooling line was created (Fig. 18) requiring three
Fig. 15. Conformal channel proposition for the cavity.
1300
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
This work is currently ongoing and the analysis will concentrate on the mechanical resistance of the bottom surface, where there is the maximum amount of pressure during the injection operation.
6. Conclusion
Fig. 16. Straight channel solution for the cavity.
A design and optimisation of conformal channels in cooling an injection-moulded component has been conducted using virtual prototypes. The method pursued involved constructing a 3D CAD model of the object, from which the core and cavity of the tool was created. The ensuing simulations showed that it was possible to optimise and predict the best location for such channels to reduce the cooling times when compared to straight-drilled channels. The study is on-going and hopefully would culminate in the suggestion of a level of proficiency required using virtual models in deciding moulding specifications for production parts. Further work is required to test core and cavity samples using FEA to check the mechanical resistance to the injection pressure and eventually modify the thickness of the mould. Some meshing of the object, reducing it to a surface model to use planar elements is required and this should lead to a better understanding of the cooling times between conformally cooled tools and conventional ones.
References
Fig. 17. Conformal cooling channel solution for the core.
drilling operations. In this case, it was impossible to fillet the corners, so losses of the liquid cooler are greater than in the previous case. The next step is the finite element analysis to check if parts can resist against injection pressure.
Fig. 18. Straight channels solution for the core.
[1] D.M. Bryce, Plastic Injection Moulding, Society of Manufacturing Engineers, Dearborn, MI, 1996. [2] Anon., Intelligent Systems Laboratory, Michigan State University, 1999 [accessed October 30, 2003]. http://islnotes.cps.msu.edu/ trp/inj/inj time.html. [3] E. Sachs, et al., Production of injection molding with conformal cooling channels using the three dimensional printing process, Polym. Eng. Sci. 40 (5) (2000) 1232–1247. [4] K.W. Delgarno, Layer manufactured production tooling incorporating conformal heating channels for transfer moulding of elastomer compounds, Plastic Rubber Compos. 30 (8) (2001) 384–388. [5] M. Ring, et al., An investigation of effectiveness of conformal cooling channels and selective laser sintering material in injection moulding tools, RPD (2002) 1–5. [6] F. Jacobs, High-conductivity Materials and Conformal Cooling Channels, Warwick Manufacturing Group, Warwick University, UK [accessed September 29, 2003]. http://www.nasatech.com/NEWS/ rpd399.xpress.html. [7] Anon., Rapid tooling/rapid prototyping, EGS Associates Corporation [accessed September 29, 2003]. http://www.esgn.com/services/ rapid tooling.htm. [8] S.J. Park, T.H. Know, Thermal and design sensitivity analyse for cooling system of injection mold, Part 1 and Part 2, J. Manuf. Sci. Eng. 120 (1998) 287–305. [9] X. Xu, et al., The design of conformal cooling channels in injection molding tooling, Polym. Eng. Sci. 41 (7) (2001) 1265–1279. [10] C.L. Li, A feature-based approach to injection mould cooling system design, Comput.-Aided Des. 33 (2000) 1073–1090. [11] O. Zollner, Optimised mould temperature control, Appl. Technol. Inform. (1997) 1104.
Design and optimisation of conformal cooling channels in injection moulding tools D.E. Dimla a, ∗ , M. Camilotto b , F. Miani b a
School of Design, Engineering and Computing, Bournemouth University, 12 Christchurch Road, Bournemouth, Dorset BH13NA, UK b DIEGM, Universit` a Degli Studi di Udine, via delle Scienze 208, 33100 Udine, Italy
Abstract With increasingly short life span on consumer electronic products such as mobile phones becoming more fashionable, injection moulding remains the most popular method for producing the associated plastic parts. The process requires a molten polymer being injected into a cavity inside a mould, which is cooed and the part ejected. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. The main objective of this study was to determine an optimum and efficient design for conformal cooling/heating channels in the configuration of an injection moulding tool using FEA and thermal heat transfer analysis. An optimum shape of a 3D CAD model of a typical component suitable for injection moulding was designed and the core and cavity tooling required to mould the part then generated. These halves were used in the FEA and thermal analyses, first determining the best location for the gate and later the cooling channels. These two factors contribute the most in the cycle time and if there is to be a significant reduction in the cycle time, then these factors have to be optimised and minimised. Analysis of virtual models showed that those with conformal cooling channels predicted a significantly reduced cycle time as well as marked improvement in the general quality of the surface finish when compared to a conventionally cooled mould. © 2005 Elsevier B.V. All rights reserved. Keywords: Tool design optimisation; Injection moulding
1. Introduction Injection moulding is one of the most exploited industrial processes in the production of plastic parts. Its success relies on the high capability to produce 3D shapes at higher rates than, for example, blow moulding. The basic principle of injection moulding is that a solid polymer is molten and injected into a cavity inside a mould; which is then cooled and the part ejected from the machine. The main phases in an injection moulding process therefore involve filling, cooling and ejection. The cost-efficiency of the process is dependent on the time spent in the moulding cycle. Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced. As in most modern industries, time and costs are strongly ∗
Corresponding author. E-mail address: [email protected] (D.E. Dimla).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.162
linked. The longer is the time to produce parts the more are the costs. A reduction in the time spent on cooling the part before its is ejected would drastically increase the production rate, hence reduce costs. It is therefore important to understand and thereby optimise the heat transfer processes within a typical moulding process efficiently. Historically, this has been achieved by creating several straight holes inside the mould (core and cavity) and forcing a cooler liquid to circulate and conduct the excess heat away so the part can be easily ejected. The methods used for producing these holes rely on the conventional machining process such as drilling. However this simple technology can only create straight holes and so the main problem is the incapability of producing complicated contour-like channels or anything vaguely in 3D space. An alternative method that provides a cooling system that ‘conforms’ to the shape of the part in the core, cavity or both has been proposed. This method utilises a contour-like channel, constructed as close as possible to the surface of the
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
mould to increase the heat absorption away from the molten plastic. This ensures that the part is cooled uniformly as well as more efficiently. The first part of this investigation concentrates on reviewing and evaluating the injection moulding process, to set the knowledge and background on the subject. Then a study of proposed methods for developing and applying conformal channels is conducted, identify the most viable method. Specific software was used to optimise the design and construction of the mould, with attention on refining the tool design through application of finite element and thermal flow analyses. Successively, a study on the effectiveness of the conformal cooling channel based on virtual models was performed using I-DEASTM software for prototyping and simulation. The study is on going and hopefully would culminate in the suggestion of the level of proficiency required using virtual models in deciding moulding specifications for production parts.
2. Brief overview of the injection moulding process The injection moulding industry, like all industries, at present needs to reduce costs to remain competitive. This need has been addressed using various technologies ranging from design software to computer numerical control machinery. After these technologies are in place and moulding begins the cost is usually based on cycle time. Adjustments can be made to the moulding machine to help reduce the time to mould but in the final analysis the time is dictated by the ability of the mould to carry the heat away from the molten polymer. Liquid is passed through cooling channels in the mould at the required temperature. This must allow the molten polymer to flow into all sections of the cavity while at the same time remove the heat as quickly as possible. Up to now these channels have been produced by drilling which can only produce straight lines. If the channels carrying the water could be conformed to the shape of the part and their crosssection changed to increase the heat conducting area then a more efficient means of heat removal could be realised. This may also help to reduce warpage when the part is ejected, as the plastic would be cooled more uniformly.
Fig. 1. Temperature history during injection moulding [2].
2.2. Pressure control Both the injection unit and the clamping system require pressure with the latter developed to resist the former (Fig. 2). Three different pressures can be distinguished in the injection unit: initial, hold and back. All these are obtained by the action of a screw. In the clamping unit the oil pump of the hydraulic system controls the pressure needed to move the mould. Holding pressure is required to finish the filling operation and maintained during solidification to supply the shrinkage. 2.3. Time control Time is the most significant parameter in the entire operation. Cost and machine efficiency can be estimated from the cycle time. The principle temporal aspects to be controlled include: gate-to-gate time, injection time and cooling time. A simple schematic illustration of a typical cycle time is shown in Fig. 3. 2.4. Thermal proprieties Despite their large diffusion, for all plastic materials temperature range is a limit to their purpose. Both high and low temperature can create damage to plastic components. It is important to study thermal proprieties to understand and predict this behaviour. Therefore cooling times in moulding ma-
2.1. Temperature control Temperatures such as those for the molten polymer, the mould, the surround temperature and the clamping system temperature need to be controlled (Fig. 1). When molten plastic is injected in the mould it must be solidified to form the object. The mould temperature is regulated by circulation of a liquid cooler, usually water or oil that flows inside channels inside the mould parts. When the part is sufficiently cooled it can be ejected. Most (95%) of the shrinkage happens in the mould and it is compensated by the incoming material; the remainder of the shrinkage takes place sometime following the production of the part [1].
1295
Fig. 2. Pressure history during injection moulding [2].
1296
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 5. Same mould as Fig. 4 with conformal channels [3].
Fig. 3. Cycle time in injection moulding [2].
chines must be set carefully to permit, first, plasticization of the thickness and secondly dissipation of melting heat. Unlike metals, the thermal capacity of plastics is high with crystalline plastics having a higher capacity than non-crystalline. Plastics have a large coefficient of thermal expansion if compared, for example, with metals. A way to modify these values is to use mineral fillers such as fibre glass. 2.5. Cooling channels As with most manufacturing fields, production time and costs (lead and lag) are strongly correlated. The longer it takes to produce parts the more are the costs, and with injection moulding production industries cooling time is often taken as the indicator of cycle time. Improving cooling systems will reduce production costs. A simple way to control temperature and heat interchange is to create several channels inside the mould where a cooler liquid is forced to circulate. Conventional machining like CNC drilling can be used to make straight channels. Herein, the main problem is the impossibility of producing complicated channels in three-dimension, especially close to the wall of the mould. This produces an inefficient cooling system because the heat cannot be taken away uniformly from the mould and the different shrinkage causes warpage and cooling time increase (Fig. 4). On the other hand, if the cooling channels can be made to conform to the shape of the part as much as possible (Fig. 5), then the cooling system the cycle time can be significantly reduced with cooling taking place uniformly in all zones. Further-
Fig. 4. Cavity (A) and core (B) of a drilled channels mould [3].
more, if the part is ejected with the same temperature in every point the subsequent shrinkage outside the mould is also uniform and this avoids post-injection warpage of parts. Another advantage is that a mould equipped with conformal channels reaches the operation temperature quicker than a normal one equipped with standard (or drilled) cooling channels [3,4]. In this way one can reduce the time required when the moulding machine is started. When the polymer is injected, it solidifies immediately touching the wall of the mould. If the volume of the part is sufficiently big and its thickness is too small, polymer solidified can obstruct the flow and hinder a complete filling of the cavity. In this case the mould must be heated to a particular temperature in order to permit the polymer to flow. Despite all these advantages it may be noticed that new technologies involved in the production of moulding tools with conformal channels can increase initial costs for the additional complexity of the construction process.
3. Conformal channels—an overview Results from an investigation of the effectiveness of conformal channels by Ring et al. [5] through the construction of three different moulds with and without conformal cooling, showed that the latter technique led to significant improvements and a general reduction of the cycle time while ameliorating heat transfer. A contribution to understanding the importance of conformal channels and the employment of new high-conductivity materials is given by Jacobs [6]. This research showed that using nickel/copper moulds with conformal channels (copper layered) led to productivity improvements of about 70% when compared to a similar mould made with conventional steel with drilled cooling channels. A comparison between conformal channels and drilled cooling channels has also been conducted by Sachs et al. [3]. They based their investigation on modelling the core and cavity coupled with software using both techniques and proceeded to construct the moulds to compare theory and experimental data. Subsequent analysis shows that the conformal channel mould reaches operational temperature faster than the conventional one, attaining a more uniform temperature distribution with efficient heat transfer capacity. A method of controlling the moulding temperature suggested by Bayer [7] presents a means of finding the right posi-
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
tion for the cooling channels conformal to the mould surface. He also analysed the heat interchange with surroundings, essential to calculating a correct cooling system. Park and Know [8] conducted a thermal analysis of the moulding process based on modified boundary element method, joined to a design sensitivity analysis, which was shown to achieve an optimised design process in moulding. Through thermal analysis the cooling time and temperature gradient on the surface could be predicted illustrating that design sensitive analysis is a natural way to obtain an optimum mould design, especially on sizing and positioning of cooling channels. With the same instrument, optimal processing conditions in the cooling operation was found, minimising functions linked to process quality and productivity. The problem of cooling channels disposition in not only to find a way to construct them, but also to look for a method to apply this technology in all kinds of cavity shapes. A solution to overcome this issue is proposed by Xu et al. [9]. The initial object is divided in small zones easy to be analysed and for each of these a cooling channel system is constructed. Then, all the information is used to build the final tool. A similar approach for solving cooling problem is proposed by Li [10], who suggests a feature-based method where complex moulds are divided into simple shapes through a recognition algorithm. Then for each shape, a specific cooling system is constructed and at the end all of these are assembled. The algorithm is based on the “superquadrics”, a family of parametrical shapes capable of modelling features, such as those used in computer graphic. The main problem in this method is selecting the best superquadric in order to approximate the whole part. Once this is done, the cooling system becomes easy to be modelled. This approach is very useful when there are complex parts to create.
4. Mould part adviser (MPA) analysis The basic idea was to construct a virtual model using Model Master in I-DEASTM and then use its Moldflow analysis option to find the best position for the runner. Then a cooling system was designed for the part. Successively the model was ready for further analysis such as finite element analysis to refine the design, etc. MPA is a tool used exclusively on the virtual solid model of the object, to help the designer to determine the manufacturability of the parts of the mould. The only requirement of the software is the choice of the material from which the object is intended to be made from.
Fig. 6. 3D solid view of the model.
with a draft angle of 8◦ . A rib was joined to the part in the internal cavity to increase the mechanical resistance and avoid the possibility of deformation. 4.2. Gate location The best position of the gate is found by trial and error and different positions for the gate are suggested and visualise via inspection of the model part in terms of quality, weld line presence, air traps and sink (Fig. 7 shows a typical scenario with weld lines). With the gate placed in the centre of the bottom surface of either of these two solutions are possible: internal position and external position. From flow analysis it emerges that the two times for the total cycle are of the same order, but with the gate positioned on the external surface the weld lines are lower than in the internal position, especially on the external surface. The criteria that can be adopted for the choice of the position can opt for the quality surface or the production time. The gate in the external position involves more time spent in cutting the eventual track of polymer, which forms on the gate part area when this is ejected, and this can be unacceptable for production purposes. Placing the runner inside the cavity can lead to problems in creating the cooling system, as there is not much space so it can be difficult to place complex shape channels and runners. An increase in the number of gates not only does not lead to an improvement in both an improvement in cooling time and quality, but obliges the creation of a complex shape runner. So the solution of one gate runner is preferred.
4.1. The model The geometry of the model used in this exercise was chosen according to specifications and characteristics required in the object such as the inclusion of a draft angle to permit the part to be ejected easily once cooled. To create this model (Fig. 6) a rectangular surface was created and than extruded,
1297
Fig. 7. Weld lines on the model surface.
1298
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 8. Reduced weld lines on the external surface.
Fig. 11. Surface temperature.
Fig. 9. Single gate position: quality prediction. Fig. 12. Freezing time.
The following observations were made from the optimised solution of the part through computer prediction: • Significantly reduced weld lines on both the external and internal surfaces (Fig. 8) compared to Fig. 7. • Improved quality prediction (Fig. 9). • Further check on quality, cooling and sink marks was conducted. Results indicated that the position of the gate was optimum as no visible marks were evidenced and the injection time reasonable at about 1 s (Fig. 10).
• Similarly, good prediction rates were achieved for both the surface temperature and freezing time (Figs. 11 and 12). Fig. 10 essentially shows that the condition chosen from the optimisation lead to a better product as the process falls within the zone that indicates a good possibility of creating the object without problems (green area). In the same form the injection time is estimated to be 1.18 s.
5. Cooling channel positioning 5.1. General considerations
Fig. 10. Results form from Moldflow window analysis.
The final aspect of the object and the precision of its shape are determined not only by the process condition, but also by the temperature of the wall of the cavity [11]. An accurate positioning of the cooling channel system is thus needed to satisfy quality standards of the production, as the temperature of the mould must be kept high in order to permit the crystallisation of the material. The problem on positioning channels is to assure a uniform and equal temperature in both core and cavity. If there is a strong gradient in the cavity be-
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
Fig. 13. Effect of the temperature gradient of the wall surface on the nonreinforced plastics [11].
tween the two halves the part may warp and distort its shape (Fig. 13). So the targets that a correct cooling system has to follow are the uniformity of the wall temperature and a gradual reduction of the polymer temperature, in order to find a compromise between the necessity of reducing cycle time and allowing for the crystallisation. 5.2. Temperature behaviour During the production cycle the temperature of the mould follows a periodic fluctuation (Fig. 14), due to several factors such as the properties of the material of the mould and the polymer. The cooling system is not able to control the amplitude of these fluctuation, but what is important is the maximum pick of temperature, reached when the flow of hot polymer arrives and touches the inside of the cavity [11]. To keep the temperature uniform some physical effects must be
Fig. 14. Fluctuation of temperature of the wall inside the cavity during the moulding cycle [11].
1299
monitored. Convex areas need high cooling because in these parts there is a concentration of heat. On the contrary concave areas need less cooling because the presence of more material helps the diffusion of heat in the mould. Thus, attention must be paid to designing corners and the cooling system in these areas. On the issue of dimensional criteria in designing cooling channels, three dimensions have to be considered: the diameter of the cross-section (or the cross-section area if not circular), the distance between channels and the distance between channel and wall of the mould. The main problems that arise when choosing these dimensions concerns the pressure losses derived from the choice of the diameter and the design of the channel. A heating/cooling relationship reported in Zollner [11] gives a guideline on the channels positioning. This states that the value resulting from the solution of the relationship should stay between 2.5 and 5% for semi-crystalline thermoplastics and between 5 and 10% for amorphous thermoplastics. 5.3. Cavity channels positioning Different solutions for the core and the cavity cooling system were suggested for this analysis, consisting of a conformal cooling system (Fig. 15) and a straight drilled cooling system (Fig. 16) for comparison. Because these parts had to be analysed after with a FEM package, only a quarter of each insert was analysed. A system of four channels was created following the surface of the object, with three channels placed to cool the lateral surface and one to cool the bottom one. 5.4. Core channels positioning The conformal channels system for the core consisted of two channels that followed the geometry of the shape (Fig. 17) with one channel cooling the upper and the short side surfaces and the second one cooling the big side surface. All corners of the channels were filleted to decrease fluiddynamic losses of the liquid cooler. In the straight channel solution one cooling line was created (Fig. 18) requiring three
Fig. 15. Conformal channel proposition for the cavity.
1300
D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300
This work is currently ongoing and the analysis will concentrate on the mechanical resistance of the bottom surface, where there is the maximum amount of pressure during the injection operation.
6. Conclusion
Fig. 16. Straight channel solution for the cavity.
A design and optimisation of conformal channels in cooling an injection-moulded component has been conducted using virtual prototypes. The method pursued involved constructing a 3D CAD model of the object, from which the core and cavity of the tool was created. The ensuing simulations showed that it was possible to optimise and predict the best location for such channels to reduce the cooling times when compared to straight-drilled channels. The study is on-going and hopefully would culminate in the suggestion of a level of proficiency required using virtual models in deciding moulding specifications for production parts. Further work is required to test core and cavity samples using FEA to check the mechanical resistance to the injection pressure and eventually modify the thickness of the mould. Some meshing of the object, reducing it to a surface model to use planar elements is required and this should lead to a better understanding of the cooling times between conformally cooled tools and conventional ones.
References
Fig. 17. Conformal cooling channel solution for the core.
drilling operations. In this case, it was impossible to fillet the corners, so losses of the liquid cooler are greater than in the previous case. The next step is the finite element analysis to check if parts can resist against injection pressure.
Fig. 18. Straight channels solution for the core.
[1] D.M. Bryce, Plastic Injection Moulding, Society of Manufacturing Engineers, Dearborn, MI, 1996. [2] Anon., Intelligent Systems Laboratory, Michigan State University, 1999 [accessed October 30, 2003]. http://islnotes.cps.msu.edu/ trp/inj/inj time.html. [3] E. Sachs, et al., Production of injection molding with conformal cooling channels using the three dimensional printing process, Polym. Eng. Sci. 40 (5) (2000) 1232–1247. [4] K.W. Delgarno, Layer manufactured production tooling incorporating conformal heating channels for transfer moulding of elastomer compounds, Plastic Rubber Compos. 30 (8) (2001) 384–388. [5] M. Ring, et al., An investigation of effectiveness of conformal cooling channels and selective laser sintering material in injection moulding tools, RPD (2002) 1–5. [6] F. Jacobs, High-conductivity Materials and Conformal Cooling Channels, Warwick Manufacturing Group, Warwick University, UK [accessed September 29, 2003]. http://www.nasatech.com/NEWS/ rpd399.xpress.html. [7] Anon., Rapid tooling/rapid prototyping, EGS Associates Corporation [accessed September 29, 2003]. http://www.esgn.com/services/ rapid tooling.htm. [8] S.J. Park, T.H. Know, Thermal and design sensitivity analyse for cooling system of injection mold, Part 1 and Part 2, J. Manuf. Sci. Eng. 120 (1998) 287–305. [9] X. Xu, et al., The design of conformal cooling channels in injection molding tooling, Polym. Eng. Sci. 41 (7) (2001) 1265–1279. [10] C.L. Li, A feature-based approach to injection mould cooling system design, Comput.-Aided Des. 33 (2000) 1073–1090. [11] O. Zollner, Optimised mould temperature control, Appl. Technol. Inform. (1997) 1104.