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 re- lies 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: dimla@bournemouth.ac.uk (D.E. Dimla). 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 drasticallyincrease the production rate, hence reduce costs. It is therefore important to under- stand 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 circu- late and conduct the excess heat awaysothe part can be easily ejected. The methods used for producing these holes rely on theconventionalmachiningprocess suchas drilling.However this simple technology can only create straight holes and so the main problem is the incapability of producing compli- cated 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 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.162 D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300 1295 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 review- ing 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 softwarewasusedtooptimisethe design and con- struction of the mould, with attention on refining the tool de- sign 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 per- formed using I-DEAS TM software for prototyping and simu- lation. 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 pro- duction 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 rang- ing from design software to computer numerical control ma- chinery. 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 nowthesechannelshave 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 cross- section 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. 2.1. Temperature control Temperatures such as those for the molten polymer, the mould, the surround temperature and the clamping system temperatureneedtobecontrolled(Fig. 1). When molten plas- tic 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]. 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 developedtoresistthe former (Fig. 2). Three different pressures can be distinguished in the injec- tion 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 opera- tion. 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 coolingtime. 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 tem- perature 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 pre- dict this behaviour. Therefore cooling times in moulding ma- 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. 3. Cycle time in injection moulding [2]. chines must be set carefully to permit, first, plasticization of thethicknessand secondlydissipationof meltingheat.Unlike metals,thethermalcapacityof plasticsis highwithcrystalline plastics having a higher capacity than non-crystalline. Plas- ticshavealargecoefficient of thermalexpansionifcompared, 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(leadandlag) arestronglycorrelated.The longerittakes to produce parts the more are the costs, and with injection moulding production industries cooling time is often taken as the indicator of cycletime.Improving cooling systems will reduce production costs. A simple way to control tempera- ture and heat interchange is to create several channels inside the mould where a cooler liquid is forced to circulate. Con- ventional machining like CNC drilling can be used to make straight channels. Herein, the main problem is the impossi- bility of producingcomplicated 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]. Fig. 5. Same mould as Fig. 4 with conformal channels [3]. more, if the part is ejectedwith the sametemperature in every point the subsequent shrinkage outside the mould is also uni- form 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 fillingof the cavity. In this casethe mould must be heated to a particular temperature in order to permit the poly- mer to flow. Despite all these advantages it may be noticed that newtechnologiesinvolved in the production ofmoulding 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 con- formal channels by Ring et al. [5] through the construction of three different moulds with and without conformal cool- ing, showed that the latter technique led to significant im- provements and a general reduction of the cycle time while ameliorating heat transfer. A contribution to understanding the importance ofconfor- mal 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 (cop- per 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 beenconductedby Sachsetal.[3]. Theybased their investiga- tion 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 anal- ysis shows that the conformal channel mould reaches opera- tional temperature faster than the conventional one, attaining a more uniform temperature distribution with efficient heat transfer capacity. A method of controlling the moulding temperature sug- gested 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 1297 tion for the cooling channels conformal to the mould surface. He also analysed the heat interchange with surroundings, es- sential 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 thesurfacecould be predicted illustratingthat de- sign sensitive analysis is a natural way to obtain an optimum mould design, especially on sizing and positioning of cool- ing 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 coolingchannelsdisposition in not only to find a way to construct them, but also to look for a method to apply this technologyin 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 simi- lar 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 para- metrical 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-DEAS TM and then use its Moldflow anal- ysis 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 ex- clusively on the virtual solid model of the object, to help the designer todeterminethemanufacturability ofthepartsofthe mould. The only requirement of the software is the choice of the material from which the object is intended to be made from. 4.1. The model The geometry of the model used in this exercise was cho- sen 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, Fig. 6. 3D solid view of the model. with a draft angle of 8 ◦ . A rib was joined to the part in the in- ternal 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 trialand 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 ontheexternalsurface.The criteriathatcanbe adoptedforthe 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. Anincreaseinthenumber ofgatesnotonlydoes notleadto 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. 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. 9. Single gate position: quality prediction. The following observations weremadefromtheoptimised 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 con- ducted. Results indicated that the position of the gate was optimum as no visible marks were evidenced and the in- jection time reasonable at about 1 s (Fig. 10). Fig. 10. Results form from Moldflow window analysis. Fig. 11. Surface temperature. Fig. 12. Freezing time. • 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 The final aspect of the object andthe 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 tempera- ture 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 1299 Fig. 13. Effect of the temperature gradient of the wall surface on the non- reinforced 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 fol- low are the uniformity of the wall temperature and a gradual reduction of the polymer temperature, in order to find a com- promise 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 fac- tors 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]. monitored. Convex areas need high cooling because in these parts there is a concentrationofheat.On the contrary concave areas need less cooling because the presence of more mate- rial 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 diame- ter of the cross-section (or the cross-section area if not circu- lar), the distance between channels and the distance between channel and wall of the mould. The main problems that arise whenchoosingthesedimensionsconcerns 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 guidelineon 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 thermo- plastics and between 5 and 10% for amorphous thermoplas- tics. 5.3. Cavity channels positioning Different solutions for the core and the cavity cooling sys- tem were suggested for this analysis, consisting of a confor- mal 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 followingthe surfaceofthe object, withthreechannelsplaced 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 fluid- dynamic losses of the liquid cooler. In the straight channel solution one cooling linewascreated(Fig.18)requiringthree Fig. 15. Conformal channel proposition for the cavity. 1300 D.E. Dimla et al. / Journal of Materials Processing Technology 164–165 (2005) 1294–1300 Fig. 16. Straight channel solution for the cavity. 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. This work is currently ongoing and the analysis will con- centrate on the mechanical resistance of the bottom surface, where there is the maximum amount of pressure during the injection operation. 6. Conclusion A design and optimisation of conformal channels in cool- ing an injection-moulded component has been conducted us- ing virtual prototypes. The method pursued involved con- structing 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 mould- ing specifications for production parts. Further work is required to test core and cavity samples using FEA tocheck the mechanical resistance tothe 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 bet- ter understanding of the cooling times between conformally cooled tools and conventional ones. References [1] D.M. Bryce, Plastic Injection Moulding, Society of Manufacturing Engineers, Dearborn, MI, 1996. [2] Anon., Intelligent Systems Laboratory, Michigan State Univer- sity, 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 cool- ing channels using the three dimensional printing process, Polym. Eng. Sci. 40 (5) (2000) 1232–1247. [4] K.W. 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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. . Journal of Materials Processing Technology 164–165 (2005) 1294–1300 Design and optimisation of conformal cooling channels in injection moulding tools D.E Ring, et al., An investigation of effectiveness of conformal cool- ing channels and selective laser sintering material in injection mould- ing tools, RPD (2002)