During a typical mould tool design for the thermoplastic injection process, consideration must be given to how the tool assembly would be cooled. The cooling time is a critical parameter as it can lead to increased productivity if kept or reduced to a minimum within the injection moulding cycle. Conventional cooling lines are often straight holes located as close to the wall as possible. If the geometrical shape of the part is complicated, drilled holes will not be suitable for adequate cooling. With the advent of additive manufacturing technologies, the production of mould inserts directly from 3D CAD models using metal powder is possible. This technique gives the designer limitless possibilities and freedom in channel creation and adaptation of configurations that conforms to the part shape being cooled. By so doing, time reduction and better quality can be achieved, as the part can be cooled uniformly and in a shorter period of time. Conformal cooling channels however give rise to structural integrity issues within the inserts. Correct placement, design, and cross section area are therefore needed to balance the mould strength.

The effectiveness and best channel disposition has been studied by Sach et al.¹ who pioneered the use of 3D printing processes to compare the effectiveness of conformal and conventional cooling. Meckley and Edwards² investigated the importance of balancing the mould strength while optimising the design of the cooling channels. Saifullah and Masood³ proposed using finite element analysis and thermal heat transfer analysis, in determining an optimum design for conformal cooling channels. Konsulova-Bakalova⁴ compared circular and elliptical cross sections of cooling channels concluding that it was possible to achieve a reduction in cooling time by up to 40%. Mohamed et al.⁵ compared the performance of different designs combining conformal and standard cooling channels. Shayfull et al.⁶ evaluated the performance of conformal cooling channels compared to straight drilled cooling channels in order to minimize warpage on the thin shallow parts of 1mm thickness. Park and Dang⁷ showed that conformal cooling channels were capable of improving the quality of a moulded part in terms of warpage, compared to the conventional straight drilled cooling channels. Au and Yu⁸ presented a scaffolding architecture for conformal cooling designed for rapid plastic injection moulding and examined the structural integrity of parts produced to validate the strength of the insert. They concluded that the scaffolding architecture provided more uniform cooling. Wang et al.⁹ designed an algorithm to create conformal channels automatically based on the geometrised part shape. They investigated the implementation of a geometric algorithm to a conformal cooling circuit, concluding that automatic generation conforms better to the shape of the product, resulting in a more uniform and accurate control of temperature. Saifullah et al.¹⁰ describe a square section conformal cooling channel system for injection moulding dies, with results showing a uniform temperature distribution with reduced freezing time and hence, a reduction in cycle time. Xu et al.¹¹ presented a systematic modular approach to the design of conformal cooling channels, with algorithms created to achieve the maximum heat transfer. Li¹² described a feature-based design synthesis approach to developing a cooling system design by first decomposing complex part shape into simpler shapes elements, and then developing an algorithm to generate cooling channels. Agazzi et al.¹³ determined the location, number and shape of cooling channels and the coolant fluid temperature, using morphological analysis to design the cooling channels based on the shapes of isotherms. Au et al.¹⁴ used a methodology called visibility-based cooling channel generation to propose an automatic preliminary cooling channel design for rapid tooling, where they considered cooling process between a mould surface and a cooling channel analogous. Jianguo et al.¹⁵ proposed an automatic design of injection-mould cooling channels using genetic algorithms, and generated shapes of the cooling channel are discussed and compared with manually designed cooling channels.

The plastic manufacturing industry is a huge market globally, and injection moulding is a consolidated process with a large history of usage. Still, important advances are constantly being introduced. As metal additive manufacturing techniques are fast becoming popular and economically affordable, inserts with conformal cooling channels are increasingly being fabricated at a reasonable price.

The aim of this study was to conduct a simulated comparative thermal analysis of conventional and conformal cooling system configurations for a non-constant threaded screw cap. Firstly, different parts are designed such that assembling them virtually creates a typical conventional cooling system. Secondly, a conformal cooling design is considered where the lines have a spiral internally bound contour without the need to assembly different parts together. The main objective was to compare parameters such as: part surface temperature, part body temperature, percentage of frozen layer vs. time, mould temperature, volumetric shrinkage and warpage, which in turn will determine the technique that would be the most appropriate for the chosen part.

Fig. 1(a) shows the screw cap with dimensions: 94 mm external diameters, 15.5 mm height, 1.5 mm uniform body thickness, thread thickness of 3 mm, volume of 17.5 mm³ and a mass of 14.7 grams. The difference in thickness of 100% around the thread area is a major problem in the cooling phase, which may cause warpage, cooling stresses, retard the cooling time and give rise to high values of volumetric shrinkage in the cap.

Fig. 1 
(a) Threaded screw cap part, (b) Representation of the mould insert and part (exploded view)

Fig. 1(b) shows the mould insert assembly consisting of 3 parts: the cavity, the core and the ejector. In this configuration, the moving insert is the cavity and the ejector is fixed. This distribution is due to the fact that the injection gate has to be in the bottom part of the plastic piece for aesthetic reasons; so that the injection point is not visible when the cap is in position. The injection is direct from a hot runner, which can also affect the temperature of the mould as it is in contact with the core which has a very high temperature at any instance. The complete mould is composed of 8 inserts and the 8 parts are filled at the same time utilising the same parameters.

2.1 Conventional Cooling Channels

The cheapest, easiest and most utilised option is to create straight drilled holes in the inserts, but usually this often is not the best cooling design for the part. It has also been common to insert a baffle or bubbler in the core but the area is used for the hot runner. This creates a need for a good and optimised conventional cooling system. Nevertheless, the cooling paths are much more expensive because the core has to be divided into three pieces and drilling the contour is complex. Three cooling channels are created: one in the core and two in the cavity. In Fig. 2 it is possible to see the optimised conventional cooling channels Fig. 2(a) and a basic representation section in the mould Fig. 2(b).

Fig. 2 
(a) Conventional cooling channels, (b) section representation of mould insert with conventional cooling channels

2.2 Conformal Cooling Channels

The conformal cooling channels are designed with a spiral contour. The spiral channel cooling structure was chosen over other techniques because it has safer powder removal, defined and orientated turbulent flow, and the channels do not block or clog easily.

As shown in Fig. 3, there are two cooling circuits, one each for the core and cavity. The core spiral conforms to the part and the hot runner while the cavity spiral only conforms to the part. The diameter of the channel is kept constant at 4 mm to ensure a good and turbulent flow. The distance within the mould surface and the centre of the channels is also 4 mm. However, the distance between channels varies depending on the part of the mould. This could have some structural integrity issues, hence the need to conduct a thorough structural analysis as future work to identify areas of possible improvements.

Fig. 3 
(a) Conformal cooling channels, (b) Representation of the mould insert with conformal cooling channels

This study has shown some of the benefits of using conformal cooling technique to improve the heat transfer in the injection moulding process, with the temperature in the inserts more accurately controlled. This means that the plastic part is cooled more homogeneous with significantly better part quality and better properties, such as less warpage. Lower residual stresses and shorten cycle times are consequently achieved. Moreover, it is possible to achieve the ejection temperature in the part faster, which increases the productivity. For high precision parts with very low dimensional tolerances, such as the screw cap used in this study, traditionally it would be cooled down much more slowly so as not to induce deflections during ejection. Nowadays, it is possible to obtain better part quality in a shorter time, and fewer parts are rejected for exceeding geometrical tolerances.

As 3D printed inserts become less expensive, the gain in the productivity and part properties will inevitably compensate the initial investment costs.

While conformal cooling channels absorbs maximum heat and provides a better cooling system, if considered in conjunction with pulse cooling, a better outcome could be expected. Pulse cooling process achieves very good mechanical proprieties of the part and a glossy surface of the plastic even in plastics with glass fibre.

A stress analysis could be carried out to simulate the pressures that affect the insert so it is possible to see if the separation between channels is wide enough. A fatigue analysis could be carried out to determine the life of the insert.