This chapter presents a comprehensive review of literature on the technologies of Rapid Prototyping (RP), Rapid Tooling (RT) and Injection Mould manufacturing with Rapid tooling with emphasis on the topic of Conformal Cooling within the mould. Tool or Tooling is a term used to describe a fabricated apparatus that helps in manufacturing one or more parts. The part material can be polymer or metal depending upon the process. For polymers, injection moulding process is utilized and for metallic parts pressure die casting process is used. In the current research main importance is given to the Injection Moulding Process and mould manufacturing with RT techniques with circular and profiled conformal cooling channels.
2.2 Rapid Prototyping (RP)
Rapid Prototyping (RP) is the name given to a range of technologies that can fabricate physical three dimensional objects directly from computer aided design (CAD) data. These methods are distinctive in that they add solid or liquid based materials in layers to form objects. These technologies are also known as
Additive Fabrication,
Three dimensional printing,
Solid freeform fabrication and
Layered manufacturing.
RP techniques have many advantages oven conventional fabrication and machining methods like turning or milling which are subtractive in nature as these technologies remove material to get the desired shape of the object.
The first commercial RP technology was Stereolithography or SLA. SLA was invented in 1984 by Charles W. Hull. He patented the SLA technology in 1986 under U.S patent No. 4,575,330 [1]. SLA is one of the oldest RP technologies. SLA can be used to make objects with complex geometry and with a surface finish that can be compared to machined parts. SLA parts are often used as masters to produce silicone moulds for vacuum or Room Temperature Vulcanizing (RTV) moulding. They are also used as sacrificial masters in the investment casting process. SLA parts have the advantage of good surface finish and accuracy but the parts need support structures that must be removed in a finishing operation and also SLA resins are hazardous and need careful handling [2].
Rapid prototyping (RP) technology can fabricate any three dimensional physical model regardless of geometric complexity using the layered manufacturing (LM) process. In general, the surface quality of a raw SLA-generated part is inadequate for industrial purposes due to the stair stepping effect created by the layer manufacturing process. Despite of the increased number of applications for SLA parts, this side effect limits their uses. In order to improve their surface quality, additional post processing, such as traditional grinding, is required, but post processing is time consuming and can reduce the geometric accuracy of a part. Therefore, a study by Dae-Keon Ahn and Seok Hee Lee [3] proposed a post-machining technology combining coating and grinding processes to improve the surface quality of SLA parts. Paraffin wax and pulp were used as the coating and grinding materials. By grinding the coating wax only up to the boundary of the part, the surface smoothness can be improved without damaging the surface.
After the introduction of SLA RP technology, many other RP techniques emerged with the same basic principle but with other materials for building parts. One such technology is known as Selective Laser Sintering or SLS which uses powder based materials and a laser to fuse or sinter the powder particles to form layers. Metallic powder can be used in the SLS process to form metal parts. SLS was developed and patented by Dr. Carl Deckard at the University of Texas at Austin in 1991 [4]. Later on it was commercialized by the DTM Corporation. SLS RP process has an advantage as compared to other methods of additive manufacturing that parts can be produced from a relatively wide range of commercially available powder materials. These include polymers such as nylon, or polystyrene, metals including steel, titanium, alloy mixtures, and composites. Mukesh Agarwala [5] et al. did an experimental research on the post processing of SLS parts to improve the structural integrity of the parts. They presented their results which show the effect of post-processing liquid phase sintering temperature and time on material properties. The process of hot isostatic pressing was also described in their work, and discusses its application to SLS metal parts. The results gained from using this process showed that it is suitable for achieving almost full-density parts.
A study by Kruth et al [6] was on the SLS materials. They found that for many materials, powders that show low fusion or sintering properties that can be laser sintered by adding a sacrificial binder material to the basic powder. After sintering the full part, the sacrificial binder can be removed from the so called green part in a furnace. The use of a sacrificial binder can enlarge the particles of laser sintered materials. However, the range of materials that can be laser sintered without sacrificial binder is quite large as compared to other rapid prototyping processes. No supports are needed for SLS as overhangs and undercuts are supported by the powder itself.
The Fused Deposition Modelling or FDM RP technology was developed by Scott Crump in the late 1980s and was commercialized in 1990. For FDM process, the build material is engineering thermoplastic like ABS and polycarbonate.
A study by Syed H. Masood [7] was on the process of Fused Deposition Modelling (FDM) RP process. In the study it was described that fused deposition offers the potential of producing parts accurately in a wide range of materials safely and quickly. In using this technology, the designer is often faced with a host of conflicting options including achieving desired accuracy, optimizing building time and cost and fulfilling functionality requirements. The study presented a methodology for resolving these problems through the development of an intelligent rapid prototyping system integrating distributed blackboard technologies with different knowledge based systems and feature based design technologies.
There has been little effort in developing metallic materials for the FDM process for developing metal parts and moulds. One such study was done by Masood and Song [8] presents the development of a new metal/polymer composite material for use in FDM process with the aim of application to direct rapid tooling. The material consists of iron particles in a nylon type matrix. The detailed formulation and characterisation of the tensile properties of the various combinations of the new composites are investigated experimentally. The feedstock filaments of this composite have been produced and used successfully in the unmodified FDM system for direct rapid tooling of injection moulding inserts. High quality plastic parts have been injection moulded using the inserts.
Kun Tong et al. [9] did a study on the software error compensation to fused deposition modelling (FDM). The purpose of this study was to extend the previous approaches and explore the possibilities to apply compensation by correcting slice files. In addition to applying the STL file-based compensation method from earlier research, a new approach using the slice file format to apply compensation was presented. A 3D part was built on the FDM machine and differences between its actual and nominal dimensions are used to estimate the coefficients of the error functions. A slice file compensation method is developed and tested on two types of parts as a means for further improving the error compensation for feature form error improvement. STL file compensation was applied to a FDM machine and the results were compared with a SLA machine. The two compensation methods were compared. The slice file compensation method theoretically allows higher compensation resolution, the actual machine control resolution of the FDM machine can be a limitation which makes the difference between STL compensation and slice file compensation indistinguishable. However, as the control resolution is increased, this method will make it possible to provide a higher degree of compensation.
2.3 Rapid Tooling (RT)
The technologies based on layered manufacturing (LM) techniques are extending their fields of application, from the building of functional prototypes to the production of tools and moulds for secondary applications like injection moulding. In particular, additive construction applied to the production of moulds, dies and electrodes, directly from digital data, is defined as rapid tooling (RT) [10].
For the fabrication of tools and dies, Rapid tooling (RT) is a technology for either indirectly utilizing a rapid prototype as a tooling pattern for the purposes of moulding production materials (thermoplastics), or directly producing a tool with a rapid prototyping system. These are known as direct and in-direct RT technologies. The major problems in the development of injection moulding tools include the progress in material technology and the developments in tool design methodology. It is vital to develop fast methods to manufacture tools for injection moulded prototype parts or mass-produced parts. Manufacturing of injection moulds with aluminium filled epoxy is reasonably faster in comparison with machined moulds. It is a relatively inexpensive and quick way to create prototype and production tools. If the moulds are designed properly, they can withstand the injection or the compression pressures with the use of aluminium frames. However, because of the poor thermal conductivity of the material, the process cycle time is higher due to longer cooling times [11].
Cheah et al. [12] did experimental study on the fabrication of injection moulding tool with aluminium-filled epoxy. In their research, an epoxy resin mould was evaluated and characteristics of the end product were presented. Mould fabrication is carried out via an indirect rapid soft tooling approach. In the indirect soft tooling approach, RP technology is employed to fabricate the master pattern of the desired final product before the mould halves are cast from tooling materials. The tooling material used for the study was MCP EP-250 aluminium filled epoxy resin.
Another research and development study of rapid soft tooling technology for plastic injection moulding was done by Ferreira and Mateus [13]. The main objective of their work was to propose some original approaches to integrate rapid prototyping (RP) and rapid tooling (RT) to manufacture plastic injection moulds with composite materials like aluminium filled epoxy and cooled by conformal cooling channels. The objective was to improve an algorithm for decision to assist the technology and materials selection. The different devices and features in soft tooling were demonstrated with some case studies applying RT technology to produce injection moulds for plastics.
Rapid tooling (RT) technology is basically the technology that adopts RP techniques and applies them to tool and die making. A comparative study on various RT techniques was done by Chua et al. [14]. In their study, several popular RT techniques are discussed and classified. A comparison was also made on these techniques based on tool life, tool development time and cost of tool development. The importance and advantages of rapid tooling were also discussed in the study. They also described that RT is most suitable for pre-series production. This involves manufacturing the product in its final material and by the intended manufacturing process, but in small numbers. Pre-series production is usually to test production equipment and tools and to test the market introduction of a product.
Some researchers used moulds in injection moulds fabricated with SLA process. SLA and aluminium moulds were compared in a study by Hopkinson and Dickens [15]. Comparisons were made with regard to the ejection forces required to push parts from the moulds, heat transfer through the tools and the surface roughness of the tools. Their results show that ejection forces for both types of tools are increased when a longer cooling time prior to ejection is used. The ejection forces required from a rough aluminium mould was higher than those from a smooth aluminium tool. Potential benefits of the low thermal properties of the tool were also discussed.
Another research by Ribeiro et al. [16] was on the thermal effects of SLA tools. In this work, the changes in SLA resin mechanical properties during the injection moulding were evaluated. A SLA mould was fabricated and used to inject small flat mouldings. Tensile test specimens made from SL resin were positioned in the recesses within the tool and plastic parts were injected. After injecting a predetermined number of mouldings, tensile tests were performed using the tensile test specimens. Tensile tests results showed that the thermal cycling encountered during the injection moulding process did not significantly affect the mechanical properties of the resin. Observations indicate that decrease in the temperatures encountered in the tool may lead to longer tool life.
A study by Rahmati and Dickens [17] was on the evaluation of rapid injection mould tools fabricated directly by SLA RP process. SLA epoxy tools were able to resist the injection pressure and temperature and 500 injections were achieved. The tool failure mechanisms during injection were investigated and it was found that tool failure either occurs due to excessive flexural stresses, or because of excessive shear stresses.
Due to the use of metallic materials, research has been done in using SLS for IM tooling. One such study by Barlow et al. [18] presented the mechanical properties of a new mould making material, proposed for producing injection mould inserts for plastics by selective laser sintering. It explains that, although the strength of this material is quite below that of the tool steel usually used to manufacture moulds, design calculations indicate that it can still be used for mould insert production. This was also pointed out that the thermal conductivity of this material is lower than that for steel but higher than that for plastic melts. From calculations, it was indicated that proper choices of conduction length and cycle time can minimize differences, relative to steel moulds, in the operational behaviour of moulds made of the new material. The durability of example moulds was also discussed.
An experimental study on hybrid moulds was done by Damir Godec et al [19]. Their research highlighted comparative experimental analysis for the influence of hybrid and classic moulds on the properties of moulded parts and the processing parameters. Such analysis enables the optimization of processing parameters in case of the hybrid mould. Moulded part and appropriate hybrid and classic moulds were designed and manufactured. The experimental work contains a screening design and the main central composite design for analyzing the performance of both moulds and moulded parts properties. In the study it was found that hybrid moulds can be successfully applied for production of thin-wall moulded parts with some limitations. The compressibility of prototype mould inserts was higher compared to classic inserts. The differences in thermal properties of mould inserts materials result in different moulded part properties and mould cavity wall temperature fields. These differences can be reduced by optimizing the processing parameters. They also implied that RT technologies can be usefully applied for fast production of moulds for injection moulding.
2.4 Injection Mould Cooling
Injection Moulding is a common plastic processing method and is a vast business in the worldwide plastics industry [20]. In the injection moulding process, the melted polymer is injected inside the mould cavity to take the shape of the cavity and become an injection moulded part. The part needs to be cooled before it can be ejected from the mould to avoid shrinkage and part warpage. Mould cooling, sometimes referred as mould Thermal Management, is a critical issue in plastic injection moulding process and has major effects on production cycle times that is directly linked with cost and also has effects on part quality. For this reason, cooling system design has great significance for plastic products industry by injection moulding. It is crucial not only to reduce moulding cycle time but also it considerably affects the productivity and quality of the product. Conventionally, injection moulding is cooled with the flow of a cooling medium usually water which is facilitated by the cooling channels in the injection mould.
2.5 Conformal Cooling Channels in Injection Moulds
Conventional cooling channels are normally fabricated with straight drilled holes in the mould, which have geometric and cooling fluid mobility limitations. The technique of conformal cooling is being introduced as effective alternatives to conventional cooling [21]. Thermal management of injection mould tools is very much enhanced with the application of conformal cooling channels as compared with conventional method of cooling with straight drilled holes.
The cross section of conventional cooling channels is circular due to the manufacturing technique of drilling. Rapid Tooling (RT) technologies have the capability of fabricating conformal channels which can have circular or non-circular geometries.
The concept of conformal cooling in injection mould tools has been experimented and published by various researchers.
Figure 2.1: Straight and Conformal Cooling Channels [22]
One of the bench mark studies on the technique of conformal cooling was done by E. Sachs et al [22]. The experimental study was on the fabrication of injection moulding tooling with conformal cooling channels using the 3D Printing RP process. Conformal cooling channels were incorporated both in the core and the cavity of the mould. The performance of this mould was compared against the performance of an injection mould with straight cooling channels. Thermocouples were buried in the core and cavity which showed that the conformal tool had no transient behaviour at the start of moulding, while the tool with straight channels took more than 10 cycles to come to a steady state temperature condition. The conformal tool was also found to maintain a more uniform temperature within the tool during an individual moulding cycle.
Another research by L. E. Rannar et al. [23] was on the fabrication of conformal cooling channels within tool inserts by Electron Beam Melting (EBM) Rapid Tooling (RT) process. Their main study was on the comparison, regarding cooling time and dimensional accuracy, of conventional injection mould cooling channel layouts, using straight holes and a baffle, and free-form fabricated (FFF) layout, manufactured by electron beam melting (EBM) technique. A test part was designed in order to reproduce an important issue of insufficient cooling in deep cores. The part and the different cooling layouts were analysed in an injection moulding simulation software and the numerical results were compared with corresponding experimental results. The analyses showed an improvement in both cooling time and dimensional accuracy in support of conformal FFF cooling channels manufactured by EBM, and they were obtained using a specific test part.
Another research conducted by Ada V. Villalon [24] was on the fabrication of injection mould tooling with Conformal Cooling Channels with Electron Beam Melting process. In this work, a new process for manufacturing rapid tools was proposed. It was found that using the EBM process, certain features in the mould tool can be optimized such as the cooling system that is of critical importance in the part cycle time of a tool. A heat transfer simulation study was carried out to find the effect of conformal cooling channels in the heat dissipation within a mould. Extensive experimentation was performed to obtain valuable guidelines for the design of conformal cooling channels in injection moulds manufactured via EBM technology The author also pointed out that, when the moulded part has curvature, spots closer to the cooling channels can give rise to differential cooling and consequent warping..
Xu et al. [25] did their study on the design of conformal cooling channels in Injection Moulding tools with Solid Freeform Fabrication technologies which have the capabilities to produce tooling with conformal cooling channels. They observed that tools with conformal cooling channels have demonstrated improvements in production rate and part quality as compared with conventional production tools. They presented their work on a systematic and modular approach to the design of conformal cooling channels. Their methodology was demonstrated through application to a complex core and cavity for injection moulding.
A study by Seungryeol Yoo [26] was on Profiled Edge Laminae (PEL) method, which is a thick-layer laminated Rapid Tooling (RT) method, intended for large-scale tooling applications. The advantage of RT methods is flexibility in building conformal cooling/heating channels within a mould for enhanced thermal control, but tool size is currently limited. They described that the ability to incorporate conformal channels of any geometry and routing into tools made by layered tooling methods will give tooling designers a unique flexibility with temperature control of tools. Their paper was mainly focused on the heat transfer performance of conformal channels for layered tools used in manufacturing applications with modest temperature and pressure conditions, such as thermoforming and composites forming.
Kin-Man Au and Kai-Ming Yu [27] presented a design study of variable radius conformal cooling channel (VRCCC) to achieve better uniform cooling performance. Thermal-FEA and melt flow analysis are used to validate the method. VRCCC is the cooling layout that conforms to the contours of moulding part geometry with various diameters along the coolant flow. It takes advantage of the Solid Free Form (SFF) technologies to produce a curvilinear geometry of conformal cooling channel (CCC) and integrate with changing diameters along the axis of cooling layout for the rapid tool. They proposed and verified the VRCCC design and fabrication based on contemporary SFF technologies with thermal FEA and melt flow analysis. Their analysis work indicated that heat transfer from the mould cavity surface to the coolant circulation via VRCCC has better cooling performance and higher part quality.
Another study done by Park and Pham [28] was on the designing of a conformal cooling system that facilitates uniform cooling over the entire mould surface with minimum cycle time. Their main objective was to minimize the cooling time ensuring a realistic design that will optimize the cooling system layout in terms of cooling channel size and location. Their work presented a systematic method for design of conformal cooling channels which leads to a more efficient and uniform control of the mould temperature through conformal cooling. They concluded that Solid Freeform Fabrication (SFF) processes can create injection moulding tools with complex cooling channels which can have significant improvement in production rate and part quality.
An experimental study by Saifullah et al. [29] was on a new square sectioned conformal cooling channel system for injection moulding. Simulation and experimental verification were conducted with these new cooling channels system. Experimental verification was done for a test plastic part with mini injection moulding machine. Their paper described a new comparative results based on temperature distribution on mould surface, cooling time of the plastic part and hardness number of the plastic part. Results provide a uniform temperature distribution and hardness number with reduced cooling time of the plastic part.
D. E. Dimla et al [30] presented work on the design and optimization of conformal cooling channels in injection mould tools. The emphasis of this study was to determine an efficient design for conformal cooling/heating channels in an injection mould tool using FEA and thermal analysis. 3D CAD model of a component suitable for injection moulding was designed and the core and cavity required to mould the part then generated. FEA and thermal analyses were used to determining the best location for the gate and cooling channels. These two factors contribute the most in the cycle time. Analysis of virtual models showed that those with conformal cooling channels had a significant reduced cycle time and improvement in the general quality in surface finish when compared to a conventionally cooled mould.
2.6 Crystallinity Measurement of Injection Moulded Polymer Parts
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. Many materials, such as glass-ceramics and some polymers, can be prepared in such a way as to produce a mixture of crystalline and amorphous regions. In such cases, crystallinity is usually specified as a percentage of the volume of the material that is crystalline [31].
Perhaps no fundamental property affects the physical properties of a polymer in such a way as the degree of crystallinity. Differential Scanning Calorimetry (DSC) provides a rapid method for determining polymer crystallinity based on the heat required to melt the polymer.
DSC is a technique that measures heat flow into or out of a material as a function of time or temperature. Polymer crystallinity can be determined with DSC by quantifying the heat associated with melting (fusion) of the polymer. This heat is reported as Percent Crystallinity by normalizing the observed heat of fusion to that of a 100 % crystalline sample of the same polymer. As authentic samples of 100 % crystalline polymer are rare, literature values are often used for this value [32].
The crystallinity percentage for the moulded parts from the four types of moulds that is CCCC, PCCC, CCCC with insert and PCCC with insert were measured with Differential Scanning Calorimetry (DSC) apparatus.
An experimental research by Y. Kong and J. N. Hay [33] was on the measurement of the degree of crystallinity of polymers by Differential Scanning Calorimetry (DSC). The degree of crystallinity is the single most important characteristic of a polymer because this determines critical mechanical properties such as yield stress, elastic modulus, and impact resistance. In their study, the procedures adopted and the inherent assumptions, made in the measurement of crystallinity of polymers by differential scanning calorimetry (DSC) are reviewed. The inherent problem in all DSC measurements is concurrent re-crystallization and melting of the polymer sample on heating to the melting point and the variation of the enthalpies of crystallization and melting, heat capacities and degree of crystallinity with temperature.
Another experimental study by T. Jaruga and E. Bociaga [34] was to test that if there is a difference in degree of crystallinity in injection moulded polyoxymethylene parts, manufactured in different cavities of a multi-cavity injection mould with geometrically balanced runners. The values of crystallinity degree were calculated on the basis of Differential Scanning Calorimetry (DSC) testing method results (DSC curves). They found that there are differences in crystallinity degree for parts from particular mould cavities. The reason of this is the difference in thermal conditions, specific for each cavity. It is supposed that the parts from each multi-cavity injection mould would have differences in properties. However, it is always dependent on cavities and runners layout. In their research, a particular injection mould was used; the conclusions cannot be directly extrapolated to other moulds. Higher crystallinity degree will occur in parts obtained from areas of higher mould temperature. They proved here that differences in parts properties can occur for multi-cavity injection moulds. In order to avoid this it is required to minimize the temperature differences in the mould. Geometrically balanced runners in the mould were supposed to assure the equal filling of all cavities. The results obtained by other researchers have shown that this is not always true. In this paper it was shown that not only weight of parts can differ but also other properties.
2.7 Aims and Objectives
The objective of this research is to further enhance the cooling rate of the mould with the use of Profiled Channels and Metal Inserts in an Injection Mould, fabricated using aluminium filled epoxy as the build material and using Rapid Tooling techniques. These techniques can further decrease the cooling time of moulds which directly effects the overall injection moulding cycle time resulting in the saving of time and cost.
Therefore, the purpose of this research is to do a study on
The influence of various cooling channel cross sectional profiles on the cooling time of the product
The use of metal inserts embedded within the epoxy moulds to further enhance the cooling rate of the mould.
Summary
In today’s competitive market, every industrial company is focused on delivering the product to the market faster. For the mould making and injection moulding industries, the competitive pressures are enormous. Competing on price is no longer a feasible option due to the availability of low-cost solutions. Competing on quality is again not viable, because in today's market quality is no longer a feature that provides benefit but now it's a standard expectation and requirement. The issue today is speed. The ability to help the customer achieve this objective results in a competitive edge. Rapid tooling enables the mould manufacturer and moulders to meet the customer's requirements and gain a competitive advantage. Without a rapid tooling solution and technology, a company's competitive position can be uncertain.
For this reason, more and more companies are turning towards RP and RT solutions for getting their products into market faster. These include automobile manufacturers, injection moulding companies, and consumer product manufacturers.
In this chapter, literature review for various RP and RT technologies were presented that are being used for industries these days.
