Evaluating the use of functionally graded materials inserts produced by selective laser melting on the injection moulding of plastics parts V E Beal1 P Erasenthiran2 C H Ahrens1 and P Dickens2 1Universidade Federal de Santa Caterina Floriano polis Brazil 2Wolfson School of Mechanical Engineering Loughborough University Loughborough UK The manuscript was received on 19 October 2006 and was accepted after revision for publication on 9 March 2007 DOI 10 1243 09544054JEM764 Abstract The demand for productivity and shape complexity on the injection moulding industry necessitates new research to improve tool design material and manufacturing A research field is the development of functionally graded materials FGMs to build injection moulds For example moulds built with the FGMs technique can have distinctive regions with higher heat conduction Higher rates of heat transfers from thicker regions of the injected part can be useful to produce better and cheaper injection moulded polymer parts It is possible to obtain moulds with differential conductivity by adding locally during the fabrication of the mould copper to the mould base material such as tool steel In this work an investigation into the effect of FGM copper Cu tool steel mould insert over polymer injected parts is presented The work is divided in two parts a numerical thermal analysis comparison between Cu tool steel graded and tool steel inserts and an injection moulding experiment with comparisons between mould surface temperature and degree of crystallinity of polypropylene parts The numerical model was used to compare different behaviour of the mould heat transfer according to the mould insert material Thereafter a bolster was built to hold FGMs and tool steel inserts obtained by a selective laser fusion process Polypropylene was injected over the inserts to compare with the numeric results To observe the effect of the cooling rate in the polypropylene parts using the graded inserts the degree of crystallinity of the parts was measured by differential scanning calorimetry DSC test The temperature of the mould was also evaluated during the injection cycles The results showed that the graded Cu tool steel inserts tested had lower capacity to store heat energy As Cu was added to the tool steel the mixture proved to transfer heat more efficiently but it had less capacity to absorb heat Keywords functionallygradedmaterials injectionmoulding rapidmanufacturing polypropylene crystallinity 1INTRODUCTION The benefit of injection moulded parts depends on three general aspects tool cost injection mould ing raw material and productivity of the tool This trio makes it difficult to change part mould design without affecting productivity and material Hence durability of the tool productivity and costs must be achieved by the optimal material raw selection and part and mould designs 1 Unfortunately there are restraints that make it difficult to find the best com promising solution As the complexity of a modern injection mould is high the mould designer s concern is how to solidify the part without causing distortions and keeping the mould with high rates of parts pro duced per hour A complex channels network is designed to enable cooling liquid to extract the heat from the mould The design of the channels is difficult as it is necessary to keep the ejection system in place Corresponding author Engenharia Meca nica CIMJECT Universidade Federal de Santa Catarina Caixa Postal 476 Campus Universita rio Trindade Floriano polis Santa Catarina 88040 900 Brazil email valterbeal JEM764 IMechE 2007Proc IMechE Vol 221 Part B J Engineering Manufacture 945 Ejector pins slides and air stream gates are used to eject the part from the mould cavity avoiding marks in the aesthetic side of the part Depending on the complexity and shape of the part the space left by the cooling system is small and it is not feasible to manufacture without leaving marks in the mould imp ression In many cases when the heat extraction equi librium for homogeneously extracting the heat from cavity and productivity are not achieved it might be necessary to redesign the part geometry to fit mould limitations An alternative to solve complex thermal issues is the use of copper beryllium Cu Be inserts 1 As Cu Be inserts have higher thermal conductivity than the usual steel alloys they are used to extract heat from regions where the cooling channels do not have an effect during the injection moulding cycles Nevertheless they are not environmentally friendly as beryllium is cited as a highly carcinogenic element 2 Another limitation is that inserts in the mould impression might leave marks in the part as the mould surface has a visible interface between the base material and the insert In addition the insert features that are needed to attach it to the base contribute to reducing the space left for the cooling channels In the mid 1980s new manufacturing technologies known as SFF solid free form fabrication emerged 3 The main difference of these technologies as oppo sed to the traditional ones was that they were based on the layer additive principle Also known as rapid prototyping RP these technologies can produce parts in low volume production in virtually any form or material The variety of available materials is limited however RP processes can build parts in metals ceramics and polymers 3 RP technologies are highly automated and they are also called three dimensional 3D printers as the machines almost print solid parts from data generated from compu ter software Designers and engineers can build and verify designed parts without misunderstandings inaccuracies and delays The basic principle of RP technologies is to build layer by layer material cor responding to the data of the designed part Raw materials can be liquid resins wires pastes powders and sheets The way to shape these materials and bond layers can be diverse including ultraviolet lasers lamps power lasers spray of glue deposition of fused material and others These additive layered manufacturing technologies LMTs have also been used to produce tools for injection moulding Depend ing on the technology and material used in cons truction the complexity of the mould impression injected part and the injected material these moulds can be competitive to traditional cast milled moulds It is possible to build moulds from 12 to 10000 parts according to the technology material and application 4 5 One interesting technique used with SFF to build injection moulds is conformal cooling channels The channels are designed in the mould impression without the concerns of the lim itations from the traditional manufacturing method The conformal cooling channels might follow the mould impression surface passing by the ejector sys tem with fewer limitations than the usual moulds Unfortunately they are still limited by the ejection system and some part features such as deep groves might not be affected by the cooling capabilities of the channels To overcome some of these constraints it is possible to use functionally graded materials FGMs to build injection moulds by SFF technologies FGMs have been the subject of research for the last 25 years 6 Most of the natural materials such as minerals and tissues have a gradual change from one functional region to another This example of nature inspires integrated form and function design all in the same component unit FGM is not comple tely new to the manufacturing processes but it was only after the 1980s that it started to receive more attention and to be classified as a specific research subject The basic idea of FGM is to improve the properties of the part by varying the quantity of an ingredient in specific regions in order to achieve dif ferential properties An ingredient could be a basic element such as carbon being used to increase the hardness of a steel part only at the surface Another example is the porosity variation from the outside to the inside of the mammal s bones The low porosity from the outside increases the stiffness of the bone but provides interconnectivity to the inside The core of the bone is porous thereby allowing weight efficiency By using this variation from one material to another optimized components can be obtained Reduced number of joints and fasteners weight reduction structural enhancement differential heat extraction thermal barriers embedded sensors and biocompatible implants are some of the potential advantages of using FGM 6 8 FGMs also could gradually join dissimilar materials with different properties in the same component The principle is similar to composite materials The difference is that composites have distinctive phases and do not vary their composition in the volume of the compo nent Despite the idea of FGMs being very simple most of the potential FGM applications are restric ted to technological limitations and high cost Diffi culties in controlling and depositing the gradient composition and producing complex shapes with computer aided design CAD computer aided man ufacturing CAM and finite element analysis FEA integration are some of the causes for restrictions of use The use of RP technologies to produce FGM parts has been investigated by many researchers 9 Since RP technologies can produce free form parts and can handle different materials it is possible to use them 946V E Beal P Erasenthiran C H Ahrens and P Dickens Proc IMechE Vol 221 Part B J Engineering ManufactureJEM764 IMechE 2007 to produce FGM components Most of the research ers investigating the fabrication of FGM by LMTs pro cess the materials with the heat source delivered by a laser beam As lasers can be easily automated and can deliver high energy densities with precision and speed they can process almost any material 10 Another aspect of FGM and RP is the frequent use of materials in the form of powder to be fused or pre sintered under a laser spot The main issue for using rapid processing and manufacturing technolo gies for producing FGM parts is the local composition control LCC This regards the principle for adding and joining the materials by controlling their percen tages on each region of the part or layers Some researchers 11 12 used miniature hopper nozzles and capillary tubes to control the deposition of pow ders in the layer Ensz et al 13 studied the optimiza tion of two powder flows in the laser engineering net shaping LENS process to build gradients from H13 to M300 steel alloys In addition computational methods to represent the graded geometry have been the subject of study Cho et al 14 investigated the LLC for the 3D printing process after obtaining geometry and material data from finite element and voxel space geometries By this method it was possi ble digitally to represent the 3D part with different volumetric gradients The idea of adding an extra functional material to a base material to produce a FGM injection mould has been researched in previous work 15 A multi compartment hopper was used to produce graded structures of H13 tool steel and Cu The H13 is commonly used as material for injection moulds as it has dimensional stability toughness and wear resistance at high temperature Nevertheless the heat conduction of this material is low compared with Cu kH13 24 3W m K kCu 385W mK 16 17 Elemental Cu powder was mixed with H13 in propor tions of 12 5 25 37 5 and 50 wt to produce FGM bars The method for producing these bars was the selective laser fusion melting SLF or SLM using a high powder Nd YAG pulsed laser The laser pro cessed the multi composition powder bed that was previously loaded with powders from the multi compartment hopper As the laser scanned the powder bed the powder was fused and bonded to the previous added layers After processing a layer the powder bed was lowered and the powders were spread over the previous layer and the laser was set to fuse the powder to form a new layer This process was numerically controlled and continued until the completion of the part The fusion process using this laser left a rough superficial aspect and required some post processing including the removal of the substrate that was used to bond the first layers of the part to the powder bed platform At the end of the process graded parts of H13 and Cu could be obtained Therefore FGMs could be used on injec tion moulds to create high heat conductivity regions to improve heat extraction As the cooling heating channels can be limited by manufacturability and the ejector system some regions of the impression could be over heated This differential heat extraction from the part might cause warpage sink and cold welding marks and poor surface quality and could reduce the production rate of the part Another application of FGM on moulds is to build the cavity edges with gradients of tool steel and tungsten car bide This could improve the part quality by red ucing defects such as flashing caused by wear in the mould edges 1 The use of FGM to obtain performance injection moulds was one of the stimuli for this research Despite the limitations of the laser and layer deposi tion systems used in this work these experiments were planned to evaluate the influence of the Cu addition to the H13 matrix The effect of the addition of Cu on the mould temperature and on the injected polymer part crystallinity degree compared with the H13 base material was analysed In theory the addi tion of Cu would increase the thermal conduct ivity of the mould The work was divided in two parts numerical modelling of the heat transfer and experimental injection moulding The first part pre sents the numerical model of the heat transfer from the injected part to the mould and the metallic inserts The model evaluated the temperature time stamp simulating mould inserts in different materi als H13 Cu and H13 50 Cu In the injection moulding experiment FGM bars mould inserts were manufactured by laser fusion and placed in a stereolithography SL mould Polypropylene PP parts were produced by injecting the polymer over these metallic inserts Two outputs were analysed from this experiment temperature of the mould sur face and crystallinity degree of the PP parts The temperature of the mould was measured by thermo couples in the exact same position taken in the numerical model The degree of crystallinity of the parts moulded with different inserts was analysed by differential scanning calorimetry DSC The DSC test was performed to identify if the parts moulded over different inserts had different cooling rates As a consequence the degree of crystallinity of the parts could be different too The lower the cooling rate the greater is the organization of the polymer chains reflecting in the crystallinity degree of the PP A rapid cooling rate helps the polymer to hold an amorphous structure When heating a plastic more heat will be necessary to dissolve the crystals more stable and lower energy state until the plastic is completely melted This phenomenon can be seen in the DSC curves measuring the energy absorbed by the sample before melting 18 Evaluating the use of functionally graded materials inserts947 JEM764 IMechE 2007Proc IMechE Vol 221 Part B J Engineering Manufacture 2METHODOLOGY 2 1Numerical model The injection moulding cycle is a transient phenom ena and thermal conductivity is not the only material property that counts when analysing the heat trans fer Density and specific heat capacity also determine the capability of the material to store or to transport energy 19 Considering volume control the energy state is obtained by the balance of the energy that is absorbed generated and lost This variation of the energy accumulated by the mass inside the volume can be modelled by equation 1 The energy that enters Ein plus the energy generated Egen inside the volume minus the energy lost Eout to the sur roundings is equal to the variation of energy E of the mass inside the volume with respect to time t Ein Eout Egen dE dt vc 1 In the case of a mould in the moments after the melted material fills the mould impression there is no heat generated in defined volume control Consid ering the heat transfer in one direction equation 1 becomes further simplified for the heat flux through an area A generating equation 2 Simplifying the area equation 3 is generated In these equations q00 represents the heat fluxes r is the material density cpsymbolizes the specific heat T is the temperature and x is the axis of the direction of the heat flux qin 00 A qout 00 A 0 Z x rcp T t Adx 2 qin 00 qout 00 Z x rcp T t dx 3 The temperature in the mould away from the impression could be considered constant Taking this into consideration in a very short period the heat fluxes can be considered constant and can be described as in equation 4 where k is the heat conduction coefficient q00 k T x x 4 There is no easy solution for solving equations 3 and 4 and a numerical model is usually necessary to solve them for complex shapes A two dimensional 2D model of an injection moulded part inside a SL mould in contact with a metallic insert is shown in Fig 1 This model considered no contact resistance between the parts moulds and insert surfaces The initial conditions were that the temperature in the nodes inside the area that represents the hot PP injected part was 195 C and the temperature for all other nodes including the connected nodes of the part with other areas was 20 C Temperature was cal culated by employing a quadrangular mesh formed by planar four node elements The nodes chosen to be analysed are indicated in Fig 1 Thermocouple temperature Ttc matched the same position in the experimental work and insert surface temperature Tis matched the region from where DSC samples were taken in the PP part The model and analysis were performed using Ansys software For inputting the material properties density specific heat and thermal conductivity in the numer ical model tabled values were used for the H13 16 However the H13 50 per cent mate
免责声明：
1. 《外文翻译--对通过塑料注射成型零件的选择性激光融化生产得到的功能梯度材料***棒的评估 英文版》内容来源于互联网，版权归原著者或相关公司所有。
2. 若《86561825文库网》收录的文本内容侵犯了您的权益或隐私，请立即通知我们删除。
