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Journal of Materials Processing Technology 171 2006 259 267 Design and thermal analysis of plastic injection mould S H Tang Y M Kong S M Sapuan R Samin S Sulaiman Department of Mechanical and Manufacturing Engineering Universiti Putra Malaysia 43400 Serdang Selangor Malaysia Received 3 September 2004 accepted 21 June 2005 Abstract This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould The technique theory methods as well as consideration needed in designing of plastic injection mould are presented Design of mould was carried out using commercial computer aided design software Unigraphics Version 13 0 The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial nite element analysis software called LUSAS Analyst Version 13 5 The software provides contour plot of temperature distribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions This uneven cooling effect at different regions of mould contributed to warpage 2005 Elsevier B V All rights reserved Keywords Plastic Injection mould Design Thermal analysis 1 Introduction Plastic industry is one of the world s fastest growing industries ranked as one of the few billion dollar industries Almost every product that is used in daily life involves the usage of plastic and most of these products can be produced by plastic injection molding method 1 Plastic injection molding process is well known as the manufacturing process to create products with various shapes and complex geometry at low cost 2 The plastic injection molding process is a cyclic process There are four signi cant stages in the process These stages are lling packing cooling and ejection The plastic injec tion molding process begins with feeding the resin and the appropriate additives from the hopper to the heating injection system of the injection plastic injection molding machine 3 This is the lling stage in which the mould cavity is lled with hot polymer melt at injection temperature After the cav ity is lled in the packing stage additional polymer melt is packed into the cavity at a higher pressure to compensate the expected shrinkage as the polymer solidi es This is followed Corresponding author E mail address saihong eng upm edu my S H Tang by cooling stage where the mould is cooled until the part is suf ciently rigid to be ejected The last step is the ejection stage in which the mould is opened and the part is ejected after which the mould is closed again to begin the next cycle 4 The design and manufacture of injection molded poly meric parts with desired properties is a costly process domi nated by empiricism including the repeated modi cation of actual tooling Among the task of mould design designing the mould speci c supplementary geometry usually on the core side is quite complicated by the inclusion of projection and depression 5 In order to design a mould many important designing factors must be taken into consideration These factors are mould size number of cavity cavity layouts runner systems gating systems shrinkage and ejection system 6 In thermal analysis of the mould the main objective is to analyze the effect of thermal residual stress or molded in stresses on product dimension Thermally induced stresses develop principally during the cooling stage of an injection molded part mainly as a consequence of its low thermal conductivity and the difference in temperature between the molten resin and the mould An uneven temperature eld exists around product cavity during cooling 7 0924 0136 see front matter 2005 Elsevier B V All rights reserved doi 10 1016 j jmatprotec 2005 06 075260 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 During cooling location near the cooling channel experi ences more cooling than location far away from the cooling channel This different temperature causes the material to experience differential shrinkage causing thermal stresses Signi cant thermal stress can cause warpage problem There fore it is important to simulate the thermal residual stress eld of the injection molded part during the cooling stage 8 By understanding the characteristics of thermal stress distribu tion deformation caused by the thermal residual stress can be predicted In this paper the design of a plastic injection mould for producing warpage testing specimen and for performing ther mal analysis for the mould to access on the effect of thermal residual stress in the mould is presented 2 Methodology 2 1 Design of warpage testing specimen This section illustrates the design of the warpage testing specimen to be used in plastic injection mould It is clear that warpage is the main problem that exists in product with thin shell feature Therefore the main purpose of the prod uct development is to design a plastic part for determining the effective factors in the warpage problem of an injection moulded part with a thin shell The warpage testing specimen is developed from thin shell plastics The overall dimensions of the specimen were 120 mm in length 50 mm in width and 1 mm in thickness The material used for producing the warpage testing specimen was acrylonitrile butadiene stylene ABS and the injection temperature time and pressure were 210 C 3 s and 60 MPa respectively Fig 1 shows the warpage testing specimen pro duced 2 2 Design of plastic injection mould for warpage testing specimen This section describes the design aspects and other consid erations involved in designing the mould to produce warpage testing specimen The material used for producing the plastic Fig 1 Warpage testing specimen produced injection mould for warpage testing specimen was AISI 1050 carbon steel Four design concepts had been considered in designing of the mould including i Three plate mould Concept 1 having two parting line with single cavity Not applicable due to high cost ii Two plate mould Concept 2 having one parting line with single cavity without gating system Not applicable due to low production quantity per injection iii Two plate mould Concept 3 having one parting line with double cavities with gating and ejection system Not applicable as ejector pins might damage the product as the product is too thin iv Two plate mould Concept 4 having one parting line with double cavities with gating system only used sprue puller act as ejector to avoid product damage during ejection In designing of the mould for the warpage testing spec imen the fourth design concept had been applied Various design considerations had been applied in the design Firstly the mould was designed based on the platen dimen sion of the plastic injection machine used BOY 22D There is a limitation of the machine which is the maximum area of machine platen is given by the distance between two tie bars The distance between tie bars of the machine is 254 mm Therefore the maximum width of the mould plate should not exceed this distance Furthermore 4 mm space had been reserved between the two tie bars and the mould for mould setting up and handling purposes This gives the nal max imum width of the mould as 250 mm The standard mould base with 250 mm 250 mm is employed The mould base is tted to the machine using Matex clamp at the upper right and lower left corner of the mould base or mould platen Dimen sions of other related mould plates are shown in Table 1 The mould had been designed with clamping pressure having clamping force higher than the internal cavity force reaction force to avoid ashing from happening Based on the dimensions provided by standard mould set the width and the height of the core plate are 200 and 250 mm respectively These dimensions enabled design of two cavities on core plate to be placed horizontally as there is enough space while the cavity plate is left empty and it is only xed with sprue bushing for the purpose of feeding molten plastics Therefore it is only one standard parting line was designed at Table 1 Mould plates dimensions Components Size mm width height thickness Top clamping plate 250 250 25 Cavity plate 200 250 40 Core plate 200 250 40 Side plate support plate 37 250 70 Ejector retainer plate 120 250 15 Ejector plate 120 250 20 Bottom clamping plate 250 250 25S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 261 the surface of the product The product and the runner were released in a plane through the parting line during mould opening Standard or side gate was designed for this mould The gate is located between the runner and the product The bottom land of the gate was designed to have 20 slanting and has only 0 5 mm thickness for easy de gating purpose The gate was also designed to have 4 mm width and 0 5 mm thickness for the entrance of molten plastic In the mould design the parabolic cross section type of runner was selected as it has the advantage of simpler machin ing in one mould half only which is the core plate in this case However this type of runner has disadvantages such as more heat loss and scrap compared with circular cross section type This might cause the molten plastic to solidify faster This problem was reduced by designing in such a way that the runner is short and has larger diameter which is 6 mm in diameter It is important that the runner designed distributes material or molten plastic into cavities at the same time under the same pressure and with the same temperature Due to this the cavity layout had been designed in symmetrical form Another design aspect that is taken into consideration was air vent design The mating surface between the core plate and the cavity plate has very ne nishing in order to prevent ashing from taking place However this can cause air to trap in the cavity when the mould is closed and cause short shot or incomplete part Suf cient air vent was designed to ensure that air trap can be released to avoid incomplete part from occurring The cooling system was drilled along the length of the cavities and was located horizontally to the mould to allow even cooling These cooling channels were drilled on both cavity and core plates The cooling channels provided suf cient cooling of the mould in the case of turbulent ow Fig 2 shows cavity layout with air vents and cooling channels on core plate In this mould design the ejection system only consists of the ejector retainer plate sprue puller and also the ejector Fig 2 Cavity layout with air vents and cooling channels plate The sprue puller located at the center of core plate not only functions as the puller to hold the product in position when the mould is opened but it also acts as ejector to push the product out of the mould during ejection stage No addi tional ejector is used or located at product cavities because the product produced is very thin i e 1 mm Additional ejec tor in the product cavity area might create hole and damage to the product during ejection Finally enough tolerance of dimensions is given consid eration to compensate for shrinkage of materials Fig 3 shows 3D solid modeling as well as the wireframe modeling of the mould developed using Unigraphics 3 Results and discussion 3 1 Results of product production and modi cation From the mould designed and fabricated the warpage testing specimens produced have some defects during trial run The defects are short shot ashing and warpage The short shot is subsequently eliminated by milling of additional air vents at corners of the cavities to allow air trapped to Fig 3 3D solid modeling and wireframe modeling of the mould 262 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 Fig 4 Extra air vents to avoid short shot escape Meanwhile ashing was reduced by reducing the packing pressure of the machine Warpage can be controlled by controlling various parameters such as the injection time injection temperature and melting temperature After these modi cations the mould produced high qual ity warpage testing specimen with low cost and required little nishing by de gating Fig 4 shows modi cations of the mould which is machining of extra air vents that can eliminate short shot 3 2 Detail analysis of mould and product After the mould and products were developed the analysis of mould and the product was carried out In the plastic injec tion moulding process molten ABS at 210 C is injected into the mould through the sprue bushing on the cavity plate and directed into the product cavity After cooling takes place the product is formed One cycle of the product takes about 35 s including 20 s of cooling time The material used for producing warpage testing speci men was ABS and the injection temperature time and pres sure were 210 C 3 s and 60 MPa respectively The material selected for the mould was AISI 1050 carbon steel Properties of these materials were important in determin ing temperature distribution in the mould carried out using nite element analysis Table 2 shows the properties for ABS and AISI 1050 carbon steel The critical part of analysis for mould is on the cavity and core plate because these are the place where the product is formed Therefore thermal analysis to study the temperature Fig 5 Model for thermal analysis distribution and temperature at through different times are performed using commercial nite element analysis software called LUSAS Analyst Version 13 5 A two dimensional 2D thermal analysis is carried out for to study the effect of thermal residual stress on the mould at different regions Due to symmetry the thermal analysis was performed by modeling only the top half of the vertical cross section or side view of both the cavity and core plate that were clamped together during injection Fig 5 shows the model of thermal analysis analyzed with irregular meshing Modeling for the model also involves assigning properties and process or cycle time to the model This allowed the nite element solver to analyze the mould modeled and plot time response graphs to show temperature variation over a certain duration and at different regions For the product analysis a two dimensional tensile stress analysis was carried using LUSAS Analyst Version 13 5 Basically the product was loaded in tension on one end while the other end is clamped Load increments were applied until the model reaches plasticity Fig 6 shows loaded model of the analysis 3 3 Result and discussion for mould and product analysis For mould analysis the thermal distribution at different time intervals was observed Fig 7 shows the 2D analysis Table 2 Material properties for mould and product Carbon Steel AISI 1050 mould ABS Polymer product Density 7860 kg m 3 Density 1050 kg m 3 Young s modulus E 208 GPa Young s modulus E 2 519 GPa Poisson s ratio 0 297 Poisson s ratio 0 4 Yield strength S Y 365 4 MPa Yield strength S Y 65 MPa Tensile strength S UTS 636 MPa Thermal expansion 65 10 6 K 1 Thermal expansion 11 65 10 6 K 1 Conductivity k 0 135 W m K Conductivity k 49 4 W m K Speci c heat c 1250 J kg K Speci c heat c 477 J kg K S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 263 Fig 6 Loaded model for analysis of product contour plots of thermal or heat distribution at different time intervals in one complete cycle of plastic injection molding For the 2D analysis of the mould time response graphs are plotted to analyze the effect of thermal residual stress on the products Fig 8 shows nodes selected for plotting time response graphs Figs 9 17 show temperature distribution curves for dif ferent nodes as indicated in Fig 8 From the temperature distribution graphs plotted in Figs 9 17 it is clear that every node selected for the graph plotted experiencing increased in temperature i e from the ambient temperature to a certain temperature higher than the ambient temperature and then remained constant at this temperature for a certain period of time This increase in tem perature was caused by the injection of molten plastic into the cavity of the product After a certain period of time the temperature is then further increased to achieve the highest temperature and remained constant at that temperature Increase in temper ature was due to packing stages that involved high pressure Fig 7 Contour plots of heat distribution at different time intervals 264 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 Fig 8 Selected nodals near product region for time response graph plots Fig 9 Temperature distribution graph for Node 284 Fig 10 Temperature distribution graph for Node 213 Fig 11 Temperature distribution graph for Node 302 Fig 12 Temperature distribution graph for Node 290 which caused the temperature to increase This temperature remains constant until the cooling stage starts which causes reduction in mould temperature to a lower value and remains at this value The graphs plotted were not smooth due to the absence of function of inputting lling rate of the molten plastic as well as the cooling rate of the coolant The graphs plotted only show maximum value of temperature that can be achieved in the cycle The most critical stage in the thermal residual stress anal ysis is during the cooling stage This is because the cooling Fig 13 Temperature distribution graph for Node 278 S H Tang et al Journal of Materials Processing Technology 171 2006 259 267 265 Fig 14 Temperature distribution graph for Node 1838 Fig 15 Temperature distribution graph for Node 1904 stage causes the material to cool from above to below the glass transition temperature The material experiences differ ential shrinkage that causes thermal stress that might result in warpage From the temperature after the cooling stage as shown in Figs 9 17 it is clear that the area node located near the cooling channel experienced more cooling effect due to fur Fig 16 Temperature distribution graph for Node 1853 Fig 17 Temperature distribution graph for Node 1866 ther decreasing in temperature and the region away from the cooling channel experienced less cooling effect More cool ing effect with quite fast cooling rate means more shrinkage is occurring at the region However the farthest region Node 284 experience more cooling although far away from cooling channel due to heat loss to environment As a result the cooling channel located at the center of the product cavity caused the temperature difference around the middle of the part higher than other locations Compressive stress was developed at the middle area of the part due to more shrinkage and caused warpage due to uneven shrinkage that happened However the temperature differences after cooling for different nodes are small and the warpage effect is not very signi cant It is important for a designer to design a mould that has less thermal residual stress effect with ef cient cooling system For the product analysis from the steps being carried out to analyze the plastic injection product the stress distribution on product at different load factor is observed in the two dimensional analysis Figs 18 21 show the contour plots of equivalent stress at different load increments A critical point Node 127 where the product experiences maximum tensile stress was selected for analysis The stress versus strain curve and the load case versus stress curves at this point were plotted in Figs 22 and 23 From the load case versus stress curves at this point plotted in Fig 23 it is clear that the product experiencing increased in tensile load until it reached the load factor of 23 which is 1150 N This means that the product can withstand tensile load until 1150 N Load higher than this value causes failure to the product Based on Fig 23 the failure is likely to occur at the region near to the xed end of the product wi