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International Journal of Machine Tools & Manufacture 43 (2003) 879887Part shrinkage anomilies from stereolithography injection mouldtoolingR.A. Harris, H.A. Newlyn, R.J.M. Hague, P.M. DickensWolfson School of Mechanical & Manufacturing Engineering, Loughborough University, Leicestershire, LE11 3TU, UKReceived 6 January 2003; accepted 4 March 2003AbstractThe use of stereolithography (SL) tooling allows plastic parts to be produced by injection moulding in a very short time due tothe speed of mould production. One of the supposed advantages of the process is that it provides a low volume of parts that arethe same as parts that would be produced by the conventional hard tooling in a fraction of the time and cost.However, this work demonstrates different rates of polymer shrinkage are developed by parts produced by SL and conventionaltooling methods. These revelations may counter the greatest advantages of the SL injection moulding tooling process as the partsdo not replicate those that would be produced by conventional hard tooling.This work identifies the different shrinkage that occurs in mouldings produced by an SL mould as compared to those producedfrom an aluminium mould. The experiments utilise two very different types of polymers and two mould geometries, which areprocessed in the same manner so that the heat transfer characteristics of the moulds are isolated as the only experimental variable.The work demonstrates how the two mould materials exhibit very different rates of expansion due to the temperature profilesexperienced during moulding. This expansion must be compensated for to establish the total amount of shrinkage incurred bymoulded parts. The compensation is derived by a mathematical approach and by modelling using finite element analysis. Bothtechniques depend upon knowledge of the thermal conditions during moulding. Knowledge of these thermal conditions are obtainedby real-time data acquisition and simulated by FEA modeling. The application of the findings provide knowledge of the completeshrinkage values relating to the mould material and polymer used which would enable the production of geometrically accurate parts.Keywords: Finite element analysis; Plastic injection moulding; Polymer shrinkage; Rapid tooling & stereolithography1. IntroductionStereolithography (SL) has shown itself to be capableof directly producing tooling cavities (inserts) for injec-tion moulding. The accuracy of the SL process resultsin epoxy inserts that require few further operations priorto their use in injection moulding. Thus, the process pro-vides a quick route to tooling that, depending on geo-metric complexity and the moulding polymer, can pro-duce up to approximately 50 parts 1. Various polymershave been successfully moulded by SL injection mould-ing. These include polyester (PE), polypropylene (PP),polystyrene (PS), polyamide (PA), polycarbonate (PC),polyether-ether-ketone(PEEK),acrylonitrile-styrene-Corresponding author. Fax: +44-0-1509-227549.E-mail address: r.a.harrislboro.ac.uk (R.A. Harris).acrylate(ASA)andacrylonitrile-butadiene-styrene(ABS) 25.SL produces an epoxy mould cavity which possessesvery different thermal properties in comparison to metalmoulds. Without the use of mould heating or cooling thedifferent mould materials impose very different rates ofpart cooling due to the inherent heat transfer character-istics of the mould material. Many polymers exhibit dif-ferent shrinkage according to the cooling conditions ofthe part during moulding 611. This work intends toestablish the effects of the thermal conditions caused bythe mould material on part shrinkage.2. AimsThis research aims to evaluate the shrinkage of mould-ings from SL tools in comparison with those from a con-880R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887ventional metal rapid tooling method for injectionmoulding.3. Methodology3.1. Experimental designThe aim of the experiments was to establish theshrinkage that occurs within 48 h of the moulding of twopolymers of very different characteristics (Polyamide 66PA66, crystalline and Acrylonitrile-Butadiene-StyreneABS, amorphous) when produced by injection mould-ing in cavities of differing materials (StereolithographySL and Aluminium AL). This would be by a directcomparison of the dimensions of the moulding cavityand the moulded parts.Besides the choice of thermoplastic polymer and toolmaterial the shrinkage of an injection moulded part canalso be dependent upon several process variables i.e. fol-low up pressure, machine type etc. However, the onlyvariables in the two experimental sets (PA66 & ABS)were the tool material type (SL & AL) and tool cavitygeometry (bar & discthese were used to identify dif-ferent shrinkage according to the polymer flow directionduring moulding and are further explained in section3.2). With respect to the injection moulding to be con-ducted, the only influential variable in each experimentalset were the thermal properties of the SL and AL mould-ing cavities. This dictated the rate at which the heat fromthe injected polymer would be conducted away from themoulded part through the cavity material. The value ofthermal conductivity of the mould materials differ gre-atly: SL = 0.2 W/m/K, AL = 200 W/m/K.3.2. Mould designThe mould design was based upon BS EN ISO 2941 and 4 12,13 and ASTM D955 14 standards forestablishing shrinkage of injection moulded polymers.Specimens of two differing geometries were mouldedin order to provide shrinkage measurements both parallel(bar shape) and perpendicular (disc shape) to the direc-tion of polymer flow. Dimensioned images of thesemould cavities can be viewed in Fig. 1.The draft angle used to ease part removal from themould was 1.5. This value has previously been shownto be an optimum value for reducing potential damageto SL tools upon part ejection 15.An open gate design was used on both moulds toavoid areas of heat and pressure concentration (such aswith fan gating) that can damage an SL tool. This gatingsystem was employed to ensure no great pressure differ-ences in the mould that may have led to unequal stressesin the part and also prevents an interruption to followup pressure caused by premature freeze-off of the gate.The depth of the gate was the same as the cross-sectionalthickness of the part (3.2 mm in both disc & bar). Thewidth of the gate size is proportional to the cavity sizein each case.The moulds did not include a part ejection system.The inclusion of steel ejector pins would have providedan area in the moulding cavity which would have cooledthe polymer at a different rate than other areas in themould due the different rates of heat transfer in theimmediate areas. Steel is of a much greater thermal con-ductivity than SL and considerably less than AL. Thiswould not have enabled the experiment to assess thecomplete effects of the heat transfer provided by SL orAL moulds on part shrinkage. The absence of an ejectorsystem caused no problems as the moulded parts werevery simple in shape and were easily removed by handfrom the mould.The SL moulds were manufactured by a 3D SystemsSLA350 machine, using Vantico 5190 resin. The buildlayer thickness was 0.05 mm, as this has previously beendemonstrated as an optimal value in extending the work-ing life of SL moulds 16.The inserts were contained within a steel bolster formoulding. These bolsters facilitated alignment on themachine platen, provided material entry into the mouldvia a tapered sprue bush and protected the inserts fromany excessive application of pressure.3.3. Tool temperature recordingIn order to establish the heat transfer characteristicsthat occured in each of the moulds, the temperature wasrecorded throughout the moulding cycle by the insertionof k-type thermocouples. The tip of the thermocoupleswere situated such that they were located 0.5 mm belowthe surface of the moulding face. The calibration of eachindividual thermocouple was checked prior to insertion.After insertion into the mould the thermocouples werecalibrated to ensure that the temperature 0.5 mm belowthe cavity surface was an accurate reflection of the actualcavity surface by comparison of their simultaneousvalues and their response to temperature change. Thedifference between the temperatures measured by thethermocouple and the actual surface temperature wasnever greater than 1 C. A data acquisition programrecorded and logged the temperature profiles over a 10minute period during the moulding of a part.3.4. Injection mouldingIn order to eliminate extra experimental variables, itwas important to find universal parameter values thatwould work with both the polymers, both the mould geo-metries and both mould material types. Other than tocompensate for the different part volumes of the bar &881R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887Fig. 1.CAD images of the mould cavity designs.disc geometries, the parameters were identical in all theexperiments conducted.The PA66 used was Bergamid A70NAT produced byPolyOne. The ABS used was Lustran Ultra 2373 pro-duced by Bayer. Both polymers were hygroscopic andwere dried immediately prior to processing.The injection moulding machine used was a Bat-tenfeld 600/125 CDC model with a Unilog 4000 controlunit. This machine consisted of a 60 tonne hydraulicclamping unit and a 125 35 mm reciprocating screwinjection unit with a conventional tapered nozzle.The following process parameters were used:? Melt temperature set at 270 C in each of the fivebarrel temperature zones.? Injection speed set at 100 mm/sec.? Injection pressure of 150 bar.? Follow-up pressure of 150 bar, held for 3 sec, on a100 mm cushion.? The ambient temperature of the mould prior to injec-tion was 23.5 C.? A cooling time of 40 sec prior to part removal. Thiswas established as the time at which parts could beremoved without distortion.? A clamping force of 15 tonnes.3.5. Shrinkage measurementBS EN ISO 291 17 was used as a standard for theenvironment in which the parts were conditioned andmeasured 48 h after moulding. Prior to injection mould-ing, the mould cavity and later the moulded specimenswere measured in the laboratory atmosphere to the near-est 0.01 mm. Twenty parts were moulded from eachexperimental set and measurements made of each. Theparts and cavities were measured across their gaugelength. Since the gate for the bar mould was centrallyplaced at the top of the gauge length, two measurementswere taken either side of the gate for each part and thecavity. Gauge length measurement for both part &mould geometries are illustrated in Fig. 2. The measure-ments taken from the specimens were compared to themeasurements of the cavities and expressed as a percent-age difference of the moulding cavity and the parts ineach experimental set after compensation for thermalexpansion of the mould.4. Results4.1. Mould temperature profilesThe data showed a strong similarity (no greater than 5% difference) between the temperature profiles forboth polymer types (ABS & PA66) and both cavity geo-metries (bar & disc). Very different temperature profileswere experienced by the SL and AL mould types. TheFig. 2.Measurement positions for disc & bar geometries.882R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887Fig. 3.Average temperature profiles for AL & SL moulds.similarity between the readings allowed an average plotto be generated that represents the temperature profilefor each tool material type (AL & SL) as shown in Fig.3. The profiles illustrate the vastly different temperatureconditions experienced in the SL and AL moulds. Thetemperature activity in the AL moulds occurred in a veryshort period of time due to the materials high thermalconductivity. The temperature profile in the SL mouldwas more extreme and protracted; without externalassistance (i.e. cooling by compressed air) the SL mouldwould take 15 minutes to return to its ambient tempera-ture.4.2. Initial shrinkage resultsThe initial shrinkage values, calculated from thepart/mould measurements, can be viewed in Table 1.However, these figures need further consideration toestablishthetotalshrinkagethatoccurs.Thisisexplained in the following section.4.3. Compensation for thermal expansionbycalculationBoth of the mould materials used expand whenheated, albeit in differing amounts. The measurementsTable 1The % size difference of part/mould, including compensation for thermal expansion by calculation & FEAPA66 Part measurementABS Part measurementMould typeAL barSL barAL discSL discAL barSL barAL discSL disc% part/mould difference ?1.3272?2.611?1.2177?2.4149?0.7524?0.6474?0.7351?0.5611% part/mould difference ?1.3434?2.7308?1.2339?2.6101?0.7687?0.7696?0.7514?0.7599including compensationfor thermal expansionby calculation% part/mould difference ?1.3841?2.7447?1.2712?2.5649?0.8097?0.7837?0.7887?0.7139including compensationfor thermal expansionby FEAtaken for shrinkage must be compensated for by theamount of cavity expansion to establish the true amountof difference between mould and part measurements.Not compensating for thermal expansion could lead toan underestimation of shrinkage that occurs duringmoulding. The value obtained for shrinkage had to becorrected by an amount corresponding to the thermalexpansion of the mould.The expansion of the mould resulted in an expansionof the cavity, not contraction. This was verified by plac-ing the mould inserts in an oven at a nominal tempera-ture of 50 C for 10 min and then measuring the cavityin the appropriate measurement axis (see Fig. 2). Allmoulds showed an increase in cavity size, demonstratingan expansion of the cavity. This was authenticatedfurther in later work (section 4.4.5).This correction factor ?SMto the part shrinkage duethe mould expansion, is given by: ?SM= am (Tm?Ta)13 where: am=linear coefficient of expansion of themould material in 10?6m/m/K Tm=maximum mouldtemperature during injection cycle in C Ta=ambienttemperature of the mould on the machine in C. Thevalues used were:? am? SL = 59 10?6m/m/K? AL = 23.8 10?6m/m/K 18Ta:? SL = 23.5 C? AL = 23.5 C.An explanation of these values is given in section 3.4.Tm:These values were derived from the thermal historyprofiles of the moulds and the injection parameters. Thetemperature at the end of the injection cycle was usedin the calculations as this was the point at which pressureapplication to the injected polymer ceased. When press-883R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887ure application ended, so did the period at which theshrinkage could be influenced. Any further expansion ofthe mould after this period would be unable to affect thepart size. The derivation of the values is illustrated inFig. 4. The maximum temperature (Tm) at the end of theinjection cycle for each mould type were:? SL disc = 57.46 C? SL bar = 44.37 C? AL disc & bar = 30.39 C.The same maximum temperature was experienced byboth the aluminium moulds. This maximum temperaturewas reached before the end of the injection cycle. Thetemperature in the stereolithography moulds continuedto rise for a long period after completion of the injectioncycle. Different maximum temperatures were experi-enced in the SL disc & bar moulds due to the differenttimes of the injection cycle required by their differentpart volume.Therefore, the calculation for thermal expansion foreach mould type was as follows:SL disc mould = 59 10?6(57.46?23.5)= 2.00364 mm/m= 0.200364%SL bar mould= 59 10?6(44.47?23.5)= 1.23133 mm/m= 0.123133%AL disc & bar mould= 23.8 10- 6(30.39?23.5)= 0.16422 mm/m= 0.016422%.The extra amount of shrinkage determined by calcu-lating the expansion of the mould was incorporated intothe percentage shrinkage of the measured parts to revealthe compensated total shrinkage. These values are shownin Table 1. However, these calculations were slightlysimplified. An assumption was made that the wholemould was at the temperature determined by the dataacquisition for tool temperatures and as such equalFig. 4.Thermal conditions in the moulds during injection cycle.expansion was experienced throughout. In reality theincrease in temperature was localised around the mould-ing cavity. Further work was required to prove whetherany differing expansion of the mould cavity occurreddue to the temperature distribution and to assess anysuch effects on the compensation values used.4.4. Compensation for thermal expansionby FiniteElement AnalysisFinite Element Analysis (FEA) was used to model themould expansion due to the localised heating caused bythe hot polymer contained within the cavity. The FEAsoftware package used in this work was Algor. Twoforms of FEA analysis from Algor were used. Firstly,a transient thermal analysis was conducted in order todetermine the temperature distribution in the mould.Then a linear elastic analysis was performed using thetemperaturedistributionresultstodeterminetheresulting expansion of the mould cavity.4.4.1. FEA modelling. Stage 1creation of modelThe FEA work began with the creation of the requiredmodels and FEA mesh. In order to reduce solution time,one quarter of the full mould was created, due to thequarterly symmetrical nature of each of the specimensgauge length. This is shown in Fig. 5.The model was extruded such that the mesh spacingbetween each node was 0.5 mm in the immediate regionbeneath the moulding cavity (4 mm deep) as these arethe elements of interest when evaluating the cavityexpansion. This was conducted to ensure that a node wasat the equivalent point to the thermocouple in the experi-ments, from which the critical time dependant tempera-tures were derived (see section 3.3). After this the meshspacing for the rest of the model was 10 mm in orderto achieve a shorter analysis time.4.4.2. FEA modelling. Stage 2allocation of materialpropertiesThe materials were assumed to be homogeneous andisotropic with constant material properties independentof temperature. The values are listed in Table . FEA modelling. Stage 3transient thermalanalysisTransient thermal analysis refers to a thermal con-dition where temperature is a function of time. Thisanalysis type was relevant to the conditions that occurredwithin the moulds during the experiments. The heat wassupplied by the injected polymer which transferred itsenergy (heat) into the surrounding mould material. Thisenergy was not limitless and the polymer correspondedby reducing in temperature as the heat was transferredinto the lower temperature mould.884R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887Fig. 5.FEA model section of bar & disc moulds.Table 2Material values used in FEA for epoxy, aluminium & polymeraEpoxySI unitsReference sourcer (density)1250 kg/m322K (thermal conductivity)0.19 W/m K22Cp(specific heat capacity)1046.7 J/kgC22E (modulus of elasticity)2.6109N/m222n (poissons ratio)0.3522a (thermal coefficient of expansion)5910?6/C18Aluminiumr2720 kg/m323K170 W/moK23Cp880 J/kgC23E68.9109N/m223n0.323a2310?6/C23Polymerr1145 kg/m322K0.2962 W/m K22Cp1625.7 J/kgC22E0.26109N /m2(a)(a)n0.3522a59 or 2310?6/C (a)18 or 23aThe value of E for the polymer was a nominal low value to reduce restraint. The value of a in the polymer was made the same as the mouldmaterial to prevent interference. The use of such values in both cases was to ensure that the presence of the polymer did not influence the expansionof the cavity in the simulationTo conduct the transient thermal analysis, certainassumptions were made:? The polymer was all initially at 270 C and was inperfect contact with the mould at all times.? There was no thermal resistance between the plasticand the mould.? The mould material was all initially at 23.5 C.The critical time dependant temperatures (see Fig. 4)at the positions equivalent to thermocouple placement inthe experiments were plotted from the FEA results andthe correct step in the solution noted. From this tempera-ture distribution it was possible to investigate the reac-tion of the model (expansion) to a chosen condition(temperature) at a pre-identified time step by performinga linear elastic analysis.885R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 8798874.4.4. FEA modelling. Stage 4linear elastic analysisThe models were fully restrained on the planes ofsymmetry, on the basis of which the FEA models weredevised (shown in Fig. 5) and the parting plane of themouldinserts.Nofurtherrestraintswereappliedallowing free expansion. The latter point may at firstseem a little odd as the mould insert was contained ina steel pocket within the injection moulding bolster.However it has been already shown that even in opti-mum conditions the maximum possible expansion of thewhole insert could be just 0.25 mm in any one direc-tion. This amount of expansion could occur freely dueto the clearance required to enable fitting and removalof the inserts in the bolster pocket. The planes ofrestraint are illustrated in Fig. . FEA modelling. Stage 5resultsAfter the linear elastic analysis was performed themovements (thermal expansion) of the model rep-resenting the planes of measurement (see Fig. 2) weredetermined by interrogating the displacement vectors ofthe nodes representing the cavity edge. An example ofthis is illustrated in Fig. 7.From these displacements an average movement(expansion) was determined. In each case the resultsdemonstrated an outwards expansion of the cavity. Todetermine the total mould expansion over the measure-ment axis, the average figure derived from the displace-ment vectors was doubled to include axial expansion inthe opposite direction, as only half of the measurementaxis was modelled. The results of these expansion valueson total part shrinkage are detailed in Table 1.5. DiscussionIt can be seen from the results (Table 1) that allowingfor thermal compensation reveals higher values ofFig. 6.Planes of restraint on bar & disc FEA models.Fig. 7.Interrogation of displacement vectors in FEA results to estab-lish amount of expansionexample shows AL bar.shrinkage in all polymer/tool combinations, albeit at dif-fering scales. Some of the values revealed quite differentshrinkage values when the mould expansion was incor-porated.The FEA values gave a good comparison with thosederived from the calculations. This gave confidence thatthe method used did give an accurate assessment of themould expansion and the resulting total shrinkage of theparts. It should be noted that the FEA method is an886R.A. Harris et al. / International Journal of Machine Tools & Manufacture 43 (2003) 879887approximate solution and relies on the accuracy of themodel, mesh density, material properties, surface contactresistance and physical restraints. Although many of thefigures used in the FEA were definitive as they wereproduced from real practice, some were the result ofassumptions that had to be made.Whencompensationforthermalexpansionisincluded, several themes in the part shrinkage resultsfromthe experimentaldatawererevealed.Theseinclude:5.1. Shrinkage directionThe crystalline polymer (PA66) demonstrated slightlygreater shrinkage differences (7% more) in the polymerflow direction (bar specimen) as opposed to the directionperpendicular to polymer flow (disc specimen). This wasalso shown with the amorphous (ABS) parts but to alesser degree (3% more). These characteristics are typi-cal of all injection moulded parts, with crystalline partsbeingmoresusceptibletoanisotropy(directionaldifferences) due to the flow direction causing greateralignment of chains within the polymer 19.5.2. Shrinkage according to mould materialPA66The results show that the shrinkage that occurred inPA66 parts from the SL moulds of both geometries wasdouble that incurred by the comparative parts from theAL moulds. An expected shrinkage range for PA66 is 12.2% 20. The parts from the AL moulds demonstratedshrinkage just above the minimum amount expected,while the parts from the SL moulds incurred shrinkageabove the maximum in the expected shrinkage range.Also the range of part of sizes measured were muchgreater than those experienced in the ABS parts. PA66part measurements differed in a range of 0.35 mm ascompared to 0.18 mm for the ABS parts. This is a typi-cal characteristic of crystalline polymers which are moredifficult to hold part tolerances, as compared to amorph-ous polymers 21.5.3. Shrinkage according to mould materialABSThe results show that the shrinkage of the ABS partsis largely unaffected by the mould material used. Ashrinkage of 0.76% was incurred by all ABS parts inthe experiments. An expected shrinkage range for ABSis 0.50.6% 20.6. ConclusionsThis work has defined that double the amount ofshrinkage occurred in PA66 (a crystalline polymer)when injection moulded in an SL tool, as compared toan AL tool. In the same experimental conditions ABS(an amorphous polymer) demonstrated no such differ-ences.The importance of compensating for thermal expan-sion of the mould in the calculation of shrinkage hasbeen demonstrated. This is critical in determining absol-ute shrinkage values in plastic tools, which expand morethan metal tools. Neglecting the mould expansion inplastic tools would lead to significant error in determin-ing the absolute part shrinkage.The establishment of differing part shrinkage in crys-talline polymers exposes a flaw in the use of shrinkagecompensation factors supplied by manufacturers. Thiswork has shown that the shrinkage of crystalline poly-mers is dependant upon process conditions which arevariable. Supplied shrinkage factors would be specificonly to the conditions under which the test pieces wereproduced. Thus, traditional shrinkage factors are insuf-ficient not only in the use of SL tools, but also any othertechniques where there is any significant process vari-ation from the norm.It has been shown that the shrinkage of an amorphouspolymer was unaffected by the cooling conditions whichwere imposed by mould material type. Consequently,where possible, it is recommended that amorphous poly-mers are used in preference to crystalline alternativeswhen using SL moulds.References1 P.D. Hilton, P.F. Jacobs, Rapid Tooling: Technologies and Indus-trial Applications, Marcel Dekker, 2000.2 T. Luck, F. Baumann, U. Baraldi, Comparison of downstreamtechniques for functional and technical prototypesfast toolingwith RP, Proceedings of Fourth European RP Conference, 1315 June, 1995, Belgriate, Italy, pp. 247260.3 J. Eschl. Experiences with photopolymer inserts for injectionmoulding, European Stereolithography Users Group meeting, 25 November, Florence, Italy, 1997.4 R.A. Harris. Direct AIM tooling. Rapid prototyping & toolingstate of the industry annual worldwide progress report, WohlersReport 2002, Published by Wohlers Associates Inc, USA, Part 3:Tooling, p. 70, 2002.5 A. Schulthess, B. Steinmann, M. Hofmann, Cibatool SL epoxyresins and some new applications. Proceedings of the 1996 NorthAmerican Stereolithography Users Group Meeting, 1014 March,San Diego, CA, 1996.6 M. Damle, S. Mehta, R. Malloy, S. McCarthy, Effect of fibreorientation on the mechanical properties of an injection moldedpart and a stereolithography-insert molded part. Proceedings ofthe Society of Plast
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