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ORIGINAL ARTICLE Surface microstructure replication in injection molding Uffe Arl Theilade shaded bubble indicates that elevated mold temperature was applied 158Int J Adv Manuf Technol 2007 33 157 166 aesthetical implications of surface topography relate to visual and tactile perception issues such as gloss color perception and general look and feel experience These parameters have a high priority in many electronic consumer products like mobile phones and audio visual equipment 16 Surface micro topography can also have aesthetical relevance when used to conceal surface defects such as sink marks and weld lines 17 18 The technical relevance of surface micro topography is comprised of a broad spectrum of performance related functions and mechanisms as demonstrated in Table 1 19 These topography dependent properties are relevant for a large number of traditional technical components With the emergence of micro engineering and nanotechnology additional functional aspects of surface topography follow Important applications in these fields where surface topography is crucial include computer components micro electro mechanical systems MEMS biomedical systems optical applications and chemical systems 20 In connec tion with injection molding many of these applications are relevant According to M nkk nen et al 3 prominent examples include TAS micro total analysis systems or lab on a chip components CDs DVDs security and decorative holograms brightness enhancement foils light collimators and DOEs diffractive optical elements As potential applications for HARMs high aspect ratio micro structures Despa et al 7 mention heat exchangers catalyst substrates and seal faces Additional examples of applications are shown in Table 2 8 MEMS and optical surfaces can generally be regarded as engineered structured surfaces and as such fall in another category than e g EDM surfaces However replication of the structured and unstructured surfaces with injection molding conceptually embodies the same problem A substantial number of articles about the replication of structured surfaces have been published but the literature on roughness replication in injection molding is quite scarce 2 3 Characterization The topographical characterization of plastic parts repre sents a challenge of its own The weakly reflecting and relatively soft plastic surfaces pose tough requirements for the characterization instruments 10 21 and contact less characterization is preferred For 2 1 2D structures the ISO 5436 step height definition lends itself well as a topographical amplitude measure Fig 2 Concerningroughnesscharacterization three dimensional topography characterization is a relatively novel area that is still being developed Standardized characterization proce dures do not exist and careful metrological considerations must begiven totheindividualcases Inthetwo dimensional regime topography parameters are well established and standardized as in ISO 4287 A similar body of standards has not yet been established for three dimensional parameters A primary set of three dimensional parameters was proposed by Stout and Blunt 19 These so called Birmingham 14 Table 1 Examples of functional implications of surface micro topography Surface use categoryFunction mechanism Translational surfacesFriction Wear Sealing Static contact surfacesAdhesion and bonding Fatigue Stress Fracture Non contact surfacesReflectivity Gloss Plating Painting Hygiene Based on 19 Table 2 Overview and examples of MEMS and MEMS like applications Type of applicationExamples Electro optical componentsSwitches Diffraction gratings Miniature lenses Mirrors Mechanical devicesWatch components Printer heads Automotive sensors Micro heat exchangers Micro pumps Medical and chemical chipsFuel cells Hearing aids Gene chips Drug delivery systems Bio sensors Compiled from 8 Fig 2 Fat lines indicate ISO 5436 step height references Axis units m Int J Adv Manuf Technol 2007 33 157 166159 parameters can be regarded as a de facto standard In the currentpaper thescopeislimitedtotheamplitudeparameters of the Birmingham 14 parameters excluding Sz as listed in Table 3 3 Micro injection molding and replication Micro injection molding can be used as the headline for injection molding of components with one of the following characteristics Very low shot weights with critical dimensions in the m range Larger products with functional features and at least one critical dimension in the m range Conceptually topographical replication quality can be defined as the degree of similarity between the plastic and the inverted mold surface As the replication process trans forms positive topography to negative perfect replication corresponds to the inverted mold surface The replication of surface microstructures in injection molding is believed to be determined by the following three main factors Driving force Material deformability Microstructure geometry The driving force is established by the cavity pressure that arises due to the cavity filling and later the holding pressure Material deformability is controlled by material properties such as viscosity and elasticity of the material which again are strongly influenced by the temperature In some cases the material deformability may be attributed to the size of the frozen layer of plastic material against the mold wall The microstructure geometry affects the repli cation in such a way that smaller structures with higher aspect ratios are increasingly more challenging to replicate 4 Replication of a specific structure 4 1 Experimental set up specific structure Theexperimentalworkwasbasedonasimple100 24 1mm ruler type part molded in a two plate mold with a conven tional cooling system and a cold runner system including a 0 6 mm film fan gate This geometrical configuration ensured an even and essentially one dimensional melt front advancement in the cavity The cavity was equipped with nickel inserts containing 2 1 2D rectangular structures with heights of 9 m and aspect ratios from 0 2 to 1 manufactured by lithography and subsequent electrochem ical plating Table 4 Production took place with an Engel ES80 25HL injection molding machine and the PP polypropylene grade Basell Moplen HP501H Type Homopolymer melt flow rate MFR 2 1 g 10 min 230 C 2 16 kg ISO 1133 heat deflection temperature B 0 45 MPa 85 C ISO 75B 1 2 The mold temperature was kept constant at approx imately 50 C while barrel temperatures of 220 250 and 280 C were employed Injection flow rate and holding pressure switch over at approximately 99 part filling were set at 35 cm3 s and 44 MPa melt respectively At all barrel temperature levels additional series were run with injection flow rates of 20 and 50 cm3 s Finally the effect of high holding pressure 89 MPa was explored at 220 C barrel temperature Table 5 Detailed process analysis was carried out with the simulation software MoldFlow MPI 4 0 mid plane Cool and Flow By using simulation measures such as plastic surface temperature and frozen layer thickness could be tracked during the molding cycle Based on pressure curve studies the software has been found to give reasonably accurate results 21 Topographical characterization of microstructures on plastic surfaces can be performed with such instruments as AFM SEM interference microscopes confocal micro scopes focus detection instruments and stylus based instru ments A comparison and evaluation of some of these techniques has been reported in 22 23 For the purpose at hand a confocal microscope was selected based on the grounds of its non contact nature fast operation and reliable results The instrument used was a Zeiss LSM 5 Pascal confocal laser scanning microscope equipped with Epiplan objectives 2 5 50 and a HeNe laser The acquired surface topographies were analyzed in the software package SPIP The total uncertainty for the measured plastic micro structures including injection molding process variance locationofreferencearea andinstrumenterrorwasestimated Table 3 The selection of Birmingham 14 three dimensional topogra phy parameters used in this investigation 19 CategorySymbolName AmplitudeSqRoot mean square deviation SskSkewness of surface height distribution SkuKurtosis of surface height distribution Table 4 Dimensions of rectangular mold microstructure profiles StructureWidth in mold m A150 0 B122 1 B223 7 C19 6 C211 3 All mold structures are 9 m deep 160Int J Adv Manuf Technol 2007 33 157 166 to 0 5 m corresponding to 10 for a typical replication height of 5 0 m 5 Results specific structure 5 1 Process condition effect The effects of process conditions are recognized in Fig 3 where parts produced from unfavorable typical and favorable process conditions are juxtaposed The effects of melt temperature injection flow rate and holding pressure are shown in Figs 4 5 and 6 In line with other studies 1 3 the experiments show that temperature is an important factor for replication by raising the melt temperature from 220 to 280 C at 35 cm3 s the achieved step height for the 20 m wide structures almost doubled from 3 9 to 7 3 m A similar step height improvement could be observed when the injection flow rate was increased from a low of 20 cm3 s to a high 50 cm3 s at melt temperature of 250 C An interesting interaction effect between injection flow rate and melt temperature seems to exist At the low injection flow rate an increase in temperature from 220 to 250 C is not adequate to improve replication and at the high injection flow rate an increase in temperature from 250 to 280 C only improves the already good replication marginally In conclusion at low and high temperature levels the temperature effect seems to overshadow the injection flow rate effect 5 2 Geometrical dependence For structure A1 with a groove width of approximately 250 m near perfect replication i e step height of approximately 9 m was achieved at all process con ditions At structure widths around 20 m structure B1 and B2 good replication achieved step heights around 7 m could be achieved for only a few process conditions P5 and P6 All process conditions performed poorly in replicating structures with widths around 10 m with a typical achieved step height of 2 m This implies that for replicating heights of around 9 m a critical width between 20 and 50 m exists Below this critical width perfect replication within a normal process window cannot be guaranteed Below this critical width a strong linear relation exits between mold groove width and achieved step height Fig 7 5 3 Structure shape The imperfectly replicated structures generally appear to consist of a base with steep walls and a rounded cap on top By relating the proportion of the base height to the total step height of the structures the trend is that taller structures have a relatively higher base Fig 3 supporting the hypothesis that the replication improvement takes place in the form of stretching the base portion of the structure Fig 3 Example of how process conditions affect replication height structure width app 20 m Fig 4 Temperature effect on achieved step height for structure B1 Table 5 Process conditions CodeTp C Q cm3 s ph MPa P12202044 P22203544 P32205044 P42203589 P52502044 P62503544 P72505044 P82802044 P92803544 P102805044 Mold temperature 50 C for all conditions Tp Barrel temperature Q Injection flow rate ph Holding pressure in plastic Int J Adv Manuf Technol 2007 33 157 166161 6 Replication of EDM roughness 6 1 Experimental set up EDM roughness A simple two plate mold with a cold runner system was applied for the experiment The cavity is a ruler type part with a 0 6 film fan gate a part thickness of 1 mm and a part flow path of 100 mm To facilitate the study the replication of different surfaces in one shot and at different distances from the gate the cavity insert segments are equipped with smaller test surface inserts The investigation in question was based on a test surface insert located near the gate The test surface insert was manufactured from Stavax ESR mold steel by EDM to an Sq roughness of around 4 6 m corresponding to approximately 30 on the Charmille scale The mold was fitted with pressure and temperature transducers located across the test surface insert The mold pressure transducer was piezo electric of the type Kistler 6157BA The pressure transducers were calibrated after mounting in the mould and uncertainty in terms of 2 was better than 1 The mold temperature transducer was thermocouple based and of the type Kistler 6992A0 4 These temperature transducers measure the temperature at the very surface of the mold The temper ature transducers were not calibrated in the lab but verified by comparison with a handheld thermocouple based device VIKING 3000 deviation between the two instruments were less than 2 C over 10 measurements The applied injection molding machine is an Engel ES80 25HL with a 18 mm screw For the experiments a standard PS grade BASF 165 H was used melt volume flow rate 3 4 cm3 10 min 200 C 5 kg ISO 1133 heat deflection temperature B 0 45 MPa 84 C ISO 75B 1 2 Barrel temperature coolant temperature injection speed and holding pressure were varied across a perceived typical process window yielding a total of 33 process conditions Of these 11 process conditions were discarded due to obvious quality issues on the macro level resulting in 22 different feasible process conditions for further analysis The most important instrument technologies for rough ness measurement are the mechanical stylus and optical techniques In the current context an optical laser focus detection instrument was chosen as the primary method of topographical characterization The instrument was chosen for its non contact properties range versatility adequately low uncertainty acceptable speed and accessibility Laser focus detection instruments LFD function by scanning in the xy plane with an optical probe During this scan the laser beam of the scanning probe is brought into focus on the specimen surface by adjusting a moveable objective lens The vertical movement of the lens being necessary to achieve focus provides quantitative information on the topography height and is obtained through a coil Specifically a UBM Microfocus was applied This instrument has a large lateral range of several millimeters and a lateral resolution of 0 5 m With the applied setting the vertical range is 1 mm with a resolution of 50 nm 24 An example of a three dimensional topographical image is shown in Fig 8 Fig 5 Injection flow rate effect on achieved step height for structure B1 at barrel temperature of 250 C Fig 6 Holding pressure effect on achieved step height for structure B1 at barrel temperature of 220 C Fig 7 Mold structure width achieved step height correlation for structure B1 B2 C1 and C2 Each curve represents one process condition 162Int J Adv Manuf Technol 2007 33 157 166 For each injection molded part a sample area of 1 1 mm was characterized With a combination of optical means and a mechanical fixture an effort was made to obtain all measurement on similar locations on the part surfaces The measurement uncertainty coverage factor 2 for Sq was estimated to be less than 0 5 In comparison the shot to shot variation in terms of 2 was around 2 6 2 Results EDM roughness In general the injection molding process replicates surface micro topography quite well and specific corresponding micron sized features can be identified on matching mold and plastic part topographical images However quantita tive differences between parts produced with favorable and unfavorable process conditions clearly stand out The results are shown graphically in Figs 9 10 11 12 13 and 14 To enable across parameter comparisons the parameters can be taken relative to the range of a given sample in the form of the range relative parameter RRP defined as RRP Sx i Sx i Sx min Sx max Sx min where Sx i A given topography parameter associated with the ith element in a given sample Sx min Lowest value of a given topography parameter in a given sample Sx max Highest value of a given topography parameter in a given sample From Fig 9 it can be seen that the parameters appear to greatly depend on both melt temperature and injection rate Higher values of melt temperature and injection rate results in higher values of the amplitude parameters In Fig 10 the somewhat same melt temperature effect as in Fig 9 can be seen With the exception of Sku at low melt temperature the parameters appear to be relatively insensitive to changes in holding pressure Figure 11 reveals a strong coolant temperature effect for all parameters As for the case of melt temperature higher values of coolant temperature result in Fig 8 Three dimensional topographical of an investigated plastic surface 1 1 mm z scale factor 3 Fig 9 Melt temperature and ram speed at low coolant tem perature effects Bands indicate operational uncertainties based on a 95 confidence interval Coolant temperature was 15 C and holding pressure hydraulic was 6 MPa for all points Int J Adv Manuf Technol 2007 33 157 166163 higher values of the amplitude parameters The combined injection rate and holding pressure factor does seem to affect all parameters When moving from low injection speed high holding pressure to high injection speed low holding pressure we generally seem to get a better replication e g higher amplitude parameters This result indicates that the injection speed overshadows any effects of holding Results from variation of one process factor at the time around the center point process condition are shown in Figs 12 13 and 14 Generally when comparing neighbor ing points pair wise the differences do not appear to be significant when the uncertainty is taken into account However in aggregate and from a qualitative perspective certain patterns do seem to emerge consistently Fig 10 Melt temperature and holding pressure effects Bands indicate operational uncertain ties based on a 95 confidence interval Coolant temperature was 15 C and

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