INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 14 (2004) R1R14 PII: S0960-1317(04)68657-8

TOPICAL REVIEW

Review on micro molding ofthermoplastic polymersM Heckele and W K Schomburg1

Forschungszentrum Karlsruhe, Institut fur Mikrostrukturtechnik Postfach 3640,D-76021 Karlsruhe, Germany

Received 21 August 2003Published 17 December 2003Online at stacks.iop.org/JMM/14/R1 (DOI: 10.1088/0960-1317/14/3/R01)AbstractMolding of micro components from thermoplastic polymers has become aroutinely used industrial production process. This paper describes both themore than 30-year-old history and the present state of development andapplications. Hot embossing, injection molding, reaction injection molding,injection compression molding, thermoforming, and various types of toolfabrication are introduced and their advantages and drawbacks arediscussed. In addition, design considerations, process limitations, andcommercially available micro molding machines are presented.

1. Introduction

Micro molding of thermoplastic polymers is one of themost promising fabrication techniques for non-electronicmicro devices. Fabrication costs of molded micro parts arehardly affected by the complexity of the design. Once amold insert has been made, several thousand parts can bemolded with little effort. The cost of the raw material inmost cases is negligibly low, because only small materialquantities are required for micro components. Therefore, partsfabricated by micro molding, even from high-end materials,are suitable for applications requiring low-cost and disposablecomponents.

Moreover, thermoplastic materials are a very largematerial class, which allows one to find a suitable polymerfor nearly every application (see table 1). There arepolymers which are stable at temperatures as high as250 C (e.g., polyetheretherketone, PEEK) and otherswhich resist aggressive chemicals such as alkaline solutions,acids, and solvents (e.g., perfluoralkoxy, PFA). Polymersare electrical and thermal insulators, but when filled withappropriate powders they can be used as electrical conductors,heat sinks, and even magnets. Molded micro structures canbe either soft and elastic such as polyoxymethylene (POM)or hard and brittle such as polysulfone (PSU). They areavailable from optically transparent materials such as (cyclo-olefin copolymer) COC and opaque ones such as polyamide

1 On leave to RWTH Aachen, Germany.

(PA) filled with graphite. Polymers such as PVDF even exhibita piezo-electrical effect.

Hence, micro molding of thermoplastic polymers shouldbe considered whenever a micro component leaves thelaboratory and enters the market.

The development of micro molding started more than 30years ago. A lot of experience has been gathered in thattime, and today micro molding machines are commerciallyavailable and routinely used in industry every day. A variety ofmicro molding tools can be purchased from several suppliers.Therefore, use of micro molding in industry and the numberof scientific institutes working in this field are expected toincrease in the forthcoming years. This will cause a growinginterest in an overview of the current situation of micromolding, which is given in this review.

2. Micro molding processes

There are five processes which are employed for micromolding of thermoplastic polymers: Injection molding,reaction injection molding, hot embossing, injectioncompression molding, and thermoforming.

2.1. Injection moldingThe well-known macroscopic injection molding can beadapted to the micro scale by employing a variotherm process[1]. It comprises the following process steps: The mold cavity

0960-1317/04/030001+14$30.00 2004 IOP Publishing Ltd Printed in the UK R1

Topical Review

Table 1. A list of thermoplastic polymers that have been used for micro molding.

Acronym Full name Temperature stability [C] Properties StructureCOC Cyclo-olefine 140 High transparency Amorphous

copolymerPMMA Polymethylmethacrylate 80 High transparency AmorphousPC Polycarbonate 130 High transparency AmorphousPS Polystyrene 80 Transparent AmorphousPOM Polyoxymethylene 90 Low friction Semi crystallinePFA Perfluoralkoxy copolymer 260 High chemical resistivity Semi crystallinePVC Polyvinlchloride 60 Cheap AmorphousPP Polypropylene 110 Mechanical properties Semi crystallinePET Polyethylene terephtalate 110 Transparent, low friction Amorphous/Semi crystallinePEEK Polyetheretherketone 250 High temperature resistivity Semi crystallinePA Polyamide 80120 Good mechanical properties Semi crystallinePSU Polysulfone 150 Chemical and temperature Amorphous

resistivityPVDF Polyvinylidenefluoride 150 Chemically inert, piezo-electric Semi crystalline

Mold insertsInjection channel

(a)

Mold insertsInjection channel

Polymer injection

(b)

Polymer injection

(c)

Figure 1. Principal process steps of micro injection molding:(a) the molding tool is closed, evacuated, and heated above the glasstransition temperature of the polymer, (b) the polymer is injectedinto the tool, and (c) tool and polymer are cooled down and thepolymer is demolded.

equipped with a micro structured tool (mold insert) is closed,evacuated, and heated above the glass transition temperatureof the polymer (see figure 1(a)), an injection unit heats thepolymer up and presses the viscous polymer into the mold(figure 1(b)), and the polymer (and the tool) is cooled downbelow its glass transition temperature and demolded from thetool (figure 1(c)). This cyclic temperature control is calledvariotherm (variothermal).

Figure 2 displays a photograph of parts made from PC asthey come from the injection-molding machine. They are stillconnected to the sprue and runners, which are the part of theinjection channel and distribution system, respectively, whichare molded together with the micro parts to be fabricated. Fromthe micro parts, shown in figure 2, optical fiber connectors [2]are assembled.

Figure 2. Typical batch of micro structures as it comes from aninjection-molding machine.

Injection molding is a technique that has been wellestablished in the macroscopic production of polymer parts fordecades. Therefore, vast know-how and machine technologyis available to be made use of in micro injection molding aswell. In general, cycle times are shortest (on the order ofsome minutes) when producing micro parts from polymersby injection molding. In almost all cases industrial seriesproduction is based on injection molding.

2.2. Reaction injection moldingReaction injection molding is similar to injection molding, butinstead of one type of plastic, two components are injected intothe closed molding tool. This technique allows fabricationof parts from polymers that are not thermoplastic, such asthermosetting materials and elastomers. The manufacture ofmicro parts by reaction injection molding was investigatedin the mid-1980s [3, 4] but turned out to be difficult toperform because a good mixture of the components needsto be achieved on the micro scale and a chemical reactionhas to take place in the micro structures of the molding tool,which requires a comparatively long time and results in longcycle times. Now, with the possibility of UV-curing insteadof thermal initiation of the polymerization, reaction injectionmolding has appeared again. Today, this process is quite fastand allows a type of rapid prototyping, because thermal cyclingis not necessary any more.

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Mold insertsThermoplastic foil

(a)

Mold insertsThermoplastic foil

(b)

(c)

Residual layerResidual layer

Figure 3. Principal process steps of hot embossing: (a) athermoplastic film is inserted into the molding machine, (b) themolding tool is evacuated and heats up the polymer above itssoftening temperature, and (c) the polymer is cooled down anddemolded.

Figure 4. Typical batch of micro structures molded by hotembossing.

2.3. Hot embossing

The principal process steps of hot embossing are [5]: Athermoplastic film is inserted into the molding machine (seefigure 3(a)), a micro-structured tool (mold insert) in anevacuated chamber is pressed with high force into the film,which has been heated above its softening temperature, and themold insert is filled by the plastic material which replicates themicrostructures in detail (figure 3(b)). Then the setup is cooledand the mold insert is withdrawn from the plastic (figure 3(c)).Figure 4 displays a photograph of a batch of housings of microvalves [6] from PSU as it comes out of the molding machine.

The embossing die and countertool are not closedcompletely, a characteristic residual layer is produced byhot embossing. The residual layer may serve as a kind ofmagazine during demolding and handling before a subsequent

dicing process. The main costs of molded micro componentstypically result from mounting them into a system. This can befacilitated significantly by arranging single micro componentson a batch in a well-ordered manner.

In contrast to injection molding, during hot embossingthe polymer flows a very short way from the foil into themicro structure only. As a result, very little stress is producedin the polymer and the molded parts are well suited as opticalcomponents, such as wave-guides and lenses. The temperaturecycle of the polymer may be smaller for hot embossing, sincethe viscous polymer does not need to be transported a long wayinto the molding tool. This reduces shrinkage during coolingand the friction forces acting on the micro structures duringdemolding. Thus more delicate micro structures with higheraspect ratios can be fabricated by hot embossing compared toinjection molding. Hot embossing is particularly suited forforming plane plates or foils, as a small amount of plastic hasto be molded only.

Hot embossing allows for a very simple setup of theplant, which is particularly advantageous if tool or plantreconstructions or modifications are necessary. This results invery short set-up times. When using standardized mold inserts,a few minutes are sufficient to exchange a tool. Moreover, foilsmade of various thermoplastic materials can be put into themachine successively without any further modifications beingrequired. Therefore, hot embossing makes the production ofsmall and medium-scale series economically more efficientand is especially suited for laboratory applications.

On the other hand, relatively long cycle times of up to30 min may be required for some components. For someapplications such a long time can be advantageous if, e.g.,inner stress is reduced by extreme slow cooling rates. Butlong cycle times are caused mainly by the fact that theheated polymer is not supplied continuously by an injectionunit. Such problems can be reduced considerably by furtherdeveloping hot embossing machines and their periphery ashas been done for injection molding machines. The principallimit for shortening the molding cycle of hot embossing is a bitlarger than for injection molding, because in injection moldingthe molten polymer can be filled into a mold insert which iscolder than the softening point of the polymer while in hotembossing the polymer needs to be heated up by the moldinsert. This means that the thermal cycle of a mold insertcan be smaller in an injection molding machine than in a hotembossing machine. On the other hand, hot embossing hasnot yet reached this limit and it is not yet clear how large thedifference in cycle time will be, due to this effect.

Cycle times are also strongly affected by the tool design.For mass fabrication a tool can be designed with heatingand cooling features adapted to the special design of thecomponent. Such an expensive tool will be not very flexibleand not be applicable for another micro component.

Originally designed for the LIGA standard format of26 66 mm2, the embossing area has been extendedconsiderably in the past few years, and this year moldingmachines for samples with a diameter of up to 200 mm arebeing introduced onto the market. With increasing area,shrinkage of the plastic component gains significance. Asophisticated execution of the process is required to preventdeformations, such as overdrawn edges of the components or

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even damage of the mold insert. Shrinkage of hot embossedsamples is smaller, and, therefore, it is especially suited for afurther extension of the format.

2.4. Injection compression moldingInjection compression molding is a combination of injectionmolding and embossing to overcome the problem of heatingthe polymer by the tool. The plastified polymer is injectedfrom a screw into the semi-closed molding tool and thenpressed into the micro structures by closing the tool. In thisway, the problem of injection through a small gap is avoidedwhen producing a micro structure on a thin carrier layer.Injection compression molding is widely used to produce CDsand DVDs. CDs possess critical dimensions of less than onemicro meter, but the aspect ratio is rather small and, therefore,demolding is no problem.

2.5. ThermoformingMicro thermoforming is employed to form thin thermoplasticfilms. The polymer film is inserted into a molding tool,which shows micro structures on one side (mold insert). Themolding tool is evacuated (see figure 5(a)), the film is clamped(figure 5(b)), heated up and pressed into the micro structure bya gas (figure 5(c)), cooled down and demolded (figure 5(d )).Figure 6 shows a 25 m thick PS film with 125 m deep and250 m wide micro channels.

Only a few papers have been published on molding ofthin thermoplastic films [7, 8]. In contrast to the processesdescribed above, the film does not fill the micro structures ofthe molding tool completely. The film may not be heated sothat it becomes too soft, because the permeability of polymersfor gases increases with temperature. As a result, and due tothe high flexibility of thin films, demolding is easy for filmsmolded by thermoforming, and even micro structures with across section similar to the form of the Greek letter can bedemolded. Thermoforming is not suitable for achieving highaspect ratios, as a thin film cannot be stressed too much, if itis not made soft enough by heating.

3. Micro molding tools

Molding tools used in micro engineering generally consist ofa micro-structured mold insert and the tool. This separationis unknown in macroscopic molding technology. It is dueto the completely different requirements to be met by themicro structure and by the tool. The tool has to fulfill theclassical tasks of encapsulating the polymer and ejecting themolded parts. In addition, the micro-molding tool needs toprovide for a vacuum and to undergo a variotherm process.Evacuated tools are required in micro molding, because themicro structures form pocket holes from which air cannotescape when the polymer is filled in. Consequently, thestructure has to be empty right from the start. In addition,the mold inserts need to be heated above the glass transitiontemperature, so the small amount of thermoplastic materialprocessed does not solidify immediately when getting intocontact with the large mass of the mold insert. For demolding,the mold insert has to be cooled down again.

Polymer film

Sealing

Mold insert

Evacuation

(a)

Polymer film

Sealing

Mold insert

Evacuation

Polymer film

Sealing

Mold insert

Evacuation

(b)

Pressurized gas

(c)

Pressurized gasPressurized gas

(d)

Figure 5. Principal process steps of micro thermoforming: (a) athermoplastic film is inserted into the molding machine and the toolis evacuated, (b) the film is fixed by clamping, (c) the molding toolheats up the polymer above its softening temperature and a gaspresses the film against the mold insert, and (d ) the polymer iscooled down and demolded.

Figure 6. Micro channels manufactured in a PS film by microthermoforming.

The requirements, which need to be fulfilled by the moldinsert, are very different. The mold insert has to provide forthe primary micro structure and, therefore, is manufacturedwith techniques appropriate for this. The micro structureshould exhibit smooth side walls to avoid friction duringdemolding and a small inclination angle is desirable, if thiscan be tolerated by the application of the micro structuresto be molded. Furthermore, the mold insert should preserve

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(a) (b) (c)

(d ) (e) (f )

Figure 7. Mold inserts fabricated by different processes: (a) milling with a CNC machine, (b) laser-manufactured tool with differentsurface roughnesses, (c) x-ray lithography and electroplating, (d ) silicon etching (courtesy of Jenoptik Mikrotechnik GmbH),(e) photolithography with SU8, and ( f ) electric discharge machining (courtesy of Institut fur Mikrotechnik Mainz).

the micro structure over many molding cycles and withstandlateral and normal forces during injection and demolding. Thisis achieved by using a material that is hard and ductile enoughfor the mold insert and by choosing a suitable design. Whichmaterial is hard and ductile enough and which design is properdepends on a lot of parameters, such as the polymer to bemolded, the molding process, etc, and no general rule can begiven.

While the tool with its closing and sealing unitsand ejection systems is manufactured by classical metalprocessing, various methods are applied for micro structuringthe mold insert. These are methods of direct structuring,including mechanical micro machining, laser structuring, andelectric discharge machining (EDM) on the one hand, andlithographic processes with x-rays or UV radiation combinedwith electroplating, on the other. Figure 7 shows someexamples of mold inserts fabricated by different techniques.

Electroplating allows fabrication of a metal mold insertfrom micro structures made of plastics, silicon, and othermaterials not suitable for use as a mold insert [911]. It maybe also advantageous to transform a metal micro structureworked from a solid soft metal into a hard alloy (e.g., FeNior CoNi) or hard metal by electroplating. Another reason foremploying electroplating is that some critical micro structuressuch as narrow grooves or sharp concave corners cannot bemilled while their inverted forms (narrow rib or sharp convexcorner) can easily be manufactured by mechanical means.Even structures in the nanometer range can be replicated byelectroplating and, therefore, this is an important techniquefor tool manufacturing. For very high microstructures moldinserts need a thick backing in order to provide the necessarymechanical stability. This results in quite long plating times

of up to some weeks. For certain purposes thinner backings(shims) are possible and electroplating is much quicker.Another problem is that the electrochemical process is verysensitive to disturbing artifacts, such as thin insulating layersand gas bubbles in the electrolyte. Control of the concentrationof chemicals in the electrolyte is another important issue,which may affect the mechanical stress in the mold insert,and thus, may result in the bending of the mold insert.Consequently, a lot of know-how is required for electroplatingmold inserts. This know-how is available at some companiesand research institutes, which offer electroplating of moldinserts as a service [12].

Mechanical micro machining is closest to traditional tooltechnology. Techniques such as turning, drilling, or milling areemployed for fabricating mold inserts (see figure 7(a)). Moldinserts fabricated by mechanical micro machining with CNCmachines are offered by companies (e.g., [13] and [14]) andavailable from research institutes e.g., [15]. The smoothestside walls of micro structures are obtained with diamond toolsbut these are not suitable for work in steel which is a favoritematerial for mold inserts. Moreover, the smallest diameterof diamond tools is approximately 200 m. When narrowergrooves need to be fabricated on a mold insert or the mold insertneeds to be made of tool steel, milling and drilling tools madeof hard metal can be used, with the requirements regardingthe smoothness of side walls being reduced. Compared tolithographic processes it is easy to fabricate mold inserts withthree-dimensional micro structures even with curved surfacesby mechanical micro machining, and sloped side walls of themicro structures can be achieved by simply using milling toolswith the required profile.

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Another technique initially developed for macroscopicuse is electric discharge machining (EDM) (see figure 7( f )).Today, it also allows micro structured mold inserts to beproduced. This is achieved by using wires as thin as 30 m forwire erosion or by sinking erosion with electrodes producedby electroplating. In this way, micro structures from nickelor other metals can be transformed into mold inserts made ofsteel. A disadvantage of this technique is that the side wallsof micro structures fabricated in this way are rough comparedto milling.

Lithographic processes are particularly suited for minutestructures. A resist is patterned with a micro structurethat is electroplated to build up a mold insert. ClassicalLIGA technology based on x-ray deep-etch lithography ischaracterized by extremely high structural heights, smallestlateral dimensions, and side walls with a roughness of lessthan 50 nm (see figure 7(c)) [16, 17]. UV lithographyis a less complex and less expensive alternative to x-raytechnology, which is able to meet less demandingspecifications. Both methods are based on the lithographicgeneration of non-conductive plastic micro structures that arefilled by electroplating. Mold inserts made by both methodsare available from companies and research institutes [12].

Mold inserts from silicon (see figure 7(d )) or glass microstructured by wet etching [1820] or reactive ion etching [21]can also be employed for micro molding, but silicon is a verybrittle material, so wafers tend to break. Therefore the waferis bonded on top of a quartz glass to get a stable compound. Ifless than 10 molding steps are to be performed for prototypingpurposes, mold inserts from silicon may be suitable. Silicontreated by wet etching is more favorable, because it showsinclined side walls and a smoother surface than dry-etchedsilicon.

Another possibility for structuring metal is lasertechnology (see figure 7(b)) [11]. This development is stillat its beginning, but promises to have enormous potential interms of aspect ratios and minimum structural dimensions.This technology is of particular interest, as it allows processingof materials such as stainless steel or tungsten carbide.

4. History

To the knowledge of the authors, the first paper on micromolding of thermoplastic polymers was published in 1970 bya group of researchers from RCA laboratories at Princeton,NJ, USA [22]. The objective of this work was to develop alow-cost reproduction technique of hologram motion picturesfor television playback [23]. A master tape was made byelectroplating nickel into photo resist patterns. The masterwas run through heated rollers together with a vinyl tape and,thus, the micro structure (see figure 8) was transferred into thevinyl.

This work was continued in Zurich, Switzerland[24], where diffraction gratings for color filtering weremanufactured by hot embossing of PVC [25]. The ratio ofthe depth of the micro structure to its width reached up to5.7 (0.4 m wide and 1.4 m high gratings). Work at RCAlaboratories was stopped, because no market success had beenachieved [26].

Figure 8. SEM of a hot-embossed micro structure from PVC for ahologram on a movie picture, 1.4 m in height [25].

The first paper on micro molding of optical waveguideswas published in 1972 [27]. A simple groove was hotembossed into PMMA with a glass fiber and the groove wasfilled with poly(cyclohexyl methacrylate) (PCHMA) whichshows a higher optical index.

Independent of this work, the development of micromolding started in Karlsruhe, Germany in the mid-1980s as areplication technology for LIGA microstructures, to providethis technique with an economic mass fabrication process.LIGA initially was an acronym for Lithografie und Galvanikwhich means lithography and electroplating [28]. Thenmicro molding was introduced in the process and the acronymwas reinterpreted as Lithografie, Galvanik und Abformungwhich means lithography, electroplating, and molding[3, 29]. First investigations had been carried out using reactioninjection molding [4] until it turned out that injection moldingis a much easier process, which can be performed with a shortercycle time. Therefore, further work focused on injectionmolding [30, 31].

During the first years the parts manufactured with micromolding were intended to demonstrate that high aspect ratios,steep side walls, and stepped profiles can be achieved andvarious materials can be used [30, 31]. In the following years,micro molding turned out to be the most important productionstep of LIGA in industrial applications, because this low-costprocess makes LIGA an economic success. On the otherhand, micro molding can not only be done with LIGA-mademold inserts. A lot of products do not need to have highaspect ratios, and inclined side walls are advantageous for thedemolding of micro structures. Mold inserts made by micromilling and electroplating of micro structures made by UVlithography in several 10 m thick resist layers were broughtup for discussion.

Meanwhile, the development of hot embossing had beenstarted at Karlsruhe [32]. The intention of this work wasto find a way to produce a molded LIGA microstructureon top of electronic circuits, i.e. to fabricate an accelerationsensor directly on top of an amplifying circuit on a siliconwafer [33]. When hot embossing turned out to be suitablefor molding micro structures with aspect ratios as highas 10 and introduction of low mechanical stress in theproducts, this process was employed for other devices as well[34, 35].

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Figure 9. Micro pump fabricated by injection molding [45].

In the same year in which the first publication was madeby the Karlsruhe group, a group from Zurich reported on theirwork related to hot embossing of integrated optical microstructures [36, 37]. Two years later, the first paper on hotembossing by a group from Mainz, Germany, was published[38] and first papers on micro molding of thermoplasticpolymers from Santa Cruz, CA, USA [26], Middlesbrough,UK [39], Dortmund, Germany [40], Stockholm, Sweden[41], Ann Arbor, MI, USA [42], Hayward, CA, USA [19],Gaithersburg, MD, USA [18], Jena, Germany [43], and Taiwan[44] followed.

In recent years hot embossing became an emergingtechnology for the fabrication of electronic devices withcritical dimensions in the nanometer range [7984].

The first components based on micro molding were madefor micro optical applications as described above [22, 27].The first micro fluidic component was a micro pump [45](see figure 9). It was fabricated by injection molding frompolysulfone (PSU). Two housing shells fabricated by moldingwere adhesively bonded to a membrane made of polyimidepatterned by photolithography. The mold had been fabricatedby milling with a CNC machine. The fabrication processof the micro pump was later called the AMANDA process[46]. Today, it is still employed to produce micro fluidicdevices. With this process and other similar processes several

Heater wireMembrane Heater wireMembrane

Figure 10. Flow sensor made of PSU with a support membrane from PI [52]. By the design of the membrane, sensitivity to temperaturechanges is reduced.

new components were developed in the next years such asmicro pumps [41, 47], micro valves [4850], and micro sensors[51].

5. Design

Polymers show properties that differ from those of silicon,glass, and metals. Of major significance are thermalexpansion, creep, diffusion, and chemical properties.Especially the large thermal expansion of polymers has to beconsidered when micro components are to be designed frompolymers. For a lot of applications it is necessary to combinedifferent materials, which results in stresses that are a functionof temperature. Even when two different types of polymers arebonded to each other the difference in their thermal expansionmay cause remarkable effects. On the other hand such effectscan be avoided with a proper design.

A 10 mm long device with a typical expansion coefficientof 100 ppm will be extended by 1 m with every degreeof temperature change. This means that polymers are notsuitable for building up high-precision components with smalltolerances. The example of an anemometric flow transducer[52] illustrates how such difficulties can be overcome.Figure 10 shows that a metal heater wire is mounted in aflow channel. The wire is supported by a membrane, which isadhesively bonded between two hot-embossed housing shells.Both the wire and the membrane are designed to be verythin (100 nm and 2.4 m, respectively) in order to achievea low heat capacity. If the membrane was spread over the fullwidth of the flow channel, thermal expansion of the housingwould result in a change of the transducer signal, becausethe wire would behave like a strain gauge. Therefore, themembrane is fixed to the housing at two points only andthe sensitive part of the wire is placed on a tongue-like partof the membrane, which is not affected by the strain of thehousing. Moreover, the heater wire is arranged on the neutralaxis where bending of the membrane does not result in anystrain. In this way, the signal of the transducer is changed by1% only when the temperature rises from 20 C to 60 C.

Polymer parts tend to creep much more than metals andglasses when subjected to a permanent load. Making housings

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from polymer does not cause any problem in most cases,because they can be designed thick enough to withstand theload. However, creep is a problem if, e.g., the elastic elementof a sensor or actuator is made of a polymer. Often, it is agood idea to fabricate the housing by micro molding and useanother material as an elastic element.

It is also known that polymers allow for the diffusion ofgases and are not tight over a longer term. The usual way ofsolving this problem is to evaporate approximately 100 nm ofa metal layer onto the polymer [53]. This increases fabricationcosts, but is no problem under most circumstances.

The design of polymer micro components needs toaccount for bonding techniques which differ very muchcompared to bonds made between silicon, glass, or metals.Adhesive bonding is quite popular, because it avoids hightemperatures that may damage micro structures or inducemechanical stresses. If adhesives are used, they need to bekept away from micro cavities by stopping grooves or sealingpartitions [54, 55]. The design of the same micro componentmay be very different when it is to be bonded by ultrasonicpower, because energy directors are necessary in this case.The lid of capillary systems is often bonded by welding[5660] or with solvents [59]. Most of these bondingtechniques are tolerant to small damages or particles onthe surface to be bonded because adhesives can fill smallholes, ultrasonic power and welding processes locally meltthe polymer and the liquid may close holes, and solvents letpolymers swell. This facilitates the design of polymer microcomponents and contributes to the success of micro molding.

Last but not the least, there are design rules caused bythe molding process. Sharp corners should be avoided in thedesign where possible, because they cause stress peaks in thepolymer, which may result in cracks.

Most problems in micro molding are not caused by thefilling of the mold, but by demolding. During demolding,micro structures may be torn apart, deformed, or destroyed.Demolding very much affects the wear of mold inserts anddelicate parts of the mold insert may even be destroyed after asingle molding process, if the micro structure is not designedproperly or unsuitable molding parameters are chosen. It ispossible to demold micro structures with vertical side walls,but an inclination angle of just 2 reduces demolding forces alot and is even more important than the roughness of the sidewalls. An important factor in demolding is the shrinkage ofthe polymer, which occurs when cooling down the polymerbetween the filling of the mold and demolding. Therefore,demolding forces also are a function of the orientation ofmicrostructures relative to the direction of shrinkage and theplacement of critical micro structures relative to the center ofshrinkage. This is why the path of the polymer into the moldhas to be chosen with care for injection molding.

Delicate micro structures, such as pins with a high aspectratio can be protected against shear forces resulting fromshrinkage and mold filling by neighboring auxiliary structureswhich are stable enough to withstand these forces.

Shrinkage of the samples may be not identical over alonger production run. For this reason, molded parts thatneed to fit together in the later production should be placed onthe same mold insert and pieces from the same molding stepshould be attached to each other.

If a mold insert is covered by areas with micro structuresof different height or varying density, these areas should bearranged symmetrically on the mold insert. In this way, tiltingof the mold insert during molding is avoided.

6. Applications

Micro molding has been employed to fabricate a variety ofpolymer components. Most applications are in the field ofmicro optics and micro fluidics, but there are also someexamples of micro (and nano) electrical and mechanicaldevices.

The most widely sold micro molding product probablyis the well-known CD and DVD for data storage, music,and videos. Another wide spread article are hologramswhich are affixed to credit cards [26]. Other moldedmicro optical components are spectrometers [35, 62](see Figure 11(a)), lenses [6567], optical switches [68, 69],optical fiber connectors [2], waveguides [27, 70, 71],anti-reflective surfaces [72], optical gratings [9, 25], andphotonic structures [73].

There is a variety of molded micro fluidic devices,such as pumps [41, 46, 47, 64] (see Figure 11(d ), valves[6, 4850], nebulizers [62], ink jets [62], degassers for HPLCsystems [74], capillary analysis systems [1820, 63, 7577](see figure 11(c)), devices for investigations of living cells[62, 78] (see figure 11(b)), pressure sensors [51], and flowsensors [52]. Today, the largest turnover is probably reachedwith nebulizers for inhalers used in asthma therapy, the largestmarket is expected for lab-on-a-chip applications.

Prospective applications of thermoplastic molding alsolie in electronics. One of them is nano imprinting [7984]which may allow for the low-cost replication of electroniccircuits with critical dimensions as small as ten nanometers.Embossing techniques provide for the transfer of the desiredgeometry into a thin resist layer on a silicon wafer, and thisgeometry is the basis for generating circuits by reactive ionetching, lift-off, and other methods. For this applicationthe polymer is used in an intermediate step only. Hence,the most important properties are a good adhesion to thesubstrate, dimensional stability, easy dissolution, and a lowglass transition temperature which allows one to keep thethermal cycle small or even to avoid it completely.

Other future electronic applications may be electronic andoptical circuit boards [71], acceleration sensors [33, 85], andsimple devices, such as electrical switches [86], may find amarket share as well.

7. Machine development

The development of micro molding and the development ofmolding machines have always been connected with eachother. The simple process technology, which mainly requiresa heating device and a pressure unit, inspired several researchgroups to start thermoplastic molding [87]. Simple opticalstructures with aspect ratios of up to 1 did not requireany special equipment. Modified laminating presses orsmall stamping presses with an electrical heating plate weresufficient to transfer microstructures from a metallic master toa plastic sheet.

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(a) (b)

(c) (d )

Figure 11. Products made by thermoplastic molding. (a) Hot-embossed micro spectrometer [35], (b) injection-molded cell chipLILLIPUT [62] (courtesy of Steag microParts GmbH and Merlin Diagnostica GmbH), (c) hot-embossed micro titer plate [63],and (d ) piezo-driven micro pump XXS2000 made by injection molding [64] (courtesy of thinXXS GmbH).

The goal to provide reliable commercial equipment formicro replication led to adaptation of the CD stampingprocess and film extrusion technology. CD technology verymuch inspired injection compression and injection moldingtechnologies, thus leading to a complete new machinegeneration [92] (see figure 13).

Film fabrication with continuous structuring from roll-to-roll was the first implementation of thermoplastic molding ofmicro structures [22] and still provides us with holographicsecurity elements and light-reflecting films for differentpurposes [8991]. In this case the demanding task is todevelop suitable rotating tools. Production machines are notcommercially available, but need to be developed for eachapplication.

The appearance of the LIGA technology led to a newchallenge in the thermoplastic molding of micro structures:The replication of high aspect ratios. As a result, moldfilling and demolding became two new issues. Groovesand pin holes of some hundred microns depth and a fewmicrons width only required a very low viscous polymer formold filling. In Karlsruhe, first attempts in the mid1980sfocused on reaction injection molding using thermosets oflow viscosity [4]. A reaction injection molding machinewas constructed (see figure 12), but process difficulties andproblematic properties like large shrinkage prevented a successsimilar to the one achieved by UV-curing and PDMS castingnowadays.

Figure 12. Home-made reaction injection machine [4].

The broader material range then gave rise to the idea ofdeveloping thermoplastic replication of micro structures, anda commercially available injection molding machine for CDproduction was employed. It needed to be adapted to themolding of micro structures, because operation at very hightemperatures close to the decomposition temperature of thepolymer and integral heating of the mold and material turnedout to be insufficient for complete mold filling. Trapped air

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(a)

(c) (d ) (e)

(b)

Figure 13. Commercially available micro molding machines. (a) Injection molding machine for CD fabrication [88] (courtesy ofFerromatik Milacron Maschinenbau GmbH), (b) hot embossing machine HEX03 [93] (courtesy of Jenoptik Mikrotechnik GmbH), (c) hotembossing machine Stamp Press MS 1 [94] (courtesy of Wickert Maschinenbau GmbH), (d ) injection molding machine microsystem 50[92] (courtesy of Battenfeld Kunststoffmaschinen GmbH), and (e) EVG R 520HE hot embossing system for embossing and nanoimprinting[95] (courtesy of EV Group).

caused defects in high aspect ratio micro structures. Therefore,a vacuum unit needed to be included.

Another constraint in high-aspect-ratio molding is a highstiffness of the machine frame to ensure precise movementsduring demolding. Steep side walls mean contact betweenthe mold and replica during the whole demolding process.Any lateral movement has to be avoided along the demoldingpath of up to some millimeters. Finally, the requirementmade on machine control is extremely high. Minimal speedof a few microns per second and position control with anaccuracy of a few microns even at high forces are necessary.Critical points are transient stages when the polymer softensand viscosity changes rapidly. At this moment machinecontrol must maintain the pressure without damaging the moldinsert.

In cooperation with Jenoptik Mikrotechnik, the first hotembossing press suitable for high aspect ratio replication wasdeveloped at Karlsruhe: The HEX02. The high accuracyof this machine allows alignment of the upper and lowercrosshead of the machine very precisely. Thus, aligned

molding on both sides of a polymer or on an alreadypatterned substrate is possible. A new concept of alignmentworking also under high load and high temperature changeswas developed under the BMBF-funded project PROBE[96] and implemented in the machine generation of HEX03(see figure 13(b)). An alignment accuracy of less than 10 mcan be achieved with this machine.

Hot embossing machines which aim at the industrialmarket are the Stamp Press MS 1 (see figure 13(c)) [94], whichwas the first machine used by industry for hot embossing ofmicro spectrometers. This year, a new type of hot embossingmachine will be introduced on the market by the samecompany. This machine reaches much shorter cycle timesand is able to produce hot-embossed layers of up to 200 mmin diameter. It can be equipped with an automatic handlingsystem, which contributes essentially to the reduction of cycletimes, because the polymer workpiece can be exchanged morequickly and at much higher temperatures.

Another direction of development are for machinessuitable for the production of smaller details. The demand

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for nano replication results in a slightly different machinetechnology. Machines for large-area replication on wafer areasof up to 200 mm with modified wafer bonding equipment areavailable from Karl Suss and EVG (see figure 13(e)) [95, 97].

8. Limitations and solutions of special problems

What are the limitations of micro molding of thermoplasticpolymers? The investigations of nano imprinting show thateven structures of only a few tens of nanometers in size arereplicated very well. This is a bit astonishing, because thesize of the macromolecules the polymer consists of is morethan that. Obviously, the macromolecules to a certain extentadapt to the form of the mold. Hence, the limit of the smallestpossible feature size has not yet been approached.

However, there is a limitation regarding the achievableaspect ratio of columns, grooves, and walls. This limit is afunction of the geometry of the microstructure, its positionon the sample, the polymer type, and the process parameters.Therefore, there is no simple rule for giving us the maximumaspect ratio which can be achieved in a particular case. Anextreme case is a groove with a width of 10 m and a depthof 100 m, which is filled and demolded with PMMA byhot embossing. Another example is the spectrometer with270 nm deep steps of an optical grid in a vertical PMMA wall of125 m in height, which are both hot-embossed and injection-molded. Finite element calculations could help solve moldingproblems of high-aspect-ratio microstructures, but FEM codesare available for mold filling only and the limiting process stepis demolding of the microstructures. Development of FEMcalculations of demolding has just started [98].

Another limitation is the overall size of a sample or batchof microstructures, which is molded in one step. Shrinkage ofthe polymer is a function of the overall size and the farther adelicate microstructure with a high aspect ratio is placed fromthe center of shrinkage, the more difficult does demoldingbecome. On the other hand, shrinkage can be reduced both bya proper design and by a proper molding process. Therefore,further development may open up the way to larger samplesand even lower costs for multiple components produced in onebatch.

Moreover, the production of through holes may bedifficult, because a direct contact of delicate microstructuresof the mold insert with the counter plate may cause damage.However, if a small gap is left in-between, it will be filledwith the polymer, because the molding process needs to beperformed such that microstructures are filled. In general, athin residual layer needs to be removed after molding to openup through holes.

An alternative to this may be hot embossing of compositelayers [99]. Interesting effects may be achieved by replacingthe simple semi-finished product by a composite of severalfoils. An example is the micro spectrometer for the UV-VIS range [35] (see figure 11(a)). By combining PMMAfoils of variable refraction coefficients, a waveguide for lighttransmission is obtained. If polymers are chosen, which arenot welded during embossing, but still adhere strongly to eachother so that they can be demolded together and separatedafterwards, through holes or separate, microstructures can begenerated (see figure 14).

Figure 14. Micro riddle (300 300 m) hot embossed into theupper layer of a stack of two polymers which were separatedafterwards [99].

The second layer may be a metal conduction path on thesurface of the plastic semi-finished product [100]. During hotembossing this conduction path follows the topography and,thus, an electric connection from the surface to the structurebase is generated. This is a way providing micro fluidicstructures with an additional functionality.

9. Conclusions and outlook

Micro molding of thermoplastic polymers today is a well-established process. Several micro molding machines aresold on the market and mold inserts fabricated with varioustechniques suitable for most applications are available. This isreflected by an increasing interest of industry in micro moldingprocesses and machines.

While injection molding is the process most frequentlyused for micro molding in industry, because short cycle timesare achieved, hot embossing is most popular on the laboratoryscale, because it is more flexible and more delicate structurescan be produced. Alternative processes are under investigationby research groups.

Further research work will focus on achieving higheraspect ratios on larger scales and on developing specialfunctionalities of molded parts, such as through holes andelectrical paths. Apart from this work, there is still a needfor the development of processes to be applied after molding,such as bonding, and for establishing reliable interconnectionsto the macroscopic world.

Research is required with respect to a proper design ofdevices and components, because design rules can neither betransferred without changes from micro components made ofsilicon or glass nor from macroscopic polymer products.

There are a variety of applications already known formicro molding of thermoplastic polymers and many more areexpected to come up in the future.

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