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Kunststoffe international 4/2009
MICHAEL STRICKER
GEORG PILLWEIN
JOSEF GIESSAUF
Optical technologies are consideredone of the most promising fields ofthe future. The opportunities for
the plastics industry are to be found inautomated production of high-qualityoptical components in large quantities [1,2]. Injection molding of such compo-nents poses greater challenges than doesconventional injection molding, specifi-cally with regard to accuracy of surfacereplication and the level of molded-instresses. Moreover, it is not unusual tohave wall thicknesses as much as ten timesgreater than those typically encounteredin injection molding (Fig. 1). When itcomes to producing high-quality opticalparts in a cost-effective manner, the con-ventional injection molding processquickly reaches its limits. For this reason,existing special processes undergo con-tinual refinement and new approaches toprocessing are introduced:■ Injection compression molding is well-
suited to produce molded parts withhigh replication accuracy and lowmolded-in stresses.
■ Multi-layer injection molding can beused to increase replication accuracy aswell. With thick-walled parts, it is fur-ther possible to reduce the cycle time[3].
■ Variotherm mold temperature controlis not limited to only improving thequality of replication of micro- ornano-structured surfaces [4].
Table 1 presents an aid to decision-mak-ing with regard to selection of the appro-priate process. The individual processesare discussed in greater detail in the fol-lowing.
Injection Compression Molding– a Well-established Process,Modern Machine Technology
The injection compression moldingprocess has been used for several decadesto successfully mold optical components[5]. In this process, the plastics melt is in-jected into an oversized cavity and sub-sequently compressed by moving moldelements during the compression phase.
For molded parts with a large flowlength/wall thickness ratio, partial fillingof the greatly oversized cavity takes placefirst. In this case, the shape is not formeduntil the compression phase; reducedmold filling pressure and thus lowermolded-in stresses are the result. Forthick-walled parts, it is useful to fill the
cavity during the injection phase and em-ploy the compression stroke to simplycompensate for shrinkage. Compared toconventional injection molding, themolded-in stress level and replication ac-curacy can also be improved in this way.
Depending on the geometry of themolded part, compression may take placeover the entire surface or only over seg-ments of the surface. To prevent the plas-tics melt from flowing into the partingline, sealing frames that enclose the cav-ity are often used. The necessary sealingforce can be applied by spring elementsor hydraulic cylinders (Fig. 2). To preventthe melt from flowing back into the in-jection unit, shutoff nozzles or slides must
Injection Molding Optical Components. With
above-average growth rates, optical technolo-
gies can be considered as setting the pace for
the 21st century. However, the challenges
when injection molding optical parts fre-
quently differ from those encountered
when injection molding conventional parts.
In the face of international competition, the
focus is shifting increasingly to innovative pro-
cessing techniques.
Deciding to take on production of optical parts requires a clear understanding of the challenges (photo: Pillwein, Engel)
Fig. 1. Thick-walled LED lens for an OP lamp,molded from PMMA (photo: Trumpf Medizin Systeme)
Focus on Precision
Translated from Kunststoffe 4/2009, pp. 30–34Article as PDF-File at www.kunststoffe-international.com; Document Number: PE110084
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be installed as a blocking mechanism [6].These should be positioned as close to thecavity as possible to minimize movementof the mass and the molded-in stressesproduced as a result.
In addition to mold design, the ma-chine and controls are essential factors insuccessful production of optical parts.Be-cause of its similarity to the injection andholding pressure phases, a newcomerquickly becomes familiar with the injec-tion compression software developed forthe all-electric “e-motion” Series (manu-facturer: Engel Austria GmbH, Schwert-berg, Austria). The operator interface isorganized into the steps■ speed-controlled compression,
transfer,■ cavity-pressure or force-controlled
compression and ■ position control (isochoric cooling)corresponding to the typical process se-quence (Fig. 3).
During the speed-controlled com-pression phase, the cavity is filled volu-metrically and the melt compressed. Theoperator can trigger transfer optionallyvia position, time, compression force orcavity pressure, with the compressionforce or cavity pressure profile beingvariable as required by the application.While the part continues to cool to theejection temperature, the molded-instress level is determined primarily by
the pressure gradients present and theextent of material movement.A high lev-el of molded-in stresses adversely affectsthe optical characteristics of lenses,among other things, and can causecracks to form in subsequently appliedcoatings [7, 8].
Isochoric cooling prevents materialmovement and helps to reduce the mold-ed-in stress level [9]. The compressiongap – and thus the volume of the cavity –is kept constant. For this purpose, a posi-tion sensor is attached to the mold andused for position control.
The Accuracy of Electric Injection Molding Machines
Surface replication accuracy require-ments are especially high for optical com-ponents. Dimensional tolerances of lessthan ± 20 µm are quite common. For thisreason, all relevant machine movements
Process
Molded part properties
Injection compression Variotherm mold temperature control Multi-layer injection molding
Thick wall (> 4 mm)
High replication accuracy by applyingholding pressure via compressionstroke; no thick sprue/gate needed
Fewer molded-in stresses due touniform holding pressure
Reduction of jetting marks
Cooling time reduction possible
Cycle time reduction possible
Improved replication accuracy, assubsequent layers compensate forshrinkage
Less risk of jetting
Stresses at interfaces and warpagepossible
Thin wall (< 4 mm)
Lower filling pressure when injectinginto oversized cavity
Reduced molded-in stresses due tolower pressure gradients during fillingand holding pressure phase
Lower filling pressure due to slowercooling of melt
Longer cycle time to be expected
Negligible benefits with thin walls
Large wall thicknessdifferences
Compression over partial surfaceincreases replication accuracy
Compression over entire surface: thinregions limit duration of compression
Limited heating optionsImproved replication accuracy, assubsequent layers compensate forshrinkage
High flow length / wall thickness ratio
Lower filling pressure when injectinginto oversized cavity
Reduced molded-in stresses due tolower pressure gradients during fillingand holding pressure phase
Lower filling pressure due to slowercooling of melt
Warpage with variotherm temperaturecontrol on one side
Further increase in flow length / wall thickness ratio, thus higher filling pressure, more molded-instresses
3-D geometryCompression motion has less effect onangled surfaces Limited heating options
Microstructures,nanostructures
More uniform replication along theflow path
High replication accuracy due to slowercooling of melt
Benefit Drawback Limitation
Table 1. Aid to decision-making for processors: benefits and drawbacks of selected processing techniques compared to conventional injection molding(source: Engel)
Engel Austria GmbHLudwig-Engel-Strasse 1A-4311 SchwertbergAustriaTel. +43 50 620-0Fax +43 50 620-3009www.engelglobal.com
Manufactureri
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must be highly accurate and reproducible[6, 8]. All-electric injection molding ma-chines are very well-suited for produc-tion of precision optical components,since the position sensors in the servomotors permit very high positioning ac-curacy in conjunction with the kinemat-ics of the toggle mechanism. In the courseof trials to investigate reproducibility, thecompression gap was observed prior tothe start of injection with the aid of aninductive position sensor. In the all-elec-tric “e-motion” Series of injection mold-ing machines, the variation was less than5 µm.
In addition to its high reproducibility,this machine line is characterized by thehigh dynamics of the clamping unit.These dynamics can be exploited, for in-stance, for thin-walled molded parts witha micro-structured surface by first form-ing the structure through an abrupt in-crease in compression force and subse-quently achieving a low molded-in stresslevel.
Better Replication with Vario-therm Temperature Control
With variotherm mold temperature con-trol, the surface of the cavity is heated pri-or to each injection cycle and then cooledagain. Compared to conventional moldtemperature control, this produces ahigher contact temperature between theplastic and cavity wall that delays solidi-fication of the melt. Depending on thegeometry and surface of the molded part,it is possible to improve different charac-teristics in this way (Table 1). In addition
to fluid-based variotherm mold temper-ature control using water, steam or oil, ad-ditional methods such as induction, in-frared radiation or electric resistancefeeding are known [4]. Cooling general-ly takes place with water.
Variotherm mold temperature con-trol permits establishment of independ-ent temperatures that have been opti-mized for the respective process step.During the cooling phase, the water sup-ply temperature can be considerablylower than is normally the case with con-ventional mold temperature control.This makes it possible to reduce the cool-ing time when producing thick-walledoptical parts.
In a simulation of temperatures usedto mold an 11 mm thick LED lens fromPMMA with both conventional and var-iotherm temperature control, a constantsupply temperature of 75°C was assumedin one case, while the temperature wasvaried between 125 and 25°C in the oth-er case. In this example, the cycle time sav-ings was 15 % (Fig. 4).
The benefits of variotherm tempera-ture control are obvious: it improvesmolded part quality and can – under cer-tain circumstances – shorten the cycletine. As drawbacks, the relatively high in-vestment, depending on the type of tem-perature control, and increased energyconsumption have to be mentioned.
Improving Quality by Layers
Multi-layer injection molding, some-times called overmolding, is a processthat is still relatively new to the field ofinjection molding of optical parts andwhich can help improve the quality ofthick-walled lenses [3]. The process in-volves first molding a preshot and thensubsequently overmolding one or morelayers of the same material. A machinewith a rotary table or a mold with an in-dexing plate is generally needed; a phys-ical transfer is also conceivable. The ma-jor benefit of the process is that the over-molding step can correct or compensatefor sink marks or other flaws in the sur-face of the preshot. A lens on which anobvious surface flaw was created fordemonstration purposes can serve as anexample (Fig. 5). This lens is overmold-ed with a 2 mm thick layer of plastic. This
Fig. 2. Schematic illustration of possible mold concepts in which the compression stroke is execut-ed by the clamping unit (illustration: Engel)
Temperature
Spec
ific
volu
me
4
4
3
3
2
2
1
1
Position control (isochoric cooling)
Cavity pressure- or force-controlled compression
Transfer
Injection and speed-controlled compression
Pres
sure
Injection Compression Process within a pvT Diagram
Fig. 3. The operator interface for the injection compression software is organized into the four stepstypical of the process sequence (illustration: Engel)
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process can thus correct even severe flawsin the surface.
The often-discussed question as to thepossible cycle time savings associatedwith multi-layer molding cannot be giv-en a generally valid answer. The potentialsavings depend largely on the part geom-etry, the relationship between the layerthicknesses and the mold temperaturecontrol requirements.
When using a rotary table, the generalconstraint is that the time required for theindividual steps should be as close toidentical as possible. Since the preshot isin contact with both sides of the mold,while subsequent molded layers arecooled largely from only one side, theconclusion drawn is that subsequent lay-ers should be approximately half as thickas the first layer. In the case of two layers,the relationship between the layer thick-nesses should be 2/3 to 1/3; in the case ofthree players, 2/4 to 1/4 to 1/4 etc. These
relationships apply exactly only for a flatplate, but can be taken as a rule of thumbfor the layer thicknesses with simple partgeometries.
Ideally, each station of the rotary tableshould have independent mold tempera-ture control. Care must be taken to en-sure that the outer surfaces essential tothe optical function are temperature-con-trolled as necessary to satisfy quality re-quirements. For the inner surfaces, thesupply temperature can be lowered to re-duce the cooling time.
To obtain useful information aboutthe cooling time, a thermal simulationbased on the actual part geometry is re-quired. Simulations were conducted un-der various constraints for a 30 mmthick, plano-convex polycarbonate lens.These simulations compared 1-, 2- and
3-layer molding processes,with two variations for thethree-layer approach: Thethree layers can be molded insuccession, or an initialpreshot can be overmoldedon both sides simultaneous-ly. It is assumed in the simu-lation that the inner surfacesare cooled more intensely (60instead of 90°C). Comparedto molding a single-layer lens,the various approaches toovermolding multiple layersin conjunction with appro-
priate temperature control shorten thecooling time (Fig. 6).
Overall, the multi-layer molding processcan not only improve surface quality con-siderably, but also reduce the cooling timeas well. This is especially the case when theinner layers are cooled more intensely. Forthe lens investigated here,the possible cool-ing time reduction amounted to as muchas 35 %.The processor must,of course,takethe additional investment costs for themore complex equipment (rotary table, in-dexing plate etc.) into consideration wheninvestigating the economic viability of anyparticular approach.
Time
300
°C
200
150
100
50
00
Tem
pera
ture
50 100 150
230s
270s
200 250 s 300
-15%
Variotherm, center of molded partConventional, center of molded part
Variotherm, cavity surfaceConventional, cavity surface
Variotherm Cycle Time Reduction
Fig. 4. Calculated temperature curves from the moment of mold filling with variotherm and conven-tional mold temperature control for an 11 mm thick LED lens molded from PMMA. Reheating of thecavity starts already during the first cycle after about 150 s (illustration: Engel)
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Fig. 5. Preshot with surface defect beforeand after overmolding of a 2 mm thick layerof plastic (mold: IKV Aachen)
0
-10
-20
-30
-40
%
2 layer
Cool
ing
time
redu
ctio
n
3 layer,sequential
3 layer,inside-outside
1 2 1 32 12 2
Fig. 6. Simulationresults for variousmolding sequences:compared to the sin-gle-layer approach,the multi-layerprocess reduces thecooling time consid-erably. The cavitywall temperaturesare color-coded(black: 60°C, red:90°C) (illustration: Engel)
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Multi-layer Injection Molding
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Beneficial Process Combination
The variety of requirements that a plastic optical component mustsatisfy may require a combination of several processes. A struc-tured optical component (Fig. 7) that permits glare-free office il-lumination may,because of its high flow length/wall thickness ra-tio,require use of injection compression molding on the one hand,while on the other the precision needed for the angles of the mi-cro-pyramids can be achieved only with variotherm mold tem-perature control. This combination of processes has been adopt-ed in actual practice.
The outcome was positive and should encourage injectionmolders: the examples presented in this article show that im-proved quality and economics are indeed compatible with oneanother where optical parts are concerned. ■
REFERENCES
1 Bundesministerium für Bildung und Forschung: Optische Technologien –Wirtschaftliche Bedeutung in Deutschland, 2007
2 N.N.: Deutsche Agenda Optische Technologien für das 21. Jahrhundert. VDI-Technologiezentrum, Düsseldorf 2002
3 Pillwein, G.: Maschinen- und Prozesstechnik zur Herstellung optischer Bauteile.IKV-Seminar: Spritzgießen hochwertiger optischer Komponenten, Aachen 2008
4 Gießauf, J.; Pillwein, G.; Steinbichler, G.: Variotherm Temperature Control Is Fitfor Production. Kunststoffe international 98 (2008) 8, pp. 57–62
5 Thonemann, O.E.: Die Herstellung optischer Linsen im „Spritz-Präge-Verfahren“aus Polymethacrylat. Plastverarbeiter 18 (1967) 10, pp. 707–714
6 Menges, G.; Michaeli, W.; Mohren, P.: Anleitung zum Bau von Spritzgießw-erkzeugen. Carl Hanser Publishers, München, Wien 1999
7 Bäumer, S.: Handbook of Plastic Optics, Wiley-VCH Publishers, Weinheim 20058 Forster, J. D.: Vergleich der optischen Leistungsfähigkeit spritzgegossener und
spritzgeprägter Kunststofflinsen. Dissertation, RWTH Aachen, 20059 Johannaber, F.; Michaeli, W.: Handbuch Spritzgießen. Carl Hanser Publishers,
München 2004
THE AUTHORS
DIPL.-ING. MICHAEL STRICKER, born in 1979, is a project manager in theProcess Technology Development department at Engel Austria GmbH, Schwert-berg, Austria; [email protected]
DR. GEORG PILLWEIN, born in 1977, is a project manager in the Process Tech-nology Development department at Engel; [email protected]
DIPL.-ING. JOSEF GIESSAUF, born in 1968, directs the Process Technology De-velopment department at Engel; [email protected]
Fig. 7. Structured optical component for glare-free light: Themicro-pyramids with a base of 1.5 × 1.5 mm2 are produced bymeans of injection compression molding in conjunction withvariotherm mold temperature control (manufacturer: Zumtobel Lighting)
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