Optical system made from trans-parent plastics are in commercial

competition with glass optics (photos/graphics: Arburg)

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I N J EC T I ON MOLD ING

© Carl Hanser Verlag, Munich Kunststoffe international 10/2009

THOMAS WALTHER

ROLF-UWE MÜLLER

The technical advantages of plasticsare the reason why they are increas-ingly replacing glass for optical ap-

plications (Title photo). Their potential assubstitutes for optical function surfacescomes from their design freedom, capa-bility for integrating several functioningparts, low materials cost and low specific

weight. Injection molding enables themto be produced in a single processing stepwith high quality and for a comparative-ly low price.

A very high level of competency in sev-eral engineering disciplines is required toproduce such articles.When optical com-ponents are being designed and devel-oped, calculation methods and polymermaterials to be processed are the majorissues. The shaping process requires con-siderable know-how in mold and process-ing technology. If consistent quality as-surance is to be achieved, a good founda-tion in the science of metrology is also

necessary. For the producers of moldedparts, optical articles represent a consid-erable challenge. Besides the usual stan-dard of dimensional accuracy, opticalfunctions have top priority.

Optical Function as a Sign of Quality

Injection molded optical parts can beclassified by their type of use. The distinc-tion here is between imaging and non-imaging optics, whereby the non-imag-ing ones can be termed illumination op-tics and the imaging ones image process-

For True InsightOptical Parts (1). Glass is being increasingly substituted with plastics in the field

of optics, too. Since the varying wall thickness of such parts does not conform to

the design guidelines applying to polymers, injection coining is coming into use

as an alternative to classic injection molding. The production of function parts

requires understanding not only of processing technology and mold making, but

of optics and metrology as well.

Translated from Kunststoffe 10/2009, pp. 72–76Article as PDF-File at www.kunststoffe-international.com; Document Number: PE110222

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I N J E C T I ON MOLD ING

>

Kunststoffe international 10/2009

Component wall thickness is a criticalparameter. It determines cooling time andthus has considerable influence onshrinkage effects. That is why componentdesigners make every effort to keep wallthickness as small as possible. Part thick-ness distribution should also be approx-imately uniform in order to controlshrinkage.

Contrary to Polymer DesignEngineering

Optical components are rarely designedfrom the point of view of polymer engi-neering. They have functionality in that

they conduct light. One of the conse-quences is products with thick walls andgreat differences in wall thickness. For theinjection molding process, that in turnmeans comparatively long cycle times.

For the resulting part geometry to con-form to reference geometry, shrinkage ef-fects have to be considered in detail whenthe part is being designed.The high mold-ing precision required plus the phenom-ena involved in thick-walled parts makeit essential to have comprehensive know-how of the entire processing chain. Clas-sic injection molding paired with conven-tional mold temperature control run upagainst their limits then. That is why con-cepts from variothemic mold control and

ing optics (Fig. 1). Depending on wallthickness and process requirements:� the demands on optical function are

greater,� injection coining becomes increasing-

ly more attractive as an alternative and � the measuring system becomes more

complex.Examples of non-imaging optics includ-ed headlights glazing or watch crystals.Outside geometry and transparency ofthe part are the main criteria. The situa-tion is similar with light conductors thattransport rays coupled into them from alight source and coupled out at one ormore pre-defined points. Imaging optics

also transport light rays, but in order ofinput. Individual image pixels are project-ed through the optics in such a way thata sharp image appears on the target.

Boundary layers where light is coupledin and out have to be very preciselyshaped in order for both imaging andnon-imaging optics to perform their op-tical function. This requires a very highdegree of molding accuracy. Internalstresses can also influence the light path,thus impairing part quality. In otherwords, not external geometry, the usualcriterion of molded part quality, but op-tical function is crucial. And the criteri-on for optical function is refractive in-dex.

Material Mold Automation

Qualityassurance &follow-upprocesses

Injectionmoldingmachine

Fig. 2. To produce optical parts by injection molding, the processing chain must be considered in itsentirety prior to startup

© Kunststoffe

Camera lenses,eyeglassesSunglasses

Projectorlenses,

eyeglass blanks

Magnifyingglasses

Automobileglazing

Advertisingglazing

Steeringangle

sensors

Watch crystals,mirrors

Rulers,optical data

carriers

Decorativeoptics,light

conductors

Light andrain sensors

Lighthouseoptics

Processing requirements

Wal

l thi

ckne

ss

Non-imaging optics Imaging optics

mCammCamCamCamera lenseseeeeeyeglassesSungl

Projector,,

aasssssssssss nknknknknkks

gnifyinglasses

leazing

verlazin

eringangle

sensors

Watchcrystals,mirrors

Rulers,optical data

carriers

corativeoptics,light

nductors

Light andrain sensors

Lighthooptics

Camunglasses

Projectorlenses,

eyeglasblan

Automobileglazing

ertisinging

Steerin

houseics

Fig. 1. Optical partscan be classifiedaccording to their ap-plication as imagingand non-imagingoptics

© Kunststoffe

injection coining are increasingly cominginto use.

The production of high-quality opti-cal components cannot do without a ho-listic approach to the production chainand the system technology involved in it(Fig. 2). In addition to the injection mold-ing step itself, it includes prior materialsconditioning and subsequent operationssuch as removal, quality assurance andother follow-up processes. They can becoating, assembly or packaging steps.

Material Leaves ProcessorsLittle Freedom

In transparent parts, any type of contam-ination, inclusions or streaking are spot-ted immediately as defects in the part. In-vestigations usually do not turn up clearcauses. That is why fundamental rulessuch as cleanliness and reproducibilitymust be maintained in all processingsteps. That is why measures derived fromcleanroom technology are utilized inmany cases.

The processor has little freedom withrespect to materials. Optical and mechan-ical raw material properties are tuned toeach particular application. Some mate-rials manufacturers supply special chargesin so-called optical grades for which theyguarantee special purity and absolutelyno dust in the material. Dust particles canlead to problems, since such particles ei-ther exhibit different plasticating behav-ior from that of the granulate, or do notmelt down at all. Then they will be trans-ported along with the melt and can, un-der certain circumstances, end up as con-taminations in the part.

For the granule drying system as wellas the materials conveyor system, it is cru-cial that their design seals out dust, andthat the conveyor system neither actsabrasively on the material nor is erodedby the same. The path from the granulatecontainer to the screw intake should beshort and clear. For applications requir-ing a high degree of cleanliness, such asthe production of ophthalmic lenses foreyeglasses, so-called dust arresters are of-ten mounted between the material sepa-rator and the intake zone.

Short Paths to Tempering

Sophisticated optical parts require a moldtechnology adapted to their quality crite-ria. High and long holding pressure phas-es as well as high temperatures are a mat-ter of course. Thus sufficient stiffness isdemanded when configuring and design-

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ing the injection molds. Both corrosionresistant and standard tool steelsequipped with chemically isolated anti-corrosion coatings are used for mold con-struction.

The precise optical function surfacescan usually be changed with mold inserts(Fig. 3). Their surfaces, such as sculpturedshapes, lens arrays or reflector prisms, areusually machined in with diamonds forprecise shaping. Although the mold in-serts are exchangeable, effective temper-ature control must be ensured.

As in cleanroom technology, superflu-ous surfaces should be avoided wherecontaminations can be deposited. Elec-tric und hydraulic lines as well as coolingcircuits should not be dragged along bymold motions. In order to supply themold through short lines, the moveableclamping platen is equipped with appro-priate interfaces (Fig. 4).

Mold temperatures and other sensorinputs are recorded and monitored in themachine control. Cooling equipment da-ta are transmitted via standard interfacesto machine control and thus to QS eval-uation. To enhance processing stability, itis advisable to control media flow. Thecooling equipment should be equippedwith a suction mechanism for use whencavities are exchanged or the mold is dis-mounted.

Coordinating all MachineQuantities

There is no typical machine concept foroptical applications. The suitable designis rather derived for each case from partsize, required coining pressure, as well asthe necessary mold and processing tech-nology.

The mold is the central item when con-figuring machine size. Mold size andholding pressure requirements determineplaten area and coining pressure andthereby primarily the size of the machine.The injection unit is modular and adapt-ed to specific requirements. The criteriahere are shot weight, the polymer to be

use, expected cycle time and necessary in-jection pressures. The shot weight heightand the expected injection pressure arerelevant when selecting screw diameter.An ideal shot weight lies between 20 and80 % of the maximum output of thescrew selected.The dwell time of the poly-mer in the melt can be determined byscrew diameter and cycle time. The latterhas to lie within the limits prescribed bythe manufacturer for the particular rawmaterial.

In actual practice, compromises oftenhave to be made especially where dwelltime is involved. When part walls arethick, long cycle times are often the resultand lead to correspondingly long meltdwell times in the plastication cylinder.Potential consequences include thermaloxidation together with increasing yel-lowing of the material, i.e., a finished partquality impaired by them.

Mainly highly wear resistant versionsof the cylinder module, screw and non-return valve are used as components inthe plastication unit. In addition, wear-protected (chromium nitride coated)screws and non-return values are utilizedfor processing polymers that tend to ad-here and can thereby form so-called blackspots in the part.

The Right ProductionSurroundings

Fundamentally speaking, optical compo-nent production is not limited to one par-ticular machine technology. State-of-the-art injection molding machines areequipped with control and sensoring sys-tems that fulfill the necessary conditionsfor the required process reproducibility.For injection coining, further going ma-chine technology is required. The drivetechnology must be capable of realizing

Fig. 3. Exchangeable mold inserts enable rapid product change; their highly accurate function surfaces are machined with diamonds for precise shaping

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Fig. 4. Cooling hoses are mounted directly onthe moveable platen to prevent them from beingmoved by mold travel and to keep the supplylines as short as possible

This is the first of two articles on this topic.In a second article, the authors will delvemore deeply into all possible versions of the mold and processing technology formaking optical parts by injection-coining. The sequel will appear in Kunststoffeinternational 11/09.

Series of Articlesi

© Carl Hanser Verlag, Munich Kunststoffe international 10/2009

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Kunststoffe international 10/2009

the envisaged coining function and befreely and flexibly programmable via ma-chine control. Generally speaking the ruleis: the design of the coining mold deter-mines the coining function on the ma-chine side.

Only in a relatively few cases is a facil-ity with cleanroom conditions a prereq-uisite for optical component production(Fig. 5). To ensure appropriately clean sur-roundings nonetheless, one of the follow-ing options/additional equipment can beselected:� A cleanroom flowbox over the locking

unit to avoid contaminations in thecavity area,

� raised machine feet for better access toclean beneath the machine,

� short supply lines for mold cooling bymounting the hoses firmly and direct-ly on the mounting plates,

� a uniform and bright machine color toease recognition of contaminants,

� liquid-cooled drive to minimize dustswirls, and

� minimized machine and mold sur-faces, e.g., by completely encapsulat-ing hydraulic blocks.

Defined Cooling to RoomTemperature

Robot systems integrated in machinecontrol for demolding the parts belong toany holistic production concept. Gripperstake the parts from the cavity, or ejectorsystems pass them to removal grippers(Fig. 6 left). Current technology includeseverything from simple pneumatic grip-per axes all the way to servo axes movingin sync with the ejector.

The main criterion for robot technol-ogy is the scope of follow-up processes aswell as the type of part buffer storage.This can vary from depositing them di-rectly onto a conveyor belt, (Fig. 6 right) to

Fig. 5. Flowboxes are integrated in the production cell for opthalmic lenses

depositing them according to a definedpattern or to fitting them in trays forstacking and destacking devices. Furtherprocessing steps can follow; not uncom-mon are defined cooling paths, for in-stance. Then the parts coming from themold at very high temperatures are setdown in defined cooling places. On these,the molded parts can cool uniformly toroom temperature before measuring de-vices check and document quality. De-pending on the application, steps can fol-low for assembly with additional parts ordeposition in suitably protected sur-roundings.

Conclusion

The manufacture of optical parts by in-jection molding is a segment developingquite rapidly at this time. However, theirproduction requires comprehensiveknowledge not only of injection moldingand mold making, but of optics and rel-evant metrology as well.

For thick-walled as well as for opticalparts, injection coining is often utilized.Vis-à-vis classic injection molding, thisprocess offers significant advantages es-pecially for such articles; however, it pre-sumes high-level, complex mold and ma-chine technology. Moreover, the process-ing technology is considerably more ex-tensive and presumes expert knowledgerequired both for working out and vali-dating processes as well as for supervis-ing volume production. �

THE AUTHORS

DR.-ING. THOMAS WALTHER, born 1969, is thehead of application technology at Arburg GmbH + CoKG of Lossburg, Germany.

DIPL.-ING. (FH) ROLF-UWE MÜLLER, born 1961, isa optics specialist engaged by Arburg as a consultantfor application technology.

Fig. 6. The parts are carefully removed by grippers (left) and deposited directly on the conveyor belt (right)

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