Journal of Scientific & Industrial ResearchVo1.61, October 2002, pp 786-791

An Experimental Approach for Mould Preparation and MouldingTechnique of Fresnel Lenses

D K BandyopadhyayMicropositioning Systems Division, Central Scientific Instruments Organization.ScctorSu, Chandigarh 160030

Received: 28 February 2002; accepted: 28 June 2002

An experimental fabrication technique of Fresnel lens master mould and the method adopted for moulding plastic Fresnel lenses isdescribed, Fresnel lenses have concentric prismatic grooves of varying depths and angles, It is a difficult process to obtain turned metallicmoulds for moulding such type of bigger size lenses. Fabrication of Fresnel lens master mould by replication technique is an attractivealternative. This paper describe how the master mould has been fabricated by copper deposition process. The selection of plastic material forlenses based on optical properties has also been enumerated.

1 Introduction

The concept of plastic Fresnel lenses is an excellentsubstitute to conventional glass-mirror system and is inuse for long. A Fresnel lens is made out of high qualitytransparent plastic. It has many concentric prismaticgrooves of varying depth on one side, while the otherside is made flat. The prism angle gradually increaseson traverse from the centre to the edge of the lens. Eachgroove is a minute refracting facet and is able to bendlight at designated focus. The lens can be used either asa condenser 1 or collimator.

These lenses have many advantages over conven-tional glass lenses". Difficulties in fabrication of hugeoptical glass lenses prompted for alternative plastic op-tics. It is estimated that 70 per cent of the moulded lensesare fabricated from plastic) and the rest are made fromglass. The Fresnel lenses are fabricated either by directturning prismatic grooves on transparent plastics or onmetallic surfaces. The turned metallic moulds are usedto get replica of the Fresnel lens profile by injection orcompression moulding. Fabrication of mould by turningis quite difficult for Fresnel lenses of bigger aperturebecause of low durability" of available tools. It is noteconomical and unsuitable approach for mass produc-tion purposes. The most cost effective and alternativemethod for mass production of large aperture Fresnellenses is fabrication by replication techniques. By thisprocess, exact impression of Fresnel lenses grooved pro-file on electro-deposited metal surface can be achieved.An experimental approach has been made to fabricate

Fresnel lens' master mould by copper electro-deposi-tion on an arranged plastic lens.

2 Process of Fabricating Master MouldThe master mould was prepared from an originally

machine turned master Fresnel lens of required opticalproperties by chemically cleaning and activating thesurface of the plastic lens. Then copper electroplatingover the surface profile of the master lens was carriedout to obtain metallic replica of the grooved surface. Thethin layer of nickel deposition over the copper platedreplica was done for durability, strength as well as toachieve smooth optically finished surface profile of themaster mould. A schematic view of the Fresnel lens pro-file is shown in Figure 1

2.1 Framing and Cleaning of Master LensThe master Fresnel lens was mounted with its flat

side to a 3 mm thick plastic (Acrylic) frame with adhe-sive. The framed lens was cleaned and washed withcleaning solution of Zonax® cleaner 50 g/L and then itwas dipped in caustic solution bath. The master lens wasfinally immersed for 5 min in a distilled water cleaningbath.

2.2 Activating ProcessActivating of the plastic lens' grooved surface was

required before silver coating operation for obtaining afine conducting surface. Conducting activator/primercoating, consisted of 2 per cent solution of stannouschloride, which was dissolved in pre-acidified tank ofwater. The master lens with grooved face down wasimmersed and gently shaken for about 5 min. This pro-

BANDYOPADHYAY: FRESNEL LENSES 787

FRESNAL LENS WITH ONCE NTR IC GROOVES

(300X300Xl8mm)

(X' ----------~----------~-,

FRESNEL LEN S PROFILE

SEC.A -' A'

DET AIL-'X'

ENLARGED VIEW

Figure I-A schematic view of the Fresnel lens profile

cess was repeated 3 to 4 times. The piece was then ready for sil verin g operation.

2.3 Silvering Process The sil vering solutio n consists of the following chemi -

cals: I 2 3 4 5

NH4N0

1

AgNOo KOH Dextrose Sucrose

7.5g in 150cc. 2.5 g in 25 cc .

12.5 g in 150 cc. 5gin 50cc.

I 0 g in I 00 cc.

The solutio n of ammoni um nitrate and sil ve r nitrate are mixed in I :6 ratio while dextrose and sucrose are mixed in I :4 proportion. These soluti ons (1 +2), 3, and (4+5) arc mixed in 7:5:3 ra ti o in a beaker and transferred in a sil vering tray. The tray contains the primer coated

lens. immersed in d isti ll ed water at 20°C. The coo led

solu tion produces a finer grain coating with hi gher me-chanical density and fewer pinholes5 . The tray is shaken gently for even d istribu tion of soluti ons over the lens ' grooved surface. The sil ver laye r coati ng process was repeated for ten times for considerabl e thi ck laye r of sil -ver over the lens surface. After silver coating was fin-

ished the coated lens was dried up and checked fo r any discontinuation of the sil ver coated layer. Any blow ho le may lead to improper e lectrop lat ing. 2.4 Copper Electroplating

It was necessary to have a solid smooth profile of grooved Fresnel lens surface o n metal fo r plastic moul-ding. For getting the true copper repli ca of the Fresnel lens' prismatic grooved profil e over the silver coated ma ter lens, copper e lectro forming process was adopted. An ac id-soluti on bath was used for deposi ting copper over the activated lens surface. To obta in finer copper deposition in the beginning, low metal and ac id contents6

were maintained in the bath. The resistivity of the bath diminished clue to no n- fo rmation of bas ic sa lts in the presence of H

2S0

4 and CuS0

4• Thi s resul ts in low cop-

per io n concentratio n7 and extends a good quality cop-per deposition on the sil ver coated Fres ne l lens su rface. An outstand ing feat ure of the acid copper e lectroform-ing process is that the rep li cat ion of the in tricate detail is ex tremely fine . The reason for using copper plating as undercoat for ni cke l8 is that the proccssis easy, accu-rate, and econom ical.

A pure copper pl ate (99.9 per cent C u) was used as anode. The size of the copper pl ate was taken same as

788 J SCI IND RES VOL 61 OCTOBER 2002

,ANODE -CA THODE

COPPER PLATE-I=-'~~

ELECTROL YTESOLUTION

FRAMESENSITISEDPLAST IC r LEN~PLA TI NG SATH

Figure 2-Copper plating arrangements on master lens

coated lens surface for even and homogeneous distribu-tion of copper deposition. Silver coated lens was takenas cathode. Both were positioned 15 em apart in theacid-bath, as shown in Figure 2. In the beginning of theelectroforming process the concentration of the acid cop-per solution is taken as CuS04 - 60 g/L, H2S04 - 2 g/Lat ambient temperature. Current density 0.54-1.0 Azdm?is maintained in the electrodes for 3 min. This processis able to generate a fine smooth layer of copper coatingon activated plastic lens surface. For thicker copper plat-ing, another acid copper bath of the following measure-ments was used:

Copper Sulphate = 200g/LSulphuric acid = 50 g/LTemperature = 25 - 30°CCurrent density = 2.2-2.7 Azdrn?

The acid bath was ceramic lined. Sulphuric acid wasadded slowly in large volume of water in the bath.The current was raised to about 25 A stepwise. Thedeposition rate was kept at 0.2 mmlh. To get about 3mm thick plating the electrolytic process continued for15 h. After the desired thickness was obtained the elec-troformed piece was removed from the bath and washedin distilled water. Removal of electroplate from originalmaster lens' surface was done by milling about 1mmfrom four sides of the copper deposits. As original mas-ter lens was made of acrylic and was self-lubricated, therewas no possibility of sticking the electroplated part withmaster lens. Then the electroformed surface was etchedin an acid bath for removing silver layer and to obtainbrighter copper surface before placing the electroformfor nickel coating.

2.5 Nickel CoatingNickel coating over copper replica of Fresnel lens

was necessary for surface hardness, durability, corrosionresistance, and shining profile of the grooved surface.

Certain percentage of brightener agent was added tonickel coating solution for better brightness and level-ing, which helps to get smoother surface.

The composition of nickel bath solution was kept as:Compound g /LNickel Sulphate = 240Nickel chloride = 30Boric acid = 30Nickel format = 45Cobalt Sulphate = 2.6Formaldehyde = 2.5Ammonium Sulphate = 0.8

The current density was maintained at 2.2-5.4 Azdm?and temperature of the bath solution kept about 45°Cfor higher efficiency of deposition. The thin layer of 12um bright nickel deposition was achieved within one-and-half-hour of electrolytic process. The nickel coatedcopper replica was taken out from nickel bath. The elec-troform was cleaned for fixing with base plate for get-ting a master mould.

2.6 Fixing of Electroform

Fixing of electroplated replica with a 12mm thickbrass plate was done by special adhesive. The adhesivehad heat curing temperature of 120°C. A thin layer ofadhesive was evenly spread over the back surface of thereplica and base surface of brass plate. For proper bond-ing with base plate the electroplated replica was keptinside an oven for 2h. under uniformly distributed pres-sure over the surface of the replica. The temperature ofthe oven was maintained at 120°C. After this the bondedreplica with base plate was removed for riveting. Theriveting was done along the ungrooved periphery of theelectroplated master mould and machining of edges wascarried out. The master mould was optically polishedbefore compression moulding of acrylic Fresnel lenses.

3 Selection of Moulding Material

It is very important to identify proper moulding ma-terial for getting moulded Fresnel lenses. The materialshould have very good optical properties. It was observedthat acrylic resins (Polymethyl methacrylate) is an ex-cellent transparent plastic with refractive index betterthan glass. This material has good dimensional stability

BANDYOPADHYAY : FR ESNEL LENSES 789

~~) ~~y ~~~y----14--PLUGGED HOLE I ' -L I ...... _:-L . .........:L _

--7 1 ---~ I ---~ I ---1 I I I I I

I I I I I I I I I I I I

I 1 I 1 1 1---1-- 1-- - CARTRIDGE HEATER I I I I I I

I I I I I I I I I I I I

I 1 I 1 I 1 I 1 I 1 I 1

I I I I I I I I I I I I

I I I I I I

TO SUPPLY

Figure 3-Schematic o f master mould heating unit

in the presence o f sunli ght9 and having a hi gh degree o f heat res istance. Thi s is a thermoplastic substance and it can be reused for moulding. This acrylic resin is gener-ally used for moulding in granular form. Phys ical properti es of po lymethyl methacrylate are :

Index of refract ion 1.49

Luminous transm ission

Specific gravi ty

Thermal conducti vi ty

Heat res istance

Moulding temperat ure

Absorption (Per cent Hp!h)

Tensile strength

Compression strength

Rockwell hardness

4 Hydraulic Press

92 per cent

1.1 8 4 to 6 kcal/gm/sq.c m/°C/c m

105°C

120°C to 270°C

0.03-0.04

5.5 x I 03 N/cm 2

I 0.5 x I 03 N/c m2

M95

The semi-automati c hydraulic press was used for the production of Fresnel lenses. The press was o f vertical four-column design with top pressure. The working speed was controlled by means of the throttle va lve, at arbi-trary pos iti on of the platen. When reducing the working speed, escape of air from the mould was enab led and thus the number of damaged moulded parts during the ejecti on is decreased.

Techni cal data of the hydraulic press used: Maximum working poweR 250 T

Return power 125 T Area of bed I 000 x I 000 mm Diameter of working pi ston Stroke Day li ght

325 Jlllll 600 Jlllll 1200 llllll

5 Moulding Arrangements

For fabrication of Fresnel lenses from acrylic ther-moplastic material , it was required to heat up the plast ic oranules over the master mould . After compression pro-o cess the mould should be cooled down to about 40°C for ejecting of the moulded parts. For these purposes, four plates were fabricated, two for heating and other two for cooling the mould.

5.1 Heating Plates

There are two heating plates one for upper hal f of the press and another for lower half. The two heating pl ates were of mild steel, having size 400x375x50 mm each. Nine holes on the plates were drilled for provision of cartridge type heating units , as shown in Figure 3. The rod shaped cartridges of I OOOW each were inserted into the plugged holes. The correct moulding tempera-ture was calculated and uniform di stribution of heat over the plate was ensured for proper formation of the moul-ded parts. The plastic granules used, needed approx i-

mately l80°C, for compression moulding. The thermo-

stat contro ller was used to contro l the des ired tempera-ture of the heating plates.

5.2 Cooling Plates Two cooling plates were fabricated exac tly of the

same size as heating plates. They were indi viduall y mounted over each heating pl ate. These plates were de-signed in such a way to expedite cooling process of the moulded parts. The cooling plates having uniformly di s-tributed cooling water supply are shown in the Fi gure 4.

790 J SCIIND RES VOL 61 OCTOBER 2002

CODLING WATERFLOW PATH PARTITION CHANNEL

WATER-OUTLET

Figure 4- Schematic of master mould cooling unit

tUPPER HALF:--------2

=4~--------3---------4

6__ 7

~====~====~~========-===--i~---------_5~~======:=:~~~12

LOWER HALF

1- RAM 6- OPTICALLY POLISHED PRESS PLATE2- BASE PLATE 7- ACRYL 1.C PELLETS3- ASBESTOS SHEET 8- GUIDE RING4- HEATING PLATE 9- FRESNEL LENS MASTER DIE5-- CDOLI NG PLATE

Figure 5 - Schematic view of the moulding setup

5.3 Press Plate

A press plate or punch plate of high chromium alloytool steel was cut as per the size of the master mould.One of the surface of the press plate was optically mir-ror polished so that there should not be any patch markduring moulding process. The optically polished sur-face facing downward was mounted on the upper halfon the cooling plate.

5.4 Mounting ProcessThe master mould was placed over the cooling plate

on the lower half of the press. The schematic view of themoulding setup is shown in Figure 5_ One brass ringwas placed around the periphery of the master mould toguide the acrylic granules. Care was taken to properlyposition the ring with the help of guide pins. Siliconspray may be applied over the grooved surface of the

BANDYOPADHYAY: FRESNEL LENSES 791

mould , before moulding processes nonsticking agent.

The measured amo unt of acry lic granules are spread

evenly over the grooved surface of the mould. The ac rylic granules were pre-heated for evaporating any

moisture.

6 Moulding Process After suitable pre-heating the press plate was a ll owed

to make contac t w ith the master mould and was heated

up to 170°C. The pressure applied was L35 T for 5 min

on 300 X 300 mm mould. All the heaters were puts off

and cooling process started. Pressure of the press plate

was then re leased but contact between mould and press plate rema ined in tact until master mould coo led down

to 40°C. The upper press plate was allowed to move

upward and the moulded plastic lens was then removed eas il y with the he lp of ejection pin. The moulded part

took the shape of orig inal grooved surface of the Fresnel

lens.

7 Conclusions

One fabricated Fresnel lens was arranged for solar energy condenser and taken as master lens for experi-

men tal purpose. The lens size was 300 sq . mm , and fo-cal length 380 mm. The moulded Fresnel lenses were then tested fo r their foca l length and image c larity. They

were found to be very good in performance.

One master mould fabricated by this process has

substantial repeatab ility and can be used for moulding

I 00 - 150 Fresnel lenses. The quality of the plastic

moulded lenses, virtually related to proper activation of the master lens, superior electroforming and overall

proper moulding technique. The fabrication processes are easy and suitable for bigger size lenses with short

foca l length . Jt can be concluded that the Fresnel lens master mould developed by e lec troform ing technique

is an excell en t substi tute for costly imported unit.

References Zhdanov D D & Pankov G Y. Plastic Fresnel lenses for proj