Christian Maier, Josef Giessauf and Georg Steinbichler

Efficient Production of Thick-Walled PartsThick-Walled Lenses

© Carl Hanser Verlag, München. 2013. All rights including reprinting, photographic reproduction and

translation reserved by the publishers.

Special reprint from Kunststoffe international 9/2013

ENGEL AUSTRIA GmbHLudwig-Engel-Straße 1 · A-4311 SchwertbergTel. +43 50 620 0 · Fax +43 50 620 3009sales@engel.at · www.engelglobal.com

2013Volume 103 www.kunststoffe-international.com

Magazine for Plastics

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© Carl Hanser Verlag, Munich Kunststoffe international 9/2013

CHRISTIAN MAIERJOSEF GIESSAUF

GEORG STEINBICHLER

The markets for plastic optical partsdiffer significantly from region to re-gion.While imaging optical parts for

mobile appliances predominate in Asia,in Europe the main focus is on the pro-duction of thicker lenses for LED lights.Since the cooling time during injectionmolding increases with the square of the

wall thickness, the biggest challenge is todevelop economic processes. With a typ-ical thickness of 30 mm for automotiveheadlamps, cycle times of at least 20 minare to be expected with standard injectionmolding processes [1].

One possibility for cycle time reduc-tion is multilayer technology, in which thethick-walled parts are built up from mul-tiple successive layers. The multilayerstructure can be generated by overmold-ing a first layer at either one or both sides,or by subsequently bonding together twopreviously independent layers by meansof an interlayer (Fig. 1).

In general, the individual layers are allproduced on the same injection molding

machine. The use of separate injectionunits for the three layers is already a firststep towards short cycle times. This en-sures that the process steps of injection,holding pressure and metering take placesimultaneously and independently of oneanother.

What are the Benefits ofMultilayer Technology?

Approximation formulas for cooling timeand productivity can be derived from the-oretical considerations (see box p. 6) and theresults illustrated in dependence on thenumber of layers (Fig. 2). The literature[1–3], however, shows that this rule of

Thick-Walled Lenses.

Injection molding is

demonstrating its versa-

tility again in the produc-

tion of challenging optical

plastic parts such as LED

lenses for automotive

headlights with toler-

ances in the micron

range. A new develop-

ment in multilayer injec-

tion molding allows pro-

ductivity for the produc-

tion of thick-walled lens-

es to be increased even

further.

Efficient Production ofThick-Walled Parts

LED headlamp of the Mercedes S Class 2013.The layers are manufactured by three-layersandwich molding on an Engel duo 600 WPpico combi injection molding machine (figure:

Automotive Lighting Reutlingen)

Translated from Kunststoffe 9/2013, pp. 148–153Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111447

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thumb provides too optimistic results forestimating the cooling time saving.

However,multilayer molding offers thepossibility of maintaining the mold re-gions for those surfaces that subsequent-ly have to be overmolded at a lower tem-perature, since, in an ideal case, the sur-face quality of the internal layers does notaffect the quality of the lens that is pro-duced. If this potential is used for themanufacture of the preform, the estimat-ed cooling time reduction agrees well withsimulated and practical results. For opti-mizing the layer thickness distribution, itis advisable to carry out simulations.

From Figure 2, the following statementscan be derived:� For the cooling time per cavity, it is

unimportant whether overmolding isperformed on one or both sides. Inboth cases the cooling time is reducedto the same degree as the number oflayers increases.

� Overmolding on one side requires alarger number of cavities, and there-fore more space in the mold, comparedto overmolding on both sides.

� Productivity is a suitable key indicatorfor an economic comparison, becauseit includes the assumption of equalnumbers of cavities.

� It is only with a large number of layersthat overmolding on one side improvesproductivity to a significant extent.However, the productivity actually ob-tainable is affected by the times formold opening, mold closing, moldtransfer/rotation and injection.

� For overmolding a preform on bothsides, on the other hand, the produc-

tivity increases significantly startingwith a three layer structure.

The sandwich option, consequently, hasan advantage over overmolding on oneside. This alternative also improves thecontour accuracy, since sink marks in thepreform due to shrinkage can be compen-sated by overmolding. Shrinkage of thethinner outer layers is thus responsible forthe contour accuracy. This effect is alsopresent for overmolding on one side, butof course only on one side. The quality ofthe other surface must therefore meet re-quirements immediately after molding,

since it does not contain a corrective toplayer.

Where there is light, there is also shad-ow: despite all the advantages of the sand-wich alternative, it must not be over-looked that it requires a more complexmold technology. Retaining the preformin the cavity and transportation from onecavity to another is challenging, while arotary table is sufficient for one-sidedovermolding. The simultaneous filling ofthe outer layers should be well balanced– pressure differences between the topand bottom layers can cause the preformto fracture.

Both methods can offer further bene-fits. Cold-runner sprues or thin-walledexterior regions limit the possible maxi-mum holding pressure time. With verythick-walled parts, the sink marks canthus only be counteracted by increasing

the holding pressure.This in turn requiresmachines with relatively high clampingforces. Extreme wall-thickness ratios can,in some cases, only be achieved throughmultilayer molding. The result is a gainin design freedom.

With the cycle time, the residence timeof the material in the barrel and hot run-ner also decreases. That has the benefit ofreduced yellowing and therefore greatertransmission. The maximum residencetimes recommended by the materialmanufacturers can be maintained. TheTitle figure shows a series application of the

Fig. 1. Possible layer sequences for the example of a thick-walled three layer lens. Left: preform 1one-side overmolded with layers 2 and 3 sequentially; center: preform 1 simultaneously overmoldedon both sides with layers 2 and 2’ (sandwich variant); right: preforms 1 and 1’ connected by injectionof a second layer 2 (figure: Engel)

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Cooling time per mold stationProductivity per cavity – overmolding on one sideProductivity per cavity – overmolding on two sides

Fig. 2. Cooling time per station in the mold, depending on the number of layers as per equation 4, andproductivity as per equations 6 and 7 (see box p. 6). The respective single-layer part serves as refer-ence (100 %) for comparing the illustrated results for overmolding on one side and on both sides(figure: Engel)

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three-layer sandwich process, productionof LED headlamps at Automotive Light-ing Reutlingen GmbH.

Shorter Cycle Times thanks toLonger Cooling Times

Investigations have shown how multilay-er molding can increase the profitabilityof the production of thick-walled parts.It takes advantage of the fact that severalthin layers will cool more rapidly than onethick layer. This can increase productivi-ty by approximately a factor of two. How-ever, the resulting cooling times of sever-al minutes are still comparatively long forinjection molding.

Considerations about the layer distri-bution generally assumed that, with athree-layer sandwich structure, the pre-form and the top layers must be cooled to

below the glass transition temperature atthe end of the cooling time.However, testshave shown that the preform can be re-moved much earlier. It must only be en-sured that its solidified outer layers aresufficiently strong to withstand the inter-nal pressure and prevent deformationduring demolding. If the preform is im-mediately overmolded in the next station,no cycle time reduction would be gained,on the contrary: the still-hot inner regionsof the preform would be further distancedfrom the mold wall and the cooling timewould be extended.

A new process therefore includes acooling stage outside the mold betweenthe injection shots. Cooling in air doestake longer than in the mold, but doesnot influence the cycle time. Depending

on the duration of the external cooling,the preform can have a lower averagetemperature during overmolding than apreform in conventional sandwich tech-nology. As a result, the preform absorbsmore heat from the top layers and there-by reduces the cooling time. This effectcan be further increased by making thepreform thicker and the top layers thin-ner.

Process Sequence with External Cooling

The new process sequence is as follows: Apreform for some cycles externally cooledto a predefined temperature is insertedinto the mold again and overmolded. Anew preform is then produced simulta-

neously or sequentially – depending onthe number of injection units. After theopening of the mold, a finished part anda preform are removed and a preform thathas previously been intermediatelycooled is inserted again. The removedpreform is deposited at a cooling station(Fig. 3).

To put a figure on the cycle time reduc-tion gained by cooling outside the mold,Bayer MaterialScience AG, Leverkusen,Germany, performed thermal simula-tions for a 20 mm-thick cuboidal part ofpolycarbonate (PC) (Fig. 4). The specialistscompared a one-layer process, a three-lay-er sandwich process and a three-layersandwich process with external interme-diate cooling. The layer thickness distri-bution was adapted to the process. In thethree-layer sandwich process, the 4 mmthick top layers required the same cool-ing time as the preform with 12 mmthickness. In a variant with external in-termediate cooling, the preform was as-sumed to have a thickness of 12.8 mm,and the top layers of 3.6 mm. Figure 5shows the maximum temperature with-in the part in dependence of time.Where-as with the one-layer alternative, the high-est temperature is always found in thecore of the part, the location of the max-imum changes for the two multilayerprocesses.

The criterion for calculating the cool-ing time of the end part is: all regions ofthe part must be cooled to below the glasstransition temperature of 150°C. In thealternative with external intermediatecooling, the preform is removed at a timewhen a 2.4 mm-thick, solidified outer lay-er has formed, but the core temperature

Fig. 4. For a process comparison, a cuboidal polycarbonate part (40 x 38 x 20 mm) was used (figure: Bayer MaterialScience)

Injection

Injection

Overmolding

Overmolding Demolding

DemoldingInsertion

Intermediate cooling outside the mold

Demolding

Time

TransferInjection

Injection

Overmolding

Overmolding Demolding

DemoldingInsertion

Intermediate cooling outside the mold

Demold

Time

Transfer

T

T > Tg

T < Tg

Fig. 3. Time sequence of the process steps for conventional three-layer sandwich molding (top) andthe variant with external intermediate cooling (bottom). The diagram shows schematically how thedifferent temperature at demolding of the preform (Tg: glass transition temperature) and the differentlayer distribution significantly shorten the overall cooling time (figure: Engel)

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is still 220°C. With this new process thetotal cooling time in the mold can be cutin half. Since the number of required cav-ities is the same compared to the knownsandwich process, the productivity in-creases by the same factor by which thecooling time decreases.

The rapid solidification rate of poly-carbonate is beneficial for short cycletimes. Simulations for polymethylmethacrylate (PMMA) with adjustedmelt and mold temperatures generated atotal cooling time that, at 314 s, was al-most twice as long. Figure 6 compares thecycle times and productivity of the threeprocesses for the processing of polycar-bonate.

Record Cycle Time

Tests and simulations demonstrate that,for the manufacture of thick-walled partswith external intermediate cooling, thecooling time in the mold can be reducedby 25 to 50 % compared to conventionalmultilayer molding – depending on thegeometry of the part. At K2013, the in-jection molding manufacturer Engel – to-gether with its project partners Bayer Ma-terialScience and the Krallmann Group –will be demonstrating the potential of thisprocess live for the first time. The produc-tion of an optical lens from PC (type:Makrolon LED 2245) in record time willbe demonstrated.�

REFERENCES1 Pillwein, G.: Maschinen- und Prozesstechnik zur

Herstellung optischer Bauteile. IKV Seminar:Spritzgießen hochwertiger optischer Komponen-ten, Aachen 2008

2 Zöllner, O.: Kunststoffoptiken im Mehrschicht-spritzguss. Dissertation, University of Erlangen-Nürnberg 2012

3 Klinkenberg, C.: Clear benefits of multi-layer.Injection World, Edition October 2012

THE AUTHORSCHRISTIAN MAIER M.SC., born in 1983, is project

head in the process technology development depart-ment at Engel Austria GmbH, Schwertberg, Austria;christian.maier@engel.at

DIPL.-ING. JOSEF GIESSAUF, born in 1968, headsthe process technology development department atEngel; josef.giessauf@engel.at

PROF. DR.-ING. GEORG STEINBICHLER, born in1955, is head of research and development in tech-nologies at Engel and director of the Institute ofPolymer-Injection Molding Technology and ProcessAutomation at Johannes Kepler University, Linz, Aus-tria; georg.steinbichler@engel.at

Cooling time

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°C

0

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pera

ture

100 200 300 400 500 s 600

>210 °C190–210 °C170–190 °C150–170 °C130–150 °C110–130 °C90–110 °C70–90 °C50–70 °C

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ture

100 200164 300 400 500 s 600

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Fig. 5. Simulated maximum temperature within the part in dependence of time for a single-layerprocess (top), a three-layer sandwich process (center) and the new three-layer sandwich processwith external intermediate cooling (melt temperature 280°C, mold temperature outer layers and sin-gle-layer variant: 120°C, mold temperature inner layers: 70°C). The graphics also include the temper-ature distribution in the part interior after 164 s (figure: Bayer MaterialScience)

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1-layer 3-layersandwich

3-layer sandwichwith intermediate

cooling

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time

Prod

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Cooling timeProductivity

Fig. 6. Simulatedcooling time per moldstation and the calcu-lated productivity fordifferent manufactur-ing processes (mate-rial: PC). The single-layer part is used asa reference (100 %)for comparison (figure:

Engel)

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What layer sequence is most appropriate, howmany layers are necessary and what savingspotential can be expected? These questions canbe answered with a few considerations that ap-ply to one-sided and two-sided overmolding,but not to the bonding of two preforms (Fig. 1).Since the heat removal from the connecting lay-ers follows relatively complex laws, this alter-native will not be considered.

First, two assumptions are made: Firstly,known mold concepts (index plate, rotary table,sliding table) are used. Ideally, a further preformis manufactured simultaneously with the over-molding of one preform. From this the require-ment, known from multicomponent injectionmolding, follows that the cooling times of alllayers must be the same.

Second, it should be noted that only the layermanufactured first is cooled on both sides (lay-er 1 in Fig. 1). The following layers only havecontact with the mold wall on one side, whilethe preform borders on the other side, which,for the sake of simplicity, is regarded as an ide-al insulator. To obtain the same cooling time inall stations, the following layers, which arecooled on one side, should be only half as thickas the first layer, which is cooled on both sides.

On the assumption that the first layer of apart produced from n layers is only half as thickas all the following layers, the layer thicknessesse of the first layer and sf of the following lay-ers are as below, where stotal describes the to-tal thickness of the part:

(1)

(2)

The cooling time is proportional to the square ofthe wall thickness. The cooling time te(n) of thefirst layer (which is cooled on both sides), corre-sponding to the cooling time tf(n) of all furtherlayers (which are cooled on one side) is de-scribed by the following equation:

(3)

During overmolding on one side, n stations inthe mold are necessary, i.e. as many stations aslayers. In the sandwich variant, in which twolayers lying one behind the other are producedsimultaneously as in a stacked mold, only(n+1)/2 stations are necessary. In this case, thenumber of layers n must be odd.

Overmolding on two sides, compared to onone side, does not at first offer a cooling timereduction, but provides the advantage of thestack mold, i.e. a saving of platen area andclamping force.

The relative cooling time per mold station, i.e.the cooling time compared to that of a single-layer part, is:

(4)

The number of parts T(n) per unit time corre-sponds to the reciprocal of the time per part,i.e. the reciprocal of the cycle time. Instead ofthe cycle time, the cooling time is used here,which is permissible to a first approximation forvery thick-walled parts. From the reciprocal ofequation 4, the relative number of parts is thusobtained:

(5)

However, the cooling time or number of partsare only conditionally suitable as values for anefficiency comparison. These values do not takeinto account the fact that multilayer processesrequire a larger number of cavities. However,increased efficiency can be expected from larg-er numbers of cavities anyway. If with single-layer processes, for example, twice the numberof cavities is available, the number of parts perunit time would also be twice as high.

The productivity is therefore used for the fur-ther assessment. It is defined as the ratio be-tween the produced parts and the productionfactors necessary for this, in this case the cavi-ties. To obtain the relative productivity of multi-layer molding compared to the single-layermethod, equation 5 only needs to be divided bythe number of cavities – i.e. by n in the case ofovermolding on one side and by (n+1)/2 in thecase of overmolding on both sides:

(6)

for overmolding on one side

(7)

for overmolding on both sides

21

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Cycle Time and Productivity – Considerations on the Layer Structure!