09. rapid pro to typing with ceramic-filled epoxy resin by op to forming

8/3/2019 09. Rapid Pro to Typing With Ceramic-filled Epoxy Resin by Op to Forming

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Rapid prototyping withceramic-filled epoxyresin by optoforming

Martin AugsburgSebastian Storch

Florian Nissen and

Gerd Witt

The authors

Martin Augsburg and Sebastian Storch are based at the

Centre of Rapid Technologies of the BMW Group, Munich,Germany.Florian Nissen is based at the Department of Industrial Forming

and Casting (UTG), University of Technology Munich, Munich,

Germany.Gerd Witt is based at the Department of Product Engineering,Gerhard-Mercator-University, Duisburg-Essen, Germany.

Keywords

Rapid prototypes, Ceramics, Automotive components industry

Abstract

Optoforming is meant to be a potential substitution for theestablished Stereolithography (SLA) process. Its potential is that

different ceramic-filled photo-curable epoxy-resins cantheoretically be used to manufacture highly loadable parts and

tools. The stiffness as well as the thermal and chemical

resistance of the material used (an epoxy resin named Tooling B)are higher than those of established SLA-materials, such asSOMOS 7120. Automotive applications, in fields where the parts

are directly used parts, such as lighting housings for prototypepurposes, as well as tools for the veneering process in smallbatch production, were successfully tested. In order to enable

precise and cost-effective fabrication, optoforming has to bedeveloped further in the field of secondary processes, such as

inline-filtration of the material and its feeding, as well as themachine software. Currently, another competitor offers a more

mature process based on upgraded SLA machines, which use aceramic-filled epoxy-resin also.

Electronic access

The Emerald Research Register for this journal is

available atwww.emeraldinsight.com/researchregister

The current issue and full text archive of this journal isavailable atwww.emeraldinsight.com/1355-2546.htm

Introduction

Stereolithography (SLA) is one of the most

common and rapid prototyping processes in which

mainly uv-curable epoxy resins are used. In the

early state of automotive product development,

prototypes made in epoxy resins are used for quick

verification of designs or as prototypes with a lowrange of functionality. These prototypes give a first

impression on a parts properties. Fully functional

prototypes with the full range of a parts properties

similar to those of its serial version cannot be built

with SLA because of the limited material

properties of epoxy resins. However many

opportunities to reinforce epoxy resins exist, one of

which is filling with ceramics, and shows promise.

The imaginable advantages of epoxy resins filled

with ceramics are better material properties and

thus possible fabrication of highly resistant parts

and tools (Dormal, 2000).

Using an established technology such as SLAfor the machine design should reduce the effort

of development. The Alphaform Corp. offers an

upgrade for conventional SLA-machines to enable

usage of ceramic-filled epoxy resins such as DSM

Somos ProtoTool20. The disadvantage with these

slightly viscous resins is the risk of decomposition.

Contained particles sink by gravitational force.

A homogenous distribution of these particles is not

possible for the long-term. Because of the problem

of decomposition, materials like somos DSM

ProtoTool20 are not processable for more than

several weeks without special treatment to revert to

a homogenous filled resin. Filled resins containingmore than 50 percent of ceramic fillers cannot be

used on such an upgraded SLA-machine because

of the increased viscosity. As a result of the flow

behaviour, sufficient recoating becomes

impossible. New concepts of feeding and recoating

are necessary. On this account, Optoform LLC, a

subsidiary of the 3D Systems Corp. is developing

the optoform-technology (Figure 1). It provides

the possibility to process different highly viscous

epoxy resins with a high ratio of fillers. As a result

of the increased amount of ceramic fillers, parts

and tools should show higher stability and stiffness

than those made of slightly viscous resins on

SLA-machines (Gebhardt, 2000).

Depending on the amount of ceramic fillers,

two alternatives for using ceramic filled epoxy

resins are possible: direct use or use after

post-sintering. A direct application of non-sintered

parts could be the fabrication of lighting housings.

For that, established SLA-materials offer good

mechanical stability but are thermally resistant

Rapid Prototyping Journal

Volume 10 Number 4 2004 pp. 225231

q Emerald Group Publishing Limited ISSN 1355-2546

DOI 10.1108/13552540410551342

Received: 18 March 2003

Revised: 1 March 2004

Accepted: 12 March 2004

225
http://www.emeraldinsight.com/researchregisterhttp://www.emeraldinsight.com/1355-2546.htmhttp://www.emeraldinsight.com/1355-2546.htmhttp://www.emeraldinsight.com/researchregister


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only for a limited range. Hence, functional

prototypes of lighting housings cannot be

fabricated using conventional SLA. A high

percentage of ceramic fillers should enable higher

thermal resistance and thus make fabrication of

functional prototypes possible.

Another application of non-sintered and

directly used parts could be the field of

aerodynamic development of racing cars.

Geometries change quickly and are examined in

computational fluid dynamics (CFD) simulations.

To verify those simulations, wind tunnel tests are

used. For these tests, parts made by rapid

prototyping technologies (such as SLA) are often

used. Many verification loops can be made in a

short time to obtain optimal flows and best

performance on the racetrack. However,

aerodynamic parts made from established rapid

prototyping materials often have a lower stiffness

than parts made of materials used for racing.

Therefore, there are differences between thesimulation and testing of aerodynamic parts.

With the use of epoxy resins filled by ceramics,

parts with higher stiffness can be achieved.

This can decrease the gap between simulation

and testing (Clarvinal et al., 2003).

The fabrication of tools made in epoxy resins

filled by ceramics is also conceivable. High

mechanical resistance to frictional wear or

compression loads is intended. Inlets for injection

moulding or other tools are possible. Post-

sintering could offer pure ceramic parts and tools

with very high mechanical and thermal resistance.

Shells for investment casting without geometricallimitation should be producable.

Characteristics of the optoformingprocess

Preparation of CAD data is similar to established

solid-freeform-fabrication. If necessary, some

parts are built with supports. Triangulation of the

parts and supports geometry is followed by slicing

them into layers.

Available layer-thickness is in a range between

75 and 200 mm. Maximum build size is

250 350 500 mm. At the end of the

preparation-process, data are converted into a

specific machine code. The supply of material is

similar to that for selective laser sintering (SLS).

A vertical piston pushes material onto the build

platform. The material is spread by a recoatingunit (Figure 1). Owing to the high viscosity of

the filled epoxy resin used, which behaves like

a paste, no expensive vat is necessary. As a

result of the high viscosity, the recoating unit

works differently to established recoating units.

To liquefy the stiff paste, rollers in front of the

recoating blades are used, which will be discussed

in detail in the next section. In general, the process

of recoating offers a high potential to reduce build

speed. The time for recoating strongly depends on

the size of the area that has to be coated. For that

reason, building platforms in different sizes are

available for optoform-technology in order tosave time.

Finally, the part needs to be removed from the

platform. It is fully embedded in uncured paste

(Plate 1).

Non-cured paste is taken off manually by

scrapers and can be reused. As a result of the lack

of transparency of the paste, careful work is

necessary to avoid damaging filigree details of the

part. The material has to be filtered before its next

use, because broken residuals pollute the uncured

paste. Filtration can be done in two ways: offline

and online. Offline-filtration is done separately

from building process. Using online-filtration,waste paste is filtered during use directly in the

machine. This process is more efficient than

offline-filtration, but currently such an online-

filtration-process does not work properly.

The screen of filtration can be blocked by residuals

which causes incorrect feeding of material.

Because of this, insufficient recoating of the build

area occurs. A wrongly recoated build area results

in poor quality of cured layers. These internal

Figure 1 Building principle of the optoform system

Plate 1 Fully embedded part

Rapid prototyping with ceramic-filled epoxy resin

Martin Augsburg, Sebastian Storch, Florian Nissen and Gerd Witt


Volume 10 Number 4 2004 225231

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material defects can negatively affect the material

properties of the whole part. Therefore, an offline-

filtration-process has to be used. A machines

piston is used to push waste material through a

fixed screen.

Storage of filtered as well as fresh paste is

affected by a high evaporation of hydrocarbons.

Unclosed storage in rooms with frequentlyexchanged air (e.g. air condition) results in drying

of the paste by removing evaporated hydrocarbons.

After storage for any more than 1-2 weeks under

those conditions, the paste is no longer not

processable. This effect is increased by

compressive stress. Therefore storage in the

machine over a period longer than 1 week without

easing the piston off the paste is not

recommended. Fresh paste has to be stored

under nearly hermetic conditions to limit the

drying-process of the material. Evaporation of

hydrocarbons cannot be stopped, so it needs to be

limited. Without the ability to escape, the gasliquefies. Thus a small puddle on the pastes

surface occurs, which can be easily re-stirred into

the paste. Manufacturing is possible for longer

than half a year, if the paste has been stored in this

fashion. Composition of Tooling B could be

re-designed to minimise these difficulties.

After pre-cleaning the parts using scrapers, the

remaining uncured material has to be flushed out.

In the respective tests with pure hot water and with

soap (at 908C), the paste is bearly soluble.

Therefore, aggressive organic solvents such as

isopropyl-alcohol (IPA) and Dovanol (TPM) have

to be applied. In order to clean automaticallyinstead of manually, tests with cleaning-units

similar to a dishwasher were performed. The

disadvantage of such machines is the poor

cleaning of filigree details and undercuts.

Uncured paste is not fully flushed out, so manual

cleaning with the help of brushes and highly

pressurised solvents offers the best results. UV

Postcuring follows to remove slight residues of

paste on the surface. Tempering at 1608C is

recommended, to get optimal material properties

(Figure 2).

Stability of the process

Precise building of parts is not yet possible. This is

due to part geometry, the material used and the

machines hardware and software. Filigree details

of parts easily break during recoating or cleaning

processes. Furthermore, fabrication of details with

thicknesses of less than 1 to 1.5 mm is bearly

possible and extensive supporting is necessary.

Therefore, the more complex the parts geometry

is, the more supports are required.

In addition to part geometry, the material used

influences the stability of the process. Currently,

the materials are produced manually under

laboratory conditions. The lack of continuity may

cause inhomogeneous properties of paste in

different batches. As a result, recoating can be

sufficient or insufficient in identical builds under

similar external circumstances.

With an increase in the number of layers, a lack

of material can be observed, although the correct

amount of material is provided by the software.

This effect relates to the ratio of cured to uncured

volumes inside a parts geometry. The more cured

volume a parts geometry has, the less this effect

occurs. A possible explanation is a deviation in

viscosity for different batches. This may result in

flowing of the uncured material. In fact, loss of

material remains low at a small number of layers,

but the loss of material is substantial after a highnumber of layers. However, cured structures inside

a parts geometry can limit this flow. In this case,

the paste that is fully embedding the part is more

stabilised, as shown by observation. For now, there

is no automatic function to solve this problem.

As long as production of paste is unstable, two

alternatives are possible. One acceptable option is

permanent monitoring to assure the correct

amount of material. Another alternative is the

building of thin walls around the parts geometry

to limit the flow of paste and guarantee the correct

amount of material.

Other frequent operational faults are based onthe character of the machine. It is a prototype-

series, which is custom-built without assembly

regulations or specific quality standards. Hence,

the performance cannot be precisely assessed.

Material properties

The first derivative of ceramic filled epoxy

resin made by Koninklijke DSM N.V. is

Tooling B which is intended for direct use

Figure 2 Tempering process of Tooling B





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(Koninklijke, 2003a). Another version named

Ceramic B, which features a higher amount of

ceramic filler and therefore is intended for a

sintering purposes, is still under development.

In general, the high amount of ceramic fillers in

the available materials is supposed to offer

improved material properties such as stiffness and

thermal resistance. However, this leads to anincreased viscosity, which was measured at

1700 cps at room-temperature. This is only 70

percent of the original viscosity of Tooling B and is

supposed to be a result of the manual production

under laboratory conditions. In comparison to

SOMOS 7120, the viscosity of Tooling B is almost

three times as high. This complicates the

processing, since the established way of recoating

by blades is nearly impossible. For this reason,

the thixotropic effect of the paste, according to

the Bingham-model, is implemented. During the

application of shearing stress, the material

becomes low in viscosity and capable of flow hence recoating becomes possible. Therefore, the

recoating unit uses rollers, which apply shearing

stress on the paste. After coating one layer, the

shearing stress diminishes, which causes

paste-solidification after a short waiting-period

(Figures 3 and 4).

The disadvantage of this thixotropic effect is the

settling down of particles during the time of low

viscosity. As a result, a slight separation of particles

and liquid epoxy resin can be observed. Regions

with higher and lower amounts of ceramic particles

exist within each layer. Therefore, the use of the

thixotropic effect has to be optimised, because themechanical properties of filled materials are

influenced more by their structure than by their

chemical composition (Nissen, 2003; Schatt and

Wieters, 1994).

Spectral analysis and analysis with scanning

electron microscopy (SEM) were applied to

analyse the structure of cured Tooling B. Results

show that Tooling B contains silica, oxygen and

carbon (Figure 5).

The epoxy resin used can be specified as a

thermoset plastic. Thermoset plastics are very stiff

materials due to their closely meshed network of

polymeric chains. Because of the high bonding-

force, nearly no plastic deformation can be

measured with an increasing state of stress.

Ruptures of polymeric chains occur at a certain

maximum state of stress. In addition, dispersing

ceramic particles in those thermoset plastics stiffen

the material. In general, ceramic particles areknown as stiff but brittle.

Figure 3 Viscosity of Tooling B at room temperature

Figure 4 Recoating unit: rollers

Figure 5 Spectral analysis: Tooling B





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SEM displayed that the ceramic fillers of Tooling B

are fully embedded in the form of differently

shaped regions bonded inside the cured epoxy

resin. The crystallites consist of SiO2 and vary in

shape, size and distribution. The particles shapes

differs from globular to angular with sharp edges,

which may be due to their method of production

(Plate 2). Their size ranges between 5 and 40 mm.An inhomogeneous shape and size can effect the

number of gliding resistances inside each ceramic

particle, and thus can cause increased brittleness

and decreased elongation at break (Weibach,

1998).

Because of the decomposition temperature of

the epoxy resin used, the level of re-enforcement

due to the filling with ceramics depends on

temperature. This marks the upper limit of

thermal range for use of parts and tools made in

Tooling B.

The supposed high stiffness can be observed

due to the use of a thermoset plastic filled withceramic particles. Chemical resistance is not

influenced by the fillers because there is only

inclusion of ceramic crystallites in an epoxy resin

instead of the sintering of an entire ceramic matrix.

Therefore, a high chemical resistance is due to the

formulation of the epoxy resin.

Measuring the material properties showed that

stiffness (Youngs and flexural modulus) and

thermal resistance of the material increase in

comparison to established epoxy resins, such as

DSM somos 7120 (Koninklijke, 2003b).

Tensile strength and elongation at break are

similar to this established epoxy resin (Figure 6).A good chemical resistance of Tooling B against

solvents such as engine oil, gasoline, coolant and

braking-fluid was detected. Tests running over

14 days showed no permeability of tempered

Tooling B. However, the use of containers made

out of Tooling B for automotive prototype

purpose is impossible due to low elongation at

break. During use, the stress of vibration would

cause damage to these containers.

Application

After analysing the characteristics of the processand the material properties, the focus was shifted

to determine opportunities for automotive

applications.

Rapid prototyping of lighting housings

Owing to the high thermal resistance of tempered

Tooling B the build of lighting housings seems to

be a feasible application. Functional prototypes of

housings are used for presentation of the designs in

which they lie. Housings made out of established

SLA-materials such as SOMOS 7120 are unable

to withstand a period of 1 h of usage because of the

heat, which may cause a deformation of the

prototype. They have to be presented for little or

no time of function.

To analyse a sample part made out of Tooling B,

the 3D-model of a taillight of the new MINI was

prepared and built on the optoform system. Three

12V braking-lights with a power of 21W each were

fixed inside the housing, whose surfaces were not

coated. A clear light cover was mounted afterwards

(Plate 3).

The lights shone for a period of 24 h

without any cooling or interruption. During the

Plate 2 SEM-Analysis: Tooling B

Figure 6 Material properties





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test, temperatures of 908C occurred on the

surfaces inside the housing. Later the

measurement showed a dimensional deviation of

less than 2 mm in each axis.

In a second step, the housing was tempered as

recommended by optoform LLC and re-tested.

Dimensional deviations of less than 0.5 mm weremeasured. In a third step, the housing was coated

with chrome and sealed with clear varnish. After a

further test-procedure of 24 h, no further

dimensional deviation of the parts geometry

occurred. This may be due to the lower

temperatures on chrome-plated surfaces and their

high reflectance of heat. The coated surfaces were

not damaged by any evaporation effect or

dimensional change (Figure 7). Instead, the clear

varnish of the housing turned less opaque due to

the low thermal resistance of the clear coat. The

test showed a good thermal resistance and low

shrinkage for tempered Tooling B.Therefore, the fabrication of lighting housings

for prototype purposes made of tempered and

coated Tooling B is possible, so long as thermal

resistance of the clear coats is improved to avoid

a colour change in the coatings surface. On the

other hand, rapid manufacturing of housings for

small volume production is not possible for two

reasons: lower dimensional deviation is needed to

achieve correct distribution of light and

additionally Tooling B is too brittle. Parts cannot

stand vibrational stress over a longer period of time

without internal or external fractures.

Rapid tooling of fittings for veneering process

The building of tools stressed by pressure and high

temperatures is conceivable with the high thermal

resistance as well as good stiffness of

Tooling B. Therefore, rapid tooling for a veneering

process was tested. In general, sandwich-builds

contain shaped layers of wood separated by a firm

glue. With the help of vacuum-driven membranes

and temperatures above 958C, the sandwich is

pressed and fixed onto prepared parts. These parts

must be fixed in a fitting (Plate 4). This fitting was

re-engineered first and fabricated in Tooling B in a

second step for testing purposes. Tempering was

the only finishing-process to show the dimensional

accuracy of parts made in Tooling B. Afterveneering five sets with three parts each, the test

was successfully completed. The frictional wear

had not damaged the fittings surface. After

completion of the test the fitting provided the same

accuracy as at the beginning. The veneered parts

featured a very good surface quality and were put

for customer use. In conclusion, depending on the

stresses present, successful rapid tooling for

prototype purposes or small batch fabrication is

possible.

Conclusion

Tooling B seems to be a promising material when it

comes to stiff and highly thermally loadable

prototypes or tools for automotive applications.

Lighting housings for prototype purpose can be

manufactured and lit without restrictions in time.

After tempering dimensional stability can be

achieved nearly without deviation. Coatings can be

used without fear of surface failure or fractures.Figure 7 Dimensional measurement

Plate 3 Assembly of the taillight

Plate 4 Veneering-parts fixed on tool





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High mechanically loadable parts can be

fabricated as well. The building of fittings for a

veneering process was shown as an example.

The fittings were used without failure and

veneered parts were used within customers cars.

Tools made in Tooling B can therefore support

small batch production for existing tool loads.

Currently, the process of building works underlaboratory conditions with a high degree of

maintenance. Precise building is not possible for

three reasons. The first reason is insufficient

feeding damaged by broken parts or supports.

The second is inhomogeneous paste as a result of

manual production. The third is unstable machine

software. This suggests that optoforming is clearly

not yet an option for cost-effective manufacturing

of ceramic-filled parts and tools. SLA-machines,

which are upgraded for the use of ceramic-filled

epoxy resins by the Alphaform Corp. are more

reliable due to the established main process and

hardware. However, highly filled epoxy resinscannot be used in this process due to the

problematical process of recoating and the danger

of decomposition by gravitational force. Regarding

the problem of decomposition, optoforming has

the advantage in comparison to the Alphaform-

process.

The clearest advantage of the optoforming

process is not presumed to be in directly

manufactured parts. The post-sintering process

removes epoxy resin in the first step and produces a

full lattice of ceramic particles in the second step,

which can be mechanically, thermally as well as

chemically stressed.

Optoforming could provide parts and tools for

rapid prototyping, rapid casting and rapid tooling

and eventually for rapid manufacturing as long as

the sintering-process, inline-filtration, a stable,

precise production of materials and machine

software are enabled or optimised. A substitution

of conventional stereolithography is possible in the

mid-term.

References

Dormal, T. (2000), Optoform a new process for rapid layermanufacturing based on paste, Rapid Prototyping &Tooling Industrial Applications, Newsletter of theRAPTIA Thematic Network, No. 4, October 2000.

Clarvinal, A-M., Carrus, R. and Dormal, T. (2003), Developmentof material for optoform process, Proceedings of the 1stInternational Conference on Advanced Research in Virtualand Rapid Prototyping, October 2003, ESTG, Leiria,Portugal, pp. 279-82.

Gebhardt, A. (2000), Rapid Prototyping, Hanser.Koninklijke DSM N.V. (2003a), Material Safety Data Sheet:

SOMOS Tooling B.Koninklijke DSM N.V. (2003b), Material Safety Data Sheet:

SOMOS 7120 Epoxy Photopolymer.Nissen, F. (2003), Erarbeitung, Erprobung und Bewertung einer

Prozesskette zur direkten Herstellung von Feingussformenmit Hilfe generativer Verfahren, Diplomarbeit, BMW AG.

Schatt, W. and Wieters, K-P. (1994), Pulvermetallurgie, VDI.Weibach, W. (1998), Werkstoffkunde und Werkstoffprufung,

Vieweg.

Further reading

Ehrenstein, W. (1999), Polymer-Werkstoffe, Hanser.Hellerich, W., Harsch, G. and Haenle, S. (1986), Werkstoff-Fuhrer

Kunststoffe, Hanser.Westkamper, E. and Warnecke, H. (2002), Einfuhrung in die

Fertigungstechnik, Teubner.





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09. rapid pro to typing with ceramic-filled epoxy resin by op to forming

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