Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to compositions produced by Stereolithography
Apparatus (SLA), generally known as a form of Rapid Prototyping, and more particularly to
a method and product allowing the protection of photoformed compositions, utilized as
mold inserts, for thermoplastic injection and blow molding. These mold inserts can be
made from start to finish without traditional machining practices.
Steroeolithography is one of several available methods of producing rapid prototype
models. The process allows complex shapes to be reproduced in polymer resin, going
from computer data to finished product within a period of 24 hours. Rapid prototyping
techniques are increasingly being used by industry to produce models in a fraction of the
time required for more conventional manufacturing methods.
To produce a stereolithography model, the product must be modelled with a
computer aided design (CAD) package. The stereolithography model is manufactured by
scanning a laser across the surface of a tank of liquid polymer resin. The CAD model
~J~I~b~.se directs this laser.The resin is sensitive to the laser light and solidifies in a thin
layer, in the scanned areas. Each scan builds up a thin layer of solidifed resin,
representing one of the cross sections of the model. Between the scanning of each layer,
the developing model is "dunked" beneath the surface of the resin, leaving a layer of liquid
to be solidified in the next scan.
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The resin is partially cured and is finally cured by placing the model in an oven.
After this, the model is cleaned up by hand and finished according to it's intended
application.
Another form of Stereolithography has been termed Solid Ground Curing (SGC).
The Cubital System uses this technology to combine layer additive and layer subtractive
process to produce patterns. The finished parts are produced in photopolymer resin,
similar to stereoltithography is appearance A typical process layer is produced in the
following steps:
1 ) An ultraviolet lamp exposes a photosensitive resin (similar to those used instereolithography) through a mask. The mask is created on a charged glass plate
using a toner similar to xerographic or laser printing techniques.
2) The unexposed resin is vacuumed up from the exposed layer.
3) The remaining voids where the unexposed resin was removed is filled with liquid
wax, and a chill plate then hardens the wax.
4) The resin/wax layer is the milled down by a cutter to a precise thickness.
5) A new layer of resin is applied, and the process continues.
When complete, the soluble wax is usually removed by dissolving it in a mild acid
solution or a solution of soap and warm water.
There has been an increasing requirement to create molds for injection molding
rather than a part using the SLA. Both the cavity and core are produced by
Stereolithography. It is an a very fast method to obtain a complicated shape with detailed
topography. Injection molding is an excellent way to repoduce the obverse of that
complicated shape in one shot. Common injection moulding processes can then produce
functional plastic parts.
There are three properties of the photoformed molds which reduce their efficiency
in injection molding of thermoplastics. The three characteristics are compressive strength,
heat deflection temperature and heat transition temperature.
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The stereolithographic molds by themselves undergo a change in mechanical
properties respective glass te",peralures. In most cases the glass temperatures are less
than 130 degrees Centigrade. Stereolithography molds tend to chip when the part is
ejected after exceeding these temperatures.
The other critical temperature to the SLA mold is the heat deflection temperature.
The heat deflection temperature of the cured photopolymer is the temperature at which the
polymer softens and begins to creep. These deflection temperatures are typically less
than 100 degrees Centigrade for stereolithograpy inserts. Heat deflection effect will
change the topography of the thermopl~.stic part produced.
Accuracy in the thermoplastic part produced by the insert is further degraded by the
compressiblity of the cured SLA resin.
In order to address these temperature problems the SLA resin core and cavity have
previously been coated with a 125 micron layer of copper used to protect the core and
cavity from heat stress during the injection moulding of relatively high temperature resins
such as Polycarbonate.
This process has produced a relative small number of parts before the delamination
of the the thin copper coating. The copper absorbs the heat more readily than the
sul,st,ale. The copper film expands at a faster rate than the sub~ te. The coppper
coating shears off the sul)slrale after approximately fifty parts are produced.
Nickel is an other protective coating material which tends to crack after a small
number of parts are produced.
In order to fully utilize the efficencies of stereolithograpy compositions for injection
mold inserts there is a need to protect the photoformed insert with an outermost layer of
the cavity or core wall consisting of a substance with excellent heat resistance low heat
conductivity high tensile strength and high elongation durability against heating cooling
cycles an abrasive resistant surface with polishability and be thin in cross section. The
protective coating must not require curing at temperatures that exceed the glass transition
and heat deflection tel"peralures of the photofo""ed insert. The high heat resistance of
the prolecti~/e layer is desireable because the greater the variety of thermoset and
thermoplastic resins can be for the injected molded part. The outermost layer must a
glass temperature greater than 200 degrees Centigrade.
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Photopolymer such as describe above are used by and not limited to the followingStereolithography Appartus:
1. 3D Systems Inc., U.S.A.
2. CMET, JAPAN - SOUP 600, 850x
3. D-MEC (Sony Group), Japan - SCS 1000HD
4. Laser 3D, France - SPL 1000, 5000
5. EOS Gmbh, Germany - STEREOS 400, 600
6. Teijin Seiki, Japan - Soliform 300, 500
7. Cubital, Isreal - Solid Ground Curing
The Disclosure
The ~ ess
The mould creation process is the main factor in creating functional and technical
prototype plastic parts. The functional prototypes, ty-pically two to five parts are needed in
the product development cycle for:
1) product planning
a) working principle verification
b) functional principle optimization
2) m~mlf~ctllring process planning
a) m~mlf~cture sequence assembly planning
b) layout pl~nning
c) resource planning
The technical prololy~es, typically three to twenty parts are needed in the product
development cycle for:
1) product planning
a) cll~tomer acceptance verification
b) fatigue strength verification
2) m~mlf~cturing process application
The goal of our research project was to produce both functional and technical
pr~tolyl,es in numbers less than fifty parts in the engineering resin identified for final
m~nllf~ctllre - Polycarbonate. The chosen test part was breast shield adapter for a mother's
breast milk pump. Two parallel three--limen.cional CAD solid models were produced, one with
PRO/engineer and the other with AutoCad R12 (Advanced Modeling Extension). The
AutoCAD produced STL file proved unsatisfactory for the Stereolithography a~al~lus
(SLA) and had to be l.,cleatt;d using a third party application working within AutoCAD R12.
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Once the part was created by the designer and all the structural and cosmetic features
have been designed into the part, it is then sent to the Stereolithography appal~us (SLA).
The SLA creates real three--limen~ional parts from the computer model. The parts which can
be made of acrylic or epoxy, address only the plu~lties of form and fit, but do not fill the
above requirements of functional and tec~ical prototypes listed previously. They are quite
brittle and will not with~t~ntl heat sterilization. The process that has been developed takes a
dirre~c.,l route from the step above. Instead of creating a part using the SLA, a mould is
created using the SLA. Previously, only one side of the mould was created by this process.
Here, both the cavity and core are produced by Stereolithography. The mould can then be
used to create functional plastic parts.
The PRO/engineer CAD model is sent to another package called PRO/mold that creates
the core and cavity of a mould. In PRO/mold, the core and cavity are adjusted for shrink
allow~ces of a plastic injection moulding process. The location of the ejector pins to strip the
plastic part, from the mould are added. Cooling lines, used to keep the mould cool during the
injection moulding process, may be added if necessary. The time to pclr~ ll this function is
about one day for the breast shield adapter.
The mould core and cavity are then sent to the SLA to be built. The building time of the
SLA parts also depends on the size of the part. This time is real time as the SLA can operate
day and night and it does not need to have a model maker present. Simlllt~neously, a mould
maker is prep~;l-g mould shoes that will hold the SLA core and cavity in the injection moulding
m~r~hine and m~king ejector pins for the mould.
The SLA core and cavity are then coated with a 125 micron layer of copper used to
protect the core and cavity in the injection moulding m~chine; This is a key part of the process.
The mold halves were ready for the next phase one week after the CAD solid model geometry
was complete.
The coated core and cavity are then fitted into the shoes, which the model maker has
made, and the shoes are placed in the injection moulding m~Ghine. The ejector pins are also
placed in the mould shoes. The injection moulding m~hine parameters, such as temperature and
s~ule, are adjusted and the first parts are made. The setup time is about two hours. Up to
forty-four functional prototypes of the breast shield adapter have been created by this process.
.
Concluslons
The prototypes of the breast shield adapter, created by this process, are functional and
could be used for hospital efficacy testing. Consumer focus groups can be provided with these
ploto~y~es for acc~ce verification. Sales staff can use these prototypes for marketing and
obtaining orders before product production commences.
Designers will gain from this process because it reduces the risk associated with the final
part design for production mould m~nllf~chlring- Mech~nic~l dçsigners can check that the parts
function propt;lly. Product integrity staffcan conduct environment~l stress analysis at the earliest
possible stage in the development process.
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The process has reduced the time to create functional plotoly~e plastic parts by a factor
of at least 15, and reduces the cost of making proto~pe moulds by 80%.
The process applies to the wide spectrwn of Thermoplastics having melt t~ el~ s
less than Polycarbonate. This group includes ap~lo~illlately 80% of all Thelmoplastics. Mould
Makers can now produce both core and cavity inserts by this process.
[1] Yeung M. and McKeen J., "Rapid M~mlf~ctllrin~ of Plastic Injection Moulds for Prototype
ABS Parts", on the proceeding of "1995 SLA User's Group Conference", Tampa, FL,
U.S.A., March 1995.
[2] McKeen J. and Yeung M., "Rapid Mould M~mlf~ctllnn~", on the procee~ling of "Rapid
Prototyping and Manufacturing '95", Dearborn, MI, U.S.A., May 1995.
In a preferred embodiment the mold insert has three component parts. The parts are
a photoformed shell a cooled heat conductive backing and a conformal protective Polyimide
exterior coating. The mould insert consists of a stereolithograpy apparatus photoformed shell
which is filled by heat conductive backing and cooling ducts. The backing material has a
co",pressive strength equal or greater than the compressive strength of the hardened
photofor",ed shell ",ate,ial. Typically this greater than Cold cure two part epoxy matrix
combined with alluminium fragments of random shape and size is used for the baking material
and copper or alluminium tubes for the cooling ducts.
Other techniques to attempt to reduce the operating temperature of the photopolymer
in a mould the core and cavity inserts have been hollowed out or or a shell is photoformed.
The shell is then filled with a conglomerate mixture of alumium fragments in a polymer. Heat
conductive ducts can be installed in the conglomerate prior to the polymer harding. These
ducts can then be cooled with water in allelllpl to keep the photoformed shell below it heat
deflection and glass transition temperatures. This water cooling is applied when photoformed
shells and backing are installed in an injection moulding machine and the photoformed shell
is in cyclic contact with a thermoplastic to produce a part with the topography of shells
obverse shape. Although the photoformed shell is cooled by the backing material it is still
subject to surface heating which reduces its number of part cycles.
The exterior of the shell is protected by a flourinated polyimide layer which is the
outermost layer of the cavity or core wall. Polyimides have been used as coatings in the
electronics field because they are high thermally stable dialectric. The other propertys that
makes Polyimides excellent for a protective coating used in conjunction with injection moulding
is the fact that are abrasive resistant there surface is slippery and can be polished.
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The p~oble", in the past has been these Polyimides have been applied in a solvent
solution such as gamma-Butyrolactone. Typically solutions such as this require final cure
te",perdlures in between 200 and 350 degrees Centigrade. Application of these Polyimide
films to a Stereolithography mould would require curing at temperatures at least 70 degrees
greater than the glass transition temperature of the Photopolymer sul,slfate. When the
temperature of the photoformed shell is exceeded the mechanical properties of the subslfate
would be degraded. The coated photopolymer would be unuseable as a mould material as
it woud be unable to withstand the pressure of injection moulding. First a photoformed shell
is made and the consisting of a substance with excellent heat resistance, low heat
conductivity, high tensile strength and high elongation, durability against heating cooling
cycles, an abrasive resistant surface with polishability and be thin in cross section. The
protecti~/e coating must not require curing at temperatures that exceed the glass transition and
heat deflection temperatures of the photofor"~ed insert. The high heat resistance of the
protective layer is desireable because, the greater the variety of thermoset and thermoplastic
resins can be for the injected molded part. The outermost layer must a glass temperature
greater than 200 degrees Centigrade.
The flourinated polyimide powder such as LARC cp-1, but not limited to, are solvent
soluble. It is mixed with solvents that are volitile at tempertatures below the glass transition
temperature of the hardened photoformed composition. Typically these solvents are Tetra
Hydro Furan, Ethyl Acetate, Acetone, Isobutal Keytone, Cloroform and a combination of Ethyl
Acetate and Acetone. These solvents are typically drawn off at room temperature. In fact this
drawing of process should not be accelerated because it may cause bubbling or bli~leri,1g in
the polyimide coating. A final heat may be applied up to the glass temperature of the
photopolymer to draw off residule solvent and to cure the polyimide coating.
The solvent mixture can be have up to 16.0% solids. The amount of solids can be
reduce to accommodate desired layer thickness and required viscosity for the application
technique.
Thickness of the conformal coating of polyimide can vary from 20 microns up to .005
inch.
The favoured application technique is eleclroslatic spray coating. This allows many
thin conformal coats to be applied one after another.
Simple hand sprayers or air brush guns can be used. Even application with a
paintbrush.
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In a second embodiment there is no backing material filling the photoformed shell. In
a third embodiment a adhesive is used between the shell and the exterior coating.
An adhesion promoter such as Hexamethyle di silazane (H.M.D.S), tradename -
H~dro~ella~e may be used. A expoxy resin mixed with a hardner may be used to promote
adhesion of the polyimide solution to the hardened photofor"led shell.
