1 Rapid Manufacturing of Non-Metallic Objects.ppt

Rapid Manufacturing of
Polymeric Objects

Outline
• Introduction
• Silicon Rubber Molding
• Epoxy Tooling
• Spray Metal Tooling
• Cast Kirksite Tooling
• 3D Keltool
• Conclusions

Rapid Manufacturing – Various Routes
RM
RM for
Polymers
Unlike the layered manufacturing approach of RP, RM would require
multi-faceted and hybrid approaches to meet the varied needs.
Material Geometry
Subtractive RM Best Best
Indirect Routes Best Ok
Additive RM Ok Ok
Hybrid RM Ok Best
RM for
Metals
RM for
Ceramics

• Subtractive RM will continue to dominate when the quality of the material and
geometry are high.
• Additive RM: Additive RP methods discussed earlier can be used for one or two
parts. Their limitations were discussed earlier.
• Hybrid RM: There are not many processes for polymers.
• Indirect RM: These are very promising. The following are the routes:
Indirect Routes for RM of Polymers
Silicone Rubber Moldling It is a Room Temperature Valcanized (RTV) rubber mold.
It is also known as Vacuum Casting.
Epoxy Tooling It can be neat epoxy or Aluminum-filled Epoxy Tooling
(AIMS) for better conductivity and strength.
Spray Metal Tooling It uses cold metal spray. HEK-MCP is a commercial
system.
Cast Kirksite Tooling Molds from soft alloys
3D Keltool From 3D Systems. It uses SLA, Vacuum casting and
post-processing of SLS to obtain injection molds.

Silicone Rubber Molding
(Vacuum Casting)

Features
• This process is also known as vacuum casting (as applied to non-
metals!).
• Silicon rubber molds can be used to make polyurethane parts such
as the bodies of telephone and other gadgets. These latest
polyurethane materials are competing with ABS, Nylon and
Polypropylene (PP) which were popular in these applications.
Table in the next slide gives a comparison of these materials.
• Room Temperature Vulcanizing (RTV) rubber is used to make the
mold.
• Typical yield is 15 to 50 parts depending on the geometric
complexity. Therefore, this is ideal for “soft tooling” and to some
extent for “bridge tooling”.
• RP has linear cost. Therefore, it will be economical to make the
first part with RP and replicate it using vacuum casting.

Properties of Polyurethane
Property SG95 SG200 SG2170 ABS Nylon6 PP
Hardness
(Shore D)
79 80 82 78 78 72
Flexural strength (kpsi) 7.2 6.9 9.0 6.3 4.7 2.9
Flexural modulus (kpsi) 396 391 495 361 284 183
Tensile strength (kpsi) 8.7 7.3 10.5 4.8 7.6 3.7
Tensile modulus (kpsi) 288 238 314 225 225 143
Notched Izod (ft-lbs/in) 0.35 1.09 0.39 1.88 1.17 0.55
• SG95, SG200, SG2170 are the latest polyurethanes. ABS, Nylon 6
and Polypropylene (PP) are given for comparison.

1 Rapid Manufacturing of Non-Metallic Objects.ppt

Steps
• A master pattern is prepared. Formerly this used to be prepared
using hand carving, manual or CNC machining. Now these master
patterns can be made using RP. Almost any RP model can be
used. SLA, SGC, SLS, LOM and FDM models have been
successfully tried. The master pattern should be nicely polished
and cleaned before use. This is because the process can faithfully
reproduce even finger prints on a glass. SLA is the most preferred.
• A sprue is mounted on the master and glued. This assembly is
wiped clean with a soft cloth moistened with isopropyl alcohol.
Hand gloves are to be used during handling these.
• A wooden or metal box is used for preparing the mold. The pattern-
sprue assembly is placed inside on top of thin supports. If feasible,
it can be suspended by a thread from the sprue.
• The liquid silicone RTV resin is mixed with its hardener under
vacuum to eliminate air bubbles.

Steps …
• It is then poured into the box over the pattern while still under
vacuum.
• The assembly is then placed in a low temperature curing oven
maintained at about 50°C for about 6-12 hours. Thus curing takes
place under vacuum at a slightly elevated temperature.
• This process is exothermic. Thus, some parts of the mold will have
more temperature than the oven. Therefore, after the curing period,
the oven should be slowly cooled to room temperature.
• The solidified RTV mold is extracted from the mold box. This is split
into two halves by cutting along an appropriate wavy parting
surface using a sharp knife or scalpel. The parting surface must be
wavy at the outside so as the locate the halves on each other.
However, the inside of the parting surface shall be smooth and
ensure extraction of the part. Cutting this parting surface is a skilled
job mastered over a period.

Steps: Part Preparation
• Master model can be
created using any one of
the current modeling
techniques.
• Establish the model’s
eventual parting line using
clear adhesive tape.

• Colour the tape edge with a
marker pen to assist in
removing master model
from mold. Alternately, a
colored tape could have
been used.
• A casting frame is
constructed. Casting gates
are attached to the model.
Steps: Mold Preparation

• Model is suspended in the
casting frame and venting
rods are attached
• De-gassed Silicone rubber is
then poured into the casting
frame
Steps: Mold Preparation …

• Further degassing in the
Vacuum Casting Chamber
the tool is moved to Oven
for further curing
• When fully cured , the
silicone rubber is then cut
following the visible parting
line marked on the tape
edge

• The master model is
removed from the mold.
• The tool is taped together
and prepared for casting.
One can also use stapling.

• Part A & B are precisely
measured by weight as per
part weight. Color pigments
may be added as required.
• Part A & B containers are
kept in the chamber, mixed
and stirred. It may also be
done manually outside.

• A + B is mixed and poured
down through the flexible
hose into the mold

• After curing in the oven the
cast resin prototype is
removed from the tool
• Casting gates and the
runners trimmed off….
Steps: Preparing Multiple Prototypes

Wavy Parting Surface in Silicon Rubber Mold

Advantages
• Fast and economical.
• As it is flexible, slight reentrant or undercut features can be
produced. There is also no need for draft. So it produces
accurate parts.
• Reproduction accuracy is extremely high. Even transparent
parts such as lamp lenses can be made. Even fine scratches
on the master get reproduced faithfully. Therefore, the master
should be polished and cleaned before used for mold making.
• Suitable for many resins.
• Used in jewelry industry for producing the wax patterns.

Limitations
• As the mold is soft and will deform under pressure, it cannot
be used for injection molding. Only vacuum casting is possible
which is labor intensive. Clamping and injection pressures
have to be low.
• Not suitable for metals.
• Long cycle times of 4-6 hours of part curing for a size of about
8 in3
. This is due to poor heat conductivity of the mold.
• Relative strength of mold limits its life. Therefore, high piece
price
• Few suppliers (Huntsman, GE Silicon etc.)

Silicon Rubber Mold Application - Golf Club

Silicon Rubber Mold Application - Bracket

Vacuum Casting Silicon Mold Made from
Kira’s Paper Master
Master Urethane Sample
Silicon Mold

Applicable materials:
• Rubber
• PP + PE
• ABS + Glass filled ABS
• Clear transparent

Features
• Silicon rubber mold is for making part that may have little
undercuts. Epoxy tooling is meant for parts that can allow
draft. This is because these molds are more rigid than the
rubber molds. Epoxy is stronger and cheaper than rubber.
Therefore, it can be used on an injection molding machine or
on a press.
• Two-part, neat or Aluminum-filled epoxy is cast onto a
pattern. When Aluminum powder is mixed with epoxy, it is
called Composite Aluminum Filled Epoxy (CAFE) tooling.
This enables the process used for making stronger parts or
more shots. AIMS is an epoxy process patented by 3D
Systems.
• Typical yield is 100 to 200 injection molded parts.

Comparison of Silicon Rubber Molds and Epoxy Molds
Silicone Rubber Mold Epoxy Mold
No need to make the mold in two
halves. As it is transparent and soft, the
parting surface is made after making
the mold.
Similar to sand mold preparation, the mold will be
made in 2 parts. Since single pattern only is
commonly available, this means double curing time
– one for each mold half.
Flexible and hence tolerant to reentrant
profiles.
Hard and hence reentrant profiles are not allowed.
These can be achieved using cores. Patterns
preferably should have allowances and drafts.
Parts are to be produced manually. Parts can be produced automatically by mounting
the halves on an injection molding machine or
press.
Typical yield is 15 to 50 injection
molded parts
Typical yield is 100 to 200 injection molded parts
Limited to producing parts of low
melting materials such as wax,
polyurethanes etc.
In addition to these materials, Aluminum filled epoxy
tools can be used for ABS, glass fiber-filled plastics
etc. and even for simple stamping of sheet metals.
Suitable only for plastic parts. Suitable only for plastic as well as sheet metal
parts.

Steps
I. Mold box preparation:
• A master pattern is prepared. It is polished to remove stair
steps and cleaned.
• A thin film of commercially available mold release agent is
applied on the pattern.
• Prepare a mold box of suitable size keeping in mind the
dimensions of the injection molding machine. Note that the
outer surfaces of the mold halves will be used for location and
hence the interior surfaces of the box shall be accurate.

Steps …
• The pattern is then placed in the mold box at the desired
orientation. By this time, a parting surface should have been
identified. If it is planar, a simple wooden parting board can be
used. However, if it is more complex, then a machined parting
board is to be used. As the entire weight of the liquid epoxy is
supported by the parting board, it must be strong enough.
• Conformal cooling ducts can be obtained by placing suitably
bent copper tubes that run close to the contours of the pattern
with adequate clearance.
• Location pins can be placed if required.

Steps …
II. Epoxy preparation:
• A suitable two-part epoxy (resin and hardener) is mixed in the
specified proportion and quantity.
• Required quantity of finely ground Aluminum powder is mixed
with this.
• This is kept in a vacuum chamber to degas the bubbles.
Note: The epoxy for each side should be prepared separately as
there is considerable time gap between the two pours. The
other alternative is to make the master pattern in two halves
as is done in sand molding – along with the parting surface in
one of them. However, this is generally not practiced as (i) RP
patterns are expensive and (ii) mold shift will affect accuracy.

Steps …
III. Pouring:
• After keeping the mold box prepared earlier, this composite
epoxy liquid is poured over the pattern under vacuum. If
possible, vibration can be provided during pouring. In order to
reduce the cost, the pieces of old molds can be added.
• It is allowed to cure for a few (6 to 12) hours.
• Then the mold is inverted and the parting board is gently
removed. The release agent is applied on the exposed surface
of the epoxy and additional coat of release agent can be given
on the pattern also.
• Pour the freshly prepared composite epoxy on this side as
before.

Steps …
III. Finishing:
• After curing, the two parts are separated and the pattern
is removed.
• Ejection holes are drilled and ejection pins/ plates are
installed. For small batch, these may not be required as
one can remove the part manually.
• Finally the mold is mounted on standard tool base. Other
assemblies such as connections to cooling channels are
made.

Advantages and Limitations
Advantages:
• Fast and economical
• Suitable for many resins
• Can be used for injection molding
Limitations:
• Relative strength of tool
• Low clamping and injection pressures
• Long cycle times
• High piece price
• Few suppliers

Epoxy Tooling of an Injection Mold

Features
• Metal spraying or plasma spraying technology, sometimes known as
Hard facing exists for a long time. This spray can be used in two
ways:
(i) making the surface conductive. This is called metallizing. The metallized
prototype can be used as a EDM electrode or polished and used in an
application directly.
(ii) making it stronger by building a thicker layer. This can be further backed up
and used as a mold.
• Metal is melted using an electric arc, atomized and sprayed directly
onto a pattern. Note that the metal is almost at room temperature
when it falls on the pattern. The resulting shell is backed with a rigid
material.
• The most commonly used spray metals are alloys of zinc (eg.
Kirksite), nickel, copper and aluminum.
• Typical yield is 100 to 1,000 injection molded parts.
• As the mold cavity is metal, it is possible to even mold metals using
this process.

Demonstration of Coldness of Metal Spray
This tin-coated balloon
is an extreme example
of rapid solidification.
The alloy dissipated
enough heat so that it
did not burst the
balloon on contact.
[http://
www.memagazine.org/
backissues/june00/
features/tools/
tools.html]
This is known as Cold
Metal Spray.

Principle
• The metal to be sprayed is available in the form of wire
similar to MIG welding. Two wires are fed from two spools.
These wires are two ends of a AC power supply.
• The wires are continuously fed to maintain the required
deposition rate. Initially a short circuit occurs causing the
ends to melt. Subsequently a constant gap is maintained in
which an arc is sustained. When compressed air is passed
through the arc, it atomizes the molten plasma and sends it
as a spray. The air also is responsible for the rapid cooling
of the jet which reaches the pattern at a sufficiently low
temperature as not to damage the pattern.

Coating Using Cold Metal Spray
This metal component on the right was produced by spray depositing tin
onto the plastic pattern on the left. Virtually any common tooling alloys, as
well as exotic materials can be used for the spray.
[http://www.memagazine.org/backissues/june00/features/tools/tools.html]

Steps
• The pattern and mold preparation is very similar to epoxy
tooling.
• The master pattern is prepared as usual. A thin film of
commercially available mold release agent is applied on the
pattern.
• The pattern is kept inside a mold box. A parting board is
placed.
• When the spray stabilized, it is sprayed so as to create a
consistent shell of about 1 to 2 mm thick on the pattern and
parting board. The temp of the spray is from 50°C to 100°C
depending on the application.

Steps …
• Subsequently it is backed up with mixed two-part resin and
solid resin particles.
• After the resin backing is cured, it is reversed and the
parting board is removed. The releasing agent is sprayed
again.
• The metal spray and backing up are repeated for the
second half also.
• Finally, the two halves can be prepared for whatever
application it is meant for. These halves can be used for
thermoforming, injection molding, blow molding, sheet metal
forming, vacuum casting etc.

Steps: Cool Spraying the Model

Steps: Backing up the Sprayed Shell
With High Temp Resin

Tool Steel Inserts Made Using Metal Spray
These P20 tool steel inserts are used for plastic injection molding. The
cavity on the left has been prepared for use in a mold base, enabling
completion of tools in a matter of days.
[http://www.memagazine.org/backissues/june00/features/tools/tools.html]

Advantages:
• Fast and economical
• Suitable for many resins and metals
• Suitable for large, highly contoured parts
• Size is not a limitation.
Limitations:
• Relative strength of tool
• Low clamping and injection pressures
• Narrow, deep features are difficult to replicate
• High piece price
• Few suppliers

Features
• Kirksite is a Zinc alloy.
• It can be used for making the tools for
– deep-drawing, forming, punching and blanking of light alloy brass
and steel sheets
– plastic injection
– thermoforming
– blowing.
• This tool is made using gravity die casting.
• Typical tool life is 500 to 1000 injection molded parts.
• Kirksite offers optimum economy for parts (sheet or plastic) production
in small and medium series and for tooling setting.

Features …
• The fluidity of Kirksite being excellent, dies can reproduce the thinnest
details and deformation during cooling is minimum, enabling a
considerable reduction or even elimination of post-foundry operations
(machining, fitting).
• To improve the mechanical functions such as handling of these tools,
lugs, reaction plates, locating pins, guide-rails etc. may be integrated as
steel inserts.
• Kirksite has good friction properties, satisfactory wear resistance, ease
of polishing, suitability for tool storage in atmospheric conditions and
recyclability (being Zinc alloy).
• Kirksite tooling generally is less accurate and more expensive than
aluminum-filled epoxy, or spray metal tooling. It has more transfer steps
than either of those methods. Kirksite tools have about the same life as
spray metal or aluminum-filled epoxy tools.

Advantages:
• Mechanical properties
• Molding temperatures and pressures
• Suitable for nearly all resins
• Recyclability of the material
Limitations:
• Accuracy
• Fine features
• Possible distortion of tool during casting process
• Limited supplier selection

Features
• This is an indirect metallic RT from 3D Systems. This was originally
developed by 3M (manufacturer of magnetic tapes, adhesive sprays etc.).
Historically 3D and 3M are together. The ground work for SLA was also
done originally by 3M.
• This is a secondary process suitable for making all kinds of tools for making
parts of all materials. This can be used for bridge tooling as well as
production tooling in “prototype timeframe”.
• The hardness of the inserts is 30-34 RC and 46-50 RC after heat-treating.
This hardness is comparable to the conventional made tools.
• This is a combination of silicon rubber molding and the post-processing
used in SLS. Hence, it involves a two-level molding.
• Typical tool life is dependent on the selected material, but capable of
producing in the 100,000's.

Steps
• The CAD models of the inserts for core and cavity are made with suitable
allowances. These patterns are then made using SLA. In principle, any RP
process can produce this pair but SLA is used for two reasons:
– KelTool is from 3D Systems and hence they will prefer SLA.
– SLA parts are the most accurate.
• Once these core and cavity patterns have been polished to the desired surface
finish, silicone rubber is cast against them to create molds. The obtained molds
will be negative of the desired molds.
• Into this silicon negative molds, a mixture of metal powder and binder is poured,
packed and cured. The metal mixture consists of finely powdered A6 tool steel
and even finer particles of tungsten carbide.
• At this point, the cast core and cavity inserts exist in a green state. These green
inserts are fired in a hydrogen-reduction furnace to burn away the binder, sinter
the metal particles and infiltrate copper into the inserts. This produces solid
metal inserts that are approximately 70 percent steel and 30 percent copper with
physical properties similar to that of P20 tool steel. The inserts are finish-
machined, drilled for ejector pins and fitted into mold bases.

Steps …
Need for a two-level molding unlike epoxy tooling:
- Excellent reproduction accuracy of silicon mold.
- Flexibility of rubber enables easy removal of the tool in
green state.

Advantages:
• Mimics properties of aluminum/steel tools
• Excellent detail and surface finish
• Presence of copper improves conductivity
Limitations:
• Maximum insert size of 5.9" x 8.5" x 4"

SLA Pattern, RTV Rubber Mold and Steel Inserts

• Very ideal for small volume production.
• Poor dimensional accuracy.
Conclusions
Silicone Rubber Moldling It is a Room Temperature Valcanized (RTV) rubber mold.
It is also known as Vacuum Casting.
Epoxy Tooling It can be neat epoxy or Aluminum-filled Epoxy Tooling
(AIMS) for better conductivity and strength.
Spray Metal Tooling It uses cold metal spray. HEK-MCP is a commercial
system.
Cast Kirksite Tooling Molds from soft alloys
3D Keltool From 3D Systems. It uses SLA, Vacuum casting and
post-processing of SLS to obtain injection molds.

Thank You!
K.P. Karunakaran
Professor
Department of Mechanical Engineering
Indian Institute of Technology Bombay
Powai, Mumbai-400076, INDIA
karuna@iitb.ac.in

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