Room Temperature Vulcanization (RTV) rubber
molds are the fastest, most accurate, and
least expensive way to create up to a dozen
or so duplicates of a prototype part. Rubber
molds can faithfully duplicate details and
textures present on the original part, and
can be very forgiving of part geometry when
it comes time to remove it from the mold.
After the RTV Molds are completed, they
are then typically used to produce limited
quantity polyurethane castings with a wide
variety of material properties.
Urethane casting material
is very popular for product prototypes.
It can be formulated to imitate elastomers,
and can have structural properties similar
to high-impact stryrene. It can mimic production
parts in material properties, thermal properties,
color, and surface texture. It can also
be machined, sanded, glued, and painted
A Rapid Prototype (usually
SLA) is used as a master pattern to form
the rubber mold. The SLA part is made 0.003
inch/inch oversize to compensate for rubber
shrinkage. The pattern is sanded and primed.
The master pattern is surrounded by a parting
surface that establishes the parting line
of the mold. The liquid RTV rubber is poured
over the pattern and parting surface combination.
Once the rubber has solidified, the parting
block is removed and rubber is poured over
the pattern and mold half combination. The
pattern is removed to yield the mold halves
of a rubber tool.
Curing time for the mold
is dependent on the product and curing agent.
Times range from 30 minutes to over 40 hours.
Adding heat will speed up the curing process
significantly. Aging the mold at room temperature
for up to 72 hours, if possible, will increase
the productive life of the mold.
Liquid urethanes, thermoset
materials, are then poured or injected into
the rubber mold. The urethane cures to solidify.
The RTV tool is then separated to remove
the cast urethane prototype. Casting methods
- Gravity casting:
The liquid urethane is poured into the
RTV mold. This method relies on gravity
to feed the material into the cavity of
- Vacuum Cast Molding
(VCM): Vacuum Cast Molding is a new
technology for producing functional thin-walled
complex prototypes parts in plastic materials
which stimulate ABS, Nylon, Polycarbonate
and other production materials. Features
as small as a finger print can be reproduced
with the vacuum casting process. Unlike
conventional RTV molding, the Vacuum Casting
process produces quantities of 15-25 parts
per week and fills complex thin-walled
parts completely void free.
- Thin Wall RIM:
Thin-wall reaction injection-molding (TW-RIM)
can produce copies of rapidly prototyped
parts at a rate of about three or four/hour.
The short cycle time comes from use of
fast-setting polyurethanes and other materials
that gel in less than a minute and which
can be demolded after about 10 min.
One factor that makes
such fast cure times possible is the
development of inexpensive disposable
mixing heads. They automatically mix
two-part resins in a vacuum and shoot
the material into the mold. Previously,
molders had to mix the materials by
hand, put the mixture in a vacuum chamber
to de-air it, then send the material
to an injector head. These time-consuming
steps made it impractical to shoot material
that gelled in less than a minute.
The cost of injection
equipment for fast-setting resins has
dropped, boosting the technique's popularity
all the more. When it first debuted,
the least expensive injection system
ran about $20,000. Newer equipment for
handling fast-setting resins carries
a price tag of around $4,000.
An additional benefit
of quick curing is that it can substantially
lengthen the life of a mold. This is
because the curing process plays a significant
role in disintegrating the mold. Thus
a mold that might normally last through
25 to 50 shots could be expected to
provide 50 to 100 shots when used with
- From pattern to first
part: 3 - 7 days
- Production Rate:
- Typically a mold can
be shot 1-3 times per day. A single mold
can sometimes produce up to 10 pieces
at once depending on the part complexity
- Types and Quantities
of Parts Made:
- Polyurethane 15 to
- Polyurea 10 to 60
- Epoxy 10 to 30
- Investment Wax Patterns
50 to 300+
- Low Melt Metal Alloy
20 to 75
- Polyurethane Foam
0 to 200+
- Silicone Rubber 20
- As accurate as the SLA
prototype (+/-0.005"); however, tolerances
of +/-0.002" have been achieved.