RESIN CASTING FOR ROCKETEERS


Introduction

Nose cones, fins, transitions, and other model rocket parts can be easily 
produced using resin casting techniques.  The time and effort that go into
fabricating a custom nose cone or a built-up fin need only be expended 
once to produce a master pattern, around which an RTV 
(room-temperature-vulcanizing) rubber mold will be poured.  The mold can 
then be used to cast many duplicates of the original part using two-part 
room-temperature-curing polyurethane resin.

This article is not intended to be a comprehensive guide, but rather a 
primer that will cover most of the unique molding and casting situations 
that model rocketeers will encounter.  Although the RTV rubbers and 
resins are neither corrosive nor highly toxic, it is advisable to use 
them only in a well-ventilated area away from open flame or sparks.  
Before using the materials, read and understand the Material Safety Data 
Sheets and the manufacturer's safety instructions that are included with 
the RTV rubbers and resins.


Creating A Master Pattern

To produce an RTV rubber mold, a high-quality master pattern must first 
be made.  A master can be made of any material that does not inhibit the 
curing of the RTV rubber.  Balsa, cardboard, hardwood, plastic, metal, 
and sulfur-free "plasticene" modeling clay are suitable materials.  
Porous materials such as balsa must be sanded and sealed before the RTV 
can be poured over them.  The master should also be clean and dry.  RTV 
silicone rubbers and polyurethane resins will not cure properly if 
moisture is present on the master or mold.

To make a nose cone master, for example, a hardwood nose cone can be turned 
on a lathe or a drill press, then sanded and sealed.  The nose should be 
sprayed with a wax-based release agent (Synair's Synlube 531, for 
example) and allowed to dry.  The dowel at the nose's rear can be pressed 
into a smooth bed of sulfur-free "plasticene" modeling clay (other clays 
can inhibit curing in the RTV rubber).  The dowel is pressed into the 
clay until the back of the tenon is flush with the clay surface.  A 
superior master can be made by turning polycarbonate, lucite, lexan, 
nylon, or aluminum rod stock on a lathe.  Masters made of these materials 
require neither sanding & sealing nor a release agent.  A transition or a 
boat-tail would be produced in a similar fashion.  Consult Peter Alway's 
"The Art of Scale Model Rocketry" (Saturn Press, 1994) for more 
information on turning parts.


Making The Mold Box

A mold box must be built around the master.  It can be cardboard, wood, 
metal, or plastic.  Porous box materials must first be sealed with a 
wax-based mold release or be covered with plastic tape like scotch tape.  
An easy way to make a small mold box is to roll a piece of sheet styrene 
into a cylinder, tape the seam, and stick it into the clay around the 
master.  There should be 1/4" to 1/2" of space between the master and the 
walls of the mold box.  The walls should be 1/4" to 1/2" higher than the 
master. 


Mixing & Pouring The RTV Rubber

The RTV silicone rubber can now be mixed.  RTV silicones consist of a 
rubber and a catalyst, and most are mixed in a 10:1 ratio (rubber to 
catalyst) by weight.  For one-piece molds, use a low-durometer (soft) 
rubber no harder than a Shore A15 (Ace Resin's RTV and Dow Corning's HS-III
RTV are A12).  The rubber and catalyst are weighed (a postal scale is 
good for this) and mixed in plastic containers (do NOT use wax-lined or 
styrofoam cups).  Pour the catalyst into the rubber and fold the rubber and 
catalyst together, being careful not to whip air into the mixture.  
Scrape the bottom and sides of the mixing container to make sure the 
rubber and catalyst are thoroughly mixed.  It is often helpful to 
transfer the material into a second container to complete the mixing.  
Any unmixed material could leave uncured sticky spots in the mold cavity.

If you have access to a vacuum chamber, it is best to de-air the rubber 
before pouring it into the mold.  De-airing prevents air bubbles from 
forming in the mold.  If a bubble is on the surface of the master, every 
casting will have a raised "bump" at that spot.  If you de-air the 
rubber, it should be in a container that is three to five times as high as 
the level of the rubber.  The rubber will foam up to this height, then 
settle back down as the air bubbles escape.

If you don't have a vacuum chamber, pour the rubber slowly in a thin 
stream into the mold box.  Pour it as far from the master as possible and 
let it rise up over the master of its own accord.  This is advisable even 
if the rubber has been vacuum de-aired.  The low durometer rubbers 
usually have low pouring viscosities, and some of them (Ace Resin RTV, 
Alumilite "Quick-Set" RTV, and Dow Corning HS-III & 3110) do not require 
vacuum de-airing.  Some RTV rubbers, such as Alumilite "Quick-Set," will 
cure in as little as four hours, but for a long mold life it is best to let 
the rubber cure for at least twenty-four hours before attempting to use 
the mold.  After the rubber has cured, carefully remove the clay from the 
master and then carefully remove the master from the mold.


RTV Rubber Versatility

The low-durometer RTV rubbers permit one-piece molds to be made to cast 
parts that would otherwise require multi-piece molds.  For example, a 
"faceted" Nike fin with a through-the-wall mounting tab can be cast in a 
one-piece mold.  The master pattern for the fin would be built-up using 
cardstock, sheet styrene, or waferglass bonded to an internal framework 
(see Peter Alway's "The Art of Scale Model Rocketry" (Saturn Press, 1994) 
for details).  A cardstock master should be sprayed with a wax-based mold 
release.  The mounting tab would be made longer than usual so that the 
excess length could be pressed into a smooth bed of plasticene modeling 
clay.  A mold box would be built around the pattern and the RTV rubber 
would be mixed and poured.  After the rubber cured, the clay would be 
removed from the mounting tab and the mold.  To remove the master from 
the mold, the mold's bottom (which would be inverted to pour a casting) 
would be carefully slit with a modeling knife along a line passing down 
the fin root.


Casting A Part

An RTV silicone rubber mold is self-releasing and requires no release 
agent.  However, mold life will be extended if a release agent is used.  
A silicone-based or wax-based release agent may be used.  Another good 
release agent is talcum powder.  It is especially useful in deep-draw 
molds such as nose cone molds.  Drop a few pinches of powder into the 
mold, cover the opening, and shake it until the mold cavity is dusted 
with the powder.

The resin is very easy to measure and mix.  Most polyurethane resins (Ace 
Resin, Alumilite, Por-A-Kast, etc.) are mixed 1:1 by volume.  Equal 
volumes of the two parts are poured into separate containers.  The "A" 
(or thinner) side can be poured into the "B" (or thicker) side and mixed, or 
both sides can be poured into a third container and mixed.  Stir 
vigorously for 10-15 seconds or until a uniform color is achieved.  
Slowly pour the resin into the mold.  The resin should cure within 2-3 
minutes.  The curing resin will get quite hot (up to 215 degrees F for 
Alumilite), so let the casting cool to room temperature before attempting 
to remove it from the mold.  The casting is still pliable while it is 
warm, so it could be distorted if it is removed from the mold before it 
cools to room temperature.


Fillers, Dyes, & Paints

The resins can be filled with any DRY filler (moisture reacts with the 
resins and produces carbon dioxide bubbles) up to 100% by volume.  
Aluminum powder can be added to give the cured part a metallic 
appearance, or microbulb filler (microballoons) can be added to lighten 
it so much that it would float on water.  Several coloring dyes are 
available from Alumilite.  The cured resins can be painted, stained, and 
plated.    


Bonding Agents

The cured resins can be cemented to themselves and to kraft paper body 
tubes, balsa, cardstock, other plastics, and fiberglass with epoxy and 
cyanoacrylate adhesives.  The resins can also be used as adhesives 
themselves.  They adhere to metal, soft wood, and plastic.


Casting One-Piece Nose Cones

To cast a solid, one-piece resin nose cone (with tenon) in a one-piece 
mold, first fabricate a nose cone master as outlined above.  Press it 
into plasticene modeling clay, build a mold box around it, then mix and 
pour the RTV rubber as described.  Nose cones can be cast with a screw eye 
already installed.  Slide the screw eye's eyelet onto a dowel or a rolled 
paper "stick."  After pouring the liquid resin into the mold, hold the 
dowel or "stick" by the ends and insert the screw eye into what will 
become the back of the tenon, holding it steady until the casting 
solidifies.  A simple jig can also be made to hold the dowel steady.  A 
transition mold and casting would be made in the same way. 


Molding A Two-Piece Hollow Nose Cone

Solid cast resin nose cones are fine for small models.  At about 0.976" 
(BT-50) size, however, even a microballoon-filled resin nose cone may be too 
heavy to be practical.  Using resin casting techniques, it is possible to 
make lightweight two-piece hollow nose cones.                              

Two masters are required for the nose cone (with tenon) and for the nose 
cone base--an outer shape and an inner "plug" for each piece.  In the 
case of the nose cone, the outer shape would be turned on a lathe or a 
drill press.  The plug would be turned in the same way, and it would have 
the same shape as the outer shape.  The plug would not have a tenon, 
but rather a cylindrical section on the back that would have the same 
length as the outer shape's tenon.  The plug's radius should be at least 
3/32" less than the radius of the outer shape, and the plug's length 
would be proportionally less than that of the outer shape.  The alignment 
pins (either turned with the piece or a dowel cemented into the blank 
before turning) on the plug and the outer shape will serve to center the 
two pieces in the next step.

Take the outer shape and press its alignment pin into a smooth bed of 
plasticene modeling clay.  Press it in until the back of the tenon is 
flush with the clay's surface.  Use a dowel or other object to make 
several depressions in the clay around the master, and build a mold box 
around these.  The rubber will flow into these holes and create keys 
which will align the two mold halves.  Mix and pour an RTV rubber as 
described above and set it aside to cure.  After the rubber has cured, 
carefully remove the clay from the master and the mold, but leave the 
master in the mold.  Next, either brush a rubber-to-rubber mold release 
ONLY on the exposed upper mold surface, or spray a wax-based mold release 
on the upper mold surface and the exposed part of the master.  After it 
has dried, build a dam around the edges of the mold (sheet styrene taped 
around the edges is sufficient).  Mix another batch of RTV and pour it 
onto the mold surface, then set it aside to cure.  After the rubber has 
cured, separate the two halves of the mold.  Remove the outer shape from 
the upper mold half.

Before casting a nose cone, insert the plug's alignment pin into the 
upper mold half and push it all the way in.  Spray the plug with a 
wax-based release agent and allow it to dry.  Mix the resin and pour it 
into the mold cavity, then slowly press the upper mold half into place.  
Allow the casting to cool to room temperature before removing it from the 
mold.

The plug must be sprayed with the release agent before casting each nose 
cone.  If desired, a second all-RTV upper mold half can be cast.  To do 
this, leave the hollow cast resin nose cone in the mold cavity.  Spray 
the exposed upper mold surface with a wax-based release agent and let it 
dry.  Build a dam around the mold edges as before.  Mix a batch of RTV 
rubber and pour it onto the upper mold surface.  Pour it away from the 
hollow nose cone and let the rubber slowly run down into the nose cone.  
After covering the upper mold surface (1/4" to 1/2" deep should be 
enough) with the rubber, set it aside to cure.  The new upper mold half will 
have an integral plug.  The nose cone base mold would be produced in the 
same way, and transition and boat-tail molds would use similar techniques.


"Pseudo Blow-Molded" Hollow Nose Cones

There is an easier way to make hollow nose cones.  It requires only one 
master pattern.  This method works best if the tenon has a truncated cone 
on its rear like blow-molded nose cones do, hence its name.  Turn the 
master on a lathe or a drill press.  It needs to have an alignment pin in 
the center of the tenon rear (the truncated cone).  Press the master into 
a smooth bed of plasticene modeling clay, all the way up to the truncated
cone/tenon interface.  Use a dowel or other object to make depressions in 
the clay around the master, then build (or place) a mold box around 
these.         

Because this casting method requires that the mold be held and rotated, 
it must be protected from being squeezed during casting.  There are two 
ways to do this.  The mold can be cast into a rigid cylinder (a length of 
plastic pipe, for example) or into a rigid box.  Another method is to 
cast the mold in a temporary mold box, then brush on a thixotropic (a 
thixotrope is a gel that will not run when applied to a surface) mold 
support shell resin.  Synair makes a mold support shell resin called 
Mother Mold, and other formulations are available from other casting 
suppliers.  If you go this route, leave the master in the mold while 
brushing on the mold shell support resin.  The mold itself will be a 
two-piece mold, and the piece covering the truncated cone will be the 
smaller of the two.  Brush the mold shell support resin onto the entire 
mold except the top of the smaller piece.  The cured mold support shell 
will be a rigid cylinder or box, open at the top so that the upper mold 
half can be removed.

Mix a batch of RTV rubber and pour it in a thin stream into the mold box.   
Pour it away from the master and let the rubber slowly rise up over the 
master.  Set aside to cure.  After the rubber has cured, carefully 
remove the clay from the master and the mold, but leave the master in the 
mold cavity.  Brush a rubber-to-rubber mold release ONLY on the exposed 
upper mold surface, or spray a wax-based release agent on the upper mold 
surface and the exposed part of the master.    

Build a dam around the upper edges of the mold (if you're not casting it 
in a rigid cylinder or a rigid box).  Mix a batch of RTV rubber and pour 
it over the mold surface.  Pour enough rubber that it rises almost to the 
top of the alignment pin on the truncated cone, then set it aside to 
cure.  After the rubber has cured, separate the mold halves and carefully 
remove the master from the mold.  The opening in the upper mold half left 
by the alignment pin is the channel into which the casting resin will be 
poured.

To cast a nose cone, assemble the mold halves and insert the mold into 
its cylinder, box, or shell.  Cut a piece of pressure-sensitive adhesive 
paper (or a pressure-sensitive mailing label) large enough to cover the 
opening in the upper mold half.  Cut or tear off a piece of masking 
tape.  The tape will be used to keep the upper mold half from falling 
out of the box or shell during casting.  Insert a funnel into the opening 
in the upper mold half.  Mix the casting resin and pour it into the 
funnel, then quickly remove the funnel and press the pressure-sensitive 
adhesive paper over the opening.  Using the masking tape, tape the upper 
mold half into place.  Gently rotate the mold in all directions so that 
the resin coats the entire surface of the mold cavity.  Rotate the mold 
until you can no longer hear the resin flowing inside, then set it aside 
to cure.  The casting will be rather thin, so make sure it has cooled to 
room temperature before attempting to remove it from the mold.  If 
desired, leave the casting in the mold and repeat the casting process to 
build up the casting's thickness.  Hollow transitions and boat-tails can 
be cast in the same way.


Molding & Casting Materials Suppliers

Several suppliers of molding and casting materials are listed in Section 
02 of the rec.models.rockets Frequently-Asked-Questions (FAQ) archive.


 
 




James J. Wentworth
d005794c@dcfreenet.seflin.lib.fl.us










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