Mold Making Guide

A Primer on Silicone RTV


RTV stands for "Room Temperature Vulcanizing". This means that the silicone will cure at room temperature, without the need of expensive oven heating. However, the higher the ambient temperature, the quicker RTV cures.

There are two basic types of silicone mold making products available. They are Polycondensation or Tin Cure and Polyaddition or Platinum Cure.

The Polyaddition/Platinum Cure is the most expensive of the two. It offers better mold stability and less shrinkage. However it is susceptible to inhibition from a wide variety of materials, and must be handled very precisely to produce a good cure.

Polycondensation/Tin Cure silicone is less expensive, has a virtually trouble-free cure, and is not sensitive to inhibition. This is the kind of silicone in TAP Silicone RTV System.

On the following pages you will see instructions using the Rhodorsil RTV2. Please note that even though the name is different, the instruction also applies to TAP Silicones. We hope you find these pages helpful in your mold making journey.

TAP Silicone RTV System for Imprint and Reproduction

TAP Silicone RTV's are used in moulding are two-component, room temperature curing elastomers.

TAP Silicone RTV's silicone elastormers have specific properties which provide users with many advantages;


Flexibility
Easy to demould
Fine reproduction detail
High tear strength
Room temperature curing, no heat release
Easy to use, without any heavy equipment
Aging resistance
Heat stability
Dielectric properties
Low shrinkage
 
There are 2 different types of RTV2;
1. Polycondensation RTV2 (PC)
2. Polyaddition RTV2 (PA)

Polycondensation type TAP Platinum Silicone is supplied as base and catalyst, polyaddition type TAP Silicone RTV System is supplied as Part A and Part B.

Each of these types have specific features that means it must be chosen according to the user's requirements.

 

POLYCONDENSATION RTV2

POLYADDITION RTV2
MIXING RATIO 100/2 - 100/10 100/10 - 100/100
SAFETY PRECAUTIONS
DURING MIXING
Glasses and gloves are required, Well ventilated working area None
POT LIFE / DEMOULDING TIME Variable depending on the catalyst, Not heat accelerated Can be independent,
Heat accelerated
RISK OF INHIBITION No Yes
SENSITIVE TO REVERSION Yes No
SHRINKAGE %0.8 - 1.2 D=%0.05 - 0.2

Curing starts once the two components are mixed at a rate that depends on the operating conditions. The ideal conditions are;
23°C ambient temperature
50% relative humidity

In order to comply with dimensions it is preferable to use;
moderate temp. 23 - 30°C and
in any case no less than 20°C
minimum 30% humidity
comply with the recommended
catalyst dosage

SILICONE RTV Application

There are numerous moulding applications for SILICONE RTV's, a few of them are mentioned below.

EXAMPLES OF USE MATERIAL







Furniture
Figurines
Reproduction of Statuettes
Frames, Mirrors, Paintings
Buttons
Souvenirs
Palaeontology
Reproduction of Old Objects
Various Decorative Items
Prototypes
Leather Goods
Shoes
Interior Car Trim
Cornices
Production of Tooling
Staff
Master Moulds for Tiles
Ceramics
Porcelain
Decorative Candles
Artistic Metalwork
Jewellery

Polyester


Polyurethane

PVC

Epoxies

Concrete

Plaster

Wax - Paraffin

Low Melting Point Alloys

Acrylic Resins

Preparing the Mold

Models are made in a wide range of materials whose surface conditions and porosity differ. Even though SILICONE RTV does not adhere to most materials, it is usual to take certain precautions before taking impressions. Various possibilities are offered below which are tested in laboratories.





TYPE OF MODEL RECOMMENDED TREATMENT
Plaster, Concrete,
Stone, Biscuit,
Terracotta, etc.

Remove dust and apply a pore filler from among the following:Polyvinyl alchol type 4/125 diluted Soap, dissolve 250g of glycerine soap in one liter of boiling water; use once cool
Wax or parafin deposit (diluted to 5% in xylene)
Vaseline deposit
Varnishing; gum lacquer or cellulose or acrylic varnish

Metal

Degreasing with a solvent or washing with a water based solution made with 5% liquid detergent, then drying
Wax

Glass, Porcelain,
Ceramics

Apply a very thin coat of vaseline

Leather

Apply a very thin coat of vaseline
Wax

Wood

Apply vaseline or wax, a cellulose pore filler or cellulose varnish

Plastic

Degrease with appropriate solvent for the plastic concerned

RTV2

Apply a very thin coat of vaseline
Wax

Resins
(Polyurethane, epoxy, polyester, acrylic, etc)

Apply polyvinyl alcohol then a very thin coat of vaseline
Wax

Wax

No treatment required

Moulding Techniques

SILICONE RTV enable moulds to be made for all types of applications using a few basic principles. In spite of the multiple variants that are possible, basic processes exist which are described below.

     

MODEL FEATURES

  MOULDING
TECHNIQUE 
APPLICATION
TECHNIQUE 
ADVANTAGES  DISADVANTAGES
Flat back,
few or no
undercuts
One Part
Block
Moulding
Casting Quick,easy to
perform
Self supporting
moulds
Low cost to
produce
Limited to
relatively simple
shapes that do
not vary much
in cross section
- Significant
material
consumption
Fully
structured,
few or no
undercuts
Two Part
Block
Moulding
Casting Quick,easy to
perform
Self supporting
moulds
Low cost to
produce
Limited to
relatively simple
shapes that do
not vary much
in cross section
- Significant
material
consumption

Flat back,
accentuated
undercuts

 

Fully
structured,
few or no
undercuts

One Part
Skin
Moulding

 

Two Part
Skin
Moulding

Casting Thin section
membranes
Economic in
materials (RTV)
Large degree
of flexibility
favouring
difficult
moulding
operations
Availability
of tooling
enabling the
production of
membranes
Longer to
implement
than the block
moulding
method
Greater cost of
production
  Little or
accentuated
undercuts,
Voluminous
model on base,
On-site
reproduction
One Side
Impression
Moulding
(Brush
Moulding)
Brush,
Spatula,
Spraying
- Possibility of
taking the
imprint on site
- Possibility of
taking vertical
or overhanging
imprints
- Adapted to
large dimension
mouldings
- Economic in
materials (RTV)
- Difficult to
perform
- Membrane
is not
interchangeable
between backing
moulds
- Longer to
implement
than the block moulding
method
Little or
accentuated
undercuts,
Fully
structured,
Voluminous
model that is
not easy or
that can not
be moved
Two Part Impression Moulding
(Brush Moulding)
Brush,
Spatula,
Spraying
- Possibility of
taking the
imprint on site
- Possibility of
taking vertical
or overhanging
imprints
- Adapted to
large dimension
mouldings
- Economic in
materials (RTV)
- Difficult to
perform
- Membrane
is not
interchangeable
between backing
moulds
- Longer to
implement
than the block moulding
method
     

Advice to Users

 

Remix each of the two components of the
SILICONE RTV everytime berfore using.

Mix the two components in the recommended proportions.

 

 

Mix by using either a spatula or a mechanical mixer on a low speed setting so as to limit the inclusion of air in the mixture, until a perfectly uniform mixture is obtained.

 

 

 

 

Degasing
The catalysed mixture is degased under a
vacuum of 20-50 mbar. The RTV will expand under the action of the vacuum and it is therefore necessary to have a container of appropriate size. Degassing is not required for TAP Platinum Silicone. It's low viscosity provides easy bubble release.

 

 

 

 

Degasing generally lasts several minutes and at least until the bubbles stop rising regularly to the surface when the vacuum is released.

However, prolonged degasing can cause loss of certain components that are required for curing.