Producing a Silicone Mold

Safety and Precautions: Please refer to the Material Safety Data Sheet which accompanied your shipment. All silicone rubber products should be used with good ventilation. Skin and eye contact, ingestion and breathing of dusts and vapors should be avoided. It is recommended you use gloves, dust masks, and eye protection. If you did not receive a Material Safety Data Sheet, please contact us before proceeding via the web at: www.quantumsilicones.com or by calling (800) 852-3147.

Supplies needed: include gloves, suitable silicone mixing containers, stirring sticks, a gram scale, moldmaking box and silicone materials. There are several web sites which can explain how to build an actual silicone moldmaking materials box. A brief summary of common mold boxes is listed in the section entitled "Types of molds".

Determine your Chemistry: Quantum Silicone produces two systems of moldmaking materials; Addition cure (platinum catalyzed) and Condensation cure (tin-catalyzed). Please refer to the technical data sheet corresponding to your silicone product of choice to determine which system was recommended for your moldmaking application. Features of each silicone chemistry are listed below. If a data sheet did not accompany your shipment a copy can be accessed via the web at: www.quantumsilicones.com or by calling (800) 852-3147.

	Polycondensation (PC)	Polyaddition (PA)
Mixing ratio	10:1	10:1
Pot life and demold time	Varies depending on catalyst; Not heat accelerated	Room temperature or can be accelerated with heat
Risk of inhibition	Unlikely	Yes*
Shrinkage	Less than 0.3	nil
Durometer range (shore A)	7 to 40	30 to 70

*refer to inhibition information under commonly used terms

Commonly used terms:

RTV Silicone: Temperature Vulcanization (room temperature cure).

Viscosity: Describes the consistency of a liquid. It is reported in centipoises (cps.). The lower the reported number, the thinner the liquid. For example; water has a viscosity of 1 cps. while chocolate syrup will measure higher at approximately 1000 cps. Catalyzed viscosities of silicone moldmaking materials range between 7,500 and 105,000 cps.

Hardness: This property can be measured with an instrument called a Durometer gauge. The gauge has a needle which is pushed into the silicone rubber. The farther the needle is able to penetrate into the rubber, the lower the reading. The Shore A scale is used for reporting durometer of most molding rubbers. The scale of measurement is 0 to 100. The higher the reading, the harder the silicone rubber. The durometer requirement of your silicone is dependant upon your applications. If you are not sure what durometer you may require, contact our technical service team at (866) 303-5327.

Pour time, working time, pot life or gel time: This is how much time the user will have before the silicone moldmaking material begins to gel. Your pour should be completed well before the end of this time to insure good flow into the details of the mold and to allow any air bubbles to rise to the surface.

Demold time: This is the soonest recommended time a cured silicone rubber part can be removed from the mold if the mold was cured at room temperature.

Cure time: Even though a silicone rubber product can be demolded and feels cured, curing may continue for up to one week.

Linear Shrinkage: This is the reduction in size which a user may experience during the life cycle of a silicone rubber mold. The shrinkage is due to the release of a cure by product in condensation cure systems, as well as "oils" being drawn out of the mold by aggressive casting resins. QSi addition cure products are noted for low shrinkage. Contact our technical team to determine which silicone moldmaking materials are best for our application if shrinkage is a concern.

Tear Strength: Tear strength is a measurement of a rubbers ability to tear.

Inhibition: This is a result of a poisoning of the cure catalyst which stops the cross linking necessary for the silicone rubber to properly cure. The result of inhibition is tackiness at the pattern interface or a total failure of the silicone elastomers to cure.

Addition Cure inhibition: Materials known to inhibit cure are sulfur-containing modeling clays, natural rubber such as latex and rubber gloves, masking tape, amine or sulfur-containing material and condensation cure (tin-catalyzed) silicones.

Condensation Cure inhibition is not common. Some sulfur containing modeling clays can slow the cure of condensation RTVs. However, the condensation cure RTV silicone will eventually cure when the clays are removed.

Inhibition can be avoided by applying a barrier coat as describe in the section entitled "Preparing the Model" section of this guide.

Types of Molds:

Practice makes perfect! For first time silicone mold makers, we recommend you make a small rubber mold for practice before attempting to mold a valuable piece.

There are four common types of molds:

• Block mold: one piece with no negative drafts of undercuts. This is probably the easiest mold to produce. The original part should be placed in the box so the silicone is able to flow around it. If the part is simple, it can be placed in a plastic cup, a disposable plastic container or a wooden box (pine wood not recommended). If you choose a wooden box make sure to seal it properly if it appears to be porous.
• Block mold-multiple piece: This silicone rubber mold can be made exactly like a block mold and simply cut in half once it cures to remove the part. If you prefer not to cut the mold, partially fill the container and allow it to cure. Then apply a thin layer of release agent, and finish pouring the mold. When the second layer is cured, the mold should easily separate.
• Glove mold-brushed onto the pattern: This requires a thixotropic agent which allows the silicone rubber to be brushed on an original piece without running off the surface. The thixotropic agent can be supplied in the catalyst or as a separate additive. This type of silicone is ideal for building restoration. The silicone rubber can be bushed on an original piece of crown molding, allowed to cure and then be removed. The mold can then be taken back to the mold shop where parts can be cast to produce replications of the original molding. This type of silicone rubber molding is also becoming popular to cast rocks or stones to create molds of fake rocks, rock climbing walls, side walks and props for theme parks.
• Cast Glove mold: This is made by creating a void around the original part in a consistent thickness. It is usually done by forming modeling clay around the pattern and then forming a hard shell around the clay. The shell can be made by using fiberglass, urethane or plaster. After the shell has cured, the clay is removed; creating a void which can be filled by silicone rubber.

Preparing the Model:

Porous surfaces, such as wood, should be dried and sealed. A clear acrylic spray lacquer is an acceptable sealer. Also applying a release agent, such as petroleum jelly will aid in easy release of a silicone mold. Because silicone rubber can sometimes bond to surfaces which contain silica, such as glass, cements and natural stone; it is recommended to do a spot test for possible adhesion.

Mixing and Curing:

Stirring individual components before use is recommended. Make sure to weigh and measure accurately. Close containers tightly after use. Products should be stored in their original, sealed containers in an environment that does not exceed 90F. Under these conditions, the product will achieve the expected shelf life. Please refer to the individual products technical data sheet to determine the products expected shelf life.

The following procedure is an example of a 10:1 mix ratio addition cure product. Please refer to the product's technical data sheet for mix ratio information:

Mixing: Charge 100 parts by weight of "Part A (base)" and 10 parts by weight of "Part B (catalyst)" into a clean, compatible metal or plastic container. The volume of the container should be 3-4 times the volume of the material to be mixed. This allows for expansion of the silicone rubber as it de-gasses.

Mix thoroughly by hand or with mixing equipment while minimizing air entrapment until a homogeneous mixture is obtained. This will occur when the silicone moldmaking material takes on a uniform color with no visible striations. Scrape the sides and bottom of the container thoroughly several times while mixing. Once mixing^* is complete it is recommended that the silicone moldmaking material be de-aired 2-3 times by intermittent evacuation for a few minutes to minimize any imperfections due to bubbles in the cured material. Typically after releasing the vacuum 2-3 times the silicone moldmaking material mass will collapse on itself at which time the vacuum should be left on only 2-4 minutes longer.

^*Dispensing machine mixed material does not normally need to be de-aired.

Pouring the Mold:

To pour the mold, begin by pouring a stream of silicone rubber into one corner of the mold box. Allow the liquid to push the air out ahead of it to avoid inducing air into the mold. If you have a highly detailed mold, some moldmakers recommend applying mixed silicone rubbers directly onto the detailed area and applying low pressure air before pouring the remainder of the mold.

Curing the Mold:

Please refer to the Cure Characteristics listed on the technical data sheet for the individual products. If a schedule is not listed, please contact our technical service team at (866) 303-5327.

Demolding: Please refer to the recommend time to demold on the technical data sheet for the individual products. Remember to carefully demold the silicone rubber at this point, since the mold has not yet achieved a full cure.

Storing the Mold:

It is best for the silicone rubber mold if it can be stored in the original mold box or on a surface that will stress them the least. If left in a distorted shape for extended periods of time, the silicone rubber mold may not be able to return to the original state. Keep of out sunlight and in a cool, dry area if possible.