High Tear Strength Tin Cured Liquid Silicone Rubber for Mold Making

Silicone Rubber - White

Silicone Rubber - Translucent

Step 1: Preparing the Master Pattern

The master pattern must be clean and dry. If the master models is friable, weak, or porous, one may need to apply a consolidant or sealant, and possibly plug deep holes or crevices.

Step 2: Applying the Release Agent

This series of silicones has excellent mold release properties, and typically does not require a mold release agent for most materials. However, if the master pattern is highly porous or rough, has numerous crevasses and undercuts, or not prone to spalling, it is recommended to apply a thin layer of mold release agent to facilitate mold release and prolong the life of the mold.

The release agent should be applied evenly over the entire master pattern, ensuring that it is distributed into all crevices and undercuts. However, it should only be applied thinly to avoid compromising the surface detail of the master pattern. If a brush is used to apply the agent, any excess should be buffed away afterwards, and any brush strokes erased. Since silicone rubber captures even microscopic details, the surface of the release agent will be recorded rather than the master pattern.

Common mold release agents include acrylic sprays, Vaseline, silicone oil and soapy water, among others.

Step 3: Measuring and Mixing

For the white tin-cured silicone, it is important to note that a small amount of powder may settle over time during storage. Therefore, it is recommended to stir Part A and shake Part B before mixing. The sediments are not reactants, so there is no issue in using them while stirring.

If you are not in a hurry, it is advisable to use the recommended dosage (100A:3B) of the standard catalyst. If you require different colors, you can add the colorant and mix it thoroughly. It is important to ensure that the base and curing agent are thoroughly mixed and stirred evenly to avoid local non-curing.

Never use more than the maximum recommended dose of the catalyst (100A:5B), as doing so may cause the liquid silicone to begin setting up before it can be properly stirred and poured. This will significantly shorten the lifespan of the silicone mold.

Step 4: Vacuum Degassing

After mixing parts A and B, it is recommended to vacuum degas the liquid silicone to eliminate any entrapped air. When subject to a vacuum, the silicone mixture should well up as air pockets rise and burst. You must pay attention to the working time of the silicone and avoid letting the silicone cure before pouring/application.

Vacuum degassing can be carried out either before or after pouring. However, if you choose to vacuum degas after pouring, your vacuum box should have a sufficiently large volume.

Techniques to Reduce Air Bubbles:

1. One is to apply a thin initial coat of silicone with a fine paintbrush, gently spreading it into all cavities and undercuts (after which more silicone may be poured).
2. Hold the silicone container high above the master pattern and allow it to flow down slowly in a thin stream. This helps to break up any air bubbles as the silicone flows.
3. Temporarily incline the master pattern at an angle and pour the liquid silicone onto the higher end, allowing it to flow down over the rest of the pattern. Once the silicone reaches the lower end, lay the pattern flat and/or tilt or rotate it as necessary to achieve even coverage.

These techniques can be used alone or in combination with each other. Additionally, reducing the temperature and decreasing the ratio of curing agent can extend the working time and help to reduce air bubbles.

Step 5: Pouring Silicone

Pour-on Process: The vacuum-degassed liquid silicone is poured carefully over the master pattern, typically covering it to a depth of 0.5-1.5 cm above the highest point. If the layer is too thin, the mold may be prone to tearing. It is also advisable to apply thicker layers of silicone in narrow or intricate sections and along the edges where extra strength is required to prevent tearing.

Brush-on Process: After the first layer of silicone begins to cure, a strip of gauze, cheesecloth, or open-weave cotton cloth can be lightly applied to increase the strength of the mold. If a second batch is applied, it should be applied while the first batch is partly cured but still tacky. Once a layer of silicone is cured completely, it is difficult to make a good bond with a subsequent layer. After the gauze is applied, apply another layer of silicone to thoroughly cover the gauze layer. Typically the finished mold should be at least 3-4mm thick even in the thinnest sections. Be sure not to push the gauze through to the master pattern surface. When selecting gauze, non-elastic types should be used.

Step 6: Demolding the Mold

After the specified curing time has passed, the outer surface of the mold feels hard and dry, and the hardness of the mold does not change after a period of time, and the mold can usually be removed or demolded. When removing the mold, gently peel up all the edges first, and then the middle section. If any portion seems too soft, immediately stop pulling and replace any lifted sections, allowing more time to cure further.