Medium Hardness Platinum Cured Silicone Rubber for Mold Making

Step 1: Model Preparation

The master pattern should be thoroughly cleaned and dried. If the master pattern is fragile, weak, or porous, it may be necessary to apply a consolidant or sealant, and possibly fill in deep holes or crevices.

Step 2: Applying Release Agent

In most cases, a release agent is not necessary when using platinum-cure silicone. However, when the master pattern is highly porous, has a rough surface, numerous cracks or undercuts, or is otherwise difficult to demold, a thin layer of release agent may be recommended. The release agent should be compatible with both the platinum-cure silicone and the material of the master pattern.

For food-contact applications, make sure that the release agent is suitable for food-contact use and complies with the applicable regulatory requirements.

Step 3: Measuring & Mixing

Mix the desired amount of Parts A and B in a 1:1 ratio, preferably by weight, and thoroughly mix them together. Accurate measurement is essential to achieve the best physical properties of the cured platinum-cure silicone rubber.

If you wish to add color, use only a pigment or colorant that is compatible with platinum-cure silicone. Add a small amount and mix thoroughly until the color is uniform. For food-contact applications, use only colorants that are suitable for such applications.

Step 4: Vacuum Degassing

You can assess the need for vacuum degassing based on the specific requirements of each project, although vacuum degassing is recommended for achieving a high-quality, bubble-free finish. If vacuum degassing is used, place the mixed silicone in a container with a capacity of at least 3–4 times the volume of the material. Apply vacuum until the silicone rises and then collapses. Continue vacuuming until most of the air bubbles have disappeared, while taking care to prevent the silicone from overflowing.

Techniques to Reduce Air Bubbles:

1. Hold the silicone container above the master pattern and allow the silicone to flow slowly in a thin stream. This helps break up air bubbles during pouring.
2. Temporarily tilt the master pattern at an angle and pour the liquid silicone onto the higher end, allowing it to flow downward over the rest of the pattern. Once the silicone reaches the lower end, place the pattern flat and/or tilt or rotate it as necessary to achieve even coverage.

These techniques can be used alone or in combination. In addition, working at a lower temperature can extend the working time and allow more time for air bubbles to escape before curing. Do not alter the Parts A:B mixing ratio to control the working time or curing time.

Step 5: Pouring Silicone

Pour the silicone evenly, making sure that there is sufficient thickness above the highest point of the master pattern to provide the mold with the necessary mechanical strength. As a general guideline, a thickness of approximately 5 mm or more is recommended. However, the appropriate thickness depends on the size, geometry, and hardness of the mold.

Step 6: Demolding the Mold

Under normal conditions, platinum-cure silicone rubber cures at room temperature. At 25 °C (77 °F), the typical demolding time is approximately 4–6 hours. Higher temperatures can accelerate curing, while lower temperatures can increase the time required for the silicone to fully cure.

The actual demolding time may vary depending on the ambient temperature and working conditions. Before demolding, make sure that the silicone has fully cured and that the surface is dry and non-tacky. To achieve the specified final physical properties, a full cure of 24 hours at 25 °C (77 °F) is recommended.