Silicone rubber is commonly used for making molds across various industries, but sometimes, silicone molds exhibit an oily residue, which can be concerning. This issue, often referred to as “bleeding oil,” is a common question when customers seek technical advice.
It’s important to note that silicone mold rubber itself does not contain oils, so the mold should not bleed oil by default. While a small amount of oil bleeding can be considered normal, it becomes problematic when large amounts of oil appear on the surface.
The primary cause of oil bleeding in silicone molds is the use of too much silicone oil or the incorporation of a compound silicone oil mixture, which includes both silicone oil and white mineral oil, during the mold-making process. White mineral oil is a petrochemical and not pure silicone oil, and some manufacturers use this mixture as a cost-saving measure since pure silicone oil is more expensive than white mineral oil.
To prevent oil bleeding and ensure high-quality silicone molds, it’s crucial to distinguish between pure silicone oil and ungraded silicone oil. Here are some key differences:
- Visual and Olfactory Inspection: Ungraded silicone oil tends to be a turbid liquid with an unpleasant smell, whereas pure silicone oil is clear and odorless.
- Burning Test: Ungraded silicone oil burns at around 100°C, while pure silicone oil has a much higher burning point of about 320°C.
- Freezing Test: When exposed to temperatures as low as -50°C, pure silicone oil remains clear and transparent, whereas ungraded silicone oil may appear cloudy or change in consistency.
Another reason for oil bleeding in silicone molds is the use of an excessive amount of curing agent (catalyst) during production. To avoid this, it’s recommended to reduce the curing agent ratio. Using too much curing agent can cause the silicone oil to bleed from the rubber.
In summary, to avoid oil bleeding issues in silicone molds, it’s important to use high-quality, pure silicone oils and carefully monitor the amounts of curing agents and additives during the mold-making process.